Abstract: The spark plug includes an insulator made from an alumina-based sintered body, and the insulator contains 90 to 98 wt % of an Al component in oxide equivalent. The insulator contains 1 to 5 wt % of an Si component, 0.1 to 1 wt % of an Mg component, 2 wt % or less of a Ca component, 0.3 to 6 wt % of a Ba component, and 0.11 to 5 wt % of a rare earth component, in oxide equivalent. In analysis using a scanning transmission electron microscope with a probe diameter of an electron beam set at 1 nm, Si and a rare earth element are detected at a crystal grain boundary having a thickness of 15 nm or less, and an alkaline earth metal at the crystal grain boundary is less than a detection limit.

Abstract: The spark plug includes an insulator formed of an alumina-based sintered body, and the insulator contains Si, Ba, and a rare earth element. In an analysis of the insulator by using a scanning transmission electron microscope having an electron-probe diameter of 1 nm, Si and a rare earth element are detected at a crystal grain boundary having a thickness of 15 nm or less, a content of an alkaline earth metal is less than a detection limit at the crystal grain boundary, and a diffraction spot is present in an electron diffraction pattern of a portion where Ba is detected.

Abstract: Disclosed is a spark plug including: an insulator having an axial hole formed along an axis of the spark plug; a center electrode disposed in a front end side of the axial hole; and a metal shell fixed around an outer circumference of the insulator, with a front end portion of the insulator protruding frontward from a front end of the metal shell. The front end portion of the insulator consists only of a first section and a second section located adjacent to and frontward of the first section and having an inner diameter larger than that of the first section. The second section has a chamfered area formed on an inner circumferential surface thereof so as to continue to a front end of the insulator.

Abstract: Disclosed is a spark plug including: an insulator having an axial hole formed along an axis of the spark plug; a center electrode disposed in a front end side of the axial hole; and a metal shell fixed around an outer circumference of the insulator, with a front end portion of the insulator protruding frontward from a front end of the metal shell. The front end portion of the insulator consists only of a first section and a second section located adjacent to and frontward of the first section and having an inner diameter larger than that of the first section. The second section has a chamfered area formed on an inner circumferential surface thereof so as to continue to a front end of the insulator.

Abstract: The spark plug includes an insulator made from an alumina-based sintered body, and the insulator contains 90 to 98 wt % of an Al component in oxide equivalent. The insulator contains 1 to 5 wt % of an Si component, 0.1 to 1 wt % of an Mg component, 2 wt % or less of a Ca component, 0.3 to 6 wt % of a Ba component, and 0.11 to 5 wt % of a rare earth component, in oxide equivalent. In analysis using a scanning transmission electron microscope with a probe diameter of an electron beam set at 1 nm, Si and a rare earth element are detected at a crystal grain boundary having a thickness of 15 nm or less, and an alkaline earth metal at the crystal grain boundary is less than a detection limit.

Abstract: The spark plug includes an insulator formed of an alumina-based sintered body, and the insulator contains Si, B a, and a rare earth element. In an analysis of the insulator by using a scanning transmission electron microscope having an electron-probe diameter of 1 nm, Si and a rare earth element are detected at a crystal grain boundary having a thickness of 15 nm or less, a content of an alkaline earth metal is less than a detection limit at the crystal grain boundary, and a diffraction spot is present in an electron diffraction pattern of a portion where Ba is detected.