Boron nitride came out more than 100 years ago, and its earliest application was hexagonal boron nitride, or h-BN, as a high-temperature lubricant. The structure and performance of h-BN are very similar to graphite, and it is white in nature, so it is commonly known as white graphite. The structure of boron nitride is hexagonal crystals, the most common is graphite lattice, and there are also amorphous variants. In addition to the hexagonal crystal form, there are other crystal forms of boron nitride, including: rhombohedral boron nitride (abbreviation: r-- BN), cubic boron nitride (abbreviation: c-BN), wurtzite type boron nitride (abbreviation: w-BN). People have even found two-dimensional boron nitride crystals like graphene (similar to MoS: two-dimensional crystals).
In 1957, Wentorf synthesized cubic BN for the first time. When the temperature is close to or higher than 1700degC and the min pressure is 11-12 GPa, pure hexagonal boron nitride (HBN) is directly transformed into cubic boron nitride (CBN). Subsequently, it was discovered that the use of a catalyst can greatly reduce the transition temperature and pressure. Commonly used catalysts are: alkali and alkaline earth metals, alkali and alkaline earth nitrides, alkaline earth fluoronitrides, ammonium borate and inorganic fluorides. Among them, the temperature and pressure required for ammonium borate as the catalyst are the lowest. The required pressure is 5GPa at 1500, and the temperature range is 600~700 when the pressure is 6GPa. It can be seen that although the addition of a catalyst can greatly reduce the transition temperature and pressure, the required temperature and pressure are still relatively high. Therefore, its preparation equipment is complicated and costly, and its industrial application is limited.

Several preparation methods of boron nitride
1.Chemical vapor synthesis
In 1979, Sokolowski successfully used pulsed plasma technology to prepare cubic boron nitride (CBN) films at low temperature and low pressure. The equipment used is simple and the process is easy to realize, so it has been developed rapidly. A variety of vapor deposition methods have emerged. Traditionally speaking, it mainly refers to thermal chemical vapor deposition. The experimental device is generally composed of a heat-resistant quartz tube and a heating device. The substrate can be heated by a heating furnace (hot-wall CVD) or high-frequency induction heating (cold-wall CVD). The reaction gas decomposes on the surface of the high-temperature substrate, and at the same time, a chemical reaction occurs to deposit a film. The reaction gas is a mixed gas of BCl3 or B2H4 and NH3.
2.Hydrothermal synthesis
This method uses water as the reaction medium in the high-temperature and high-pressure reaction environment in the autoclave to dissolve the normally insoluble or insoluble substances, and the reaction can also be recrystallized. The hydrothermal technology has two characteristics, one is its relatively low temperature, and the other is that it is carried out in a closed container to avoid the volatilization of the components. As a low-temperature and low-pressure synthesis method, it is used to synthesize cubic boron nitride at low temperatures.
3.Benzene thermal synthesis
As a low-temperature nanomaterial synthesis method that has emerged in recent years, benzene thermal synthesis has received widespread attention. Because of its stable conjugated structure, benzene is an excellent solvent for solvothermal synthesis. Recently, it has successfully developed into benzene thermal synthesis technology, such as the reaction formula: The reaction temperature is only 450degC, and the benzene thermal synthesis technology can produce a metastable phase that can only be prepared under extreme conditions at relatively low temperature and pressure and can only exist under ultra-high pressure. This method realizes the preparation of cubic boron nitride at low temperature and low pressure. However, this method is still in the experimental research stage, and it is a synthetic method with great application potential.
4.Self-propagating technology
The necessary energy from the outside is used to induce high exothermic chemical reactions, and the system reacts locally to form a chemical reaction front (combustion wave). The chemical reaction proceeds quickly with the support of its own heat release, and the combustion wave spreads across the entire system. Although this method is a traditional inorganic synthesis method, it has only been reported in recent years for the synthesis of boron nitride.

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