Multi-layer gas sensor element and gas sensor comprising the same

A multi-layer gas sensor element comprising a solid electrolytic member, a substrate, and a porous member, co-fired together, wherein each of the substrate and the porous member has a thickness of at least 1.5 times larger than that of the solid electrolytic member with respect to a lamination direction;

• the substrate and the porous member face each other and sandwich the solid electrolytic member;
• a ceramic component constituting the substrate in the highest volume percent thereof is the same as the ceramic component constituting the porous member in the highest volume percent thereof;
• the solid electrolytic, member also contains said ceramic component which is predominately contained in said substrate in an amount of 20 to 70 mass %;
• and the volume percent (R2) of the ceramic component contained in the porous member is 80% or more of the volume percent (R1) of the ceramic component contained in the substrate,
• wherein when the mean grain size of crystals constituting the substrate is referred to as “a1” and the mean grain size of crystals constituting the porous member is referred to as “a2”a value A represented by the following equation (1) falls within a range of from 0.9 to 5 inclusive:
• A=a1/a2 (1).