A Common Method for Growing Sapphire Crystals—Czochralski Method

The pulling method, also known as the Czochralski method, was invented by J. Czochralski as a method of pulling high quality single crystals from the melt. This method has been demonstrated to be effective in the growth of a number of important gem crystals, including colorless sapphire, ruby, yttrium aluminum garnet, gadolinium gallium garnet, alexandrite, spinel, etc.. In the 1960s, the pulling method was further developed into a more advanced method for crystal growth, known as the method of guiding mode by the melt. It represents a growth technology that enables the direct production of crystals with diverse cross-sectional shapes from the melt. This approach offers the advantage of eliminating the heavy mechanical processing of artificial crystals in industrial production. Additionally, it effectively reduces the consumption of raw materials and the production costs.

Basic principles of Czochralski method

The Czochralski method involves melting the raw materials comprising the crystal in a crucible, and then raising a seed crystal to the surface of the melt. Under controlled conditions, atomic or molecular rearrangement occurs at the interface between the seed crystal and the melt, leading to gradual solidification of a single crystal through cooling.

Synthesis devices of Czochralski method

First of all, the raw material for the crystal is placed in a high-temperature resistant crucible where it can be heated and melted. The temperature in the furnace is then adjusted so that the upper part of the melt is in a supercooled state. After that, a seed crystal is placed on the seed crystal rod, allowing it to contact the surface of the melt. After the seed crystal surface is slightly melted, the seed crystal rod is lifted and rotated, so that the melt keeps a supercooled state and crystallized on the seed crystal. In the process of continuous pulling and rotating, a cylindrical crystal is formed.

Advantages of of Czochralski method

• It can be directly tested and observedduring the crystal growth process, which is conducive to controlling the growth conditions;

• The use of high-quality oriented seed and “necking” technology can reduce crystal defects and obtain the desired orientation of the crystal;

• High speed ofcrystal growing;

(4) Low crystal dislocation density and high optical uniformity.

Disadvantages of of Czochralski method

• The material of crucible may pollute the crystal;
• The liquid flow action of the melt, the vibration of the transmission device,and the fluctuation of temperature all affect the quality of the crystal.