TITLE: Heat management device using inorganic foam

FIELD OF THE INVENTION

The present invention relates to heat management devices such as a heat pipe that uses a component made of inorganic porous material. The heat pipe can be used to provide cooling in a wide range of applications through repeated evaporation and condensation cycles of a working liquid. In one specific example the heat pipe can be used to provide cooling to electronic devices such as Central Processing Units (CPUs) . The invention also extends to a method for making metallic porous material and to the resulting product thereof.

BACKGROUND OF THE INVENTION

Many different applications exist, in particular in the electronics industry where components need to be cooled such as to maintain them within a temperature range in which they can reliably operate. An example of a cooling device that has found wide acceptance is the heat pipe. A heat pipe is capable of transferring thermal energy very effectively allowing to maintain the surface of an electronic component, such as a CPU, relatively cool.

A typical heat pipe has a closed chamber with a hot side and a cold side. The hot side is the side that receives the heat to be removed while the cold side transfers that heat to an adjoining medium acting as a heat sink. Working fluid, such as water is provided in the closed chamber. When the hot side receives thermal energy, that energy vaporizes the working fluid that is in the vicinity of the hot side. The vapor naturally flows in the chamber to the cold side where it condenses. The latent heat

of vaporization that is released during the condensation process is transmitted to the cold side and to the adjoining medium. A wick is provided between the cold and the hot sides such that condensed liquid is continuously supplied to the hot side where it can be vaporized again.

The wick can be made from sintered metallic material that owing to its porous structure pulls the liquid by capillary action toward the hot side. Alternative arrangements can also be used such as an array of fine channels machined on the bottom wall of the chamber that also develop capillary pressure capable of transporting the working liquid.

As an alternative to the wick, the geometry and orientation of the heat pipe may be such that liquid is allowed to flow back to the hot side by gravity. In such case, a wick structure to transport the liquid may not be required.

The continuous phase change cycle of the working liquid from liquid to vapor and then back to liquid confers a very good heat transport characteristics to the heat pipe. Depending on the design and configuration of the heat pipe, a number of factors determine the ultimate heat transport capacity. The key factors are (1) the ability of the structure to allow the vaporized liquid to escape from the boiling liquid and (2) the ability to supply continuously sufficient amounts of working liquid to the hot side for the evaporation to be maintained. If any one of these two mechanisms is interrupted the heat transport cycle stops and in the case of key factor (1) the heat pipe reaches the so called critical heat flux value and in case of key factor (2) the heat pipe reaches the so called dry out state. At

the critical heat flux value water vapor is essentially- trapped under the pool of working liquid and the heat pipe essentially ceases to function. In the dry out state, there is no more working liquid to be evaporated and the heat pipe essentially ceases to function.

While current heat pipe designs are effective, there is a need in the industry to raise their heat transport effectiveness even further such as to allow the heat pipes to be made smaller and/or capable of handling more intense cooling requirements.

SUMMARY OF THE INVENTION

As embodied and broadly described herein the invention provides a heat pipe having an enclosed chamber with a hot side and a cold side and an inorganic porous structure between the hot side and the cold side. The inorganic porous structure transports working liquid by capillary action from the cold side toward the hot side and having a wicking speed in excess of about 0.005m/s.

As embodied and broadly described herein the invention provides a heat pipe having an enclosed chamber with a hot side and a cold side and an inorganic porous structure between the hot side and the cold side. The inorganic porous structure transports working liquid by capillary action from the cold side toward the hot side and having an absorption capacity of at least 200kg/m ³ .

As embodied and broadly described herein the invention provides a heat pipe having an enclosed chamber with a hot side and a cold side. The heat pipe also has an inorganic porous structure between the hot side and the cold side

for transporting working liquid by capillary action from the cold side toward the hot side, the inorganic porous structure having a porosity distribution, characterized by: i) a first pore group:

(1) having an average pore size in the range from about 200μm to about lOOOμm;

(2) having a pore size standard deviation in the range from about lOOμm to about 500μm (3) constituting in the range from about

30% to about 80% of the void volume of the inorganic porous structure; ii) a second pore group having:

(1) having an average pore size in the range from about 40μm to about 120μm;

(2) having a pore size standard deviation in the range from about 30μm to 80μm;

(3) constituting at least 20% of the void volume of the metallic porous structure; iϋ) a third pore group:

(1) having an average pore size in the range from about 250nm to about 20μm;

(2) having a pore size standard deviation in the range from about 200nm to lOμm; (3) constituting in the range from about

10% of to about 40% the void volume of the metallic porous structure.

As embodied and broadly described herein the invention provides a heat pipe having an enclosed chamber having a hot side and a cold side. An inorganic porous structure is provided between the hot side and the cold side for transporting working liquid by capillary action from the

cold side toward the hot side. The inorganic porous structure has a porosity distribution, characterized by: i) a first pore group:

(1) having an average pore size in the range from about 20μm to about 200μm;

(2) having a pore size standard deviation in the range from about lOμm to about lOOμm

(3) constituting in the range from about 50% to about 80% of the void volume of the inorganic porous structure; ii) a second pore group:

(1) having an average pore size in the range from about 250nm to about 15μm;

(2) having a pore size standard deviation in the range from about 200nm to lOμm;

(3) constituting in the range from about 20% of to about 50% the void volume of the metallic porous structure.

As embodied and broadly described herein the invention provides a heat pipe having an enclosed chamber with a hot side and a cold side and an inorganic porous structure for receiving working fluid that is evaporated on the hot side and condensed on the cold side. The inorganic porous structure having a specific surface area in the range from about 10,000 m ² /m ³ to about 100,000 m ² /m ³ .

As embodied and broadly described herein the invention provides a heat pipe having an enclosed chamber with a hot side and a cold side, at least a portion of the enclosed chamber defining a conduit for conveying working fluid. The conduit has a direction of longitudinal extent and having walls defining in cross section a figure having a

closed boundary. An insert made of inorganic porous material is provided within the conduit, the insert being in contact with the wall and in cross-section having a shape that follows a shape of the wall at least along a portion thereof.

As embodied and broadly described herein the invention provides a process for manufacturing a component for use in a heat management device, the process comprising: a) providing a sheet of heat conductive material having: iii) a pair of generally opposite main faces; iv) a pair of opposite edge portions; b) placing on one of the main faces a body of inorganic porous material; c) rolling the sheet into a tube while the body of metallic material is located on the one of the main faces, the rolling bringing the opposite edge portions in a face-to-face relationship; d) joining the opposite edge portions.

As embodied and broadly described herein the invention provides a heat pipe having an enclosed chamber having a hot side and a cold side. The heat pipe also having: a) working fluid in the enclosed chamber; b) an inorganic porous structure for receiving thermal energy input at the hot side and for boiling the working fluid, the inorganic porous structure: i) having a porosity distribution, characterized by: (1) a first pore group having an average pore size in excess of about 20μm; (2) a second pore group having:

(a) having an average pore size in the range from about 250nm to less than about 15μm;

(b) having a pore size standard deviation in the range from about 200nm to about 10 μm;

(c) constituting in the range from about 20% to about 50% of the void volume of the inorganic porous structure; ii) having at least one main surface including a plurality of projections.

As embodied and broadly described herein the invention provides a heat pipe having an enclosed chamber having a hot side and a cold side. The heat pipe also having: a) working fluid in the enclosed chamber; b) an inorganic porous structure for receiving thermal energy input at the hot side and for boiling the working fluid, the inorganic porous structure having: i) a base layer; ii) a plurality of projections extending from the base layer and being integrally formed with the base layer, the projections being spaced apart and defining valleys therebetween; iii) the projections and the base layer being porous; iv) the pores in the projections having an average pore size that is larger than an average pore size of portions of the base layer that register with the valleys.

As embodied and broadly described herein the invention provides a method for manufacturing a metallic porous structure for boiling working fluid in a heat pipe, the method including providing a metallic porous blank having a pair of main faces opposite to one another and embossing one of the main faces to create a plurality of spaced apart projections.

As embodied and broadly described herein, the invention further includes method for making an open cell porous body, the method comprising the steps of: a. providing a dry flowable powder mixture comprising: l. a first predetermined amount of inorganic particles having a first melting temperature; li. a second predetermined amount of a binding agent having a decomposition temperature, the decomposition temperature being lower than the first melting temperature; b. heating the mixture at a temperature lower than the decomposition temperature to solidify or cure the binding agent to obtain a solid continuous mixture; and c. heating the solid continuous mixture at the decomposition temperature to decompose cleanly the binding agent and obtain an non-sintered open cell porous body; and d. heating the non-sintered open cell porous body at a temperature lower than the first melting temperature to sinter the inorganic particles and obtain a solid low density open cell porous body.

As embodied and broadly described herein, the invention further provides a dry flowable powder mixture for making open cell porous bodies. The mixture comprising a first predetermined amount of inorganic particles having a first melting temperature and a second predetermined amount of a binding agent having a decomposition temperature, the decomposition temperature being lower than the first melting temperature.

BRIEFDESCRIPTION OF THE DRAWINGS

A detailed description of examples of implementation of the present invention is provided hereinbelow with

reference to the following drawings, in which:

Figure 1 a longitudinal cross-sectional view of heat pipe constructed according to a non-limiting example of implementation of the invention;

Figure 2 a transverse cross-sectional view of the heat pipe of the example shown in Figure 1;

Figure 3 is cross-sectional view of a heat-pipe according to another non-limiting example of implementation of the invention;

Figure 4 illustrates a flat sheet of metallic porous material for use in making the wicking structure of a heat pipe;

Figure 5 shows the flat sheet of the metallic porous material of Figure 4 formed into a tube for insertion in a heat pipe;

Figure 6 shows the rolled tube of metallic porous material placed inside a heat pipe;

Figure 7 shows a flat sheet including several layers for use in making a heat pipe;

Figure 8 illustrates the tube structure obtained by rolling the flat sheet of Figure 7;

Figure 9 is a cross-sectional shape of a heat pipe according to yet another non-limiting example of implementation of the invention;

Figure 10 is perspective view of the heat pipe shown in Figure 9, some elements being shown only partially to expose underlying structures;

Figure 11 is a perspective view of a metallic porous structure for use in boiling working liquid in a heat pipe;

Figure 12 is an enlarged cross-sectional view of the metallic porous structure shown in Figure 11; and

Figure 13 illustrates in cross-section yet another non-limiting example of implementation of a heat pipe;

In the drawings, embodiments of the invention are illustrated by way of example. It is to be expressly understood that the description and drawings are only for purposes of illustration and as an aid to understanding, and are not intended to be a definition of the limits of the invention.

DETAILED DESCRIPTION

Figure 1 is a longitudinal cross-sectional view of a heat pipe, according to a non-limiting example of implementation of the invention. The heat pipe 10 is used to provide cooling to a heat generating component (not shown) . Typically, such a component is an electronic component such as a Central Processing Unit (CPU) . Heat pipes are effective cooling devices since they rely on the vaporization and condensation of a working fluid, as a heat transport vehicle.

The heat pipe 10 has an enclosed chamber 12, which in this example is in the form of an elongated cylinder. The enclosed chamber has, therefore cylindrical walls closed by end caps 14. Inside the enclosed chamber 12 is provided a wick structure in the form of tubular liner 16. The tubular liner 16 is an inorganic porous structure that will be described in greater detail later. As shown in the drawings, the tubular liner 16 extends almost the full length of the heat pipe. It should be expressly noted that this is merely an example of implementation and many variations as to the placement, structure, shape and size of the wick structure are possible.

The heat pipe 12 contains working fluid. The working fluid is capable to accumulate thermal energy by undergoing phase transition (from liquid to vapor) and then releases that energy to an external medium. The energy release causes the vapor to condensate. This cycle repeats itself as long as there is heat to dissipate.

The heat pipe 12 has a hot section at which the thermal energy is received and a cold section from which the thermal energy is dissipated. The hot section and the cold section can be located on any area of the heat pipe structure 12 as long as they are sufficiently spaced apart to allow the phase transitions in the working liquid to take place. For instance the hot section can be designated as the end portion 18 while the cold section can be designated as the end portion 20. Thus, the hot section 18 will, in use be in contact with the component to be cooled, while the cold section 20 will release heat to the surrounding medium. This surrounding medium may be air, water or any other suitable material that acts as a heat sink. To provide a more efficient heat release from

the cold side 20, the cold side 20 may be provided with any suitable heat dissipation structure such as fins, for instance (not shown in the drawings) .

The heat pipe 12 contains working fluid. That fluid is in a liquid phase in the area of the hot section 18. As a result of heat received at the hot section 18 the working liquid boils and converts to vapor, the phase change storing a significant amount of thermal energy. The working vapor is allowed to flow toward the cold section 20 via the lumen of the tubular liner 16. As the vapor reaches the cold section 20 it condensed and released heat to the surrounding medium. The condensation effect reduces the vapor pressure in the cold section 20 and as a result creates a lower pressure which has the effect of pulling vapor near the hot section 18 toward the cold section 20. Accordingly, the boiling and the condensation of the working fluid creates a pressure gradient in the heat pipe 12 that naturally causes the vapor to flow from the hot section 18 toward the cold section 20.

The vapor condensed into liquid at the cold section 20 at the surface or to some limited depth within the tubular liner 16. As indicated previously, the tubular liner 16 is a porous structure. The porous structure that will be discussed in greater detail later defines a certain void volume within the "solid" area of the tubular liner 16 (the lumen 22 is not considered to be part of the solid area) . The void volume is capable to take-up the condensed liquid and to carry that liquid toward the hot section 18. The liquid transport is the result of capillary pressure in the porous structure. Since the liquid in the tubular liner 16 is boiled out at the hot

section 18, the void volume or pores in that area are dry. Accordingly, they pull by capillarity the liquid that is building up in the porous structure near the cold section 20. Accordingly, the liquid migrates from the cold section 20 to the hot section 18, in a direction opposite the flow of vapor, such as to sustain the phase transitions of the working fluid.

Gravity also has an effect over the movement of the liquid in the heat pipe 12. In the example shown in

Figure 1, where the flow path of the liquid is horizontal, the gravity effect is largely minimized. In this case, gravity only creates a non-uniform liquid loading of the tubular liner 16, where more liquid will tend to accumulate in the lower part of the tubular liner 16 than in the upper part. However, this non-uniformity is also dependent on the actual pore size and the attendant capillary pressure exerted on the liquid. When the pore sizes are relatively small, the capillary pressure is higher and can counterbalance the gravity effect.

Accordingly, smaller pore sizes of the tubular liner will tend to favor a more uniform liquid distribution (in a vertical plane) in the tubular liner 16.

Gravity will have a more pronounced effect when the geometry of the heat pipe 12 is such that the hot section 18 is at a different elevation than the cold section 20 (not shown in the drawings) . For example, if the cold section 20 is located at a higher elevation than the hot section 18, then gravity will assist the movement of working liquid toward the hot section 18. In contrast, when the cold section is located at a lower elevation than the hot section 18, then the capillary effect will have to combat gravity in order to transport the liquid toward the

hot section 18 .

The amount and type of working liquid in the heat pipe 12 will vary according to the intended application. Since the heat transport mechanism is based on phase transition, it is not necessary to form in the heat pipe 12 a pool of liquid that will submerge a portion of the tubular liner 16. It is usually sufficient to provide enough liquid such as to create within the tubular liner a continuum of liquid that extends from the cold section 20 to the hot section 18. As to the type of working liquid used, it depends on the temperature range within which the cooling is to be provided and also the compatibility of the liquid and the materials used to make the heat pipe 12, including the tubular liner 16. In one specific example, the working liquid is water and the enclosed chamber 12 and the tubular liner 16 are both made of copper. Other possibilities exist. In operating temperature under water's freezing point, ammonia (NH3) is used as the working liquid and nickel as the constituting material for the enclosed chamber 12 and the tubular liner 16.

The tubular liner 16 is bonded to the inner wall of the enclosed chamber 12 in order to provide a good thermal conductivity. Such thermal conductivity is important to allow heat to easily enter the hot section 18 and boil the liquid and also easily egress the cold section 20. Good thermal conductivity is created by forming an intimate physical contact between the tubular liner 16 and the inner wall of the enclosed chamber 12. Examples of bonding techniques which would work well when the tubular liner 16 is made of metal and that would provide an intimate physical contact include sintering or soldering

connections between the tubular liner 16 and the enclosed chamber 12.

Soldering is a bonding process whereby a filler metal or alloy is heated to or above its melting temperature, which is generally referred to its liquidus temperature and below the melting temperature or solidus temperature of the base material to be joined. The molten filler metal or alloy flows between two or more close-fitting parts of the material to be joined by capillary action. At its melting temperature, the molten filler metal or alloy wets the base material and interacts with a thin layer of the base material, cooling to form a sealed joint. Note that for the purposes of this specification, soldering encompasses brazing techniques which use non-ferrous filler materials that have a relatively high melting point, generally above 450 degrees Celsius.

Sintering is a method for making objects, generally from powdered material, by heating the material until its particles adhere to each other. Sintering does not melt the material particles to create the bond between them: the material particles adhere to each other through a bond mainly created by solid-state diffusion. Effective solid- state diffusion occurs between material particles when they are heated, for a certain time, at temperatures slightly under the melting temperature of the material particles .

The efficiency of the heat pipe 12 is determined largely by the rate at which it can pump heat out of the hot section 18. One way to increase the efficiency of the heat pipe 12, without altering its size, is to design the heat pipe 12 such as the phase transitions occur at a

faster rate. In other words, more working fluid is boiled and condensed per unit of time such as to carry more heat. One factor that limits the rate at which working liquid can be boiled is the ability of the tubular liner 16 to replenish the hot section 18 with sufficient amounts of liquid. When the amount of heat applied at the hot section 18 is such that the rate at which the working liquid is boiled exceeds the rate at which the tubular liner can replenish the hot section 18, a dry-out occurs and the heat pipe 12 ceases to function.

The material used for making the tubular liner 16 is designed such that its porosity induces liquid to travel relatively quickly such as to be able to feed the hot section 14 adequately. In a specific and non limiting example of implementation the tubular liner has a wicking speed in excess of about 0.0005m/s, more preferably in excess of about 0.00075m/s, even more preferably in excess of about 0.001m/s, yet even more preferably in excess of about 0.0015m/s and most preferably in excess of about 0.002m/s. A test for determining the wicking speed is provided later in this specification.

Metallic porous materials manufactured according to methods described later in this specification and tested for wicking speed have yielded the following results:

• Metallic porous structure made of pure titanium - wicking speed of 0.00108m/s

• Metallic porous structure made of pure nickel - wicking speed of 0.00256m/s

• Metallic porous structure made of pure copper - wicking speed of 0.00262m/s

In terms of absorption capacity the tubular liner has an absorption capacity of at least about 200kg/m ³ , more preferably of at least 300kg/m ³ , even more preferably of at least about 400kg/m ³ , and most preferably of at least about 500kg/m ³ . A test for determining the absorption capacity is provided later in this specification.

Metallic porous materials manufactured according to methods described later in this specification and tested for absorption capacity have yielded the following results :

• Metallic porous structure made of pure titanium - absorption capacity of 301.67kg/m ³ • Metallic porous structure made of pure nickel - absorption capacity of 491.57kg/m ³

• Metallic porous structure made of pure copper - absorption capacity of 568.44kg/m ³

The tubular liner 16 is made of inorganic material. The inorganic material comprises metallic material, metallic alloy material, ceramic material, carbon based material, coated material and/or a combination thereof. Note that among the inorganic materials that are best suited for heat management applications, metals are usually the best candidates because they have a good thermal conductivity. Carbon based inorganic materials are also a possibility since they tend to conduct heat also well.

For the purposes of this specification "metallic" in "metallic porous structure", "metallic porous material" or any other similar expression is meant that the porous structure or material includes at least 50% of metallic

component. The metallic component can be a pure metal or an alloy or an amalgamation of pure metal and alloy. The metal or metals are preferably transition metals (e.g. copper, nickel, iron) as defined by the periodic table of elements) . A high metal concentration is preferred in heat management applications since metal has a good thermal conductivity, hence it will transmit heat readily between the interior or the heat pipe 12 and the external environment .

The resulting material has a porosity distribution which is characterized by at least two pore groups. In a first example of implementation the metallic porous material has three pore groups, namely a first pore group, a second pore group and a third pore group.

The first pore group has an average pore size in the range from about 200μm to about lOOOμm, preferably in the range from about 200μm to about 750μm and most preferably from about 200μm to about 500μm. In each case the standard deviation is in the range from about lOOμm to about 500μm. The first pore size group constitutes from about 30% to about 80% of the void volume of the metallic porous structure .

The second pore group has an average pore size in the range from about 40μm to about 120μm, preferably in the range from about 40μm to about 90μm and most preferably from about 40μm to about 60μm. In each case the standard deviation is in the range from about 30μm to about 80μm. The second pore size group constitutes at least 20% of the void volume of the metallic porous structure.

Finally, the third pore group has an average pore

size in the range from about 250nm to about 20μm, preferably in the range from about 500nm to about 15μm and most preferably from about 500nm to about lOμm. In each case the standard deviation is in the range from about 200nm to about lOμm. The third pore size group constitutes from about 10% to about 40% of the void volume of the metallic porous structure.

The first pore group which has the largest pores is the result of the foaming agent used during the manufacturing of the metallic porous structure, as it will be discussed later. The second pore group, which contains smaller pores, are created by inter-pore interstices or voids in the structure between large pores that belong to the first group. In other words, the pores of the first group communicate between them via inter-pore interstices, which behave from the perspective of interaction between the material and liquid, as smaller pores. In other words, the inter-pore interstices can store liquid and also can induce liquid to migrate through the porous structure via capillary action.

The third pore group contains the finest pores of the material. Those pores are defined between the individual metal particles that are bonded via sintering. Since the sintering process does not actually melt the metal particles, those particles bond to adjoining particles at the respective physical contact points, leaving some void spaces between them.

In a second example of implementation the metallic porous material has two pore groups, namely a first pore group and a second pore group.

The first pore group has an average pore size in the range from about 20μm to about 200μm, preferably in the range from about 40μm to about 150μm and most preferably from about 60μm to about lOOμm. In each case the standard deviation is in the range from about lOμm to about lOOμm. The first pore size group constitutes from about 50% to about 80% of the void volume of the metallic porous structure .

The second pore group has an average pore size in the range from about 250nm to about 15μm, preferably in the range from about 500nm to about 15μm and most preferably from about 500nm to about lOμm. In each case the standard deviation is in the range from about 200nm to about lOμm. The second pore size group constitutes from about 20% to about 50% of the void volume of the metallic porous structure .

Without intent of being bound by any particular theory it is believed that the presence of two or more pore groups in the metallic porous structure contributes to obtain a good liquid wicking speed which allows liquid to quickly travel from the cold side 20 to the hot side

14. In this fashion, the liquid at the hot side 14 can be boiled at a faster rate without creating a dry-out.

The technique for manufacturing the heat pipe 12 will be described in connection with Figures 4 to 8. In a first example of implementation shown in Figure 4, a metallic porous structure in the form of a flat sheet 400 is manufactured according to the method described later. The flat sheet 400 has a pair of main faces 402 and 404, end edges 406 and 408 and side edges 410 and 412. The flat sheet 400 is then rolled into a tube, as shown in

Figure 5. The rolling operation can be performed by using any appropriate method. For instance a mandrel can be provided (not shown) shaped as a rod and having a diameter that corresponds to the lumen of the tubular liner to be formed. The flat sheet 400 is then rolled over the mandrel to obtain the tube 414 of Figure 5. The rolling operation causes the metallic porous structure to bend permanently and acquire the tubular shape. When the tube is formed, the side edges 410 and 412 are brought in a face-to-face relationship.

If desired the side edges 410 and 412 can be secured to one another by sintering or soldering. Note that this operation is not strictly necessary since the tube 414 is housed in an enclosed chamber, as described below.

The tube 414 forms an insert that is placed in an outer conduit 600, as shown in Figure 6. The conduit 600 has walls which in cross-section define a closed figure, namely a circle. The tube 414 is simply inserted in the cylindrical conduit 600. Note that in Figure 6, the conduit 600 is shown as having a significantly larger diameter than the tube 414. This is shown for clarity only. The tube 414 is designed to be tight fitting and as such it contacts the internal wall of the cylindrical conduit 600.

In a possible variant, the tube 414 is constructed such as to leave a small gap between the side edges 410 and 412. Also the diameter is selected such as to be slightly larger than the inner diameter of the cylindrical conduit 600. In this fashion, when the tube 414 is to be inserted into the cylindrical conduit 600, it should be resiliently deformed to bring the side edges 410 and 412

closer to one another to allow the tube 414 to fit within the cylindrical conduit 600. Once inserted, the tube 414 is released and the resilience of the material will cause the tube to spring back against the inner walls of the cylindrical conduit 600. In this fashion a tighter fit can be obtained between the cylindrical conduit 600 and the inner tube 414.

The inner tube 414 can be bonded to the inner wall of the cylindrical conduit 600 by using any appropriate technique such as sintering or soldering.

In the example of implementation described above, the resulting heat pipe structure has a tubular liner formed by the tube 414. The tube 414 is in contact and follows the shape of the conduit 600 wall. In instances where it is not desirable or necessary to provide a liner that follows the wall of the conduit 600 along its complete periphery, it is possible to use a smaller insert having a cross-section that follows only a portion of the conduit 600 wall. For example, the insert may shaped in cross- section as a half-circle or as a quarter of a circle, thus establishing contact with the conduit 600 wall over a smaller area.

Also note that while a cylindrical conduit 600 is shown, conduits having other shapes can be used. For instance, the conduit can be rectangular in cross-section and the insert of porous material can be made as a rectangular tube as well to allow a full perimeter contact with the conduit. Alternatively, the insert can be made as a portion of a rectangle in cross-section when such full perimeter contact is not desired or necessary.

Figure 7 illustrates a variant. In this case a flat sheet 700 is provided that is made from metallic porous material. The flat sheet 700 is similar to the flat sheet 400 described earlier. The flat sheet 700 has a pair of main faces 702 and 704, end edges 706 and 708 and side edges 710 and 712. The flat sheet 700 is laminated with a layer 714 that after a forming operation will constitute the enclosed chamber of the heat pipe. In this specific example of implementation, the layer 714 is made of solid copper. The layer 714 has side edges 716 and 718, opposite to one another. The flat sheet 700 and the layer 714 are laminated by using any suitable technique. Preferably, sintering or soldering is used to create a physically strong bond and also enhance the thermal conductivity between the two layers .

The laminate is then rolled into a tube using a mandrel, as discussed above. The resulting tube is shown in Figure 8 and designated by the reference numeral 800. The outer surface of the tube 800 is formed by the layer 714 whose side edges 716 and 718 meet face-to-face along a joint area 802 that extends along the longitudinal axis of the tube 800. The inside of the tube is formed by the metallic porous structure whose side edges 710 and 712 also meet at the joint area 802.

In order to seal the joint area 802, the side edges 716 and 718 are joined to one another. This can be done by welding or soldering, for example.

A possible variant that can be applied to any one of the heat pipe examples discussed above, a layer of inorganic porous material, such as metallic porous material is provided on the outer wall of the enclosed

chamber. This is shown in Figures 9 and 10. More specifically, an outer jacket 900 made of metallic porous material is applied on the outer wall of the enclosed chamber 12. In this fashion, the wall of the enclosed chamber is sandwiched between two metallic porous layers.

The purpose of the outer porous layer 900 is to enhance the heat transfer to and from the heat pipe. The metallic porous layer 900 can have a porosity that is identical to the porosity of the tubular liner 16 or it can be different. It is advantageous to provide a porosity which has a high specific area and at the same time is open enough to allow a cooling medium to readily flow though the outer porous layer 900. This allows increasing the heat transfer between the surrounding medium and the heat pipe. As discussed in connection with other examples of implementation, the outer metallic layer 900 should be bonded to the outer wall of the enclosed chamber such as to create a bond allowing a good thermal conductivity. Examples of bonding methods include sintering or soldering, among others. The sandwich structure can be made in a similar fashion as the rolled structures described earlier. Specifically, the two flat layers of porous materials are bonded to the central non- porous sheet that is also flat. The resulting laminate is rolled and the meeting ends jointed to one another as deemed appropriate.

Figure 3 illustrates another example of implementation of a heat pipe. In this example, the heat pipe 300 functions conceptually in the same fashion as the heat pipe 10, except that it uses a larger amount of working fluid that forms a pool at the bottom of the heat

pipe 300 and that is boiled to provide the heat transfer effect .

The heat pipe 300 defines an enclosed chamber 302 having a hot section 304 at the bottom and a cold section

306 at the top. The cold section 306 is formed integrally with cooling fins 308 to facilitate the transfer of heat from the cold section 306 to the surrounding medium. If desired a cooling aid can also be provided, such a fan to force air to pass through the cooling fins 308 and thus further enhance the heat transfer.

The lower part of the enclosed chamber 302 is provided with a metallic porous structure 310 that is in contact with a pool of working liquid. The amount of working liquid present can vary according to the application but in most cases the metallic porous structure will either be entirely submerged or partially submerged such that in use a pool of liquid is always in contact with the metallic porous structure 310.

In this example of implementation the purpose of the metallic porous structure is generally two fold. First it enhances the heat transfer to the liquid body in order to facilitate the boiling process. Second it also acts as a wick to receive and distribute the condensed liquid that returns to the hot section.

The metallic porous structure is characterized by a high specific surface area to increase the contact surface between the metallic porous structure and the body of liquid. The specific surface area is in the range from about 10,000 m ² /m ³ to about 100,000 m ² /m ³ , preferably of about 15,000 m ² /m ³ to about 80,000 m ² /m ³ , even more

preferably from about 18,000 m ² /m ³ to about 70,000 m ² /m ³ , yet even more preferably from about 20,000 m ² /m ³ to about 60,000 m ² /m ³ and most preferably from about 20,000 m ² /m ³ to about 50,000 m ² /m ³ . The specific surface area is defined as the available contact surface the metallic porous structure with the body of liquid with relationship to the bulk volume of the metallic porous structure.

In this specific example, the metallic porous structure has a pore distribution that is characterized by at least two pore groups, namely a first pore group and a second pore group. The first pore group has an average pore size in excess of 20μm. The second pore group has an average pore size in the range from about 250nm to about 15μm. Preferably, the second pore group has an average pore size in the range from about 500nm to about 15μm, and most preferably an average pore size in the range from about 500nm to about lOμm. In each case the standard deviation is in the range from about 200nm to about lOμm. The second pore size group constitutes from about 20% to about 50% of the void volume of the metallic porous structure .

In one even more specific example, the first pore group has an average pore size in the range from about

20μm to about 200μm, preferably in the range from about

40μm to about 150μm and most preferably in the range from about 60μm to about lOOμm. In each case the standard deviation is in the range from about lOμm to about lOOμm. The first pore size group constitutes from about 50% to about 80% of the total void volume of the metallic porous structure .

In yet another specific example of implementation the

metallic porous structure has, in addition to the first and second pore groups a third pore group. The first pore group has an average pore size in the range from about 40μm to about 120μm, preferably from about 40μm to about 90μm and most preferably from about 40μm to about 60μm. In each case the standard deviation is in the range from about 30μm to about 80μm. The first pore size group constitutes from about 5% to about 30% of the total void volume of the metallic porous structure. The third pore group contains the largest pores and it has an average pore size in the range from about 200μm to about lOOOμm, preferably from about 200μm to about 750μm and most preferably from about 200μm to about 500μm. In each case the standard deviation is in the range from about lOOμm to about 500μm The third pore group constitutes from about 30% to about 80% of the total void volume of the metallic porous structure.

Figure 11 illustrates a possible variant of the metallic porous structure. More specifically, the metallic porous structure 1100 has a length dimension a, a width dimension B and a thickness dimension C. In this case the thickness C is significantly less than anyone of the length and width dimensions A and B. Note that since the metallic porous structure 1100 is shaped as a disk, the length dimension A is equal to the width dimension B.

Also note that the disk shape is merely exemplary and many other shapes are possible without departing from the spirit of this invention.

Therefore, the metallic porous structure 1100 has a pair of main faces 1102 that are opposite one another and a narrow side surface 1106. The main face 1102 is bonded to a substrate 1108, which in this example is made of

copper. The purpose of the substrate is to provide a support for the metallic porous structure and allow the metallic porous structure to be handled during the manufacturing of the heat pipe, without breakage. Copper, or another metals in general would be the material of choice for manufacturing the substrate 1108 since it provides good thermal conductivity. Alternatively, the substrate 1108 can also be made out of a carbon based material that could provide acceptable thermal conductivity.

The metallic porous structure 1100 is bonded to the top surface of the substrate 1108 via any suitable technique that would provide good thermal conductivity. Examples include sintering and soldering.

An enlarged cross-sectional view of the metallic porous structure and the underlying substrate 1108 is shown in Figure 12. The metallic porous structure has a plurality of projections 1200 that extend upwardly from a base layer 1204. The projections 1200 and the base layer 1204 are integrally formed. The projections 1200 are spaced apart and define between them valleys 1202. The projections have a density in the range of 9 to about 10,000 per square inch. Preferably the projection density is in the range of about 25 to about 2,500 per square inch. Most preferably the projection density is in the range of about 25 to about 1000 per square inch.

The projections may or may not be distributed uniformly on the top surface 1102. The method for measuring the projection density is generally a two step approach. The first is to measure the surface area of the top surface 1102. This is done by using any standard

measurement techniques. The second is to count the number of projections 1200 that are formed on the top surface 1102. Finally, the count is divided by the surface area in square inches to determine the number of projections 1200 in a single square inch. When the projections 1200 are uniformly distributed over the top surface 1102, an alternative method is to count the number of projections 1200 formed within an area of one square inch, instead of counting the total number of projections 1200 on the top surface 1102.

The average projection height is in the range of about 250μm to about 10mm, preferably from about 500μm to about 5mm and most preferably from about 750μm to about 3mm. The method for assessing the average height is to first count the number of projections 1200 on the top surface 1102 and then measure the height of each projection 1200. All the height values are summed and the result is divided by the total number of projections 1200. Note when the projections 1200 are all of the same height, then it suffices to measure the height of a single projection 1200 in order to determine the average projection height.

The height of a projection 1200 is the height as measured from the base of the projection up to its tip. This is dimension Z shown in Figure 12. In other words, the projection height does not include the thickness of the base layer 1204. The average thickness of the base layer is in the range from about 50μm to about 2mm, preferably from about 50μm to about lmm and most preferably from about lOOμm to about lmm. The average thickness is determined by measuring the dimension X, as shown in Figure 12, associated with each projection 1200,

summing up the results and dividing by the number of projections 1200. If the thickness is constant across the metallic porous structure then a single measurement anywhere will suffice to determine the average thickness.

The projections 1200 are formed on the top surface 1102 by an embossing process. The process starts by providing a metallic porous blank which has two opposite main faces and a constant thickness. In other words, the thickness dimension measured between the two main faces is the same across the blank. The porous metallic blank is then embossed by using a die (not shown in the drawings) . The die has a relief surface that is the exact opposite of the projections and valleys profile desired to be obtained. In other words, for each valley 1202 and projection 1200 to be formed, a corresponding projection and valley are provided on the die. The die is then pressed against one of the main surfaces of the porous metallic blank in order to emboss the porous metallic blank and thus transfer over the surface the die relief.

The embossing operation alters somewhat the pore distribution profile of the metallic porous structure. More specifically, the localized compression of the structure that creates valleys has the effect of partially crushing the pores in the material in the areas at which that compression is applied. Accordingly, the pores that are found in the regions of the base layer 1204 between two adjacent projections 1200, which corresponds to the bases of the valleys 1202, are reduced in size. Those regions 1206 will therefore contain pores that have an average pore size that is somewhat smaller than the average size of the pores located in the projections 1200.

This pore distribution profile is beneficial in terms of liquid evaporation characteristics. Without intent of being bound by any particular theory it is believed that during the operation of the heat pipe, liquid that submerges the metallic porous structure 1100 is boiled off primarily at the areas that correspond to the bottoms of the valleys 1202. The bubbles that form in the bottoms of the valleys 1202 float up through the liquid and then reach the surface. Some boiling also occurs on the sides of the projections 1200, however most of the liquid is boiled off at the bottoms of the valleys 1202. This is so because those areas are closer to the source of heat . Since the heat propagation path is short, enough thermal energy reaches the liquid residing at the valley bottoms to cause the liquid there to boil first.

Fresh liquid that replenishes the liquid being boiled off enters the metallic porous structure via the projections 1200. Since those projections are porous, that porosity allows liquid to migrate through the projections 1200 and then reach the base layer 1204 area where it is evaporated. In this fashion, the vapor released from the metallic porous layer and the fresh liquid that enters the metallic porous layer move along separate paths. This limits their interaction and allows vapor to be released more easily from the boiling liquid. Also, it limits the blocking effect that escaping bubbles may have on the liquid penetration in the metallic porous structure .

The pore distribution profile that manifests smaller pores at the valley bottoms assists the liquid transport from the projections 1200 to the valley bottoms. The smaller pores in the areas 1206, by virtue of their

increased capillary effect then to pull the liquid from the remainder of the metallic porous structure 1100 precisely in the regions where the boiling occurs. Accordingly, the pore distribution profile is such as to modulate the capillary attraction exerted on the liquid by pulling the liquid in the areas from which the liquid is being dissipated by evaporation.

Figure 13 shows another variant of the heat pipe. The heat pipe 1300 is generally similar to the heat pipe 300 discussed above with the difference that the metallic porous structure 1302 is located vertically. This vertical structure can be used for cooling an electronic component that is vertical instead of being installed horizontally. For reference the electronic component, such as a CPU 1304 is shown in dotted lines.

In light of the vertical orientation of the metallic porous structure 1302, it is only partially submerged in the pool of liquid. However in light of the porosity of the structure, which acts as a wick, liquid can be more effectively drawn from the pool and distributed throughout the metallic porous structure where it is evaporated.

Metallic porous structures according to the examples described earlier are produced by a method which involves dry-mixing inorganic particles, binder and optionally a foaming agent, removing a binder and then sintering the inorganic particles. Two specific examples of the method are provided. Example 1 produces a metallic foam that has a porosity distribution characterized by two pore groups, while example 2 produces a porosity distribution characterized by three pore groups.

Example 1

The porous material according to one or more examples provided above can be produced from a dry flowable powder mixture comprising a base material and a binding agent, all provided in predetermined amounts. The base material includes inorganic particles having a first melting temperature, the binding agent is preferably, but not exclusively, an organic binder having a decomposition temperature lower than the first melting temperature and having clean burn out characteristics.

As it will be readily understood, the exact amount of each constituent of the mixture is determined, prior to the execution of the method of the present invention, based on the physical and chemical properties of the inorganic particles and of the binding agent, and based on the desired properties of the finished open cell porous body. Consequently, the exact composition of the mixture will vary according to the nature of the base material and of the binding agent .

The inorganic particles comprise metallic particles, metallic alloy particles, ceramic particles, coated particles and/or a combination thereof. In the case of metallic and metallic alloy particles, the metal or metals are preferably transition metals (e.g. copper, nickel, iron) as defined by the periodic table of elements. The inorganic particles will have a first melting temperature. Though the inorganic particles content may vary from about 10 to about 90 wt % of the total weight of the mixture (preferably from about 40 to about 90 wt % for metal particles and from about 10 to about 60 wt % for ceramic

particles) , the exact amount of the inorganic particles and the choice thereof will be determined by the skilled addressee depending on the requirements of the application for which the open cell porous material is being manufactured.

The binder used in the mixture is preferably an organic binder provided in a dry flowable powdered form and with clean burn out characteristics. The binder can be a thermoplastic polymer, a thermoset resin and/or a combination thereof. The binder can also be an inorganic, a synthetic binder or a mixture of organic and/or inorganic and/or synthetic binders. The binder may be provided in solid form (preferably powder particles), in semi-solid form, in liquid form, in gel form or in semi- liquid form. The binder has a decomposition temperature lower than the first melting temperature of the inorganic particles in order to prevent premature melting of the inorganic particles during the decomposition step. Though the binder content in the mixture may vary from about 10 to about 90 wt % of the total weight of the mixture and preferably from about 20 to about 70 wt %, the exact amount thereof will be determined by the skilled addressee depending on the nature of the inorganic particles and on the requirements of the application for which the open cell porous material is being manufactured. Most preferably, the binder should not leave decomposition products that may negatively affect the final properties of the porous structure. However, some residues can be accepted if they have no impact on the final product or if they improve some of its properties.

Optionally the mixture may comprise a cross-linking agent that may induce faster curing of the binder during

or after the curing step and, by the way, improve the mechanical strength of the cured structure before the decomposition of the binder. Optionally, the mixture may also comprise other additives such as a lubricant to ease shaping, molding or demolding or flowing agents to improve the flowability of the powder when all the constituents are in powdered form.

The organic binder can be blended with the other constituent using various techniques such as but not limited to mixing, milling, mixing the binder in suspension or in solution in a liquid, blending the binder in molten, liquid, gel or semi-liquid form with the inorganic particles and the other additives. Whichever mixing technique is used, the resulting product should be a curable mixture.

In other variants, spacing agents may be added to the mixture for providing additional porosity and to improve pore connectivity. The spacing agents are removed after curing to leave voids in the structure after decomposition of the binder or after sintering. The spacing agent can be removed by thermal decomposition after curing or by leaching after curing, decomposition of the binder or sintering. The spacing agent can be particles or a scaffold. When particles are used, they are admixed with the rest of the mixture. In one non limitative example, the spacing agent can be polymeric particles admixed with the mixture. In this case, the spacing agent concentration can vary from about 5 to 50 wt %, but preferably between 10 and 30 wt %. When a scaffold is used, its porous structure is filled with the mixture used to produce the porous material. The scaffold can be, for example and in no limiting fashion, a porous structure,

like a polymeric foam, that can be filled with the mixture and removed by thermal decomposition or by leaching.

It is also contemplated to add additional binder in amount varying between 0.05 wt % to 5 wt %, but preferably between 0.05 wt % to 1 wt %, in the mixture. This additional binder may be generally used to glue different constituents of the mixture together in such a way that the final product is less prone to segregation and/or dusting. This additional binder can also be used to improve the flowability of the mixture should all the constituents be provided in powdered form. The additional binder may be added at different steps of the mixing procedure, either before mixing the inorganic particles with the binder, after the binder addition, after the lubricant addition, after the flowing agent addition or after the addition of any combination of those constituents. Whichever mixing technique is used, the resulting product should be a curable mixture.

The resulting mixture may be shaped using methods such as molding, deposition, lamination or extrusion. The product is then heated at a moderate temperature to melt the binder, if the latter is not already in liquid, gel or semi-liquid form, and to initiate the curing of the mixture. Optionally, pressure may be applied to the mixture before or during heating the mixture.

The resulting open cell porous material porosity and structure will depend on the particle size, shape, density and content of the inorganic particles; the content and viscosity of the binder, as well as the processing conditions .

Materials can be cured in a mold to provide three- dimensional porous structures. The mixture can be cured on or in a substrate to produce a coating or to produce composite structures. Curing can be done for example on a plate, on a rod, in or outside a tube or cylinder, in or on other porous structure (mesh, beads, foam for example) or any other substrate. The material can be machined after curing, decomposition of the binder or sintering.

Functionally graded materials can be produced using mixtures with variable composition. Graded layered structures can be produced for example by deposing layers of mixtures with different composition. Functionally graded materials can also be produced by controlling the thermal gradient during curing in order to control material curing and pore size distribution.

Optionally, the mechanical strength of the cured structure may be further increased, before decomposition of the binder and sintering, by using externally assisted cross-linking techniques such as irradiation or light exposure .

After curing and optionally cross-linking, the cured mixture is treated at higher temperature to decompose the binder. The atmosphere (with or without the presence of oxygen) , duration and temperature of the thermal treatment should preferably allow a clean decomposition of the binder. Binder decomposition should preferably not deteriorate the three-dimensional structure of the cured mixture. If gas pressure generated during binder decomposition is too important, cracking may occur in the still unsintered structure. Oxidizing or reducing conditions during the thermal treatments may be chosen to

optimize binder decomposition. After decomposition, the cured mixture is composed of open cell metal, and/or metal alloy, and/or ceramic material particles.

Sintering is done after the decomposition of the binder to create bonds between the inorganic particles of the cured mixture. Sintering conditions (temperature, time and atmosphere) should be such that the inorganic particles do not melt to create the bond between them: conditions should be such that the material particles adhere to each other through a bond mainly created by solid-state diffusion to form a strong metallurgical joint between them. Effective solid-state diffusion occurs between material particles when they are heated, for a certain time, at temperatures slightly under the melting temperature of the material particles. Sintering is generally done in reducing atmosphere for metal particles to avoid the formation of surface oxides on the foam.

Mechanical strength may be adjusted for the application. The choice, size, nature and/or physical state of the inorganic particles and of the binder content will have a substantial influence of the physical properties (e.g. mechanical strength) of the produced open cell porous material.

Additional treatment can be done on the porous material produced. The internal surface of the foam can be modified for example by heat treatment, chemical treatment or deposition of coatings using various state of the art deposition techniques. The external surfaces of the foam can be modified for example by a stamping, etching, embossing, or grooving technique and by state of the art surface coating techniques. The foams can be integrated in

other products and/or to other structures using different state of the art techniques such as diffusion bonding, press fitting, welding, brazing, sintering or gluing. The invention is not so limited.

In a very specific example, a metallic porous structure, with copper (Cu) as the base material, was produced with the formulation presented in Table 1. The different constituents were dry-mixed together until the mixture became homogeneous. After mixing, the mixture was poured into a mould and cured at 110 ⁰ C in air for 2 hours. After curing, the material was submitted to the decomposition of the binder in a furnace at 650 ⁰ C for 4 hours in a dry air stream. Finally, the specimens were sintered in an Ar-25%H2 atmosphere for 2 hour at 1000 ⁰ C.

TABLE 1 - Formulation used for the production of the Cu based foam

Example 2

Metallic porous structures with copper (Cu) as the base material were produced with the formulation presented in Table 2 and in accordance with the procedure described in U.S. Patent No. 6,660,224. The different constituents were dry-mixed together until the mixture became homogeneous. After mixing, the mixture was poured into a mould and

foamed at 110 ⁰ C in air for 2 hours. After foaming, the material was submitted to the decomposition of the binder in a tube furnace at 650 ⁰ C for 4 hours in a dry air stream. Finally, the specimens were sintered in an Ar-25%H2 atmosphere for 2 hours at 1000°C. Note that example 2 differs primarily from example 1 in that foaming agent is used to form some of the pores of the material. In the case of example 1 no such foaming agent is used.

TABLE 2 - Formulation used for the production of the Cu foam

The wicking speed and absorbent capacity of the porous structure according to anyone of the examples discussed above are assessed according to the test procedure described below.

A disc shaped sample with a 2cm diameter and a lcm thickness ("Reference Sample") is manufactured. A solution made of 85% ethanol and 15% methanol is used as the wicking fluid. The measurements are done in standard atmosphere conditions, i. e. 23°C and 101.3 kPa.

Before the wicking test starts, the Reference Sample is :

Weighted to measure its dry weight.

• The bulk volume of the reference sample is computed.

The Reference Sample is then deposited in a large reservoir filled with the wicking fluid so that one of its main faces (disc shape surface) is in full contact with the bottom of the reservoir. The Reference sample is not supported in any way by an external apparatus; it is directly deposited inside the reservoir. The lateral dimensions of the reservoir are such that there is a 1 mm thick layer of wicking liquid inside the reservoir, with the total volume of wicking fluid inside the reservoir being sufficiently large so that the lmm thickness stays relatively constant throughout the wicking test. Hence, once deposited in the reservoir, one end of the Reference Sample is immersed in lmm of fluid.

Immediately after the Reference Sample is deposited in the reservoir, a timer is started. Visually, the migration of wicking liquid through the sample is observed and when the Reference sample is completely saturated throughout its volume with wicking liquid, the timer is stopped. On the basis of the counted time and the vertical distance traveled (1 cm), the wicking speed (m/s) is computed.

This process is repeated ten times and the wicking speed results averaged. The resulting average wicking speed value, is therefore considered for the purpose of this specification to be the wicking speed of the sample.

The Reference Sample is then quickly removed and placed on a nonabsorbent surface to be weighted to measure the fluid saturated weight of the Reference Sample. The

difference in weight between the fluid saturated weight and the dry weight of the Reference Sample is divided by the computed volume of the Reference Sample. This ratio is used as a measure of the absorbent capacity of The Reference Sample. The absorbent capacity is expressed as weight of the test liquid (kg) per volume (m ³ )

This process is repeated ten times and the absorbent capacity results averaged. The resulting average absorbent capacity is therefore considered for the purpose of this specification to be the absorbent capacity of the sample .

Although various embodiments have been illustrated, this was for the purpose of describing, but not limiting, the invention. Various modifications will become apparent to those skilled in the art and are within the scope of this invention, which is defined more particularly by the attached claims.