GB678718A - Improvements in or relating to ceramic electrical devices and the manufacture thereof - Google Patents

Abstract

678,718. Insulators; resistance coatings; condensers. CLARK, C. H. W., and STEATITE & PORCELAIN PRODUCTS, Ltd. Aug. 4, 1950 [May 19, 1949], No. 13425/49. Drawings to Specification. Classes 36 and 37. [Also in Group XXIII] A ceramic electrical device such as a H.T. insulator, capacitor, or resistor, is provided on at least part of its surface with a conducting or semi-conducting coating the effective conducting constituent of which comprises blue titania. Blue titania is formed by firing titania under reducing conditions or in the presence of certain oxides, but reverts to the non-conducting form due to oxidation during normal cooling. The TiO 2 is therefore applied in a slip and the base then coated with a glazing composition which matures to a substantially impermeable state above that at which the TiO 2 assumes its conducting form, and the double coating fired so that the TiO 2 becomes conducting and is prevented from re-oxidation by the cover glaze. Alternatively a coating of slip comprising TiO 2 and fluxing ingredients is applied and fired, when the TiO 2 is converted and protected by the matured coating. The conductivity may be controlled by the addition to the slip of up to 10 per cent of the TiO 2 content of one or more of the oxides of Be, Cr, Ca, Co, Ni, Mn, Mo, W, V or substances such as magnesium thorate or zirconate which can inhibit or promote the conversion of the TiO 2 . Other substances such as diluents, e.g. clay, MgO, Al 2 O 3 , CeO 2 , SiO 2 , or alkaline earth oxides in amounts up to 20 parts diluent to one part TiO 2 may also be added to control the conductivity. The diluent is normally chosen to give a coefficient of expansion similar to that of the ceramic base, e.g. by adding steatite to form a 93/7 porcelain/steatite mix of percentage composition, clay 35, SiO 2 36, Feldspar 23, soapstone 6. The firing conditions must be carefully controlled, but even so the control of the conductivity is not a close control, however unsuitable coatings may be subjected to reducing and oxidizing treatment at a temperature sufficient to render the glaze permeable. To join a conductor to the coating the overlying glaze is removed locally by sand-blasting &c. The temperature coefficient of the glaze is comparatively low. Tables and graphs are given showing the resistivities of various coatings.