Zinc oxide varistor structure

A varistor device comprised of a varistor matrix containing a plurality of first and second metallizations, said varistor matrix being comprised of more than three layers with a portion of each matrix layer being directly bonded to another matrix layer, said metallizations being spaced from each other by at least substantially a layer of matrix and being in direct contact with said matrix, each said metallization being present at substantially the region between two layers of matrix, each said metallization having a proximal end portion and a distal end portion wherein the proximal end portion is closest to the edge of the matrix and opposite the distal end portion which is farthest away from the edge of the matrix, each said metallization being totally surrounded by said matrix except for its proximal end portion, each said first metallization having its proximal end portion electrically connected to form a first electrode, each said second metallization having its proximal end portion electrically connected to form a second electrode, said first and second metallizations being offset from each other, at least a pair of said spaced first and second metallizations being overlapping, each said metallization being continuous and being comprised of elemental silver, said varistor matrix being comprised of zinc oxide grains isolated from each other by continuous amorphous glassy phase, said varistor device having a porosity of less than about 10%.

A process for producing a sintered multi-layer composite having a porosity of less than about 10% by volume of the composite, said composite being comprised of at least a three-layer varistor matrix of zinc oxide grains isolated from each other by continuous amorphous glassy phase wherein said matrix totally surrounds each of at least a first and second silver metallization, each said metallization having a proximal end portion and a distal end portion wherein the proximal end portion is closest to the edge of the matrix and opposite the distal end portion which is farthest away from the edge of the matrix, said metallizations being continuous, spaced from each other and having a configuration enabling said composite to be useful for forming a varistor device, which comprises:

• (a) providing a varistor-forming powder comprised of a mixture of zinc oxide and glassy phase-forming additive;
• (b) admixing said varistor-forming powder with an organic binding material;
• (c) forming the resulting mixture into tape;
• (d) providing a silver metallization-forming material;
• (e) forming a layered structure of at least three of said tapes containing said metallization-forming material therewithin between said layers, said metallization-forming material being present in a configuration and amount sufficient to produce said metallizations;
• (f) laminating the layered structure producing a laminated layered structure wherein none of said metallization-forming material is exposed;
• (g) firing said laminated structure to thermally decompose its organic component at an elevated temperature below about 600.degree. C. leaving no significant deleterious residue in the resulting fired structure, said firing being carried out in an atmosphere or vacuum which has no significant deleterious effect on said composite;
• (h) sintering the resulting fired structure at a temperature ranging from about 1000.degree. C. to about 1400.degree. C. in an oxygen-containing atmosphere to produce a sintered product having the composition of said composite, said fired structure having a sufficient open volume available to accommodate said silver during sintering;
• and
• (i) cooling said sintered product to produce said composite, said sintering and cooling being carried out in an atmosphere which has no significant deleterious effect on said composite.