WO2002043439A1

WO2002043439A1 - A method of producing electrically resistive heating elements having self-regulating properties

Info

Publication number: WO2002043439A1
Application number: PCT/GB2001/005148
Authority: WO
Inventors: Jeffery Boardman
Original assignee: Bdsb Holdings Limited
Priority date: 2000-11-21
Filing date: 2001-11-21
Publication date: 2002-05-30
Also published as: AU2002223863A1

Abstract

A method of producing an electrically resistive heating elemet comprising depositiong between an electrically conductive metal substrate (12) and a contact layer (14) successive layers (10a, 10b) of different metal oxides having both different compositions and degrees of oxidation such that the combined characteristics of the various different metal oxides results in the metal oxide deposit exhibiting an effectively constant resistivity and resistance over the temperature range from ambient to a predetermined operating temperature limit and a substantial increase in resistivity and resistance at temperatures above the predetermined operational temperature limit, and an electrically resistive heating element so formed.

Description

A METHOD OF PRODUCING ELECTRICALLY RESISTVE HEATING ELEMENTS HAVING SELF-REGULATING PROPERTIES

This present invention relates to electrical heating elements for use in domestic and other appliances which, by virtue of their configuration, construction and the inherent properties of the materials used, will not continue to heat up beyond a predetermined limiting temperature when electrical power is applied to them, even under overload or unusual operating conditions.

In this respect such elements exhibit a self-controlling property and function by virtue of the characteristics of the materials utilised, their method of construction and configuration, a property which is not available from conventional heating elements and consequently renders them safer for use in domestic and other appliances than conventional elements.

Conventional electrical heating elements do not have any self-regulating properties and when connected to an electrical power source will continue to heat up with increasing rise in temperature until they fail by burning out and self-destruct.

The safe use of these conventional elements is achieved by combining them with some form of temperature sensitive control device which effectively cuts off the electrical supply when a predetermined temperature level has been reached.

Generally these temperature sensitive control devices incorporated bimetals in various configurations and rely on the ability of the bimetallic components to deflect at or around a predetermined temperature to provide a mechanical action which "makes" or "breaks" the electrical supply contacts, thus interrupting the electrical power supply to the elements concerned.

Whilst such temperature sensitive bimetallic and other similar control devices are widely used and produced to high quality standards they are generally mechanical devices, and like all mechanical mass produced devices subject to the probability of failure, this failure probability increasing with increased usage. The operational failure of such temperature sensitive control devices will result in the over-heating and self-destruction of the associated elements, with potentially catastrophic results for users of the appliances incorporating such items.

Electrical heating elements are available which have self-controlling characteristics and are manufactured from various compositions of barium titanate, where the resistance increases by several powers of ten when the temperature is raised to the vicinity of the Curie Point, also known as the "switching" temperature.

However, such heating elements have several basic disadvantages which currently severely limit their widespread application and usage, some of which are set out as follows.

The major disadvantage lies with the inherent property of almost all the compositional variations of barium titanates, in that the resistivity of such materials is not constant over the temperature range from ambient to the "switching" temperature, or Curie Point, but reduces progressively with increasing temperature before increasing to a high value.

The consequence of this is that elements manufactured from such compositions will have operational resistances which reduce significantly from that measured at ambient temperature, to that just prior to the "switching" temperature, or Curie Point, a reduction which can be as high as half the original resistance.

This presents the manufacturers and users with the problem of applying such elements and deciding which ambient resistance to produce such elements to, in order to maximise the power output.

In explanation of this, consider the use of a conventional element in a domestic kettle operating with a single phase 230 volt a.c. supply. The maximum current allowed for 230 volt appliances is 13 amps and by Ohm's Law this defines the maximum power output of such appliances to circa 3 kilowatts, and consequently the minimum resistance of the heating element employed to 17.7 ohms.

In general the resistance of such elements increases slightly with increase in operating temperature by 1-2%.

Consequently the generation of heat by the element and transfer of this energy to the water is a maximum when the temperature is at a minimum and only slightly reduced from this as the boiling point is reached.

The same power and current limitations apply to barium titanate elements such that the minimum resistance of 17.7 ohms would need to be at a temperature near the "switching" or Curie Point, resulting in a higher resistance at ambient temperature. Assuming a resistance decrease over the appropriate temperature range of, say, 25%, a typical barium titanate element would need to be produced with an ambient resistance of 23.6 ohms.

Using Ohm's Law it can be shown that at the start of the water heating cycle the thermal energy available is only 2.24kw, rising to 3kw only when the boiling point is reached.

This is the opposite of that required by the domestic appliance manufacturers and an example of the resistance-temperature characteristic of a barium titanate composition with the Curie Point "switching" temperature at 120°C is shown in Fig.5.

A second disadvantage with barium titanate elements arises from the method used to produce them. Barium titanates derive their particular temperature/resistance properties mainly from the characteristics of the grain boundaries between the individual particles making up the bulk matrix of any particular piece.

Accordingly, objects made of barium titanates are produced by pressing together the required amount of fine powder particles of the appropriate composition in a press, usually with a binding agent, to the appropriate size and shape' of the required finished object and then sintering the pressed mass in a furnace at the requisite temperature to produce a homogeneous product.

Whilst this is an adequate manufacturing process it may result in products which are not fully dense from the pressing stage, and therefore do not exhibit uniform operating characteristics or have residual stresses form the sintering stage, which may result in cracking and failure during subsequent thermal operational cycles.

The present invention seeks to overcome, or very substantially reduce, the problems previously described by providing an electrical heating element which has the required self-controlling characteristic, in that the resistance increases greatly when the operating temperature is raised to a predetermined limit, but additionally has a nearly constant resistivity and resistance in the temperature range from ambient to the predetermined operational limit.

In accordance with a first feature of the present invention defined hereinafter there is provided a supporting substrate which may be comprised of an electrically conductive metal or metal alloy, or an electrically insulating material with an electrically conductive layer applied onto an area of it, a thermally sprayed resistive metal oxide deposit applied to an appropriate area of one surface of the conductive substrate or to an appropriate area of the electrically conductive layer situated onto one surface of an electrically insulating substrate, and a contact area disposed over the majority of the thermally sprayed electrically resistive metal oxide deposit such that an electric current may be passed from the contact area on one side through the thickness of the thermally sprayed resistive metal oxide deposit to the conductive metal substrate, or the conductive layer applied to the insulating substrate, on the other, electrical connection being made firstly to the contact area and secondly to the conductive metal substrate, or the conductive layer applied to one surface of an insulating substrate, and that heat is generated within the volume of the thermally sprayed resistive metal oxide deposit matrix as a result of the passage of said electrical current.

The thermally sprayed resistive metal oxide deposit is comprised of successive layers of different metal oxides having both different compositions and degrees of oxidation such that the combined characteristics of the various different metal oxides result in the thermally sprayed resistive metal oxide deposit exhibiting an effectively constant resistivity and resistance over the temperature range from ambient to a predetermined operating temperature limit and a substantial increase in resistivity and resistance at temperatures above the predetermined operational temperature limit, the rate of increasing resistivity and resistance being greater than the rate of temperature increase. The latter structure enables a self-regulating property to be achieved.

The property of the thermally sprayed resistive metal oxide deposit, whereby the resistivity and resistance increase dramatically, for example by a factor of at least ten, at temperatures above a predetermined operational limiting temperature, is achieved by making one or more of the successive layers of different metal oxides comprising the prementioned resistive metal oxide deposit from one of a variety of ferro-electric materials which have crystalline structures of the perovskite type and of the general formula 'A.B.O₃\ where 'A' may be a mono-, D1-, or trivalent cation, and 'B' may be a penta tetra-, or trivalent cation, and O₃ is the oxygen anion.

Such mixed metal oxides of the prementioned types are also generally known as oxygen-octahedra-ferro-electrics, and the characteristics of these materials, such as initial resistivity, variation of resistivity with temperature and Curie Point, or "switching" temperature, may be varied by variations in composition.

Barium titanates are the most well known, researched and widely used form of the oxygen-octahedra-ferro-electrics and is utilized in the forthcoming example of an element type which is the subject of this present invention.

All the oxygen-octahedra-ferro-electric metal oxides exhibit the characteristic of reducing resistivity with increasing temperature up to the Curie Point, or "switching" temperature, and this is compensated for in the element type which is the subject of the present invention by making one or more of the successive layers of the different metal oxides comprising the prementioned resistive metal oxide deposit from metal oxides which exhibit the characteristic of increasing resistivity and resistance with increasing temperature.

It is known that metal alloys comprised of the nickel-chrome type when oxidised and thermally sprayed exhibit a characteristic of increasing resistivity/resistance with increasing temperature, generally as described in patents EU 302589, US 5039840 and PCT/GB96/01351 to which reference is hereby directed, precursor operation, prior to being thermally sprayed as one or more layers of the resistive metal oxide deposit, as described in GB 2344042, GB 0028286.3 and PCT/GBOO/1885, or may be oxidised to the required degree during the thermal spraying operation, generally as described in patents EU 302589, US 5039840 and patent application No. PCT/GB96/01351.

In order that the prementioned thermally sprayed resistive metal oxide deposit exhibits the characteristic of a constant resistivity and resistance over the temperature range from ambient to a predetermined operating temperature limit, the rate of increase of resistivity with temperature of the one or more layers of thermally sprayed resistive metal oxide deposit comprised of oxides produced by the processing of alloys of the nickel-chrome type should closely match the rate of decrease of resistivity with temperature of the one or more layers of the oxygen-octaliedra-ferro- electric oxides over the same temperature range.

Accordingly a second feature of this present invention provides an electrically resistive heating element having the properties of effectively constant resistivity and resistance over an operating temperature range from ambient to a predetermined limiting temperature and thereafter a significant increase of resistivity/resistance with temperature at values above the limiting temperature, preventing the element from exceeding the predetermined limiting temperature by any amount other than that small temperature rise due to thermal momentum, and that such properties are achieved from a combination of different metal oxides as previously described and that in exhibiting a temperature limiting property the element has a self-controlling characteristic by the inherent properties of the different metal oxides comprising the thermally sprayed resistive metal oxide deposit applied to the prementioned supporting substrates.

The resistance in ohms of the electrical element described above will be dependent upon the' resistivity of the several layers of the different metal oxides comprising the thermally sprayed resistive metal oxide deposit, the number and thickness of the several layers of the different metal oxides and the area of the thermally sprayed resistive metal oxide deposit comprised of several layers of different metal oxides. It is known from empirical work with flame sprayed metal oxides described in the prementioned patents that the resistive properties of the different metal oxides derive mainly from the grain boundary effects at the junctions between successive oxidised metal particles, and that the smaller the size of the oxidised particles the greater the number in any given volume of the thermally sprayed resistive metal oxide deposit the greater will be the resistivity of the thermally sprayed resistive metal oxide deposit, and consequently the smaller the volume required to achieve the optimum resistance for a heating element designed to produce a given output of heating energy from conventional electrical supplies.

It is similarly known from empirical work that the thermally sprayed resistive metal oxide is comprised of "lines" of interconnecting metal oxide particles which act as current carrying mechanisms, and that the greater the number of such "lines" of interconnecting metal oxide particles the higher is the current carrying capacity of any thermally sprayed resistive metal oxide deposit comprised of such "lines". Such "lines" of interconnecting metal oxide particles may be considered as resistive wires in parallel and consequent^ the overall resistance of the thermally sprayed resistive metal oxide deposit is the sum of all the resistances of the parallel lines of interconnecting metal oxide particles which in turn will be dependent upon the area of the thermally sprayed resistive metal oxide deposit to which a contact area, as mentioned in the first aspect, is deposed and additionally the size of the metal oxide particles comprising the current carrying lines.

It is possible to determine by calculation the dimensions and relationship between the various components comprising one type of electrical heating element which is the subject of this present invention, for example utilising the following basic procedures.

For the purpose of this example it is assumed that a typical kettle electrical liquid heating element is required to produce 2.5 kilowatts of heating energy from a conventional domestic single phase electrical supply of 230 volts with maximum current of 13 amps.

From considerations of Ohm's Law a power output of 2500 watts at 230 volts requires a current of 10.87 amperes with an element having a minimum resistance of 21.16 ohms.

Such an element will utilise a metal substrate 110mm diameter with a thermally sprayed resistive metal oxide deposit 80mm diameter applied to it and a contact area 70mm diameter disposed over it. The metal substrate will be comprised of 2mm thick aluminium plate, such material having excellent electrical and thermal

_» conductive properties.

Empirical work has shown that for water to boil at the surface of such a metal substrate the metal-liquid interface temperature must be a minimum of 101°C and uniform over the whole of the 110mm diameter metal surface.

Using the values of density, specific heat and thermal conductivity for aluminium it is possible to calculate the operating temperature of the thermally sprayed resistive metal oxide deposit necessary to generate 2500 watts of thermal energy with a liquid-metal interface temperature of 101 °C as being 160°C.

In order that the element exhibits the required self-controlling characteristics it should not exceed a temperature more than 10°C above the operating level to allow for variations in operating conditions of such liquid heating elements. Consequently the safe maximum operating temperature of the element may be fixed at 170°C.

Examination of the well known properties of the various oxygen-octahedra- ferro-electric metal oxides enables a suitable composition to be chosen, having a Currie Point, "switching" temperature, at or close to 170°C. Such bibliographic sources will also detail the ambient resistivity and rate of decrease of resistivity with temperature over the temperature range from ambient to the maximum safe operating temperature of 170°C.

Utilising the data of ambient resistivity and rate of decrease of resistivity with temperature of the chosen oxygen-octahedra-ferro-electric metal oxide, the required ambient resistivity and rate of increase of a second metal oxide may be calculated such that a combination of the oxygen-octahedra-ferro-electric metal oxide and the second metal oxide provide the thermally sprayed resistive metal oxide deposit with the properties required and previously described of constant resistivity over the operating temperature range and a very rapidly increasing resistivity with increase in temperature above the predetermined safe limit.

Utilising details of the ambient resistivities of the two component oxides and their rate of change or resistivity with temperature and the design heating area of the thermally sprayed resistive metal oxide deposit, the overall thickness of said thermally sprayed resistive metal oxide deposit may be calculated. Similarly, the same details will enable the proportion of the different oxides within the total thickness of the thermally sprayed resistive metal oxide to be determined.

As a mathematical illustration of the preceding aspects of the present invention and prementioned combinations of different oxides, an element construction is envisaged which utilises a combination of barium titanate and oxidised NiCrFe alloy as described in patent application GB0028286.3, PCT/GB00/01885 and GB2344042.

There is a barium titanate composition with a Curie Point, "switching" temperature, of 160-170°C, which from empirical work and bibliographical data has an ambient resistivity of circa 4000 ohm cms, a rate of decrease of resistivity with temperature circa 40%, giving a resistivity at 160-170°C of 2400 ohm cms.

Similarly, as stated in patent application GB0028286.3, PCT/GB00/01885 and GB2344042, it is known that an alloy of 75% Ni, Cr 15% and Fe 10%, when oxidised to circa 15%, has an ambient resistivity circa 24000 ohm cms increasing with temperature by 25% to give a resistivity at 160-170°C circa 30,000 ohm cms.

The combination of the several layers of the prementioned two different oxides comprising the thermally sprayed resistive metal oxide deposit are considered as resistances in series, and the overall prescribed element resistance of 21.16 ohms is comprised of the resistance of the barium titanate component, defined as R_l5 added to the resistance of the oxidised NiCrFe alloy, defined as R₂, such that 21.16 ohms ^ R) ohms + R₂ ohms [1]

The conductive area of the thermally sprayed resistive metal oxide deposit is defined by the contact area disposed over it and established as having a diameter of, say, 70mm and area of 38.5cm² by calculation. The resistances R, and R₂ of the two oxides comprising the resistance of the thermally sprayed resistive metal oxide deposi may be defined in terms of the respective resistivities, thicknesses and areas as set out in equation [2].

Resistance R = Resistivity p x Thickness t [2]

Area A

Consequently, equation [1] may be re-written in the form:

21.16 = p₁-xJι x p₂-x t₂ [3]

A_j A₂

Where p_l3 tj and A[ are respectively the resistivity, thickness and area of the barium titanate component of the oxide deposit and p₂, t₂ and A₂ are respectively the resistivity, thickness and area of the oxidised NiCrFe component of the same thermally sprayed resistive metal oxide deposit, the areas A_! and A₂ are equal and have the value of 38.5cm² as defined by the contact area.

For the element to have the same effective overall resistance for the intended operating temperature range of ambient to 160°C equation [3] should have the same value for both temperature points and may consequently be re-written inserting the resistivity values for each oxide component at the respective upper and lower temperatures of ambient and 160°C as equations [4] and [5], where

21.16 = 4000 1_! + 24000 t, [4]

38.5 38.5 and

21.16 = 2400 x tr + 30.000 [5]

38.5 38.5

The solution to equations [4] and [5] gives a relationship between the thickness of the barium titanate component t, and the NiCrFe oxide component t₂ where

Substituting the values of t or t₂ into either of equations [4] or [5] gives values for t, of0.0783cms and t₂of 0.0209cms respectively, with a total thickness for the thermally sprayed resistive metal oxide deposit of:

0.0783 + 0.0209cms = 0.0992cms Thus, a heating element may be produced which has an effectively constant resistivity over an operational range from ambient to a predetermined limiting upper temperature and which also has a self regulating property whereby the resistivity increases greatly above the predetermined limiting temperature by virtue of the inherent properties of the different metal oxides comprising it, the configuration of one oxide with respect to a second and the constructional techniques employed to manufacture liquid heating elements, or other different types of heating elements.

Whilst the preceding mathematical example set out assumes that only one alternative and second metal oxide is required to combine with the chosen oxygen- octahedra-ferro-electric oxide, with the appropriate resistivity and rate of resistivity increase at the predetermined operational limiting temperature, it is envisaged that at other limiting temperatures and operational temperature ranges, combinations of two or more metal oxides may be required to balance the properties of the chosen oxygen-octahedra-ferro-electric oxide or oxides.

The degree of oxidation and methods of combining metal oxides deriving from different metals or alloys is substantially as set out in PCT/GB00/01885 and GB2344042.

It is a further feature of this present invention that the different oxides of different compositions comprising the thermally sprayed resistive metal oxide deposit may be applied to the supporting substrate in a variety of ways and using different techniques.

One first methodology can be to deposit the metal oxides produced from the NiCrFe or similar alloys as one complete layer, thermally sprayed to the required calculated thickness, area and configuration, subsequently followed by the application of the appropriate oxygen-octahedra-ferro-electric oxide component, thermally sprayed to the required calculated thickness, area and configuration, to produce the required combined properties and characteristics of the heating element concerned.

Alternatively, the reverse of this first methodology may be utilised, whereby the oxygen-octahedra-ferro-electric oxide component is firstly applied to the supporting substrate followed by the second component metal oxide. An alternative third methodology can be to apply the different metal oxides as alternative and successive layers until the required thickness of the thermally sprayed resistive metal oxide deposit is reached.

The thermal spraying operation required to apply the different oxides to the supporting substrate may be achieved using any of a variety of techniques, including flame spraying, where the heat source is provided by the combustion of oxygen and various hydrocarbons, plasma equipment, high velocity oxy fuel, and wire spraying.

For particular compositions of the oxygen-octahedra-ferro-electric oxides, other deposition techniques may be utilised and incorporated and these may include chemical and physical vapour deposition techniques.

It is a still further feature of this present invention that the configuration of the liquid and other different types of heating element precedingly described may be of any shape appropriate to the end use of said heating elements ranging from square rectangular or rhomoidal with re-entrant or projecting areas as determined by design, calculation or usage.

A further aspect of this present invention is embodied in the procedures used in the deposition of the layers of the different oxides comprising the thermally sprayed resistive metal oxide deposit, that such electrically resistive self-controlling heating elements may be manufactured within close tolerance limits for the required final operating resistance.

The present invention thus enables the provision of a method of producing electrically resistive heating elements having self-regulating properties by virtue of their configuration, construction and the inherent properties of the materials utilised and which will not continue to heat up beyond a predetermined limiting temperature when electrical power is applied to them, even under overload or unusual operating conditions.

The invention is described further hereinafter, by way of example only, with reference to the accompanying drawings, in which: -

Fig. 1 is a diagrammatic plan of an example of a resistive heating element in accordance with the present invention; Fig. 2 is a section on 1-1 in Fig. 1;

Figs. 3 and 4 are similar sections of second and third examples of resistive heating elements in accordance with the present invention; and

Fig. 5 is a graph of ohmic resistance versus temperature for a barium titanate composition.

The embodiment of Figs. 1 and 2 comprises a multilayer electrically resistive oxide deposit 10 formed on a conductive metal substrate 12 and carrying an electrically conductive contact layer 14. In this case, the multilayer resistive deposit 10 and contact layer 14 are both rectangular and the conductive metal substrate is a flat/planar plate. In other embodiments, the substrate could equally well be tubular or indeed any shape. Again, the overall shape of the substrate could by any desired configuration, eg. square, rectangular, round, but is preferably one definable by a mathematical equation.

The current flow from the contact layer to the conductive substrate, or vice versa, can be considered to be by way of a plurality of generally parallel, linear paths of oxide covered metal particles as indicated diagrammatically by the parallel lines 16.

Fig. 3 illustrates an example where the oxide layer comprises the prementioned first methodology whereby the metal oxides produced from the N,C_RFe or similar alloys and having a positive temperature coefficient of resistance is applied to the conductive substrate 12, as layer 10b, and the appropriate oxygen-octahedra- ferro electric component oxide flame sprayed as the second layer 10a, the metal contact layer 14 being applied to the top surface of 10a.

In other embodiments this first methodology may be reversed and the oxygen- octahedra-ferro electric component applied as the first layer onto the conductive substrate, followed by the N_JC_RFΘ or similar alloy oxides.

In further embodiments, a first resistive oxide layer may be combined with a further layer having particular properties.

For example as shown in Fig. 4, a layer 18 comprised of oxygen-octahedra- ferro-electric materials in layered combination with other metal oxides can be applied to the NlCRFe resistive oxide layer 10, where the oxide combinations comprising layer 18 are those materials which are reactive to external stimuli such as electromagnetic radiation in various forms.

The resistance of the oxygen-octahedra-ferro-electric materials in the prementioned layers increase dramatically, typically by 1000-10000 times, at a temperature corresponding to the Curie Point, at which point/temperature the crystalline structure changes from tetragonal to cubic and effectively acts as a "switch" mechanism, limiting the power outputs and performances of the combined layers lOa/lOb and 10 and 18.

Although the current path is shown in the sketches going from contact layer to substrate, it can equally flow the other way by reversing the polarity of the current supply.

Claims

1. A method of producing an electrically resistive heating element comprising depositing on an electrically conductive metal substrate successive layers of different metal oxides having both different compositions and degrees of oxidation such that the combined characteristics of the various different metal oxides result in the metal oxide deposit exhibiting an effectively constant resistivity and resistance over the temperature range from ambient to a predetermined operating temperature limit and a substantial increase in resistivity and resistance at temperatures above the predetermined operational temperature limit.

2. A method as claimed in claim 1, wherein at least one of said successive layers is of a metal oxide from one of the variety of ferro-electric materials which have crystalline structures of the perovskite type and of the general formula 'A.B.O₃', where 'A' may be a mono-, D1-, or trivalent cation, and 'B' may be a penta-tetra-, or trivalent cation, and O₃ is the oxygen anion, and having the property of greatly increasing resistivity at their Curie Point.

3. A method as claimed in claim 2, wherein said at least one of said successive layers is of barium titanate.

4. A method as claimed in claim 2 or 3 in which at least one other of said successive metal oxide layers is an oxide of a nickel-chrome type alloy.

5. A method as claimed in any of claims 1 to 4, wherein at least one of the metal oxide layers is formed by

(a) oxidising particles of a metal powder to produce a layer of metal oxide on the surface of each metal particle such that the oxide so produced has the property of increasing resistivity with increasing temperature;

(b) heating the pre-oxidised metal particles to a temperature at which the particles become at least partially molten or softened; and

(c) depositing the at least partially molten or softened pre-oxidised particles to form an electrically resistive layer.

6. A method as claimed in claim 5, wherein the metal powder is produced by the water or gas atomising process or by crushing from a solid.

7. A method as claimed in claim 5 or 6, wherein the metal powder comprises any metal or metal alloy which will react with oxygen to produce an oxide having a resistivity greater than that of the original metal or alloy.

8. A method as claimed in claim 5, 6 or 7, wherein the method of oxidising the metal particles comprises a system which exposes the particle surface to the presence of oxygen at a temperature level at which that metal will react to form a surface oxide layer.

9. A method as claimed in claim 8, wherein the surface oxide layer on the metal particles is obtained by passing the particles through an oxidising combustion flame where the ratio of oxygen to the fuel is up to twice or more that required for stoichiometric combustion in volumetric terms and then into a receptacle containing a quenching medium whereby the oxidation reaction is stopped.

10. A method as claimed in claim 9, wherein the fuel combining with oxygen in the combustion process to oxidise the metal particles comprises either liquid or gaseous hydrocarbons.

11. A method as claimed in any of claims 5 to 10, wherein the procedure for producing the surface oxide layer on the metal particles is repeated to progressively increase the thickness of the surface oxide layer to a desired level whilst maintaining the presence of original metal within the oxide layer.

12. A method as claimed in any of claims 5 to 11, wherein the oxidation process is performed to the extent necessary to achieve an electrically resistive oxide layer of bulk resistivity value in the range of 100 to 200,000 ohm centimetres when deposited on the metal substrate.

13. A method as claimed in any of claims 5 to 12, wherein the method used to heat the pre-oxidised particles to the at least partially molten or softened condition and to deposit the at least partially molten or softened particles comprises hot isostatic pressing or hot spraying.

14. A method as claimed in claim 13, wherein the molten or semi-molten pre- oxidised particles are applied at velocities in excess of 100 metres per second.

15. A method as claimed in claim 13 or 14, wherein for establishing a pre- determined thickness of resistive metal oxide layer, a plurality of high-speed passes of a stream of semi-molten or softened oxide particles are conducted over a relevant area.

16. A method as claimed in claim 15, wherein the thickness of the deposited metal oxide layer is in the range of 50 to 1250 microns.

17. A method as claimed in any of claims 2 to 16, wherein the Curie Point of the metal oxide of said at least one of the successive layers is variable by chemical composition.

18. An electrically resistive heating element comprising an electrically conductive metal substrate having deposited there on successive layers of different metal oxides having different compositions and degrees of oxidation such that the combined characteristics of the various different metal oxides result in the metal oxide deposit exhibiting an effectively constant resistivity and resistance over the temperature range from ambient to a predetermined operating temperature limit and a substantial increase in resistivity and resistance at temperatures above the predetermined operational temperature limit.

19. An electrically resistive heating element as claimed in claim 18, wherein at least one of said successive layers is of a metal oxide from one of the variety of ferro-electric materials which have crystalline structures of the perovskite type and of the general formula ^ςA.B.O₃', where 'A' may be a mono-, D1-, or trivalent cation, and 'B' may be a penta-tetra-, or trivalent cation, and O₃ is the oxygen anion, and having the property of greatly increasing resistivity at their Curie Point.

20. An electrically resistive heating element as claimed in claim 19, wherein said at least one of said successive layers is of barium titanate.

21. An electrically resistive heating element as claimed in claim 19 or 20, wherein at least one other of said successive metal oxide layers is an oxide of a nickel-chrome type alloy.

22. An electrically resistive heating element as claimed in any of claims 19 to 21, wherein a required safe operating/controlling temperature is determined by the inclusion of the oxygen-octahedra-ferro electric material having a Curie Point or temperature of the same magnitude as the required safe operating temperature and dependent upon the chemical composition of the said oxygen-octahedra-ferro electric material.

23. An electrically resistive heating element as claimed in any of claims 19 to 22, wherein the operating properties of resistance variation or resistance with temperature, safe operating or controlling temperature, heat dissipation area and proportion of resistive metal oxide and oxygen-octahedra-ferro electric material comprising the electrically resistive layer may be determined by calculation.

24. An electrically resistive heating element as claimed in any of claims 19 to 23, wherein the layer of oxygen-octahedra-ferro electrical material is deposited as a first layer onto the electrically conductive substrate followed by a second layer of resistive N,C_RFe metal oxide, or vice versa, and finally by a conductive contact layer.

25. An electrically resistive heating element as claimed in any of claims 19 to 24, wherein the layer of oxygen-octahedra-ferro electric material applied to the conductive substrate is combined with other metal oxides such that the resistive oxide deposit is sensitive to stimulation by electro-magnetic radiation, so developing particular properties for specific applications.

26. An electrically resistive heating element as claimed in any of claims 19 to 25, wherein oxygen-octahedra-ferro electric material and the resistive metal oxides are applied to the conductive substrate as alternate and successive layers until a required thickness of the thermally sprayed oxide layer is achieved.

27. An electrically resistive heating element as claimed in any of claims 19 to 26, wherein the thickness of the oxygen-octahedra-ferro electric material layer is in the range of 10 to 500 microns.

28. An electrically resistive heating element as claimed in any of claims 18 to 27, having an electrically conductive contact layer which comprises an electrically conductive metal, non-metal polymers or combination of the said metals, non-metals or polymers.

29. A method as claimed in any of claims 1 to 17, wherein a conductive contact layer is applied to the resistive oxide deposit by any of chemical vapour deposition, magnetron sputtering, hot flame spraying, chemical electrolytic or mechanical means or combinations of said means.

30. A method as claimed in any of claims 1 to 17 and 29, wherein combinations of said electrically resistive deposit and electrically conductive contact layers are applied to suitably prepared supporting metal substrates either in flat, tubular or spherical form, or of any shape.

31. A method as claimed in claim 30, wherein combinations of said electrically resistive deposit and electrically conductive contact layers are applied to metal substrates of a shape for which a mathematical equation may be derived and used in a computer program 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.

32. A method as claimed in any of claims 1 to 17 and 29 to 31, wherein a thermal spraying operation for applying the different oxides to the supporting substrate is achieved using any of a variety of techniques, including flame spraying, where the heat source is provided by the combustion of oxygen and various hydrocarbons, plasma equipment, high velocity oxy fuel, and wire spraying.

33. A method as claimed in any of claims 2 to 17 and 29 to 32, wherein for particular compositions of the oxygen-octahedra-ferro electric oxides, other deposition techniques may be utilised and incorporated and these may include chemical and physical vapour deposition techniques.

34. A method as claimed in any of claims 1 to 17 and 29 to 33, wherein the conductive metal substrate comprises any of copper, aluminium, mild and stainless steels, brasses and bronzes.