| 摘要 |
In insulator including alumina as a main component, a Na component in the insulator is set in the range of 0.07 to 0.5 wt % as Na2O. While the Na component content is as high as to have conventionally been regarded to be beyond the common sense, with this range of the Na component content, insulation resistance, mechanical strength and the like at high temperature are unexpectedly not reduced and an insulator with performances comparable to those of an insulator from conventional low soda alumina lower in Na component content than the above described range can be obtained. As a result, medium soda alumina and regular soda alumina that are much lower in cost than conventionally used low soda alumina can be used instead of the low soda alumina, so that dramatic reduction in production costs of insulator 2 for a spark plug 100 and in addition, of the spark plug 100 using the insulator 2 are realized.
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