Boron nitride, chemical formula BN, a non-oxide material, is the isoelectronic body of carbon (C2), its crystal structure is similar to graphite, is currently one of the more studied and applied nitride ceramic materials. According to the crystal structure type, BN mainly includes hexagonal boron nitride (H-BN), cubic boron nitride (C-BN), rhombic boron nitride (R-BN), wurtzite boron nitride (W-BN).
What are the properties of boron nitride? As a structural analogue of graphite, BN has better physical and chemical properties than graphite. The properties of BN are as follows: 1, thermal properties (1) high heat resistance: the oxidation temperature in the air is up to 900℃, and the inert environment begins to decompose at 1800-2000℃; (2) High thermal conductivity: good thermal conductivity, so that BN becomes one of the best thermal conductivity materials in ceramic materials; (3) Low thermal expansion coefficient: the expansion coefficient is 10-6, second only to quartz, which is the smallest among ceramics. 2, electrical properties (1) high temperature insulation; Resistance at 2000℃ is 104 ω /cm, the best high temperature insulating material in ceramics, (2) good dielectric properties: dielectric constant is 4, can penetrate microwave, commonly used as the outer protective layer of radar antenna. 3, chemical stability (1) BN does not react with most oxidants, inorganic acids/bases, showing chemical inertia, showing good corrosion resistance; (2) Neither chemical reaction nor wetting occurs with most metals.
1. Isotropic BN nanomaterials isotropic BN nanomaterials mainly refer to solid and hollow nanospheres with central point symmetry and similar nanostructures. Figure 2 shows the microstructure of BN nanospheres. As early as 1990, there was a patent claiming that using BCl3 and ammonia as raw materials, using CVD method to prepare spherical BN particles at low temperature. BN hollow nanospheres were synthesized by solvothermal method using Zn and KBH4/NH4BF4 as raw materials, but it is obviously a dangerous synthesis route.
Since the preparation of Carbon Nanotube (CNT), researchers have focused on the preparation, performance, structure and application of Boron Nitride Nanotube (BNNT). See Figure 3. The structure of boron nitrode nanotubes is similar to that of carbon nanotubes. B atoms and N atoms replace C atoms in graphite sheets alternately to form nanotube-like structures, which can be divided into single-walled tubes and multi-walled tubes. Boron nitride nanotubes come in three structural types: armchair, serrated and chiral. The commonly used preparation methods mainly include arc discharge, laser burning, ball milling, plasma injection, carbon nanotubes replacement, chemical vapor deposition and so on
(2) Boron Nitride nanowire (BNNW) — usually produced in the process of preparation of Boron Nitride nanotubes. In the early 1970s, boron nitride nanowire was synthesized from ammonia and boron oxide, but the quality and ammoniation process of precursor nanowire were quite difficult to control. Although boron nitride nanowires have better oxidation resistance and dielectric properties than carbon fiber, compared with carbon fiber, research on boron nitride nanowires is much less. Currently, research is also underway on a polymer precursor that can be used to synthesize boron nitride nanowires. The precursor is a borazane. (3) The structure of two-dimensional hexagonal boron nitride nanosheets is similar to that of graphite, which can be regarded as the substitution of B and N atoms for C atoms in graphite. FIG. 4 shows the structure comparison of graphene and monolayer hexagonal boron nitride. Monolayer boron nitride nanosheets have only been paid attention in recent years, but compared with graphene, which has a long history of research, monolayer boron nitride is still in its infancy. In the graphite structure, each carbon layer is stacked together by relatively weak van der Waals force, while in the multilayer BORON nitride sheet, there is a force between each BN layer due to the polarization between the B-N bond. This is one of the reasons why monolayer boron nitride nanosheets are difficult to obtain, while multilayer boron nitride nanosheets are conducive to the stability of the overall structure.
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