In 1955, Taylor of the United States published the first report on sintering boron nitride by hot pressing. Since the 1960s and 1970s, it has been put into industrial production abroad. Domestically, Professor Li Minchao of Fuzhou University has systematically studied the process of hot-pressed and sintered boron nitride ceramics, and analyzed from the perspectives of crystal morphology, thermodynamics and kinetics in the hot-press sintering process. It is believed that the densification of hot-press sintering is mainly due to the effects of plastic flow and atomic diffusion. Boron powder properties and additives are tested and tested to reveal their influence on sintering properties. However, Chen Guangle and others further studied the compactness of high-purity h-BN prepared by hot pressing and sintering, and obtained the following conclusions: ① Under the same hot pressing conditions, as the purity of the material increases, the bending strength tends to decrease, but the densification The degree does not change much; ② The degree of densification of h-BN ceramics can be promoted under higher pressure. Hexagonal boron nitride is difficult to sinter hexagons through covalent bonds. In order to synthesize hexagonal boron nitride ceramics at a lower temperature and improve the sinterability of densification, additives are usually added in the thermal sintering process. Boron trioxide, sintered aluminum oxide, yttrium oxide, silicon nitride, calcium carbonate, calcium fluoride.
For example, Liu Zhiguo uses a small amount of boron nitride powder as an additive, and the hexagonal boron nitride sintered body is obtained by hot pressing. The boron trioxide may not have a great impact on the performance of the product. When there is boron trioxide, the product is easy to be dense, but the stability to water is poor; when there is no boron trioxide, the opposite is true. Therefore, the raw materials used for the corresponding bonding agent that are added in a high temperature treatment or a special atmosphere should be suitable. Ye Naiqing et al. added the second phase of aluminum oxide and yttrium oxide to the material and found that aluminum oxide can react with yttrium oxide to form yttrium aluminum oxides such as Al5Y3O12 and YAlO3, and yttrium aluminum oxide can promote particles under high temperature conditions. Rearrangement and diffusion and migration of substances promote the densification of ceramics. Although additives can improve the degree of densification, it was later discovered that it has a certain negative impact on the performance, so someone improved the synthesis process. In the absence of a sintering aid, hot isostatic pressing was performed in an argon atmosphere to prepare a high-purity and high-density hexagonal boron nitride ceramic with a density of 2.21g/cm3, but its application is limited by the high cost of this method. Because a single bn material is too "soft", even if a sintering aid is added, the performance of bn cannot be fully utilized. The current research on h-bn ceramics is mainly to add a second phase, and use bn and other materials to prepare composite ceramics to obtain better overall performance. High composite ceramics have a wide range of applications.
