EP0188370B1

EP0188370B1 - Electrical wire with refractory coating

Info

Publication number: EP0188370B1
Application number: EP86300208A
Authority: EP
Inventors: Richard John Penneck; James Martin O'brien; Stephen John Duckworth
Original assignee: Raychem Ltd
Current assignee: Raychem Ltd
Priority date: 1985-01-14
Filing date: 1986-01-14
Publication date: 1989-05-10
Also published as: JPS61165910A; ATE43024T1; DE3663317D1; CA1295890C; EP0188370A3; EP0188370A2

Abstract

An electrical wire comprises a copper conductor and an electrical insulating refractory coating. At least part of the coating and preferably all the coating has been formed by a sol-gel method. In addition, the conductor includes a keying layer formed from a metal other than copper in order to increase the adhesion of the refractory coating to the conductor. The keying layer may be formed for example by electroplating, rolling or wire drawing methods.

Description

This invention relates to electrical wire and cables and to electrical conductors suitable for use therein.
Numerous forms of electrical cable have been proposed for use in environments where there is a risk of fire and accordingly where fire retardency of the cable is required. These cables may make use of specific, highly effective, halogenated polymers or flame retardant materials such as polytetrafluoraethylene, polyvinyl chloride, or polyvinylidine fluoride as polymers or decabromodiphenyl ether as flame retardant additives. Halogenated systems, however, suffer from the disadvantage that when they are heated to high temperatures during a fire, they liberate toxic and corrosive gases such as hydrogen halides, and a number of halogen free insulating compositions have therefore been proposed, for example in U.S. patent specification No. 4 322 575 to Skipper and in U.K. patent specification Nos. 1 603 205 and 2 068 347A, the disclosures of which are incorporated herein by reference.
In certain fields where cables are used, for example in military, marine, mass transit or offshore applications, it is desired to use cables which are capable of functioning at relatively high temperatures. In other instances it is desired to use cables which not only do not burn, or, if they burn, do not liberate toxic or corrosive gases, but also are capable of functioning after having been subjected to a fire, or preferably for a period of time during a fire without shorting or otherwise failing. Cables that are capable of functioning for a period of time during a fire have been called circuit integrity cables or signal integrity cables depending on their use. The previously proposed circuit and signal integrity cables have generally used the principle that the individual conductors should be separated from one another by mica tapes or by large volumes of packing materials or silicones or by combinations thereof in order to prevent the formation of short circuits during a fire, with the result that the previously proposed cables are relatively heavy or large or both. There is therefore a need for a cable that will function at relatively high temperatures or will function after it has been subjected to a fire, and which preferably will retain its integrity for a period of time during a fire but which is smaller or lighter than the previously proposed cables.
The present invention provides an electrical wire which comprises a copper conductor and an electrically insulating refractory coating at least part of which has been deposited on the conductor by a sol-gel method, the conductor including a keying layer formed from a metal other than, copper for increasing the adhesion of the refractory coating to the conductor. According to the invention it is possible to form articles having a refractory coating which, although being relatively thick (that is to say, in the order of 3 to 15 micrometres) and so having good electrical insulation characteristics, also exhibits good adhesion to the underlying metal even when subjected to mechanical or thermal stresses.
The use of a sol-gel method for depositing the refractory coating has the advantage that the refractory coating is substantially contaminant-free, that is to say, it contains only those species that are intended in order for the layer to fulfill its intended function, and contains substantially no species that result from the manufacturing process. An important feature of the refractory coating is good control of composition to optimise the high temperature performance of the wire. The composition is totally inorganic and therefore does not rely on conversion processes to occur during exposure to normal or emergency high temperature service, as is the case for example in many mica filled or glass filled silicone resin systems. The composition is also improved by removing the use of polymeric binders to support inorganic materials which may be consolidated by firing processes to form the inorganic insulation. Similarly, wires in which the refractory coatings have been formed by electrochemical conversion of metal layers e.g. by anodising an aluminium layer, and which do not form part of the invention, usually have coatings that are porous and often heavily contaminated with ionic residue from the electrolytic solutions e.g. sulphates from sulphuric acid anodisation processes.
Preferably the insulating refractory coating is formed from an electrically insulating infusible or refractory metal or semi-metal oxide or nitride and the invention will be described below in many cases with respect to oxides and nitrides although other refractory coatings are included. By the term "infusible" or "refractory" is meant that the coating material in its bulk form should not fuse or decompose when subjected to a temperature of 800°C, for 3 hours. Preferably the oxide or nitride should be able to withstand higher temperatures also, for example it should be able to withstand a temperature of 1000° C for at least 20 to 30 minutes. The preferred oxides and nitrides are those of aluminium, titanium, tantalum and silicon or mixtures thereof with themselves or with other oxides or nitrides. Thus, for example, the use of mixed metal oxides for the refractory coating are also encompassed by the present invention.
The wires according to the present invention are particularly applicable for use in systems in which they need to be capable of functioning at high temperatures for significant lengths of time without failure, e.g. circuit and signal integrity cable and magnet wire. The conductor may be single, solid conductor or it may be a stranded conductor in which individual strands are laid together to form a bundle which preferably contains 1; 19 or 37 strands. Where the conductor is stranded it is preferred for the bundle to be coated rather than the individual strands, that is to say, the refractory coating extends around the circumference of the bundle but not around the individual strands so that substantially only the outwardly lying surfaces of the outer-most layer of strands are coated.
This form of conductor has the advantage that the inter strand electrical contact is retained and the dimensions of the bundle are kept to a minimum (since the thickness of the coating may constitute a significant proportion of the strand dimensions for fine gauge conductors) and also it aids the formation of good electrical connections, e.g. crimp connections, to the conductor because a large proportion of the surface of the strands, and the entire surface of the strands in the central region of the conductor, will be inncoated by the refractory coating.
If a circuit or signal integrity cable is formed according to the invention from a stranded conductor, it has the advantage that it is very flexible as compared with other signal and circuit integrity cables, especially if a stranded conductor is used. The ability of the wire to be bent around tight bends (small bend radii) without deleterious effect is partly due to the fact that the layer providing the integrity is thinner than with other signal and circuit integrity cables. However, when the conductor is a stranded conductor it may be bent around tight bends without undue stress on the surface of the strands because the strands are displaced from a regular hexagonal packing at the apex of the bend thereby exposing uncoated areas of the strands to the eye. It is highly surprising that even though uncoated strands may be exposed when the wire conductor is bent there is no electrical contact between adjacent stranded conductors. It is believed that in this case the integrity is retained because the profile of a stranded conductor is not cylindrical but rather is in the form of a hexagon that rotates along the length of the conductors, so that adjacent stranded conductors will touch one another only at a few points along their length, which points are always provided by the outwardly oriented part of the surface of the strands in the outer layer of the conductors. It is these points of contact that are always provided with the refractory coating.
The refractory coating preferably has a thickness of at least 0.5, more preferably at least 1, especially at least 2 and most especially at least 3 micrometres but preferably not more than 15 and especially not more than 10 micrometres, the most preferred thickness being about 5 micrometres depending upon specific operational requirements. The exact thickness desired will depend on a number of factors including the type of layer and the voltage rating of the wire, circuit integrity cables usually requiring a somewhat thicker coating than signal integrity cables and sometimes above 15 micrometres. The lower limits for the layer thickness are usually determined by the required voltage rating of the wire whilst the upper limits are usually determined by the time, and therefore the cost, of the coating operation.
As stated above, the conductor includes a keying layer formed from a metal other than copper for increasing the adhesion of the refractory coating to the conductor. The keying layer may be bonded directly to the copper or may be located on a further, intermediate metal layer. The metal of the keying layer or the further layer is preferably one which forms a good bond between the underlying metal and the refractory coating and also, as described in our copending British Application entitled "Temperature Resistant Coated Article" filed on even date herewith (Agent's Ref. RK263) (corresponding to European Application No. 85304871.8) one which acts as a barrier to diffusion of oxygen or copper or both or which acts to reduce stress in the refractory layer imposed by substrate strain, resulting from mechanical or thermal stress. Preferred metallic layers include those formed from aluminum, titanium, tantalum, chromium, manganese, silicon or nickel although other metals may be used. Examples of articles in which they may be used are described in our copending British Patent Application entitled "Electrical Wire & Cable" (Agent's Reference RK264) filed on even date herewith (corresponding to European Application No. 85304872.6) the disclosures of which is also incorporated herein by reference.
It has been found that, in such cases the metal forming the keying layer eliminates or substantially reduces the mechanisms by which failure occurs, thus extending the high temperature lifetime of the article. Thus, for example in the case of ciruit or signal integrity cables the time required to cause circuit failure in a fire would be substantially increased. The metal forming the keying layer for this purpose may be one which acts as a barrier to diffusion of either the underlying substrate to the outer surface of the article or to the diffusion of oxygen into the substrate. It may restrict diffusion in its elemental form or it may hinder diffusion processes, by formation of oxide scales when exposed to air, as is the case with for example aluminium or nickel. Such scales are most effective if they are stable on formation and exhibit low growth rates. The keying layer may be formed of metals which will alloy with the underlying substrate on exposure to high temperatures but which would still preferentially oxidise to form stable scales on exposure to air, or may be formed from metallic alloys which exhibit high oxidative stability e.g. titanium/aluminium alloys. The metal forming the keying layer may also be selected to take advantage of physical or chemical compatibility with the substrate and refractory layers to maximise adhesion.
In addition it has been found that in many cases the provision of a relatively thick keying layer significantly reduces the formation of cracks in the refractory layer when the article is subjected to mechanical abuse. It is believed that the reduction in formation of cracks is due to the reduction of stress in the refractory layer when the article is subjected to strain by virtue of the deformation of the intermediate layer, and accordingly it is preferred for the keying layer to be formed from a metal having a lower modulus than that of copper as described in the European Application No. 85304871.8.
The metallic keying layer may be formed in a number of ways, for instance by electroplating, standard wire cladding techniques such as roll bonding, and by vacuum deposition techniques e.g. sputtering, evaporation, flame spraying, plasma assisted chemical vapour deposition (CVD) or other techniques.
The refractory coating may provide the entire electrical insulation or one or more additional insulating layers may be provided thereon. The additional insulating layer may be inorganic or organic or a combination of inorganic and organic layers may be provided. For example polymeric insulation may be provided in order to provide additional insulation to the conductor during normal service conditions and also to enable the wire to have the desired dielectric properties and other properties e.g. mechanical properties, scuff resistance, colour coding ability etc. However, an important advantage of the present invention is that since a significant proportion of or all the service insulating properties are provided by the refractory coating, the electrical properties of the polymeric insulation are not as critical as with other wire contructions in which the polymeric insulation provides the sole insulation between the conductors. Of the known polymeric materials that are used for electrical insulation, polyethylene probably has the most suitable electrical properties but is highly flammable, and has poor mechanical properties. Attempts to flame retard polyethylene have either required halogenated flame retardants which, by their nature, liberate corrosive and toxic hydrogen halides when subjected to fire, or have required relatively large quantities of halogen-free flame retardants which have a deleterious effect on the electrical properties and often also the mechanical properties of the polymer. Accordingly, an acceptable wire has in the past only been achieved by a compromise between different properties which is often resolved by using a relatively thick-walled polymeric insulation and/or dual wall constructions. Although such forms of polymeric insulation may be used with the wire according to the present invention, the presence of the refractory layer does obviate these problems to a large extent since the polymer used for the insulation may be chosen for its flammability and/or its mechanical properties at the expense of its electrical properties. As examples of polymers that may be used to form the polymeric insulation there may be mentioned polyolefins e.g. ethylene homopolymers and copolymers with alpha olefins, halogenated polymers e.g. tetrafluoroethylene, vinylidene fluoride, hexafluoropropylene and vinyl chloride homo or copolymers polyamides, polyesters, polyimides, polyether ketones e.g. polyarylether ketones, aromatic polyether imides and sulphones, silicones, alkene/vinyl acetate copolymers and the like. The polymers may be used alone or as blends with one another and may contain fillers e.g. silica and metal oxides e.g. treated and untreated metal oxide flame retardants such as hydrated alumina and titania. The polymers may be used in single wall constructions or in multiple wall constructions, for example a polyvinylidine fluoride layer may be located on for example a polyethylene layer. The polymers may be uncrosslinked but preferably are crosslinked, for example by chemical cross-linking agents or by electron or gamma irradiation, in order to improve their mechanical properties and to reduce flowing when heated. They may also contain other materials e.g. antioxidants, stabilizers, crosslinking promotors, processing aids and the like. The polymeric insulation may, if desired, contain a filler e.g. hydrated alumina, hydrated titania, dawsonite, silica and the like, and especially a filler that has the same chemical composition, at least under pyrolys, is conditions, as the refractory coating, so that the filler in the polymeric insulation will provide additional insulation when the wire or cable is subjected to a fire. A preferred type of polymeric insulation is one that will char, for instance certain aromatic polymers mentioned above, or that will ash e.g. a silicone polymer, when subjected to a fire so that the char or ash, together with the refractory coating, will provide the necessary insulation during a fire. Examples of polymers, compositions, their manufacture and wires using them are described in U.S. Patent Specifications Nos. 3 269 862, 3 580 829, 3 953 400, 3 956 240, 4 155 823, 4 121 001 and 4 320 224. British Patent Specifications Nos. 1 473 972, 1 603 205, 2 068 347 and 2 035 333, 1 604 405 and in European Patent Specification No. 69 598, the disclosures of which are incorporated herein by reference. Preferably the wire is substantially halogen free.
The polymeric insulation may be applied onto the conductor by any appropriate method, for example by extrusion, tape winding or dip coating. In some instances, for example when certain aromatic polymers are used, it may be appropriate to form the insulation on the conductor by a plasma or thermal polymerisation process.
It has been found that it is possible to form articles according to the invention that are highly resistant to high temperatures and that the integrity of the refractory coating is not destroyed by exposure to high temperatures for relatively long periods of time. By examination of articles in accordance with the present invention and articles in which no metal keying layer is present, by means of a scanning electron microscope, it has been observed that the predominant failure mechanism of articles having no keying layer is through spalling. When articles are provided with a thin metal keying layer the spalling is reduced and failure occurs through a mechanism in which the underlying copper appears to migrate through the refractory layer and appear at the outer surface of the refractory layer, in the form of small globules or a network of "dykes" or in other cases, in the form of "blisters". This form of failure may occur at temperatures as low as 5000° C, well below the melting point of copper. The particular reason why this failure occurs is unclear and it is likely that more than one mechanism is responsible for the failure in different cases. One theory as to the failure mechanism is that, at elevated temperatures, the underlying copper is oxidized by ambient oxygen which has penetrated the refractory layer, either by diffusion or through cracks that may have been caused by mechanical or thermal stresses in the refractory layer, to form copper oxide (C_U20 or CuO) which are relatively electrically conductive. Growth of the copper oxide scale would proceed by outward diffusion of copper through the copper oxide to combine with inwardly diffusing oxygen until it reached the outer surface of the refractory layer. In the case of circuit integrity wires electrical integrity of the system would be significantly deleteriously affected.
Whatever the precise failure mechanism is, and whether the underlying copper migrates through the refractory layer in its elemental form or in the form of its oxide, it has been observed that this migration may be significantly reduced or prevented by the provision of a relatively thick metal layer which acts as a barrier to diffusion of oxygen or copper or both. For this reason, amongst others, the keying layer preferably has a thickness of at least 0.5, more preferably at least 1, especially at least 2 and most especially at least 3 micrometres.
It has also been observed that thick intermediate layers (e.g. aluminium layers) can act to reduce or eliminate crack formation resulting from the thermal expansion mismatch between copper and the refractory layer, and so improve the temperature resistance of the article.
Preferably a major part and most preferably substantially all the refractory coating is deposited on the conductor by a sol-gel method. The sol-gel process involves the hydrolysis and polycondensation of a metal alkoxide, for example, silicon tetraethoxide, titanium butoxide or aluminium butoxide to produce an inorganic oxide gel which is converted to an inorganic oxide glass by a low temperature heat treatment. The metal alkoxides can be used as precursors to inorganic glass preparation via the sol-gel route. The alumina gel can be prepared by adding an alkoxide of aluminium, such as aluminium secondary butoxide, to water which is heated to a temperature above 80° C and stirred at high speed. Approximately two litres of water per mole of alkoxide are suitable quantities. The solution is maintained at 90°C and approximately 0.5 - 1 hour after the addition of, the alkoxide a quantity of acid, for example 0.07 moles of hydrochloric acid per mole of alkoxide, is added to peptise the sol particles. The sol is maintained at the boiling temperature to evaporate excess butanol and reflux conditions are established and maintained until peptisation is complete. The sols can be reduced in volume by removal of water until a viscosity suitable for wire coating is reached.
Wires are provided with the alumina gel for subsequent conversion to an inorganic insulation by a dip or extrusion process. In this process the wire is drawn through the gel prepared to a suitable viscosity, as described above, such that a controlled thickness of gel adheres to the wire. The thickness is best controlled by wiping excess gel from the wire using sizing dies. The gel coated wire then undergoes suitable drying and firing stages to convert the coating into an inorganic oxide glass. The precise conditions with respect to temperature and residence time in the various stages of conversion are dependent upon the gel composition prepared and its tolerance to relatively rapid changes in its environment. Porosity and integrity of the coating can be significantly affected by these stages. A suitable conversion process would include drawing the wire through drying ovens in which the temperature is controlled at a temperature of approximately 80c C and subsequently through progressive heat treatment stages which expose the wire for a few minutes to temperatures of 300°C to 500° C. The required exposure times are dependent upon the initial thickness of the gel coating, but general guidelines are used with the recommendation that the drying process is carried out as slowly as practical. It may be desirable to build thickness in a multipass process in which several thin layers are deposited sequentially.
Although wires in which the entire refractory coating has been deposited by a sol-gel method have the advantage that they allow relatively rapid manufacturing operations, it may, be preferred in some instances to form part of the refractory coating by a different technique. For example the underlying part of the refractory coating adjacent to the metal keying layer may be formed by a slower deposition method such as a vacuum deposition process in order to improve further the adhesion of the refractory coating to the conductor. Examples of such methods include sputtering, evaporation, ion plating and chemical vapour deposition, and are described in our copending British Patent Applications entitled "Temperature resistant Coated Article" (Agent's reference RK263) and "Refractory coated Articles" (Agent's reference RK265), filed on even date herewith, the disclosures of which are incorporated herein by reference.
After the refractory coating has been deposited on the wire conductor it may be desirable to coat it with a thin coating of a polymeric resin or lacquer in order to provide mechanical protection and a barrier against water or electrolytes during service.
In order to form a circuit or signal integrity on cable the appropriate wires according to the invention may simply be laid together and be enclosed in a jacket. If desired the wires may be provided with a screen or electromagnetic interference shield before the cable jacket is applied. Thus a cable may be formed in a continuous process by means well known in the art by braiding the wire bundle and extruding a cable jacket thereon. Any of the materials described above for the wire polymeric insulation may be used although halogen-free compositions e.g. compositions as described in the U.K. Patent Specifications Nos. 1 603 205 and 2 068 347A mentioned above are preferred.
It is of course possible to employ additional means for providing integrity of the cable such as mica tape wraps, but these are not necessary nor are they desirable in view of the increased size and weight of the cable. The present invention is also suitable for forming flat cables which, as will be appreciated, are not susceptible to being wrapped with mica tape. Thus it is possible by means of the present invention to form flat cables that are capable of functioning as circuit and signal integrity cables.
Several embodiments of the invention and a method of production thereof will now be described by way of example with reference to the accompanying drawings in which:

Figure 1 is a cross-section through one form of wire according to the present invention,
Figure 2 is a cross-section through a signal integrity cable employing the wires of figure 1,
Figure 3 is a cross-section through, part of a flat conductor flat cable and,
Figure 4 is a schematic section through part of the thickness of article in accordance with the invention.

Referring to figure 1 of the drawings a 26 AWG stranded copper conductor formed from 19 copper strands 1 is coated with a 3 micrometre thick keying layer of aluminium by a vacuum evaporation technique, and a refractory aluminium oxide layer 2 having a thickness of 6 micrometres by the sol-gel method described above. A coating 3 based on a polyetherimide sold under the trade name "ULTEM" or a polyether ether ketone or polyether ketone is then extruded on the oxide coated conductor to form a polymeric "insulating" layer of mean wall thickness 0.2 mm.
Figure 2 shows a signal integrity cable formed by laying together seven wires shown in figure 1, forming an electromagnetic interference screen 4 about the bundle by braiding and then extruding thereon a jacket 5 based on a halogen-free composition as described in British Patent Specification No. 2 068 347 Example 1A.
The cable so formed is particularly lightweight and has a relatively small overall diameter in relation to the volume of the copper conductor.
Figure 3 shows a flat conductor flat cable comprising an array of flat copper conductors 1 with a 100 mil (2.54 mm) spacing. Each copper conductor 1 is provided with a 3 micrometre thick aluminium keying layer and a 6 micrometre thick alumina coating thereon as described above, and the coated conductors are embedded in a single polymeric insulating layer formed for example from the polyether imide sold under the trade name "ULTEM" or from a polyether ether ketone or polyether ketone.
Figure 4 is a schematic section through parts of an article according to the invention showing a typical arrangement of layers that may be formed on the copper substrate, the thickness of the layers being exaggerated for the sake of clarity.
A copper substrate 21 is provided with a thick (e.g. 1 to 3 micrometres) layer 22 of nickel followed by a layer 23 of aluminium metal. A layer 24 of nonstoichiometric aluminium oxide A120, and a layer 25 of stoichiometric aluminium oxide AI₂0₃ may optionally be deposited on the aluminium layer e.g. by a sputtering method. An additional, relatively thick layer 26 of aluminium oxide (e.g. of about 5 to 15 micrometres thickness) is deposited on the layer 25 by a sol-gel method or may be deposited directly onto the aluminium layer 23.
The following Examples illustrate the invention.

Examples 1 to 3

In Example 1 a copper conductor was provided with a 12 micrometre thick alumina coating by the sol-gel process described above, the coating being deposited directly onto the copper surface.
In Example 2, a copper conductor was provided with a 3.3 micrometre thick aluminium keying layer by means of a sputtering technique described in our copending British patent Application entitled "Refractory Coated Article" filed op even date herewith (Agent's reference RK265). The sputtering conditions were as follows: the wire 4 was precleaned by vapour degreasing in 1,1,1-trichloroethane prior to deposition. The cleaning was achieved by passing the wire through a vapour degreasing bath such that a residence time of 3 minutes was achieved. The wire 4 was then loaded into the vacuum chamber. The chamber was then evacuated to a pressure of 1 x 10-⁶ mbar prior to starting the process. At this stage argon was admitted to attain a pressure of 1.5 x 10-² mbar whereupon a high frequency (80 kHz) bias potential was applied to the wire handling system which was isolated from ground. A bias potential of -850 V was achieved, and the wire was transferred from reel 3 to reel 4 such that a residence time of 10 minutes was achieved. On completion of the cleaning cycle the pressure was reduced to 8 x 10-³ mbar and the deposition process started.
3 kW of DC power was applied to the aluminium target 5. The wire passed from reel 2 to reel 3 being coated as it passed the target 5. Residence time in this region was controlled by wire speed and adjusted to give the required thickness. The roller mechanism alternated the wire face exposed to the target as it progressed down the target length.
The aluminium coated conductor was then provided with an alumina coating as described with respect to Example 1.
In Example 3 a copper conductor was provided with a 3.3 micrometre aluminium keying layer as described with respect to Example 2 and was subsequently coated with aluminium oxide in a similar process. For this second coating, an aluminium oxide target powered with an RF power supply was, used. The wire residence time and target power were adjusted to give a constant thickness of aluminium oxide, being about 0.2 micrometres. During deposition of both aluminium and aluminium oxide the copper conductors were held at a bias potential relative to the chamber to promote adhesion.
The aluminium and alumina coated conductor was then provided with a sol-gel deposited alumina coating as described with reference to Example 1.
The samples were then tested to determine the adhesion of the top coat as follows. A fixed length of wire was subjected to a tensile strength whilst the strain was continuously recorded. During testing the wire sample was viewed through an optical microscope. When the coating was seen to significantly spall the strain was recorded. The strain value recorded at this point gave a measure of the adhesion of the coating. The composition of the samples and the results obtained are shown in Table No. 1.
The results show a clear improvement in adhesion of the gel derived alumina coating with the aluminium layer and a further improvement in adhesion with the vacuum deposited aluminium oxide layer.

Examples 4 and 5

The electrical performance of wires prepared as those in Example 3, were tested by twisting pairs of identical wires (2 twists per 2.5 cms length) to form a twisted pair cable of 1.5 m in length, connecting one end of the wires to a 1 MHz, 30 V square wave source and observing the wave across a 200 ohm load at the other end of the wires by means of an oscilloscope. The twisted pair cables were subjected to heating in a propane gas burner having a flat flame 8 cm wide. The temperature of the flame just below the twisted pairs was maintained at the required temperature and the time to failure recorded.
In Example 4 the sample was found to survive for 70 seconds in a flame at 900° C. In Example 5 the wires had still not failed after a flame exposure time of 72 minutes at 650°C. The substrate material onto which the sol-gel derived aluminium oxide was deposited for Examples 4 and 5 had a dense 0.2 micrometres coating of vacuum deposited aluminium oxide on its surface. Although this layer is insulating, it was incapable of supporting 30 V at room temperature.

Claims

1. An electrical wire which comprises a copper conductor and an electrically insulating refractory coating, the conductor including a keying layer formed from a metal other than copper for increasing the adhesion of the refractory coating to the conductors, obtainable by depositing at least part of the refractory coating by a sol-gel method.

2. A wire as claimed in claim 1, wherein a major part of the refractory coating, and preferably substantially all the refractory coating, has been deposited on the conductor by a sol-gel method.

3. A wire as claimed in claim 1 or claim 2, wherein the refractory coating has a thickness greater than 1 micrometre preferably greater than 2 micrometres.

4. A wire as claimed in any one of claims 1 to 3, wherein the refractory coating comprises a number of layers that have been deposited by a sol-gel method.

5. A wire as claimed in any one of claims 1 to 4, wherein the refractory coating comprises a metal oxide.

6. A wire as claimed in any one of claims 1 to 5, wherein the refractory coating comprises a compound of silicon, aluminium or titanium or tantalum.

7. A wire as claimed in any one of claims 1 to 6, wherein the keying layer comprises nickel, aluminium, titanium, manganese, tantalum, chromium, or an alloy thereof.

8. A wire as claimed in any one of claims 1 to 7, wherein the keying layer comprises the same metal as that present in the refractory layer.

9. A wire as claimed in any one of claims 1 to 8, wherein the keying layer has a thickness of at least 0.5, preferably at least 1, more preferably at least 2, and especially at least 5 micrometres.

10. A wire as claimed in any one of claims 1 to 9, wherein the keying layer has been formed by a vacuum deposition technique, preferably, by a metal rolling method, an electroplating method or by drawing the wire through a metal melt.

11. A wire as claimed in any one of claims 1 to 10, wherein the metal from which the keying layer is formed has a higher ductility than that of copper.

12. A wire as claimed in any one of claims 1 to 11, wherein the keying layer is formed from a metal that acts as a barrier to diffusion of copper or oxygen or both.

13. A wire as claimed in any one of claims 1 to 12, which has one or more additional layers on top of the refractory coating or between the refractory coating and the keying layer.

14. A wire as claimed in any one of claims 1 to 13, wherein the conductor is a stranded conductor and the refractory coating extends around the conductor but not around the individual strands thereof.

15. A wire as claimed in any one of claims 1 to 14, which is provided with an additional layer of polymeric insulation.