| http://www.w3.org/ns/prov#value | - In the present invention, it is preferred that the thermal expansion coefficient of the material constituting the cap-like metal member and/or ring-shaped metal member at a temperature within the range of operating temperature is preferably 10???10???6/??? C. or smaller, more preferably 9???10???6/??? C. or smaller, and especially preferably 8???10???6/??? C. or smaller.
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