INSULATOR AND SPARK PLUG

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PROBLEM TO BE SOLVED : To improve strength and dielectric strength performance of an insulator used in a spark plug. SOLUTION : An insulator is used in a spark plug and comprises a sintered compact whose main constituent is aluminum. In the sintered compact, the content ratio of a rare earth element (R. E.) and a group 2 element (2A) satisfies 0.1≤R.E./2A≤1.4, and the content ratio of the rare earth element (R. E.) and a barium oxide (BaO) satisfies 0.2≤BaO/R.E.≤0.8. At least one virtual rectangular frame of 7.5 μm×50 μm surrounding crystals containing the rare earth element lies within an arbitrarily-defined region of 630 μm×480 μm in a cross section of the sintered compact. The occupancy rate of an area of the crystals containing the rare earth element to an area of the rectangular frame is 5% or more, and in each of divided regions when the rectangular frame is equally divided into three in a direction of a long side, the ratio of a maximum area occupancy rate and a minimum area occupancy rate of the area occupancy rates of the crystals containing the rare earth element is 5.5% or less. COPYRIGHT : (C)2015, JPO&INPIT