Oxygen sensor element

An oxygen sensor element comprising:

• a solid electrolyte having a side surface at one side thereof, said side surface being contactable with a gas to be measured;
• a skeletal electrode provided on said side surface and having a plurality of pore portions, each of said pore portions passing through said skeletal electrode up to said solid electrolyte;
• and
• a reactive electrode made of a porous film provided in each of said pore portions and disposed directly on said solid electrolyte, a thickness of said porous film being smaller than that of said skeletal electrode,
• wherein 80% or more of said pore portions in which said reactive electrode exists have an average area of 100 .mu.m.sup.2 or less,
• an average percentage (SH/SZ) which is a ratio of a total area (SH) of said reactive electrode provided in said pore portions having an area of 100 .mu.m.sup.2 or less to a total area (SZ) of said skeletal electrode and said reactive electrode provided in said pore portions having an area of 100 .mu.m or less is in a range from 10 to 50%,
• a film thickness of said skeletal electrode is in a range from 1.5 to 4 .mu.m, and
• said film thickness of said reactive electrode is in a range from 0.6 to 1.5 .mu.m.

A method of manufacturing an oxygen sensor element having a solid electrolyte, said method comprising:

• a step of forming a first metal film on a surface of said solid electrolyte at the side of a gas to be measured;
• a step of forming pore portions on said first metal film by coagulating said first metal in such a manner that said pore portions pass from a surface of said first metal film to said solid electrolyte;
• and
• a step of forming a second metal film on said surface of said first metal film, which have been covered with said pore portions.