Boron carbide, which has a high melting point, outstanding hardness, good mechanical properties, low specific weight, great resistance to chemical agents and high neutron absorption cross-section (10BxC, x>4) is currently used in high-technology industries—fast-breeders, lightweight armors and high-temperature thermoelectric conversion.

The contents of this review are: (1) introduction; (2) preparations—industrial preparative routes, powders, sintering (additives, pressureless, hot pressing, HIP); laboratory methods of synthesis (CVD, PVD, plasma, crystal growth); (3) analytical characterization; (4) phase diagram—a peritectic, nearly pure boron, and a wide phase homogeneity range (B4C-B10·5C); (5) rhombohedral crystal structure—a comprehensive model of the whole solid solution is proposed; (6) chemical properties; (7) physical properties—density, mechanical (strength, hardness, toughness) and thermo-electrical properties; (8) main industrial applications; (9) conclusion.

Boron carbide ― a comprehensive review

The field of thermoelectric energy conversion is reviewed from both a theoretical and an experimental standpoint. The basic theory is introduced and the thermodynamic and solid state views are compared. An overview of the development of thermoelectric materials is presented with particular emphasis being placed on the most recent developments in high-temperature semiconductors. A number of possible device applications are discussed and the successful use and suitability of these devices for space power is manifest.

https://iopscience.iop.org/article/10.1088/0034-4885/51/4/001/pdf

Materials for thermoelectric energy conversion

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 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

http://www.ibmc.up.pt/projectos/FJMonteiro/Researchers/AHDeAza/08_AHdeAza_Biomaterials_23%203%20937-945%202002.pdf

Crack growth resistance of alumina, zirconia and zirconia toughened alumina ceramics for joint prostheses.

Electrical conductivity, thermoelectrical power and thermal conductivity measurements on boron carbide samples show that the electrical conductivity follows the small polaron hopping theory and that thermal conductivity occurs by phonon diffusion. The evolution of these properties with carbon content illustrates the particular role played by the 13.3 at% C compound in the phase homogeneity range B10.5C to B4C. The value of the figure of merit (0.85×10−3 K at 1250 K) proves that this particular boron carbide compound could be a very interesting candidate material for high-temperature thermoelectrical conversion.

The correlation between the thermoelectric properties and stoichiometry in the boron carbide phase B4C−B10.5C

In this paper we study the phase composition of hot-pressed boron carbide specimens containing 0–60 at.% C with the help of various analytical techniques, namely chemical analysis of boron and carbon, chemical analysis combined with lattice parameter measurements, addition method adapted for quantitative X-ray phase analysis and electron microprobe analysis.

The homogeneity range of rhombohedral boron carbide was determined. It was found that the composition range extends from B10.4C (8.8 at.% C) on the boron-rich side to stoichiometric B4.0C (20.0 at.% C) on the carbon-rich side.

Analytical investigations in the BC system

The specimens examined were completely characterized within the homogeneity range of the carbide phase (8.8–20.0 at.% C, i.e. B10.4CB4C). An X-ray study of the lattice dimensions together with accurate measurements of the density of the compounds examined allowed us to determine the number n of atoms contained in the unit cell. The value of n varies linearly with the carbon content between B12(CBC)0.666B1.333 (“B10.4C”; n = 15.33 atoms per unit cell) and B12−xCxC¦BxC1−xC) (“B10.4C”; 0

The properties and structure of the boron carbide phase

The structures of the B6O samples prepared by oxidizing boron with ZnO at temperatures of 1350–1500°C in an argon atmosphere have been refined by the Rietveld method. The B6O samples were found to contain an amorphous phase from their X-ray diffraction profiles in addition to B6O diffraction peaks. The results indicate that the B6O samples, space group R¯3m, no.166, ahex = 0.5367(1) nm and chex = 1.2328(2) nm in the hexagonal unit cell, have oxygen deficiencies with 0.76(6) oxygen occupancy. A radial distribution function method applied to an extracted portion of the amorphous phase, suggests that each amorphous phase has nearly similar short-range order structure to that of the α-tetragonal boron type rather than to those of any other related boron phases.

Structure of B6O boron-suboxide by Rietveld refinement

The vaporization behaviour of pure Al 2 O 3 , Y 2 O 3 and SiC as well as SiC–Al 2 O 3 and SiC–Al 2 O 3 /Y 2 O 3 mixtures has been analysed by thermodynamic calculations in an open system. Pure Al 2 O 3 and Y 2 O 3 evaporate congruently in the 1200–2300 K temperature range. Pure SiC vaporizes in a non-congruent manner leading to graphite formation as by-product. A SiC–Al 2 O 3 mixture evaporates congruently according to the main vaporization reaction, 2 SiC(s) + Al 2 O 3 (s) +Al 2 O(g) ⇆ 2 SiO(g) + 2 CO(g) +4 Al(g), but the overall composition changes: for SiC rich samples, the mixture tends towards pure SiC in time, and for Al 2 O 3 rich samples towards pure Al 2 O 3 . A SiC–Al 2 O 3 /Y 2 O 3 mixture shows similar behaviour.

François Thévenot

Papers

The correlation between the thermoelectric properties and stoichiometry in the boron carbide phase B4C−B10.5C

Analytical investigations in the BC system

The properties and structure of the boron carbide phase

Structure of B6O boron-suboxide by Rietveld refinement

High temperature sintering of SiC with oxide additives: I. Analysis in the SiC–Al2O3 and SiC–Al2O3–Y2O3 systems