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The first application of hexagonal Boron Nitride, or hBN, was as a lubricant for high temperatures. hBN has a similar structure to graphite and performs similarly. Because it’s white in color, it is sometimes called “white graphite”.There are many different crystal forms of boron, but the hexagonal form is most common. Other crystal forms of Boron Nitride exist, in addition to hexagonal boronnitride. These include: rhombohedral, cubic, and wurtzite boronnitride. 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 600-700 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.
Several methods of preparation for 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 easy to use and the process simple, so this technology has developed rapidly. There are many vapor deposition techniques. In the past, this term was used to describe primarily 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.
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 volatilization. This method is used at low temperatures to synthesize cubic Boron Nitride.
3.Benzene Thermal Synthesis
The benzene thermochemical synthesis, which is a new method for synthesis of nanomaterials at low temperatures that emerged in the last few years, has been widely praised. The conjugated structure of benzene makes it an ideal solvent for solvothermal syntheses. It has been successfully developed in recent years into benzene thermo synthesis technology.
The reaction temperature for this technology is 450degC. It can create a metastable phase, which can only exist at ultra-high pressure and under extremely low temperatures and pressures. This method is able to produce cubic boron at low temperatures and 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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