GB2374786A

GB2374786A - Self regulating heating element

Info

Publication number: GB2374786A
Application number: GB0100232A
Authority: GB
Inventors: Jeffery Boardman
Original assignee: BDSB HOLDINGS Ltd
Current assignee: BDSB HOLDINGS Ltd
Priority date: 2001-01-05
Filing date: 2001-01-05
Publication date: 2002-10-23
Also published as: GB0100232D0

Abstract

An electrically resistive heating element exhibiting constant resistivity and resistance from ambient to preset operational temperature and increase in resistivity and resistance above preset operational temperature comprises successive layers of different metal oxides deposited on an electrically conductive metal substrate the layers of metal oxides having both different composition and degrees of oxidation on an electrically conductive metal substrate such that the combined characteristics of different metal oxides result in desired overall characteristics. The element may be adapted for use in electric irons.

Description

PATENT PATENT APPLICATION FOR A SELF REGULATING ELECTRICAL HEATING ELEMENT FOR USE IN DOMESTIC APPLIANCES UTILISED FOR THE PRESSING OR IRONING OF CLOTHING.

APPLICANT : MR JEFFERY BOARDMAN BSc (Eng) 32 BURFIELD DRIVE APPLETON NR WARRINGTON CHESHIRE WA4 5DA ASSIGNEE : B. D. S. B. HOLDINGS LTD LAB T20 DARESBURY LABORATORY DARESBURY WARRINGTON CHESHIRE WA44AD REFERENCE: JB S. C. H. E.-5 TITLE SELF REGULATING ELECTRICAL HEATING ELEMENTS FOR USE IN DOMESTIC IRONS AND OTHER RELATED APPLICATIONS

This present invention relates to electrical heating elements for use in domestic appliances utilised for pressing or ironing clothing 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. The method of construction and the configuration, a property which is not available from conventional liquid heating elements and consequently renders them safer for use in domestic appliances than conventional elements.

Conventional electrical liquid heating elements of the tubular sheathed variety or screen printed type 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 in domestic appliances 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 incorporate 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 domestic 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 disadvantagé 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"temper- ature, 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 domestic appliance manufacturers with the problem of using 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 iron 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 clothing material and the water used to produce steam 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 heating cycle the thermal energy available is only 2. 24kw, rising to 3kw only when the requisite operating temperature 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 1200C is shown in Fig 1.

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 from 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 by powers of ten 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 aspect of the present invention 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 contact layer may be comprised 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 metal oxide deposit so as to leave a distance between the outer edge of the metal oxide deposit and the corresponding outer edge of the contact layer, sufficient to prevent an electric current passing directly from the contact area to the conductive substrate or conductive layer on an insulating substrate when a voltage is applied between contact and substrate.

For the conductive contact layer the thickness should be such that it will carry the maximum current required and allow it to distribute evenly over the whole of its surface such that the current passing through the thermally sprayed metal oxide deposit from contact to substrate is uniform in density for each unit area of the metal oxide.

This provision ensures that the heat energy generated within the volume of the resistive metal oxide is uniformly distributed, producing a uniform temperature over the appropriate area of the supporting substrate without any localised hot spots.

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

The supporting substrate may be comprised of any electrically conductive metal or metal alloy, or electrically insulating material and of a sufficient thickness to provide dimensional stability for the element system during production and subsequent operational use.

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 increase in resistivity and resistance by powers of ten at temperatures above the predetermined operational temperature limit, the rate of increasing resistivity and resistance being greater than the rate of temperature increase.

The property of the thermally sprayed resistive metal oxide deposit, whereby the resistivity and resistance increase by powers of ten a temperatures above a pre- determined 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. 03', where'A'may be a mono-, D1-, or trivalent cation, and'B'may be a pentatetra-, or trivalent cation, and 03 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 utilised 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 the required characteristic of increasing resistivity/ resistance with increasing temperature, generally as described in patents EU302589, US5039840 and PCT/GB96/01351.

Such nickel-chrome type metal alloys may be oxidised to the required degree, as a precursor operation, prior to being thermally sprayed as one or more layers of the resistive metal oxide deposit, as described in GB2344042, or may be oxidised to the required degree during the thermal spraying operation, generally as described in patents EU302589, US5039840 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- octahedra-ferro-electric oxides over the same temperature range.

Accordingly a second aspect 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 an increase of resistivity/resistance with temperature at values above the limiting temperature by powers of ten 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.

T, ktAc.

It is a seecpd aspect of this present invention that the resistance in ohms of the

electrical element described in the first aspect 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 liquid heating element designed to produce a given output of heating energy from conventional single phase domestic electrical supply.

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 inter-connecting 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 consequently 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 disposed, and additionally the size of the metal oxide particles comprising the current carrying lines.

Utilising the previously described first and second aspects of the present invention it is possible to determine by calculation the dimensions and relationship between the various components comprising the type of electrical iron heating element which is the subject of this present invention utilising the following basic procedures set out

in patent application 0030623. 3, ref JB S. C. H. E.-2.

Thus it is claimed that a losm heating element may be produced in accordance with

the preceding aspects of this present invention 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 by powers of ten 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 such liquid heating elements.

Whilst the prementioned mathematical example set out in patent application 0030623.3, ref JB S. C. H. E.-2, 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 patent application No 9821020.6 published as GB2344042.

FEMMM It is a tbt) M) aspect 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 liquid 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 alternate 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 ranging from 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.

FfP7J. l It is a fS : 1 aspect of this present invention that the configuration of the iron heating

element precedlngly described may be of any shape appropriate to the end use of said liquid heating element ranging from square, rectangular or rhomoidal with reentrant 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 iron heating elements may be manufactured within close tolerance limits for the required final operating resistance.

There is another aspect of this present invention in that a layer of glass ceramic material may be applied to that surface of the element substrate which is in contact with the clothing materials being pressed or ironed using the techniques described

and advantages claimed in patent application GB9820062.9, published as GB2341603A, the prementioned glass ceramic layer being utilised to provide electrical insulation between the clothing material and the metal element substrate, or an abrasion or wear-resistant surface, or a form of decoration and visual enhancement of the iron element, or a combination of such properties.

As an additional safety feature the glass ceramic layer may also be applied to the surface of the metal substrate onto which have been deposited the resistive oxide layers, serving as a dielectric medium and thus preventing accidental and possible dangerous contact with the "live" part of the iron element, and permitting water to be in intimate contact with the hot oxide without becoming "live".

Claims

CLAIMS FOR PATENT APPLICATION NO 0100232.8 REF JB S. C. H. E-5

1. Forms of self-controlling electrically resistive heating elements for use in appliances utilised for the pressing or ironing of clothing which will not continue to heat up beyond a pre-determined limiting temperature when electrical power is applied even under overload or unusual operating conditions and exhibit a self-controlling property and function by virtue of the characteristics of the different metal oxides utilised in their construction whereby the combination of one type of metal oxide having a positive temperature coefficient of resistance with a second metal oxide which substantially increases in resistivity and resistance at temperatures above the pre-determined operational temperature limit provide a heating element having a constant resistance from ambient to the pre-determined operating temperature and a very substantial increase in resistance at the pre- mentioned operational temperature limit such that the power output decreases rapidly to a fraction of that produced below the operational temperature limit, consequently the elements will not exceed the operational temperature limit and require no external devices or controls to provide this self-controlling property as with current conventional heating elements of the tubular sheathed variety or screen printed types.

2. Heating elements of the type described in claim 1 whereby the construction consists of a conductive metal substrate, or an insulating substrate to which is applied an electrically conductive layer, and onto the said substrate the two different resistive metal oxides are applied as two layers in intimate contact with successively the conductive substrate and each other and to the upper surface of one of the resistive oxide layers is applied an electrically conductive contact layer such that when a voltage is applied between the conductive substrate and the contact layer a current flows through the resistive metal oxide layers generating heat within them by virtue of their resistance (generally as described in PCT/GB01/05148).

3. Heating elements of the type and configuration described in claims 1 and 2 whereby the different resistive metal oxide layers and electrically conductive layers may be applied to the appropriate substrate by any means and method ranging from thermal deposition, whereby powders of metal oxides are passed through a heat source comprising a plasma arc or the combustion of oxygen and hydrocarbon gases to soften them prior to impact onto the substrate, such methodology being known generally as thermal spraying, or electrolytic processing, or hot isostatic pressing utilising powder compression and sintering combinations or chemical or vapour deposition techniques.

4. Elements of the type and configuration described in claims 1 to 3 wherein one of the resistive metal oxide layers is one of a variety of ferro electric materials having crystalline structures of the perovskite type and of general formula AB. 03 where'A'may be a mono - D1 or trivalent cation and'B'may be a penta, tetra or trivalent cation and 03 is the oxygen anion and generally known as oxygen-octahedr-ferro electric materials whereby the characteristics of initial resistivity and variation of resistivity with temperature and"switching"temperature of Curie Point may be varied by variations in composition as described in PCT/GB01/05148.

5. Elements of the type and configuration as described in claims 1 to 4 inclusively whereby the second resistive metal oxide is produced from nickel- chrome type metal alloys which may be oxidised to the required degree as a precursor operation prior to being applied in layer form to the substrate or maybe oxidised to the required degree during the application operation and which exhibit the required characteristic of increasing resistivity/resistance with increasing temperature generally as described in Patents EU302589, US5039840, PCT/GB96/01351, GB2344042 and PCT/GB01/05148.

6. Elements of the type and configuration as described in claims 1 to 5 inclusively whereby the resistive metal oxide portion of the element may comprise firstly a layer of resistive metal oxides derived from metal alloy or the nickel-chrome type applied to the substrate and secondly a layer of the oxygen-octahedra-ferro electric oxide material to which is deposited a final electrically conductive contact layer as described in PCT/GB01/05148.

7. Elements of the type and configuration as described in claims 1 to 6 inclusively whereby the resistive metal oxide portion of the element may comprise a layer of the oxygen-octahedra-ferro electric oxide material applied firstly to the substrate and secondly a layer of resistive metal oxides derived from alloys of the nickel-chrome type to which is deposited a final electrically conductive contact layer as described in PCT/GB01/05148.

8. Elements of the type and configuration as described in claims 1 to 7 inclusively whereby the resistive metal oxide portion of the element may be comprised of alternate layers of the oxygen-octahedra-ferro electric material and resistive metal oxide derived from alloys of the nickel-chrome type as required by element usage and to the top surface of which is deposited a final electrically conductive contact layer as described in PCT/GB01/05148.

9. Elements of the type and configuration as described in claims 1 to 8 inclusively whereby the oxygen-octahedra-ferro electric materials may have ambient resistivities in the range 100-100,000 ohm cms and the resistive metal oxides derived from alloys of the nickel-chrome types have ambient resistivities in the range of 2,000-80, 000 ohm cms.

10. Elements of the type and configuration as described in claims 1 to 9 inclusively whereby for a design of known heating area and operating power level the relative thicknesses of the different resistive oxides of the oxygen- octahedra-ferro electrical materials and those derived from alloys of the nickel-chrome type may be derived by calculation from a knowledge of the resistivities of the materials to be utilised.

11. Elements of the type and configuration as described in claims 1 to 10 inclusively whereby the area of the finally applied electrically conductive contact layeris made smaller by 2%-5% than the area of the combined resistive oxide layers so as to leave a resistive peripheral track thus ensuring that when a voltage is applied between the conductive substrate and the contact layer the current flow is directly through the resistive oxide from contact to substrate and not laterally from contact edge to substrate which may cause short circuiting of the resistive path.

12. Elements of the type and configuration as described in claims 1 to 11 inclusively whereby the conductive substrate may be comprised of any electrically conductive metal or metal alloy comprised of aluminium, copper, mild steel or stainless steel or of any type of electrically insulating material comprised of plastic, ceramics, glass or composites of the same to which is applied an electrically conductive layer to which a contact may be made.

13. Elements of the type and configuration as described in claims 1 to 12 inclusively whereby the finally applied contact layer may be comprised of any type of electrically conductive material to which a contact may be made.

14. Elements of the type and configuration as described in claims 1 to 13 inclusively whereby the techniques utilised to apply the final electrically conductive contact layer to the resistive oxide layer may compnse any form of thermal deposition, electrolytic deposition, adhesion utilising glues or solders, mechanical attachment or chemical and physical vapour deposition.

15. Elements of the type and configuration as described in claims 1 to 14 inclusively whereby the element may have any size, shape or form for which a mathematical equation may be derived.