Structure of an Amorphous Boron Carbide Film: An Experimental and Computational Approach

Abstract

International audienceAn amorphous boron carbide ceramic is prepared via hot wall chemical vapor deposition at 1000 °C using a BCl3/CH4/H2 mixture. Its elemental composition is assessed by electron probe microanalysis (EPMA) and its structure studied by Raman spectroscopy, transmission electron microscopy (TEM), both X-ray diffraction (XRD) and neutron diffraction, 11B magic angle spinning nuclear magnetic resonance (MAS NMR), X-ray absorption spectroscopy (XAS), and ab initio modeling. The atomic structure factor and pair distribution function derived from neutron diffraction data are compared to those deduced from an atomistic model obtained by a liquid quench ab initio molecular dynamics simulation. The good agreement between experimental data and simulation shows that the as-prepared material is essentially made of a random arrangement of icosahedra (B12, B11C, and B10C2) embedded in an amorphous matrix rich in trigonal (BC3 or BC2B) and tetrahedral (CB4) sites. The existence of trigonal boron environments is clearly confirmed by a peak at 50 ppm in both the experimental and simulated 11B MAS NMR spectra, as well as a 190.0 eV component in the XANES-B(1s) spectrum. The intericosahedral linear C–B–C chains observed in crystalline B4C are absent in the as-processed material. Free hexagonal carbon and B4C crystallites appear in the ceramic when heat-treated at 1300 °C/2 h/Ar, as evidenced by high-resolution TEM and Raman spectroscopy. Comparing the pair distribution functions of the heat-treated material with the crystalline B4C model allows confirming the apparition of C–B–C chains in the material. Indeed, two new peaks located at 1.42 and 2.35 Å can only be attributed to a first-neighbor distance between the B and C atoms in the chain and a second-neighbor distance between a chain-boron atom and an icosahedron-boron atom, respectively

Carbon-rich icosahedral boron carbides beyondB4Cand their thermodynamic stabilities at high temperature and pressure from first principles

“…III B. Below 40 GPa, B 2.5 C is unstable and decomposes into B 4 C and diamond, which is in line with the experimental observations that synthesizing boron carbide with at.% C 20 generally results in a mixture of boron carbide and free carbon [11,25,26]. The stability range of B 2.5 C under very high pressures is limited, as it decomposes into γ -boron and diamond, for example at p = 60 GPa and T = 1000 K. Figure 6(b), on the other hand, displays the stability of the B 4 C composition of boron carbide.…”

“…1(a). Even though synthesis of boron carbide beyond the carbon-rich limit has been reported one time, ∼24 at.% C by Konovalikhin et al [24], synthesizing boron carbide, where at.% C 20, generally results in a mixture of boron carbide and free graphitelike carbon [11,25,26]. Recently, Jay et al [27] suggested a possibility that the solubility range of boron carbide could be extended to 21.43 at.% C, in which, instead of the three-atom C-B-C chain, a structural model of B 11 C p (C-C), as shown in Fig.…”

Disordered Icosahedral Boron-Rich Solids: A Theoretical Study of Thermodynamic Stability and Properties

“…Under high pressures, ranging between 40 and 67 GPa, depending on the temperature, B 2.5 C is thermodynamically stable with respect to B 4 C, γ-boron and diamond. Below 40 GPa, B 2.5 C is unstable with respect to B 4 C and diamond, which is in line with the experimental observations that synthesizing boron carbide with at.% C 20 generally results in a mixture of boron carbide and free carbon [36,79,80]. The stability range of B 2.5 C under very high pressure is limited, as it decomposes into γ-boron and diamond.…”

“…Synthesizing boron carbide with high carbon content beyond the solubility limit, in which at.% C 20, generally results in a mixture of boron carbide with the composition, close to B 4 C, and free graphite-like carbon [36,79,80], although a synthesis of boron carbide beyond the carbon-rich limit (at.% C ∼ 24) were reported one time in the literature [81]. A recent theoretical study of boron carbide [82,83] suggested also a possibility of extending the solubility limit of carbon in boron carbide to 28.57 at.% C. [20,36,45,57,81,82].…”

“…Since the temperature for depositing the material is limited by the melting temperature of the substrate, in particular aluminium, as-deposited B 4 C films are practically amorphous (a-B 4 C). It is also of interest to investigate atomic structure of the a-B 4 C thin films, which has so far been rarely addressed in the literature [80]. Indeed, determination of atomic structure of amorphous materials is a challenge for both theoreticians and experimentalists due to the lack of long-range crystal order.…”