Spark plug, alumina insulator for spark plug, and method of manufacturing the same

A spark plug comprising:

• a metal shell arranged outside a center electrode;
• a ground electrode arranged so as to be opposed to the center electrode, an end of the ground electrode being connected to the metal shell;
• and
• an insulator, which resides between the center electrode and the metal shell, and which surrounds the outside of the center electrode, wherein
• the insulator includes alumina as a main component and further a Na component in the range of 0.07 to 0.5 wt % as a value converted into Na2O, and
• insulation resistance that is measured by passing a current through the insulator between a metal terminal and the metal shell while the entire spark plug is held at about 500° C. is 200 MΩ or higher, and wherein
• the number of pores each having a size equal to or larger than 10 μm that are observed in a sectional structure of the insulator is equal to or less than 100 as average counts per 1 mm2 of the section.

A spark plug comprising:

• a metal shell arranged outside a center electrode;
• a ground electrode arranged so as to be opposed to the center electrode, an end of the ground electrode being connected to the metal shell;
• and
• an insulator, which resides between the center electrode and the metal shell, and which surrounds the outside of the center electrode, wherein
• the insulator includes alumina as a main component and further a Na component in the range of 0.07 to 0.5 wt % as a value converted into Na2O, and
• insulation resistance that is measured by passing a current through the insulator between a metal terminal and the metal shell while the entire spark plug is held at about 500° C. is 200 MΩ or higher, and wherein,
• the insulator has an insulation withstand voltage of 35 kV/mm or higher at 20° C.