B11C Icosahedra Research Articles

In Situ Observation of Fracture along Twin Boundaries in Boron Carbide.

The observation of fracture behaviors in perfect and twinned B4 C crystals via in situ transmission electron microscopy (TEM) mechanical testing is reported. The crystal structure of the synthesized B4 C, composed of B11 C icosahedra connected by boron-deficient C-▫-C chains in a chemical formula of B11 C3 , is determined by state-of-the-art aberration-corrected scanning TEM. The in situ TEM observations reveal that cracking is preferentially initiated at the twin boundaries (TBs) in B4 C under both indentation and tension loading. The cracks then propagate along the TBs, thus resulting in the fracture of B4 C. These results are consistent with the theoretical calculations that show that TBs have a softening effect on B4 C with amorphous bands preferentially nucleated at the TBs. These findings elucidate the atomic arrangement and the role of planar defects in the failure of B4 C. Furthermore, they can guide the design of advanced superhard materials via planar defect control.

Low temperature specific heat anomaly with boson peak in isotope-enriched boron carbides B4·3C – B10C

The low temperature specific heat of boron carbide exhibits characteristic deviations from the classical Debye model (c ∝ T3). Including α-rhombohedral boron, the degree of deviations from this model is proportional to the concentration of B(3) voids in the structure and to the content of B11C icosahedra, respectively. The observed effects of specific structural defects can be described in accordance with the model of voids in amorphous Si proposed by Nakhmanson and Drabold. Results of β-rhombohedral boron, largely corresponding to the Debye model, are used for comparison.

Microstructural characterization of boron-rich boron carbide

Boron carbide has a wide range of solubility, but the effects of stoichiometry on its microstructure and mechanical response are not well understood. In this study, detailed microstructural characterization was carried out on three hot-pressed B-rich boron carbide samples. Lattice parameter measurements from XRD identified the compositions to be B4.2C, B5.6C and B7.6C. Local substitutional disorder was observed by Raman spectroscopy, particularly for more B-rich samples. Electron energy loss spectroscopy observations suggest that excess boron preferentially substitutes for carbon atoms in the B11C icosahedra; after which additional boron modifies the CBC chains. Moreover, the boron content has salient effects on boron carbide densification and microstructure. Improved densification was observed in the more B-rich samples (B5.6C and B7.6C), and there is a transition from few or no intragranular planar defects (B4.2C), to numerous stacking faults (B5.6C), to copious twins (B7.6C). Nanoindentation experiments revealed that the highest value for B4.2C is statistically larger than that for B5.6C or B7.6C, suggesting that the hardness of boron carbide is reduced by boron substitution.

Boron carbide: Consistency of components, lattice parameters, fine structure and chemical composition makes the complex structure reasonable

The complex, highly distorted structure of boron carbide is composed of B12 and B11C icosahedra and CBC, CBB and B□B linear elements, whose concentration depends on the chemical composition each. These concentrations are shown to be consistent with lattice parameters, fine structure data and chemical composition. The respective impacts on lattice parameters are estimated and discussed. Considering the contributions of the different structural components to the energy of the overall structure makes the structure and its variation within the homogeneity range reasonable; in particular that of B4.3C representing the carbon-rich limit of the homogeneity range. Replacing in B4.3C virtually the B□B components by CBC yields the hypothetical moderately distorted B4.0C (structure formula (B11C)CBC). The reduction of lattice parameters related is compatible with recently reported uncommonly prepared single crystals, whose compositions deviate from B4.3C.

A Free Energy Model of Boron Carbide

The assessed phase diagram of the boron-carbon system contains a single nonstoichiometric boron-carbide phase of rhombohedral symmetry with a broad, thermodynamically improbable, low temperature composition range. We combine first principles total energy calculations with phenomenological thermodynamic modeling to propose a revised low temperature phase diagram that contains two boron-carbide phases of differing symmetries and compositions. One structure has composition B4C and consists of B11C icosahedra and C-B-C chains, with the placement of carbon on the icosahedron breaking rhombohedral symmetry. This phase is destabilized above 600 K by the configurational entropy of alternate carbon substitutions. The other structure, of ideal composition B13C2, has a broad composition range at high temperature, with rhombohedral symmetry throughout, as observed experimentally.

Advanced microstructure of boron carbide

The rhombohedral elementary cell of the complex boron carbide structure is composed of B12 or B11C icosahedra and CBC, CBB or B□B (□, vacancy) linear arrangements, whose shares vary depending on the actual chemical compound. The evaluation of the IR phonon spectra of isotopically pure boron carbide yields the quantitative concentrations of these components within the homogeneity range. The structure formula of B4.3C at the carbon-rich limit of the homogeneity range is (B11C) (CBC)0.91 (B□B)0.09 (□, vacancy); and the actual structure formula of B13C2 is (B12)0.5(B11C)0.5(CBC)0.65(CBB)0.16 (B□B)0.19, and deviates fundamentally from (B12)CBC, predicted by theory to be the energetically most favourable structure of boron carbide. In reality, it is the most distorted structure in the homogeneity range. The spectra of natBxC make it evident that boron isotopes are not randomly distributed in the structure. However, doping with 2% silicon brings about a random distribution.

Boron carbides from first principles

In this work, we focus on the understanding gained from the investigation of the physical properties of boron carbides with theoretical methods based on density functional theory (DFT). Together with the examination of the DFT total energies of various atomic configurations in the unit cell, comparison with the experiments of the theoretical vibrational or NMR spectra has led to the determination of the atomic structure of B4C as C-B-C chains linking mostly B11C icosahedra, and a few percents of B10C2 icosahedra. In the icosahedron, the carbon atom is found to be in the polar site (B4Cp). When there are two carbon atoms, they are found to be in antipodal polar positions. At carbon concentrations other than 20%, we find that only four structural models have a negative formation energy with respect to a formation from α-boron + diamond. Moreover, they all have a positive formation energy with respect to B4Cp, showing a tendency to decompose into B4Cp + α-boron or B4Cp + diamond. This metastability explains actual difficulties in the synthesis of clean samples, in particular for B13C2. Finally, the idea of combining high hardness and superconductivity in the same material by doping boron-rich solids has emerged. We show results on the strength of the electron-phonon coupling constant obtained with DFT-based methods in B13C2.

Synthesis and photoluminescence property of boron carbide nanowires

Large scale, high density boron carbide nanowires have been synthesized by using an improved carbothermal reduction method with B/B2O3/C powder precursors under an argon flow at 1100°C. The boron carbide nanowires are 5–10 μm in length and 80–100 nm in diameter. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) characterizations show that the boron carbide nanowire has a B4C rhombohedral structure with good crystallization. The Raman spectrum of the as-grown boron carbide nanowires is consistent with that of a B4C structure consisting of B11C icosahedra and C-B-C chains. The room temperature photoluminescence spectrum of the boron carbide nanowires exhibits a visible range of emission centred at 638 nm.

Defect Formation in Boron Carbide-An ab-initio Electronic Structure Study

ABSTRACTThe electronic structure of a crystalline boron carbide has an energy forbidden gap of ∼ 3 eV and is hence a good insulator. But, on the other hand, the electrical conductivity of boron carbide is measurable. It is therefore believed that the defects formation in boron carbide is responsible for its electrical conductivity and a theory of hopping conduction of bipolaron through localized defects were developed, accordingly. Although the bipolaron electrical conductivity model does not rely on any specific type of defect, the bipolaron formation in boron carbide is believed to be a defective CBB intraicosahedral chain in connection with an B11C icosahedron. The current study examined the existing theory of bipolaron electrical conductivity by performing a systematical study on the formation energies of the defects in boron carbide using a state-of-the-art ab-initio electronic structure method. The studied defects cover a) stoichiometric variations of carbon concentration, b) missing boron atoms, and c) distribution of carbon atoms in the materials. It is found that the ground state of a fully carbonated boron carbide consists of B11C icosahedra connected by CBC intraicosahedral chains, i.e. consistent with the reported structural model of B4C. When carbon concentration is reduced, however, the population of CBC chain is found to be intact, while the population of B11C icosahedron is reduced by the replacements of B12 icosahedron. This observation is fundamentally different from the existing model of boron-rich boron carbide. The localized states associated with missing boron atoms are identified and the electrical conductivity through these localized defects states is studied.

IR-Active Phonons and Structure Elements of Isotope-Enriched Boron Carbide

The IR phonon spectra of 10B, 11B, and 13C isotope-enriched boron carbide with the compositions B4.3C, B6.5C, and B10C are presented. Specific phonons are attributed to the stretching and the bending mode of a small concentration of CCC chains, whose occurrence seems to depend on the specific preparation conditions. Based on the assumption that the electron deficiency is compensated by structural defects, the concentrations of CBC, CBB, CCC, and B□B arrangements are determined for the whole homogeneity range. Then the concentrations of B12 and B11C icosahedra result from the compound stoichiometry. The concentration of B□B arrangements, calculated this way, excellently agrees with the concentration of B(3) vacancies obtained by neutron scattering.

Carbon in β-rhombohedral boron

Carbon atoms in the β-rhombohedral boron structure substitute for boron atoms at the polar sites of the B12 icosahedra. The donor level of the excess electron of carbon coincides with the upper valence band, thus raising the hopping probability within this band. This assumption is consistent with various experimental results. The ionisation energy of the intrinsic electron trapping levels is slightly different for B12 and B11C icosahedra. The photoconductivity and quotient spectra of the reflectivities of differently doped samples indicate carbon-induced changes in the density of states within the energy bands.

Bipolaron Formation in B12 and (B11C)+ Icosahedra

ABSTRACTBoron carbides, B1-xCx with 0.085 ≤ × ≤ 0.200, generally contain both B12 and B11C icosahedra. However, the electronic transport with 0.1 ≤ × ≤ 0.2 is believed to occur by means of bipolaron hopping between only B11 C icosahedra [i]. We have calculated the changes in energy, atomic positlions and charge distribution when a pair of electrons is added to the isoelectronic icosahedral clusters B12 and (B11C)+. We simulate an icosahedron in a neutral lattice by bonding the icosahedral atoms to hydrogenic atoms which we constrain to be neutral. The computations are performed with a self-consistent molecular-orbital method, PRDDO.We find a total energy reduction of -3.7 eV for two electrons added to a B12 icosahedron. Of this, -2.7 eV arises from the electrons filling the icosahedron's bonding orbitals. The remaining -1.0 eV comes from the contraction of the icosahedron's radius by -0.09 Å. For two electrons added to a (B11C)+ icosahedron we find a total energy reduction of -18.2 eV. Of this, -16.5 eV arises from filling the icosahedron's bonding orbitals. The remainder arises frop a -0.09 Å contraction of the icosahedron's radius. Thus, we find (B11C) icosahedra to be strongly energetically favored over B12 icosahedra as bipolaron sites.The positive charge associated with a (B11C)+ icosahedron is distributed over the eleven boron atoms. Concomitantly, we find the added two electrons of the bipolaron to be distributed over all twelve sites of the B11C icosahedron. We find the energy difference between an electron pair added to B12 and (B11C)+ icosahedra to arise principally from the increased Coulomic attraction provided by the extra positive charge of the (B11C) icosahedron.