US2745928A

US2745928A - Heater bodies and their production

Info

Publication number: US2745928A
Application number: US359343A
Authority: US
Inventors: Frank W Glaser
Original assignee: American Electro Metal Corp
Current assignee: American Electro Metal Corp
Priority date: 1952-10-06
Filing date: 1953-06-03
Publication date: 1956-05-15
Anticipated expiration: 1973-05-15

Description

May 15, 1956 w, GLASER 2,745,928

HEATER BODIES AND THEIR PRODUCTION Filed June 5, 1953 v2 [Af j/ (5% 70 5%)63: a

IN VEN TOR. E 41/. 6'4 45x5 HEATER BODIES AND THEIR PRODUCTION Frank W. Glaser, New York, N. Y., assignor to American Electro Metal Corporation, Yonkers, N. Y., a corporation of Maryland Application June 3, 1953, Serial No. 359,343 Claims priority, application Austria October 6, 1952 3 Claims. (Cl. 201 613) This invention relates to electric resistor heater bodies, and more particularly to such bodies which are formed with molybdenum disilicide and to the production of such heater bodies.

It has long been known that molybdenum disilicide MoSiz has unusually desirable properties which would make it an ideal material for electric resistor heater bodies.

However, in the past, electrical heater rod bodies for electric furnaces and the like have in the past been made substantially out of silicon carbide, such heater bodies being known commercially as Globar bodies. The known silicon carbide heater bodies have a maximum operating temperature of about 1400 C., and they are made with a resistivity of 2000 to 5000 microhm centimeters because with lower resistivity it is diflicult to supply such heater bodies with the much larger currents that would be required to bring them to the desired high operating temperatures.

Because of its relatively low electrical resistivity of about 22 microhm centimeters at room temperatures, no practical way was found in the past for utilizing molybdenum disilicide as electrical heater bodies. Cemented bodies of molybdenum disilicide are very brittle and fragile and proposals to increase the resistivity of molybdenum disilicide by additions of molybdenum aluminides, and/or high melting oxides, such as zirconium oxide, thorium oxide, which become conducting at high temperatures, and/ or other high melting oxides, such as aluminum oxide, beryllium oxide, silicon oxide, likewise failed because these additions result in a further increase in the objectionable brittleness of molybdenum disilicide.

One phase of the present invention is based on the discovery that by combining molybdenum disilicide with chromium boride CrBz, there is obtained a material of considerably smaller resistivity than molybdenum disilicide, the new material being also of a lower order of brittleness than molybdenum disilicide.

The foregoing and other objects of the invention will be best understood from the following description of exemplifications thereof, reference being made to the accompanying drawings, wherein the single figure is an elevational view with parts in section showing one form of heater body exemplifying the invention.

Bodies of pure molybdenum disilicide MoSiz which melts at about 1880 C. can be readily molded from MoSiz powders by powder metallurgy and ceramic techniques. Because of their high stability and resistance to corrosion in oxidizing atmospheres at 1700 to l800 C., many proposals have been made in the past for using cemented MOSiz material in electric resistance heaters for operation at heating temperatures of about 1700 C. However, because MOSiz has a relatively low electrical resistivity of only 22 microhm centimeters at room temperature, heater bodies of this material would have to be made with a very thin cross-section, such as thin tubing or ribbon.

In addition, such cemented MoSiz material is very brittle and accordingly, thin heater bodies of such material are i nited States Patent O also impractical because of their excessive critical brittleness.

One phase of the invention electric heater bodies of similar physical characteristics as MoSiz, but about 10 times higher resistivity will be obtained by combining molybdenum disilicide with chromium boride CrBz in a critical range of proportions and that such combined new material is of a lower order of brittleness than any other combination of molybdenum disilicide with other additional ingredients that might reduce the electrical conductivity thereof, including additions of the oxides, nitrides, silicides or other borides of the transition metals of the fourth, fifth and sixth groups of the periodic table.

An investigation has shown that MoSiz and chromium boride are mutually soluble over the range containing 5% to about 50% CrBz and that by combining MOSiz with 5% to 50% CrBz, there is obtained a heater resistance material of a lower order of resistivity without increasing the brittleness of the material compared to MoSiz. (Throughout the specification and claims, all proportions are given by weight, unless otherwise stated.) Superior results are obtained by combining 5% to 50% CrBz with MoSiz. Within this critical range of proportions the stability of either of the components MoSiz and CrBz is not affected by any side reaction and the combined material may be readily produced in a well controlled manner.

As distinguished therefrom, if molybdenum disilicide is combined with other metal borides or with the oxides, nitrides and/ or silicides of the metals of the fourth, fifth and sixth groups of the periodic table, the resulting materials are of such high brittleness as to render them utterly impractical for use in electrical heater bodies.

Investigations have shown that electrical resistivity of MOSiz at room temperature, increases from 22 microhm centimeters to about 200 microhm centimeters, or by a factor of 10, as the content of CrBz is increased from 0 to 5 and that throughout this range, the MoSiz and CrBz form a series of solid solutions which exhibit metallic characteristics.

If the content of CrBz is increased above 50%, the resistivity of the resulting material decreases. On the other hand, if the molybdenum disilicide is combined with other elements for decreasing its resistivity, side reaction takes place which lead to the decomposition of the molybdenum disilicide compound. By way of example, if aluminum oxide A1203 is added to MOSiz and heated with it to elevated temperatures, side reaction takes place leading to the decomposition of the molybdenum disilicide into MoAlSi-O type compounds wherein the Si-O phase is the most stable. The strength of such other body aggregate decreases from the original strength of pure molybdenum disilicide. Thus, whereas, pure MoSiz has a transverse rupture strength of 60,000 p. s. i. (pounds per square inch) when it is combined with aluminum oxide, its transverse rupture strength decreases to about 13,000 to 15,000 p. s. i. Furthermore, the equilibrium conditions for such aggregate can never be achieved, and the value of the transverse rupture strength is in a continuous flux.

An outstanding advantage of bodies of the invention which combine MoSiz with 5% up to 50% CrBz is the fact that such combined solid solution body exhibits essentially the structure of the CIBz compound. This factor is of great advantage since at elevated temperature, the temperature coefficient of resistivity of the MoSiz increases linearly with temperature. On the other hand, MoSi2, if alloyed or combined in accordance with the invention with CrBz over the range of the proportions up to 50% CrBz, gives a body with a temperature coefficient of resistance which approaches 0 at about 1200 C. and does not change with a further increase of the temperature up to about 1700 to 1900 C., the new material melting only at about is the discovery that superior w 2000" C.v The negligible change in the resistivity of the combined material of the invention over the temperature range from about 1200 to 1900 C. and higher, is of great practical importance for electric heater bodies formed of such material.

By way of example, there will now be described one satisfactory. procedure for. producing. such body of' the invention. A powder mixture containing 10% CrBzwith 90% MOSis is hot pressed. at 1500 C. into a cylindrical compact. The compact is then heated. in hydrogen for 15 hours at 1300 C. and thereafter comminutcd, yielding powder particles which contain the desired proportions of CrBz and MoSiz in completely alloyed solid solution form.

The alloy powder particles so obtained. are then ready for forming heater bodies. To this end, the alloy powder particles may be cold pressed into the desired shape, presintered, then further shaped to give it final shape, followed by sintering into the desired shaped heater body, such as a heater rod. The alloy powder may be also shaped into the desired heater body by extruding it after first mixing it with an extrusion. vehicle such as methyl cellulose, followed by a sintering in vacuum. When making the desired heater body by the extrusion process, the extruded rods should be sintered while they are suspended in a hanging position to avoid distortion.

In producing out of t .e alloy powder the desired heater bodies by extrusion, it is important that the. heating cycle of the sintcring operation should be very rapid. In other words, the final sintering temperatures should be reached from room temperature within about 10 minutes. If the heating cycle is slow, the organic. binder which is mixed with the alloy powder for carrying on the extrusion, will depart rapidly, leaving a relatively weak porous structure, thus endangering the production of satisfactory heater rod bodies. such as methyl cellulose, should be de-aired before mixing it with the alloy powder, as the entrapped air interferes with the rapid heating of the rods and also causes blowups. In lieu of the methyl cellulose, other binders may be used such as ethyl cellulose, cellulose acetate with appropriate solvents.

The single figure indicates schematically the body exemplifying the invention. An elongated cylinder of MoSi-z combined with 40% CrBz forms the heater body 11 which is provided at its ends with water-cooled electrode terminals 12.

Inasmuch as the addition of to 50% of CrBz to MoSiz increases the resistivity of the resulting combined material only by a factor of 10, it is desirable to form heater bodies made of such material of only thin cross-section so that no excessively large currents be required for bringing the heater body material to the operating temperature of about 1700 C. According to the invention, desired heater It is also important that the extrusion vehicle,

bodies having such thin heater body layer of MOSiz con taining 10 to CrBz are made as follows: A relatively rigid supporting body, such as a rod of ceramic material is covered with a surface layer of allowed powder particles of MoSiz containing in solid solution 5 to 50% CrBz. The material for the applied coating layer may be prepared by mixing the alloy powder with a suspension medium such as methyl cellulose and the powder mixture so prepared is then applied as a uniform coating layer to the ceramic rod. The coated ceramic rod is then sintered at elevated temperature to solidify the applied coating layer and drive off the methyl cellulose in the manner analogous to that described above is preparing heater rods by extrusion.

Heaters of the invention formed in the manner described above, are supen'or to conventional silicon carbide heaters because they may be operated for prolonged periods of time without corrosion at temperatures much higher than the maximum temperature of 1400 at which such silicon carbide heaters may be used, the heaters of the invention cost only a fraction of the cost of such silicon carbide heaters.

The features and principles underlying the invention described above in connection with specific exemplifiestions, will suggest to those skilled in the art many other modifications thereof. It is accordingly desired that the appended claims be construed broadly and that they shall not be limited to the specific details shown and described in connection With exemplifications thereof.

I claim:

1. In an electric heater device, a heater resistor body having spaced metallic electrodes for passing heating current therethrough, said body consisting essentially of molybdenum disilicide MoSiz containing 5% to 50% CrBz in solid solution.

2. A heater device as claimed in claim 1, the molybdenum disilicide MoSiz of said body containing 10% to 50% CrBz and having a negligibly low thermal coefficient of resistivity at temperatures between about 1300 to 1700 C.

3. A heater device as claimed in claim 1, the molybdenum disilicide MoSiz of said body containing 40% to 50% CrBz and having a negligibly low thermal coefiicient of resistivity at temperatures between about 1300 to 1700 C.

References Cited in the file of this patent UNITED STATES PATENTS 2,412,373 Wejnarth Dec. 10, 1946 2,622,304 Cofit'er Dec. 23, 1952 2,650,903 Garrison et al. Sept. 1, 1953 2,665,474 Beidler Jan. 12, i954

Claims

1. IN AN ELECTRIC HEATER DEVICE, A HEATER RESISTOR BODY HAVING SPACED METALLIC ELECTRODES FOR PASSING HEATING CURRENT THERETHROUGH, SAID BODY CONSISTING ESSENTIALLY OF MOLYBDENUM DISILICIDE MOSI2 CONTAINING 5% TO 50% CRB2 IN SOLID SOLUTION.