Boron nitride and some of its preparation methods

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Boron nitride The first application of hexagonal Boron Nitride, or hBN, was as a lubricant for high temperatures. The structure of hBN is very similar to graphite. Because it is white and has a similar performance, it is known as white graphite.The hexagonal structure of boron is amorphous, but there are also graphite-like variants. Other crystal forms of Boron Nitride exist, in addition to hexagonal crystals. These include: rhombohedral Boron Nitride (abbreviation, r—- BN), Cubic Boron Nitride (abbreviation, c-BN), Wurtzite Type Boron Nitride. There are even two-dimensional boron crystals that look like graphene.
Wentorf created cubic BN first in 1957. Pure hexagonal boron nitride can be directly converted into cubic boron nitride when the temperature is near or above 1700degC, and the pressure is between 11-12 GPa. The use of a catalyst can reduce both the transition pressure and temperature. The most common catalysts include alkali metals and alkaline nitrides. Ammonium borate is the least expensive catalyst, but also requires the lowest pressure and temperature. At 1500, the required pressure is 5GPa and at 600700 degrees Fahrenheit the pressure is 6GPa. Although the addition of catalysts can significantly reduce the temperature and pressure required for the transition, they are still quite high. The preparation equipment for this catalyst is complex and expensive, and the industrial applications are limited.

Various preparation methods of Boron Nitride
1.Chemical vapor synthesis
Sokolowski was the first to use pulsed-plasma technology in 1979 for the preparation of cubic boron (CBN), at low temperature, and under low pressure. Equipment is simple, and the process can be easily realized. This has led to rapid development. There are many vapor deposition techniques. In the past, the term was used to describe mainly thermal chemical vapour deposition. The experimental device generally consists of heat-resistant quartz tubes and heating devices. The substrate may be heated using a hot-wall CVD furnace or by high-frequency electromagnetic induction (cold-wallCVD). The reaction gases decompose on the surface the substrate at high temperatures, while a chemical reactions occurs to deposit film. The reaction is a gas mixture of BCl3 and B2H4 or NH3.
Hydrothermal synthesis
Water is used as a reaction medium to dissolve insoluble and insoluble substances in an autoclave environment of high temperature and high pressure. The reaction can be recrystallized. It is also carried out within a sealed container, which prevents the components from escaping. This method is used as a low-temperature, low-pressure synthesis to produce cubic boron oxide at low temperatures.
3.Benzene Thermal Synthesis
As a low temperature nanomaterial synthesis technique that has been developed in recent times, the benzene thermochemical synthesis has attracted widespread attention. The conjugated structure of benzene makes it an ideal solvent for solvothermal syntheses. It has been successfully adapted to benzene thermo synthesis, including the reaction formula.
The reaction temperature for this technology is only 450degC. A metastable phase can be created only in extreme conditions of low pressure and temperature, and it can only exist at ultra-high pressure. This method is able to produce cubic boron at low temperature under low pressure. The method is still under experimental research and has great application potential.
4. Self-propagating technology
The energy needed from the outside is used for high exothermic reactions. The system then reacts locally, forming a chemical reaction wave (combustion front). The chemical reactions are accelerated with its own heat and spread across the entire system. It is an inorganic method that has been around for a long time. However, this particular method was used to synthesize boron oxide only in the last few years.

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