Technical Field

The present disclosure relates to twisted multi-conductor cables and, in particular, but not exclusively, to thermocouples comprising twisted multi-conductor cables.

Background

A mineral insulated (Ml) multi-conductor cable is a type of multi-conductor cable in which two or more wires are housed within a metal sheath, with the space between the two or more wires and the spaces between the wires and the sheath being packed with an insulator core of a refractory or ceramic material, typically magnesium oxide. The use of a refractory material for the insulating core means that Ml cables are generally able to withstand high temperatures and ionising radiation, making them suitable for use in a wide range of industrial environments. One application of Ml cables is therefore in thermocouple devices that need to operate at high temperatures and/or in radioactive environments. Figure 1 shows an axial cross section through an exemplary Ml cable 100, such as may be used in a thermocouple, which has two parallel wires 102A, 102B spaced apart from one another and surrounded by an insulator core 104 that has a surrounding metal sheath 106.

A twisted multi-conductor cable is another type of multi-conductor cable in which two or more insulated wires extend (generally) parallel to one another and are twisted about each other to form intertwined helices. The twisted arrangement means that, on average, none of the wires is closer to any sources of interference than any of the other wires. Any voltages or currents induced in the wires by magnetic, electric and/or electromagnetic fields outside the wires are therefore equal, to a first approximation, which allows them to be removed by appropriate data processing, such as by measuring differences between the signals carried by the wires. The wires in twisted multiconductor cables are generally insulated using a flexible, plastics material, e.g. a polyimide (such as Kapton®) or a fluoropolymer such as perfluoroalkoxy (PFA) or polytetrafluoroethylene (PTFE) polymer, which can be twisted without breaking. Such materials are, however, generally unable to withstand temperatures above 250-400°C, making them unsuitable for high temperature applications. Twisted multi-conductor cables that have two insulated wires twisted about each other to form a double helix, are commonly referred to as “twisted pair cables”.

Manufacturing Ml cables in which the wires are twisted about each other is challenging, particularly for smaller Ml cables, because of difficulties associated with twisting the wires within the metal sheath and avoiding electrical shorts. Conventional Ml cables may therefore be more sensitive to interference than similar twisted multi-conductor cables. Conventional Ml cables are also difficult to miniaturise without compromising the integrity of the insulating core.

An alternative design for multi-component cables for high temperature use involves surrounding each of the wires in the cable (and the cable itself) with braided glass or ceramic fibres. However, the thickness of the braids may make such “braided” cables unsuitable for applications for which there are demanding size constraints. Braided cables may also be unsuitable for use under vacuum because of high outgassing rates caused by the high surface area of the fibres and the use of organic binder materials.

Summary

According to a first aspect of the present disclosure, there is provided a twisted multiconductor cable comprising two or more wires twisted about each other. One or more of the wires has a silica coating to insulate the or each of the one or more wires from the other wires. Surprisingly, it has been found that the use of a silica coating - in contrast to coatings with ceramic materials such alumina or zirconia - provides sufficient elasticity and adhesion to the wire to prevent (or at least substantially reduce) damage to the coating caused by the wires being twisted about each other. The silica coating means that the multi-conductor cable is suitable for use at high temperatures, e.g. temperatures above 400°C, such as 600-1400°C, 800-1200°C and/or radioactive environments.

In some implementations, the silica coating may have a thickness from 4 microns to 20 microns, from 6 microns to 15 microns, or from 8 microns to 12 microns. For some applications, the thickness of the silica coating may be in a range from 0.5 microns to 10 microns, or from 2 microns to 8 microns. Such thicknesses have been found to provide effective insulation for the wires even after the wires have been twisted about each other. For coatings deposited using physical vapour deposition (PVD) and/or Chemical Vapour Deposition (CVD), thicker coatings may tend to become stressed, which in some cases may lead to the coating failing or delaminating from the wire.

The coating preferably extends continuously around the outer surface of the wire (i.e. the coating encircles the wire without any breaks in the coating) to ensure that there is good adherence to the wire and to avoid any gaps in which contaminant materials or gases may be trapped. A continuous coating around the wire may also be provided to ensure that no unintended junctions (e.g. between exposed sections of the two or more wires) are formed which might, for example, affect the output readings if the cable is used in a sensor, such as thermocouple.

The silica coating may be formed by Physical Vapour Deposition and/or Chemical Vapour Deposition. Coatings formed by PVD and/or CVD may have less stress compared to other coating methods, such as coatings formed by drawing molten glass. The risk of damaging the wires during coating may also be reduced because little or no manipulation of the wire is needed, thereby avoiding or minimising defects that may limit the lifetime of the multi-conductor cable, the maximum temperature at which the cable can be used and/or the accuracy of a sensor (e.g. thermocouple) that incorporates the cable. PVD and/or CVD may be preferred in some cases because very good control over the thickness of the silica layer(s) can be achieved.

The thickness of the silica coating may have a tolerance of less than 25%, preferably less than 10%, more preferably less than 5%. For example, the coating may have a mean thickness of 5 microns and a tolerance of less than 0.5 microns along the length of the coated wire (or at least a region of the coated wire in contact with another wire). The tolerance may be defined by a standard deviation or some other measure of the variation of a distribution of thicknesses, such as a Full Width at Half Maximum (FWHM), for example.

In some implementations, the purity (e.g. by mass) of the silica coating may be greater than 75%, preferably greater than 90% and more preferably greater than 95%, or even greater than 99%. Higher purities of silica may be preferred to ensure that the coating is elastic and/or adheres strongly to the wire(s), such that the multi-conductor cables (particularly those with thin coatings) can be twisted. Preferably, the cable is suitable for use in a temperature range from 800°C to 1200°C, or from 900°C to 1100°C. The ability to operate at high temperatures such as these may allow the multi-component cable to be used in hotter environments than existing multicomponent cables. For example, materials other than silica (e.g. borosilicate glass) may soften at temperatures approaching 800°C, making them unsuitable for coating multicomponent cables operating at higher temperatures.

The twisted multi-conductor cable may have a twist pitch length in a range from 1 mm to 50 mm, preferably from 2 mm to 10 mm. The twist pitch length may depend on the diameter of the wires. For example, the twist pitch length may in some cases be greater than 2 mm for wires having a diameter of around 0.2 mm or more. The twist pitch length refers to a length measured along the cable (i.e. axially) over which the wires make a complete revolution about each another, i.e. the distance over which the relative orientations of the wires repeats. The cable may be suitable for use in devices that are subject to large magnetic fields as the twisting of the wires about each other reduces the amount of noise/interference that the cable picks up. Low twist pitch lengths (e.g. less than 10 mm) may be preferred in such cases.

One or more (e.g. all) of the wires have a diameter of less than 1 mm, preferably less than 0.5 mm. One or more (e.g. all) of the wires may have a diameter of greater than 100 microns. For example, one or more (e.g. all) of the wires may have a diameter from 100 microns to 1000 microns, or from 200 microns to 500 microns. The overall diameter of the cable may therefore be relatively small compared to pre-existing twisted multiconductor cables.

The twisted multi-conductor cable may further comprise a conductive conduit surrounding the two or more wires to electrically shield the two or more wires from sources of electromagnetic interference external to the cable. Such shielding, in addition to the twisting of the wires, further reduces the sensitivity of the cable to sources of electromagnetic interference, which may for example facilitate data processing of signals carried by the wires in the cable. The silica coating(s) is or are preferably configured to insulate each of the wires from the conduit, although other insulating materials may be provided between the wires and the conduit in addition to the silica coating if desired. Each of the two or more wires may have a silica coating. The silica coating of each wire may have (substantially) the same thickness.

The cable may be a twisted pair cable comprising two wires twisted about each other, e.g. a twisted pair cable. The cable may be used to transfer electrical signals to or from a sensor, and/or provide power to the sensor. The sensor may be a temperature sensor, such as a thermocouple or a resistance temperature detector (RTD), or another type of sensor, such as a strain gauge or a magnetic field probes (e.g. a Hall probe). A resistance temperature detector is a temperature sensor that measures the resistance of a resistive element, such as a length of platinum, and provides an output signal indicative of the temperature of the resistive element.

At least one of the one or more wires having the silica coating may be made from a different conductor material from another of the wires, whereby forming a junction between the different electrical conductor materials generates a temperature dependent (thermoelectric) voltage. The different electrical conductor materials may be chromel and alumel, for example. The cable may therefore be used to make a thermocouple junction for a thermocouple sensor that is suitable for use at high temperatures and is less sensitive to sources of electromagnetic interference than other high temperature thermocouples that use conventional Ml cables. Thus, according to another aspect of the present disclosure there is provided a thermocouple sensor comprising the twisted pair cable, wherein the wires made from different electrical conductor materials are electrically connected to form a thermocouple junction. The combination of a thermocouple sensor and the cable may, for example, allow temperature measurements to be made with less electromagnetic interference than when similar non-twisted cables are used. A resistance temperature detector (RTD) may alternatively be used with the cable for making temperature measurements as RTDs may, in some cases, be more immune to electromagnetic interference than thermocouple sensors. However, a thermocouple sensor may be preferred for measuring high temperatures (e.g. temperatures greater than 1000°C), for which RTDs may be unsuitable, or for applications in which the relative fragility of RTDs and/or the size of protective housings often used for RTDs is an issue.

The thermocouple junction may be fixed to a surface by a brazing alloy, preferably a gold-based brazing alloy. The thermocouple sensor may be suitable for (e.g. adapted for) use in a temperature range from 800°C to 1200°C

According to a further aspect of the present disclosure, there is provided a method of manufacturing a twisted multi-conductor cable (which may be performed as part of a method of manufacturing a thermocouple, for example). The method comprises coating a wire with silica; and twisting the wire and one or more further wires about each other to form a twisted multi-conductor cable.

Coating the wire with silica may comprise depositing silica on the wire using Physical Vapour Deposition (PVD) or Chemical Vapour Deposition (CVD). PVD may comprise sputtering and/or evaporation to deposit the silica, for example. The silica coating may be deposited until it has a thickness in a range from 0.5 microns to 10 microns, preferably from 2 microns to 8 microns, or more preferably from 4 microns to 6 microns. Greater thicknesses may be preferred to ensure the coating is not damaged by twisting the wires. For example, a thickness of greater than or equal to 4 microns, greater than or equal to 6 microns, or greater than or equal to 8 microns, may be preferred in some instances. In some implementations, the thickness is from 4 microns to 20 microns, 6 microns to 15 microns, or 8 microns to 12 microns.

The method may further comprise coating the one or more (and preferably all of) the further wires with silica prior to said twisting.

Brief Description of the Drawings

Figure 1 is a schematic axial cross section view of a prior art Ml cable;

Figure 2 is a schematic side view of a twisted pair cable according to the present disclosure; and

Figure 3 is a schematic axial cross section of the twisted pair cable of Figure 2 taken along the line A-A’.

Detailed Description The present disclosure addresses or at least alleviates the issues described above with existing multi-conductor cables.

Figure 2 shows a portion of a twisted pair cable 200 comprising a first wire 202 and a second wire 204 that are twisted about one another to form an intertwined double helix arrangement. The first and second wires 202, 204 are shown in Figure 2 as being spaced apart from one another for clarity, but generally the exterior surfaces of the wires are in contact along most of or all of the cable, as illustrated in the axial cross section through the cable 200 taken along the line A-A’ shown in Figure 3.

The first and second wires 202, 204 each comprise a core 202A, 204A made of an electrical conductor material (e.g. a metal or metal alloy) surrounded by a coating of silica 202B, 204B. The respective electrical conductor materials of the cores 202A, 204A of the first and second wires 204, 204 may be the same material or different materials can be used. For example, the two electrical conductor materials may be dissimilar metals, such that a thermocouple junction can be formed by bringing the two materials into electrical contact, with a thermoelectric voltage being generated between the first and second wires 202, 204 when the temperatures of the two materials are different (a voltage generated by the Seebeck effect, for example).

In the cable 200 of Figure 2, the electrical conductor materials are chromel (an alloy consisting of around 90% nickel and 10% chromium by weight) and alumel (an alloy consisting of approximately 95% nickel, 2% aluminium, 2% manganese, and 1 % silicon). The alloys may of course comprise small (e.g. trace) amounts of other materials. The region of each of the wires 202, 204 at an end of the cable (shown at the left hand side of Figure 2) do not have a surrounding layer of silica so that the two exposed regions together can be joined (e.g. welded or crimped) together to form a thermocouple junction. In some cases, the exposed regions may be formed by removing the layer of silica from the wire (either all the way or partially around the circumference of the wire) to expose the electrical conductor material beneath.

The silica coatings 202B, 204B electrically insulate the cores 202A, 204A of the wires 202,204 from one another. In the present example, the silica coatings 202B, 204B each have a thickness (i.e. perpendicular distance from the perimeter of the core 202A, 204A to the edge of the coating 202B, 204B) from 2.5 to 8 microns. Although silica is conventionally regarded as a brittle material, it has been found that silica coatings applied to the cores 202A, 204A of the wires 202, 204 remain sufficiently elastic, and adhere sufficiently well to the cores 202A, 204A, that the wires 202, 204 can be twisted about each other to form a cable 200 without the coatings being damaged (i.e. without the insulating properties of the coatings being degraded significantly). The use of silica coatings 202A, 204A also mean that the cable 200 is compatible with ultrahigh vacuum (UHV) conditions and will not, for example, “outgas” any molecules that might contaminate sensitive industrial processes (e.g. semiconductor fabrication processes).

As the silica coatings 202B, 204B can be applied to even fine gauge wire (e.g. wire with a diameter of 500 microns or less), the overall thickness of the cable 200 may be very small, i.e. the thickness of the cable may have a characteristic length scale (e.g. diameter) of less than 1 mm in some cases. Such miniaturised twisted pair cables are attractive for many applications, including vacuum feedthroughs and sensors used in space-constrained environments, e.g. sensors located that need to be located partially or wholly within a thin layer (e.g. a layer than is less than 1 mm thick) of a multi-layered component. The cable 200 may also be used in scenarios where the mass of the cable 200 needs to be minimised, such as space-based applications, e.g. satellites, planetary exploration devices (e.g. rovers) and robotic spacecraft (e.g. space probes).

As mentioned above, the cable 200 may be also used to manufacture a thermocouple junction, and in this case, the use of fine gauge wires may reduce the heat capacity of the junction and increase the sensitivity and response time of the thermocouple to temperature changes. The twisted arrangement of the wires 202, 204 means that the thermocouple can be used in environments in which there are high temperatures (e.g. 600-1200°C) and/or sources of interference, such as large magnetic and/or radiofrequency (RF) fields. In such cases, the cable 200 may preferably be enclosed in a metal conduit to shield the wires 202, 204 electrically from such sources. In embodiments where the wires are made from chromel and alumel, the thermocouple may be referred to as a K-type thermocouple.

The silica coating can be formed using physical vapour deposition (PVD) or chemical vapour deposition (CVD), as is known in the art, e.g. using a mixture of silane and oxygen to generate silicon dioxide within a plasma. Other methods can also be used, however, such as oxidation of a layer of silicon applied to the wires. As the silica coating is chemically inert, the cable 200 can be used in many “harsh” chemical environments that would be liable to damage other insulators. PVD may be preferred in many cases because it is a clean, vacuum process and the thickness of the silica coating can be carefully controlled.

In one example, the wires 202, 204 were coated by reactive PVD magnetron sputtering using a silicon target in a controlled oxygen plasma at around 0.3 Pa (2 mTorr) to form the silica (silicon oxide, SiCh) coatings 202B, 204B. The thickness of the coatings may be controlled by varying the duration of the coating process. In some cases, coatings of greater than 4 microns were formed. The coatings 202B, 204B were found to provide effective insulation for the wires even after contact with a flame from a butane torch.

In some applications, one or more of the wires 202, 204 of the cable 200 may be brazed on to a surface to make electrical and/or thermal contact with the surface (although other methods of fixing can be used). For example, a thermocouple junction as described above may be brazed onto a surface so that the temperature of the surface can be measured. This may be achieved, for example, by denuding and joining the ends of the wires 202, 204 to form a junction and then brazing the junction to the surface. Brazing alloys such as BNi-2, BNi-6 and BAu-4 may be used. However, gold-based brazing alloys (e.g. BAu-4) may be preferable as they result in negligible erosion of the silica coating 202B, 204B on the wires 202, 204 adjacent to the thermocouple junction. By contrast, alloys such as BNi6 and BNi2 may be less preferable as they may in some cases cause inter-granular cracking of the wires as a result of the brazing alloy permeating into metallic grain boundaries within the wires. High gold-content alloys such as BAu-4 have been found to undergo less permeation and their flow characteristics during brazing are such that high strength joints can be formed, even with very fine wires.

Whilst the twisted multi-conductor cable 200 described above has two wires 202, 204 (i.e. it is a twisted pair cable), similar twisted multi-conductor cables can be manufactured that comprise more than two wires, e.g. 3, 4, or 5 or more wires. Such cables can be formed by arranging the ends of the wires evenly around a circle and collectively twisting the wires about each other to form an intertwined plurality of helices. Multi-conductor cables comprising a plurality of twisted multi-conductor cables (e.g. a plurality of the twisted pair cables 200) may also be formed, e.g. by enclosing multiple twisted multiconductor cables in a conduit, or even by twisting the twisted multi-conductor cables about each other. The twisted multi-conductor cable 200 may be used in apparatuses that are operated at high temperatures such as ovens, kilns, furnaces, engines, hot gas filters and turbines for electricity generation, or other apparatuses that need to withstand high temperatures for some amount of time, such as vacuum chambers that need to be baked to high temperatures prior to use.

While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example, and not limitation. It would be apparent to one skilled in the relevant art(s) that various changes in form and detail could be made therein without departing from the spirit and scope of the invention.