Silicon Boride Research Articles

Effect of non-boron element with the different electronegativity of binary boride on oxidative dehydrogenation of propane

Boron-based catalysts are active and selective in the oxidative dehydrogenation reaction of propane (ODHP) to propylene. Herein, the binary boron-based compounds (boron nitride, boron phosphide, boron carbide, and silicon boride) composed of boron elements and one type of other non-boron elements, and the elemental boron were systematically compared to elucidate how the non-boron elements of binary borides influence their catalytic activity in ODHP. By combined experimental and computational studies, we demonstrated that the non-boron elements can modulate the performance of catalysts by altering the ability of the B-OH site to activate dioxygen and the subsequent activation of propane. In particular, the turnover frequency (TOF) of the catalyst exhibited a high negative linear correlation with the electronegativity of the non-boron element, except for the boron phosphide. This study highlights the importance of the chemical environment around the active boron sites and provides insight into the design of efficient ODHP catalysts.

Icosahedral silicon boride: A potential hybrid photovoltaic-thermoelectric for energy harvesting

Boron-rich compounds have attracted significant attention due to their promising and diverse physical properties, which include ultrahardness, resistance to oxidation and corrosion, and even superconductivity. Here, using a crystal structure search method based on first-principles calculations, we find a boron-rich silicon compound $\mathrm{Si}{\mathrm{B}}_{12}$ that is stable under moderate pressure of around 20 GPa, and which we predict is recoverable to ambient pressure. This silicon boride, with space group Pnnm, is structurally related to the $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{B}}_{28}$ boron phase. Specifically, the $\mathrm{Si}{\mathrm{B}}_{12}$ structure is formed by replacing the ${\mathrm{B}}_{2}$ pairs in $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{B}}_{28}$ with silicon atoms. Our calculations show that this Pnnm $\mathrm{Si}{\mathrm{B}}_{12}$ phase exhibits good thermal stability at moderate pressures above 20 GPa and temperatures to 900 K. We suggest this structure has dynamic stability at ambient pressure and remains stable to temperatures as high as 2000 K. Impressively, this $\mathrm{Si}{\mathrm{B}}_{12}$ phase possesses good light absorption and thermoelectrical properties, which are enhanced by its small and indirect band gap, doubly degenerate bands, and low lattice thermal conductivity. Our predictions should stimulate further investigations of this class of boron-rich semiconductors, especially in view of their superior photovoltaic and thermoelectric properties which may be beneficial in energy applications.

Preparation of silicon boride SiBx (x = 3, 4, 5, 6) powders by chemical oven self-propagating combustion synthesis

Abstract A new method was developed for quickly preparing a highemissivity silicon boride compound of SiBx (x = 3, 4, 5, 6) by highly exothermic Ti-TiO2-Si-Al chemical oven preheating. The SiBx combustion synthesis process and adiabatic combustion temperature were investigated. A large exothermic reaction occurred at the combustion temperature of 1 700 K. X-ray diffraction results indicate that an SiBx phase and a substantial amount of unreacted Si were identified in the products. By increasing the boron content until the Si-B ratio reached to 1 : 6, the diffraction peaks primarily indicated SiB6, SiB4, and Si11B33 in the final product. According to the spectra and quantitative results, the atomic chemical composition ratio of Si and B was close to the nominal composition. Thus, this method offers an efficient way to produce Si-B compounds with less time and energy consumption than current methods.

Preparation of silicon boride SiBx (x = 3, 4, 5, 6) powders by chemical oven self-propagating combustion synthesis

Abstract A new method was developed for quickly preparing a highemissivity silicon boride compound of SiBx (x = 3, 4, 5, 6) by highly exothermic Ti-TiO2-Si-Al chemical oven preheating. The SiBx combustion synthesis process and adiabatic combustion temperature were investigated. A large exothermic reaction occurred at the combustion temperature of 1 700 K. X-ray diffraction results indicate that an SiBx phase and a substantial amount of unreacted Si were identified in the products. By increasing the boron content until the Si-B ratio reached to 1 : 6, the diffraction peaks primarily indicated SiB6, SiB4, and Si11B33 in the final product. According to the spectra and quantitative results, the atomic chemical composition ratio of Si and B was close to the nominal composition. Thus, this method offers an efficient way to produce Si-B compounds with less time and energy consumption than current methods.

Radiative lifetimes of several doublet and quartet states of silicon boride

In this study, the potential energy curves were calculated for the X4Σ–, A4Π, B4Δ, C4Σ+, D4Σ–, E4Π, a2Π, b2Δ, c2Σ–, d2Σ+, and e2Π states of silicon boride. The transition dipole moments were computed for all dipole–allowed transitions between these states. The complete active space self-consistent field method was used for calculations, followed by the valence internally contracted multireference configuration interaction approach. The radiative lifetimes were approximately 1 – 10 μs, 0.1 – 1 μs, 1 – 10,000 ms, 1 – 1000 ms, and 1 μs for the A4Π, D4Σ–, B4Δ, C4Σ+, and E4Π states, respectively. Of these transitions between the six lowest–lying quartets, the spontaneous emissions from the A4Π – X4Σ–, D4Σ– – X4Σ–, E4Π – X4Σ–, and E4Π – A4Π systems were strong, whereas those from the B4Δ – A4Π, C4Σ+ – A4Π, and E4Π – B4Δ transitions were weak. The radiative lifetimes were approximately 0.1 – 1000 ms, 0.1 – 100 ms, 10 – 100 μs, and 1 – 10 μs for the b2Δ, c2Σ–, d2Σ+, and e2Π states, respectively. Of the transitions between the a2Π, b2Δ, c2Σ–, d2Σ+, and e2Π states, the spontaneous emissions from the e2Π – b2Δ and e2Π – c2Σ– systems were relatively strong, whereas those from the b2Δ – a2Π transition were relatively weak. The transition frequencies, Einstein A coefficients, and Franck–Condon factors of all spontaneous vibronic bands from these transitions were calculated. The results obtained in this study were compared with experimental and other theoretical values. The radiative–lifetime distribution varying with rotational angular quantum number J was calculated when J ≤ 50.5 for a particular vibrational level of these states.

Prediction of superconductivity in pressure-induced new silicon boride phases

The crystal structures and properties of boron-silicon (B-Si) compounds under pressure have been systematically explored using particle swarm optimization structure prediction method in combination with first-principles calculations. Three new stoichiometries, ${\mathrm{B}}_{2}\mathrm{Si}$, BSi, and ${\mathrm{BSi}}_{2}$, are predicted to be stable gradually under pressure, where increasing pressure favors the formation of silicon rich B-Si compounds. In the boron-rich compounds, the network of boron atoms changes from ${\mathrm{B}}_{12}$ icosahedron in the ambient phases to the similar buckled graphenelike layers in the high-pressure phases, which crystalize in the same $P\overline{3}m1$ symmetry but with different numbers of boron layers between adjacent silicon layers. Phonon calculations show that these structures might be retained to ambient conditions as metastable phases. Further electron-phonon coupling calculations indicate that the high-pressure phases of boron-rich compounds might superconduct at 1 atm, with the highest ${T}_{c}$ value of 21 K from the Allen-Dynes equation in $P\overline{3}m1$ ${\mathrm{B}}_{2}\mathrm{Si}$, which is much higher than the one observed in boron doped diamond-type silicon. Moreover, further fully anisotropic Migdal-Eliashberg calculations indicate that ${\mathrm{B}}_{2}\mathrm{Si}$ is a two-gap anisotropic superconductor and the estimated ${T}_{c}$ might reach up to 30 K at 1 atm. On the silicon-rich side, ${\mathrm{BSi}}_{2}$ is predicted to be stable in the ${\mathrm{CuAl}}_{2}$-type structure. Our current results significantly enrich the phase diagram of the B-Si system and will stimulate further experimental study.

Mysterious SiB3: Identifying the Relation between α- and β-SiB3

Binary silicon boride SiB3 has been reported to occur in two forms, as disordered and nonstoichiometric α-SiB3–x, which relates to the α-rhombohedral phase of boron, and as strictly ordered and stoichiometric β-SiB3. Similar to other boron-rich icosahedral solids, these SiB3 phases represent potentially interesting refractory materials. However, their thermal stability, formation conditions, and thermodynamic relation are poorly understood. Here, we map the formation conditions of α-SiB3–x and β-SiB3 and analyze their relative thermodynamic stabilities. α-SiB3–x is metastable (with respect to β-SiB3 and Si), and its formation is kinetically driven. Pure polycrystalline bulk samples may be obtained within hours when heating stoichiometric mixtures of elemental silicon and boron at temperatures 1200–1300 °C. At the same time, α-SiB3–x decomposes into SiB6 and Si, and optimum time-temperature synthesis conditions represent a trade-off between rates of formation and decomposition. The formation of stable β-SiB3 was observed after prolonged treatment (days to weeks) of elemental mixtures with ratios Si/B = 1:1–1:4 at temperatures 1175–1200 °C. The application of high pressures greatly improves the kinetics of SiB3 formation and allows decoupling of SiB3 formation from decomposition. Quantitative formation of β-SiB3 was seen at 1100 °C for samples pressurized to 5.5–8 GPa. β-SiB3 decomposes peritectoidally at temperatures between 1250 and 1300 °C. The highly ordered nature of β-SiB3 is reflected in its Raman spectrum, which features narrow and distinct lines. In contrast, the Raman spectrum of α-SiB3–x is characterized by broad bands, which show a clear relation to the vibrational modes of isostructural, ordered B6P. The detailed composition and structural properties of disordered α-SiB3–x were ascertained by a combination of single-crystal X-ray diffraction and 29Si magic angle spinning NMR experiments. Notably, the compositions of polycrystalline bulk samples (obtained at T ≤ 1200 °C) and single crystal samples (obtained from Si-rich molten Si–B mixtures at T > 1400 °C) are different, SiB2.93(7) and SiB2.64(2), respectively. The incorporation of Si in the polar position of B12 icosahedra results in highly strained cluster units. This disorder feature was accounted for in the refined crystal structure model by splitting the polar position into three sites. The electron-precise composition of α-SiB3–x is SiB2.5 and corresponds to the incorporation of, on average, two Si atoms in each B12 icosahedron. Accordingly, α-SiB3–x constitutes a mixture of B10Si2 and B11Si clusters. The structural and phase stability of α-SiB3–x were explored using a first-principles cluster expansion. The most stable composition at 0 K is SiB2.5, which however is unstable with respect to the decomposition β-SiB3 + Si. Modeling of the configurational and vibrational entropies suggests that α-SiB3–x only becomes more stable than β-SiB3 at temperatures above its decomposition into SiB6 and Si. Hence, we conclude that α-SiB3–x is metastable at all temperatures. Density functional theory electronic structure calculations yield band gaps of similar size for electron-precise α-SiB2.5 and β-SiB3, whereas α-SiB3 represents a p-type conductor.

Two-dimensional silicon boride on ZrB2(0001)

A two-dimensional silicon boride phase, ``siliborophene,'' was formed on a ${\mathrm{ZrB}}_{2}$(0001) single-crystal surface, which coincided with the $2\ifmmode\times\else\texttimes\fi{}2$ periodicity of the substrate. The phonon dispersion relations were measured using high-resolution electron energy-loss spectroscopy and were compared with theoretical predictions based on first-principles density functional theory. Among many theoretically derived surface models, only one structure made of ${\mathrm{Si}}_{3}{\mathrm{B}}_{6}$ can reproduce details of the measured phonon dispersion. That model consists of a cyclic boron ring ($c{\mathrm{B}}_{6}$) capped by a Si atom, ${\mathrm{SiB}}_{6}$ group, and $s{p}^{2}$-like Si atoms connecting them. This structure is much more robust than that of silicene: it shows no order--disorder transition until 1300 K; above that temperature, Si is desorbed gradually. Electron band calculation of the isolated film of ${\mathrm{Si}}_{3}{\mathrm{B}}_{6}$ displays a band crossing at the $\overline{\mathrm{K}}$ point, which makes this material promising for use with Fermi-level engineering.

Metal-free silicon boride catalyst for oxidative dehydrogenation of light alkanes to olefins with high selectivity and stability

Due to the free coking and no equilibrium limitation in alkanes conversion, the oxidative dehydrogenation of light alkanes to olefins offers an attractive route, but encounters the difficulty in selectivity control for olefins because of the over-oxidation reactions that produce a substantial amount of CO2. Here we report silicon boride exhibiting good oxidation resistance property at high temperature, as a metal-free catalyst efficiently catalyzed dehydrogenation of light alkanes to olefins with a high selectivity at a given reaction conditions. The stability was evidenced by the operation of a 100-h test with steady conversion and product selectivity. When the conversion of ethane, propane and isobutane reached 18.8%, 19.1% and 6.0%, the selectivity of olefins were up to 98.0%, 94.4% and 96.4%, respectively. This work experimentally supports that the catalytic origin of the boron species from the metal-free catalyst initiate the oxidative dehydrogenation of light alkanes.

THERMODYNAMIC ASSESSMENT OF CAPABILITY FOR DEPOSITION OF SILICON BORIDES FROM THEIR HALOGENIDES

The paper presents the results of predictive calculation of boron silicide thermodynamic properties (enthalpy, entropy, heat capacity) required for the thermodynamic analysis of the Si–B–Cl–H system conducted by means of the TERRA software. The paper considers cases of SiB4 and SiB6 condensed phase formation in the reaction mixture. The chemical vapor deposition parameters (temperature, pressure, the ratio of initial reagents) of silicon borides were evaluated by thermodynamic calculations of the Si–B–Cl–H system formed by the SiCl 4 , BCl 3 , and H 2 for a temperature range of 1000–2200 K and a pressure range of 0,00001–0,1 MPa. The paper demonstrates that thermodynamic stability of higher chlorides in the Si–B–Cl system decreases with decrease in pressure, while the proportion of lower chlorides increases, i.e. degradation of initial silicon and boron chlorides occurs, but condensed phases SiB 4 and SiB 6 do not form – their appearance is facilitated by hydrogen introduction. It was determined that both single-phase and multiphase coatings can be produced by varying the chemical vapor deposition parameters. The results obtained in this paper are of scientific and practical interest for the developers of various silicon boride production processes (gas phase, liquid phase, etc.).

Efeito da adição de coating de cromita de ferro na emissividade de concreto refratário de alta alumina

Resumo Coatings de alta emissividade têm sido utilizados em aplicações industriais há mais de 40 anos com o objetivo de reduzir as perdas térmicas em processos de aquecimento. Com a aplicação de um coating de alta emissividade na superfície dos revestimentos internos de um forno industrial é possível aumentar a eficiência nas trocas térmicas entre a atmosfera e as paredes do revestimento, reduzindo a perda de calor e também o consumo de combustíveis. Em geral são utilizados compostos cerâmicos como óxido de cério, carbeto de boro, boreto de silício, siliceto de molibidênio ou óxido de cromo como agentes de emissividade para se obter as propriedades termo-ópticas desejadas nos coatings. Entretanto tais compostos muitas vezes inviabilizam sua aplicação, devido ao seu elevado valor comercial ou ainda sua escassez. Neste contexto, o presente trabalho buscou avaliar os efeitos da cromita de ferro, um óxido mineral abundante e de valor comercial acessível, em sua utilização como agente de emissividade em coatings refratários de alta emissividade. Por meio de um método indireto de medição de emissividade, foram avaliadas de maneira comparativa composições com e sem a presença da cromita de ferro visando sua aplicação como cobertura de revestimentos isolantes e refratários de fornos de aquecimento industrial. Com adições de 7,5% de cromita de ferro, observaram-se aumentos na emissividade dos coatings da ordem de 8%, sugerindo seu potencial para essa aplicação.

Enhancement of the power factor in two‐phase silicon–boron nanocrystalline alloys

In previous publications it was shown that the precipitation of silicon boride around grain boundaries may lead to an increase of the power factor in nanocrystalline silicon. Such an effect was further explained by computational analyses showing that the formation of an interphase at the grain boundaries along with high boron densities can actually lead to a concurrent increase of the electrical conductivity σ and of the Seebeck coefficient S. In this communication we report recent evidence of the key elements ruling such an unexpected effect. Nanocrystalline silicon films deposited onto a variety of substrates were doped to nominal boron densities in excess of 1020 cm−3 and were annealed up to 1000 °C to promote boride precipitation. Thermoelectric properties were measured and compared with their microstructure. A concurrent increase of σ and S with the carrier density was found only upon formation of an interphase. Its dependency on the film microstructure and on the deposition and processing conditions will be discussed.

Novel Refractory Phase, Ta7Si2(SixB1–x)2

X-ray single crystal (XSC) and powder diffraction data (XPD) were used to elucidate the crystal structure of a new refractory silicon boride Ta7Si2(Si(x)B(1-x))2 (x = 0.12). Tetragonal Ta7Si2(Si(x)B(1-x))2 (space group P4/mbm; a = 0.62219(2) nm, c = 0.83283(3) nm) with B atoms randomly sharing the 4g site with Si atoms is isotypic with the boride structure of (Re,Co)7B4. The architecture of the structure of Ta7Si2(Si(x)B(1-x))2 combines layers of three-capped triangular metal prisms (Si,B)[Ta(6+2)(Si,B)] alternating with double layers of two-capped Si[Ta(8+1)Si] Archimedian metal antiprisms. Consequently, the metal framework contains (B/Si) pairs and Si-Si dumbbells. These two types of coordination figures around the nonmetal atoms are typical for the system-inherent structures of Ta2B (or Ta2Si) and Ta3B2. DFT calculations showed strong B(Si)-B(Si) and Si-Si bonding and represent Ta7Si2(Si(x)B(1-x))2 as a covalent-ionic compound. This bonding behavior is reflected in the high hardness value of 1750 HV. The Sommerfeld constant, γ = 7.58 mJ/mol K(2), as derived from the electronic density of states, calculated at the Fermi level, suggests typical metallic behavior.

Phase relations and thermodynamic property of boron in the silicon-tin melt at 1673 K

In order to optimize the removal of B during the Si refining process using a Si–Sn solvent, thermodynamic property of B in the melt is crucial. In this study, the isothermal section for the Si–Sn–B system at 1673K was measured by the equilibration of the liquid phase with B solid solution or silicon boride. The activity coefficient of B in the Si–Sn melt at 1673K was evaluated, which was found to be three orders of magnitude larger in the Sn-rich side than that in the Si-rich side. This result suggests the removal of B can be achieved easily from the Si–Sn melt with the larger Sn content.

The electronic emission spectrum of SiB

The gas phase spectrum of the silicon boride radical has been observed for the first time. Two electronic transitions were observed in emission from a corona excited supersonic expansion source. The D 4Σ −– X 4Σ − system consists of emission from v′ = 0 to v″ = 0–3, while the A 4Π– X 4Σ − system consists of numerous bands with v′ = 0–5 and v″ = 0–11, although only the strong 0–0 and 0–1 bands have been analyzed so far.

Investigation of Boron Diffusion into Silicon Using a Liquid Boron Tribromide Source and Its Application to Buried-Gate-Type Static-Induction Thyristors

We systematically examined thermal diffusion processes of boron into silicon including the predeposition and the drive-in diffusion using a liquid boron tribromide source, in order to enable its application to buried-gate-type static-induction thyristors (SIThys). A low sheet resistance of with a good in-wafer uniformity of within and with a good wafer-to-wafer uniformity of within was successfully achieved in the predeposition process using the liquid source. The effect of the silicon boride phase on the stability of the boron-diffused layers after drive-in diffusion was also investigated, and it was concluded that the removal of the silicon boride phase is important for obtaining stable drive-in diffused layers. The characteristics of junctions in SIThys fabricated using the liquid source exhibited better properties than those fabricated using a solid boron nitride (BN) source. The results of the recombination-lifetime measurement indicated that high-quality junctions were realized for boron-diffused wafers prepared using the liquid source, in comparison with those prepared using the solid BN source.

Preparation of ZrB2 and HfB2 by Metallothermic Reduction of ZrO2, ZrSiO4, and HfO2

ZrB2 and HfB2 were prepared by metallothermic reduction (MTR) of ZrO2, ZrSiO4, and HfO2 with Mg or Mg and MgO mixtures. We understood that the theory of function-building fine particle preparation was able to be applied to synthesize nano-atomized fine particles of ZrB2. ZrB2 can also be obtained directly from mixture of ZrSiO4, B2O3, and Mg by MTR method. This sample has no borides such as silicon boride except for ZrB2. Therefore, this result shows the possibility to obtain ZrB2 easily from natural source. In addition, HfB2 can also be synthesized by MTR method using HfO2 which was calcined over 750°C.

Modelisation of boron diffusion from ultra-low-energy implantation in crystalline silicon

We have investigated and modeled the boron diffusion in silicon following ultra-low-energy implantation (500eV). It is well known that reducing implant energies is an effective way to eliminate transient enhanced diffusion due to the excess of interstitials from the implant. However, for sub-keV B implants diffusion remains enhanced. This enhancement is linked to the presence of a silicon boride layer located at the silicon surface which creates interstitials. This phenomenon is named “boron enhanced diffusion” (BED). The BED effect is of obvious interest since it counteracts the advantage obtained by reducing the ion implantation energy. For these reasons, we have investigated the diffusion of low-energy boron implanted in crystalline silicon and tested a complete simulation program, which takes into account the effect of boron precipitation and the effect of the silicon boride layer as a source of self-interstitials. Experimental results have been simulated and consistent parameters have been found to fit the data. BED effect has been studied. Model parameters extractions have been discussed.

ＳｉＢ６‐Ｂ４Ｃ系セラミックスの焼結性

Silicon boride (SiB6) was reported that have the characteristics of oxidization resistance and excellent hardness in the high temperature. On the other hand, Boron carbide (B4C) was known that have high hardness after the diamond, and high See-beck coefficient. It is able to expect that the SiB6-B4C composite ceramics have the characteristics of hardness, resistance oxidization, and high See-beck coefficient in the high temperature. SiB6-B4C ceramics were prepared by hot-press sintering method. The sintered body's chemical components were determined by using X-ray diffraction analysis and their relative density and electrical conductivity were measured.

Boron Vapour Phase Doping of Silicon for Bipolar Device Applications

Boron vapour-phase doping (VPD) has been investigated as a possible processing tool for defect-free applications in shallow, vertical and deep junctions in devices. From literature and experiments it has become clear that silicon boride (SiB3) layers can form during VPD. Thermodynamical calculations and analytical techniques were used to establish boride-free doping conditions. SiB3 formation and decomposition have been investigated as a function of temperature, time and the presence of O-containing compounds in the gas system. The hydrogen anneal after boron VPD not only decomposes boride layers on the Si surface, but it is also a means to control dose and depth of a boron junction. Boride-enhanced boron diffusion with buried SiB3 layers has been demonstrated. Effects of patterned Si windows in masking layers and polycrystalline Si layers on the VPD process are described. Application of the boron VPD technique to make a p-type base in bipolar npn transistors shows excellent device and high-frequency characteristics.