Boron carbide is characterized by a unique combination of properties that make it a material of choice for a wide range of engineering applications. Boron carbide is used in refractory applications due to its high melting point and thermal stability; it is used as abrasive powders and coatings due to its extreme abrasion resistance; it excels in ballistic performance due to its high hardness and low density; and it is commonly used in nuclear applications as neutron radiation absorbent. In addition, boron carbide is a high temperature semiconductor that can potentially be used for novel electronic applications. This paper provides a comprehensive review of the recent advances in understanding of structural and chemical variations in boron carbide and their influence on electronic, optical, vibrational, mechanical, and ballistic properties. Structural instability of boron carbide under high stresses associated with external loading and the nature of the resulting disordered phase are also discussed.

Boron Carbide: Structure, Properties, and Stability under Stress

Boron is an element of fascinating chemical complexity. This arises from frustration: situated between metals and insulators in the periodic table, boron has only three valence electrons that could in principle favour metallicity, yet they are sufficiently localized to give rise to an insulating state. This delicately balanced electronic structure is easily modified by pressure, temperature and impurities, making it difficult to establish boron's structure and properties. Oganov et al. have now explored the high-pressure behaviour of boron and uncovered a previously unknown ionic phase consisting of negatively charged icosahedral B12 clusters and positively charged B2 pairs. The ionicity of the new phase strongly affects many of its properties, and arises from the different electronic properties of the B12 clusters and B2 pairs and the resultant charge transfer between them. This paper has explored the high-pressure behaviour of boron and uncovered a new phase that consists of negatively charged icosahedral B12 clusters and positively charged B2 pairs. The ionicity of the new phase strongly affects many of its properties, and arises from the different electronic properties of the B12 clusters and B2 pairs and the resultant charge transfer between them. Boron is an element of fascinating chemical complexity. Controversies have shrouded this element since its discovery was announced in 1808: the new ‘element’ turned out to be a compound containing less than 60–70% of boron, and it was not until 1909 that 99% pure boron was obtained1. And although we now know of at least 16 polymorphs2, the stable phase of boron is not yet experimentally established even at ambient conditions3. Boron’s complexities arise from frustration: situated between metals and insulators in the periodic table, boron has only three valence electrons, which would favour metallicity, but they are sufficiently localized that insulating states emerge. However, this subtle balance between metallic and insulating states is easily shifted by pressure, temperature and impurities. Here we report the results of high-pressure experiments and ab initio evolutionary crystal structure predictions4,5 that explore the structural stability of boron under pressure and, strikingly, reveal a partially ionic high-pressure boron phase. This new phase is stable between 19 and 89 GPa, can be quenched to ambient conditions, and has a hitherto unknown structure (space group Pnnm, 28 atoms in the unit cell) consisting of icosahedral B12 clusters and B2 pairs in a NaCl-type arrangement. We find that the ionicity of the phase affects its electronic bandgap, infrared adsorption and dielectric constants, and that it arises from the different electronic properties of the B2 pairs and B12 clusters and the resultant charge transfer between them.

Ionic high-pressure form of elemental boron

Boron carbide is a strategic material, finding applications in nuclear industry, armour for personnel and vehicle safety, rocket propellant, etc. Its high hardness makes it suitable for grinding and cutting tools, ceramic bearing, wire drawing dies, etc. Boron carbide is commercially produced either by carbothermic reduction of boric acid in electric furnaces or by magnesiothermy in presence of carbon. Since many specialty applications of boron carbide require dense bodies, its densification is of great importance. Hot pressing and hot isostatic pressing are the main processes employed for densification. In the recent past, various researchers have made attempts to improve the existing methods and also invent new processes for synthesis and consolidation of boron carbide. All the techniques on synthesis and consolidation of boron carbide are discussed in detail and critically reviewed.

Synthesis and consolidation of boron carbide: a review

Many of the fundamental questions regarding the solid-state chemistry of boron are still unsolved, more than 200 years after its discovery. Recently, theoretical work on the existence and stability of known and new modifications of the element combined with high-pressure and high-temperature experiments have revealed new aspects. A lot has also happened over the last few years in the field of reactions between boron and main group elements. Binary compounds such as B(6)O, MgB(2), LiB(1-x), Na(3)B(20), and CaB(6) have caused much excitement, but the electron-precise, colorless boride carbides Li(2)B(12)C(2), LiB(13)C(2), and MgB(12)C(2) as well as the graphite analogue BeB(2)C(2) also deserve special attention. Physical properties such as hardness, superconductivity, neutron scattering length, and thermoelectricity have also made boron-rich compounds attractive to materials research and for applications. The greatest challenges to boron chemistry, however, are still the synthesis of monophasic products in macroscopic quantities and in the form of single crystals, the unequivocal identification and determination of crystal structures, and a thorough understanding of their electronic situation. Linked polyhedra are the dominating structural elements of the boron-rich compounds of the main group elements. In many cases, their structures can be derived from those that have been assigned to modifications of the element. Again, even these require a critical revision and discussion.

Boron: elementary challenge for experimenters and theoreticians.

This review focuses on the discussion of the latest progress and remaining challenges in selected metal-free photocatalysts for hydrogen production. The scope of this review is limited to the metal-free elemental photocatalysts (i.e. B, C, P, S, Si, Se etc.), binary photocatalysts (i.e. BC3, B4C, CxNy, h-BN etc.) and their heterojunction, ternary photocatalysts (i.e. BCN) and their heterojunction, and different types of organic photocatalysts (i.e. linear, covalent organic frameworks, microporous polymer, covalent triazine frameworks etc.) and their heterostructures. Following a succinct depiction of the latest progress in hydrogen evolution on these photocatalysts, discussion has been extended to the potential strategies that are deemed necessary to attain high quantum efficiency and high solar-to-hydrogen (STH) conversion efficiency. Issues with reproducibility and the disputes in reporting the hydrogen evolution rate have been also discussed with recommendations to overcome them. A few key factors are highlighted that may facilitate the scalability of the photocatalyst from microscale to macroscale production in meeting the targeted 10% STH. This review is concluded with additional perspectives regarding future research in fundamental materials aspects of high efficiency photocatalysts followed by six open questions that may need to be resolved by forming a global hydrogen taskforce in order to translate bench-top research into large-scale production of hydrogen.

Metal-free photocatalysts for hydrogen evolution.

From fine-structure investigations it is well known that in many icosahedral boron-rich solids the occupation densities of specific atomic sites are considerably reduced. Investigations of the electronic properties have proved that the electronic properties of these semiconductors are strongly influenced by high densities of intrinsic states in the band gaps. For -rhombohedral boron and boron carbide, the best investigated icosahedral boron-rich solids, it is shown that the concentrations of structural defects and electronic gap states are quantitatively correlated, and that this way the electron deficiencies theoretically calculated for the valence bands of corresponding idealized structures are compensated. Obviously, the structural defects in these crystals are the necessary consequence of the valence electron deficiency. It is suggested that this correlation holds for the icosahedral boron-rich solids in general.

https://iopscience.iop.org/article/10.1088/0953-8984/11/35/316/pdf

Correlation between structural defects and electronic properties of icosahedral boron-rich solids

Distribution of carbon atoms on the boron carbide structure elements

Thermoelectric energy conversion is a very reliable way of generating electrical power, for example from solar heat or from waste industrial thermal energy. Boron-rich solids are shown to be very promising candidates for high efficiency thermoelectric energy conversion. The outstanding high melting points and extraordinary chemical stability allow their use under extreme conditions which are not accessible by most other materials. Some boron-rich semiconductors exhibit very favourable transport properties such as high Seebeck coefficients increasing monotonically up to very high temperatures, electrical conductivities with values typical of semiconductors, and very low thermal conductivities.

Boron-rich solids : a chance for high-efficiency high-temperature thermoelectric energy conversion

Some properties of single-crystal boron carbide

β-rhombohedral boron doped with carbon up to about 1 at% is investigated by X-ray diffraction, by optical transmission spectroscopy in the spectral range of the absorption edge and its low energy tail, by I R reflection spectroscopy in the lattice vibration range, and by Raman scattering. The carbon atoms substitute for boron sites in the icosahedra shrinking as a consequence of the increased intraicosahedral bonding force. Hence the lattice constants decrease with increasing carbon content. Within the accuracy of measurement the interband transition energies are not influenced by the carbon content, and the p-type conductivity remains qualitatively unchanged. Additionally to the series of six ionization energies of B12 electron trapping levels, which are equidistantly arranged below the conduction band in pure boron, a second series of B11C trapping levels develops. The traps are esscntially generated by the interaction between one electron each and up to six pairs (probably triples) of defined phonons. These phonons are quenched as a consequence of the occupation of these traps. In correlation with the new optical results ESR investigations of other authors can now be interpreted. Important details of the electronic structure and its modification by carbon doping are determined. The relevance for other boron-rich solids with icosahedral structure is discussed.



β-rhomboedrisches Bor, dotiert mit Kohlenstoff bis zu etwa 1 At%, wird rnit Rontgenbeugung, mit optischer Transmissionsspektroskopie im Spektralbereich der Absorptionskante und ihres Auslaufers, mit Reflexionsspektroskopie im Gitterschwingungsbereich und mit Ramanstreuung untersucht. Die Kohlcnstoffatome substituieren Borplatze in den Ikosaedern, die infolge der wachsenen Bindungsstarke schrumpfen. Dies fuhrt zu einer Verkleinerung der Gitterkonstanten. Die Interbandubergange werden durch den Kohlenstoff im Rahmen der Mesgenauigkeit nicht verandert, und die p-Leitung bleibt qualitativ erhalten. Zusatzlich zu der Serie von sechs B12-Elektronenhaftniveaus, die in reinem Bor aquidistant unter dem Leitungsband angeordnet sind, tritt eine zweite Serie von B11 C-Haftstellen auf. Die Haftstellen entstehen durch Wechselwirkung zwischen je einem Elektron und bis zu sechs Paaren (moglicherweise Tripeln) definierter Phononen. Diese werden bei Besetzung der Haftstellen gequencht. In Zusammenhang mit den neuen optischen Ergebnissen konnen nun ESR-Untersuchungen anderer Autoren interpretiert werden. Wichtige Details der elektronischen Struktur und ihre Veranderung bei Kohlenstoffdotierung werden bestimmt. Die Bedeutung der Ergebnisse fur andere borreiche Festkorper wird diskutiert.

Helmut Werheit

Papers

Correlation between structural defects and electronic properties of icosahedral boron-rich solids

Distribution of carbon atoms on the boron carbide structure elements

Boron-rich solids : a chance for high-efficiency high-temperature thermoelectric energy conversion

Some properties of single-crystal boron carbide

Structural and Electronic Properties of Carbon-Doped β-Rhombohedral Boron