GB409031A - Improvements in or relating to non-metallic resistors and the manufacture thereof - Google Patents

Abstract

409,031. Resistance materials. CARBORUNDUM CO., Ltd., Trafford Park Road, Trafford Park, Manchester.- (Globar Corporation; Niagara Falls, New York, U.S.A.) Oct. 26, 1932, No. 30038. Drawings to Specification. [Class 37.] In order to increase the specific resistance of non-metallic resistors, particularly those composed principally of silicon carbide, a comminuted metal such as aluminium powder is added to the non-metallic mixture and the resistor is subsequently cured at a temperature of 1800‹ C. or greater in an atmosphere which is non-oxidizing with respect to the principal non-metallic conducting ingredient. In an example, a small quantity, usually from 1 to 5 per cent of aluminium powder is mixed with dry silicon carbide grain and a small quantity of sodium silicate solution is added to serve as a temporary binder, the mixture then being moulded to the required shape. The moulded resistors are then baked at a low temperature, e.g. 600‹ C., dipped in a slip or slurry consisting of an aqueous suspension of very finely divided sand and carbon in equal proportions, and dried. The resistors are placed end to end between electrodes in a bed consisting of a mixture of 3 parts of fine silica and one part of carbon by weight in a furnace as described in Specification 366,605, current being passed through the resistors so that they are heated to about 2000‹ to 2500‹ C. in an atmosphere containing carbon monoxide and silicon vapour generated by the interaction of the silica and carbon of the bed. A small proportion (about 2.5 per cent.) of carbon flour may be added to the mixture before moulding. Other easily oxidizable metals and alloys, such as magnesium and magnesium-aluminium alloys, may be used instead of aluminium. Other non-metallic resistance materials, such as carbon or graphite, may be employed instead of silicon carbide. For example, powdered aluminium may be added to crushed particles of carbon, a carbonizable binder such as tar or pitch added, and the formed resistor cured at a temperature of 1800‹ C. or greater. Specification 15390/05, [Class 37], also is referred to.