Zirconium diboride is a highly covalent refractory ceramic material with a hexagonal crystal structure. ZrB₂ is an ultra high temperature ceramic with a melting point of 3246 °C. This along with its relatively low density of ~6.09 g/cm³ and good high temperature strength makes it a candidate for high temperature aerospace applications such as hypersonic flight or rocket propulsion systems. It is an unusual ceramic, having relatively high thermal and electrical conductivities, properties it shares with isostructural titanium diboride and hafnium diboride. ZrB₂ parts are usually hot pressed and then machined to shape. Sintering of ZrB₂ is hindered by the material's covalent nature and presence of surface oxides which increase grain coarsening before densification during sintering. Pressureless sintering of ZrB₂ is possible with sintering additives such as boron carbide and carbon which react with the surface oxides to increase the driving force for sintering but mechanical properties are degraded compared to hot pressed ZrB₂