Patent ID: 6822378
Filing Date: 2004-11-23
Classification: C03C,H01T

Abstract:
A spark plug comprising:a center electrode; a metal shell; and an alumina ceramic insulator disposed between the center electrode and the metal shell, wherein at least part of the surface of the insulator is covered with a glaze layer comprising oxides, wherein the glaze layer comprises: 30 to 60 mol % of a Si component in terms of SiO2; 20 to 50 mol % of a B component in terms of B2O3; 0.5 to 25 mol % of a Zn component in terms of ZnO; 0.5 to 15 mol % in total of at least one of Ba and Sr components in terms of BaO and SrO, respectively; 2 to 12 mol % in total of at least two alkaline metal components of Na, K and Li, in terms of Na2O, K2O, and Li2O, respectively, wherein K and Li is essential; 0.1 to 10 mol % of a F component in terms of F2; wherein the glaze layer has a Pb content not more than 1 mol % in terms of PbO; and wherein the insulator comprises an alumina insulating material comprising 85 to 98 mol % of an Al component in terms of Al2O3, and the glaze layer has an average thermal expansion coefficient at the temperature ranging 20 to 350Â° C. of 5Ã—10âˆ’6/Â° C. to 8.5Ã—10âˆ’6/Â° C.