UHTCs all exhibit strong covalent bonding which gives them structural stability at high temperatures. Metal carbides are brittle due to the strong bonds that exist between carbon atoms. The largest class of carbides, including Hf, Zr, Ti and Ta carbides have high melting points due to covalent carbon networks although carbon vacancies often exist in these materials; indeed, HfC has one of the highest melting points of any material. Nitrides such as ZrN and HfN have similarly strong covalent bonds but their refractory nature makes them especially difficult to synthesize and process. The stoichiometric nitrogen content can be varied in these complexes based on the synthetic technique utilized; different nitrogen content will give different properties to the material, such as how if x exceeds 1.2 in ZrNx, a new optically transparent and electrically insulating phase appears to form. Ceramic borides such as HfB2 and ZrB2 benefit from very strong bonding between boron atoms as well as strong metal to boron bonds; the hexagonal close-packed structure with alternating two-dimensional boron and metal sheets give these materials high but anisotropic strength as single crystals. Borides exhibit high thermal conductivity (on the order of 75–105 W/mK) and low coefficients of thermal expansion (5–7.8 x 10−6 K−1) and improved oxidation resistance in comparison to other classes of UHTCs. Thermal expansion, thermal conductivity and other data are shown in Table 2. The crystal structures, lattice parameters, densities, and melting points of different UHTCs are shown in Table 1.
