Rhenium diboride

ReB2 can be made by at least three different methods at standard atmospheric pressure: solid-state metathesis, melting in an electric arc, and direct heating of the elements. In the metathesis reaction, rhenium trichloride and magnesium diboride are mixed and heated in an inert atmosphere and the magnesium chloride byproduct is washed away. Excess boron is needed to prevent the formation of other phases such as Re7B3 and Re3B. In the arc-melting method, rhenium and boron powders are mixed and a large electric current is passed through the mixture, also in an inert atmosphere. In the direct reaction method, the rhenium-boron mixture is sealed in a vacuum and held at a high temperature over a longer period (1,000 °C for five days). At least the last two methods are capable of producing pure ReB2 without any other phases, as confirmed by X-ray crystallography. == Hardness ==

Rhenium diboride is occasionally, and controversially, cited as a "superhard material" due to its high hardness level. However, tested in the asymptotic-hardness region, as recommended for hard and superhard materials, In another study, a fully dense B4C-ReB2 ceramic composite nanopowder was fabricated by spark plasma sintering. It has exhibited a microhardness of 50 ± 3 GPa under a 49 N load in the asymptotic-hardness region and had a 3.2 g/cm3 density, comparable with the hardness and density of the c-BN. The hardness of ReB2 exhibits considerable anisotropy because of its hexagonal layered structure, being greatest along the c axis. Two factors contribute to the high hardness of ReB2: a high density of valence electrons, and an abundance of short covalent bonds. Rhenium has one of the highest valence electron densities of any transition metal (476 electrons/nm3, compare to 572 electrons/nm3 for osmium and 705 electrons/nm3 for diamond). The addition of boron requires only a 5% expansion of the rhenium lattice because the small boron atoms fill the existing spaces between the rhenium atoms. Furthermore, the electronegativities of rhenium and boron are close enough (1.9 and 2.04 on the Pauling scale) that they form covalent bonds in which the electrons are shared almost equally. ==See also==