A/F sensor element and method of manufacturing the same

An Air/Fuel (A/F) sensor element comprising:

• a substrate made of an insulating ceramic having a bottomed cylindrical shape with a closed distal end and an opened rear end;
• an electrolyte part made of a solid electrolyte;
• and
• a pair of electrode portions;
• wherein, the insulating ceramic is made of a material having a higher thermal conductivity than the solid electrolyte;
• the electrolyte part is embedded in at least a portion of the side wall of the substrate to constitute a part of the sidewall;
• the pair of the electrode portions is formed on an inner surface and an outer surface of the side wall, respectively, and is formed at positions sandwiching the electrolyte part;
• the A/F sensor element is used by inserting a rod-like heater in the substrate having the bottomed cylindrical shape;
• the substrate is formed of the insulating ceramic at a contact position to the heater within the substrate;
• a difference in thickness between the substrate and the electrolyte part in a radial direction of the substrate and electrolyte part forms a level difference in the radical direction;
• a boundary section forms a joint portion in circumferential direction of the substrate and the electrolyte part;
• and
• the level difference at the boundary section between the substrate and the electrolyte part is 10 μm or less.