Varistors with upturn at high current level
AssigneeGeneral Electric Company
InventorJoe Wong
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Abstract
A method for producing varistors and varistor powder is disclosed in which the zinc oxide powder is predoped with certain desired impurities in order to keep these impurities out of the intergranular phase which is known to exist in sintered metal oxide varistors. In particular, certain trivalent dopants are added to increase the zinc oxide grain conductivity without the usually concomitant increase in higher leakage current. In particular, the method disclosed herein is especially important for the doping of varistor material with aluminum at low levels of concentration.
Claims16
A method of producing doped zinc oxide varistor powder useful in forming sintered varistors comprising the steps of:
- (A) dissolving a conductivity increasing dopant in a solvent;
- (B) forming a slurry by thoroughly mixing zinc oxide powder with the solvent containing the dopant to the extent necessary to achieve the selected concentration of dopant;
- (C) removing the solvent from the slurry, whereby the zinc oxide powder particles are coated with the dopant;
- and
- (D) heating the coated zinc oxide powder particles for a time and at a temperature sufficient to cause thermal diffusion of the dopant into the zinc oxide powder particles, but not at a temperature high enough to initiate zinc oxide grain growth.
A method of producing doped metal oxide varistors comprising the steps of:
- (A) dissolving a conductivity-increasing dopant in a solvent;
- (B) forming a slurry by thoroughly mixing zinc oxide powder with the solvent containing the dopant to the extent necessary to achieve the selected concentration of dopant;
- (C) removing the solvent from the slurry, whereby the zinc oxide powder particles are coated with the dopant;
- (D) heating the coated zinc oxide powder particles for a time and at a temperature sufficient to cause thermal diffusion of the dopant into the zinc oxide powder particles, but not at a temperature high enough to initiate zinc oxide grain growth;
- (E) mixing the dopant diffused zinc oxide powder particles with selected amounts of selected metal oxide varistor additives;
- and
- (F) sintering the varistor powder from step E at a temperature of between approximately 900.degree. C. and approximately 1000.degree. C. for between approximately one and approximately twelve hours.
A sintered zinc oxide varistor comprising:
- a granular phase composed substantially of zinc oxide grains;
- an intergranular phase;
- and
- a conductivity-increasing dopant contained substantially within said granular phase and being substantially absent from said intergranular phase.
Dependencies are inferred from the claim text.