GB828123A - Improvements in or relating to electric resistance elements - Google Patents

Abstract

828,123. Heating resistors. KANTHAL, A. B. Jan. 29, 1958 [Feb. 5, 1957], No. 3008/58. Class 39(3) [Also in Groups II and XXXVI] A resistor comprises an electrically insulating heat resistant base member e.g. a ceramic having applied there to by flame-spraying an electrically conductive layer consisting mainly of silicides of the transition elements of Group IV, V and VI of the periodic system i.e. of the elements Ti, Zr, Hf, Ta, Cb, V, Mo, Wand Cr. The conductive layer comprises preferably MoSi 2 , VSi 2 , WSi 2 , TiSi 2 and Mo 5 Si 3 and is 5-500 microns thick. On annealing the layer in an oxidizing temperature e.g. at 1400‹C., a gas-tight protective layer of quartz glass forms on the surface and is 2-30 microns thick. The silicide layer may be formed as meanders or coils by spraying through thin steel masks on to a base of refractory brick, e.g. of corundum, silicone carbide, glass, porcelain or on to coatings of enamel or flame sprayed oxide, e.g. of aluminium on a metal base. The resistor layer may be treated in an atmosphere of SiCl 4 , B Br, or Mo(Co) 6 . The layer may be porous and may be covered by a layer rich in SiO 2 e.g. of glass glaze or enamel which may also enter the pores. Thus a heater may be formed by fusing together two glass plates one forming a base for the resistor and the other the protective layer; or enamelled metal plates may be used in place of glass to form a radiator or heatstorage device. The resistor may be attached only at spaced points to the ceramic base by partly coating the latter with a destructible material before spraying. The resistor may contain other heat resistant silicides e.g. TiSi 2 and small amounts of oxidation resistant borides e.g. CrB 2 or carbides e.g. SiC and oxides e.g. of Al. The centre part of the resistor may be of MoSi 2 and heated to incandescence, the outer sections being heated to 300- 700‹C. and comprising TiSi 2 possibly with CrSi 2 . The terminals may be of silver or gold or a mixture of TiSi 2 and CrSi 2 . The initial silicide is embedded as a powder preferably with 1% of aluminium or magnesium in a plastic such as polythene to form a flexible wire for spraying. Helical resistance coatings are sprayed on to ceramic tubes e.g. or corundum for use in furnaces or on to the surface of a ceramic melting crucible for use up to 1500‹C. The tube may carry a thin glaze of clay and further clay may be applied to the resistors. The resistors may be used on immersion heaters, cooking and frying vessels and covered with enamel or silicone resins and have a positive temperature coefficient of resistivity. Fig. 6 shows a cooking plate comprising a metal plate 4 with an insulating enamel layer 5 on which is sprayed a spiral pattern 6 of MoSi 2 0.05 mm. thick covered with a quartz coating, and resting on a ceramic filler 9 in a casing. Spherical or ellipsoidal bodies of porcelain flamesprayed with the silicide are placed in mutual contact in a vessel between two electrodes, and a gas or liquid is heated by flowing over them. The spheres may be of copper covered with enamel and then the silicide. The silicide grains sprayed do not exceed 50Á or preferably 15Á; and aluminium powder may be added from 1 to 10% of a grain size 10 to 150Á. Flame spraying includes processes in which particles are blown or projected at high temperatures on to a base member, the particles being simultaneously heated or pre-heated to a temperature at which they are ductile when they hit the base; and the energy may be supplied to the particles by the continuous combustion of a gas, by detonations in a gaseous mixture, by an electric arc, by the use of magnetic forces or high tension electric fields or of gas under pressure for atomizing previously fused material. As the conductive layer is somewhat porous, a ceramic material melting at 1100-1700‹C. may be infiltrated into the pores and may be a glass or enamel e.g. a composition comprising 0-10% Na 2 O or K 2 O, 0-60% Al 2 O 3 and 40-100% SiO 2 . Alloys used as resistive coatings comprise 50% MoSi, and 50% Mo 5 Si 3 ; or 55% MoSi 2 , 40% Mo 5 Si 3 and 5% oxides; or equal parts of TiSi 2 and CrSi 2 ; or 80% VSi 2 and 20% TaSi,; or 90% MoSi 2 and 10% SiO 2 ; or 40% MoSi 2 , 50% Mo 5 Si 3 and 10% SiC.