Aerosol dynamics modeling of silicon nanoparticle formation during silane pyrolysis: a comparison of three solution methods (original) (raw)
Related papers
2011
The aim of this work is to present a new detailed multivariate population balance model to describe the aerosol synthesis of silica nanoparticles from tetraethoxysilane (TEOS). The new model includes a chemical representation of the silica particles to facilitate a detailed chemical description of particle processes. Silica nanoparticles are formed by the interaction of silicic acid monomers (Si(OH) 4) in the gas-phase as reported in a previous study. A multidimensional population balance model is developed where each particle is described by its constituent primary particles and the connectivity between these primaries. Each primary, in turn, has internal variables that describe its chemical composition, i.e., the number of Si, free O and OH units. Different particle processes, such as inception, surface reaction, coagulation, sintering, and intra-particle reactions, are formulated from first-principles that alter the particle ensemble and are two-way coupled to the gas-phase. The free parameters in the model are estimated by fitting the model response to experimental values of collision and primary particle diameters using low discrepancy Sobol sequences followed by the simultaneous perturbation stochastic approximation algorithm. The simulation results are finally presented at different process conditions. A strong dependence of particle properties on process temperature and inlet concentration is observed. The desirable operating conditions for different industrial applications are also highlighted. This work illustrates the significance of adopting a multidimensional approach to understand, and hence control, complex nanoparticle synthesis processes.
Aerosol Science and Technology, 2009
In this work, a two-dimensional model was developed for silicon nanoparticle synthesis by silane thermal decomposition in a sixway cross laser-driven aerosol reactor. This two-dimensional model incorporates fluid dynamics, laser heating, gas phase and surface phase chemical reactions, and aerosol dynamics, with particle transport and evolution by convection, diffusion, thermophoresis, nucleation, surface growth, coagulation, and coalescence processes. Because of the complexity of the problem at hand, the simulation was carried out via several sub-models. First, the chemically reacting flow inside the reactor was simulated in three dimensions in full geometric detail, but with no aerosol dynamics and with highly simplified chemistry. Second, the reaction zone was simulated using an axisymmetric two-dimensional CFD model, whose boundary conditions were obtained from the first step. Last, a twodimensional aerosol dynamics model was used to study the silicon nanoparticle formation using more complete silane decomposition chemistry, together with the temperature and velocities extracted from the reaction zone CFD simulation. A bivariate model was used to describe the evolution of particle size and morphology. The aggregates were modeled by a moment method, assuming a lognormal distribution in particle volume. This was augmented by a single balance equation for primary particles that assumed locally equal number of primary particles per aggregate and fractal dimension. The model predicted the position and size at which the primary particle size is frozen in, and showed that increasing the peak temperature was a more effective means of improving particle yield than increasing silane concentration or flowrate.
Journal of Aerosol Science, 2003
This paper discusses an experimental and numerical study of the nucleation and growth of particles during low-pressure (∼1:0 Torr) thermal decomposition of silane (SiH 4 ). A Particle Beam Mass Spectrometer was used to measure particle size distributions in a parallel-plate showerhead-type semiconductor reactor. An aerosol dynamics moment-type formulation coupled with a chemically reacting uid ow model was used to predict particle concentration, size, and transport in the reactor. Particle nucleation kinetics via a sequence of chemical clustering reactions among silicon hydride molecular clusters, growth by heterogeneous chemical reactions on particle surfaces and coagulation, and transport by convection, di usion, and thermophoresis were included in the model. The e ect of pressure, temperature, ow residence time, carrier gas, and silane concentration were examined under conditions typically used for low-pressure (∼1 Torr) thermal chemical vapor deposition of polysilicon. The numerical simulations predict that several pathways involving linear and polycyclic silicon hydride molecules result in formation of particle "nuclei," which subsequently grow by heterogeneous reactions on the particle surfaces. The model is in good agreement with observations for the pressure and temperature at which particle formation begins, particle sizes and growth rates, and relative particle concentrations at various process conditions. A simpliÿed, computationally inexpensive, quasi-coupled modeling approach is suggested as an engineering tool for process equipment design and contamination control during low-pressure thermal silicon deposition. ?
Journal of The Electrochemical Society, 2000
The growth of silicon films via chemical vapor deposition (CVD) is of considerable importance in the microelectronics and photovoltaics industries. This process often involves the thermal decomposition of silane, which is achieved by heating the wafer or rod to be coated to a suitable temperature. A wide range of geometries and conditions are employed. For example, low-pressure chemical vapor deposition (LPCVD), at pressures around 1 Torr (133 Pa), is used with a stagnation-point flow geometry to deposit thin films of silicon in the fabrication of integrated circuits, while cylindrical polysilicon rods are grown on a heated filament using atmospheric-pressure CVD (APCVD).
Coagulation and Aggregation Model of Silicon Nanoparticles from Laser Pyrolysis
Aerosol Science and Technology, 2006
For spherical silicon nanoparticles produced by laser pyrolysis in a temperature range below the melting point of the bulk phase, the average particle size measured by transmission electron microscopy has been found to be larger than the crystallite size derived from profile fitting of broadened X-ray diffraction peaks. This result, interpreted in terms of aggregation and solidification of particles during the coagulation process, has been explained by a sectional model suitably developed to describe coagulation and aggregation of particles. The model predicts the evolution of the size distribution of both macroscopic polycrystalline particles and of crystallites composing the aggregates. Single crystal particles are supposed to form not only by collisions between liquid particles but also by collisions between a larger solid and a smaller liquid particle coexisting in the system due to capillarity effects which are responsible for the decreasing of the melting point of nanometresized particles with respect to the bulk phase. On the other hand, polycrystalline aggregates are supposed to form by collisions between solid particles. The spheroidal shape of particles found in the analysed powders has been explained by observing that the characteristic sintering time was at least one order of magnitude smaller than the characteristic time for coagulation. The good predictive capabilities of the model for the average particle size of both the macroscopic size distribution and that of crystallites have confirmed that the hypothesis of aggregation is able to quantitatively reproduce the experimental findings.
The Journal of Physical Chemistry B, 1999
Product contamination by particles nucleated within the processing environment often limits the deposition rate during chemical vapor deposition processes. A fundamental understanding of how these particles nucleate could allow higher growth rates while minimizing particle contamination. Here we present an extensive chemical kinetic mechanism for silicon hydride cluster formation during silane pyrolysis. This mechanism includes detailed chemical information about the relative stability and reactivity of different possible silicon hydride clusters. It provides a means of calculating a particle nucleation rate that can be used as the nucleation source term in aerosol dynamics models that predict particle formation, growth, and transport. A group additivity method was developed to estimate thermochemical properties of the silicon hydride clusters. Reactivity rules for the silicon hydride clusters were proposed based on the group additivity estimates for the reaction thermochemistry and the analogous reactions of smaller silicon hydrides. These rules were used to generate a reaction mechanism consisting of reversible reactions among silicon hydrides containing up to 10 silicon atoms and irreversible formation of silicon hydrides containing 11-20 silicon atoms. The resulting mechanism was used in kinetic simulations of clustering during silane pyrolysis in the absence of any surface reactions. Results of those simulations are presented, along with reaction path analyses in which key reaction paths and rate-limiting steps are identified and discussed.
The Journal of Physical Chemistry A, 2004
Thermal decomposition of silane can be used to produce silicon nanoparticles, which have attracted great interest in recent years because of their novel optical and electronic properties. However, these silicon nanoparticles are also an important source of particulate contamination leading to yield loss in conventional semiconductor processing. In both cases, a fundamental knowledge of the reaction kinetics of particle formation is needed to understand and control the nucleation of silicon particles. In this work, detailed kinetic modeling of silicon nanoparticle formation chemistry was carried out using automated reaction mechanism generation. Literature values, linear free-energy relationships (LFERs), and a group additivity approach were incorporated to specify the rate parameters and thermochemical properties of the species in the system. New criteria for terminating the mechanisms generated were also developed and compared, and their suitability for handling an unbounded system was evaluated. Four different reaction conditions were analyzed, and the models predicted that the critical particle sizes were Si 5 for an initial H 2 /SiH 4 molar ratio of 90:10 at 1023 K and Si 4 for the same initial composition at 1200 K. For an initial H 2 /SiH 4 molar ratio of 99:1, the critical particle size was larger than or equal to Si 7 for both temperatures, but it was not possible to determine the exact critical particle size because of limitations in computational resources. Finally, the reaction pathways leading to the formation of nanoparticles up to the critical size were analyzed, and the important species in the pathways were elucidated.
Modelling the flame synthesis of silica nanoparticles from tetraethoxysilane
Chemical Engineering Science, 2012
This work proposes a kinetic model and an inception pathway for the flame synthesis of silica nanoparticles from tetraethoxysilane (TEOS). The kinetic model for the decomposition of TEOS is developed by generating reactions involving species that were reported in high concentrations at equilibrium. Flux and sensitivity analyses are then performed to identify the main reaction pathways. The parameters for these reactions are systematically fitted to experimental data using low discrepancy (LD) sequences and response surfaces. The main product of TEOS decomposition is deduced to be silicic acid (Si(OH) 4). To increase computational efficiency, the kinetic model has been reduced by determining the level of importance (LOI) of each species and retaining only the important ones. This reduced kinetic model is then coupled to a detailed population balance model using an operator splitting technique. New particle inception and surface growth steps have been incorporated into the particle model in which particles form and grow by the interaction of Si(OH) 4 monomers. Coagulation and sintering of particles are also included in the model and the material dependent sintering parameters have been determined by fitting the model to experimental values of collision and primary particle diameters using LD sequences. The particle size distributions and computer-generated TEMstyle images have been generated and good agreement with experiments is observed. The gas-phase reactor composition and the temporal evolution of particle size at different temperatures are also presented.
Journal of Aerosol Science, 2012
Flame aerosol synthesis is one of the commonly employed techniques for producing ultra fine particles of commodity chemicals such as titanium dioxide, silicon dioxide and carbon black. Large volumes of these materials are produced in industrial flame reactors. Particle size distribution of product powder is the most important variable and it depends strongly on flame dynamics inside the reactor, which in turn is a function of input process variables such as reactant flow rate and concentration, flow rates of air, fuel and the carrier gas and the burner geometry. A coupled flame dynamicsmonodisperse population balance model for nanoparticle synthesis in an aerosol flame reactor is presented here. The flame dynamics was simulated using the commercial computational fluid dynamics software CFX and the particle population dynamics was represented using a monodisperse population balance model for continuous processes that predicts the evolution of particle number concentration, particle volume and surface area. The model was tested with published experimental data for synthesis of silica nanoparticles using different burner configurations and with different reactor operating conditions. The model predictions for radial flame temperature profiles and for the effects of process variables like precursor concentration and oxygen flow rate on particle specific surface area and mean diameter are in close agreement with published experimental data.
Modeling and simulation of nanoparticle production in an aerosol flame reactor
2017
Aerosol flame synthesis is one of the commonly used methods for producing nanoparticles on a large scale. Particle size distribution (PSD) is one of the important variables that determines the end use of product nanoparticles. The PSD strongly depends on flame dynamics inside the reactor, which in turn, is a function of input process variables such as reactant flow rate and concentration, flow rates of air, fuel, carrier gas and the burner geometry. A coupled flame dynamics-population balance model (PBM) for nanoparticle synthesis in an aerosol flame reactor is presented here. Various case studies for process design and scale up of a lab scale flame reactor and pilot scale furnace reactor for synthesis of titania, silica and carbon black will be presented. The model predictions were tested with published experimental data for flame temperature and particle size distributions. The model presented here will be useful for simulation of fine particle production in industrial furnace rea...