Electrical Heating Element, Related Composites, and Composition and Method for Producing Such Products Using Dieless Micropyretic Synthesis

1. Field of the Invention

This invention uses a novel technique to make electrical heating elements which may be used up to 900.degree. C. This technique also provides new methods for manufacturing ceramic composites, which may be used as both electrical heating elements and oxidation resistant materials.

2. Description of the Prior Art

Generally, heating elements are made from metals like Mo and W, and alloys like Fe-Cr-Al and Ni-Cr, SiC and metal silicides. The alloy heating element is produced by melting in the electric induction or arc furnaces (sometimes alloys have to be melted several times to achieve homogeneity) and then is pressed or extruded. Silicon carbide and molybdenum disilicide heating elements are made by powder metallurgy and have to be sintered at high temperatures and extended times in H.sub.2 and CO atmospheres. In the powder metallurgy industry, sintering is often the most costly step of the total manufacturing process, especially when high temperature sintering is involved. Melting and casting are also costly. In addition, because the manufacture of conventional materials is limited by processing, heating elements can be produced only within a predetermined resistivity range.

It is evident that the prior art methods exhibit serious disadvantages, and it is a primary object of the present invention to obviate these by providing a micropyretic synthesis method and composition for production of electrical heating elements and oxidation resistant materials.

Micropyretic synthesis (also called combustion synthesis) is a technique which uses the heat release from a reaction for synthesis of a product. This exothermic reaction can become self-sustaining and will propagate throughout the reactant mixture in the form of a combustion wave. When the combustion wave advances, the reactants are converted to the products. There are three main parameters important to the micropyretic synthesis process: (1) combustion temperature, (2) ignition temperature at which a sample will become self-propagating, and (3) the velocity of wave propagation. This invention utilizes micropyretic synthesis to make the desired product.

In comparison to prior art methods, micropyretic synthesis provides: (i) energy-saving because of the use of self-sustaining reactions, (ii) simplicity, (iii) reliability of the process, (iv) relative purity of the products, (v) low cost of original powders, and (vi) very low processing costs when compared to conventional methods. It is advantageous if a cheap powder and self-sustaining reactions can be used during the preparation of electric heating elements.

According to the invention there is provided a composition comprising a filler material, at least one reactive system, and optionally a plasticizer (or extrusion agent). Fillers may be SiC, MoSi.sub.2, Cr.sub.2 C.sub.3, WC, Al.sub.2 O.sub.3, SiO.sub.2, SnO.sub.2, C, Be, La, Co, Ni, rare earths, ZnO, Y.sub.2 O.sub.3, ZrO.sub.2, Cu, Ni-Co based superalloys, Sb.sub.2 O.sub.3, CuO, Fe.sub.2 O.sub.3, GeO, Fe.sub.3 O.sub.4, V.sub.2 O.sub.5, FeO, Mo, Nb, Cr, Al, Si, Y, Fe, Si.sub.3 N.sub.4, B, or alloys and mixtures thereof. Fillers may also be naturally occurring minerals such as sand or clay. The content of fillers may range up to 95% (of the total weight). The reactive system comprises at least two combustible materials which will react exothermically with one another by micropyretic synthesis and are present in such proportion to one another that combustion will occur when ignited. The reactive systems and ceramic phases formed thereby may be any of those in Table I. Various combinations of these combustible materials may be used. Non-stoichiometric weights may also be chosen as long as combustion can be made to occur. The content of the reactive system can range from about 5% to 95% by weight of the total composition. Plasticizers or extrusion agents such as polyvinyl butyral, polyurethane, colloidal silica, 2%-5% aqueous chemical cellulose solution, phosphoric acid, bentonite, fused silica and its activator, may form 0-90% of the total weight of the composition.

Preferably the composition of the invention contains, in weight percent based on the total weight, from about 20% to about 85% filler material, about 15% to about 85% reactive system, and 0% to about 25% plasticizer.

Any one or more of the preferred ranges set forth above can be used with any one or more of the broad ranges for the remaining components indicated above.

The proportion of combustible material to filler in the electrical heating element is critical, since the combustible content will change the composition of final products and combustion temperature. For instance, the combustion temperature of the MoO.sub.3 +2Al+2Si system is as high as 3300.degree. K. It is known that the melting point of its reaction product (MoSi.sub.2) is 2293.degree. K. and that the final products may, therefore, melt. Enough filler or diluent is thus necessary in order to keep the original shape of the product and a crack free surface. However, too low a combustible material content will lead to low combustion temperatures so that the final product will not be well-bonded and will display weak room temperature strength. Fillers have three important effects on the products: (1) the fillers act as diluent of the combustion process; (2) they form a part of the final composition and act as a reinforcement to the combustion products, (for instance, MoSi.sub.2 shows very low room temperature fracture toughness and low yield strength levels at elevated temperatures. It must therefore be reinforced by some kind of filler such as ZrO.sub.2, Al.sub.2 O.sub.3, SiO.sub.2, SiC, etc.); and (3) fillers may also act as sintering aids, (e.g., Y.sub.2 O.sub.3 addition will enhance the sintering during combustion). Filler contents have significant effects on electrical resistance, room temperature fracture toughness, and oxidation resistance at elevated operating temperatures of final products. In the exemplary embodiments disclosed hereinafter filler and combustion sources were chosen according to the following criteria: (1) adequate mechanical strength, (2) high oxidation resistance at designed working temperature, (3) thermal stability between filler and combustion source at working temperature, (4) resistance to thermal shock when subject to heating and cooling, (5) slow grain growth, (6) combustion temperature, and (7) electrical resistance.

The present invention further provides a method of preparing composite articles, in particular electrical heating elements, comprising the steps of:

(a) blending a mixture comprising up to 95% by weight of a particulate filler material, between about 5% and 95% by weight of at least one reactive system, wherein said reactive system comprises at least two particulate combustible materials which will react exothermically with one another by micropyretic synthesis and are present in such proportion to one another that combustion will occur when ignited, up to 90% of a plasticizer, and a sufficient amount of solvent in order to form a slurry;

(b) fashioning said slurry into a final desired article shape; and

(c) combusting said shape by ignition at a temperature between about 150.degree. C. and 1250.degree. C.

An electrical heating element or ceramic composite article in accordance with the invention, which may be used at temperatures upto 1900.degree. C., comprises a ceramic composite formed by micropyretic synthesis of a composition containing: (a) up to 95% by weight of a filler material; and (b) between about 5% and about 95% by weight of at least one reactive system, wherein said reactive system comprises at least two combustible materials which will react exothermically with one another by micropyretic synthesis and are present in such proportion to one another that combustion will occur when ignited.

Ignition may be effected by a heat source such as a flame, laser, electron beam or welding electrode, or by passing the shaped article through an induction coil or a furnace heated to ignition temperature.

Since the method of the invention permits preparation of composites with different volume fractions of constituent phases, it is readily possible for the first time to control the electrical resistance, and the change in electrical resistance with temperature changes. For example, silicon carbide has a negative slope of resistance vs. temperature, whereas MoSi.sub.2 has a positive slope. Judicious combination of these filler materials may thus provide a very slight change in resistance vs. temperature. The room temperature resistivity of heating elements of the invention can be tailored to range from 30.mu. ohm cm to 20 ohm cm.

As will be evident from the compositions set forth hereinafter, the best known mode of carrying out the invention includes the use of the following compositions, all percentages being by weight.

A - A filler material comprising at least one of from about 20% to about 80% MoSi.sub.2, up to about 30% chromium, up to about 15% iron, up to about 6% molybdenum, up to about 2% titanium, up to about 1.2% niobium, up to about 0.7% yttrium, up to about 2.5% aluminum, up to about 10% silver, up to about 42% silicon carbide, up to about 12% Y.sub.2 O.sub.3, up to about 2.5% Al.sub.2 O.sub.3, up to about 8% SiO.sub.2, and up to about 2.5% MgO; a reactive system comprising from about 12% to about 35% nickel, and about 3% to about 13% aluminum; and a plasticizer which when present comprises about 8% to about 12% of a 2.5% aqueous chemical cellulose solution.

B - A filler material comprising at least one of from 0% to about 75% MoSi.sub.2, about 8% to about 10% SiO.sub.2, up to about 2% silicon, about 0.8% to about 40% silicon carbide, up to about 0.5% boron, up to about 8% Y.sub.2 O.sub.3, and up to about 2% Si.sub.3 N.sub.4 ; a reactive system comprising from about 7% to about 28% Cr.sub.2 O.sub.3, about 2.5% to about 10% aluminum, and about 0.7% to about 3% carbon; and a plasticizer comprising at least one of from about 4% to about 5% polyvinyl butyral, and about 8% to about 12% of a 2.5% aqueous chemical cellulose solution.

C - A filler material comprising at least one of from about 1% to about 50% silicon carbide, up to about 71% MoSi.sub.2, up to about 10% SiO.sub.2, up to about 10% Y.sub.2 O.sub.3, up to about 10% Si.sub.3 N.sub.4, up to about 0.5% BN, up to about 1% chromium, up to about 1% boron, up to about 0.5% aluminum, up to about 10% Al.sub.2 O.sub.3, up to about 0.5% silicon, and up to about 7% Zr.sub.2 ; a reactive system comprising from about 7% to about 30% MoO.sub.3, about 2.5% to about 11% aluminum, and about 2.5% to about 38% silicon and up to about 11% carbon; and a plasticizer comprising at least one of from about 10% to about 15% polyvinyl butyral, about 8% to about 15% of a 2.5% aqueous chemical cellulose solution, about 8% to about 10% fused silica and its activator, and about 4% to about 10% bentonite.

D - A filler material comprising at least one of from about 35% to about 40% silicon carbide, about 7% to about 8% Y.sub.2 O.sub.3, about 1.7% to about 2% Al.sub.2 O.sub.3, about 7% to about 8% SiO.sub.2, and about 1.7% to about 2% MgO; a reactive system comprising from about 25% to about 30% titanium, and about 9% to about 11% silicon; and a plasticizer comprising from about 8% to about 12% of a 2.5% aqueous chemical cellulose solution.

Compositions embodying the invention are as follows, it being understood that these are illustrative and not limiting:

Processing in accordance with the invention may include the following procedures:

Step 1. Powders and polyvinyl butyral were weighed according to desired compositions.

Step 2. The weighed powders and polyvinyl butyral were mixed in acetone by ball milling for 2-10 hours with ZrO.sub.2 milling media.

Step 3. The thin slurry was then transferred to a large glass container, dried in a 70.degree. oven, and solvent was allowed to evaporate.

Step 4. Dried powder was ground in a mortar for one hour and acetone was added to this powder to form a thick slurry.

Step 5. This thick slurry was ground for one hour to form a plastic mass.

Step 6. This plastic mass was forced through a die at high pressure (5-300 MPa) to produce wires.

Step 7. The green wire was formed into various shapes, e.g., coil, U-shape or straight.

Step 8. The wires from step 7 were dried in air for 1 hour, (these wires were no longer flexible at this time), and then dried in a 70.degree. oven.

Step 9. The wires were combusted in a furnace in air or argon atmosphere in the temperature range of 150.degree. C.-1250.degree. C.

Step 1. Powders and bentonite were weighed according to desired compositions.

Step 2. The weighed powders and bentonite were mixed in water by ball milling for 2-10 hours with ZrO.sub.2 milling media.

Step 3. This thin slurry was moved to a large glass container, dried in a 100.degree. C. oven, and the water was allowed to evaporate.

Step 4. Dried powder was ground in a mortar for one hour and water was added to this powder to form a thick slurry.

Step 5. This thick slurry was ground for one hour to form a plastic mass.

Step 6. This plastic mass was forced through a die at high pressure to produce wires.

Step 7. The green wire was formed into various shapes, e.g., coil, U-shape or straight.

Step 8. The wires from step 7 were dried in air for 2-4 hours (these wires were no longer flexible at this time), and then dried at 110.degree. in the oven, for 2-5 hours.

Step 9. The wires were combusted in a furnace with air or argon atmosphere in the temperature range of 150.degree.-1250.degree. C.

Step 1. Powders and polyurethane and thinner were weighed according to desired compositions.

Step 2. The weighed powders, polyurethane and thinner were mixed for one half hour.

Step 3. This thin slurry was coated on a porous polyurethane polymer. Coated products were dried in air for about 10 minutes and then coated again; coating thickness was controlled by coating time and slurry viscosity.

Step 4. Coated products were dried in air for 1-2 hours and then at a temperature of 300.degree. C. in an oven for 0.5-2 hours.

Step 5. Coated products were combusted in the range of 150.degree. C.-1200.degree. C. in a furnace, or coated products from step 4 were combusted by a torch.

Step 1. Powders and colloidal silica or phosphoric acid were weighed according to desired compositions.

Step 2. The weighed powders and silica were mixed for half an hour.

Step 3. This thin slurry was coated on porous polymer or oxidized TiB.sub.2 porous base. Coated products were dried in air for about 10 minutes and then coated again; coating thickness was controlled by coating time and slurry viscosity.

Step 4. Coated products were dried in air for 1-2 hours.

Step 5. Coated products were combusted at 150.degree. C.-1200.degree. C. in a furnace, or coated products from step 4 were combusted with a torch.

Step 1. Powders and polyvinyl butyral were weighed according to desired compositions.

Step 2. The weighed powders and polyvinyl butyral were mixed in acetone by ball milling for 2-10 hours with ZrO.sub.2 milling media.

Step 3. This thin slurry was moved to a large glass container, and dried in a 70.degree. C. oven. Acetone was allowed to evaporate.

Step 4. Dried powder was ground in a mortar for one hour.

Step 5. This powder was pressed in a die to form various kinds of samples, for instance, sandwich samples.

Step 6. The products were combusted in a furnace with air or argon atmosphere in the temperature range of 150.degree.-1250.degree. C.

Step 1. Powders and polyvinyl butyral were weighed according to desired compositions.

Step 2. The weighed powders and polyvinyl butyral were mixed in acetone by ball milling for 2-10 hours with a ZrO.sub.2 milling media.

Step 3. This thin slurry was moved to a large glass container, and dried in a 70.degree. C. oven. The solvent was allowed to evaporate.

Step 4. Dried powder was ground in a mortar for one hour, and acetone was added to this powder to form a thin slurry.

Step 5. This thin slurry was mixed mechanically for another hour to form a slip.

Step 6. This slurry was cast in a die to form products with various shapes.

Step 7. The sample from step 6 was dried in air for about 10 hours, and then heated at 250.degree. C. in an oven.

Step 8. This sample was combusted in a furnace with air or argon atmosphere in the temperature range of 150.degree.-1250.degree. C.

Step 1. Powders were weighed according to desired compositions.

Step 2. The weighed powders were mixed in acetone by ball milling for 2-10 hours with a ZrO.sub.2 milling media.

Step 3. Mixed powders were ground in a mortar for one hour.

Step 4. This powder was pressed in a die to form various kinds of samples, for instance, sandwich sample, dog bone shaped samples, etc.

Step 5. The products were combusted in a furnace in air or argon atmosphere in the temperature range of 150.degree.-1250.degree. C.

Step 1. "750 Cotronics" fused silica was ball milled for two days and then sized by -325 mesh sieve.

Step 2. Powders and sieved "750 Cotronics" fused silica were weighed according to desired compositions.

Step 3. The weighed powders and fused silica were mixed in water by ball milling for 2-10 hours with ZrO.sub.2 milling media.

Step 4. The thin slurry was moved to a large glass container, dried in a 100.degree. C. oven, and the water was allowed to evaporate.

Step 5. The dried powder was ground in a mortar for one hour, and liquid silica activator was added to the powder to form a thick slurry.

Step 6. The thick slurry was ground for 30 minutes to form a plastic mass.

Step 7. The plastic mass was forced through a die at high pressure to produce wires.

Step 8. The green wire was formed into various shapes, e.g., coil, U-shape or straight.

Step 9. The wires from step 8 were dried in air for 2-4 hours (these wires were no longer flexible at this time), and dried in an oven at 110.degree. C. for 2-5 hours.

Step 10. The wires were combusted in a furnace with air or argon atmosphere in the temperature range of 750.degree.-1250.degree. C.

Step 1. Powders were weighed according to desired compositions.

Step 2. The weighed powders were mixed in water by ball milling for 2-10 hours with ZrO.sub.2 milling media.

Step 3. The thin slurry was moved to a large glass container, dried in a 110.degree. C. oven, and water was allowed to evaporate.

Step 4. The dried powder was ground in a mortar for one hour and 2.5 wt % chemical cellulose solution in water was added to this powder to form a thick slurry.

Step 5. This thick slurry was ground for another hour to form a plastic mass.

Step 6. This plastic mass was forced through a die at high pressure (5-300 MPa) to produce wires.

Step 7. The green wire was formed into various shapes, e.g., coil, U-shape or straight.

Step 8. The wires from step 7 were dried in air for 1 hour, (these wires were no longer flexible at this time), and then dried at 100.degree. C. in an oven for 2 hours, and then the oven temperature was increased to 400.degree. C. to burn the plasticizer out.

Step 9. The wires were combusted in a furnace in air or argon atmosphere in the temperature range of 700.degree.-1250.degree. C.

Step 1. Powders were weighed according to desired compositions.

Step 2. The weighted powders were mixed in water by ball milling for 2-10 hours with ZrO.sub.2 milling media.

Step 3. This thin slurry was moved to a large glass container, dried in a 100.degree. oven, and the water was allowed to evaporate.

Step 4. Dried powder was ground in a mortar for one hour and 2.5 weight percent aqueous chemical cellulose solution was added to this powder to form a slurry.

Step 5. This slurry was ground for a half hour to form a homogenous mass.

Step 6. This mass was slip cast by molding to form different shapes, e.g., cast plates, or by pressing the mass to form plates, or by working the mass with clay-sculpturing tools to obtain a shape.

Step 7. The green articles from step 6 were dried in air for 2-19 hours (these articles were no longer flexible at this time), and dried at 110.degree. C. in an oven for 2-5 hours.

Step 8. The articles were combusted in a furnace with air or argon atmosphere in the temperature range of 750.degree.-1250.degree. C.

Final products were prepared in accordance with the following non-limiting examples:

Composition U and Process II were used to make heating elements. The final products (1-10 mm wires) showed very high strength at room temperature and could be used as high temperature heating elements. Samples were run at 1600.degree. C. for 40 hours without any degradation.

Composition W and Process I were used to make an electrical heating element.

After combustion, the products showed excellent room temperature strength. According to this invention, this high room temperature strength comes not only from filler reaction joining among SiO.sub.2, MoSi.sub.2, SiC and the reaction product Al.sub.2 O.sub.3, but also from reaction bonding between MoSi.sub.2 reaction products and these fillers. It was found that an increase of the combustible (MoO.sub.3 +2Al+2Si) content up to a value of 45% by weight of the total composition substantially enhanced the room temperature strength. But if this combustible content were more than 50% by weight, the combustion reaction would become too strong, so that the final products were broken and cracks could form on the surface of the products. The adiabatic temperature of MoO.sub.3 +2Al+2Si reaction is as high as 3300 K, which is higher than the melting point of MoSi.sub.2. In this reaction, therefore, at least 50% filler and plasticizer were necessary. According to this embodiment, the MoO.sub.3 +2Al+2Si reaction is extremely useful in making high temperature heating elements, and oxidation resistance composites. In addition, the fillers such as Y.sub.2 O.sub.3 and Al.sub.2 O.sub.3, enhance sintering during combustion. It is essential, in order to obtain the best products, that different particle sizes be used in the sample. The products made from the processing were in the form of wires 1 mm-10 mm in diameter or flat plate 5 mm thick.

These products could be used at high temperatures. Testing was carried out between 1200.degree. and 1600.degree. C. The sample surface was noted to be coated with a protective layer of SiO.sub.2 due to the reaction between MoSi.sub.2 and oxygen. This thin quartz layer also sealed any of the pores on the surface. On account of the formation of this silica layer the product could be used at high temperatures. The wires were tested in the form of heating elements by passing 5-50 amps through the wires for long times and allowing the samples to attain temperatures between 1200.degree. C. and 1600.degree. C. At 1600.degree. C. the wire ran for 100 hours without any sign of deterioration. The test was discontinued because of the terminals becoming too hot. At 1200.degree. C. the samples ran for over 1400 hours, and the test is still continuing. In this test the terminals were cooled with cooling water. The room temperature resistivity of these samples averaged 90.mu. ohm cm before the test and remained 90.mu. ohm cm after 1400 hours when the test was briefly interrupted.

Process III was used to mix 70 grams Ti and 30 grams boron powder by ball milling. 100 ml polyurethane was used as a liquid media and mixed with the Ti and B powder. This slurry was coated on porous polyurethane polymer 3-5 times, and then dried in air for 2 hours and 300.degree. C. oven for 1 hour, respectively. Samples were combusted in the furnace at 800.degree. C., and oxidized at 950.degree. C. for 3 hours. This formed an oxidized TiB.sub.2 surface.

Eighty-seven grams of Ni powder (-100 mesh) and 13 grams of Al (-325 mesh) were mixed with ball milling in accordance with Process III. After milling, the mixed powders wre mixed with 100 ml polyurethane. This thin slurry was coated for 1-3 times on the surface of an oxidized TiB.sub.2 porous base. The samples were combusted at 1000.degree. C. This porous heating element could be used as a low temperature heater in the temperature range of 300.degree. to 500.degree. C.

Composition M and Process V were used to make a sandwich sample. A sandwich sample is one which contains layers of different compositions of pressed powders or slurry. A powder mixture with 69 grams of Cr.sub.2 O.sub.3, 24 grams of Al and 7 grams of carbon were mixed as a combustible source and used as the core of the sandwich. Samples were pressed into a sandwich. After combustion, the core of the sandwich is a composite of Cr.sub.2 O.sub.3 and Al.sub.2 O.sub.3 which are porous materials and insulators. The two outside layers were the composite resulting from Composition M. This sample showed high strength for this kind of product. When used as a heating element the sample was noted to remain stable at 1300.degree. C.

Composition Y and Process I were used to make heating elements. The ZrO.sub.2 (partially stabilized) is advantageous in reinforcing MoSi.sub.2 since its coefficient of thermal expansion is close to that of MoSi.sub.2. It was found that partially stabilized ZrO.sub.2 significantly toughened MoSi.sub.2, and the final products could be used at temperatures up to 1600.degree. C.

Composition R and Process I were used to make heating elements. The properties of the final products were comparable to those of Example 1. However, the combustion temperature is lower than that of Composition W used in Example 1.

Composition Z and Process VIII were used to make heating elements. The "750 Cotronics" fused silica was ball milled for 2 days to decrease the particle size to less than 40 micrometers before mixing with the other powdered material. The fused silica and activator functioned very well as a plasticizer. The plastic mass could be extruded into shapes of various kinds. After drying in air and an oven at 110.degree. C., the samples showed good green strength. The green samples were combusted in the range of 750.degree. to 1200.degree. C. Final products exhibited excellent room temperature strength and could be used as high temperature heating elements in the range of 1000.degree. to 1700.degree. C.

Composition v and Process II were used to make heating elements. The combustible material comprised 45% by weight of the total composition. The combustion temperature was higher than that noted in compositions having 40% or less combustible material. Composition V could be ignited at relatively low temperatures, on the order of 750.degree.-950.degree. C. At such temperature levels crack-free products were obtained. The final products had very high room temperature strength and could be used as high temperature heating elements.

Composition R and Process I were used to make heating elements. However, extra Al and Si in the combustible, and Cr and B in the filler, were added to increase the density of the composition. It is believed that the B addition may decrease the melting point of the Si O.sub.2 in the mixture, so that the products may be liquid sintered during the combustion step.

Composition E and Process VIII were used to make heating elements (with omission of steps 1 and 2 since Composition E contained no plasticizer). Samples were combusted in the temperature range of 1000.degree. C. to 1150.degree. C. The final products showed reasonable room temperature strength and could be used as heating elements at temperatures of 500.degree.-900.degree. C.

Composition AA and Process VIII were used to make high temperature heating elements. Pure SiO.sub.2 powder was used as the plasticizer, with "750 Cotronics" liquid silica activator. Since impurities were reduced in the final products by use of pure SiO.sub.2, the working temperature range of the heating elements was raised.

Composition BB and Process VIII were used to make high temperature heating elements, again with pure SiO.sub.2 powder and "750 Cotronics" liquid silica activator. These were found to work very well as a plasticizer. The working temperature of the heating elements was increased in comparison to products using bentonite as a plasticizer, due to reduction of the impurity phase.

Composition CC and Process I were used to make high temperature heating elements and oxidation resistant composites. SiC was used (in place of SiO.sub.2) in this composition as part of the filler material, and it was found that the final products could be used at temperatures as high as 1700.degree. C.

Composition DD, or Composition JJ, and Process X were used to make plate-like heating elements and oxidation resistant composite articles. The final products showed improved room temperature strength and could be used as heating elements in room heaters in place of conventional alloy heating elements or ceramic heating elements. The resistivity of the element prepared from Composition DD was measured at room temperature and found to be 0.2 ohm cm.

Average particle sizes used in the above examples, obtained from commercially available sources, are set forth in Table II. No representation is made that these particle sizes are optimum, but they were found to be operable and hence constitute the best mode now known of carrying out the invention.

The materials made in accordance with this invention remain stable mechanically and remain resistant to oxidation attack at high temperatures. Consequently they may be also used not only as heating elements but also as materials where high temperature oxidation prevention is a service requirement. Such uses may be in furnaces, aero-space propulsion vehicles, in engines where high temperatures are produced such as jet engines and car engines, or for chemical and electrochemical uses.