化学气相沉积（CVD）Chemical vapor deposition (CVD)

化学气相沉积（CVD）

CVD是Chemical Vapor Deposition的简称，是指高温下的气相反应，例如，金属卤化物、有机金属、碳氢化合物等的热分解，氢还原或使它的混合气体在高温下发生化学反应以析出金属、氧化物、碳化物等无机材料的方法。这种技术最初是作为涂层的手段而开发的，但目前，不只应用于耐热物质的涂层，而且应用于高纯度金属的精制、粉末合成、半导体薄膜等，是一个颇具特征的技术领域。

其技术特征在于：

1、高熔点物质能够在低温下合成；

2、析出物质的形态在单晶、多晶、晶须、粉末、薄膜等多种；

3、不仅可以在基片上进行涂层，而且可以在粉体表面涂层，等。特别是在低温下可以合成高熔点物质，在节能方面做出了贡献，作为一种新技术是大有前途的。

例如，在1000℃左右可以合成a-Al2O3、SiC，而且正向更低温度发展。

CVD工艺大体分为二种：一种是使金属卤化物与含碳、氮、硼等的化合物进xing 气相反应；另一种是使加热基体表面的原料气体发生热分解。

CVD的装置，由气化部分、载气精练部分、反应部分和排除气体处理部分所构成。目前，正在开发批量生产的新装置。

CVD是在含有原料气体、通过反应产生的副生气体、载气等多成分系气相中进行的，因而，当被覆涂层时，在加热基体与流体的边界上形成扩散层，该层的存在，对于涂层的致密度有很大影响。这样，由许多化学分子形成的扩散层虽然存在，但其析出过程是复杂的。粉体合成时，核的生成与成长的控制是工艺的重点。

作为新的CVD技术，有以下几种：

（1）采用流动层的CVD；

（2）流体床；

（3）热解射流；

（4）等离子体CVD；

（5）真空CVD，等。

应用流动层的CVD，可以形成被覆粒子（例如，在UO2表面被覆SiC、C），应用等离子体的CVD同样也有可能在低温下析出，而且这种可能性正在进一步扩大。

CVD is the abbreviation of chemical vapor deposition, which refers to the gas-phase reaction at high temperature, such as the thermal decomposition of metal halides, organic metals, hydrocarbons, etc., hydrogen reduction or chemical reaction of its mixture gas at high temperature to precipitate metal, oxide, carbide and other inorganic materials. This technology was originally developed as a means of coating, but at present, it is not only applied to the coating of heat-resistant materials, but also used in the refining of high-purity metals, powder synthesis, semiconductor films, etc., which is a characteristic technology field.

Its technical features are as follows: 

1. High melting point materials can be synthesized at low temperature;

2. The forms of precipitated substances are in single crystal, polycrystalline, whisker, powder, thin film, etc.;

3. Coating can not only be carried out on the substrate, but also on the surface of powder, etc. In particular, it can synthesize high melting point materials at low temperature, which has made contribution to energy saving and has great prospects as a new technology.

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 and boron in the Xing gas phase; the other is to decompose the raw gas heated on the substrate surface.

The CVD unit is composed of gasification part, carrier gas refining part, reaction part and exhaust gas treatment part. At present, new devices for mass production are being developed.

CVD is carried out in the gas phase containing raw gas, secondary gas produced by reaction, carrier gas and so on. Therefore, when the coating is coated, a diffusion layer is formed at the boundary between the heated substrate and the fluid. The existence of this layer has a great influence on the density of the coating. In this way, although the diffusion layer formed by many chemical molecules exists, the precipitation process is complex. In the process of powder synthesis, the control of nuclear formation and growth is the key point.

As new CVD technologies, there are the following: 

(1) CVD with flow layer; 

(2) fluid bed;

(3) pyrolysis jet;

(4) plasma CVD; 

(5) vacuum CVD.

CVD with flowing layer can form coated particles (for example, SiC and C coated on the surface of UO2). CVD with plasma may also precipitate at low temperature, and this possibility is further expanded.