| http://www.w3.org/ns/prov#value | - As mentioned above, when the hollow sheath 12 is made of Si3 N4, Al2 O3 molybdenum, tungsten or an alloy composed mainly of these elements, the thermal expansion is made substantially equal among the heater 1, the hollow sheath 12 and the cementing medium 13 to prevent occurrence of cracks.
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