BN fiber has the characteristics of ultra-high temperature resistance, excellent mechanical properties at high temperature and neutron absorption. Its oxidation resistance temperature is higher than that of carbon fiber and boron fiber. BN fiber can be used in oxidation atmosphere below 900 ℃ and inert atmosphere below 2800 ℃ for a long time. It has been widely concerned in the application of ultra-high temperature resistant materials such as hypersonic missiles. The preparation methods of BN fiber mainly include inorganic precursor method and organic precursor method.

Preparation of BN fiber by inorganic precursor method
In the United States, emery company first started the research on the preparation of BN fiber by inorganic precursor method, and successfully prepared BN fiber with B2O3 fiber as the precursor. After that, the former Soviet Union, Japan, China and so on have carried out BN fiber research, and there are a lot of reports. In 1967, the American economy reported that B2O3 precursor fibers were prepared from boric acid. The fibers were transformed into BN fibers at high temperature in NH3 (over 1000 ℃) and N2 (less than 2000 ℃), and h-BN fibers with a diameter of 10 μ m, a strength of about 830mpa and directional microcrystalline were prepared by this inorganic transformation method, After that, BN fiber with high modulus (up to 210 GPA) was developed. Xiao Yongdong of Shandong Research and Design Institute of industrial ceramics began to study this technology in 1976, and made BN fiber with good performance index and continuous BN fiber products.

Preparation of BN fiber by organic precursor method
Inorganic precursor method is simple and easy to operate, but there are many disadvantages, such as the BN fiber has crystal orientation difficulties, B2O3 fiber as a precursor is very easy to absorb moisture, easy to lead to product defects, so the properties of the prepared fibers are generally low. Therefore, the preparation of BN fiber by organic precursor conversion has become a new method to improve the properties of the final prepared fiber. The organic precursor method not only makes up for the shortcomings of the inorganic precursor method, but also produces products with good processing performance, and the mechanical properties and thermal properties of the fiber are stable, so this method has attracted great attention in recent decades Taniguchi, a Japanese scholar, was the first to use organic precursor method to prepare BN fiber. He carried out a series of researches on the preparation of BN fiber by organic precursor method, and successfully prepared BN fiber with better mechanical properties by pyrolysis using n-phenyl-b-aminoborazine as precursor, but the disadvantage is that its results can not be repeated. Since then, Wade has developed and prepared a fusible spinning BN precursor. Miele group in France used BCl3 and NH4Cl as raw materials to react at low temperature to obtain cyclic trichlorobronzane, and then reacted with dimethylamine to prepare organic precursor 2,4-bis (monomethylamine) - 6-monomethylamine borane monomer. BN fiber with good comprehensive properties was successfully prepared by melt spinning and high temperature nitriding treatment of organic precursor in inert atmosphere. The diameter of BN fiber is 7.5 μ m, the tensile strength is over 1400MPa, and the elastic modulus is up to 400gpa.