Patent ID: 7824531

Claim:
A multilayered air-fuel ratio sensor having a plurality of stacked layers comprising: a plurality of substrate layers comprising at least one solid electrolytic substrate layer and at least one insulating substrate layer; and a boundary layer interposed immediately between said solid electrolytic substrate layer and said insulating substrate layer without any other intervening layer; wherein each of said solid electrolytic substrate layer, said insulating substrate layer, and said boundary layer is obtained by sintering original particles of a source material so as to change the original particles to sintered particles, and an average size of the sintered particles of said boundary layer is adjusted to be larger than an average size of the sintered particles of each of said solid electrolytic substrate layer and said insulating substrate layer, wherein said boundary layer comprises a component selected from the group consisting of alumina, spinel, and steatite, and wherein said boundary layer is made primarily from α-alumina with an average sintering particle diameter of 3 to 4 μm.