Decomposition Of Silane Research Articles

Effects of macrostructure of carbon support in preparation of C/Six/C anode materials for lithium-ion batteries via silane decomposition

Effects of macrostructure of carbon support in preparation of C/Six/C anode materials for lithium-ion batteries via silane decomposition

Numerical simulation of granular silicon growth and silicon fines formation process in polysilicon fluidized bed

Operating conditions strongly affect the yield and quality of polysilicon in a polysilicon fluidized bed. In this study, a new model of polysilicon fluidized bed was established using the Euler–Euler model coupled with population balance model (PBM), which was combined with fluid flow, heat, and mass transfer models, while considering the scavenging effect of silicon fines. The effects of different operating conditions on the deposition and formation rates of silicon fines were investigated. Results show that the model can correctly describe the particle growth process in the fluidized bed of polysilicon. The silicon fines and the interphase velocity difference show “N”- and “M”-shaped distributions along the axial direction, respectively. The particle temperature and concentration near the wall are higher than those in the central region. The decomposition of silane in the bottom region of the bed is dominated by heterogeneous deposition. The scavenging of silicon fines occurs in the dilute-phase region. The effects of operating conditions, i.e. inlet gas temperature, silane composition, and gas velocity, on the reactor performance were also explored comprehensively. Increasing the inlet gas composition and velocity enhances the formation rates of solid silicon and fines. Increasing the inlet gas temperature promotes the growth of solid silicon and inhibits the formation of silicon fines. High fluidization ratio, low inlet silane concentration, and high inlet gas temperature enhance the selectivity of silicon growth.

Multifunctional coating on magnesium alloy: Superhydrophobic, self-healing, anti-corrosion and wear-resistant

Superhydrophobic coating (SHC) was fabricated on magnesium (Mg) alloy by sequential surface etching, in-situ growth of MgAl-layered double hydroxides (LDHs), and modification of octadecyl-trimethoxy-silane (OTMS). The so-fabricated SHC possessed high contact angle above 150° and low sliding angle below 10° by using the probe droplet of aqueous solution of HCl (pH = 2), NaOH (pH = 13), NaCl (3.5 wt.%) or others such as milk, cola, and tea. The lubricant behavior as well as wear resistance was enhanced by such a SHC as compared with the bare Mg alloy. Moreover, corrosion of Mg alloy was greatly slowed by such a SHC which maintained long-term hydrophobic property after corrosion immersion in aqueous solution of NaCl for 6 weeks. The scratch on the SHC could be self-healed by immersing into water due to the hydrolytic polycondensation of silane and corrosion decomposition of LDHs within 8 days.

Gas Phase Reaction of Silane with Water at Different Temperatures and Supported by Plasma.

The interaction of silane and water is discussed controversially in literature: some authors suggest monosilane and water react kinetically and sufficiently fast enough to remove water, while others state the reaction occurs only at elevated temperatures. This question is of technological interest for the removal of unavoidable water residues in Ar gases. Thermodynamic calculations show that virtually complete removal of water is expected with superstoichiometric silane addition. However, mass spectrometric and infrared spectroscopic experiments give evidence that the addition of monosilane to such an Ar gas at room temperature is unable to remove residual water, which disagrees with some current hypotheses in the literature. This holds even for very high SiH4 concentrations up to 2 vol.-%. Silane reacts with water above temperatures of 555 °C, initiated by the thermal decomposition of silane. A cold dielectric barrier discharge-plasma used for silane and water dissociation enhances reactivity similar to elevated temperatures. Fourier-transformed infrared spectroscopy points toward silanol generation at temperatures between 400 and 550 °C, while quadrupole mass spectrometry indicates the creation of SiOH+, SiHOH+, SiH2OH+, and SiH3OH+. Cold plasmas generate smaller amounts of SiOH+, SiHOH+, and SiH2OH+ compared to elevated temperatures. Reaction products are hydrogen and nanoscaled particles of non-stoichiometric silicon oxides. The silicon-oxide particles produced differ in elemental composition and shape depending on the prevailing water content during decomposition: SiO x generated with residual water appears with relatively smooth surfaces, while the addition of water supports the formation of significantly rougher particle surfaces. Higher initial water contents correlate with higher oxygen contents of the particles.

Boron Induced Crystallization of Silicon on Glass: an Alternate Way to Crystallize Amorphous Silicon Films for Solar Cells

Demand for efficient window layer in thin film solar cells with high crystallinity is ever increasing that finds important application in multi-junction/tandem silicon solar cells. Doping of diborane (B2H6) in hydrogenated silicon films using plasma discharge decomposition of silane (SiH4) and (B2H6) gases were analyzed. The boron flow (FB) to silane ratio was varied from 0 to 0.30. Variation in film characteristics with B2H6 gas-phase ratio were analyzed, and concluded that doping boron induces crystallization in hydrogenated amorphous silicon (a-Si: H) film structure. The Raman and field emission scanning electron spectroscopy (FESEM) confirmed the boron induced crystallinity effect in silicon films at different diborane flow. The results showed that as boron content increases beyond certain ratio, silicon crystallization suppresses and the crystallite sizes were also reduced. From results, it was observed that crystallinity in FB = 0.05 is 79% and decreases to 77% when films are slightly higher doped (FB = 0.10) and further decreases when the films were heavily doped. These results validate that boron suppresses silicon crystallization due to local deformations caused by the impurities. Infra-red absorption studies and their analysis also confirm the crystallization in boron doped films with additional band appears at ~ 611 cm−1. This band is named as boron induced crystallinity mode of vibrational spectra. The estimated hydrogen content (CH) decreases confirmed crystallinity in the silicon structure with boron doping. Further, the energy dispersive spectroscopy (EDX) indicates the presence of boron and other impurities in deposited silicon films. The effect of boron on crystallinity and crystallite size as well as the mechanism were presented in detailed.

Direct gas phase synthesis of amorphous Si/C nanoparticles as anode material for lithium ion battery

Abstract High specific capacity of silicon is very attractive for its application as anode material in lithium ion batteries. However, the implementation of silicon is challenging due to its large volume expansion on lithiation leading to pulverization and buildup of a solid-electrolyte interphase. Nanostructuring and design of silicon alloys are a promising strategy to circumvent these challenges. Here we demonstrate an industrially scalable gas phase synthesis method using thermal decomposition of silane and ethylene to produce novel amorphous silicon/carbon-based particles with enhanced electrochemical performance. Fundamental principles and kinetics considerations for the design of high-performance silicon/carbon-based materials are discussed.

Self-Catalyzed Vertically Aligned Carbon Nanotube-Silicon Core-Shell Array for Highly Stable, High-Capacity Lithium-Ion Batteries.

Here, we report on the simple, catalyst-free preparation and application of carbon nanotube-silicon core-shell composite anodes on stainless steel. The stainless steel mesh structure acts as a self-catalyzing agent for the plasma-enhanced chemical vapor deposition (PECVD) growth of vertically aligned, dense, multiwalled carbon nanotube arrays. The carbon nanotube array then serves as a bed for silicon deposition by the decomposition of silane through chemical vapor deposition (CVD). This approach leads to the formation of highly conductive and stable composite anodes. Silicon deposition on the substrate is controlled in terms of the optimal silicon shell thickness, thus enhancing the performance of the cell. These extremely stable, binder-free composite electrodes were characterized as potential anodes in Li-ion batteries, exhibiting long cycle life (>700 cycles), high gravimetric capacity (>4000 mAh/gSi), low irreversible capacity (<10%), and high Coulombic efficiency (>99.5%). These composite anodes meet the requirements of Li-ion batteries for future portable electronics and electric vehicle applications.

Si(Li) detector with ultra-thin entrance window on the diffusive lithium side

Present work reports the results of activities intended to reach thin Si(Li) detector entrance window on the diffusive lithium layer side. It was established that the new n-contact represented by a heterostructure of unalloyed amorphous n-type silicon a-Si:H allows one to achieve the entrance window thickness 3 – 4 orders of magnitude smaller than the lithium-side entrance window of standard Si(Li) detectors. The films of amorphous silicon were synthesized with MASD (magnetron assisted silane decomposition) method in mixture of SiH4 (25%) and Ar (75%) gases. Lithium layer surface resistivity and silicon target type (n- or p-) affection on electrical properties of Si(Li) detector contact produced were studied. The investigation performed had led to a technology of Si(Li) detector production with thickness of the entrance window on the diffusive lithium layer side below 0.1 µm.

Fabrication of doxorubicin-loaded monodisperse spherical micro-mesoporous silicon particles for enhanced inhibition of cancer cell proliferation

Spherical micro-mesoporous silicon (mSi) particles were fabricated by thermal decomposition of monosilane (SiH4) within the mesopores of monodisperse silica particles (MSP) and subsequent etching out of the template material. The thermal destruction of SiH4 occurs within the whole pore volume uniformly because in the thermal chemical vapor deposition method we developed, the limiting stage of the process in which MSP pores are filled with silicon is the reaction in which monosilane is decomposed, rather than its diffusion within template mesopores. The porosity of the mSi particles synthesized can be varied without changes in the shape and outer diameter of the particles by varying the parameters of the technological process of silane decomposition. The particles have large Brunauer-Emmett-Teller specific surface area (250–350 m2 g−1) and pore volume (0.3–0.5 cm3 g−1). For the first time the obtained submicron spherical porous silicon particles are monodisperse with a size scatter not exceeding 10%, which provides the identity of their hydrodynamic and adsorption properties so important for biomedical applications. We introduced the cytostatic drug doxorubicin (DOX) into mSi pores by the physical adsorption method. It was shown that DOX is desorbed from the mSi pores and penetrates into K-562 cancer cells, with local DOX concentration maxima observed in the intracellular space. The cytotoxicity of the mSi and mSi/DOX particles was studied, and the inhibition capacities of the cancer-cell proliferation by doxorubicin adsorbed by mSi particles and a DOX solution were compared in vitro. It was shown that the half-maximal inhibitory concentration (IC50) is four times lower under the action of DOX-loaded mSi particles as compared with free doxorubicin.

Influence of Water-Cooled Jacket on Polysilicon CVD, Fines Formation and Thermal Loss in Komatsu Reactor

A water-cooled jacket enclosing a silicon rod could reduce the fines formation during silane pyrolysis in Komatsu reactor. However, it would result in energy loss. Especially, as the diameter of the silicon rod increases, the free space between the jacket and the silicon rod will vary, and the jacket's effect would be complicated. This work presents a numerical study of the polysilicon chemical vapor deposition (CVD), fines formation, and thermal loss during silane pyrolysis in a Komatsu reactor. The CFD software FLUENT with detailed chemical reaction model of silane decomposition was applied to predict the silicon deposition rates, fines formation and thermal loss produced by a water-cooled jacket. The effects of water-cooled jacket temperature, rod wall temperature, and the distance between water-cooled jacket and rod surface on the polysilicon deposition rate and the fines formation have been investigated. The model was verified by comparing the model predictions with the experimental measurements in the literature. The results show that the jacket wall temperature and rod diameter have a complex influence on the fines formation, CVD growth rate, and thermal loss. The jacket wall temperature should be adjusted as the rod diameter enlarges.

Thermal properties of Bentonite Modified with 3-aminopropyltrimethoxysilane

Chemical modifications of Bentonite (BNT) clay have been carried out by using 3-aminoprophyltrimethoxysilane (APS) in various solvent media. The degradation properties of products (BNTAPS) were characterized by thermogravimetric analysis (TGA). Samples were heated at 30 to 700°C with a heating rate 10 deg/min, and the total silane-grafted amount was determined by calculating the weight loss at 200 – 600°C. The thermogram of TGA showed that there were three main decomposition regions which are attributed to the elimination of physically adsorbed water, decomposition of silane and dehydroxylation of Bentonite. High weight loss attributed to the thermal decomposition of silane was observed between 200 to 550°C. Quantitative analysis of grafted silane results high silane loaded using a solvent with high surface energy, which indicates the type of solvent affected interaction and adsorption of APS in BNT platelets.

Production of polycrystalline silicon from silane pyrolysis: A review of fines formation

Silane is a major silicon source gas for production of polycrystalline silicon. Production of polysilicon from silane has several benefits: lower energy consumption, much easily obtaining electronical polysilicon and less impact on the environment. There is, however, a major challenge that silane homogenous decomposition may result in amorphous silicon powders (fines) formation. In this review we focus on the fines formation with silane as feed gas. Firstly, this paper reports the situation of the production of silane source gas, the mechanism of the silane decomposition and fines formation. The reactor design, operation parameters and carrier gas, all of these would affect fines formation. This study also reviews on emerging trends in these aspects in outcome studies.

Kinetics of Silane Decomposition in High-Pressure Confined Chemical Vapor Deposition of Hydrogenated Amorphous Silicon

We study the kinetics of silane pyrolysis via confined high-pressure chemical vapor deposition (HPCVD) at pressures of 20–33 MPa in a microcapillary of 5.9 μm inner diameter. We find the growth rate to be first order with respect to silane concentration, with an activation energy of 53.7 ± 2.9 kcal/mol and a pre-exponential factor of 1.5 × 1010 m/s. The obtained activation energy is in the range of activation energies reported for hydrogen desorption from c-Si surfaces, suggesting that hydrogen desorption from the surface is the rate-limiting step in film growth. To further investigate this finding, reactive molecular dynamics simulations of thermal decomposition of silane on clean and hydrogen-passivated c-Si were performed. Homogeneous reactions were not observed in any of the simulations, in support of the hypothesis that heterogeneous silane decomposition on the silicon surface is the dominant mechanism for film deposition. In silane pyrolysis simulations on clean c-Si surfaces, almost all available s...

Effect of operation parameters on fines formation during thermal decomposition of silane

Abstract This work presents a systematic numerical investigation of the effects of the operation parameters on silane pyrolysis in a chemical vapour deposition (CVD) reactor. A detailed chemical kinetic model of fines nucleation was coupled with the fluid flow, heat and mass transfer model. The commercial computational fluid dynamic (CFD) software FLUENT was used to predict the silane decomposition and fines formation. The effects of total reaction pressure, wall temperature, inlet gas composition, inlet gas velocity and specific surface area on the silane conversion and the fines formation have been investigated. The model was verified by comparing the simulated fines formation with the experimental data in the open literature. Results show that the total reaction presssure, inlet silane concentration and the wall temperature have vital effects on the fines formation, while the inlet gas velocity and the specific surface area have only influence in a certain range.

Outlier analysis for a silicon nanoparticle population balance model

We assess the impact of individual experimental observations on a multivariate population balance model for the formation of silicon nanoparticles from the thermal decomposition of silane by means of basic regression influence diagnostics. The nanoparticle model is closely related to one which has been used to simulate soot formation in flames and includes morphological and compositional details which allow representation of primary particles within aggregates, and of coagulation, surface growth, and sintering processes. Predicted particle size distributions are optimised against 19 experiments across ranges of initial temperature, pressure, residence time, and initial silane mass fraction. The influence of each experimental observation on the model parameter estimates is then quantified using the Cook distance and DFBETA measures. Seven model parameters are included in the analysis, with five Arrhenius pre-exponential factors in the gas-phase kinetic rate expressions, and two kinetic rate constants in the population balance model. The analysis highlights certain experimental conditions and kinetic parameters which warrant closer inspection due to large influence, thus providing clues as to which aspects of the model require improvement. We find the insights provided can be useful for future model development and planning of experiments.

Diagnostics of impurity composition of high-purity monosilane by results of the analysis of a test silicon single crystal

Elements the 3 and 5 of groups of the Periodic System and carbon are the most important impurities in silicon. The estimation technique of carbon, boron and phosphorus impurity content in high-purity monosilane has been proposed. The technique involves the preparation of polycrystalline silicon by silane decomposition, growing a test single crystal by the floating zone melting method and analysis of single crystal samples by IR spectroscopy. Calculation of impurity concentration in polycrystalline silicon has been performed using results on their content in the test single crystal samples, data on impurity distribution in the liquid-solid system and sample coordinates along the ingot length. Effective impurity distribution coefficients in the solid-liquid system for specific growing conditions have been calculated using the Burton-Prim-Slichter equation. Results for the test silicon samples with natural isotopic composition and 28Si isotope enriched ones obtained from monosilane samples with different impurity contents have been reported. Results on IR spectroscopic studies of impurity composition for the test silicon single crystal are in agreement with the concentration data obtained by chromatography. The concentration of 3 and 5 group impurities in monosilane were in the range 4x10−9−2x10−6 at. %, and for carbon concentration was 2x10−6–6x10−4 at.%. The measurement uncertainty by IR spectroscopy does not exceed 15% for carbon impurity and 20% for boron and phosphorus. We show that the upper limit of carbon content in monosilane detected using this method is determined by its solubility in silicon, while the bottom limit depends on the detection accuracy of the IR spectroscopy technique and possible background contamination.

Multiple mapping conditioning for silica nanoparticle nucleation in turbulent flows

In this paper a study of the sparse-Lagrangian multiple mapping conditioning (MMC) model for the synthesis of silica nanoparticles is presented. This is the first attempt using the model to simulate the formation of kinetically limited solid state species. A simplified differential diffusion model, involving a modified mixing timescale, is used to account for the much lower diffusion of the solid species. The model is validated against DNS of a counterflowing double shear layer of silane and hot combustion products. The reduced reaction mechanism involves silane decomposition and clustering leading to silica. The model is implemented in the context of large eddy simulations (LES). The MMC model allows a sparse distribution of stochastic particles for computing the reactive scalar field, including silica number density, by enforcing the property of localness in mixture fraction space onto the mixing model. Gas phase mixture fraction and fast reactive species are predicted with acceptable accuracy. The prediction of silica number density is qualitatively correct although the magnitude is underpredicted somewhat and the rms is greatly underpredicted. The effects of differential diffusion are clearly visible in the modelling and the simplified model correctly predicts the trend of increasing silica formation when differential diffusion is included.

Core-shell amorphous silicon-carbon nanoparticles for high performance anodes in lithium ion batteries

Abstract Core-shell silicon-carbon nanoparticles are attractive candidates as active material to increase the capacity of Li-ion batteries while mitigating the detrimental effects of volume expansion upon lithiation. However crystalline silicon suffers from amorphization upon the first charge/discharge cycle and improved stability is expected in starting with amorphous silicon. Here we report the synthesis, in a single-step process, of amorphous silicon nanoparticles coated with a carbon shell (a-Si@C), via a two-stage laser pyrolysis where decomposition of silane and ethylene are conducted in two successive reaction zones. Control of experimental conditions mitigates silicon core crystallization as well as formation of silicon carbide. Auger electron spectroscopy and scanning transmission electron microscopy show a carbon shell about 1 nm in thickness, which prevents detrimental oxidation of the a-Si cores. Cyclic voltammetry demonstrates that the core-shell composite reaches its maximal lithiation during the first sweep, thanks to its amorphous core. After 500 charge/discharge cycles, it retains a capacity of 1250 mAh.g −1 at a C/5 rate and 800 mAh.g −1 at 2C, with an outstanding coulombic efficiency of 99.95%. Moreover, post-mortem observations show an electrode volume expansion of less than 20% and preservation of the nanostructuration.

Comparative Study of Hydrogen and Argon Dilution Effects in Amorphous SiC Thin Films Deposited by Plasma Enhanced Chemical Vapor Deposition.

Amorphous SiC thin films were deposited from a mixture of silane and methane gas with the separate hydrogen and Argon dilution by using the plasma enhanced chemical vapor deposition process. The effects of different dilution gas on the chemical composition, bonding states, microstructure and optical property of the films were studied and compared. For the films with H2 dilution, the higher hydrogen content passivated more Si dangling bonds so that the formation of nanocrystalline silicon was inhibited and the films were kept in amorphous state. The amorphous nature along with the higher carbon content of the films led to the larger optical band gap E(opt), of 2.63 eV. In contrast, for the films with Ar dilution, they presented a relative higher deposition rate due to the rapid decomposition of silane accelerated by Ar. The Si-Si bonds were formed and they had a higher tendency to precipitate silicon quantum dots.

CFD–PBM coupled simulation of silicon CVD growth in a fluidized bed reactor: Effect of silane pyrolysis kinetic models

The coupled numerical code of Eulerian–Granular model and population balance model was applied to simulate silane-derived silicon chemical vapor deposition growth in a three-dimensional fluidized bed reactor. Two silane homogenous kinetic models were used to obtain the gaseous concentration of silicon cluster for further calculation on cluster scavenging rate, and two heterogeneous kinetic models implemented to evaluate the rate of silane surface deposition. The applicability of silane pyrolysis kinetic models on gaseous species and silicon growth were analyzed in detail after the verification of fundamental flow behavior. The results showed that the growth rate agreed well with Hsu’s experimental data under the combination kinetic models of Hogness’s homogenous and Furusawa’s heterogeneous ones, in which a complete silane decomposition was almost achieved, but the impractical growth result was obtained as inlet silane mole fraction higher than 0.5. Moreover, the interphase mass transfer of silane surface deposition and cluster scavenging was also focused, which showed that silane surface deposition tended to occur at the solid dense bottom region with higher mass transfer rate and cluster scavenging happened in almost the whole solid phase.