| 摘要 |
PROBLEM TO BE SOLVED: To increase the infrared-ray transmission factor at a low cost by fitting an infrared-ray transmitting material made of polycrystalline alumina. SOLUTION: Polycrystalline alumina having a thickness of about 30-300μm is used for the infrared-ray transmitting material 2 of the infrared-ray detecting device. When the thickness is below 30μm, the strength of the infrared-ray transmitting material 2 is low, and the durability of the infrared-ray detecting device becomes poor. When the thickness is larger than 300μm, the infrared-ray transmission factor is reduced. The thickness of the infrared-ray transmitting material 2 is preferably set to 30-300μm, particularly about 50-200μm. The polycrystalline alumina having a thickness of 300μm or below has the infrared- ray transmission factor near that of monocrystalline alumina, and the infrared- ray transmission factor of the polycrystalline alumina having the thickness of 200μm is very high in particular. This polycrystalline alumina preferably has a purity of about 90% or above, particularly about 96% or above, as Al2 O3 .
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