What is CVD

                                                                                The knowledge about CVD

This technology was originally developed as a means of coating, but it is not only applied to the coating of heat-resistant substances, but also applied to the refining of high-purity metals, powder synthesis, semiconductor films, etc., which is a quite characteristic technical field.
Its technical features are: ⑴ High melting point materials can be synthesized at low temperature; ⑵ The forms of precipitated substances are single crystal, polycrystalline, whisker, powder, film, etc.; ⑶ It can not only be coated on the substrate, but also on the powder surface. Especially at low temperature, it can synthesize materials with high melting point, which has made contributions to energy saving. As a new technology, it has great prospects.

For example, a-Al2O3 and SiC can be synthesized at about 1000 ℃ and are developing towards lower temperature.
CVD process can be divided into two types: one is to make metal halides react with compounds containing carbon, nitrogen, boron, etc. in gas phase; The other is to make the raw gas heating the surface of the substrate undergo thermal decomposition.
The device of CVD is composed of gasification part, carrier gas refining part, reaction part and exhaust gas treatment part. New devices for mass production are being developed.

CVD is carried out in the gas phase of multi-component system including raw gas, by-product gas generated by reaction, carrier gas, etc. Therefore, when coating, a diffusion layer is formed on the boundary between the heated substrate and the fluid. The existence of this layer has a great impact on the density of the coating. Figure 2 shows the schematic diagram of this diffusion layer. In this way, although the diffusion layer formed by many chemical molecules exists, its precipitation process is complex. During powder synthesis, the control of nucleus formation and growth is the key point of the process.