High temperature and high-pressure synthesis

In 1957, Wentorf synthesized cubic BN for the first time. When the temperature is close to or higher than 1700 ℃ and the minimum pressure is 11-12gpa, pure hexagonal boron nitride (hBN) is directly transformed into cubic boron nitride (CBN). Subsequently, it was found that the use of catalysts can significantly reduce the transition temperature and pressure. The commonly used catalysts are alkali and alkaline earth metal, alkali and alkaline earth nitride, alkaline earth Fluor nitride, ammonium borate, and inorganic fluoride. The temperature and pressure required for ammonium borate as catalysts are the lowest, which is 5 GPa at 1500 ℃ and 600-700 ℃ at 6 GPa. It can be seen that although adding catalysts can greatly reduce the transition temperature and pressure, but the required temperature and pressure are still high. Therefore, the preparation equipment is complex and the cost is high, so its industrial application is limited.

Chemical vapor synthesis

In 1979, Sokolowski successfully prepared cubic boron nitride (CBN) films by pulsed plasma technology at low temperature and low pressure. The equipment used is simple and the process is easy to realize, so it has been developed rapidly. Many methods of vapor deposition have been developed. Traditionally, it mainly refers to thermochemical vapor deposition. The experimental device is generally composed of a heat-resistant quartz tube and heating device. The substrate can be heated by the heating furnace (hot-wall CVD) or high-frequency induction heating (cold wall CVD). The reaction gas decomposes on the surface of the high-temperature matrix, and the film is deposited by chemical reaction. The reaction gas includes a mixture of BCl3 or b2h4 and NH3.

Hydrothermal synthesis

In this method, water is used as the reaction medium in the high temperature and high-pressure reaction environment in the autoclave, so that the usually insoluble or insoluble substances are dissolved, and the reaction can be recrystallized. Hydrothermal technology has two characteristics: one is its relatively low temperature; the other is that it is carried out in a closed container to avoid component volatilization. Cubic nitriding is used as a low-temperature synthesis method.

Benzene thermal synthesis

As a new synthesis method of low-temperature nanomaterials, benzene thermal synthesis has been widely concerned. Due to its stable conjugated structure, benzene is a good solvent for solvothermal synthesis

BCl3+Li3N→BN+3LiCl

Or BBr3 + Li3N → BN + 3LiBr

The reaction temperature is only 450 ℃. The metastable phase, which can be prepared under extreme conditions and exists under ultra-high pressure, can be prepared at relatively low temperature and pressure. In this way, cubic boron nitride was prepared at low temperature and low pressure. However, this method is still in the experimental research stage, and it is a promising synthetic method.

Carbothermal synthesis technology

This method is based on the surface of silicon carbide, boric acid as raw material, carbon as a reducing agent, ammonia nitriding method to obtain boron nitride, the product purity is very high, has great application value for the preparation of composite materials.