GB2344042A

GB2344042A - Method of producing resistive heating elements on an uninsulated conductive substrate

Info

Publication number: GB2344042A
Application number: GB9821020A
Authority: GB
Inventors: Jeffery Boardman
Original assignee: Individual
Current assignee: Individual
Priority date: 1998-09-29
Filing date: 1998-09-29
Publication date: 2000-05-24
Also published as: GB9821020D0

Abstract

A resistance element is produced by oxidising particles of metal powders to form a surface oxide layer, preparing a metal substrate to receive the particles, Heating the previously oxidised particles so they become at least semi-molten and depositing them on the substrate to form a resistive layer and depositing a conductive contact layer onto the surface of the deposited oxide layer. Current carrying paths, formed from lines of interconnected oxide particles extending though the resistive layer, interconnect the conductive layer and substrate, the area of the conductive layer determining the power output of the heating element. Ni Cr Fe alloys may be employed and the deposition processes may be plasma, arc or flame spraying. Parameters are disclosed.

Description

The present invention concerns a method of producing electrically resistive heating elements applied directly onto an electrically conductive substrate without the need for an electrically insulating intermediate layer. The invention also encompasses electrical heating elements when produced by the new method.

There are two conventional methods of producing electrical elements directly onto conductive substrates.

The first method is to screen print a resistive track in a variety of configurations onto a suitably prepared conductive substrate, which in this case is invariably metal.

In this process an insulating layer is firstly applied to the conductive surface which is to receive the resistive track. The insulating layer is generally of a material type compatible in properties with both the conductive metal substrate and the resistive element. It may be applied to the conductive metal substrate in a variety of ways but is generally done so by screen printing using two or more steps, each consisting of a printing, drying and firing operation.

The use of multiple steps in the application of the dielectric insulating layer to the conductive supporting substrate is intended to eliminate the chance of defects in any one layer coinciding with defects in either a preceding or succeeding layer, and causing the dielectric layer to lose its insulating properties.

Wth the successful provision of a dielectric insulating layer onto the electrically conductive supporting substrate, the required electrically resistive tracks may be screen printed onto the dielectric layer to form an electrical element of the required configuration. To ensure uniformity of properties for the resistive element the track configuration is generally applied in several stages. The material comprising the matrix within which the resistive component is suspended needs to match the properties of the preceding insulating layer.

The second method comprises the deposition, by flame spraying, of a metal oxide or oxides onto an electrically conductive supporting substrate. Such substrate also incorporates an electrically insulating dielectric layer, applied to the surface to which the electrically resistive oxide is to be applied by flame spraying to form the electrical heating element, generally as described in patents EU302589, US5039840 and patent application No PCT/GB96/01351.

A supporting substrate is required for both types of elements produced by the precedingly described processes as the materials forming the electrically resistive elements do not have sufficiently high intrinsic strengths to be selfsupporting.

Whilst both processes may be used to produce elements using electrically non-conductive materials such as fired ceramics as the supporting substrates, experience has shown that such systems are both more expensive and less robust in use than those employing insulated electrically conductive metal substrates.

The requirement for an electrically insulating dielectric layer between the element and conductive metal substrate arises almost entirely from the low resistivities of the materials used to form the electrically resistive element components.

As an example, the resistive materials used in the firstly described process, that of multi layer screen printing, are generally based on silver palladium compounds, with resistivities in the region of 10 to 160mQ square for thicknesses of 20pm.

This requires the elements produced from this process to be configured in the form of tracks of appreciable length.

Whilst the resistivities of the metal oxides produced by the second method are higher, ranging from 100 to 3000 ohm mms, the elements so produced do need to have a track length greater than their thickness by a large ratio.

The deposition of either type of electrically resistive material previously described directly to a supporting electrically conductive metal substrate would result in failure on the application of an electrical supply. The electrical current would flow from one contact point directly through the resistive layer to the metal substrate and subsequently along the shortest path through the metal and up through the resistive layer to the other point of contact.

This catastrophic form of failure may be readily seen in either type of element where the dielectric layer between resistive track and substrate metal is sufficiently defective to allow the passage of current in the form of a small hole whose surroundings show evidence of high temperature.

Whilst the two aforementioned methods are effectively and successfully used to manufacture electrical elements they are subject to various constructional disadvantages and the elements so produced to several operational disadvantages, some of which are listed below.

For both methods, the material used to form the insulating dielectric layer must be compatible with both the type of metal used for the supporting substrate and the resistive layer applied to it.

This compatibility usually requires the metal and dielectric material to have matching, or nearly matching, coefficients of thermal expansion and good adhesion one to the other.

With the oxidised flame spray method the metal substrate material may be aluminium, copper, mild or stainless steel with alumina, alumina titania, magnesia, or any combination of insulating metal oxides, or even an enamel or glass ceramic used as the dielectricAlnsulating layer.

However the screen printed element technology is restricted to a glass ceramic dielectric material, which in turn is compatible with virtually only one type of ferritic stainless alloy.

For all the above metal and insulation material combinations, the adhesion is dependent upon some form of metal surface pre-treatment and chemical bonding mechanism. Failure to achieve the requisite metal to insulation bond will result in element failure where separation occurs.

Similarly a mis-match in the coefficients of thermal expansion between the supporting metal substrate and the dielectric layer material will induce tensile stresses in the less ductile layer during thermal cycling whilst in use. The least ductile material is inevitably the dielectric layer and the effect of the stresses resulting from thermal cycling is to cause micro cracking of the insulating layer, with consequent loss of dielectric properties and subsequent failure of the element system.

The prime requirement of the intermediate layer is that it provides sufficient electrical insulation between the resistive element track and the metal substrate to meet the appropriate requirements of the various standards used to determine the safe operating conditions and properties of the various types of elements and associated applications.

Whilst such insulating materials may have high dielectric properties, a defect or hole in one part or area beneath the resistive element track will result in either failure in service or non-compliance with the appropriate regulations and standards.

To avoid such defects it is customary to apply the insulating material to the metal substrate in a series of thin layers. As a result, the deposition of the dielectric layer is a multi-stage process, generally requiring high energy input at each stage.

In consequence, the production of the insulating layer is comparatively expensive and can constitute the major cost component for the manufacture of the appropriate element system.

In general, materials with good dielectric properties inevitably have low thermal conductivities. As a result they act as barriers to the transmission of heat energy from the point of origin at the resistive element layer to the point of dissipation and utilisation at the outer surface of the metal substrate.

For some metal and dielectric systems the thermal conductivity of the insulating layer effectively determines the operating conditions for the whole element system. It is not unknown for a metal substrate to water interface to be at only 104 C whilst the element operating temperature is in excess of 250 C, due entirely to the poor thermal conductivity of the insulating layer.

This effect has deleterious operational implications for the efficiencies and use of such elements. High operating temperatures can limit the types of materials to be used to contain them or require the provision of thermal barriers. Where such elements may be used with low melting point plastic containment materials, there is a fire and safety risk if uncontrolled.

The conflict of requirements for a dielectric material thick enough to meet the insulation standards and yet thin enough to provide good thermal conductivity is a continuing problem for manufacturers of the two aforementioned types of elements.

The present invention seeks to overcome or substantially reduce the problems described above associated with the known element systems and manufacturing techniques.

In accordance with a first aspect of the present invention there is provided an electrically resistive heating element comprising a substrate of an electrically and thermally conducting metal, a thermally sprayed resistive oxide layer applied to an appropriate area of one surface of the conductive metal substrate, and a contact area disposed over the majority of the electrically resistive area such that an electric current may be passed from the contact area on one side through the thickness of the electrically resistive layer to the metal substrate on the other, electrical connection being made firstly to the contact area and secondly to the metal substrate.

The contact layer may consist of any electrically conductive material such as copper, nickel, aluminium, gold, silver, brass or conductive polymers, applied by means of flame spraying, chemical vapour deposition or magnetron sputtering techniques, electrolytic or chemical processes, or a solid piece held in place with adhesives, mechanical pressure or magnetic means.

Such contact layer is smaller in area than the resistive oxide layer so as to leave a distance between the outer edge of the contact layer and the outer edge of the oxide layer, sufficient to prevent an electrical current passing directly from the contact area to the metal substrate when a voltage is applied between contact and substrate.

The metal substrate may consist of any electrically conductive metal or metal alloy having either a flat two dimensional or three dimensional curved form and of a sufficient thickness to provide dimensional stability for the element system during the production process and subsequent operational use.

For the conductive contact layer the thickness should be such that it will carry the maximum current required for the element and allow it to distribute evenly over the whole of its surface such that the current passing through the oxide layer from contact to metal substrate is uniform in density for each unit area of oxide. This provision ensures that the heat energy generated per unit area is uniform and consequently the element operates at a uniform temperature and does not develop localised hot spots.

It is preferable but not necessary to make that area of the contact layer to which the extemal power supply point is to be fixed thicker than the remaining areas to assist in the even distribution of the current.

The resistive oxidised layer may be considered to consist of strings of interconnecting oxidised particles extending through the oxide layer. Each string of oxidised particles may be considered as a'wire'and hence the resistive oxidised layer may be considered as being composed of a multitude of parallel'wires', each wire carrying an appropriate fraction of the overall current.

The measured resistance of the element system is effectively the sum of the resistances of all the parallel'wires', or particle strings, connecting the contact area to the metal substrate.

It is a requirement of the present invention that the resistivities of the metal oxides comprising the electrically resistive layer are very substantially greater than the values obtainable by the method set out in patents EU302589, US5039840 and patent application No PCT/GB96/01351.

As an example, an element having a resistive oxide layer with a thickness of 100 microns, a heated area of 40cms2 and resistance of 24 ohms will require the resistivity of the material constituting the resistive layer to be of the order of 100,000 ohm centimetres.

Similarly, the same element resistance and thickness, but with a heated area of 60cms2 would require the resistivity of the material constituting the resistive layer to be in the region of 150,000 ohm centimetres.

There is an economic benefit to be derived from using such high resistivity oxide materials in that the higher the resistivity of the material comprising the resistive oxide heating layer, the less weight of material is required to produce elements at a specified resistance with consequent reduction in manufacturing costs.

It is also a requirement of the present invention that the resistive oxide heating layer comprised of the high resistivity oxide materials be completely free of any defects such as porosity or holes.

The presence of any such defects will allow the electrical current to pass directly from the contact layer to the electrically conductive metal substrate with resulting catastrophic failure of the mode previously described for the failure of the dielectric insulating layer with the two existing methods.

In order to meet this requirement the method utilised to apply the high resistivity oxidised material to the electrically conductive substrate must be such that the oxidised particles have sufficiently high thermal and kinetic energies that they deform on impact with the electrically conductive substrate to produce dense homogeneous layers with the optimum degree of interparticulate adhesion.

Experience has shown that it is not possible to achieve such conditions with the method described in patents EU302589, US5039840 and patent application No PCT/GB96/01351. Attempts to do so result in resistive oxide deposits which have high degrees of porosity and suffer the mode of failure by shorting out of the electrical current previously described.

It is also considered that the resistive materials utilised to produce elements by screen printing do not have resistivities of the magnitude required to meet the requirements of the present invention. In addition, such materials as are used need to be enclosed within a glass ceramic matrix to avoid any deleterious reaction with oxygen from their surroundings which will result in failure.

It is a requirement of the present invention that the production of the high resistivity oxide materials used to form the electrically resistive heating layer in a uniform, dense, defect-free layer may only be achieved by pre-oxidation of the metal alloy powder particles.

The method utilised to achieve the necessary degree of pre-oxidation of the metal alloy particles may include exposure of the particles to oxygen within a heated furnace or similar heated enclosure, or the passage of the metal particles through an oxidising flame produced by a combination of oxygen and a combustible fuel gas.

The latter method is preferred as the heated reacting particles may be projected into a vessel containing water or any other quenching medium which will effectively stop the oxidation reaction which occurs at the outer surface of the heated particles.

For example, it is known that particles of a conventional alloy having the composition Ni 75%, Cr 15%, Fe 10% will oxidise to the extent of 18-22% by weight when passed through an oxidising flame having the ratio of oxygen to the fuel gas of the order of 5: 1 as measured by rates of gas flows at a powder flow rate of approximately 10 grammes per minute and at a distance of 25cm from the surface of the quenching medium.

It is further known that subsequent repetitions of the above process with the initially oxidised metal particles will increase the degree of oxidation by successive amounts. However, the amount by which the degree of oxidation of the particles increases with repetition of the oxidising process decreases with subsequent repetition.

For the powder composition mentioned previously, a second oxidation produces an increase in the region of a further 10-12% by weight, a third oxidation giving an increase in the region of 7-8%.

Whilst the percentage increase in the degree of oxidation of the metal particles with successive exposure to the oxidising process progressively decreases, the bulk resistivity values for the oxidised material rises by increasing amounts for each successive oxidising step.

For example, the bulk resistivity for a deposit comprised of oxidised particles having a degree of oxidation by weight of 18-20% of the previously mentioned Ni Cr Fe alloy will be in the region of 30,000 ohm cms, but subsequent oxidation steps increase the resistivity of the material to values of 80,000 ohm cms and 120,000 ohm cms.

It is also known that alloy metal powders of different compositions will oxidise to greater or lesser extents than the previously mentioned Ni Cr Fe composition resulting in higher or lower bulk resistivity properties.

It is further known that the temperature coefficient of resistance of the electrically resistive layer formed from the high resistivity pre-oxidised particles is dependent by both type and degree on the amount of particle preoxidation.

For example, it is known that for the Ni Cr Fe alloy previously mentioned the temperature coefficient of resistance is positive for degrees of oxidation up to 10-15% by weight in that the resistance of an electrically resistive layer formed from such particles increases with increase in temperature, the rate of change increasing with the degree of oxidation.

Where the degree of metal particle pre-oxidation by weight is increased beyond 15% then the temperature coefficient of resistance becomes increasingly negative in that the resistance of an electrically resistive layer formed from such particles decreases with increasing temperature, the rate of change increasing with the increasing degree of oxidation.

It is further known that the change in the type of temperature coefficient of resistance from positive to negative occurs at greater or lesser degrees of particle pre-oxidation depending on the degree to which alloys of different compositions react in the oxidation process.

For example, particles of an alloy comprised only of Ni and Cr, such as 80% Ni 20% Cr, undergo lower degrees of oxidation with successive repetitions of the process previously described and the temperature coefficient of resistance for electrically resistive layers formed from such pre-oxidised particles is invariably positive.

It is also known that alloy metal particles having compositions of Ni Cr Fe whereby the Fe component reduces with increases in Ni and Cr, demonstrate reducing degrees of oxidation for each repetitive step of the oxidising process, and require higher degrees of oxidation before the temperature coefficient of resistance changes from positive to negative.

It is a requirement of this present invention that the degree of particle preoxidation is not such that the whole mass of each particle is oxidised, but that there remains a metallic region within the surrounding oxidised layer or at the nucleus of each particle.

Whilst the precise effect of the metal nucleus on the mechanism by which electrical charge carriers move between individual oxidised particles is not accurately established, it is considered to be of importance to the operation of these resistive oxide elements in a manner set out as follows.

In reviewing the properties of these resistive oxide layers it may be stated that: a) The resistivity of the oxide layer produced around each oxidised metal particle is far higher than that of the original metal alloy. For an electrically resistive layer made up of such oxidised particles the resistivity of the oxide layers and not the resistivity of the original metal alloy will determine the resistance value measured for such a layer. b) The greater the degree of oxidation of the individual particles which is assumed to be effectively an increase in the thickness of the oxidised layer, the greater will be the measured resistivity of a resistive layer so formed and the greater the resistance measured. c) The conductive paths along which the charge carriers move consist of metal nuclei surrounded by an oxide matrix. d) Whilst the exact composition of the individual oxides formed during the oxidation of Ni Cr and Ni Cr Fe alloy particles has not yet been precisely determined, it may be assumed that they are of the composition NiO, CrO and FeO. It is known that NiO and CrO are non conductive at normal temperatures and act as insulators.

It may be considered that under normal conditions resistive layers formed from oxidised particles as above would be non-conducting, which is not the case with the resistive layers which are the subject of this present invention.

It is considered that the grain boundary interfaces between successive oxidised particles become electronically charged by virtue of the method of deposition. This electronic charge at the oxide grain boundaries causes the free electrons in the metal nuclei to diffuse into the oxide layer so creating a space charge within it.

With the application of a potential to these grain boundaries, such as when an extemal electrical supply is made to an electrically resistive element, then the electrons will flow through the oxide layer and the resistance to their passage results in the generation of thermal energy.

With increase in element temperature and consequent increase in vibration of the atoms in the atomic lattices comprising the oxide grain boundaries, there is increased resistance to the flow of electrons, which is detectable as a positive temperature resistance coefficient.

It is considered that this mechanism may well explain the observed behaviour of resistive layers composed of oxidised particles of 80% Ni 20% Cr and the lesser degrees of oxidation for Ni Cr Fe alloy particles.

Empirical work has demonstrated that increasing percentages of Fe in the alloy powders promotes the exothermic oxidation reaction and it may be that the FeO produced can not be considered to be non-conductive.

It is considered that the presence of additional charge carriers in the oxide grain boundary layer, due to the presence of Fe above a certain level, reduces the charge barrier at the oxide grain boundary to the extent that on the application of an external potential there is an avalanche effect of charge carriers across the. grain boundary, which increases with increasing temperature and is detectable as a negative resistance temperature coefficient.

It is a requirement therefore of this present invention that the interparticulate contact between successive oxidised particles be as close as possible, to the extent that the outermost atoms of successive oxide layers may diffuse.

From the preceding statements it is also considered that by utilising preoxidised particles with varying properties, namely differing temperature coefficients of resistance, it is possible to produce electrically resistive element layers with temperature resistance coefficient values which are either positive, negative or neutral.

As an example, pre-oxidised particles of Ni Cr Fe alloy having a negative temperature coefficient of resistance may be deposited in combination with a second oxide having a positive temperature resistance coefficient, such as Barium Titanate, to produce a resistive layer with varying performance characteristics. It is known that the temperature resistance characteristic of Barium Titanate changes by four or five magnitudes at or about the Curie point, and this effect can act as a self-controlling mechanism for electrically resistive heating elements utilising this material.

Whilst a range of techniques may be used to prepare these electrically resistive oxide layers, the optimum method to ensure good adhesion to the metal substrate, uniform thickness and minimum porosity, is considered to be that of thermal spraying.

The thermal spraying technique consists of passing a stream of particles through a heat source, during which process the particles become semi molten or molten, and when projected onto an appropriate substrate form a homogeneous layer.

The heating source may take the form of a plasma device, or an arc struck between two wires or rods, such that the rods or wires melt and molten droplets are projected as a stream onto a surface, or a flame source formed from the reaction of oxygen with hydrocarbon gases or liquids.

The most cost effective devices are of the flame spray type utilising combustion gases, such as hydrogen, propane or acetylene, or liquids such as kerosene in combination with oxygen.

As the melting points of oxidised particles are in excess of those of the metal alloys from which they originate, conventional metal flame spraying equipment is not fully suitable in meeting the required conditions and will only do so when modified.

In conventional equipment the flame is unrestrained and loses a great deal of its heat to the surrounding air, and in addition the powder stream passing through the combustion source forms a divergent cone, whereby the outer streams of metallic particles reach a lower temperature than those in the centre.

It is known that the optimum conditions for the successful deposition of the appropriate pre-oxidised metal particles onto a conductive metal substrate are obtained by using stochiometric, or marginally less than stochiometric, oxygen to combustion gas or liquid ratios, in volumetric terms and by utilising a heat resistant tube to surround the combustion flame as set out in patent No 93/26052 and patent application PA9125927.5.

It is an objective of this present invention that the power outputs of the elements produced by the deposition of an electrically resistive layer formed from previously oxidised metal particles onto a conductive metal substrate be capable of variation, not only by virtue of their area and/or thickness, but also and more conveniently by varying the area, shape or configuration of the contact layer applied to that surface not in contact with the conductive metal substrate.

As an example of this, consider a resistive layer having the properties previously described, area 40cm2, thickness 100 microns and a resistance of 24 ohms for the appropriate bulk resistivity of the pre-oxidised metal particles.

It is known that the resistive layer is composed of strings of interconnecting oxidised particles which may be considered to be comparable to a multitude of wires in parallel.

Accordingly, if a contact layer is applied to the whole area of one side of the resistive layer, then all the strings of interconnecting oxidised particles may be considered to be carrying the current and the heat energy generated and consequently the power output will be maximised.

If however a contact layer is applied to less than the full area of the resistive layer, then a lesser number of interconnecting oxidised particle strings will be available to carry the electrical current with consequent reduction in the amount of current carried, heat energy generated and power output.

Accordingly, by varying the area and configuration of the contact layers applied to a series of resistive layers of identical area and thickness, electrical elements of varying power outputs may be produced.

It is considered that this facet of the process may be economically beneficial in the high volume production of elements.

It is also an objective of this present invention that the process so described may be used to produce electrically resistive elements utilising not only flat conductive metal substrates, but also curved or cylindrical metal substrates, and in fact any shape or configuration of conductive metal substrate for which a mathematical equation may be derived and used in a computer programme to control a robotic device capable of holding and moving the heat source through which the pre-oxidised metal particles may be caused to pass and subsequently deposited onto the appropriate metal substrate.

According to the present invention there is provided a method of producing electrically resistive oxide layers directly onto conductive metal substrates which may subsequently be used to generate heat from the passage of electrical current, the method comprising the steps of: a) Oxidising particles of metal powders to produce a layer of oxide on the surface of each particle and some of the original metal within the interior. b) Preparing the surface of a suitable electrically conductive metal supporting substrate such that the surface is substantially chemically clean and roughened to the extent that molten pre-oxidised particles will adhere to it. c) Heating the pre-oxidised particles to a temperature at which the particles become molten or partially molten and depositing the molten particles onto said surface of the supporting conductive metal substrate to form an electrically resistive layer to the thickness required. d) Depositing an electrically conducting contact layer onto the surface of the resistive oxide layer to form an electrically resistive element, the current carrying paths of which extend through the thickness of the resistive layer from the electrically conductive substrate to the electrically conducting contact layer.

Claims

CLAIMS 1. A method of constructing electrically ~ resistive heating elements comprised of an electrically conductive substrate, a resistive oxide layer and an electrically conductive contact layer such that the current carrying paths are made up of lines of interconnecting oxidised metal particles extending through the thickness of the resistive layer from the electrically conductive metal substrate to the electrically conductive contact layer, the method comprising the steps of: a) Oxidising particles of metal powders such that a layer of oxide is formed on the surface of each particle which surrounds the original metal within. b) Preparing the surface of a suitable metal supporting substrate such that the surface is substantially chemically clean and roughened to the extent that molten pre-oxidised particles will adhere to it.

c) Heating the previously oxidised metal particles to a temperature at which they become molten or semi molten and depositing the heated particles onto said surface of the supporting substrate to form an electrically resistive layer. d) Depositing an electrically conductive contact layer onto the surface of the resistive layer such that on the application of a voltage at the contact layer the conductive metal substrate current will flow through the resistive layer from contact layer to conductive substrate.
2. A method as claimed in claim 1, wherein the metal powders to be used are produced by the water or gas atomising process and may be of any shape ranging from spherical to those having re-entrant angles.
3. A method as claimed in claims 1 and 2, wherein the metal powders may be of any metal alloy which will react with oxygen to produce an oxide having a resistivity greater than that of the original metal or alloy.
4. A method as claimed in claims 1,2 and 3, wherein the method of oxidising the metal particles may be any system which exposes the particle surface to the presence of oxygen at a temperature level at which the particular metal will react to form a surface oxide layer.
5. A method as claimed in claims 1-4, wherein the preferred method for producing a surface oxide on the metal particles being to pass the said particles through an oxidising combustion flame where the ratio of oxygen to the fuel may be twice that required for stochiometric combustion in volumetric terms and then into a receptacle containing a quenching medium whereby the oxidation reaction is stopped.
6. A method as claimed in claims 1-5, wherein the fuel combining with oxygen in the combustion process to oxidise the metal particles may be either liquid or gaseous hydrocarbons.
7. A method as claimed in claims 1-6, wherein the procedure for producing the surface oxide layer on the metal particles may be repeated to progressively increase the thickness of the surface oxide layer whilst maintaining the presence of the original metal within the oxide layer.
8. A method as claimed in claims 1-7, wherein the number of repetitions of the oxidising process required to increase the thickness of the surface oxide layer on the metal particles is dependent upon the rate of reaction of the metal or alloy with oxygen and the desired thickness of the surface oxide layer.
9. A method as claimed in claims 1-8, whereby the surface oxide layer produced on the metal particles when subjected to the oxidising process will produce electrically resistive oxide layers with bulk resistivity values in the region of 50,000 to 200,000 ohm centimetres when deposited onto a suitably prepared metal substrate.
10. A method as claimed in claims 1-9, wherein the supporting metal substrate may be any metal or metal alloy which is electrically conductive.
11. A method as claimed in claims 1-10, wherein the preferred supporting metal substrates are those which combine good electrical and thermal conductivities and which are readily and economically available, for example copper, aluminium, mild and stainless steels, brasses and bronzes.
12. A method as claimed in claims 1-11, whereby the process used to prepare the surface of the electrically conductive metal substrate may be any chemical or mechanical technique which after processing produces a chemically clean metal surface with a surface roughness roughly equivalent to that on 60 grit emery paper.
13. A method as claimed in claims 1-12, whereby the method used to heat the pre-oxidised particles to a molten condition and to deposit the said molten particles as a uniform electrically resistive layer onto a supporting substrate may range from processes using combinations of heat and pressure such as hot isostatic pressing to hot spraying techniques.
14. A method as claimed in claims 1-13, wherein the preferred method for heating the oxidised particles and depositing them as an electrically resistive layer onto the conductive metal substrate is a hot spraying technique as described in patent application W0093/26052.
15. A method as claimed in claims 1-14, wherein the velocity of the molten oxidised particles on impact with the metal supporting substrate is in excess of 200 metres per second.
16. A method as claimed in claims 1-15, whereby the thickness of the resistive oxide layer is the result of a plurality of high speed passes of the stream of molten oxide particles over the appropriate area such that any minor defects or porosity in one sub-layer is not coincident with any defect in any preceding or succeeding sub-layer and is generally of the order of 50 to 350 microns.
17. A method as claimed in claims 1-16, whereby the resistive oxide layer may be composed of two or more sub-layers of oxides of different metals or alloys having different types of temperature resistance coefficients such that the resulting temperature resistance coefficient of the combined layer may be positive, negative or neutral, dependent upon the end use of the said combined resistive oxide layer.
18. A method as claimed in claims 1-17, whereby the resistive oxide layer may be composed of two or more sub-layers which have either positive or negative temperature resistance coefficients, resulting from different degrees of oxidation of particles of the same metal or alloy such that the resulting temperature coefficient of the combined layer may be positive, negative or neutral, dependent upon the end use of the said combined resistive oxide layer.
19. A method as claimed in claims 1-18, whereby the resistive oxide layer may be composed of a mixture of oxidised particles of a given metal or alloy which have been processed according to claims 1-9 and a second metal oxide combination produced by other means such that the properties of the combined resistive layer change by a substantial degree under pre-determined conditions.
20. A method as claimed in claims 1-19, whereby combinations of oxidised particles within the electrically resistive deposit are utilised to impart self-regulating characteristics to the said electrically resistive deposit under operating conditions of temperature, applied electrical fields, magnetic influence or stimuli from extemal radiation.
21. A method as claimed in claims 1-20, whereby the techniques used to deposit the molten oxidised particles result in an electrically resistive oxide layer composed of lines of intimately bonded particles comprising a metal nucleus and an outer layer of oxide such that the current path from contact layer to conductive substrate is from successive metal centres via the oxide interfaces.
22. A method as claimed in claims 1-21, whereby the"lines"of intimately bonded oxidised particles may be considered to be compared to resistive wires arranged in a parallel configuration and that the power output of the electrically resistive layer comprised of these"lines"is dependent upon that proportion of the total which carries electrical current at any one time.
23. A method as claimed in claims 1-22, whereby the proportion of the "lines"of oxidised particles comprising the electrically resistive layer which may carry electrical current is dependent upon the area and configuration of the electrically conductive contact layer applied to the resistive oxide layer.
24. A method as claimed in claims 1-23, whereby for any given value of extemal electrical supply the power output and heat generating capacity of the electrically resistive layer operating as a heating element is governed by the proportion of the area of the resistive film to which the electrically conducting contact layer is applied and the configuration of the said contact layer.
25. A method as claimed in claims 1-24, whereby the electrically conductive contact layer may consist of electrically conductive metals, non-metals, polymers or combinations of the said metals, non-metals or polymers.
26. A method as claimed in claims 1-25, whereby the processes used to apply the conductive contact layer to the resistive layer may range from chemical vapour deposition, magnetron sputtering, hot flame spraying, chemical electrolytic or mechanical means or combinations of said means.
27. A method as claimed in claims 1-26, whereby combinations of resistive oxide and conductive contact layers may be applied to suitably prepared supporting metal substrates either in flat, tubular or spherical form, or of any shape for which a mathematical equation may be derived and used in a computer programme to control a robotic device capable of holding the heat source used to deposit the oxidised particles onto the surface of said suitably prepared supporting metal substrate.
28. A method as claimed in claims 1-27, whereby the electrically resistive elements produced by the process previously detailed are not constrained by the need to have an intermediate insulating dielectric layer between the resistive layer and conductive substrate, and in consequence will have better thermal conductivity characteristics, may be made with lower thermal mass, operate at higher watts densities, be constructed from combinations of more cost effective materials, and be more tolerant to damage.