Process for improving the stability of sintered zinc oxide varistors

#The present invention relates generally to processes for manufacturing sintered varistors which are composed primarily of zinc oxide, and pertains particularly to heat treating of such varistors after they are sintered.

#Zinc oxide varistors are highly non-linear and are especially suitable for overvoltage protection devices, such as overvoltage surge or lightning arresters. They contain mostly zinc oxide with certain selected additives for controlling the mechanical and electrical characteristics of the varistor. The varistors are generally in the form of rods or discs which are provided with metal electrode layers on the end faces.

#The manufacturing process for zinc oxide varistors includes pressing a powder mixture of zinc oxide and the desired additives in a die to form a self-supporting body. Then the body is sintered in a furnace at about 1200.degree. C. (Celsius) for a time until a non-porous ceramic is formed, and is then cooled and provided with metal electrodes.

#FIG. 1 is an elevational view of a varistor in accordance with the preferred embodiment of the invention.

#FIG. 2 is a graphical representation of the stability of a number of trial sample varistors.

#FIG. 3 is a graphical representation of the stability of a number of additional trial sample varistors.

#A preferred embodiment of the invention is the varistor disc 10 of FIG. 1. The disc 10 is about 6.9 cm (centimeters) in diameter and about 2.25 cm thick. It is pressed from a powder having the following composition, in mole percent:

#______________________________________ 95.7% ZnO (zinc oxide) 0.5% Bi.sub.2 O.sub.3 (bismuth trioxide) 0.5% Co.sub.2 O.sub.3 (cobalt trioxide) 0.5% MnO.sub.2 (manganese dioxide) 1.0% Sb.sub.2 O.sub.3 (antimony trioxide) 0.5% Cr.sub.2 O.sub.3 (chromic oxide) 0.1% BaCO.sub.3 (barium carbonate) 0.1% B.sub.2 O.sub.3 (boron oxide) 0.1% SiO.sub.2 (silicon dioxide) 1.0% NiO (nickel oxide) 0.003% Al(NO.sub.3).sub.3.9H.sub.2 O (aluminum nitrate) ______________________________________

#The disc 10 is now sintered at 1200.degree. C. for 5 hours in air. At the end of the sintering time, the disc 10 is cooled slowly, at a rate of about 100.degree. C. per hour by, for example, leaving it in the cooling furnace. When it has cooled to a temperature of about 400.degree. C. or less, aluminum is flame sprayed on the faces to form electrodes 12, only one of which is shown in FIG. 1. Next, the disc 10 is reheated in a furnace at 580.degree. C. in air for one hour and again slowly cooled at 400.degree. C. or less at an average rate of for example 100.degree. C. per hour. The reheating cycle to 580.degree. C. and slow cooling is repeated at least once, preferably several times. The disc 10 is now stable with a low leakage current and may be incorporated into an overvoltage surge arrester either alone or as one of a number of arrester valve discs.