CA1295890C - Electrical wire with refractory coating - Google Patents

Abstract

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 addi-tion, 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

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ELECTRICAL WIRE WITH REFRACTORY COATIl!IG
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 accord-ingly 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 polytetrafluoro-ethylene, polyvinyl chloride, or polyvinylidine fluoride as poly-mers or decabromodiphenyl ether as flame retardan-t additive~.
Halogenated systems, however, suffer from the disadvantage that when they are heated to high temperatures during a Eire, 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 NoO 4,322,575 to Skipper and in U.R. patent specification Nos. 1,603,205 and 2,068,347A, .~ .

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In certain fields where cables are used for example in military, marine mass transit or offshore applications, it is desired to use cables which a~e capable of functioning at relatively high temperatures In other instances i~ is desired to use cables which not onl~ do not burn, or :if they burn, do not liberate toxic or corrosive gases but also are capable of functioning after having been su~jected to a fire or preferably for a period of time during a fire without shorting or otherwise failing. Cables tha~ are capable of functioning for a period o~
time during a fire hav~ been called circuit integrity cables or signal integrity cables depending on thelr use. The previously proposed circui~ and signal integrity cables have yenerally used the princlple that the individual aonductors should be separated from one another by mica tapes or by large volumes of packing materials or silicones or by combinations thereo~ 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 ~unction after it has been subjected to a fire and which preferably will retain its integrity for a period of time duriny a fire but which is smaller or lighter than the previously proposed cables.
The present invention provides an electrical wire which aomprises 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 increasin~

- 2a - 27065--124 the adhesion of the refractory coating to the conductor, and the wire heing further provided with an additional outer layer of polymeric insulation.

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' _ 3 _ RK266 According to the invention it i5 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 mech-anical or thermal stresses.

The use of a sol-gel method for depositing the refractory coating has the advantage that the refrac-tory coating is substantially contaminant-free, that is to say, it contains only those species that are inten-ded 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 perform-ance 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 pa~t of the invention,~usually have coatings that are porous and oft~en heavily contaminated with ionic residue from the electrolytic solutions e.g. sulphates from sulphuric acid anodisation processes.
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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 refrac-tory 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 800C, 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 1000C for at least 20 to 30 minutes. The preferred oxides and nitrides are those of aluminium, titanium, tantalum and silicon or mix-tures thereof with themselves or with other oxides ornitrides. 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 temper-atures for significant lengths of time without failure, e.g~ circuit and signal integrity cable and magnet wire. The conductor may be a single, solid conductor or it may be a stranded conductor in which individual strands are laid together to form a bundle which preferably contains 7, 19 or 37 strands. Where the conductor is stranded it is preferred for the bundle to be coated rather than the individual s~rands, that is to say, the re,fractory coating extends around the ; circumference of the bundle but not around the individ-ual strands so that substantially only the outwardly ,' :: ' :~

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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 5 dimensions o~ the bundle are kept to a minimum-(since the thickness of the coating may constitute a signif-icant 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 un-coated by the re~ractory 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 ex~osing uncoated areas of the strands to the eye. It is highly surpr~sing that even though uncoated strands may be exposed when the wire conductor is bent there is no electrical contact between adjacent stranded con-.

ductors. It is believed that in this case the in-tegrity 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 oonductors. so that adjacent stranded conductors will touch one another only at a few points along their length, which points are always provided by the out-wardly 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 O.S, 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 reguired voltage rating of the wire whilst the upper limits are usually deter-mined- by the time, and therefore the cost, of the coating operation.

As stated above, the~conductor includes a keying layer formed fr~m a metal other than copper for in-creasing the adhesion of ~he refractory coating to theconductor. The ~eying 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 iB preferably one which forms a good bond between the underlying metal and the refractory coating and al~o as described in Canadian Patent No. 1,264,616, one which acts as a barrier to diffusion of oxygen or copper or both or ~which acts to reduce stre~s in the refractory layer imposed by gubstrate ~train re~ulting from mechanical or thermal ~treæ Preferred metallic layers include those formed from aluminium, titanium, tantalum, chromium, manganese, silicon or nickel although other metals may be used. Examples o~ articles in which they may be used are degcribed in Canadian Patenk No. 1,241,395.
It has been found that, in such cases the metal forming the keying layer eliminates or Yubstantially reduces the mechanisms by which failure occurs, thus extending the high temperature lifetime o~ the arkicle. Thus, for example in the case of circuit or signal integrity cables the time required to cau6e circuit failure in a fire would be subs~antially 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 artlcle or to the diffusion of oxygen lnto the substrate. It may restrict diffugion in its elemental form or it may hinder difiusion processes, by~formation of oxide ~cales when exposed to air, as i~ the case with for example aluminium or nicXel. Such scaleg are most effective i~
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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 preferenkially oxidise to form stable scales on exposure to air, or may be formed from metallic alloys which exhibit high oxidative stability e.g. tita-nium/aluminium alloys. The metal forming the keying layer may also be selected to take advan-tage of physical or chemical compat-ibility 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 Eormation of cracks in the reEractory layer when the article is subjected to mechanical abuse. It is believed that the reduc-tion 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 accord-ingly 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 tech-niques such as roll bonding, and by vacuum deposition techniques e.g. sputtering, evaporation, flame spraying, plasma assisted chemica1 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 inor-;~ ~ ganlc or organic or a ' ~ ' , '` `` ~_2~5~`63~

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 an~ also to enable the wire to have the desired dielectric properties and other properties e.g. mechanical proper-ties. scuff resistance, colour coding ability etc.
However, an important advantage of the present inven-tion 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 ln 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 f~lame retard polyethylene have either required haIogenated 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 const'ruotions~. Although such forms of polymeric insulation may be used with the wire accord-ing to the present invention, the presence of the ~ ~ ref~ractory layer does o~viate these problems to .~
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a large extent since the polymer used for the insul-ation 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. tetra-fluoroethylene, vinylidene fluoride hexafluoropropy-lene and vinyl chl~ride homo or copolymers polyamides.
polyesters. polyimides. polyether ketones e.g. poly-arylether 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 ~luoride 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 mechan-ical properties and to reduce flowinq when heated.
They may also contain other materials e.g. anti-oxidants, stabilizers. orosslinking promotors. process-ing 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 fil;er that has the same chemical comp-osition, at least~under pyrolysis conditions, as the refractory coating, so that the filler in the polymeric insulation will provide~additional insulation when the ,~ :
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wire or cable is subjected to a fire. A preferred type of poly-meric insulation is one that will char, for instance certain aro-matic polymers mentioned above, or that will ash e.g. a silicone polymer, when subjected to a fire so that the char or ash, to-gether 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. Pre-ferably the wire is substantially halogen free.
The polymeric insulation may be applied onto the conduc-tor by any appropriate method, for example by extrusion, tape winding or dip coating. In some instances, for example when cer-tain 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 tem-peratures 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 keyin~ layer the spalling is reduce~ and failure occurs through a mechanism in which the under-lying 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 500C, well below the melting point of copper. The particular reason why this failure occurs is unclear and it is likely that more than one mechanLsm is responsible for the failure in different cases. One ;15 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 (Cu20 or Cu0) 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 o~ 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 throu~h the refractory layer in its elemental form or in the form of its oxide, it ha~s been-observed that this migration may be significantly reduced or prevented by ~he ': ~

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R K 2 6 6 ~i provision of a relatively thick metal layer ~hich acts as a barrier to diffusion of oxygen or copper or both.
For this reason, amongst others, the ~eyîng layer preferably has a thickness of at least 0.5, more S preferably at least 1, especially at least 2 and most especially at least 3 micrometres.

It has also been observed that thiFk 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 al-koxide, 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 ~lass preparation via the sol-gel route. The alumina gel can be prepared by adding an al-koxi~ of aluminium, such as aluminium secondary butoxide, to water which is heated to a temperature above 80C and stirred at high speed. Approximately two litres of water per mole of alkoxide are suitable quantities. The solution is maintained at 90C and approximately 0.5 - 1 hour after the addition of~the al-koxide a quantity of acid, for example 0.07 moles of hydrochloric acid per mole of alkoxide, is added to peptise the sol particles. ~he sol is maintained at the boiling temperature to evapor-ate excess butanol and~reflux conditions are estab-~: ' ',' .

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lished 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 ~he gel prepared to a suitable viscosity, as described above, such that a controlled thickness of gel adheres to the wire. The thickness is best con-trolled by wiping excess gel from the wire using sizingdies. 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 integ-rity 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 and subsequently through progressive heat treatment stages which expose the wire for a few minutes to temperatures of 300C to 500C. The required exposure times are dependent upon the initial thickness of the gel coating, but general guidelines are us~ed wlth 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 sequen-tially.

Although wires in which the entire refractory coating~has been~deposited~by a sol-gel meth~d bave the . ~ :
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.~ . , advantage that they allow relatively rapid manufactur-ing operations. it may be preferred in ~ome instances to form part of the refractory coating by a different technique. For example the underlying part o~ the refractory coating adjacent to the metal keying layer may be formed by a ~lower 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 sputter-ing. evaporation. ion plating and chemical vapourdeposition, and are described in Canadian Patent No.
1,264,616 and Canadian ~a~ent A~plicati~ N~ 499,446.

After ~he refractory coating has been deposited on the wire conductor it ~ay be desirable to coat it with a thin coating of a polymeric resin or lacquer in order to provide mechanical protection and a barrier agains~
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 ~ac~et. If desired the wires may be provided with a ~creen or elec~romagnetic i~terference ~hield before tbe cable jacket is applied. Thus a cable may be formed in a continuou~ 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 polym~ric insulation may be used .,~ .~ .

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although halogen-free compositions e g. compositions as described in the ~.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 desir-able 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 ` 25 conductor flat cable and Figure 4 is a schematic section through part of ` ~ the thickness of article in accordance with the invention.

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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 ~eying layer of aluminium by a vacuum evaporation techni~ue, and a refractory aluminium oxide layer 2 having a thickness of 6 micrometres by the sol-gel method described~above.
A coating 3 based ~n a polyetherimide sold under the trade name "ULTEM" or a polyether ether ~etone or polyether -ketone is then extruded on the oxide coated tO conductor to orm 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 t5 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 IA.

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 com-prising an array of flat copper conductors t with ~; a 100 mil (2.54 mm) spacing. Each copper conductor 1 is provided with a 3 micrometre thick aluminium keying ~ayer and a 6 m~icrometre thick alumina coating thereon as. described above, and the ccated 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 -~etone or polyether -ketone.
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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 thick-ness 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 non-stoichiometric aluminium oxide A120x and a layer 25 of stoichiometric aluminium oxide A1203 may optionally be deposited on the aluminium layer e.g. by a sputter-ing method. An additional relatively thick layer 26 oE aluminiumoxide (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 sur-;~ face.
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 on 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-.

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trichloroethane prior to deposition. The cleaning was achieved by passing the wire through a vapour degreas-ing 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 1.5.10 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 -850V was achieved, and the wire was transferred from reel 3 to reel 4 such that a residence time oE 10 minutes was achieved. On completion of the cleaning cycle the pressure was reduced to 8.10 3 mbar and the deposition process started.

3 kW of ~C 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.
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In Example 3 a copper conductor was provided with a 3.3 micrometre aluminium keying layer as described with respect t;o,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 micro-metres. During deposition of both aluminium and aluminium oxide the copper conductors were held at a bias potential rela~ive to the chamber to promote adhesion.

The aluminium and alumina coatea 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.

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Table 1 Example 1 2 3 Substrate22 awg Stranded 22 awg Str,anded 22 awg Stranded Copper Cu ~u Metallic Aluminium layer (micrometres) 0 3.3 3~3 Vacuum deposited Aluminium Oxide Layer (micro-metres) 0 0 0.2 Sol gel deposited Aluminium Oxide Layer (micro-metres) 12 12 12 Adhesion (arbitary units) 0* 415 600 * The adhesion of the coating to bare copper was very poor, rendering the samples unable to be tested due ~ to immediate spalling.

:~ : 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.

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Examples 4 and 5 . _ _ The electrical performance of wires prepared as those in Example 3, were tested by t:wisting pairs of identical wires (2 twists per 2.5 cms length) to form a twisted pair cable of 1.5 m in length. connectihg one end of the wires to a 1 MHz, 30V 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 8cm 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 900C. In Example 5 the wires had still not failed after a flame exposure time of 7~
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 30V at room temperature.

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Claims (23)

1. 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, and the wire being further provided with an additional outer layer of polymeric insulation.

2. A wire as claimed in claim 1, wherein a major part of the refractory coating has been deposited on the conductor by a 801-gel method.

3. A wire as claimed in claim 2, wherein substantially all the refractory coating has been deposited on the conductor by a sol-gel method.

4. A wire as claimed in claim 1, wherein the refractory coating has a thickness greater than 1 micrometre.

5. A wire as claimed in claim 4, wherein the refractory coating has thickness greater than 2 micrometres.

6. A wire as claimed in claim 1, wherein the refractory coating comprises a number of layers that have been deposited by a sol-gel method.

7. A wire as claimed in claim 1, wherein the refractory coating comprises a metal oxide.

8. A wire as claimed in claim 1, wherein the refrac-tory coating comprises a compound of silicon, aluminium or titanium or tantalum.

9. A wire as claimed in claim 1, wherein the keying layer comprises nickel, aluminium, titanium, manganese, tantalum, chromium, or an alloy thereof.

10. A wire as claimed in claim l1, wherein the keying layer comprises the same metal as that present in the refractory layer.

11. A wire as claimed in claim 1, wherein the keying layer has a thickness of at least 0.5 micrometres.

12. A wire as claimed in claim 11, wherein the keying layer has a thickness of at least 1 micrometre.

13. A wire as claimed in claim 12, wherein the keying layer has a thickness of at least 2 micrometres.

14. A wire as claimed in claim 13, wherein the keying layer has a thickness of at least 5 micrometres.

15. A wire as claimed in claim 1, wherein the keying layer has been formed by a vacuum deposition technique.

16. A wire as claimed in claim 15, wherein the keying layer has been formed by a sputter ion plating method.

17. A wire as claimed in claim 1, wherein the keying layer has been formed by a metal rolling method, an electroplating method or by drawing the wire through a metal melt.

18. A wire as claimed in claim 1, wherein the metal from which the keying layer is formed has a higher ductility than that of copper.

19. A wire as claimed in claim 1, wherein the keying layer is formed from a metal that acts as a barrier to diffusion of copper or oxygen or both.

20. A wire as claimed in claim 1, which has one or more additional layers on top of the refractory coating or between the refractory coating and the keying layer.

21. A wire as claimed in claim 1, wherein the conductor is a stranded conductor and the refractory coating extends around the conductor but not around the individual strands thereof.

22. A wire as claimed in claim 1, wherein the polymeric insulation will char when subjected to a fire.

23. A wire as claimed in claim 1 or claim 22, wherein the polymeric insulation has been deposited by a pyrolytic or plasma deposition process.