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Additional recommended knowledge
The production method of this material does not involve high pressures as with other hard synthetic materials, such as cubic boron nitride, which makes production cheap.
The compound is formed from a mixture of rhenium, noted for its resistance to high pressure, and boron, which forms short, strong covalent bonds with rhenium. The resulting material is hard enough to scratch diamond.
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 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 (80 amperes) 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 for a long time (1000 °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.
Two factors contribute to ReB2's great hardness: 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 (0.476 electrons/Å3, compare to 0.572 electrons/Å3 for osmium and 0.705 electrons/Å3 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.
Although ReB2 is harder than diamond along certain directions, it exhibits considerable anisotropy because of its layered structure.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Rhenium_diboride". A list of authors is available in Wikipedia.|