Pyrolysis Of Silane Research Articles

CFD-DEM analysis of particle polydispersity on the performance of fluidised bed reactor during silane pyrolysis

A CFD-DEM model is developed for chemical vapour deposition of silane in a fluidized bed reactor to investigate the effect of polydispersity on local phase dynamics, mass and heat transport, as well as the rate of Si deposition. Both gas-phase fines formation and heterogeneous deposition on the seed particle, as well as the scavenging effect for Si particle growth, are incorporated. The method is first validated against the experimentally measured Si deposition rate, percentage of fine production and minimum fluidisation velocity. Subsequently, the properties, primarily particle intermixing, fluidisation behaviour and bubble dynamics along with the deposition process, are analyzed for several polydisperse beds under various operating conditions. The effect of polydispersity of the bed on a fraction of the bubble size, dense phase expansion and interchange coefficient between various phases is thoroughly investigated to establish a correlation with reactor performance. The results show that broad-Gaussian particle size distribution (PSD) exhibits excellent improvement in fluidisation behaviour, resulting in a high deposition rate, and minimum formation of Si fines.

A comparative theoretical and experimental investigation on thermal parameters and initial pyrolysis mechanism of methyl-phenyl-dimethoxy silane modified phenolic resin

In this paper, classical molecular dynamics and reactive molecular dynamics are used to study the glass transition temperature, thermal conductivity, and initial reaction of pyrolysis of methyl-phenyl-dimethoxy-silane-modified phenolic resins. The simulation results showed that silanes increased the glass transition temperature and thermal conductivity of phenolic resins, resulting in an increase in glass transition temperature from 426.4 K to 449.8 K and thermal conductivity from 0.21 W/(m·k) to 0.226 W/(m·k). To verify the accuracy of the simulation calculations, we experimentally tested the glass transition temperature and thermal conductivity of silane-modified phenolic resins. The results showed that the glass transition temperatures of phenolic resin and silane-modified phenolic resin were 333.5 K and 353.5 K, respectively, while the thermal conductivities were 0.221 W/(m·k) and 0.235 W/(m·k), respectively. The error between the simulated and experimental thermal conductivity values was 5 %. At the same time, the glass transition temperature did not match the experimental value due to the difference in cooling rate between the simulation and experiment. Then, it was corrected using the WFL equation at a low cooling rate, resulting in an error of about 1.6 % between the glass transition temperature and the experimental value. In addition, the silane-doped phenolic resin produced a large amount of protective gas during the pyrolysis process, which carries away heat as it evaporates and forms a reaction barrier. Since a silicon-oxygen bond replaces the phenolic hydroxyl group, the production of hydroxyl groups during pyrolysis is reduced. At the same time, the silicon radicals produced by silane pyrolysis form silicon hydroxides with the hydroxyl groups. This dramatically reduces the likelihood of hydroxyl groups oxidizing other functional groups, slowing down the formation of CO and increasing the carbon residual rate.

Electrochemical scissoring of disordered silicon-carbon composites for high-performance lithium storage

Practically adapted physical integration of silicon and carbon predominates as a viable solution to realize high energy density batteries, however, the composite structure is vulnerable to fracture. Here we report a molecular-level mixed silicon-carbon composite anode through thermal pyrolysis of silane and subsequent mechanical mill, entailed by electrochemical dissociation and reclustering of such disordered silicon-carbon bonds during the cycles. Lithium insertion induces heterolytic fission of the bonds into sub-nanometre silicon particles segregated by redox-active carbon framework validated by microscopy analysis and reactive molecular dynamics simulation. The embedded structure with a high packing density of silicon prevents detrimental electrochemical coalescence and direct contact to a liquid electrolyte to stabilize the interfaces, while three-dimensional (3D) carbon framework buffers large volume expansion of silicon to enable an extended full battery cycling.

Current status of the high-temperature kinetic models of silane: Part I. Pyrolysis

Abstract The present work compares the performance of seven reaction models with respect to a large experimental dataset relevant to the high-temperature pyrolysis of both silane (SiH 4 ) and disilane (Si 2 H 6 ). Their performances were established based on different validation criteria that account for the shape and the amplitude of the validation profile. Then, the model performances were quantified with a global error, which accounts for the experimental uncertainties. The most satisfactory model has a global error as low as 3.1 (i.e., meaning 3.1 times higher than the experimental uncertainty) and the highest fraction (74%) of criteria with a low error ( 2 ), while most of the models have large discrepancies with the validation dataset, global error near 8 and up to 110 for the less accurate model. The origins of these discrepancies are identified with reaction pathway and sensitivity analyses. Among the seven tested model, three main decomposition pathways are evidenced, including one more specific to the models presenting the lowest errors. Based on the global error values, the ability to reproduce all the experimental conditions, and the model analyses, the reaction pathways relevant to the high-temperature pyrolysis of silane and disilane are determined. In addition, the present study provides experimental and numerical guidance for the future developments of silicon hydride reaction models. The limited performance of most of the oldest reaction models may have a significant impact on our current understanding of the pyrolysis and oxidation kinetics of silane.

Crystallinity of Silicon Nanoparticles: Direct Influence on the Electrochemical Performance of Lithium Ion Battery Anodes

AbstractThe use of silicon (Si) in the form of nanoparticles is one of the most promising routes for boosting the capacity of modern Li‐ion batteries. Many parameters influence the performance of Si making the comparison of materials complicated. The present work demonstrates a direct comparison of Si nanoparticles with amorphous and crystalline structures prepared through the same chemistry with the same particle size and morphology. The amorphous Si nanoparticles with an average diameter of 100 nm were synthesized through silane pyrolysis, and their crystalline analogues were obtained through subsequent annealing not altering size or morphology of the nanoparticles. Such direct comparison allows evaluation of the specific impact of crystallinity on the material's performance. From electrochemical analysis of these materials, the electrodes prepared from amorphous nanoparticles were found to exhibit improved cycle life compared to electrodes prepared from crystalline nanoparticles when the delithiation capacity of the anode was limited to 1000 mAh/gSi.

Correction of reaction models using collision limit violation analyses: Application to a silane reaction model

This study presents a detailed methodology to verify and correct the chemical consistency of any reaction model based on collision limit violations (CLV) analyses, thermochemical data calculations, and reaction rates reffiting. As a study case, an a priori well validated reaction model representative of silane chemistry is selected to highlight the benefit of this methodology. In the final mechanism, 30 new thermochemical data obtained using a high theory composite method (G4//B3LYP/6-311++G(3df,2p)) are corrected, as well as 13 reaction rates initially above the collision limits. In addition to the suppression of unphysically high reaction rates, the collision limit violation analyses allow to identify and correct important inconsistencies of the initial reaction model such as species mislabeling or discontinuities in some thermodynamic data, which are not easily detectable without an extensive review of each submodels. The novelties of the present work include (i) the discussion of the CLV uncertainty, (ii) the use of this uncertainty to identify CLVs, and (iii) the establishment of the relationship between high CLV values and species mislabeling. As a results of this study case on a specific mechanism, this work also provides a more consistent reaction model for silane chemistry, relevant for both high temperature combustion and flame relevant conditions, as well as a large number of ab initio calculation results, relevant to the SixHyOz chemical system. Based on the present analyses, the predictions of the final reaction model are mainly improved for silane pyrolysis, while the benefits are more limited for silane oxidation.

Measurement of sub-2 nm stable clusters during silane pyrolysis in a furnace aerosol reactor.

The initial stages of particle formation are important in several industrial and environmental systems; however, the phenomenon is not completely understood due to the inability to measure cluster size distributions. A high resolution differential mobility analyzer with an electrometer was used to map out the early stages of Si particle formation from pyrolysis of SiH4 in a furnace aerosol reactor. We detected for the first time subnanometer stable clusters from silane pyrolysis, and the diameter was measured to be about 0.7 nm. This diameter is within the range of probable sizes that the reported families of critical silane clusters could have based on their actual molecular structure. The size distributions of negative clusters are also mapped out. In addition, gas chromatography mass spectrometry, and transmission electron microscopy characterizations of the clusters and primary particles are used to assess their mechanistic roles in aerosol dynamics of the initial stages of particle formation.

Best Performing SiGe/Si Core‐Shell Nanoparticles Synthesized in One Step for High Capacity Anodes

AbstractSilicon‐germanium nanostructures are promising anode materials for high stability, high capacity, and fast cycling Li‐ion batteries. In this work, we report on the outstanding performance of new SiGe/Si core@shell nanoparticle heterostructures synthetized in one step by laser pyrolysis of silane and germane. By tuning the silane to germane ratio, the composition of Si100‐xGex alloy was readily adjusted. Nanoparticles with x=0, 20, 47, 77, and 100 were investigated and the composition of each alloy (including internal mixed phases) was confirmed by X‐ray diffraction and energy‐dispersive X‐ray spectroscopy. The electrochemical performances of the Si100‐xGex alloys were evaluated by cycling half cell batteries from C/5 to 5 C. The optimal trade‐off between stability and capacity was obtained in Si53Ge47 core shell nanoparticles alloy. This material exhibits the best performance reported so far for SiGe compounds, with a reversible specific capacity of 1695 mAh.g−1 after 60 cycles (90 % of its initial value). The (de)alloying properties of this optimal Si53Ge47 heterostructure were followed by operando synchrotron WAXS measurements, suggesting sequential lithiation of the various phases present in the material. The alloying process, combined with the realization of peculiar nanostructures composed of a Ge‐rich core and a Si‐rich shell, therefore allow to reach electrochemical properties suited for a practical application in energy storage device.

Relative kinetics of nucleation and condensation of silane pyrolysis in a helium atmosphere provide mechanistic insight in the initial stages of particle formation and growth

Chemical nucleation for the synthesis of solid nanoparticles from gas phase precursor is a fundamental process of technological relevance, yet, not well understood. In this work, a silane pyrolysis system was built to obtain insights in particle formation and growth. Primary particle analysis was used to determine the relative condensation and nucleation rates in the aerosol process of silane pyrolysis. TEM, XRD and SMPS allow the comparison between primary particle and agglomerate dynamics. The analysis reveals that nucleation is correlated to silane concentration to the power of 2/3. This relationship and other observations are used to assess the impact of magnetic interactions in classical Brownian agglomeration in the initial stages of primary particle dynamics in silane pyrolysis. Furthermore, this mechanism is consistent with key observations reported in the literature regarding nanoparticles of various materials, suggesting the proposed mechanism could be valid for more materials and deserves further attention.

Laser-Processed Nanosilicon: A Multifunctional Nanomaterial for Energy and Healthcare.

This review describes promising laser-based approaches to produce silicon nanostructures, including laser ablation of solid Si targets in residual gases and liquids and laser pyrolysis of silane. These methods are different from, and complementary to, widely used porous silicon technology and alternative synthesis routes. One can use these methods to make stable colloidal dispersions of silicon nanoparticles in both organic and aqueous media, which are suitable for a multitude of applications across the important fields of energy and healthcare. Size tailoring allows production of Si quantum dots with efficient photoluminescence that can be tuned across a broad spectral range from the visible to near-IR by varying particle size and surface functionalization. These nanoparticles can also be integrated with other nanomaterials to make multifunctional composites incorporating magnetic and/or plasmonic components. In the energy domain, this review highlights applications to photovoltaics and photodetectors, nanostructured silicon anodes for lithium ion batteries, and hydrogen generation from water. Application to nanobiophotonics and nanomedicine profits from the excellent biocompatibility and biodegradability of nanosilicon. These applications encompass several types of bioimaging and various therapies, including photodynamic therapy, RF thermal therapy, and radiotherapy. The review concludes with a discussion of challenges and opportunities in the applications of laser-processed nanosilicon.

WITHDRAWN: Relative kinetics of nucleation and condensation of silane pyrolysis in a helium atmosphere provide mechanistic insight in the initial stages of particle formation and growth

WITHDRAWN: Relative kinetics of nucleation and condensation of silane pyrolysis in a helium atmosphere provide mechanistic insight in the initial stages of particle formation and growth

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.

Identification of higher order silanes during monosilane pyrolysis using gas chromatography-mass spectrometry

There is a deficit of ways to detect higher order silane isomers during silane pyrolysis. Thus, a novel instrument utilizing gas chromatography-mass spectrometry (GC-MS) for detection of higher order silanes has been developed. The instrument enables us to separate higher order silane species using gas chromatography before they are introduced to the mass spectrometer, thereby obtaining spectra of separate isomers, rather than overlaid spectra. In this contribution we describe the details of the GC-MS system. We compare our GC-separated mass spectra of mono-, di- and trisilane to mass spectra of these species available in the literature. Further, we present mass spectra of the tetrasilane isomers n-tetrasilane (n-Si4H10), silyltrisilane (i-Si4H10) and cyclotetrasilane (cyclo-Si4H8) and of the pentasilane isomers n-pentasilane (n-Si5H12), silyltetrasilane (i-Si5H12) disilyltrisilane (neo-Si5H12) and cyclopentasilane (cyclo-Si5H10). Six of these mass spectra are previously unpublished. Based on the fragmentation pattern in the tetra- and pentasilane mass spectra, we are able to acquire mass spectra of silanes with up to eight silicon atoms. Finally, we apply the novel detection technique to a silane pyrolysis reactor to track the outlet concentration of higher order silanes as function of reactor temperature. We believe that the detection technique that we present here may open the door for validation of monosilane pyrolysis models, and thus constitute a roadmap for future research in this field.

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.

Sub-oxide passivation of silicon nanoparticles through rapid mechanical attrition

Inks produced from the inclusion of silicon nanoparticles in a suitable polymer present the best opportunity for true printability of silicon with purely additive patterning under ambient conditions with no thermal post-processing. For large scale production of silicon nanoparticles, the most appropriate methods are vapor phase synthesis by pyrolysis of silane or the milling of bulk material. In this work, we compare low energy ball milling using zirconium ball with a rapid attrition of bulk silicon using a high-energy vibratory disk mill and show that the later process passivates the further growth of surface oxide on the nanoparticles. XPS results indicated a significant surface oxidation for silicon nanoparticles produced using low energy ball milling relative to the rapid attrition using the high-energy vibratory disk. HRTEM shows a pronounced surface oxide thickness on the nanoparticles produced via low energy ball mill. This surface oxide resulted in charge trapping and drastically effected the carrier transport of the compacted powder. The powder produced from the high-energy mill, in contrast, does not show a distinct surface oxide layer under HRTEM and the carrier transport was generally ohmic.

Silane Pyrolysis to Silicon Rod in a Bell-Jar Reactor at High Temperature and Pressure: Modeling and Simulation

Chemical vapor deposition (CVD) at high temperature and pressure in a unique bell-jar reactor has been widely applied for high-pure polysilicon production using trichlorosilane (TCS) as a precursor. Silane is an alternative to TCS and used for ultrapure polysilicon. Nevertheless, silane is so reactive that results in significant homogeneous nucleated fines which lead to low yield and quality of crystalline silicon product. In this paper, we aimed to minimize the homogeneous nucleation by modulating flow pattern and temperature field in reactor. A novel bell-jar reactor with cooling jacket for each rod was modeled and simulated based on a three-dimensional CFD model. Contribution of homogeneous nucleation to rod growth on a basis of kinetic theory was included. The result showed that, in the novel reactor, the fresh feed was restricted in channels between cooling jacket and rod in which the favorable temperature and velocity profiles were achieved, and the gas phase near the rod was cooled so that the homo...

Effects of Synthesis Parameters on Silicon Nanopowders Produced by CO2 Laser‐Driven Pyrolysis of Silane

A well‐designed and assembled apparatus for producing silicon nanoparticles by CO2 laser‐driven pyrolysis of SiH4 is shown. The effects of process parameters (chamber pressure, laser power, gas composition) on the nano‐silicon characteristics (average particle size, size distribution and shape) are systematically investigated. The produced silicon nanopowders are characterized and analyzed, demonstrating the produced particles are much smaller and much more uniform in size than the commercial products and those previously reported. The impressive productivity and yield are also discussed. This research allows a better understanding of the influences of processing parameters on silicon nanopowders, shows a controllable way of producing the desired powders, and paves the way to commercialization.

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.