US2966430A

US2966430A - Electric resistance elements

Info

Publication number: US2966430A
Application number: US713777A
Authority: US
Inventors: Schrewelius Nils Gustav
Original assignee: Kanthal AB
Current assignee: Kanthal AB
Priority date: 1957-02-05
Filing date: 1958-02-03
Publication date: 1960-12-27
Anticipated expiration: 1977-12-27

Description

Dec. 27, 1960 N. G. SCHREWELIUS 2,965,430

ELECTRIC RESISTANCE ELEMENTS Filed Feb. 5, 1958 4 Sheets-Sheet 1 Fig.1

lNVE/VTOR. NILS GUSTAV SOHREWELIUS %ez {gr/ W arromvsrs Dec. 27, 1960 N. e. SCHREWELIUS 2,965,430

ELECTRIC RESISTANCE ELEMENTS Filed Feb. :5, 1958 4 Sheets-Sheet 2 Fig. 4

Fig.5

Fig.6

INVENTOR NILS GUSTAV SCHREWELIUS ATTORNEYS ELECTRIC RESISTANCE ELEMENTS Filed Feb. 5, 1958 4 Sheets-Sheet 3 Fig.7

INVENTO/F NILS GUSTAV SCHREWELIUS ATTORNEYS Dec. 27, 1960 N. e. SCHREWELIUS 2,966,430

ELECTRIC RESISTANCE ELEMENTS Filed Feb. 5, 1958 4 Sheets-Sheet 4 lrll llllllI-llllllllllll'allli'l INVENTOR- NILS GUSTAV SCHREWELIUS ATTORNEYS United States Patent ELECTRIC RESISTANCE ELEMENTS Nils Gustav Schrewelius, Hallstahammar, Sweden, assignor to Aktiebolaget Kanthal, Hallstahammar, Sweden, a corporation of Sweden Filed Feb. 3, 1958, Ser. No. 713,777

Claims priority, application Sweden Feb. 5, 1957 '12 Claims. (Cl. 117--212) The present invention generally relates to electric resistance elements, and more particularly to resistance elements of the kind comprising an electrically insulating, refractory base member having applied thereto by a socalled flame-spraying operation a thin, electrically conductive layer.

The invention has for its object to realize certain new and useful improvements in or relating to resistance elements of this general kind, as will be stated in detail hereinafter.

In accordance with the present invention, an electric resistance element of the kind referred to is mainly characterized in that said electrically conductive layer mainly consists of silicides of the transition elements belonging to groups IV, V and VI of the periodic system, i.e. the elements Ti, Zr. Hf, Ta, Cb (Nb), V, Mo, W and Cr.

By the term flame spraying in this connection is to be understood a process in which particles are blown or projected at high velocities on to a base member, these particles being simultaneously heated, or pre-heated to a temperature at which they are ductile when they impinge against the base member. The energy required to heat the particles and to impart high velocities to the same, can be supplied in any one of several different ways, such as, for instance, by the continuous combustion of a gaseous fuel, by detonations brought about in a gaseous mixture, by electrical arcing, by the utilization of magnetic forces or high-tension electric fields, or of gases uinder pressure for the atomization of previously fused materials, etc.

It is well known that silicides of the transition elements in the form of solid, non-porous bodies provide a good oxidation resistance at elevated temperatures. This is true to a particularly high degree for the compound MoSi but also TiSi VSi and WSi are highly oxidation resistant, which depends on the formation of a gastight layer of silicon dioxide which protects the underlying silicide against any continued oxidation.

It has now been found, quite surprisingly, that it is possible, by flame spraying thin conductive layers of one or more of the silicides onto a ceramic base member or carrier, to produce articles which, to advantage, can be used as electric resistance elements even at elevated temperatures in an oxidizing atmosphere, even though the thin silicide layers have a substantial degree of porosity, which is inherent in flame spraying operations in gen- .eral. As compared to previously known resistance elements of the coating-conductor type, those of the present invention involve no, or unnoticeable, aging, satisfactory resistance to various kinds of corrosion and strong adherence. The thermal responsiveness of their resistance value can be varied over a wide range. Practical ex.- periments have shown that the thickness of the sprayed coating should preferably be kept within the range of from 5 to 500 micron, preferably from 10 to 200 micron. Layers thicker than 500 micron may tend to scale off due to great temperature changes. coating'thickness is determined substantially by the fact The lower limit of the Patented Dec. 27, 1960 that the relative variation in thickness will increase with decreasing average coating thicknesses. Thus, for instance, if in the flame spraying operation an average thickness of 30 micron is maintained for the coating, then the thickness will vary from point to point between about 10 micron and about 50 micron. Such local variation in coating thickness will be comparatively more important in respect of thin coatings, and this is the reason why the lower limit of the coating thickness has been fixed to be 5 micron.

In the course of the flame spraying operation, the conductive silicide layer may be applied in the form of coils, meanders and the like by using masking arrangements, suitably made of thin, polished sheet-steel to which the sprayed silicide layer will not adhere.

The material to be used for the base element employed in the flame spraying of conductive silicide layers may be any material having a substantially lower electrical conducitivity than the silicides and not being susceptible of deterioration through the flame spraying operation, i.e. generally materials of a ceramic nature. Among ceramics which may be employed the following materials may be mentioned: refractory bricks of various qualities, silicon carbide, glazed or unglazed porcelain, heat resistant glass and enamel coatings or flame sprayed oxide coatings. The last-mentioned two examples offer an interesting possibility of utilizing also metallic supporting members, in that the metallic object or article will have applied onto it first an electrically insulating, thin superficial oxide coating of enamel or flame sprayed oxide; this insulating coating can then be used as a base which has applied thereto by flame spraying electrically conductive coils of silicide material.

To be taken into account when choosing the insulating base is not only its refractory characteristic, but also its coefficient of thermal expansion, for example. Corundum (Al' O offers interesting possibilities in this connection, since its coeflicient of thermal expansion equals that of MoSi Base members, or backings, of A1 0 may be in the form of flame sprayed coatings, sintered corundum bricks, or porous, ceramic-bonded grinding wheels with abrasive grains of fused corundum.

It is to be noted that there is no necessity for the final conductive layer or coating of the invention to be of the same composition or structure as the layer produced directly through flame spraying. As a matter of fact, the former layer may be subjected to subsequent treatment, for instance by being annealed in an oxidizing atmosphere so as to obtain a gas-tight protective skin consisting mainly of silicon dioxide and having a thickness of 2 to 30 micron, preferably 5 to 15 micron. In conjunction with this oxidation process, the conductive silicide layer may change as regards its composition or structure. The characteristics of the flame sprayed layer or coating may be modified othelwise, as well, such as bybeing subjected to treatment in an atmosphere containing SiCl BBrg M0(CO) or other volatile compounds which will easily undergo thermal decomposition.

Another way of establishing a protective layer or skin rich in silicon dioxide would be to cover the conductive layer completely or partly with an electrically insulating material, such as glass, a glaze or an enamel. In this case also, when the protective coating is being applied, such material will tend to penetrate into the pores of the conductive layer and thus to alter the composition thereof, whereby an improved resistance to corroding attacks can be obtained. i e

For certain special applications, it may be suitableto protect the current conducting flame sprayed layer with a thicker ceramic coating. Thus, a corresponding resistance element, for example, may comprise twoplatesof heat-resistant glass, one of these plates forming the base member for a conductive flame-sprayed silicide layer, and the other a protective layer, the two plates being fused together. The similar procedure may be applied using enamelled metal plates instead of glass plates.

According to a further embodiment of the present invention, the flame-sprayed conductive layers are attached only partially to the ceramic base member. As a matter of fact, it may be of advantage in order to avoid such tensile stresses which may arise upon cooling down of the resistance elements, that the flame-sprayed silicide layer be partially free from its ceramic support, and this may be accomplished by coating areas of the ceramic surface of the base member with a destructible material before the flame spraying operation takes place.

The current conducting layer, as stated hereinbefore, should consist, to at least 50% by weight, of one or more silicides of the transition elements of groups IV, V and VI, i.e. Ti, Zr, Hf, Ta, Cb (Nb), V, Cr, Mo and W. A layer containing mainly or exclusively MoSi has been found to be particularly advantageous. In addition, the layer may contain other heat resistant silicides, such as, for instance, TiSi VSi Mo Si and others. A further advantageous combination has been found to be that of MoSi and Mo Si The sprayed layer may contain, in addition to oxidation resistant silicides, relatively small amounts of oxidation resistant borides, for instance CrB carbides, for instance SiC, and oxides. For resistance elements intended to be used in air at elevated temperatures, for instance above 1200 (3., however, it is necessary that the sprayed layer be capable of forming a gas-tight protective coating or skin of quartz glass. This may be accomplished according to the invention by forming the incandescent zone of the sprayed conductive layer mainly of one'or more of the silicides Mosi VSi WSi and TiSi The thickness of this quartz glass skin or coating will increase in the course of heat treatments carried out in an oxidizing atmosphere, so that, for example, in respect of the composition 50% MoSi and 50% Mo Si the thickness of the quartz glass coating will be 2 micron after 1 hour and micron after 7 days at 1400 C. A

further extension of the treatment time at this temperature will only cause a very slow increase in the thickness of the SiO -layer.

The skin of SiO -glaze will be determinative of the oxidation resistance, and thus of the life-time at elevated temperatures, of the conductive surface layer. Its thickness growth due to oxidation will make inroads upon the current conducting silicide layer, thus causing a considerable increase of its electrical resistance if the silicide layer is very thin. Therefore, as a matter of fact, surface-layer resistors are more susceptible to ageing than are solid resistance elements. In order to counter-act such ageing, it is suitable to preoxidize the element at a sufficiently high temperature to cause the formation of a rather substantial protective coating or skin, preferably of a thickness of the order of 10 micron. Subsequently, the element can be used for a long time at a lower temperature without any noticeable ageing.

A certain inconveniency in flame-sprayed elements according to the invention, where the conductive layer contains molybdenum disilicide, resides in the fact that, in spite of its high oxidation resistance at very high temperatures this molybdenum disilicide is subject to rapid oxidation at moderate temperatures, of the order of 300 to 700 C. Such desintegration is sometimes termed the molybdenum silicide plague. It has been found possible partly to inhibit such plague by pre-oxidizing the layer in the above-described manner at a sufiiciently high temperature to cause the formation of a gas-tight coating or skin of silicon dioxide. For use over extended periods, however, such protective skin or coating has sometimes been found to be insufiicient, and in such cases it may be suitable to employ for the sections situated at either sides of the incandescent zone, where the temperature may be expected to range between 300 and 700 C., a material other than such containing molybdenum disilicide. One such material is TiSi for example, possibly in combination with CrSi Also the lower silicides of the stoichiometric type M Si where M stands for one of the abovementioned transition metals, are satisfactorily adapted for this purpose.

The above-mentioned molybdenum silicide plague may be inhibited or at least retarded by applying a protective coating of a suitable ceramic, such as enamel. It is possible also to use for the terminal sections of the elements an inert metal, such as silver or gold, which is incapable of reacting chemically with the silicides used.

To be used in the flame spraying of silicide layers in accordance with the invention are suitably powdered initial materials, possibly together with a strongly positive metal, such as aluminium or magnesium, for example, the powder being then used either as such or embedded in a plastic material, such as polyethylene, so as to form a flexible wire or string in a well-known manner. Aluminium powder added to the powder mixture will be almost completely oxidized into A1 0 which is blown away as losses. The aluminium content of the layer will be low, less than 1%.

For certain applications spraying by means of a spray gun fed with solid, straight, powder-metallurgically produced rods containing silicides may offer advantages, since the yield will be higher than in the case of powder spraying where the material is fed to the spray gun in a powdered state.

The flame-sprayed resistance elements according to this invention may find a lot of practical applications. In electric heating apparatus of all kinds the resistance wires, to advantage, may be replaced by flame-sprayed surfacelayer elements which are cheaper and less sensitive to mechanical strains and stresses. Straight refractory ceramic tubes provided with helical resistance coils may replace elements based on silicon carbide in industrial furnaces designed for element temperatures up to approximately 1400 C. Thin-walled ceramic melting crucibles having a resistance layer applied directly on their external wall surface can be used for relatively short periods at temperatures up to 1550" C. Among electric heating apparatus designed for more moderate temperatures, which may, to advantage, be provided with flame-sprayed resistance layers, flat irons, toasters and other household appliances, as well as electric radiation heaters, may be mentioned. In immersion heaters, cooking and frying vessels and other electric heating appliances designed for lower temperatures, in which both the ceramic and the current conducting layer have been provided with a protective or shielding skin or coating, for instance of enamel, it is possible to take advantage of the high permissive surface load of the sprayed conductive layer to attain a more rapid heat transfer. Said shielding coating or skin, of course, can react with the quartz glass skin formed by oxidation at elevated temperatures. Where lower-temperature applications are concerned, in addition, silicon resins and the like may be used as insulation.

Panels made of heat resistant glass and provided with a conductive layer of aluminium have been used previously for electrical room-heating purposes. In accordance with this invention, said conductive layer may, to advantage, consist of a silicide whereby the unfavourable aging characteristics inherent in aluminium layers will be avoided. It is made possible according to modern methods to take advantage of a cheaper night rate of electric power consumption by causing well heat-insulated heat absorbing bodies to absorb heat electrically by night, and then extract such heat in daytime by circulating air past the heated body. Suitable to be used for such applications are supporting sheet-metal plates having enameled thereon a base or carrier layer and superimposed thereon a flamesprayed conductive silicide layer having on it a superficial shielding coating rich in silicon dioxide, such assemincense 5 bly being adapted to withstand an operating temperature of 900 C. or higher.

Furthermore, the resistance elements according to this invention are adapted for use as pure electric resistors, i.e. without utilizing any heat generated therein in use. In this case, the favourable anticorrosion characteristics of the silicides used will inhibit aging of the resistance material. Resistors of resistance values upto 100,000 ohms and with any desired coefficient of thermal resistivity can readily be produced.

A few practical embodiments of the invention will now be described in conjunction with the accompanying drawings, in which:

Fig. 1 shows one embodiment in side-view, while Fig. 2 is a cross-section taken along the line II-II in Fig. l, and

Fig. 3 is an end-view of the same.

Fig. 4 illustrates a second embodiment in bottom plan view, while Fig. 5 is a fragmentary and enlarged section taken along the line V-V in Fig. 4, and

Fig. 6 is a complete diametrical section taken along the line VIVI in Fig. 4.

Fig. 7 is a perspective view of a third embodiment, and

Fig. 8 is a section through a fourth embodiment intended for water heating purposes.

Referring first to Figs. 1 to 3, in the embodiment illustrated therein a refractory ceramic tube 3 being 200 mm. in length and 12 mm. in diameter, is provided with a helically coiled layer 1 being 2 mm. in width, 0.06 mm. thick and 760 mm. in length. Its thickness is built up by a 0.03 mm. conductive silicide layer and a 0.03 mm. shielding or protective coating of quartz glass. The adjacent turns of the conductive layer 1 should be spaced as closely as possible longitudinally of the tube, preferably by l to 2 mm. Otherwise, since the thermal conductivity of the electrically conductive layer 1 is substantially higher than that of the ceramic base or carrier member 3, troublesome temperature differences will arise between the flame sprayed portions 1 and the exposed intervening surface portions of the base member 3, thus giving rise to thermal stressing. Said conductive layer contained 55% MSi 40% Mo Si and 5% oxides. At 1400 C. and 240 volts a current of 5 A. was measured, corresponding to 1200 w. and 48 ohms. The surface load on the element coil corresponds to 80 w./cm. An element of this type has the advantage of a high resistance and, consequently, a low amperage, which enables the same to be connected directly to the power supply. The mechanical strength of elements of this type will be determined entirely by the ceramic base material of the tube 3 and thus can be made as high as desired. The consumption of costly silicide material is extremely low; thus, for example, the element just described contains 0.4 g. molybdenum silicide. The element further has an extension or terminal layer 2 which has a thickness of 0.04 mm. and consists of equal parts of TiSi and CrSi The rod thus described may, to advantage, replace prior-art rods of silicon carbide. As regards the element thus described it is to be noted that particularly the resistance of the incandescent zone layer thereof displays a surprising characteristic. For freshly sprayed material the resistance at room temperature is 7 ohm/mmF/rn, the same increasing with increasing temperature up to 13 ohms at 800 C. At 900 C. a structural transformation takes place, the resistance value then rapidly decreasing irreversibly. After this transformation, the resistance value shows a thermal responsiveness approximately the same as that of pure MoSi being 0.8 ohm at room temperature and 3.2 ohms at 1500 C. Since the practical consequence of this sudden resistance decrease at 900 C. will be that, the voltage applied being unchanged, the temperature of the element will rapidly increase, it will be suitable to subject flame sprayed resistance layers of this type to heat treatment at 900 C. before they are taken into use.

Figs. 4 to 6 illustrate an electric rapid-cooking plate 4 made of metal, the bottom surface thereof being provided with an electrically insulating layer 5 of a highly heat resistant enamel and superimposed on this enamel a spirally extending conductive layer 6 consisting of only MoSi and forming a resistance coil. The radial clearance between the turns of the coil should not exceed 1 to 2 mm. for the same reason as stated in the foregoing example in conjunction with Fig. 1. The resistance coil has a layer thickness of 0.05 mm., a length of 2.4 m. and a width of 2 mm. This thickness includes a 0.04 mm. conductive silicide layer and a 0.01 mm. protective quartz glass coating. At a maximum operating temperature of 800 C. the power output of the plate is 2.15 kw., at 220 v., corresponding to 45 w./cm. The terminal sections 7, being of larger width, viz., 8 mm., consist of a mixture of VSi and 20% TaSi Packed in beneath the heat radiant layer is a heat insulating ceramic filler material 9. It would be of advantageat least as regards the incandescent zone proper-for the conductive coil 6 to be attached in part only to the enamelled base member 4, the remaining portions thereof being free from the base member 4 so as to rest loosely thereon to accommodate differential thermal expansion and contraction.

Referring now to Fig. 7, a plate 12 consisting of heat resistant glass and having the dimensions 250 x 100 x 3 mm. is provided with a resistance element in the form of a meander or serpentine strip 10 being 0.03 mm. in thickness, 3.0 mm. in width, and 800 mm. in overall length. The thickness includes a 0.02 mm. thick conductive silicide layer as well as a 0.01 mm. thick protective coating of an easily fusib'e glaze. layer is MoSi and 10% SiO At an element temperature of 70 C. and 56 v. a current of 8 A. was measured, which corresponds to 7 ohms and 440 w. The silicide layer is protected additionally by a thin silicone coating not illustrated in the drawing.

Referring now to Fig. 8, the resistance element illustrated therein consists in a great number of ceramic bodies 13, being for instance spherical or ellipsoidal in shape and preferably made of porcelain, these bodies forming base members for flame sprayed electrically conductive layers composed by 40% MoSi 50% Mo Si and 10% SiC. Subsequent to the flame spraying operation, these balls are subjected to tumbling in order to rub away any oxide film present thereon. The halls are placed in a container 14 having upper and lower electrodes, 15 and 16, respectively. This will provide for a composite element having a high resistance value. The balls are about 10 mm. in diameter, and it is possible by varying the layer thickness, the number of balls and the overall diameter and height of the complete element, to vary the resistance value in accordance with any requirement. The extremely high chemical corrosion-resistance of the molybdenum silicide layer ensures that the-re will be no danger of chemical attacking thereof by the water or impurities. The very high power output per unit area and the intimate contact between the surface layer and the water enable the heating up to occur very rapidly.

In order to increase their heat conductivity, the balls 13 may be made of metallic material, such as copper, thus forming metallic supporting bodies having thereon an enamel coating forming a base or carrier surface having flame sprayed thereon the electrically conductive layer.

The embodiment illustrated in Fig. 8 could, of course, be employed also for heating gaseous fluids.

According to a fifth embodiment (not illustrated in the drawings, a high-ohmic resistor is produced from a sintered-corundum tube having an outer diameter of 12 mm. and a length of mm. and forming the base member. Sprayed onto this base member is 12 micron thick layer constituted by a conductive layer of pure MoSi 10 micron in thickness, and a protective coating The composition of this conductive of quartz glass 2 micron in thickness, and forming a helix 10 m. in length and 0.15 mm. in width. The resistance value of this helix is approximately 35,000 ohms at 20 C. Such resistor can be used up to 1000 C. without suifering from any ageing, and will withstand satisfactorily even the greatest temperature changes owing to the fact that the coefficient of thermal expansion of MoSi equals that of corundum (Al O In a further embodiment associated with that of Figs. 1 to 3, the tube is made of a high grade ceramic and has a diameter of 8 mm. and a length of 120 mm. The external surface of this tube is sandblasted and coated with a thin glaze of a baked, easily fusible clay mineral. The glaze is securely attached by being heat treated during 15 minutes at 1400 C. and then forms the base surface. By flame spraying powdered MoSi together with 3% aluminium powder, the conductive layer will result, and this layer is coated in its incandescent zone again with baked, easily fusible clay which is securely attached by burning for 15 minutes at 1400 C. so as to form a protective coating or skin rich in silicon dioxide. The first glazing of the porcelain tube has for its purpose to obturate the pores in the tube wall, since otherwise the glaze coating applied after the flame spraying operation would be absorbed by the porcelain whereby the latter would loose is favourable influence on the useful life of the element.

The silicide grains of the powder mixture used for flame spraying of the conductive layer have preferably a size not exceeding 50 micron and advantageously at most 15 micron, and the quantity of aluminium powder is preferably from 1 to 10 percent by weight but with best advantage from 2 to 6 percent at a grain size of from 10 to 150 micron, preferably from 40 to 70 micron.

It is generally remarked that the flame-sprayed conductive layer will be somewhat porous the porosity varying from to 30% by volume. It is preferable to infiltrate in said pores a ceramic material, such as glass, enamel or the like, of a type having a melting point of 1100 to 1700 C., or most advantageously, from 1300 to l600 C. As a preferred example of composition of said ceramic material may be cited:

0 to by weight Na O or K 0, 0 to 60% by weight A1 0 and 40 to 100% by weight SiO What is claimed is:

1. An electric resistance comprising an electrically insulating heat resistant and oxidation proof support and a thin electrically conducting porous layer deposited by means of flame-spraying on said support, the conducting layer consisting essentially of at least one silicide of the E5 transition elements of groups from IVVI in the periodic system, characterized in that the conducting layer is coated at least in part with a protective film of an insulating substance rich in silica.

2. An electric resistance as described in claim 1 wherein said substance fills'the pores of the conducting layer at least in part.

3. An electric resistance as described in claim 2 wherein the thickness of the conducting layer is 5 to 500 microns and the thickness of the protecting film 2-20 microns.

4. An electric resistance as described in claim 3 wherein the thickness of the conducting layer is 10 to 200 microns and the thickness of the protecting film is 5-15 microns.

5. An electric resistance as described in claim 1, wherein the conducting layer consists substantially of molybdenum disilicide.

6. An electric resistance as described in claim 2, wherein the conducting layer consists substantially of molybdenum disilicide.

7. An electric resistance as described in claim 1, wherein the protecting film consists substantially of quartz glass which has been provided by heat-treating the conducting layer in an oxidizing atmosphere.

8. An electric resistance as described in claim 2, wherein the protective film consists substantially of quartz glass which has been provided by heat-treating the conducting layer in an oxidizing atmosphere.

9. An electric resistance as described in claim 1, wherein the protecting film is obtained by heating an easily fusible clay mineral while in contact with the conducting layer.

10. An electric resistance as described in claim 2, wherein the protecting film is obtained by heating an easily fusible clay mineral while in contact with the conducting layer.

11. An electric resistance as described in claim 1, wherein the electrically insulating support is a material of the group consisting of a thin ceramic layer such as enamel or glaze, and an oxide film such as A1 0 12. An electric resistance as described in claim 2, wherein the electrically insulating support is a material of the group consisting of a thin ceramic layer such as enamel or glaze, and an oxide film such as A1 0 References Cited in the file of this patent UNITED STATES PATENTS 869,013 McOuat Oct. 22, 1907 1,473,107 Kohn Nov. 6, 1923 2,546,142 Watson Mar. 27, 1951

Claims

1. AN ELECTRIC RESISTANCE COMPRISING AN ELECTRICALLY INSULATING HEAT RESISTANT AND OXIDATION PROOF SUPPORT AND A THIN ELECTRICALLY CONDUCTING POROUS LAYER DEPOSITED BY MEANS OF FLAME-SPRAYING ON SAID SUPPORT, THE CONDUCTING LAYER CONSISTING ESSENTIALLY OF AT LEAST ONE SILICIDE OF THE TRANSITION ELEMENTS OF GROUPS FROM IV-VI IN THE PERIODIC SYSTEM, CHARACTERIZED IN THAT THE CONDUCTING LAYER