​Growing sapphire crystals from the gas phase (vapor)

Growing sapphire crystals from the gas phase (vapor)

Bulk, disc-shaped crystals, whiskers and epitaxially grown thin films can be grown from the gas phase. Some methods are particularly specialized. For example, molecular beam sputtering and deposition from vacuum are used to obtain thin films, while larger The crystals are mainly grown by crystallization in sealed ampoules. Other methods have wider fields of application. In particular, the method of chemical deposition in airflow is suitable for obtaining thin films, larger crystals and whiskers.

Crystallization from the gas phase has many advantages, such as low process temperature, no need for oversaturation, easy composition control, weak interaction between the vessel and the material to be crystallized, and high crystal perfection. However, the size of the grown crystals is not Big. 

The methods of crystallization from the gas (vapor) phase belong to the category of physics and chemistry. Some of the previous methods, based on the effect of concentration, include the crystallization of substances from their own vapor, also called "sublimation" and "molecular beam methods". From In-vapor crystallization is particularly suitable for substances that cannot achieve a solid-gas phase transition through the liquid phase. In closed and open systems and in vacuum or gas-phase media, sublimation is used for crystal growth, and the crystallization process is often carried out in sealed ampoules. Both halves of the ampoule are heated to the same temperature, and a saturated vapor pressure is set corresponding to that temperature. One end of the ampoule is then cooled until nuclei form. The temperature is then raised again to prevent the formation of new nuclei, and the crystal grows The process is carried out under constant conditions.

In crystal growth by chemical reaction methods (reduction, thermal decomposition, oxidation, etc.), the gas phase composition is different from that of the growing crystal. The basic principle of chemical transfer method is as follows: When interacting with gas phase input by reverse reaction, the solid phase or The liquid-phase species forms gas-phase products. After the gas-phase products are transferred to another part of the system, when the equilibrium conditions are changed, the gas-phase products decompose into subsequent precipitates of crystalline species. For the estimation of the reaction transfer characteristics, the most important factor is the crystalline species The partial pressure difference (ᇞP) between the initial state and the second phase. Chemical transfer in a closed system or gas flow occurs by convection and diffusion. An open system with a pressurized hydrodynamic scheme is more efficient. In this case , if the flow rate is optimized, the transfer is not the growth rate controlling stage. The commonality of these methods is the transfer of material from the source to the crystallization site due to the low concentration of the material to be crystallized in the medium.

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