Boron suboxide (chemical formula B₆O) is a solid compound with a structure built of eight

icosahedra In geometry, an icosahedron ( or ) is a polyhedron with 20 faces. The name comes and . The plural can be either "icosahedra" () or "icosahedrons". There are infinitely many non- similar shapes of icosahedra, some of them being more symmetrica ...

at the apexes of the

rhombohedral In geometry, a rhombohedron (also called a rhombic hexahedron or, inaccurately, a rhomboid) is a three-dimensional figure with six faces which are rhombus, rhombi. It is a special case of a parallelepiped where all edges are the same length. It c ...

unit cell. Each icosahedron is composed of twelve

boron Boron is a chemical element with the 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 th ...

atoms. Two

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...

atoms are located in the interstices along the 11rhombohedral direction. Due to its short interatomic bond lengths and strongly covalent character, B₆O displays a range of outstanding physical and chemical properties such as great hardness (close to that of

rhenium diboride Rhenium diboride (ReB2) is a synthetic superhard material. It was first synthesized in 1962 and re-emerged recently due to hopes of achieving high hardness comparable to that of diamond. The reported ultrahigh hardness has been questioned, althou ...

and

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

), low mass density, high

thermal conductivity The thermal conductivity of a material is a measure of its ability to conduct heat. It is commonly denoted by k, \lambda, or \kappa. Heat transfer occurs at a lower rate in materials of low thermal conductivity than in materials of high thermal ...

, high chemical inertness, and excellent wear resistance. and references therein B₆O can be synthesized by reducing B₂O₃ with boron or by oxidation of boron with

zinc oxide Zinc oxide is an inorganic compound with the formula . It is a white powder that is insoluble in water. ZnO is used as an additive in numerous materials and products including cosmetics, food supplements, rubbers, plastics, ceramics, glass, cemen ...

or other oxidants. These boron suboxide materials formed at or near ambient pressure are generally oxygen deficient and non-

stoichiometric Stoichiometry refers to the relationship between the quantities of reactants and products before, during, and following chemical reactions. Stoichiometry is founded on the law of conservation of mass where the total mass of the reactants equal ...

(B₆O_x, x<0.9) and have poor crystallinity and very small grain size (less than 5 μm). High pressure applied during the synthesis of B₆O can significantly increase the crystallinity, oxygen stoichiometry, and crystal size of the products. Mixtures of boron and B₂O₃ powders were usually used as starting materials in the reported methods for B₆O synthesis. Oxygen-deficient boron suboxide (B₆O_x, x<0.9) might form

icosahedral In geometry, an icosahedron ( or ) is a polyhedron with 20 faces. The name comes and . The plural can be either "icosahedra" () or "icosahedrons". There are infinitely many non- similar shapes of icosahedra, some of them being more symmetrica ...

particles, which are neither single

crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...

s nor

quasicrystal A quasiperiodic crystal, or quasicrystal, is a structure that is ordered but not periodic. A quasicrystalline pattern can continuously fill all available space, but it lacks translational symmetry. While crystals, according to the classical cr ...

s, but twinned groups of twenty

tetrahedral In geometry, a tetrahedron (plural: tetrahedra or tetrahedrons), also known as a triangular pyramid, is a polyhedron composed of four triangular faces, six straight edges, and four vertex corners. The tetrahedron is the simplest of all the o ...

crystals. B₆O of the α-rhombohedral boron type has been investigated because of its ceramic nature (hardness, high melting point, chemical stability, and low density) as a new structural material. In addition to this, these borides have unique bonding not easily accessible by the usual valence theory. Although an

X-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nb ...

emission spectroscopic method indicated a probable parameter range for the oxygen site of B₆O, the correct oxygen position remained open to question until Rietveld analysis of X-ray diffraction profiles on B₆O powders were first carried out successfully, even though these were preliminary investigations.

Preparation

B₆O can be prepared by three methods: * (1) solid state reaction between B and B₂O₃, * (2) reduction of B₂O₃ and * (3) oxidation of B. The high vapor pressure of B₂O₃ at elevated temperatures would cause the B excess composition in the process of the solid state reaction between B and B₂O₃. In the reduction of B₂O₃, reductants that can be used include, but not limited to, Si and Mg which remain in B₆O as an impurity in the process. While in the oxidation process of B, oxidants such as ZnO would contaminate B₆O in the process.

Physical properties

B₆O has a strong

covalent 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 atoms ...

nature and is easy to compose at temperatures greater than 1,973 K. Boron suboxide has also been reported to exhibit a wide range of superior properties such as high hardness with low density, high mechanical strength, oxidation resistance up to high temperatures as well as its high chemical inertness. Preliminary first-principle

ab initio ''Ab initio'' ( ) is a Latin term meaning "from the beginning" and is derived from the Latin ''ab'' ("from") + ''initio'', ablative singular of ''initium'' ("beginning"). Etymology Circa 1600, from Latin, literally "from the beginning", from ab ...

density functional calculations of the structural properties boron suboxide (B₆O) suggest that the strength of bonding in B₆O may be enhanced by the presence of a high electronegativity interstitial in the structure. The computational calculations confirm the shortening of covalent bonds, which is believed to favor higher elastic constants and hardness values.

Applications

The potential applications of B₆O as a wear-reduction coating for high-speed cutting tools,

abrasive An abrasive is a material, often a mineral, that is used to shape or finish a workpiece through rubbing which leads to part of the workpiece being worn away by friction. While finishing a material often means polishing it to gain a smooth, reflec ...

s, or other high-wear applications, for example, have been an object of intense interest in recent years. However, despite intensive research efforts commercial applications have yet to be realized. This is partly because of the low fracture toughness of the hot-pressed material and the considerable practical challenges associated with densifying

B₆O material with good crystallinity. Furthermore, numerous mechanical properties of the material were until recently rather poorly understood. Boron suboxide is also a promising

body armor Body armor, also known as body armour, personal armor or armour, or a suit or coat of armor, is protective clothing designed to absorb or deflect physical attacks. Historically used to protect military personnel, today it is also used by variou ...

material, but its testing is still in the early stages and no commercial deployment is known as of 2019.

References

{{DEFAULTSORT:Boron Suboxide Boron compounds Oxides

Preparation

Physical properties

Applications

See also

References