GB1387674A - Electrical resistors - Google Patents

Abstract

1387674 Varistors MATSUSHITA ELECTRIC INDUSTRIAL CO Ltd 25 Aug 1972 [27 Aug 1971 (4) 17 Sept 1971 (2) 22 Sept 1971] 39710/72 Heading H1K A sintered body consisting mainly of zinc oxide and exhibiting a voltage dependent resistivity is treated by diffusing lithium and/or sodium ions into its surface to improve the stability of its electrical characteristics against temperature, humidity and aging. In addition to zinc oxide the body may include oxides of one or more of bismuth, cobalt, manganese, antimony, barium, strontium, tin, lead and uranium. The lithium or sodium is preferably diffused in from a paste comprising its oxide, or one of a wide range of materials which under the oxidizing firing conditions used to cause diffusion will convert to the oxide, together with resin binders and solvent. Specified pastes include, in addition to lithium oxide, one of more of the oxides of cobalt, manganese, silver, chromium, nickel, boron, silicon, bismuth, and lead but the preferred paste includes 1-2À5 parts by weight lithium oxide and 1-3 parts potassium oxide. The body is produced by conventional milling, drying, preheating and pulverizing of the components which are then press-moulded or extruded to the required form, sintered at 1000-1450‹ C. for 1-5 hours and cooled in the furnace or air quenched. Surfaces not be be provided with electrodes are coated with paste and heated at 600-1000‹ C. for long enough to give a diffusion depth >À01 mm. Metal electrodes, e.g. of silver, copper, nickel, tin, zinc, or aluminium are subsequently applied, e.g. by flame spraying or vacuum, electroless or electrolytic deposition. Alternatively, silver paste electrodes are fired prior to the diffusion step. Finally wire leads are attached to the electrodes with solder or conductive paste.