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Publication numberUS2729880 A
Publication typeGrant
Publication dateJan 10, 1956
Filing dateSep 23, 1948
Priority dateSep 23, 1948
Publication numberUS 2729880 A, US 2729880A, US-A-2729880, US2729880 A, US2729880A
InventorsMiller William A
Original AssigneeRca Corp
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Aluminum oxide semi-conductors
US 2729880 A
Abstract  available in
Previous page
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Claims  available in
Description  (OCR text may contain errors)

United States Patent ALUMINUM OXIDE SEMI-CONDUCTORS William A. Miller, Port Jefferson, N. Y., assignor to Radio Corporation of America, a corporation of Delaware No Drawing. Application September 23, 1948,

Serial No. 50,883 Y 7 Claims. (Cl. 29-182-1) This invention relates to improvements in that class of materials known generally as semi-conductors and having a negative temperature coefiicient of resistance. The resistance of this class of materials is relatively high but not nearly as high as that of materials classed as good insulators.

In certain applications of semi-conducting materials, it is desirable to employ a form having a low temperature coefficient of resistance. The present invention relates more particularly to materials of this nature.

One object of the present invention is to provide improved materials having relatively high resistance and a relatively low temperature coeflicient of resistance.

Another object of the invention is to provide a novel.

product comprising aluminum oxide and aluminum, which product has a negative temperature coefiicient of resistance.

Another object of the invention is to provide improved semi-conducting materials which can be operated at high temperatures.

These and other objects will be more apparent and the invention will be better understood from the following specification.

In general, the products of the present inventi n are made by combining certain proportions of aluminum and aluminum oxide at sintering temperatures in the presence of a non-oxidizing atmosphere. By a nonoxidizing atmosphere is meant a vacuum, a reducing at mosphere or an inert atmosphere. It is preferable to exclude water vapor from the atmosphere.

Aluminum, itself is, of course, a good conductor of electricity while aluminumoxide is normally a good insulator. If so much aluminum is added to the aluminum oxide that a continuous conducting path around the insulating particles is provided by the aluminum, the mixture would then become a fairly good conductor. However, this type of composition does not come within the scope of the present invention. It has been found that aluminum will disperse freely throughout a body of aluminum oxide. It is not possible to mix any randomly selected metal with a randomly selected ceramic material and have the metal disperse freely throughout the ceramic. Also, in those compositions in which there is free dispersal, the resulting product is not always a semi-conductor having a negative temperature coefficient of resistance. It, therefore, appears to be an individual problem as to whether any particular metal and a particular ceramic will produce the desired type of product.

Example I One part of aluminum was mixed with three parts aluminum oxide and the mixture molded into a cylinder one-sixteenth inch in diameter and one-fourth inch long.

The cylinder was fired in vacuo at about l,500 C. for

,. lce

Example II A cylinder similar to that above described was made up containing one part aluminum to twenty-five parts aluminum oxide and fired at the same temperature and for the same time as the material in Example I. When tested under similar conditions, the product had a resistance or about seventy-five ohms.

The complete range of proportions of aluminum oxide to aluminum has been found to be from 3 to l toabout 40 to 1 by weight.

The firing temperatures may range from about 1,450" C. to temperatures just below the melting point of aluminum oxide (2,050 C.). The time of firing varies inversely with firing temperature. Times of twenty minutes up to one and one-ha1f hours have proved satisfactory depending upon the temperature of firing. There does not appear to be any critical upper limit of time after the product has once become stabilized. Further heating neither causes deterioration of the product nor improvement. If vacuum heating is used, the vacuum must be good enough so that all appreciable amounts of oxygen are excluded. The inert atmosphere may be any of the usual inert gases such as helium, nitrogen or argon.

Since aluminum vaporizes readily in a vacuum, the product may be prepared by first forming a compressed body of aluminum oxide and then maintaining the com.- pressed body in an atmosphere of aluminum vapor until sufficient diffusion has taken place of the metal throughout the ceramic.

Another way to combine the two substances is to immerse a compressed pellet of the ceramic in a bath of molten aluminum.

The principal advantages of the improved materials of the present invention are:

First, that they are highly refractory and, therefore, eminently suited to those uses where large amounts of power must be dissipated at high temperatures, and

Second, that they are useful where it is desired to use a material having a very low negative temperature coefiicient of resistance.

There has thus been described a new and useful type of semi-conducting material comprising aluminum and aluminum oxide ceramically combined. It is desired that the invention be limited only as defined in the appended claims.

I claim as my invention:

1. A semi-conductor having a negative temperature 00- efiicient of resistance, consisting essentially of a compact porous body of sintered aluminum oxide and aluminum, said body being made by diffusing aluminurm metal homogeneously throughout the pores of a compact body of aluminum oxide at sintering temperatures of said oxide above about l,450 C. but below the melting temperature of said oxide and in a non-oxidizing atmosphere, and the ratio by weight of said oxide to said aluminum being within the range of about 40 to 1 and about 3 to l.

2. A material according to claim 1 in which said ratio is 25 to l. v

3. A material according to claim 1 in which said ratio is 3 to 1.

4. A method of making a semi-conducting material comprising diffusing aluminum metal homogeneously throughout the pores of a compressed body of aluminum oxide, that is, above about 1450 C. at sintering temperatures of said oxide but below the melting point of said oxide and in a non-oxidizing atmosphere, the ratio by weght of said oxide to said aluminum being within the range of about 40 to 1 and about 3 to l.

5. A method according to claim 4 in which the oxide and the aluminum are mixed together before the oxide is sintered.

6. A method according to claim 4 in which the oxide is first formed into a compressed body, simered and their immersed in molten aluminum.

7. A method according to claim 4 in which the oxide is first formed into a compressed body, sintered and then exposedto an atmosphere of aluminum va or.

References Cited in the file of this patent UNITED STATES PATENTS 4 Bethel Oct. 17, 1933 'MAfifiiif 061. 28, 1941 Goldschmidt et a1 May 19, 1942 Preis Apr. 27, 1943 Colbert et a1. Dec. 16, 1947 Alexander 2.22;; July 6, 1948 McKinley Mar. 1, 1949 Hend'ren s s== s Mar. 29, 1949 FOREIGN PATENTS Great Britain 11113: 8, 1920 OTHER REFERENCES Metal industry, May 14, 1948, pa es 405-407.

Patent Citations
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US1738141 *Aug 11, 1928Dec 3, 1929Charles F NoftzgerElectrical resistance unit and method of manufacturing same
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Referenced by
Citing PatentFiling datePublication dateApplicantTitle
US2958936 *May 20, 1955Nov 8, 1960Eberhard Meyer-HartwigElectrical semi-conductors and method of manufacture
US3025441 *Sep 19, 1958Mar 13, 1962Gen ElectricElectrical capacitor
US3378498 *Jan 25, 1965Apr 16, 1968Metal Diffusions LtdProcess for diffusing metal into a refractory or ceramic oxide using a nitrate promoter
US3718441 *Nov 18, 1970Feb 27, 1973Us ArmyMethod for forming metal-filled ceramics of near theoretical density
US4673435 *Apr 24, 1986Jun 16, 1987Toshiba Ceramics Co., Ltd.Alumina composite body and method for its manufacture
U.S. Classification428/539.5, 75/235, 419/19
International ClassificationH01C7/04, H01B1/08
Cooperative ClassificationH01C7/04, H01B1/08
European ClassificationH01C7/04, H01B1/08