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Brief introduction of Ultra-high temperature borides

 Ultra-high temperature borides mainly include hafnium boride (HfB2), zirconium boride (ZrB2), tantalum boride (TaB2) and titanium boride (TiB2), etc. Currently, zirconium boride (ZrB2) and hafnium boride (HfB2) research is the most concentrated. Boride ultra-high temperature ceramics (UHTCs) are composed of strong covalent bonds and have the characteristics of high melting point, high hardness, high strength, low evaporation rate, high thermal conductivity and electrical conductivity. It has the disadvantage of being difficult to sinter and densify. In order to improve its sintering performance and increase the density, it can be solved by increasing the surface energy of the reactants, reducing the grain boundary energy of the product, increasing the bulk diffusivity of the material, accelerating the transfer rate of the material, and improving the mass transfer kinetics.

Single-phase zirconium boride (ZrB2) and hafnium boride (HfB2) have good oxidation resistance below 1200 ℃, which is because the liquid boron oxide (B2O3) glass phase is formed on the surface, which plays a good role in oxidation protection. For example, in the oxidation process of zirconium boride (ZrB2), zirconium boride (ZrB2) is oxidized to form zirconium oxide (ZrO2) and boron oxide (B2O3), forming an anti-oxidation protective layer, preventing the oxidation of zirconium boride (ZrB2), When the temperature exceeds the melting point (450 °C) of boron oxide (B2O3), boron oxide (B2O3) slowly evaporates. The higher the temperature, the greater the evaporation rate of boron oxide (B2O3), and the lower its role as an oxygen diffusion barrier. , resulting in a decrease in the antioxidant properties of borides.

Properties of common carbides and borides