Patent ID: 7951277

Claim:
A prismatic multilayer gas sensor element having a substantially rectangular cross section, comprising: a gas-sensing cell portion disposed at a distal end portion of the prismatic gas sensor element, comprising a zirconia solid electrolyte layer and a gas-detecting electrode and a reference electrode disposed on top and bottom planar surfaces of the zirconia solid electrolyte layer, respectively; a posterior lead portion adjoining to the gas-sensing cell portion and extending to a rear end portion of the prismatic gas sensor element, comprising the zirconia solid electrolyte layer longitudinally extending from the gas-sensing cell portion, a first metallic lead and a second metallic lead connected to the gas-detecting electrode and the reference electrode, respectively, the first and second metallic leads being disposed on the top and the bottom planar surfaces of the solid electrolyte layer and extending from the posterior lead portion, respectively, a first non-porous alumina layer covering the top planar surface of the zirconia solid electrolyte layer and the first metallic lead disposed thereon, and a second non-porous alumina layer covering the bottom planar surface of the zirconia solid electrolyte layer and the second metallic lead disposed thereon; and a heating resistor sandwiched by insulating layers and positioned in the vicinity of the gas-sensing cell portion so as to heat the gas-sensing cell portion to a temperature of more than 600° C.; wherein longitudinal lateral surfaces of the posterior lead portion, which longitudinal lateral surfaces are substantially perpendicular to top and bottom planar surfaces of the prismatic multilayer gas sensor element, and which longitudinal lateral surfaces are heated by the heating resistor to a temperature not exceeding 600° C., are coated with a third non-porous alumina layer, wherein the third non-porous alumina layer comprises a joining layer joined with a lateral surface of the solid electrolyte layer and a surface layer joined with the joining layer, and wherein the third non-porous alumina layer is non-porous against carbon intrusion such that carbon particles are prevented from passing through the third non-porous alumina layer and reaching the solid electrolyte layer.