Homogeneity range boron carbide

Starting from fine boron and carbon powders, the direct synthesis of stoichiometric boron carbide is possible -, either under vacuum at 2073K in an electric furnace, or at 2273 K by hot pressing under Ar. This method is inefficient economically and finds no practical application. The compositions in the phase homogeneity range B10.4C-B4C can be obtained by hot pressing mixtures of B C with boron . The diffusion diagram between B and C is established. ... 

In the B-C system one binary phase, boron carbide, is established. This phase shows an extensive homogeneity range and to distinguish it from stoichio-... 

Froment et al. [164,166] measured the carbon activities in the homogeneity range of boron carbide by electromotive force method (emf, potentiometric... 

A thermodynamic optimization of the system was performed by Domer (1982) [167]. This dataset was later refined by Lim and Lukas (1996) [36]. Due to additional crystallographic information concerning the extended homogeneity range of the boron carbide phase [152, 168] a further assessment was necessary [33, 34,169]. Data for the calculated invariant reactions are given in Table 13. Boron carbide of composition 16.4 at.% C melts congruently at 2731 K. 

In orientation to the crystallographic results [152, 168], Kasper et al. (1996) [33, 34] and Seifert et al. [169] used for the model description of the homogeneity range of boron carbide the sublattice description (hi2> BiiC)(CBC, CBB, BVaB). The sublattice model (B)93(B,C)i2 was used to describe the carbon solubility in j8-boron. The Redlich-Kister parameters for the liquid phase and graphite (ss) and general formula descriptions for the solution phases were accepted from [36]. The calculated optimized phase... 

The homogeneity range of this boron carbide results from a substitution of some boron atoms of the chains and/or of the icosahedra by carbon. Also, some related... 

Together with the assessment of the binary B-C phase diagram the homogeneity range of boron carbide and the other compounds was modeled by means of the Compound Energy Formalism using the following sublattice models (Table 1) where vacancies are denoted as V ... 

The physical and chemical properties of boron carbide have been reviewed by Lipp [159], Thevenot and Bouchacourt [256], Thevenot [164,165], and Schwetz (1999) [223]. Special problems while presenting the physical properties arise from the large homogeneity range of boron carbide. Furthermore, its poor sinterability requests additives that are usually unspecified and results in residual porosity and various grain sizes which are often also not considered in the publications. Most variation and discrepancies in the properties reported come from the undefined composition of the materials studied. 

However, on the one hand, low ESR signals alone are a weak argument for the assumption of hole bipolarons. On the other hand, several experimental results are in contradiction of this model. For example, (a) the electrical conductivity of boron carbide is maximum at the minimum concentration of BnC icosahedra in the homogeneity range (b) polaron-type effects are restricted to one electron per icosahedron and no corresponding electron-phonon interaction with holes, in particular not with hole pairs in icosahedra, has been proved experimentally (c) the distortion of the icosahedra in boron carbide depends to only a small degree on electron-phonon interaction and (d) the electronic transport in boron-rich solids is due to classical band-type conduction and hopping processes side by side. Hence, the hole bipolaron theory for boron-rich solids can hardly be maintained. 

Boron carbide can be synthesized within a large homogeneity range extending from B43C at the carbon-rich limit (118) to about B12C at the boron-rich limit (119-121). X-ray diffraction... 

The problem was solved quantitatively by the decomposition of the IR optical stretching mode of the three-atom chain by model calculations taking the possible compositions and the frequency shift depending on the mass distribution in natural and isotope-enriched boron carbide into account (57). The determined concentrations of B12 and BnC icosahedra and C—B—C and C—B—B chains are shown in Fig. 26. Other chain compositions can be excluded. Toward the boron-rich limit of the homogeneity range, an increasing number of unit cells without chains arise. Two alternative models are compatible with the optical spectra completely chain-free unit cells and unit cells in which single B atoms saturate the outer bonds of the equatorial atoms of the adjacent icosahedra. Theoretical calculations of reaction kinetics based on the second version (132) satisfactorily confirm the results in Fig. 26. 

Figure 26 demonstrates that at none of the compositions in the homogeneity range does a completely homogeneous structure exist. This disproves, e.g., the idealized structure model of boron carbide Bg.sC [structural formula (Bi2)C—B—C], which is preferably used in theoretical band structure calculations (79,135,136). Obviously, the most homogeneous structure of... 

It is well known that excess carbon in boron carbide precipitates as graphite (see, e.g. Refs. 159-162) if the carbon content sufficiently exceeds the carbon-rich limit of the homogeneity range. However, there seems to be a narrow transition region close to the carbon-rich limit of the homogeneity range where carbon is atomically dispersed when the samples are carefully prepared, e.g., by melting. For example, in a sample with the atomic ratio C/(C -i- B) = 0.199 (see Refs. 147 and 148) there is an excess carbon concentration of 0.155 C atoms per unit cell... 

The thermal conductivity is important for many applications in semiconductor technology, e.g., for thermoelectric devices. In Fig. 41 the thermal conductivity of boron carbide is plotted versus chemical composition (163-165). The strong decrease from B4.3C at the carbon-rich limit of the homogeneity range toward more boron-rich compositions can easily be explained by the change of the most homogeneous structure at 64,30 to the most distorted one at about B6.5C shown in Fig. 26. For temperature dependence, see Ref. 166. 

J Conard, M Bouchacourt, T Thevenot, G Hermaim. C and "B nuclear magnetic resonance investigations in the boron carbide phase homogeneity range A model of solid solution. J Less Common Met 117 51, 1986. 

H Werheit, U Kuhlmann, R Eranz, W Winkelbauer, B Herstefl, D Fister, H Neisius. Electronic transport properties of boron carbide within the homogeneity range. In D Emin, T Aselage, AC Switendick, B Morosin, CL Beckel, eds. Boron-Rich Solids, AIP Conference Proceedings 231. Albuquerque AIP, 1990, p 104. 

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