Spark plug for an insulator of alumina-based sintered body

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PROBLEM TO BE SOLVED : To obtain an alumina base sintered compact having sufficient voltage endurance and slight unevenness of voltage endurance at a high temperature even in reduction in thickness of alumina insulation layer by making the sintered compact include La, Pr and Nd among rare earth elements (RE) and specifying the area ratio of pores having a specific major axis and a major axis distribution, exposed to an arbitrary mirror finished polished surface. SOLUTION : Alumina raw material powder is formulated with about 0.01-20, calculated as RE, of at least one kind of oxide of La, Pr and Nd among RE and a fixed sintering auxiliary, sintered in an atmosphere, subjected to HIP treatment and densified. Consequently, an alumina base sintered compact having = about 60 kV/mm average voltage endurance value at approximately 700 deg.C and <= about 20 kV/mm voltage endurance unevenness and is a suitable material for a ceramic product having high insulating properties.