Cadmium-free silver and metal oxide composite useful for electrical contacts and a method for its manufacture

Claim 1

A process for preparing a composite of about 12 to 25 volume percent of a first metal oxide and about 0.1 to 2 volume percent of a second metal oxide highly dispersed in a remaining volume percent of silver, the first oxide being selected from zinc oxide or tin oxide; and the second oxide being selected from bismuth oxide, lead oxide, copper oxide, indium oxide or a mixture of two or more of said second oxides, which comprises: (a) atomizing a melt of an alloy of silver, first metal and second metal to form substantially spherically shaped solid alloy particles of less than 0.2 mm in diameter, the first metal being selected from zinc or tin in an amount calculated to yield said volume percent of the first metal oxide when oxidized, and the second metal being selected from bismuth, lead, copper, indium or a mixture of two or more of said second metals in an amount calculated to yield said volume percent of the second metal oxide when oxidized; (b) milling the spherically shaped alloy particles to reduce their size and deform them into elongated, platlet shaped particles having a thickness substantially less than and a length greater than the spherically shaped alloy particle diameter; (c) oxidizing the platlet shaped particles by heating in an oxygen containing atmosphere first at a temperature of about 673° K. to about 773° K. for about 2 to 6 hours and then at a temperature of about 873° K. to about 1073° K. for about 0.5 to 2 hours; said milling and said oxidizing in two steps producing a metal oxide-silver dispersion, in which the first and second metal oxide particles are extremely small and highly dispersed; (d) pressing the oxidized particles to combine them into a unitary solid; and (e) sintering the unitary solid in an air-like atmosphere or a non-oxidizing atmosphere at a temperature of about 973° K. to 1173° K. for about 0.5 to 2 hours to form the composite.