Metal oxide varistors and methods therefor

Claim 1

A varistor comprising a primary varistor metal oxide, aluminum in an amount of about 1 to about 30 parts per million (ppm), about 0.1 to about 0.5 mole % bismuth, about 0.1 to about 1.5 mole % antimony, about 0.1 to about 1 mole % chromium, about 0.1 to about 1.0 mole % manganese, about 0.1 to about 2.0 mole % cobalt, and about 0 to about 1.0 mole % boron, each of said elements being present in the form of its oxide and said varistor having a DC leakage current at 80% of V 1ma/cm .spsb.2 of less than 1.0×10 -7 amp/cm 2 .

Claim 11

A method of making a metal oxide varistor comprising at least one primary metal oxide and one or more additive metal oxides and having a DC leakage current at 80% of V 1ma/cm .spsb.2 of less than 1.0×10 -7 amp/cm 2 , which method comprises the steps of; (a) forming an aqueous solution comprising up to about 25 mole % (based on the additive metal oxides plus the primary metal oxides) of at least one soluble precursor of an additive metal oxide; (b) mixing in said aqueous solution up to about 75 mole % of at least one primary metal oxide powder having an average particle size up to about 5 microns to form a suspension or slurry of the primary metal oxide powder in said solution; (c) adding to the suspension or slurry a sufficient amount of a precipitation reagent to cause one or more of the dissolved additive metal oxide precursors to convert to an oxide or hydrous oxide and precipitate from said solution in the presence of said primary metal oxide powder in the form of an oxide or hydrous oxide; (d) removing water and by-product salts from the suspension of primary metal oxide powder and precipitate of additive metal oxide or hydrous oxide; (e) drying the powder and precipitate to form a metal oxide varistor precursor powder; (f) shaping said precursor powder in a green body having a desired shape and a thickness of less than or equal to 0.3 inch; and (g) sintering said green body to produce a varistor having less than or equal to 0.5 mole % bismuth and a DC leakage current at 80% of V 1ma/cm .spsb.2 of less than 1.0×10 -7 amp/cm 2 .

Claim 12

A method of making a metal oxide varistor comprising at least one primary metal oxide and one or more additive metal oxides and having a DC leakage current at 80% of V 1ma/cm .spsb.2 of less than 1.0×10 -7 amp/cm 2 , which method comprises the steps of; (a) forming an aqueous solution comprising up to about 25 mole % (based on the additive metal oxides plus the primary metal oxides) of at least one soluble precursor of an additive metal oxide; (b) mixing in said aqueous solution up to about 75 mole % of at least one primary metal oxide powder having an average particle size up to about 5 microns to form a suspension or slurry of the primary metal oxide powder in said solution; (c) adding to the suspension or slurry a sufficient amount of a precipitation reagent to cause one or more of the dissolved additive metal oxide precursors to convert to an oxide or hydrous oxide and precipitate from said solution in the presence of said primary metal oxide powder in the form of an oxide or hydrous oxide; (d) removing water and by-product salts from the suspension of primary metal oxide powder and precipitate of additive metal oxide or hydrous oxide; (e) drying the powder and precipitate to form a metal oxide varistor precursor powder; (f) shaping said precursor powder in a green body having a desired shape and a thickness of less than or equal to 0.3 inch; and (g) sintering said green body to produce a varistor having less than or equal to 0.5 mole % bismuth and a DC leakage current at 80% of V 1ma/cm .spsb.2 of less than 1.0×10 -7 amp/cm 2 .