FIELD OF THE INVENTION

The present invention provides a heat exchange element which facilitates efficient heat transfer. The invention also provides a method of transferring heat to or from a fluid using a heat exchange element of the invention. Processes suitable for producing heat exchange elements, including the heat exchange element of the invention, are also provided, in particular an electroless flow deposition process. The invention also provides a heat exchange element obtained or obtainable by the electroless flow deposition process.

BACKGROUND TO THE INVENTION

The transfer of heat across surfaces is of importance in many products and systems.

Examples of such systems include cooling systems (such as air conditioning systems or refrigeration systems) and heating systems such as boilers. Other examples of such systems include heat recovery systems. A typical configuration of an apparatus for heat exchange in such a system involves the transfer of heat between a heat exchange element and a fluid in contact with the surface of that element. A wide variety of sources may be used to provide heat to the heat exchange element. Examples of such configurations include, for example, heat exchangers (where the source of heat to the heat exchanger is a second fluid in contact with the reverse side of the heat exchanger element), boilers, radiators, refrigerators and so on.

It is therefore desirable to provide a heat exchange element which has very good heat transfer properties. It is particularly desirable to provide a heat exchange element which can efficiently transfer heat to a fluid such as a liquid in contact with the said element. However, the processes involved in heat transfer across a surface, and particularly from a solid surface to a liquid, are complex and poorly understood. It is therefore not a straightforward matter to produce a heat exchange element which has good heat transfer properties, or to optimise existing surfaces to improve their heat transfer properties.

Previous efforts have been made in this field. The approach taken has typically involved maximising surface area of an object intended for use in heat transfer. Some previous workers have attempted to control the heat transfer ability of an object by providing its surface with a specific wettability. In one example, WO 2011/149494 describes a heat exchange surface having a preselected contact angle with a particular liquid. The surface is produced by providing hydrophilic nanostructures on a substrate. The surface nanostructure is formed by depositing oxide-based nanomaterials on a substrate, and the nanostructures have an average root-mean-square roughness, or height, of 200 to 600 nm. A surface thus produced is said to be useful in pool boiling experiments.

Other previous workers have attempted to control the heat transfer ability of a surface by providing precisely engineered structures thereon. The engineered structures are usually made of silicon. An example is found in "Surface structure enhanced microchannel flow boiling", Zhu et al, Journal of Heat Transfer, Vol. 138, pp 091501-1 to 091501-13. A microchannel is provided having an array of silicon micropillars, and is said to promote heat transfer in a flow boiling regime.

The present inventors have previously provided a nano-rough surface having a hierarchical nanostructure for use in heat transfer, described in WO2014/064450. It was found that, by carrying out electroless deposition for a limited period of time, a coating having a hierarchical nanostructure could be produced on a substrate. Typically, the hierarchical nanostructure comprised a first-level structure coated with a second level of structure of ten or one hundred times smaller size. Typically, the first-level structures were up to 500 nm high and the second level of structure comprised features up to 50 nm high. These surfaces were shown to effect heat transfer in flow boiling experiments.

It is an object of the invention to provide a heat exchange element having heat transfer properties comparable to or better than the heat exchange elements discussed above, and that is suitable for heat transfer to a wide range of fluids. Improved heat exchange properties allow a heat exchange element of the invention to cool a heat source faster and via a smaller element, saving space and weight.

In addition to providing a heat exchange element with good heat transfer properties, it is an object of the invention to provide a process for producing a heat exchange element. Many known methods of producing heat exchange elements are laborious and costly. These known methods are commonly methods for increasing the surface area of an object intended for use in heat transfer

One previous example of a method of producing a heat transfer surface is described in "Surface structure enhanced microchannel flow boiling", Zhu et al., Journal of Heat Transfer, Vol. 138, pp 091501-1 to 091501-13. In that method, engineering of silicon structures on a surface was used to increase the surface area of an object. The methods used to engineer the silicon structures include ion etching of a silicon substrate and bonding of a silicon wafer to a silicon surface.

The present inventors have previously described the use of electroless deposition to create a coating on a heat exchange element (WO2014/064450). In that case, the electroless deposition process created a nano-rough surface and the method involved placing a substrate in a bath of electroless deposition solution. However, the electroless deposition of a metal in a bath process suffered from a bubble adhesion issue. Electroless deposition of a metal on a substrate usually produces bubbles of hydrogen gas at the surface of the substrate. It was found that, during the bath process, bubbles of hydrogen produced during electroless deposition stuck to the substrate surface and caused the coating to form around the bubbles. This had two particular adverse effects. Firstly, the rough structure of the coating formed by the electroless deposition process was disrupted by the presence of bubbles, causing gaps in the coating and/or portions of the coating not having the desired rough structure. Secondly, the coating formed by electroless deposition was formed over the bubbles, leading to portions of the coating not in contact with the substrate. These non-adhered portions of the coating were found to be fragile and frequently peeled away from the substrate over time, for instance during use of the coated substrate in heat exchange. This was undesirable as it reduced the heat exchange efficacy of the coating. Moreover, the exposed portions of the substrate caused by the hydrogen bubbles were subject to corrosion during use of the coated object as a heat exchange element.

Electroless deposition methods remain desirable for producing heat exchange coatings as they can produce rough structures at low temperature, which reduces the cost of the process. Electroless deposition is also desirable as it can be used to provide a coating containing metal, and hence having good heat exchange properties, which is desirable in a coating for a heat exchange element. Moreover, electroless deposition processes require less material than hot dip galvanising processes (which are also referred to as galvanic deposition processes) which are performed in a bath of liquid metal.

It is an object of the present invention to provide a process which can be performed cheaply and quickly, ideally at a low temperature to minimise energy costs. It is also desired to provide a process which can be performed on an existing heat exchanger in situ, so that the process may advantageously be used to retro-fit the heat exchange element of the invention in an existing heat exchanger. Further, it is an object of the invention to provide a process for providing a coating suitable for a heat exchange element (that is, a rough coating comprising a metal) to a substrate, said process having the advantages of an electroless deposition process but avoiding the above-mentioned difficulties.

SUMMARY OF THE INVENTION

The inventors have found that a heat exchange element having a coating comprising sharp spikes in the micrometre size range, wherein the length of the spikes varies over a surface of the heat exchange element, has particularly advantageous heat transfer properties. The invention therefore provides a heat exchange element comprising a substrate and a coating, wherein the heat exchange element defines a flow path for flow of fluid, and wherein at least a part of the flow path is coated with the coating, wherein:

the coating comprises a metal;

the coating comprises a plurality of spikes of a length of up to 100 μπι;

the coating comprises a first region at an end of the flow path in which the average spike length is Si and a second region on the flow path in which the average spike length is S2; and

Si is greater than S ₂ .

The heat exchange element of the invention is particularly well suited to promoting efficient heat transfer between the spiky surface and a fluid. Accordingly, the invention provides a method of transferring heat to or from a fluid which comprises passing the fluid along a flow path of a heat exchange element.

A coating as comprised in the heat exchange element of the invention can conveniently be formed by electroless deposition. The invention therefore further provides a process for producing a heat exchange element of the invention wherein the process comprises providing an electroless deposition solution to a surface of a substrate.

The inventors have further surprisingly found that flowing an electroless deposition solution comprising a metal ion over a substrate removes hydrogen bubbles rapidly from the surface and hence reduces the issues of fragility and/or peeling of the deposited coating which are associated with the bath processes. Moreover, the flow of electroless deposition solution unexpectedly still provides a rough surface suitable for promoting heat transfer from the surface. This finding is unexpected as it was previously thought that the flow process would lead to an irregular structure of the coating, that would not have such good heat exchange properties. Moreover, this electroless flow deposition process is found to be capable of producing a heat exchange element having regions of differing spike length according to an embodiment of the heat exchange element of the invention. Particularly advantageously, this electroless flow deposition process may be used to retrofit an electrolessly deposited coating to an existing heat exchanger in situ and without disassembly. The deposition solution can be provided only to those parts of a heat exchanger which require coating, thus minimising waste of material.

The invention therefore provides a process for producing a heat exchange element comprising a substrate and a coating, wherein:

the coating comprises a metal; and

the process comprises flowing an electroless deposition solution over a surface of the substrate.

This electroless flow deposition process produces an object having a rough coating, the coating comprising a metal. The coating comprising a metal is a good conductor of heat and has a large surface area, which increases contact between the heat exchange element and the surroundings, promoting heat transfer between the element and the surroundings. Therefore the heat exchange element produced by this process is suitable for use in heat exchange.

The invention also provides a heat exchange element obtained or obtainable by this electroless flow deposition process. BRIEF DESCRIPTION OF THE FIGURES

Figure 1 illustrates the different modes of heat transfer from a surface to a fluid.

Figure 2 illustrates diagrammatically the coating on the heat exchange element of the invention. Figure 2a shows model spikes in differing orientations. Figure 2b shows a coating (1) on a substrate (2). Figure 2c shows an arrangement of clusters on a surface and the pore there between. Figure 2d shows a coating (1) on a substrate (2), the coating having a graduating spike length across the substrate surface. Figure 2e illustrates diagrammatically a heat exchange element comprising a coating (1) on a substrate (2), the coating formed on a substrate by the electroless flow process of the invention. Figure 2f shows a cross-section of a heat exchange element (5) comprising a coated flow channel and an uncoated flow channel.

Figure 3 contains SEM images of coatings according to the invention. Figure 3(a) shows a coating comprising spikes of 1 to 3 μιη in length; Figure 3(b) shows a coating comprising spikes of 4 to 5 μπι in length; and Figure 3(c) shows a coating comprising spikes of 8 to 10 μηι in length.

Figure 4 is an SEM image of a coating applied to the inside of a heat exchanger, comprising spikes which are approximately 3 μιη in length. This coating was produced by the electroless flow deposition process of the invention.

Figure 5 contains SEM images of coatings according to the invention wherein the coatings comprise clusters. Figure 5(a) shows a coating comprising spikes of approximately 7 μιη in length arranged in clusters. Figure 5(b) also shows a coating of spikes arranged in clusters, at lower resolution.

Figure 6 is an SEM image of a coating according to the invention comprising spikes approximately 7 μηι in length applied to a 75 μm-diameter wire mesh.

Figures 7 and 8 show the heat flux in kW m ^"2 from a surface to an organic refrigerant as a function of wall superheat (ΔΤ _ς ) for various different surfaces. In Figure 7, one surface is a polished surface and the other is a surface coated according to the electroless flow deposition process defined herein. Figure 9 shows the heat transfer coefficient in W m ^"2 K ^"1 for a surface coated according to the invention (by the electroless flow deposition process), and an uncoated surface, at a variety of refrigerant flow rates.

Figure 10 shows the approximate spike height (dashed upper line) and spike base radius (solid lower line) achieved by an electroless flow deposition process according to the invention over time.

Figure 1 1 shows a test rig used to compare the heat exchange performance of an evaporator (heat exchanger) to that of an uncoated evaporator. The coated evaporator was coated in accordance with the electroless flow deposition process of the invention to provide a heat exchange element according to the invention.

Figure 12 shows the heat exchange coefficients as a function of heat transfer rate for the coated and uncoated evaporators tested in the rig shown in Figure 1 1.

DETAILED DESCRIPTION OF THE INVENTION

Heat transfer across a surface

In the accompanying drawings Figure 1 (upper image) illustrates how the heat transfer across a surface into a fluid (in this case, a liquid) varies with temperature. The Figure illustrates the several different modes of heat transfer available. At lower temperatures heat transfer from a hot (e.g. metal) surface to a fluid (e.g. water) works well through natural convection, especially if there is complete wetting of the surface by the fluid. As the temperature of the heat transfer surface increases, the formation of bubble nuclei leads to heat transfer occurring by nucleate boiling. The temperature at which the onset of such nucleate boiling occurs is affected by surface roughness and the existence of regions that are strongly hydrophobic. As temperature increases further there is a transition to stable film boiling. In stable film boiling a layer of vapour exists next to the surface and heat transfer through this film is thought to occur by conduction. In the transition from nucleate boiling to stable film boiling some areas of the surface display film boiling and some nucleate boiling. Because the thermal conductivity of the vapour is lower than the liquid, the heat flux across the surface tends to reduce in the transition boiling region before reaching a minimum at the onset of stable film boiling, and then increasing again with temperature. The lower image of Figure 1 also illustrates how the heat flux from the surface into the fluid (q _w ) varies with the surface superheat (ΔΤ _ς , which is the difference in temperature between the temperature of the surface and the temperature of the fluid).

Without wishing to be bound by theory, it is speculated that the advantages of the heat exchange element of the invention may be attributable to various aspects of the structure of the coating.

It is believed that in the nucleate boiling regime, an important factor contributing to the efficiency of heat transfer is the sharpness of the spikes in the coating. It is speculated that the sharper the spikes, the more efficiently the processes required for heat transfer by nucleate boiling occur, including:

- formation of a bubble at a spike tip;

- transfer of the bubble down the side of the spike into a cavity or pore;

- growth of the bubble in a cavity or pore by addition of vapour; and

- detachment of the bubble and re-wetting of the surface.

The spikes at the surface of the heat exchange element of the invention are sharp, and hence promote the efficient performance of the above steps and hence of heat transfer. Further, the spike concentrates the flow of heat to the tip of the spike, promoting the above processes.

Further preferred features of the coating also contribute to improvements in heat transfer, in particular in the nucleate boiling regime. These include the size and shape of the spikes, the density of the spikes, the pore or cavity sizes on the surface and the density of pores/cavities (pores or cavities are spaces between spikes; these are discussed in more detail below). The surface is believed to have an advantageous balance between the presence of enough spikes to create bubbles and the presence of enough pores/cavities to store and grow bubbles.

Bubble nucleation is believed to occur at or near the tips of spikes and so the high density of spikes in the coating advantageously provides a large number of nucleation sites, enabling the efficient creation of bubbles. During bubble growth, heat is transferred to the bubble by boiling of liquid to produce the gas in the bubble. Bubble growth is advantageous for heat transfer and is promoted by the existence of bubble growth sites such as pores and cavities. The coating of the invention has a density of pores/cavities large enough to promote bubble growth, but not so large that the number of bubble nucleation sites is compromised. The size of the pores/cavities in the coating of the invention is also believed to be suitable for promoting efficient bubble growth. The surface is also easily re-wettable to detach bubbles from the surface easily. Control of the size of the spikes, and the size of the pores or cavities between the spikes, is therefore believed to be advantageous in promoting heat transfer by a boiling regime.

The heat exchange element of the invention provides variation in the length of the spikes at different regions on a flow path defined by the said element. A flow path is a route along which a fluid can flow over a surface of the heat exchange element. In one region, at an end of the flow path, the average spike length is longer than in another region at a different point on the flow path. The different regions are believed to be suited to different kinds of heat transfer. The first region, having a longer average spike length and therefore deeper cavities/pores, is better suited to promoting nucleate boiling of a fluid passing along the flow path of the heat exchange element. The second region is less suited to promoting nucleate boiling and more suited to promoting film boiling of a fluid passing along the flow path of the heat exchange element. Thus, the heat exchange element of the invention advantageously provides regions suited to at least two kinds of heat transfer regime.

It is further speculated that the arrangement of these regions along the flow path of the heat exchange element of the invention assists in promoting heat transfer during flow boiling. The region of longer spikes is beneficially arranged at an end of the flow path, where a fluid to be cooled may commence its flow along the flow path. Nucleate boiling may be initiated in this region. As the fluid flows along the flow path, it encounters the second region of shorter spikes which may assist the establishment of an efficient film boiling regime further along the flow path.

Heat exchange element

It should be noted herein that reference to the "heat exchange element of the invention" indicates a heat exchange element as defined in claim 1. Reference to a heat exchange element indicates a heat exchange element which can be formed according to the electroless deposition process of the invention. In preferred embodiments, the heat exchange element comprises all the features of claim 1 and is a heat exchange element of the invention.

By "heat exchange element" is meant a solid obj ect suitable for transferring heat from itself to its surroundings. The surroundings may be, for example a solid or fluid adjacent to (and usually in contact with) the heat exchange element. The heat exchange element is capable of absorbing heat from a source. The heat source may be, for example, a solid or fluid adjacent to (and usually in contact with) the heat exchange element. Thus, the heat exchange element of the invention is a solid object capable of transferring heat from a source, via itself, to its surroundings. In particular, the heat exchange element of the invention is suitable for transferring heat to a fluid, particularly to a liquid because a liquid is capable of boiling.

The heat exchange element of the invention defines a flow path for flow of fluid. A heat exchange element which is not a heat exchange element of the invention may also define a flow path for fluid. The flow path for flow of fluid, also referred to as a flow path, is a route along which a fluid can flow over a surface of the heat exchange element. Thus, the flow path comprises at least part of an exposed surface of the heat exchange element. An exposed surface of the heat exchange element is one which may come into direct contact with the surroundings. The flow path must be exposed in order that fluid may come into contact with it. For example, where the heat exchange element is in the form of a plate, the flow path may be any route over a surface of the plate. In some embodiments, the flow path may pass through the heat exchange element. For instance, the heat exchange element may comprise one or more channel(s) (including an open channel or a closed channel, i.e. a tube) to allow fluid to flow through the element; in that case, the flow path may include at least a part of the one or more channel(s) through the heat exchange element.

The flow path is typically all of or part of the transfer area of the heat exchange element. By "transfer area" is meant the area of the heat exchange element which may contact a fluid to which heat is to be transferred (such a fluid is referred to as a working fluid or a refrigerant). In some embodiments, the flow path is the whole surface area of the heat exchange element which can contact a fluid to which heat is to be transferred (a refrigerant). In other embodiments, the flow path may comprise only part of the surface area of the heat exchange element which can contact a fluid to which heat is to be transferred. Where the heat exchange element comprises a channel (or flow channel) for carrying the fluid to which heat is to be transferred, in some embodiments the flow path comprises part of the surface of the flow channel. In other embodiments the flow path comprises all of the surface of the flow channel.

Where the heat exchange element comprises a flow channel, an end of the flow path may be situated at an end of the flow channel. For instance, an end of the flow path may be situated at an inlet to the flow channel. Alternatively or additionally, an end of the flow path may be situated within a flow channel (not at its end), e.g. distant from an inlet to the flow channel. Typically, an end of the flow path is at a position on the heat exchanger which is first contacted by a fluid to which heat is to be transferred. Typically, an end of the flow path is situated where the fluid to which heat is to be transferred first comes into contact with the coating described herein.

A cross-section of a heat exchange element (5) is illustrated in Figure 2f. The heat exchange element (5) has a first flow channel and a second flow channel, each having an inlet (3). The first flow channel comprises a coating (1) while the substrate (2) is not coated in the second flow channel. The first flow channel defines a flow path (4) therethrough.

The flow path is usually continuous, meaning that it is a path along which fluid can flow while in continuous contact with the heat exchange element. The flow path may comprise more than one surface of the heat exchange element, e.g. an internal surface and an external surface.

It should be noted that a flow path does not necessarily constitute a path along which fluid is directed to flow. Rather, the flow path constitutes at least part of an exposed surface of the heat exchange element that fluid can contact with, and hence that fluid can flow over.

The heat exchange element (e.g. the heat exchange element of the invention) may be incorporated into a product. In one embodiment of the invention, the heat exchange element is incorporated into a heat exchanger. In another embodiment of the invention, the heat exchange element is incorporated into an air conditioning unit, refrigerator, heat recovery system, radiator, heat sink, solar collector, boiler, or heat exchanger such as a mini heat exchanger or microchannel heat exchanger. Coating

The heat exchange element comprises a coating which promotes heat transfer from the heat exchange element. The heat exchange element is therefore able to transfer heat to its surroundings efficiently via the coating. Thus, typically the coating of the heat exchange element is present on a heat transfer surface of the heat exchange element. A heat transfer surface is a surface of the heat exchange element suitable for transferring heat to the surroundings. A heat transfer surface may contact the surroundings directly or indirectly; for example, a heat transfer surface may have one or more layers thereon which separate it from direct contact with the surroundings. Typically, the coating is present directly on the heat transfer surface.

An exposed surface of the heat exchange element is a surface which is exposed to the surroundings. Typically, at least a part of the coating is an exposed surface. However, in some embodiments a further layer is present on the coating such that the further layer(s) form the exposed surface

In the heat exchange element of the invention, at least a part of the flow path is coated with the coating. In some embodiments, the entire flow path is coated with the coating. This embodiment may be preferred as it enables the heat exchange element to maximise heat transfer to a fluid in contact with, e.g. flowing along, the flow path.

The coating may also be present elsewhere on the heat exchanger (i.e. other than along the flow path). Advantageously, the coating may be present only on those parts of the heat exchange element which are for transferring heat to a fluid. This minimises waste of material.

The coating may be a continuous coating, wherein the entire coated portion or portions of the substrate are covered with the coating. Alternatively, the coating may be discontinuous, such that there are gaps in the coating on the coated portion of the substrate where the coating is not present on the substrate. Preferably, the coating is a continuous coating, as illustrated in Figure 2b. In the heat exchange element of the invention, the coating comprises spikes. However, not all of the material of the coating is necessarily arranged in spikes; the coating may also comprise material distributed over the substrate surface. Thus a continuous coating does not require spikes to be arranged side-by-side without gaps.

In the heat exchange element of the invention, the coating comprises a plurality of spikes. By "spike" is meant a structure having a thicker portion at one the end of the structure which tapers to a thinner portion at the other end of the structure. A spike may therefore be described as a structure having a pointed top, or as a tapering structure. The thickest part of the spike (base) is typically at the end of the spike nearest to the substrate and the thinnest part of the structure (tip) is typically at the end of the spike furthest from the substrate.

The base of the spike is defined as the smallest cross-section of the spike which intersects the end of the shortest side of the spike. For instance, when a spike extends at right angles to a flat surface, all sides of the spike (that is, distances from one end of the spike to the other) will have the same length and the tip of the spike lies along a line forming a right angle with the base. The base of the spike is therefore the cross-section of the spike at its plane of contact with the substrate. This is illustrated in the left-hand image of Figure 2a. However, when a spike extends at 45° from a flat surface, its tip may not lie over the base at all. In that case, the length of the sides of the spike as measured from the substrate to the spike tip will vary depending on where they are measured. The base of the spike is therefore positioned at the base of the shortest side and tilted at 45° to the base (right-hand image of Figure 2a).

A spike is usually approximately cone-shaped. That is, the spike may be approximated to a cone having a circular base and a tip lying along an axis extending at right angles to the base. The approximately circular base is taken to be the smallest circle which contains the true base.

• The length of a spike is taken to be the distance from the centre of the circular base to the spike tip in this approximated cone shape.

• The base radius of a spike is the radius of the approximated circular base.

• The cone angle is the angle the sides of the cone make with its central axis, measured at the spike tip. The coating comprises a plurality of spikes of a length of up to 100 μιη. In general, the coating comprises spikes having a length of at least 1 μηι. The coating also generally comprises spikes having a length of up to 50 μηι. Thus, typically the coating comprises a plurality of spikes having a length of at least 1 μηι and no more than 50 μιη. Preferably, the spikes have a length of 1 to 15 μιη, e.g. 2 to 10 μηι, for example 3, 4, 5, 6 or 7 μιη.

The cone angle of the spikes in the plurality of spikes is typically small. The cone angle is generally less than 40°. In some embodiments, the cone angle is from 2° to 30°, e.g.

approximately 5° or approximately 10° or approximately 20°. In the context of cone angle, an approximate value may vary by ± 5°, for instance ± 2°.

The cone angle of the spikes stays within the above-specified ranges regardless of spike length. Accordingly, the spike base radius of the spikes in the plurality of spikes increases with spike length. Typically, the spike base radius is less than 5 μιη. For example, the spike base radius may be from 0.05 μηι to 3 μιτι, preferably from 0.1 μηι to 2 μηι or from 0.2 μιη to 1 μηι. In some embodiments, the spike length is from 1 to 15 μιη and the spike base radius is from 0.2 to 3 μιη. In some embodiments, the spike length is from 1 to 10 μιη and the spike base radius is from 0.1 to 2 μιη. In some embodiments, the spike length is from 2 to 10 μηι and the spike radius is from 0.2 μιη to 1 μιη.

The spike length, cone angle and spike base radius may all be calculated by taking an SEM image of the coating and fitting approximate cones to the spike or spikes observed in that image. The fitting may be done by eye or by computer modelling.

The spikes are generally arranged close to one another. The density of spikes in the coating (that is, the number of spikes per unit area) will usually vary with the spike base radius, as a smaller base radius will allow spikes to be more closely packed. In general, the coating comprises 5 or more spikes per 100 μπι ² , e.g. at least 10 spikes per 100 μιτι ² , or at least 20 spikes per 100 μπι ² . The base radius of the spikes also imposes an approximate upper limit on the density of spikes; however, where the spikes are arranged in clusters (see below) the density of spikes may be increased. Thus, in general the coating comprises no more than 500 spikes per 100 μπι ² , e.g. no more than 200 spikes per 100 μπι ² . Preferably, the coating comprises from 5 to 500, e.g. from 5 to 200, spikes per 100 μπι ² . The coating comprises a first region wherein the average spike length is Si and a second region wherein the average spike length is S ₂ . By "average spike length" is meant "mean spike length". Average spike length may be calculated by establishing the length of each spike in a region and calculating the mean therefrom. More conveniently, average spike length may be calculated on the basis of a representative sample of spikes in a region. Si and S2 may take values up to up to 100 μιη. Si and S2 are generally at least 1 μιη. Also, Si and S2 are generally 50 μιη or less. Thus, typically Si and S2 are at least 1 μιη and no more than

50 μπι. Preferably, Si and S2 are from 1 to 15 μιη, e.g. 2 to 12 or 2 to ΙΟμιη.

By "region" is meant an area of the coating on the surface of the substrate. A region is typically an area of at least 20 μιτι ² , for example an area of at least 50 μπι ² .

The average spike length in the first region (Si) is greater than the average spike length in the second region (S2). Typically, S2 is 95% of Si or less. In some embodiments, S2 is 90% of Si or less, e.g. 80% of S2 or less. Usually, S2 is at least 10% of Si, for instance at least 40% of Si. For example, S ₂ may be from 10% of Si to 95% of Si or from 50% to 90% of Si.

51 may be up to 100 μιη. Generally, Si is from 1 to 50 μιη. In a preferred embodiment, Si is from 1 to 20 μπι, e.g. from 2 to 15 μπι. S2 may be up to 100 μπι. Generally, S2 is from 0.1 to 50 μπι. In a preferred embodiment, S2 is from 0.2 to 10 μηι, e.g. from 0.5 to 10 μιη. In one embodiment, Si is from 1 to 20 μπι and S2 is from 0.2 to 12 μιη, e.g. Si is from 2 to 12 μιη and S2 is from 0.5 to 10 μιτι. In some embodiments, the difference between Si and S2 is 0.1 μπι or more, e.g. 0.5 μπι or more or 1 μπι or more. For instance, the difference between Si and S2 may be from 0.1 to 5 μιη.

The first region is located at an end of a flow path defined by the heat exchange element. For instance, where the flow path comprises a flow channel (e.g. a tube) through a heat exchanger, the first region may be located at the end of that flow channel. However, in some embodiments the flow path may not begin precisely at the end of such a flow channel and so the first region may be located some way inside the flow channel. The second region is located elsewhere on the flow path to the first region. For instance, the second region may be located towards the centre of a flow channel. The first and second regions may be isolated from one another. For instance, in a heat exchange element of the invention the first and second regions may be located on different plates or fins, or on in different flow channels. That is, in some embodiments of the invention, the first and second regions may exist in unconnected portions of the coating. In other embodiments of the invention, the first and second regions may be located in the same portion of coating.

In a particular embodiment, the heat exchange element of the invention comprises a coating wherein the coating comprises a first region at an end of the flow path in which the average spike length is Si and wherein the average spike length decreases along at least a part of the flow path, starting at the first region. In this embodiment, the second region may be any region (other than the first) along the said part of the flow path. In a preferred aspect of this embodiment, the average spike length is graduated along at least a part of the flow path such that the longest average spike length occurs at the end of the flow path, and the average spike length decreases along the flow path away from that end. This aspect is illustrated in Figure 2d. A coating according to this aspect of the invention may conveniently be achieved by the method of the invention. For instance, the time for which a substrate is exposed to an electroless deposition solution may be varied along the flow path by very slowly dipping the substrate into the solution. Alternatively, an electroless deposition solution having a concentration gradient that varies along the flow path may be provided to the substrate.

Thus, in one embodiment, the average spike length is graduated along all or part of the flow path. By "graduated" is meant that the average spike length shows a change in a constant direction, i.e. a gradual or incremental rather than step-change. For example, the average spike length measured at a series of neighbouring positions along the flow path may be successively larger at each position. In one aspect of this embodiment, the average spike length increases from one end of the flow path to another. Where a flow path is coincident with a flow channel or part of a flow channel, the average spike length may increase from one end of the flow channel to another.

The coating of the heat exchange element of the invention may comprise a third region in which the average spike length is S3. The third region may in one example lie on the flow path defined by the heat exchange element. S3 may be up to 100 μιτι. Generally, S3 is from 1 to 50 μιη. Preferably, S3 is from 1 to 20 μιη, e.g. from 1 to 15 μιη, e.g. 2 to 12 or 2 to 10 μπι. The average spike length in the third region (S3) is typically of a similar order of magnitude to the first region (Si) and greater than the average spike length in the second region (S ₂ ). Typically, S2 is 95% of S3 or less. In some embodiments, S2 is 90% of S3 or less, e.g. 80%) of S3 or less. Usually, S2 is at least 10% of S3, for instance at least 40% of S3. For example, S2 may be from 10% of S3 to 95% of S3 or from 50% to 90% of S3. S3 is typically from 95 to 105%, for example from 99 to 101%, of Si.

In one embodiment, the third region is located at an end of the flow path. In this

embodiment, the coating comprises a first region at an end of the flow path in which the average spike length is Si, a second region on the flow path in which the average spike length is S2, and a third region at another end of the flow path in which the average spike length is Si, wherein Si is greater than S ₂ . Si and S2 may be as defined above. In this embodiment, the average spike length shows a graduated decrease and a graduated increase along the flow path.

The plurality of spikes comprised in the coating have sharp tips. As explained above, the sharpness of the spikes promotes nucleate boiling. Thus, the spikes are usually thin at the tip. Generally, a thickness at the tip of the spikes is 100 nm or less. This means that the maximum diameter of the spike, which occurs at the end of the spike (and at the tip of the approximate cone) is 100 nm or less. For instance the thickness at the tip of the spikes may be from 0.1 to 100 nm. Preferably the thickness at the tip of the spikes is 60 nm or less. For example the thickness at the tip of the spikes may be from 1 to 50 nm.

In some embodiments of the invention, the spikes are arranged in clusters. The invention therefore provides a heat exchange element comprising a substrate and a coating as described herein, wherein the coating comprises a plurality of spikes arranged in one or more clusters. Each cluster comprises two or more spikes. Particularly good heat transfer efficiencies have been observed using heat exchange elements comprising clusters.

The number of spikes above two in a cluster is not particularly limited. Preferably, a cluster comprises five or more spikes. Typically, a cluster comprises between 5 and 500 spikes.

A cluster is typically a flower-like arrangement of spikes. A cluster comprises two or more spikes protruding from a node. A node is a volume of coating material from which the spikes of that cluster protrude outward. The node may be approximately spherical or hemispherical in shape. The spikes protrude from the node in an approximately radial fashion (that is, approximately in directions along the radii of a spherical or hemispherical node). However, significant deviations in radial orientation are possible and so each spike may not lie perfectly along the radius of the sphere or hemisphere. A cluster and node are illustrated in Figure 2c.

The node may usually have a diameter (corresponding to a diameter of a sphere or hemisphere approximating to the node) of up to 50 μπι. Generally the diameter of the node is from 0.05 to 50 μιη, e.g. from 0.1 to 20 μιη, or 0.5 to 10 μιη. Where the node is very small, it may be more conveniently thought of as a point from which the spikes protrude.

A cluster may be described as having a height and a diameter. The height is the longest distance perpendicular to the substrate. The diameter is the diameter of the smallest circle in the plane of the substrate which encloses the cluster when viewed from perpendicularly above the cluster. The height and diameter of a cluster may be determined by taking an SEM image of the coating comprising the cluster and fitting the height and diameter thereto, either by eye or via computer modelling.

The diameter of a cluster is generally less than 200 μιη. The diameter of a cluster is typically from 1 to 200 μιη. Preferably, the diameter of a cluster is from 2 to 100 μιτι, more preferably from 5 to 50 μπι or 10 to 50 μιτι, e.g. from 10 to 40 μιτι.

The height of a cluster is generally less than 200 μπι. The height of a cluster is typically from 0.5 to 150 μιτι. Preferably, the height of a cluster is from 1 to 100 μιτι, more preferably from 2 to 50 μιτι, e.g. from 5 to 30 μιη.

The density of clusters (that is, the number of clusters per unit area) will vary with cluster diameter. Where the coating comprises clusters, the density of clusters is generally up to 100 clusters per 100 μηι ² . Preferably, the density of clusters is from 0.5 to 50 clusters per 100 μπι ² , e.g. from 1 to 25 clusters per 100 μιη ² .

Where the coating comprises clusters, the first region may comprise a greater density of clusters than the second region. Similarly, where the coating comprises clusters, the average diameter of the clusters in the first region may be larger than the average diameter of clusters in the second region. Average diameter in this context means mean diameter. The average cluster diameter may be calculated by establishing the diameter in a region and calculating the mean therefrom. More conveniently, average cluster diameter may be calculated on the basis of a representative sample of clusters in a region.

The presence of spikes and/or clusters in a coating gives rise to cavities and pores. By "pore" is meant a space between adjacent clusters; by "cavity" is meant a space between adjacent spikes. Cavities are therefore typically smaller than pores. The shape of a pore is not particularly limited. A pore may be described as having a depth, a width and a length. The depth of the pore is the maximum distance (in a direction perpendicular to the substrate) from the part of the pore closest to the substrate to the maximum height of a neighbouring cluster. The length of the pore is the greatest straight linear extent of the pore in the plane of the substrate surface. The width of the pore is the greatest straight linear extent of the pore perpendicular to its length in the plane of the pore. As with the other characteristics of the surface, these parameters may be determined by taking an SEM image of the coating and fitting the parameters, either by eye or via computer modelling.

Generally, the depth of a pore is approximately equivalent to the height of the adjoining clusters. Thus, the depth of a pore is generally less than 200 μπι. The depth of a pore is typically from 0.5 to 150 μιη. Preferably, the depth of a pore is from 1 to 100 μπι, more preferably from 2 to 50 μπι, e.g. from 5 to 30 μπι.

Generally, the length of a pore is less than 500 μπι. The length of a pore is typically from 2 to 250 μπι. Preferably the length of a pore is from 10 to 100 μπι, e.g. from 20 to 85 μιη.

Generally, the width of a pore is less than 100 μιη. The width of a pore is typically from 0.5 to 50 μιη. Preferably the width of a pore is from 1 to 25 μπι, e.g. from 4 to 20 μιη.

It is speculated that different kinds of coatings (e.g. varying in spike length or the presence or absence of clusters) may be suited to the most efficient heat transfer to different fluids. The density of spikes and cavities/pores, and their sizes, determine the density of bubble formation and the sites into which those bubbles move and grow. This affects heat transfer properties. The presence of clusters provides pores, and so heat exchange elements having clusters are best suited for heat transfer to fluids to which heat may be efficiently transferred by the formulation and growth of larger bubbles. Thus the coating in the heat exchange element of the invention may be varied to provide optimised heat transfer to a range of different fluids having different heat transfer characteristics. The fluids may include varied species such as organic refrigerants, water, liquid N2 or CO2 and so on. For instance, the organic refrigerants may include hydrofluoroolefins (HFOs), hydrofluorocarbons (HFCs), fluorocarbons (FCs) and hydrocarbons. Another exemplary fluid is ammonia.

The thickness of the coating in the heat exchange element is not particularly limited. The thickness of the coating may be defined as the largest perpendicular distance from the substrate to the edge of the coating material. Typically the thickness of the coating is at least Ι μιη, e.g. at least 2 μιτι. Generally the thickness of the coating is 200 μπι or less. Preferably, the thickness of the coating is from 1 μιη to 100 μιη, e.g. from 2 to 50 μιη. In one embodiment, the invention provides a heat exchange element wherein the thickness of the coating is 10 μηι or more. In another embodiment, the invention provides a heat exchange element wherein the thickness of the coating is from 2 to 50 μιτι.

The exact weight of the coating per unit area of the substrate will depend on the structure of the coating and the materials therein. Generally, the weight of the coating per unit area of substrate is at least 10 g m ^"2 . Generally, the weight of the coating per unit area of substrate is no more than 900 g m ^"2 . Usually, the weight of the coating per unit area of substrate is from 20 to 500 g m ^"2 , preferably from 30 to 400 g m ^"2 .

The coating comprises one or more metals. Generally, the coating comprises one or more transition metals. Preferably, the coating comprises one or more of vanadium, chromium, manganese, cobalt, nickel, and copper. In a preferred embodiment, the coating comprises copper, nickel or an alloy of copper and nickel. In a particularly preferred embodiment of the invention, the coating comprises copper. In another particularly preferred embodiment, the coating comprises an alloy of copper and nickel.

The coating usually has a high metal content, i.e. it is primarily metallic. Usually, the coating contains at least 50% metal by weight of the coating. In a preferred embodiment, the coating contains at least 70% metal by weight of the coating. In one embodiment of the invention, the coating comprises 80% metal by weight of the coating. In a particularly preferred embodiment, the coating has a very high metal content e.g. at least 90% or at least 99% metal by weight of the coating.

The coating of the heat exchange element of the invention having a structure and properties as described above may conveniently be obtained by electroless deposition. Thus, in one embodiment of the invention, the coating is obtainable by electroless deposition. In an aspect of this embodiment, the coating is obtained by electroless deposition. For example, the coating is typically obtained or obtainable by an electroless flow deposition process as defined herein.

In one embodiment of the invention, the coating comprises one or more, e.g. one or two, surface layers on the coating. By "surface layer" is meant a layer of material on the surface of the coating. A surface layer is therefore a layer of material on the side of the coating opposite to the side of the coating which is in contact with the substrate.

The material of a surface layer is not particularly limited. For example, a surface layer may comprise one or more metal(s) or one or more polymer(s). A surface layer may comprise one or more hydrophobic material(s) and/or one or more hydrophilic material(s) to adjust the wettability of the heat exchange element. A surface layer may comprise one or more protective material(s) to protect the coating from wear and tear or the influence of harsh refrigerants such as ammonia.

Preferably a surface layer comprises one or more transition metal(s), particularly nickel or titanium. In a preferred embodiment, a surface layer consists of nickel, titanium, or an alloy comprising nickel and/or titanium. Preferably a surface layer is a nickel layer. Typically the surface layer comprising a transition metal is on an exposed surface of the heat exchange element, i.e. it comes into direct contact with the fluid flowing along the flow path.

Typically, a single surface layer is present, preferably a single layer comprising a transition metal as discussed above.

The surface layer(s) are thin layer(s) in order to preserve the advantageous structure of the coating. Generally the total thickness of any surface layer(s) present is 500 nm or less. For example, the total thickness of the surface layer(s) present may be from 1 to 250 nm, e.g. from 10 to 200 nm. Substrate

The substrate is a solid object. The substrate generally takes the form of a typical heat exchange element or a part thereof or a heat exchanger or part thereof which is then coated according to the invention.

Examples of suitable substrates include: shell and tube heat exchangers, plate heat exchangers, brazed plate heat exchangers, gasketed heat exchangers, plate and shell heat exchangers, adiabatic wheel heat exchangers, plate fin heat exchangers, pillow plate heat exchangers, fluid heat exchangers, dynamic scraped surface heat exchangers, mini heat exchangers and microchannel heat exchangers. Other examples of suitable substrates include parts of heat exchangers such as a fin, plate, coil or tube that is part of a heat exchanger. Still other examples of suitable substrates include heat exchangers or parts of heat exchangers suitable for incorporation in a boiler, air conditioner, refrigerator, radiator, heat sink, solar collector or other type of thermal transfer component.

The substrate is preferably a conductor of heat. The substrate may therefore comprise metal. In one embodiment, the substrate is a metal object including a metallic or metal alloy. For example, the substrate may be an object such as a heat exchange element made of one or more of carbon steel, austenitic stainless steel, martensitic steels, aluminium and its alloys such as aluminium bronzes, aluminium silicon etc., copper and its alloys, titanium and zirconium. Preferably the substrate is an object comprising stainless steel or titanium, or the substrate consists of stainless steel or titanium. These metals are preferred as they resist corrosion.

The substrate may be non-metallic, and comprise a semiconductor such as silicon, or gallium nitride. It may, for instance, comprise a carbon composite that has a high thermal conductivity. In one embodiment, the substrate may be made of a carbon composite.

The substrate may comprise one or more outer layer(s). In the heat exchange element (e.g. the heat exchange element of the invention), where an outer layer is present, all or part of an outer layer of the substrate is located between the body of the substrate and the coating. In one embodiment, a single outer layer is present which is in contact with the body of the substrate and the coating. In other embodiments, two or more outer layers are present.

Typically if an outer layer is present, a single outer layer is present. More preferably, there is no outer layer such that the substrate is in direct contact with the coating.

An outer layer, when used, typically comprises one or more metal(s) or metal alloy(s). For instance, an outer layer may be a metallic layer. An outer layer is useful in improving corrosion resistance of the substrate, particularly where the outer layer comprises titanium, nickel or stainless steel. An outer layer may also improve formation of the coating during the manufacture of the heat exchange element of the invention, and may improve adhesion of the coating to the substrate in the heat exchange element.

The heat exchange element is suitable for transferring heat from its surface to a fluid in contact with its surface. As discussed above, the structure of the coating promotes efficient heat transfer by nucleate boiling and/or film boiling of a liquid, and so the heat exchange element of the invention is particularly suitable for transfer of heat to a liquid. Often, therefore, the substrate is an object suitable for or adapted for transferring heat to a liquid. In some embodiments, the substrate is a heat exchange element, heat exchanger or part of a heat exchanger that is designed for the transfer of heat to a liquid. In one embodiment, the substrate is a heat exchanger suitable for transferring heat to a liquid.

In a particular embodiment of the invention, the heat exchange element may be suitable for fluid to fluid heat transfer, for example gas to liquid heat transfer or liquid to liquid heat transfer. Thus, the substrate may be a heat exchanger or part of a heat exchanger designed for fluid to fluid heat transfer, for example gas to liquid heat transfer or liquid to liquid heat transfer.

A heat exchange element suitable for transfer of heat to a fluid such as a liquid has a surface or surfaces suitable for contacting a fluid. A fluid to which heat is transferred by a heat exchange element may be referred to as a "working fluid" or "refrigerant". Typically but not essentially, a heat exchange element suitable for transfer of heat to a fluid such as a liquid comprises one or more flow channel(s) suitable for carrying a fluid to which heat is to be transferred. Typically but not essentially, therefore, the substrate comprises one or more flow channel(s) suitable for carrying a fluid such as a liquid to which heat is to be transferred. A fluid from which heat is transferred to a heat exchange element may be referred to as a "heat transfer fluid" or "heating fluid". A heat exchange element that is suitable for receiving heat from a fluid such as a liquid typically comprises one or more flow channel(s) suitable for transfer of heat from a fluid to the heat exchange element. Thus, typically but not essentially, the substrate comprises one or more flow channel(s) suitable for carrying a liquid from which heat may be transferred to the substrate.

In one aspect of the invention, the heat exchange element (e.g. the heat exchange element of the invention) is a fluid to fluid heat exchanger, preferably a fluid to liquid heat exchanger such as a gas to liquid or liquid to liquid heat exchanger, or part thereof. Typically, therefore, the substrate comprises one or more flow channel(s) suitable for carrying a fluid (e.g. a liquid) from which heat may be transferred to the substrate, and one or more flow channel(s) suitable for carrying a fluid (preferably a liquid) to which heat may be transferred from the substrate.

By "flow channel" is meant a channel along which fluid can pass through the substrate. A flow channel comprises one or more openings via which fluid may enter and/or leave the flow channel. Such an opening may be referred to as an inlet. In most configurations, a flow channel is enclosed on all sides along its length (i.e. a tube) and comprises openings at one or more ends. However, as the skilled person will appreciate, other configurations of a flow channel are possible.

In one embodiment of the heat exchange element, the flow path comprises a flow channel and the coating is present on at least a part of the surface of said flow channel. Typically the coating substantially covers the surface of the flow channel. By the surface of the flow channel is meant the internal surface of the flow channel. The internal surface of a flow channel will come into contact with fluid flowing through the flow channel. The flow path may sit entirely within the flow channel. That is, the flow channel may extend beyond the flow path. Alternatively, the flow path may extend to or even beyond one or more openings (inlets) to the flow channel.

Since the coating is particularly useful for promoting heat transfer to a refrigerant (e.g. a liquid), the coating is advantageously present on the surface or surfaces (such as the surface of a flow channel) that a refrigerant will contact when the element is in use. In order to avoid wastage of material, the coating may be present only on the surface or surfaces of the heat exchange element which may contact a refrigerant.

The heat exchange element may comprise one or more flow channel(s) not having coating present on their surface. Such uncoated flow channels may be useful for carrying a heat transfer fluid when the heat exchange element is in use. Such flow channels for heat transfer fluid are termed "first flow channels" herein. Typically, the heat exchange element is arranged such that the heat transfer fluid can transfer heat to the refrigerant, via the heat exchange element.

Where the heat exchange element of the invention comprises a flow channel having coating present on the surface of the flow channel, the first region (having an average spike length Si therein) and the second region (having an average spike length S2 therein) may both be located within the flow channel. Alternatively, one region may be located in the flow channel and the other may not. In one embodiment, the first region is located at or near to an inlet to the flow channel and the second region is located at a greater distance from the inlet than the first region. For instance, where the first and second regions are within a flow channel, the coating may comprise longer spikes at or near the inlet and shorter spikes further along the flow channel, e.g the spike length may gradually decrease from at or near the inlet to a point further along the channel. A heat exchange element according to this embodiment may be conveniently produced by the process of the invention.

In a preferred embodiment of the invention, the heat exchange element is suitable for transferring heat to a refrigerant, preferably to a liquid refrigerant. When the heat exchange element is in use, it may comprise one ore more refrigerant(s); for instance, a refrigerant(s) may be present in one or more flow channels in the heat exchange element. Thus, in one embodiment, the invention provides a heat exchange element that contains a refrigerant. That is, the invention provides a working heat exchange element wherein the working heat exchange element comprises a heat exchange element of the invention and a refrigerant. In a further aspect of this embodiment, the heat exchange element also comprises a heat transfer fluid (for instance in one or more flow channels of the heat exchange element).

The refrigerant is a fluid, preferably a liquid, suitable for receiving heat. A wide variety of liquids are suitable including CO2, nitrogen, ammonia, water, aqueous solutions, organic liquids including halogenated alkanes such as CFCs, and sulphur-based refrigerants. The heat transfer fluid is a fluid, usually a liquid capable of providing heat to the heat exchange element. A wide variety of liquids are suitable including water and organic liquids such as oils.

Heat transfer efficiency of the heat exchange element of the invention

The heat exchange element of the invention is capable of highly efficient heat transfer. The heat transfer efficiency of the heat exchange element of the invention is similar or better than the heat transfer efficiency of a comparable substrate having a sintered surface. However, advantageously, much less metal is needed to create the coated heat exchange element of the present invention than is needed to prepare a comparable substrate having a sintered surface.

The heat exchange element of the invention facilitates more efficient heat transfer than a comparable substrate having a polished surface. In one embodiment, the heat exchange element of the invention has a heat transfer coefficient at least 20% higher than a comparable substrate having a polished surface, for instance at least 30% higher or 50% higher than a comparable substrate having a polished surface. The heat transfer coefficients are typically calculated for the same system {i.e. having the same heat source and the same refrigerant and at the same temperature or temperatures) in order to make the comparison. The heat transfer coefficients for the purpose of this comparison are typically calculated in a flow boiling regime and at a heat flux of less than 200 kW m ^"2 , e.g. at 100 kW m ^"2 . A polished surface is a surface polished with emery paper of grade 1200. A polished surface typically has an average roughness of 0.04 μπι or less, measured on a Taylor Hobson surface profiler (Taylor Surf Series 2, using Taylor Hobson ultra software).

In one embodiment, the heat exchange element of the invention has a heat transfer of 7000 W m ^"2 K ^"1 or more at a heat flux of approximately 80 kW m ^"2 .

In one embodiment, the heat exchange element of the invention displays a superheat that is 50%) or less of that displayed by a comparable substrate having a polished surface, under the same test conditions. Preferably, the heat exchange element of the invention displays a superheat that is 30%o or less of that displayed by a comparable substrate having a polished surface, under the same test conditions. Typical test conditions include a pool boiling experimental regime, and a heat flux of up to 500 kW m ^"2 , e.g. 20 kW m ^"2 . The superheat is the difference (in Kelvin) between the temperature of the surface from which heat is being transferred to a fluid and the temperature of the fluid.

In one embodiment, at a heat flux of up to 500 kW m ^"2 the heat exchange element of the invention displays a superheat of 10 K or less. In an aspect of this embodiment, at a heat flux of up to 200 kW m ^~2 the heat exchange element of the invention displays a superheat of less than 10 K. Typical test conditions include a pool boiling experimental regime.

Method of heat transfer

The heat exchange element of the invention is capable of facilitating efficient heat transfer to its surroundings. In particular, the heat exchange element is capable of facilitating efficient heat transfer to a fluid in contact with the element, and particularly to a fluid in contact with the coating of the element. The heat exchange element defines a flow path along which fluid may contact the element and which is coated with the coating, along all or part of its length. Thus, the invention provides a method of transferring heat to or from a fluid which comprises providing the fluid to a flow path of a heat exchange element of the invention.

As discussed above, the coating is adapted to promote heat transfer by facilitating boiling of a liquid (in a nucleate and a film boiling regime). Accordingly, in preferred embodiments of the method of the invention, the method comprises transferring heat to a liquid. In these embodiments, the method comprises providing a liquid to a flow path of a heat exchange element of the invention. In one embodiment, the method is a method of transferring heat from a solid to a liquid. In another embodiment, which is preferred, the method is a method of transferring heat from a fluid to a liquid, e.g. from a gas to a liquid or a liquid to a liquid. In a particular embodiment, the method is a method of transferring heat from a liquid to a liquid.

In some embodiments, the method of heat transfer comprises passing a fluid (preferably a liquid) along a flow path of a heat exchange element of the invention.

Generally, the method comprises transferring heat to a refrigerant, the method comprising passing a refrigerant along the flow path of a heat exchange element of the invention. For example, the method may comprise passing a refrigerant through a first flow channel (which defines the flow path) of the heat exchange element of the invention. In some embodiments, the method comprises transferring heat from a heat transfer fluid by passing a heat transfer fluid along a second flow channel of the heat exchange element of the invention. In a preferred embodiment, the method is a method of transferring heat from a heat transfer fluid to a refrigerant, the method comprising passing said heat transfer fluid through a second flow channel of the heat exchange element and passing said refrigerant through a first flow channel of the heat exchange element. The flow channels are typically arranged to conveniently enable heat transfer from the heat transfer fluid via the heat exchange element to the refrigerant.

The temperature at which the method of the invention is performed is typically less than 500 °C. The temperature at which the method of the invention is performed will depend on the refrigerant (if used). Typically, where a refrigerant is used, the method is performed at a temperature within 20 °C of the boiling temperature of the refrigerant, e.g. from 0 to 10 °C above the boiling temperature of the refrigerant.

Also provided by the invention is the use of a heat exchange element according to the invention as a heat exchanger. In one embodiment, the invention provides the use of a heat exchange element in a method of heat exchange as described herein.

Process for producing heat exchange element

The coating of the heat exchange element of the invention may conveniently be created by electroless deposition. Thus, the invention provides a process for producing a heat exchange element according to the invention, the process comprising providing an electroless deposition solution to a surface of a substrate. In one aspect, the electroless deposition process is a bath process. In another aspect, the electroless deposition process is a flow process. These specific aspects will be described in further detail in later sections; the following comments concerning electroless deposition processes apply equally to bath processes and flow processes. The heat exchange element of the invention and substrate are as defined herein. The electroless flow deposition process of the invention, however, can provide heat exchange elements which are not heat exchange elements of the invention. That is, the electroless flow deposition process can provide coatings which differ from that of the heat exchange element of the invention. The invention therefore provides an electroless flow deposition process for producing a heat exchange element, and heat exchange elements obtained or obtainable by that process. In a preferred aspect, the electroless deposition process of the invention is a process for producing a heat exchange element of the invention. The heat exchange element and substrate are as defined herein.

Electroless deposition involves the reduction of metal ions in solution to produce metal atoms deposited on the substrate surface to form a coating comprising metal, as described above. The electroless plating process is a non-electrolytic process. The electroless plating process does not require molten metal, unlike hot dip galvanising processes.

Advantageously, electroless deposition may be performed at low temperatures. Typically, the electroless deposition process is performed at a temperature of from 20 °C to 120 °C. In one embodiment, the electroless deposition process is performed at room temperature.

Preferably, the electroless deposition process is performed at a temperature of 100 °C or less, for example from 20 °C to 100 °C or 50 °C to 100 °C, e.g. at approximately 60 °C, 70 °C or 80 °C. It should be noted that performing electroless deposition within this temperature range means that the electroless deposition solution is maintained within the aforementioned temperature range, typically 20 °C to 120 °C, during electroless deposition. Preceding and subsequent method steps may be performed at temperatures within or outside the above- mentioned range.

The structure of the coating produced by the electroless deposition process is influenced by the electroless deposition conditions at the surface of the substrate. In particular, varying the deposition time, the concentration of ions in the electroless deposition solution and the temperature will influence the structure of the coating formed by the electroless deposition process.

The electroless deposition solution comprises one or more metal ions. The metal ion(s) are deposited onto the surface of the substrate to form a coating comprising metal during the electroless deposition process. The metal ion(s) are generally selected from one or more of a vanadium, chromium, manganese, cobalt, nickel, or copper ion. Preferably, the electroless deposition comprises copper and/or nickel ions. Particularly preferably, the electroless deposition solution comprises copper ions. For example, the electroless deposition solution may comprise one or more of Cu ²⁺ , Cu ⁺ and Ni ²⁺ .

The electroless deposition solution typically comprises a reducing agent. The choice of reducing agent will depend on the one or more metal ions in the solution and on the nature of the substrate. Suitable reducing agents include, for example, one or more of iodate;

oxyphosphorus ions such as phosphate, phosphite and hypophosphite; or borate ions.

The electroless deposition solution is typically an aqueous solution. However, the electroless deposition solution may comprise solvents other than water such as alcohols or ethers.

Usually, the primary solvent in the electroless deposition solution is water. The electroless deposition solution may also comprise one or more complexing agent(s) and/or one or more stabiliser(s) and/or one or more modifier(s), the choice of which depends on the substrate and the materials to be deposited.

The structure of the coating formed by the electroless deposition process is influenced by the concentration of metal ions and reductant at the surface of the substrate. A higher concentration of metal ions tends to increase the amount of material deposited by electroless deposition. (By "a higher concentration of metal ions" is meant a higher concentration of the metal ions that are deposited onto the surface during the electroless deposition process). A higher concentration of metal ions also tends to increase the thickness of the coating formed. A higher concentration of metal ions also favours the formation of larger surface features. For example, a higher concentration favours the formation of longer spikes and/or a greater density of spikes on the surface. Thus, the formation of a coating according to the invention (comprising a first region wherein the average spike length is longer than that in a second region) may be achieved by providing variation in concentration of the electroless deposition solution over the surface of the substrate. Thus, in some embodiments, the invention provides an electroless deposition process which comprises:

providing an electroless deposition solution having a concentration Ci to a first region of the substrate;

providing an electroless deposition solution having a concentration Ci to a second region of the substrate; wherein Ci is greater than C ₂ .

Ci and C ₂ are the concentrations of metal ions which may be deposited on the substrate in the electroless deposition solution or solutions.

The structure of the coating formed by the electroless deposition process is also influenced by the time for which the electroless deposition is allowed to occur. Generally, the longer the time allowed for electroless deposition, the thicker the coating will be. Similarly, a longer deposition time favours the formation of larger surface features. For instance, a longer deposition time favours the formation of longer spikes and/or a greater density of spikes on the surface. Usually, the process comprises providing the electroless deposition solution to a surface of a substrate for at least 15 minutes. Also usually, the process comprises providing the electroless deposition solution to a surface of a substrate for period of 12 hours or less. In one embodiment, the process of the invention comprises providing the electroless deposition solution to a surface of a substrate for a period of from 0.5 hours to 10 hours. For example, the period may be from 1 hour to 5 hours. Evidently, the electroless deposition process can coat a heat exchange element in an advantageously short time.

In the context of an electroless flow deposition process, reference to "providing the electroless deposition to a surface" should be taken as meaning "flowing the electroless deposition solution over a surface". That is, "providing" should be taken to mean "flowing" or "flowing over". For example, where the process is a flow process, the process usually comprises flowing the electroless deposition solution over a surface of a substrate for at least 15 minutes. Also usually, the process comprises flowing the electroless deposition solution over a surface of a substrate for period of 12 hours or less. In one embodiment, the process of the invention comprises flowing the electroless deposition solution over a surface of a substrate for a period of from 0.5 hours to 10 hours. For example, the period may be from 1 hour to 5 hours.

In some embodiments, the electroless deposition solution is not refreshed during the electroless deposition process. If the electroless deposition solution is not refreshed, the electroless deposition solution will become depleted over time during the process of the invention. By "depleted" is meant that the concentration of metal ions and reductants in the solution have fallen below their initial value (the value at the start of the process). The electroless deposition solution may be said to be depleted by 5% once the concentration in the solution of metal ions to be deposited has fallen by at least 5%, to 95% or less of its original value. That is, if the electroless deposition solution is suitable for depositing copper ions, the solution is said to be depleted by at least 5% once the concentration of copper ions in solution has fallen to 95% or less of its original value.

It should be noted that localised variations in concentration may occur. Depletion is therefore considered in terms of the depletion of the electroless deposition solution taken as a whole. For instance, multiple samples of the electroless deposition solution may be taken in order to indicate the depletion in the solution taken as a whole. For instance, in a bath process, multiple samples may be taken from the bath while the electroless deposition solution is agitated. In another example, where the process comprises flowing electroless deposition solution from a reservoir over the surface and back into the reservoir, one or more samples may be taken from the reservoir. Similarly, one or more samples may be taken from the solution flowing from the reservoir to the substrate. Alternatively or additionally, one or more samples may be taken from the solution flowing from the substrate back into the reservoir.

Once the electroless deposition solution is considerably depleted (for instance by 50% or more), the concentration of ions in solution may be low and hence the deposition process will become undesirably slow. Accordingly, the electroless deposition process of the invention is usually continued until the electroless deposition solution is depleted by 50% or more.

In one embodiment, the process for producing a heat exchange element comprises providing an electroless deposition solution to a surface of the substrate for a time T, wherein T is the time taken for the electroless deposition solution to become depleted by 5 to 50%. That is, until the concentration of metal ion to be deposited has fallen to 50% to 95% of its original value. Preferably, T is the time taken for the electroless deposition solution to become depleted by 10 to 40%, or by no more than 30%. For instance, the process for producing a heat exchange element (e.g. a heat exchange element of the invention) may comprise flowing an electroless deposition solution over a surface of the substrate for a time T.

The extent of depletion may be determined by, for example, noting the change over time in ion concentration or conductivity of the electroless deposition solution. Thus, in one embodiment, the process comprises measuring the ion concentration of the solution. In another embodiment, the process comprises measuring the conductivity of the solution. "Measuring" may comprise monitoring the change in a particular value over the course of the electroless deposition process.

The skilled person would appreciate that a wide variety of methods are suitable for measuring the ion concentration of the electroless deposition solution, and hence for determining the extent of depletion. These methods may be performed on one or more sample(s) taken from the electroless deposition solution. Alternatively they may be performed in-line, e.g. within a bath or reservoir of electroless deposition solution that is used for electroless deposition. Suitable methods include optical methods such as colorimetry. Colorimetric methods are particularly well suited for solutions of Cu ²⁺ ions which are highly coloured. Other methods include anodic stripping voltammetry, ion chromatography and ion emission spectroscopy (e.g. inductively coupled plasma optical emission spectroscopy, ICP-OES). Thus in some embodiments the process comprises measuring the ion concentration in the electroless deposition solution, for example by any of the above methods. In the context of this measurement, "ion concentration" includes the concentration of metal ions which are deposited on the substrate during electroless deposition.

Where the process comprises flowing electroless deposition solution over a substrate, ICP- OES may be used to analyse the electroless deposition solution flowing over the substrate both before and after it contacts the substrate. This can reveal the maximum and minimum ion concentrations to which the substrate is exposed. The ion concentration of the solution after it has contacted the substrate may be used to determine whether the solution flowing from the substrate back into a reservoir needs to be dosed with additional ions to adjust its concentration. In the flow process of the invention, the ion concentration in the reservoir may be measured periodically to determine whether the reservoir itself needs to be dosed with additional ions to adjust its concentration. Similarly, the ion concentration of a bath determined during a bath process may be used to determine if dosing is required to increase ion concentration.

The desired spike length in the coating of the heat exchange element of the invention may be achieved by adjusting the initial ion composition of the electroless deposition solution, and/or the length of time for which the deposition process is allowed to occur. The amount of time for which electroless deposition is allowed to occur affects not only the spike length in the coating formed by the process, but also the incidence of clusters of spikes in the coating. The longer the time for which electroless deposition is allowed to occur, the greater the likelihood that clusters of spikes will form. Thus, the process for producing a heat exchange element may be adjusted to form clusters by increasing the amount of time for which electroless deposition is allowed to occur.

The exact amount of time needed to form clusters will vary with the substrate, the composition of the electroless deposition solution, and the temperature.

The process may also be adapted to promote the formation of clusters by repeating the process more than once. A process for producing a heat exchange element of the invention having a coating comprising clusters may therefore comprise:

(i) performing the electroless deposition process of the invention as described herein;

(ii) activating the surface of the coating of the heat exchange element thus produced, for instance by immersing the heat exchange element in an a solution of PdCh to form an activated heat exchange element; and

(iii) repeating the electroless deposition process of the invention.

Bath process

In one aspect, the process of the invention is a bath process (for producing a heat exchange element of the invention). In a bath process, the electroless deposition solution is provided to the surface of a substrate by placing the substrate in a bath of electroless deposition solution. In some embodiments, the electroless deposition solution is agitated during the electroless deposition process. Agitation of the electroless deposition solution reduces variation in the composition of the electroless deposition solution (such as variation in local metal ion concentration) throughout the bath.

In some embodiments, the entire surface or surfaces of the substrate are coated during the bath process. In other embodiments, part of the surface or surfaces of the substrate are protected before the substrate is placed into the bath such that the protected parts are not coated. In still further embodiments, part of the surface or surfaces of the substrate are not capable of adhering to the coating and hence are not coated during immersion in the bath.

In some embodiments, multiple heat exchange elements according to the invention may be produced simultaneously by placing two or more substrates into a bath.

In one aspect, the bath process may be used to provide a coating having a first region wherein the spike length is Si and a second region wherein the spike length is S2 by adjusting the length of time of the electroless deposition process. For example, the substrate may be dipped into the bath slowly such that the part or parts of the substrate surface which enter the bath first experience a longer exposure to the electroless deposition solution than part or parts of the substrate surface which enter the bath subsequently. The coated part or parts of the heat exchange element thus produced which spent the longest time exposed to the electroless deposition solution will typically have the longest average spike length. If the substrate is dipped into the bath at a constant slow speed, the coating produced on the substrate may show a smooth variation in average spike length. In another example, a protective covering may be removed from a portion of the substrate part way through the electroless deposition process, leaving that portion of the substrate to be coated during the remaining immersion time.

In another aspect, the bath process may be used to provide a coating having a first region wherein the spike length is Si and a second region wherein the spike length is S2 by allowing the concentration of the electroless deposition solution to vary across the surface. Where a substrate comprises a channel into which the electroless deposition solution can flow, solution will usually flow into said channel when the substrate is dipped into the bath of solution. As the solution flows into the channel, electroless deposition occurs and the concentration of metal ions to be deposited in the solution flowing through the channel is decreased. Fresh solution flows into the channel from the bath over time, but always has the greatest concentration of metal ions to be deposited (corresponding approximately to the higher metal ion concentration in the surrounding bath) at the entrance to the flow channel. Thus a concentration gradient is formed, a higher concentration of metal ions to be deposited being found at the entrance or entrances to the flow channel and a lower concentration of metal ions to be deposited occurring further into the flow channel. This effect is particularly pronounced where the flow channel is long and/or narrow. The longest spikes form where the concentration of metal ions to be deposited is highest; shorter spikes form elsewhere.

Flow process

In one aspect, the process for producing a heat exchange element comprises flowing an electroless deposition solution over a surface of a substrate. In some embodiments, this electroless flow deposition process produces a heat exchange element according to the invention.

Flowing an electroless deposition solution over a surface of a substrate comprises providing a flow of electroless deposition solution moving over a surface of the substrate at a non-zero flow rate. The flow of solution is understood to be "over" a surface of the substrate if it is in contact with said surface of the substrate. The flow may be provided over one or more surfaces of the substrate, for example external and/or internal surfaces of the substrate, an internal surface being, for example, the surface of a channel (e.g. a tube) passing through the substrate. The flow of solution may be provided over the entire substrate or over only part of the substrate.

In an electroless deposition process that occurs in a static environment such as a bath, there is usually no net direction of flow of electroless deposition solution over any point on the substrate surface. The solution is typically agitated to provide movement of solution within the bath, but that movement typically has no net direction over the course of the static electroless deposition process (e.g. bath electroless deposition process). In a static environment the direction of flow of electroless deposition solution over any point on the substrate surface may change frequently and at random during the course of the electroless deposition process.

The flow electroless deposition process provides a net direction of flow of electroless deposition solution over any point on the substrate surface subjected to the flow. The direction of flow of electroless deposition solution over any point on the substrate surface subjected to the flow typically does not change during the deposition process. The direction of flow is usually constant during electroless flow deposition. However, the direction of flow provided to the surface of the substrate may be deliberately changed during the course of the flow process.

As the skilled person will appreciate, the precise details of the process used to provide a flow of electroless deposition solution over a substrate (for example the apparatus) may vary. In a particularly simple setup, the process may comprise pouring the electroless deposition solution from a container over a surface of a substrate. More usually, the process will comprise generating flow of electroless deposition solution using a flow generator (e.g. a pump), and providing the flow of electroless deposition solution to a surface of the substrate via a conduit.

In the flow process of the invention, the electroless deposition solution flows over the surface of a substrate. The path taken by the flow of solution over the substrate surface is referred to as "the solution flow path". In some embodiments, the coating is formed over the whole of the solution flow path. In other embodiments, such as embodiments wherein part of the substrate on the solution flow path is masked to prevent electroless deposition, the coating is formed over part of the solution flow path. Preferably, the coating is formed over the whole of the solution flow path.

The flow of electroless deposition solution over a surface of a substrate creates a

concentration gradient over the said surface of the substrate. The electroless deposition solution provided to the surface contains metal ions to be deposited. The concentration of such metal ions in the electroless deposition solution before it is provided to the surface of the substrate may be referred to as Ci. Once the electroless deposition solution contacts the substrate (that is, once the solution contacts a part of the surface of the substrate which is susceptible to being coated by electroless deposition) electroless deposition will occur. This causes metal ions to come out of solution and reduces the concentration of metal ions to a value lower than Ci. Thus as the electroless deposition solution flows over the substrate, it is depleted. A concentration gradient forms. The concentration gradient lies along the solution flow path. The concentration of metal ions in the solution decreases as the distance the solution has travelled over the substrate surface increases.

As discussed above, the concentration of metal ions tends to affect the thickness of the coating and the size of surface features formed by the process of the invention. Thus the process of the invention may provide a coating that varies in thickness, and/or that shows varying size of surface features, along the path taken by the flow of solution over the substrate surface. A surface feature is a structure in the coating such as a spike. In one embodiment, the thickness of the coating decreases along the solution flow path. The coating may be thickest at or near an end of the solution flow path, usually the end at which the electroless deposition solution first contacts the surface of the substrate during the process of the invention. In another embodiment, the size of the surface features decreases along the solution flow path. The surface features may be largest at or near an end of the solution flow path, usually the end at which the electroless deposition solution first contacts the surface of the substrate during the process of the invention.

The formation of longer spikes is favoured by a higher concentration of metal ions in the electroless deposition solution. Thus, the electroless flow deposition process can produce a coated heat exchange element, the coating comprising a first region wherein the average spike length is Si and a second region wherein the average spike length is S2. The electroless deposition flows along a route across the surface or surfaces of the substrate, that route being the flow path of the electroless deposition solution. Typically, the first region is at or near the end of that flow path, and the second region is also on the flow path.

In some embodiments, the process comprises flowing an electroless deposition solution over a surface of the substrate along a single flow path in a single direction. In other

embodiments, the flow process comprises a step of changing the flow direction. The flow process may additionally or alternatively comprise a step of changing the flow path of the electroless deposition solution over the surface or surfaces of the substrate.

Where the process comprises changing (e.g. reversing) the direction of flow or the flow path of the electroless deposition solution, the process comprises altering the direction of the concentration gradient over the surface of the substrate. Thus, the position or region on the surface of the substrate at which the thickest coating or largest features (e.g. the longest spikes) form will move to the new position, corresponding to the new position of the substrate surface which is in contact with the electroless deposition solution having a maximum concentration, Ci. This may cause the creation of a further region, a third region, having an average spike length different to or the same as that in the first or second regions. For example, where the flow of electroless deposition solution is provided along a flow channel, the flow channel may comprise a thicker coating and/or larger features at or near the end of the flow channel where the electroless deposition solution entered the channel, and a thinner coating and/or smaller features further along the channel. If the process comprises flowing an electroless deposition solution into one end of a flow channel of a substrate (the inlet) and out of another end of the flow channel (the outlet), the thickest coating/largest features will be located at or near the inlet and the thinnest coating/smallest will be located at or near the outlet. However, if the direction of flow is reversed during the electroless deposition process, the thinnest coating/smallest features will be located at or near the middle of the channel and the thicker coating/largest features will be located at or near each end of the channel.

For example, where the flow of electroless deposition solution is provided along a flow channel, the first region may be located at or near an end of the flow channel and the second region may be located further along the flow channel. Thus, the flow channel may comprise longer spikes at or near the end of the flow channel where the electroless deposition solution entered the channel, and shorter spikes further along the channel. If the process comprises flowing an electroless deposition solution into one end of a flow channel of a substrate (the inlet) and out of another end of the flow channel (the outlet), the longest spikes will be located at or near the inlet and the shortest spikes will be located at or near the outlet.

However, if the direction of flow is reversed during the electroless deposition process, the shortest spikes will be located at or near the middle of the channel and longer spikes will be located at or near each end of the channel.

The coating thickness and/or feature size, e.g. the spike length, produced in the electroless deposition process can be adjusted by controlling the flow rate. The faster the flow rate, the thinner the coating and/or the smaller the surface features produced. For instance, the faster the flow rate, the smaller the spikes produced. Additionally, increasing the flow rate reduces the size of the concentration gradient and hence reduces the variation in coating thickness and/or feature size (e.g. spike length) along the flow path of the electroless deposition solution. In some embodiments, the process of the invention comprises continually providing fresh solution to the substrate. In other embodiments, the electroless deposition solution is recycled. In one such embodiment, the process comprises:

flowing the electroless deposition solution from a reservoir of electroless deposition solution over the surface of the substrate; and

returning the electroless deposition solution to the said reservoir.

By "reservoir" is meant a volume of electroless deposition solution. Typically the composition of the electroless deposition solution in the reservoir is not adjusted by an external source. For example, the reservoir is typically not topped up from an external source of electroless deposition solution (during the electroless deposition process).

Generally, the electroless deposition solution is provided to the surface of the substrate by pumping.

During electroless deposition, the process generally comprises flowing electroless deposition solution over the substrate surface at a flow rate of at least 10 mL/min. Typically, the process comprises flowing electroless deposition solution over the substrate surface at a flow rate of at least 50 mL/min or at least 100 mL/min, preferably at least 1 L/min. The aforementioned flow rates are generally maintained for at least one minute, for instance for at least ten minutes, e.g. for at least 30 minutes or 1 hour.

The process may involve varying the flow rate of the electroless deposition solution. In one embodiment, the process comprises:

flowing an electroless deposition solution over a surface of the substrate at a first flow rate Fi; and

flowing an electroless deposition solution over the said surface of the substrate at a second flow rate F ₂ .

Fi and F2 are typically different. Fi and F2 are typically at least 10 mL/min. For example, Fi and F ₂ may be at least 50 mL/min or 100 mL/min. In one embodiment, F ₂ is greater than Fi. For example, F ₂ may be twice as large as Fi. e.g. F ₂ may be 2 to 50 times as large as Fi. In another embodiment, F ₂ is greater than Fi. For example, Fi may be twice as large as F ₂ . e.g. Fi may be 2 to 50 times as large as F ₂ . This latter embodiment may be useful to ensure that the substrate is rapidly covered with electroless deposition solution, before the flow rate is adjusted to a suitable deposition flow rate.

In some embodiments, F2 is sufficiently large to reduce the adherence of hydrogen bubbles from the substrate during electroless deposition. F2 may be large enough to force hydrogen bubbles away from the substrate surface during electroless deposition. In some

embodiments, both Fi and F ₂ are sufficiently large to reduce the adherence of hydrogen bubbles from the substrate during electroless deposition, and/or to force hydrogen bubbles away from the substrate surface during electroless deposition. In some embodiments, Fi and/or F2 are sufficiently large to force electroless deposition solution into a flow channel of the substrate.

In one aspect, the process comprises:

pumping an electroless deposition solution from a reservoir over a surface of the substrate at a first flow rate Fi and returning the electroless deposition solution to the reservoir; and

pumping an electroless deposition solution from a reservoir over the said surface of the substrate at a second flow rate F2 and returning the electroless deposition solution to the reservoir.

In some cases, the process comprises flowing an electroless deposition solution over a surface of the substrate at a first flow rate Fi for an initiation period before changing the flow rate to F ₂ . For instance, the solution may be provided at a flow rate Fi until all the surfaces of the substrate which are to be coated are covered with the solution.

In some embodiments, the process comprises pumping the electroless deposition solution to cause the electroless deposition solution to flow over a surface of the substrate. For example, the process may comprise pumping solution from a reservoir to create a flow of electroless deposition solution over a surface of the substrate. Alternatively or additionally the process may comprise pumping electroless deposition solution away from the substrate, for instance out of one or more flow channels in the substrate. A suitable pump is any kind of device suitable for creating a flow of a fluid, particularly of liquid. In some embodiments, the substrate comprises one or more flow channel(s), and the process the process comprises flowing an electroless deposition solution through said one or more of said flow channel(s). That is, the solution flow path comprises one or more of the said flow channel(s). In some embodiments, the substrate comprises a flow channel, and the process the process comprises flowing an electroless deposition solution through said flow channel.

By flow channel is meant a path by which fluid can pass through the substrate. In these embodiments the process comprises contacting the electroless deposition solution with the surface of said flow channel, usually the inner surface of said flow channel.

In one aspect of this embodiment, the process may involve coating one or more flow channel(s) that are suitable for and/or intended for carrying refrigerant. In a preferred aspect of this embodiment the process comprises flowing an electroless deposition liquid only over a surface or surfaces of the substrate which are for contacting a refrigerant. For instance the substrate may be suitable for use as a fluid-fluid heat exchanger having a region for carrying the refrigerant fluid (a fluid to which heat is transferred from the heat exchange element) and a region for the heat transfer fluid (a fluid which transfers heat to the heat exchange element), and the process comprises flowing electroless deposition solution only over one or more surfaces which are suitable for and intended for contacting a refrigerant (i.e. the refrigerant fluid). This embodiment is advantageous as it ensures that no electroless deposition solution is wasted in coating surfaces not intended for heat transfer.

The electroless flow deposition process is advantageous as it enables the flow electroless deposition solution to be controlled such that only the surfaces of the substrate which are intended to be coated may come into contact with the solution. This reduces wastage of the solution. The flow of solution may be controlled by, for example, connecting a reservoir of solution to the inlet or inlets of those flow channels which are intended to be coated. Other flow channels may be blocked.

Advantages of the electroless flow deposition process (flow process)

As explained above, the flow process of the invention reduces the effect of hydrogen bubbles on the coating of a substrate by electroless deposition. The flow process therefore provides a heat exchange element which is robust, durable and resistant to corrosion. The flow process has various other advantages. For example, the flow process can advantageously be used to coat substrates having small depressions (e.g. small holes or channels therein) which may not be conveniently coated by a bath electroless deposition process. Where a substrate having small depressions is placed into a bath of liquid, pockets of air may be trapped in those depressions, preventing the electroless deposition solution from contacting the substrate hidden beneath the trapped air. The flow process of the invention is generally performed at a sufficiently high flow rate to force such air pockets away from the substrate surface and thus to ensure that all parts of the substrate which are exposed to the electroless deposition solution are coated. The flow process of the invention is therefore suitable for coating, for instance, substrates comprising very narrow channels, particularly substrates suitable for use in electronics cooling.

Another advantage of the electroless flow deposition process is that it reduces wastage of deposition solution. In the flow process of the invention, the flow path of the electroless deposition solution over the substrate surface may be controlled. Consequently, electroless deposition solution may be provided only to those parts of the surface which are to be coated in the process of the invention. By contrast, in a bath process, the entire substrate is usually immersed in the electroless deposition solution and any parts of the substrate surface which are not intended to be coated are masked by a protective coating. This may result in electroless deposition occurring on the protective coating, wasting material.

A further advantage of the flow process of the invention is that various parameters of the process may be controlled in order to adjust the surface structure obtained. For example, the flow rate of the electroless deposition solution over the surface, the temperature at which the process is performed, the composition of the electroless deposition solution and so on may all be altered to adjust the structure of the coating produced by the process. Particularly advantageous structures for heat exchange are achieved where the electroless deposition solution comprises copper ions, due to the high conductivity of the resulting copper coating.

The process can create a wide array of coatings suitable for heat transfer in various conditions. For instance, it may be used to produce a heat exchange element suitable for transferring heat to a fluid, preferably to a liquid. In one embodiment the process is for producing a heat exchange element with a coating suitable for use in an evaporative heat exchanger (a heat exchanger which cools a fluid by heating another fluid to the point of evaporation). In another embodiment the process is for producing a heat exchange element with a coating suitable for transferring heat by boiling a liquid.

A particular advantage of the flow process of the invention is that it may be used to retrofit an electrolessly deposited coating to an existing heat exchanger in situ and without disassembly.

Aspects of electroless flow deposition process

The following specific aspects of the electroless flow deposition process are provided.

1. A process for producing a heat exchange element comprising a substrate and a coating, wherein:

the coating comprises a metal; and

the process comprises flowing an electroless deposition solution over a surface of the substrate.

2. A process according to aspect 1 wherein the process is performed at a temperature of from 20 °C to 120 °C.

3. A process according to aspect 1 or aspect 2 wherein the electroless deposition solution is an aqueous solution.

4. A process according to any preceding aspect wherein the electroless deposition solution comprises copper and/or nickel ions.

5. A process according to any preceding aspect wherein the process comprises:

flowing the electroless deposition solution from a reservoir of electroless deposition solution over the surface of the substrate; and

returning the electroless deposition solution to the said reservoir.

6. A process according to any preceding aspect wherein the process comprises flowing the electroless deposition solution over the surface of the substrate for a period of from 0.5 hours to 10 hours. 7. A process according to aspect 5 or aspect 6 wherein the process comprises flowing the electroless deposition solution over the surface of the substrate for a time T, wherein T is the time taken for the electroless deposition solution to become depleted by 5 to 50%.

8. A process according to any preceding aspect wherein the process comprises monitoring the ion concentration in the electroless deposition solution.

9. A process according to any preceding aspect wherein the process comprises:

flowing an electroless deposition solution over a surface of the substrate at a first flow rate Fi; and

flowing an electroless deposition solution over the said surface of the substrate at a second flow rate F ₂ .

10. A process according to aspect 9 wherein F2 is greater than Fi.

11. A process according to any preceding aspect wherein the process comprises pumping the electroless deposition solution to cause the electroless deposition solution to flow over a surface of the substrate.

12. A process according to any preceding aspect wherein the substrate comprises a flow channel, and the process the process comprises flowing an electroless deposition solution through said flow channel.

13. A process according to any preceding aspect wherein the process comprises:

(i) providing an acid to a surface of the substrate, and/or

(ii) activating a surface of the substrate,

wherein steps (i) and/or (ii) are carried out prior to flowing an electroless deposition solution over the surface of the substrate according to any preceding aspect.

14. A process according to any preceding aspect wherein the process comprises applying a surface layer, preferably a surface layer comprising nickel, to a heat exchange element produced according to any one of aspects 1 to 13. 15. A heat exchange element obtainable or obtained by a process according to any preceding aspect.

Additional process steps

Usually, prior to electroless deposition the surface of the substrate is prepared by exposure to acid and to an activation solution. Thus, in one embodiment, the process for producing a heat exchange element comprises:

(i) providing an acid to a surface of the substrate; and/or

(ii) activating a surface of the substrate,

wherein steps (i) and/or (ii) are carried out prior to providing an electroless deposition solution to the surface of the substrate.

The function of the acid is typically to clean and optionally also to etch the surface. Suitable acids include sulphuric acid, hydrochloric acid or nitric acid. Where the substrate is a steel substrate, the acid used is preferably sulphuric acid. Where the substrate is a copper substrate, the acid used is preferably hydrochloric acid. The acid used is typically strong acid, for instance 20% acid or above. Generally, the step of exposure to acid is performed at room temperature or above, for instance from 20 °C to 120 °C, usually from 50 °C to 100 °C. Usually the substrate is exposed to acid for a minute or more, preferably from 1 minute to 1 hour. The substrate is usually rinsed with water after exposure to acid.

The step of activating the surface may involve providing a metal-containing solution to the surface, for instance an aqueous solution comprising metal ions. An exemplary activating solution is PdCh solution. Activation is generally performed at a temperature of between 0 °C and 100 °C, usually at room temperature. Typically the substrate is rinsed after the activation step and prior to electroless deposition.

Other steps which may be performed prior to the electroless deposition process include, for instance, applying a protective mask to a part of the substrate surface to prevent application of coating to that part of the substrate.

In some embodiments, the process for producing a heat exchange element of the invention comprises one or more steps performed after the electroless deposition process. In one embodiment, the process comprises applying a surface layer, preferably a surface layer comprising a metal, e.g. nickel or tin (preferably nickel), to a heat exchange element produced by a method as described herein.

Experimental protocol

Exemplary methods of applying a coating to a substrate to produce a heat exchange element of the invention are described below.

1. Bath process i. Any part of the substrate which is not intended to be coated is protected, for instance by applying stop-off lacquer thereto. This step may be unnecessary if it is intended to apply coating to all parts of the substrate that will be placed in the bath.

ii. The substrate is pre-heated to a temperature of 80 °C by submerging the substrate in a water bath at 80 °C.

iii. The substrate is then transferred to a 20% sulphuric acid bath at 80 °C and left for 15 minutes. The substrate is then rinsed in deionised water.

iv. The substrate is placed in a PdCk solution (1 g L ^"1 ) for 2 minutes. The substrate is then rinsed in deionised water.

v. The substrate is then re-heated to 80 °C as in (ii).

vi. The substrate is then placed in a bath of nanoFLUX electroless deposition solution at 75 °C for two hours. The nanoFLUX solution comprises 0.01M to 0.1M CuSC , 0.001M to 0.01M NiS0 ₄ , 0.1M to 0.5M NaH ₂ P0 ₂ , 0.001M to 0.1M Na ₃ C ₆ H ₅ 07, O. lM to IM HBO3, Janus Green 0-700ppm, PVP 0-200ppm, CTAB 0-300ppm, SBS 0-500ppm and 0 to 200 ppm PEG. The solution in the bath is agitated continuously during this time. The bath contains at least 10 litres of solution per square metre of substrate surface to be coated.

vii. The coated substrate is removed from the bath and rinsed in deionised water.

viii. The coated substrate is dried in an oven.

The above experimental protocol can be modified to produce a coating comprising clusters by adjusting step (vi) such that: vi. The substrate is then placed in a bath of nanoFLUX electroless deposition solution at 75 °C for four hours. The solution in the bath is agitated continuously during this time. The bath contains at least 50 litres of solution per square metre of substrate surface to be coated.

2. Flow process

This flow process protocol is for coating one square metre of a substrate's surface. i. Any part of the substrate which is not intended to be coated is protected, for instance by applying stop-off lacquer thereto. This step may be unnecessary if the intended flow path of the electroless deposition solution contacts only the part or parts of the substrate which are intended to be coated.

ii. The substrate is pre-heated to a temperature of 80 °C by submerging the substrate in a water bath at 80 °C.

iii. The substrate is then transferred to a 20% sulphuric acid bath at 80 °C and left for 15 minutes. The substrate is then rinsed in deionised water.

iv. The substrate is placed in a PdCh solution (1 g L ^"1 ) for 2 minutes. The substrate is then rinsed in deionised water.

v. The substrate is then re-heated to 80 °C as in (ii).

vi. At least 10 litres of nanoFLUX electroless deposition solution as defined above are heated to 75 °C in a reservoir. The electroless deposition solution is continuously pumped from the reservoir over a surface of the substrate and returned to the reservoir for a period of two hours.

vii. The coated substrate is rinsed in deionised water.

viii. The coated substrate is dried in an oven.

The above experimental protocol can be modified to produce a coating comprising clusters by adjusting step (vi) such that: vi. At least 50 litres of nanoFLUX electroless deposition solution are heated to 75 °C in a reservoir. The electroless deposition solution is continuously pumped from the reservoir over a surface of the substrate and returned to the reservoir for a period of four hours. 3. Flow process for coating a flow channel of a heat exchanger

This flow process protocol is for coating a flow channel of a heat exchanger. The heat exchanger comprises a first flow channel having two ends (which is intended to be coated) and a second flow channel having two ends (which is for carrying a heat transfer fluid). In this protocol, the flow channel to be coated has a surface area of one square metre. i. The second flow channel of the substrate (i.e. the flow channel that is not to be

coated) is attached at each end to a water source that is maintained at 80 °C. Water is pumped through the second flow channel continuously.

ii. The first flow channel of the substrate (i.e. the flow channel that is to be coated) is attached at each end to a reservoir of 20% sulphuric acid bath at 80 °C. Sulphuric acid from the reservoir is pumped through the first flow channel for 15 minutes. iii. The pumping of water and acid is stopped.

iv. All acid is drained out of the first flow channel.

v. The first flow channel is then connected to a source of deionised water. Deionised water is pumped through the channel for 5 minutes or until the water exiting the first flow channel runs clear. All water is then drained out of the first flow channel.

vi. The first flow channel is connected at both ends to a source of PdC . solution (1 g L ^"1 ) at room temperature. PdCk solution is pumped into the first flow channel and left for 2 minutes. All PdCb. solution is then drained from the first flow channel.

vii. The substrate is then rinsed in deionised water as in step (v).

viii. The pumping of water at 80 °C through the second flow channel is restarted.

ix. The first flow channel is connected at both ends to a reservoir containing at least 10 litres of nanoFLUX electroless deposition solution (as defined above) maintained at 75 °C. The electroless deposition solution is continuously pumped from the reservoir into the flow channel slowly, such that the first flow channel is filled with the electroless deposition solution after a period of five minutes or more (for instance 0.1 L min ^"1 for a heat exchanger having a volume of 1 L).

x. The pumping rate of electroless deposition solution is increased by a factor of ten (for instance to 1.0 L min ^"1 ). This pumping rate is maintained for two hours.

xi. The pumping of electroless deposition solution is stopped and all electroless

deposition solution is drained out of the first flow channel. xii. The coated substrate is rinsed in deionised water as in step (v).

xiii. The coated substrate is dried in an oven.

The above experimental protocol can be modified to produce a coating comprising clusters in the first flow channel by:

- adjusting step (ix) such that the first flow channel is connected at both ends to a

reservoir containing at least 50 litres of nanoFLUX electroless deposition solution maintained at 75 °C; and

- adjusting step (x) such that the higher pumping rate is maintained for a period of four hours.

The skilled person will appreciate that the specific features of the above processes, such as temperature and pump rate, can be varied.

Examples

1. Preparation of sample coatings.

Small test pieces of copper were coated according to the method of the invention using the bath process protocol (protocol 1), above. The coating times were varied to achieve different spike lengths. At least 100 ml of nanoFLUX solution was used in each case. The resulting heat exchange elements according to the invention were images using scanning electron microscopy (SEM). The results are shown in Figure 3 (ordinary protocol) and Figure 5 (protocol modified to produce clusters).

Figure 3(a) shows a coating comprising spikes of 1 to 3 μπι in length, obtained by subjecting the test piece of copper to a bath of nanoFLUX solution for up to 1 hour. Figure 3(b) shows a coating comprising spikes of 4 to 5 μπι in length, obtained by subj ecting the test piece of copper to a bath of nanoFLUX solution for approximately 2 hours. Figure 3(c) shows a coating comprising spikes of 8 to 10 μπι in length, subjecting the test piece of copper to a bath of nanoFLUX solution for up to 4 hours. Figure 5(a) shows a coating comprising spikes of approximately 7 μιτι in length arranged in clusters. This coating was obtained by subjecting the test piece of copper to a bath of nanoFLUX solution for approximately 3 hours. This was reactivated and then coated for another 3 hours. Figure 5(b) also shows a coating of spikes arranged in clusters.

A test piece of steel wire mesh comprising wires 75 μπι in diameter was also coated according to the method of the invention using the bath process protocol (protocol 1), above. The wire mesh was subjected to the bath of nanoFLUX solution for approximately 3 hours. The product was imaged using SEM and the results are shown in Figure 6.

2. Preparation of a heat exchanger.

A brazed plate heat exchanger made of 316 stainless steel with a copper brazing, having a channel for a heat transfer fluid and a channel for a refrigerant, was coated according to the method of the invention using the flow process for coating a channel of a heat exchanger (protocol 3 above). This produced a heat exchanger having a coating comprising spikes approximately 3 μηι in length along the inside of the channel for a refrigerant. An SEM image of the inside of that channel is shown in Figure 4.

This example shows that the process of the invention may be used to retro-fit a heat exchange element of the invention into an existing heat exchanger. The process may be used to coat all or part of an existing heat exchanger to prepare a product according to the invention.

3. Efficiency of heat transfer of the heat exchange element in pool boiling.

A heat exchange element was produced according to the process of the invention, by coating a test piece of copper according to protocol 2, above, except that in step vi the test piece was placed in a bath of nanoFLUX solution at 75 °C for up to 4 hours. The heat exchange element was similar to that shown in Figure 3(c) (comprising spikes of 8 to 10 μπι in length). It was tested for its ability to transfer heat to an organic refrigerant in a pool boiling experiment. The pool boiling experiment is described in "Compound effect of EHD and surface roughness in pool boiling and CHF with R-123", Ahmad et al. , Applied Thermal Engineering, vol. 31, pp. 1994-2003, 2011, and in "Pool boiling on Modified Surfaces Using R- 123", Ahmad et al, Heat Transfer Engineering, Vol 35, Issue 16-17, 2014. The results are shown in Figures 7 and 8.

Figure 7 shows the heat flux in kW m ^"2 from a surface to an organic refrigerant as a function of wall superheat (ΔΤ _ε ). Results are given for a coated copper heat exchange element such as that shown in Figure 3(c) and for a polished oxygen- free copper surface. The results for the heat exchange element according to the invention are shown in blue and appear on the steeply-sloping line to the left of the graph. The results for the polished surface are shown in black and appear on the flatter line along the bottom of the graph. Figure 7 shows that the element of the invention can achieve a high heat flux from element to refrigerant while maintaining a low superheat. Even when a high heat flux is provided to the surface, the element transfers heat to the refrigerant so efficiently that its temperature does not rise far above that above the refrigerant. The surface worked so well that the heating units providing heat to the surface could not keep up with the rapid loss of heat from the surface. By contrast, the polished surface transfers heat to the refrigerant slowly. Heat flux to the surface heats the polished surface to a temperature high above that of the refrigerant as heat dissipates out of the surface and into the refrigerant slowly.

Figure 8 shows the heat flux in kW m ^"2 from a surface to an organic refrigerant as a function of wall superheat (ΔΤ ₀ ) for various different surfaces:

(i) a polished surface (the black line with the flattest slope).

(ii) a surface coated with a copper coating such as that described in

WO2014/064450, comprising copper surface structures of the order of 500 nm high (green line, almost flat up to a AT _C of approximately 11 °C and then rising sharply).

(iii) a heat exchange element according to the invention comprising a copper substrate coated with a coating comprising copper spikes 1 μιη long (red line, almost flat up to a AT _C of approximately 8 °C and then rising sharply).

(iv) a heat exchange element according to the invention comprising a copper substrate coated with a coating comprising copper spikes 10 μιη long (blue line, rising sharply from a ΔΤ _ς of approximately 2 °C). The heat exchange elements of the invention produced a very high heat flux, up to the point where the test rig failed. Moreover, the heat exchange elements of the invention (particularly that having spikes 10 μηι long) maintained a very low superheat even at high heat flux, illustrating the excellent efficiency of the heat transfer from surface to refrigerant.

4. Efficiency of heat transfer of the heat exchange element in flow boiling.

A heat exchange element according to the invention comprising a thin metal tube as a substrate was prepared by the electroless flow deposition process of the invention. A coating was provided on the inner surface of the tube such as the coating shown in Figure 4. An organic refrigerant was flowed through the tube whilst it was heated. The flow rate of the organic refrigerant was set to be 200 kg m ^"2 s ^"1 , 300 kg m ^"2 s ^"1 , 400 kg m ^"2 s ^"1 , and 500 kg m ^"2 s ^"1 in turn. The heat transfer coefficient at each flow rate was measured. The heat transfer coefficient of an uncoated tube at each flow rate was also measured. The experimental protocol for measuring the heat transfer coefficient is described in "Flow Boiling Heat Transfer In A Vertical Small-Diameter Tube: Effect Of Different Fluids And Surface Characteristics", Al-Gaheeshi et al. , Conference: Proceedings of the 4th International Forum on Heat Transfer, IFHT2016, November 2-4, 2016, at Sendai, Japan. The heat transfer coefficient in W m ^"2 K ^"1 is a measure of a measure of the efficiency of heat transfer. A large heat transfer coefficient shows that the heat exchange element transfers heat more efficiently.

The results are shown in Figure 9. As can be seen in the Figure, the coated tube had a higher heat transfer coefficient than the uncoated tube at each flow rate. The coating to produce a heat exchange element according to the invention therefore improved heat flux across the surface.

5. Variation in deposition time.

The electroless deposition process of the invention can be adjusted to vary the size of the deposited structures. Figure 10 shows the approximate spike height (dashed upper line) and spike radius at the base (solid lower line) achieved by varying the time for which a substrate is subjected to electroless deposition solution, e.g. to a flow of electroless deposition solution. The spike length increases with time, as does the base radius. The spikes are approximately conical in shape. The length and radius above are obtained by approximating each spike to a cone whose axis passes through the cone' s base at a right angle. The base of the cone is the circle in the plane at the base of the cone's shortest side. Thus, spike length is the length of the approximate cone axis from base to tip, and base radius is the radius of the circular base in this approximation.

It was noted while performing these experiments that the cone angle (that is, the angle the side of the cone makes with the base) is not affected by the length of time for which electroless deposition is performed. Thus, the process of the invention can produce long, sharp spikes; the spikes do not become less sharp as they become longer during the process of the invention.

6. Comparison of heat transfer efficiency of coated vs uncoated heat exchanger

A coated evaporator (that is, a heat exchanger) was prepared according to Protocol 3 above. As explained in that protocol, the flow channel of the evaporator for receiving refrigerant was coated according to the process of the invention. However, the other flow channel of the evaporator for receiving water was not coated.

An uncoated evaporator was also obtained for comparative purposes. This uncoated evaporator was identical in structure to the other evaporator, but neither flow channel of the uncoated evaporator was coated in accordance with the invention.

The coated and uncoated evaporators were incorporated in turn in a test rig. The test rig is shown in Figure 11.

The system used as the working refrigerant R245fa. The ability of the evaporator to transfer heat from water to the refrigerant was then evaluated. Refrigerant and water were circulated through the evaporators in accordance with normal usage of a heat exchanger. During the experimental tests performed in this regard, the rate of heat transfer from water to the refrigerant was assessed. Multiple experiments were carried out to test the coating of the coated evaporator under different flow rates. The experiments were repeated using the uncoated evaporator but the same flow parameters to enable a comparison. Based on the experiments performed, the heat transfer coefficient (UA) was calculated. This is the proportionality constant between the heat flux across a surface and the temperature differential that existed across the surface. Thus, where the surface has a large heat transfer coefficient, it can efficiently transfer heat even where the difference in temperature across the two sides of the surface is small. Surfaces with smaller heat transfer coefficients require a larger difference in temperature across the surface (i.e. a larger driving force) before the surface will permit an appreciable flow of heat Methods of calculating the heat transfer coefficient are given by Fernando et al. in "Propane heat pump with low refrigerant charge: design and laboratory tests", International Journal of Refrigeration, 27(1), pp.761-773, 2004, and by Dutto et al. in "Performance of brazed plate heat exchanger set in heat pump

Proceedings of the 18th International Congress of Refrigeration, new challenges in refrigeration", Montreal, Quebec, Canada, vol. 3 (10-17 August 1991).

The heat transfer coefficient as a function of temperature for the two evaporators is shown in Figure 12. The refrigerant flow rate through the evaporators in both cases was 0.0121 kg/s.

The UA of the uncoated evaporator is shown by the square markers, and is in the region of 200 W/K. However, the coated evaporator, shown by circular markers, achieves heat transfer coefficients in the region of 300 W/K. For example, comparing the heat transfer coefficient calculated at a rate of heat transfer on the water side of the evaporators of 2.25 kW shows that the UA of the coated evaporator is 51.74% higher than that of the uncoated evaporator. This is a remarkable improvement.

Moreover, this was achieved with very little pressure drop on the refrigerant-coated side of the evaporator during operation. The pressure drop is the difference in flow pressure along the flow path (for example, between the point of entry to the heat exchanger and the point of exit). The pressure drop is caused by turbulence in the flow of fluid along the flow path. A high pressure drop means that the pump pumping fluid (e.g. refrigerant) through the heat exchanger needs to work harder to force the fluid through. It is therefore advantageous that a high pressure drop is avoided.