Rhenium diboride (ReB₂) is a synthetic high-

hardness In materials science, hardness (antonym: softness) is a measure of the resistance to plastic deformation, such as an indentation (over an area) or a scratch (linear), induced mechanically either by Pressing (metalworking), pressing or abrasion ...

material that was first synthesized in 1962. The compound is formed from a mixture of rhenium, noted for its resistance to high pressure, and

boron Boron is a chemical element; it has symbol B and atomic number 5. In its crystalline form it is a brittle, dark, lustrous metalloid; in its amorphous form it is a brown powder. As the lightest element of the boron group it has three ...

, which forms short, strong

covalent bond A covalent bond is a chemical bond that involves the sharing of electrons to form electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs. The stable balance of attractive and repulsive forces between atom ...

s with rhenium. It has regained popularity in recent times in hopes of finding a material that possesses hardness comparable to that of diamond. Unlike other high-hardness synthetic materials, such as the c-BN, rhenium diboride can be synthesized at

ambient pressure The ambient pressure on an object is the pressure of the surrounding medium, such as a gas or liquid, in contact with the object. Atmosphere Within the atmosphere, the ambient pressure decreases as elevation increases. By measuring ambient atmosp ...

, potentially simplifying a

mass production Mass production, also known as mass production, series production, series manufacture, or continuous production, is the production of substantial amounts of standardized products in a constant flow, including and especially on assembly lines ...

. However, the high cost of rhenium and commercial availability of alternatives such as

polycrystalline A crystallite is a small or even microscopic crystal which forms, for example, during the cooling of many materials. Crystallites are also referred to as grains. Bacillite is a type of crystallite. It is rodlike with parallel longulites. S ...

c-BN, make a prospect of large-scale applications less likely.

Synthesis

ReB₂ can be synthesized by at least three different methods at standard atmospheric pressure: solid-state metathesis, melting in an

electric arc An electric arc (or arc discharge) is an electrical breakdown of a gas that produces a prolonged electrical discharge. The electric current, current through a normally Electrical conductance, nonconductive medium such as air produces a plasma ( ...

, 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 Magnesium chloride is an inorganic compound with the formula . It forms hydrates , where ''n'' can range from 1 to 12. These salts are colorless or white solids that are highly soluble in water. These compounds and their solutions, both of which ...

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 X-ray crystallography is the experimental science of determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to Diffraction, diffract in specific directions. By measuring th ...

Hardness

Rhenium diboride is occasionally, and controversially, cited as a "

superhard material A superhard material is a material with a hardness value exceeding 40 gigapascals (GPa) when measured by the Vickers hardness test. They are virtually incompressible solids with high electron density and high Covalent bond, bond covalency. As a res ...

" due to its high hardness level. However, tested in the asymptotic-hardness region, as recommended for hard and superhard materials, rhenium diboride demonstrates a Vickers hardness of only 30.1 ± 1.3 GPa at 4.9 N, well below the generally-accepted threshold of 40 GPa or more needed to classify it as "superhard". Another research has estimated the of full-dense ReB₂ at about 22 GPa under an applied load of 2.94 N, comparable to that of

tungsten carbide Tungsten carbide (chemical formula: ) is a carbide containing equal parts of tungsten and carbon atoms. In its most basic form, tungsten carbide is a fine gray powder, but it can be pressed and formed into shapes through sintering for use in in ...

silicon carbide Silicon carbide (SiC), also known as carborundum (), is a hard chemical compound containing silicon and carbon. A wide bandgap semiconductor, it occurs in nature as the extremely rare mineral moissanite, but has been mass-produced as a powder a ...

, titanium diboride or zirconium diboride. Values greater than 40 GPa have been observed only in tests with very low loads, which is not a suitable testing method for this type of solids. In one test, the lowest tested load of 0.49 N yielded the ''average'' hardness of 48 ± 5.6 GPa and a ''maximum'' hardness of 55.5 GPa, which is comparable to the hardness of

cubic boron nitride Boron nitride is a thermally and chemically resistant refractory compound of boron and nitrogen with the chemical formula B N. It exists in various crystalline forms that are isoelectronic to a similarly structured carbon lattice. The hexago ...

(c-BN) under an equivalent load. Such phenomenon of inverse relationship between the applied load and hardness is known as the indentation size effect. In recent times, there has been a significant amount of research into improving the hardness and other properties of the ReB₂. In one study, the hardness for the ReB₂(R-3m) polymorph was estimated at 41.7 GPa, while for the ReB₂(P6₃/mmc) it was placed at c.a. 40.6 GPa. 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 Anisotropy () is the structural property of non-uniformity in different directions, as opposed to isotropy. An anisotropic object or pattern has properties that differ according to direction of measurement. For example, many materials exhibit ve ...

because of its hexagonal layered

structure A structure is an arrangement and organization of interrelated elements in a material object or system, or the object or system so organized. Material structures include man-made objects such as buildings and machines and natural objects such as ...

, being greatest along the ''c'' axis. Two factors contribute to the high hardness of ReB₂: a high density of

valence electron In chemistry and physics, valence electrons are electrons in the outermost shell of an atom, and that can participate in the formation of a chemical bond if the outermost shell is not closed. In a single covalent bond, a shared pair forms with b ...

s, and an abundance of short

s. Rhenium has one of the highest valence electron densities of any transition metal (476 electrons/nm³, compare to 572 electrons/nm³ for

osmium Osmium () is a chemical element; it has Symbol (chemistry), symbol Os and atomic number 76. It is a hard, brittle, bluish-white transition metal in the platinum group that is found as a Abundance of elements in Earth's crust, trace element in a ...

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.

References

{{Borides Borides Rhenium compounds Superhard materials Substances discovered in the 1960s

Synthesis

Hardness

See also

References