At present, the crucibles used in the sapphire single crystal furnace are iridium crucible, tungsten crucible, and molybdenum crucible. From the perspective of practical application, iridium crucible has the least pollution to sapphire, but the price is too expensive and the cost is higher; while tungsten crucible and molybdenum crucible are relatively cheaper. But the pollution is relatively large. Different crystal growth methods will use different crucibles.

Spinning crucible

The heat exchange method (HEM) generally uses thin-walled spinning molybdenum crucibles. This crystal growth method requires smashing a single crystal at the end, and relatively speaking, molybdenum crucibles are the cheapest. It is understood that the crystal growth furnace in China that adopts the heat exchange method is basically imported from GT Company of the United States and uses spinning molybdenum crucibles. The mainstream sizes of spinning molybdenum crucibles currently in use are 381 and 431.8mm, and some sapphire manufacturers have begun to try to use 508mm molybdenum crucibles.

The spinning forming method is to fix a flat or hollow blank on the mold of a spinning machine. While the blank is rotated by the main shaft of the bed, a rotating wheel or rod is used to pressurize the blank to cause local plastic deformation. Under the combined action of the feed motion of the rotary wheel and the rotating motion of the blank, the local plastic deformation is gradually extended to the entire surface of the blank and close to the mold to complete the spinning processing of the parts.

When the molybdenum crucible is prepared by the spinning forming method, the process of shear spinning of cone-shaped parts or flow spinning of cylindrical parts is adopted. First, the molybdenum slab is prepared by powder metallurgy. The molybdenum slab is hot-rolled into a thin plate of the design thickness through a hot rolling process, and the thin plate is processed into a disc shape, and then placed on a hot spinning machine (the molybdenum plate and the mold are kept concentric ), use a mixed flame of hydrogen and compressed air to heat during spinning, the heating temperature is about 900 °C, and the thickness reduction of each pass is controlled at about 0.5 mm. In order to prevent cracking during spinning, when the processing rate reaches 50%~70%, the annealing treatment is generally adopted 800~920 °C hydrogen gas heat preservation for 1 h to eliminate internal stress and increase plasticity for further spinning. After many times of spinning, the shape of the molybdenum plate becomes a crucible shape, the wall thickness of the molybdenum plate is reduced and it is close to the mold, and the spinning molybdenum crucible is obtained after cooling.

Austrian Plansee company has successfully produced molybdenum crucibles by spinning process, and they have been put on the market. Domestic research on the production of tungsten and molybdenum products by spinning process began in the 1980s, and good results have been achieved in small batches of experimental production. There is still a lot of room for development in this field in the future.