Ngk Spark Plug Company, Limited

Nobuyoshi Araki

Kuniharu Tanaka

Yutaka Yokoyama

Haruki Yoshida

Jumpei Isasa

Toshiki Kon

Yusuke Nomura

Hirokazu Kurono

Hironori Uegaki

Toshimasa Saji

Toshitaka Honda

Katsuya Takaoka

Shun Kondo

A spark plug according to one embodiment of the present invention includes an insulator formed of an alumina-based sintered body, wherein the insulator contains 90 wt % or more of an aluminum component in terms of oxide, and wherein crystal grains of the insulator has an average grain size of 1.5 mm or smaller and a grain size standard deviation of 1.2 &#x3bc;m or smaller.

Spark plug

An alumina-based sintered body includes 92 mass % to 96 mass % of Al in terms of an oxide, and at least three elements selected from Group II elements, one of the at least three elements being 1.90 mass % or more of Ba in terms of an oxide. The alumina-based sintered body includes the following phases in a grain boundary phase present between alumina crystal grains: a first crystal phase containing Si and at least one of the Group II elements, and a second crystal phase containing Al and at least one of the Group II elements. In X-ray diffraction of the alumina-based sintered body, the maximum relative intensity of the first crystal phase and the maximum relative intensity of the second crystal phase are both 2 or above relative to the maximum diffraction intensity of the alumina crystal.

A spark plug includes an insulator, a center electrode, a metallic shell, and a ground electrode. The insulator contains not less than 90.0 mass % and not greater than 98.1 mass % of AlOwith respect to a total mass of elements, as reduced to oxides, contained in the insulator, and has a porosity not greater than 5%. Assuming that a porosity of the insulator after a heat treatment is B, a difference (B&#x2212;A) of the porosities before and after the heat treatment is not greater than 3.5%. In the heat treatment, the insulator is placed in a furnace, and a temperature in the furnace is increased from a room temperature to 1400&#xb0; C. at a temperature increase rate of 7&#xb0; C./min and is held at 1400&#xb0; C. for 30 minutes, and thereafter, the temperature is decreased from 1400&#xb0; C. to 400&#xb0; C. at a temperature decrease rate of 10&#xb0; C. or less per 5 minutes and is decreased to the room temperature.

A spark plug includes an insulator made of an alumina sintered body containing AlOas a principal component and additional components including; an Si component, a Ba component, an Mg component, a Ca component, an Sr component, and a rare earth element component. When the additional components are expressed as oxides including R, R, R, R, R, and R, contents (mass %) of the sub-component satisfy expressions (1) to (6) as follows: (1) 1.0&#x2266;R&#x2266;5.0; (2) 0.5&#x2266;R&#x2266;5.0; (3) 0&#x2266;R&#x2266;0.18; (4) 0&#x2266;R/R&#x2266;0.36; (5) 0.3&#x2266;(R+R+R)&#x2266;1.8; and (6) 0&#x2266;R&#x2266;0.1.

A spark plug including an insulator containing not less than 92 mass % and not greater than 96 mass % of Al component in terms of oxide, wherein the insulator is formed from an alumina sintered body comprising alumina crystal and a grain boundary phase present between crystal grains of the alumina crystal. Assuming that mass contents of an Si component, an Mg component, a Ba component, and a Ca component in terms of oxide are represented by M, M, M, and M, respectively, and a sum of M, M, M, and Mis represented by Mt, the grain boundary phase contains these components so as to satisfy conditions (1) to (4) as follows: (1) 0.17&#x2266;M/Mt&#x2266;0.47; (2) 0.005&#x2266;M/Mt&#x2266;0.07; (3) 0.29&#x2266;M/Mt&#x2266;0.77; (4) 0.03&#x2266;M/Mt&#x2266;0.19.

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Nagoya, Japan

Nobuyoshi Araki