Insulator for spark plug and spark plug using the same

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PROBLEM TO BE SOLVED : To obtain both an insulating material for spark plug having high voltage endurance characteristics at a high temperature in the vicinity of 700 deg.C and a spark plug provided with the insulating material. SOLUTION : This insulating material 2 for a spark plug comprising alumina (Al2O3) to be used for a spark plug 100 as a main component is constituted of an alumina base sintered compact which contains =95% theoretical density ratio. The insulating material 2 for a spark plug controls the occurrence of dielectric breakdown affected by residual pores existing at an alumina crystal grain boundary and a low-melting glass phase and has excellent voltage endurance characteristics at a high temperature of about 700 deg.C by the constitution.