| 摘要 |
PROBLEM TO BE SOLVED: To make the cloudiness or discoloring scarcely occur and provide a part and an apparatus excellent in optical characteristics using a fluoride crystal heated in a heat-treating furnace in which the interior is an inert gas under atmospheric pressure or an applied pressure or a fluorine-based gas atmosphere by heating the fluoride crystal. SOLUTION: A fluoride crystal 100 is housed in a crucible 5 of a heat-treating apparatus and a solid scavenger, as necessary, is placed therein. A heat-treating furnace 1 is hermetically sealed and evacuated to about 1.33×103 Pa and a heater 2 is energized. An inert gas, fluorine gas or a mixed gas thereof is introduced and the pressure in the heat-treating furnace 1 is kept in a state of atmospheric pressure or a pressurized state. The crucible 5 is heated at a high temperature so as to regulate the crucible 5 to a prescribed temperature or above and kept at the heat-treating temperature for a prescribed time. The quantity of the energization for the heater 2 is then gradually reduced to naturally cool the crucible 5. When the temperature is reduced to room temperature, the crucible 5 is lowered to take out the fluoride crystal 100. In the case of calcium fluoride, the preferred heat treating temperature is >=1,000 deg.C and the melting point of the calcium fluoride or below. |
