Patent ID: 6771009
Filing Date: 2004-08-03
Classification: C03C,H01T

Abstract:
A spark plug comprising: a center electrode; a metal shell; and an insulator comprising an alumina ceramic and disposed between the electrode and the shell,wherein at least part of the surface of the insulator is covered with a glaze layer, the glaze layer contains a lead component content of 1 mol % or lower of PbO, the glaze layer comprises: 35 to 80 mol % of a first ingredient comprising 5 to 60 mol % of silicon component SiO2 and 3 to 50 mol % of boron component B2O3; and 12 to 60 mol % of a second ingredient comprising at least one of a zinc component ZnO and an alkaline earth metal component R, in which R is at least one selected from the group consisting of calcium, strontium, and barium, of ZnO and empirical formula RO, respectively, the total content of the first ingredient and the second ingredient is from 65 to 98 mol %, the total content of the zinc component ZnO and at least one of the barium component BaG and strontium component SrO is from 12 to 30 mol %, the glaze layer further contains at least one alkali metal component selected from the group consisting of sodium, potassium and lithium in a total amount of from 2 to 15 mol % of Na2O, K2O, and Li2O, respectively, the insulator includes, in an axially central position thereof, a projection part protruding from the outer circumferential surface thereof and extending in a circumferential direction, the insulator includes a main body located adjacent to the projection part on the rear side thereof, which is the side opposite to the front side facing the center electrode in the axial direction, and the main body of the insulator includes a base portion having a cylindrical outer circumference, and the outer circumference of the base portion is covered with the glaze layer which gives a surface roughness curve having a maximum height Ry of 10 &mgr;m or smaller, wherein the insulator gives a surface roughness curve having a maximum height Ry of 15 to 35 &mgr;m, and the glaze layer has thickness of from 10 to 50 &mgr;m.