JPM. 1. Introduction. Excerpt from the Proceedings of the 2016 COMSOL Conference in Munich. requirement is the. through. efficient design of

Transcription

1 Multiphysics Modelling of a Microwave Furnace for Efficient Silicon Production N. Rezaii 1, J.P. Mai 2 JPM Silicon GmbH, j-p.mai@jpmsilicon.com Abstract: Silicon must be extremely pure, above % be used in solar and semiconducr applications. A novel approach meet this requirement is use of microwave furnaces and high-purity raw materials without going through successive purification processes. Hence, it is desirable develop computer simulation methodologies identify efficient design of microwave furnace. This is done by modelling internal processes taking place within microwave. For this purpose, governing equations and boundary conditions are solved for several combinations of materials and geometries. The resulting profiles provide valuable information on performancee of system. Keywords: Solar Silicon, Microwave Furnace, Radio Frequency, Heat Transfer, Computational Fluid Dynamics 1. Introduction The demand for silicon as main raw material in communications and phovoltaics is steadily increasing. The JPM silicon GmbH presents a novel method for production of solar grade silicon in a microwave oven. This method can specially reduce energy costs and increase efficiency of process. Figure 1 shows a schematic view of components of system. Principally, core of system is a magnetron, h generates electromagnetic microwaves. The generated waves are transmitted through waveguide reaction chamber or resonar. The reaction chamber holding sample is main areaa for concentration of electrical field, heats sample up. The tuner guarantees better absorption of microwaves generated by magnetron. Finally, system is equipped with a circular, dissipates reflective microwave energy in a water bath as a heat sink in order protect magnetron against possible overheating. Electromagnetic Resonar Power Source Tuner Circular Sample Water Cooling System Waves Generated by Magnetron Waves Reflected from Sample Figure 1. Schematic representation of microwave system of JPM Silicon GmbH. Basically, microwave heating allows for selective heating of certain materials on a volumetric heat input, thus reducing heat loss through a temperature drop from inside out, over conventional processes. Due faster warming and shorterr residence time, diffusion of impurities in sample is also reduced. In order optimize existing processes, knowledge of three-dimensional temperature distribution and local heat flux is required. A measurement of temperature with rmocouples in microwave processs is not possible, since presencee of metallic objects leads a local elevation of electromagnetic field. Instead, a numerical model is developed, depicts physical, chemical and electromagnetic phenomena of silicon production process. Using COMSOL Multiphysics software a dynamic coupling between electromagnetic field distribution and temperature profile can be established, particularly takes strong temperature dependency of electromagnetic properties of some materials in account. In order increase efficiency of system, it is important identify relevant influencing parameters and estimate ir uncertainties. By variation of component s materials and design of system, model is optimized and extended for a multicomponent system.

2 2. Governing Equations in Microwave Heating The equations that govern microwave heating of a material are Maxwell s Equations, govern propagation of microwave radiation, and Forced Heat Equations, govern absorption and diffusion of heat by materials. 2.1 Maxwell s Equations For calculating heat input in a volume element, calculation of electric field () and magnetic field () is required. The Maxwell equations describe relationship between different electromagnetic field sizes. = + = = =0 In order solve Maxwell equations, additional equations are needed that describe material dependency behavior. These equations can be simplified as: = 0 = 0 Where and μ describe electromagnetic behavior in vacuum. It should be noted that permittivity is a complex quantity, composing of a real ( ) and imaginary part ( ). = + Where real part is a measure of polarizability of a material, while imaginary part characterizes quantity of associated heat release. 2.1 Forced Heat Equations Thermal Conduction: By means of conduction, heat is transferred through a medium as a result of interactions between molecules or ams. In this form of heat transfer, no microscopic material flow is required, thus it is mainly specified solid media. For a sufficiently large body, heat conduction can be calculated through Fourier s law. This law states that time rate of heat transfer through a material is proportional negative gradient in temperature and area, at right angles that gradient, through heat flows. = = Thermal Convection: Convective heat transfer is one of major types of heat transfer between a solid and a fluid. When re exist a temperature difference between a body and a fluid in contact, heat is transferred from surface of body fluid by means of conduction. Individual particles of fluid take rmal energy and carry it from surface in main stream flow. The general approach description and calculation of heat supplied by convection is: = Heat Radiation: In heat transfer by radiation, no transport medium is required. All bodies can emit energy in form of electromagnetic radiation. The intensity of this radiation depends on one hand on temperature of body and on or hand, on its nature and geometry. Heat radiation from a solid or a liquid usually possesses a continuous spectrum with a characteristic curve, can be described by Planck's radiation law. However, gases emit radiation in range of characteristic line spectrum of material. The oretical maximum heat flux of a so-called black body is described by Stefan-Boltzmann law: = 3. Theory Silicon is product of reduction of quartz (SiO2) with carbon (C) at high temperatures resulting in silicon (Si) and carbon monoxide (CO)

3 In fact, conventional silicon production methods are not only energy intensive and costly but also experienced operars are required. Furrmore, produced silicon metal has a purity of 98..5% and has be refined reach required purity of %. In day s industry this is done via gas-phase. Thus, silicon metal forms a gaseous silicon compound with hydrogen chloride. This compound is distilled high- silicon. The process itself originates in 1950s and is technically complex and energy intensive. On contrary, JPM Silicon starts with high- purity raw materials and uses a contamination- free microwave furnace directly obtain solar silicon. This reduced energy consumption and purity and decomposed afterwards give solar subsequent refining costs results in low-cost solar silicon. 4. Model Properties As shown in Figure 2, structure of microwave oven can be dividedd in main three parts: 1) reaction zone, 2) waveguide, and 3) electromagnetic power source. Heat Source Inlet Outlet The reaction chamber encompasses crucible, isolation, waveguide ports for coupling microwave field, and gas inlets or outlets. The reaction zone refers area that is in direct contact with reaction mixture, must be heated. The waveguide structure is used transmit generated electromagnetic field in resonar chamber. The distribution of electromagnetic field should be in way that predetermined power input is achieved. The maximumm power be provided by furnace is 2.4 kw at a nominal frequency of 2.45 GHz. For illustration of relevant processes within microwave oven, Heat Transferr (HT) module, Radio Frequency (RF) module, and Computational Fluid Dynamics (CFD) module are used. In RF module, calculation of electromagnetic field intensity and distribution is carried out, is originated from intended port power (p) and electromagnetic boundary conditions are set according Maxwell s Equations. The tal dissipated heat through polarization effects is interpreted as volumetric heat source in HT module. The HT module forms heat transfer mechanisms in different material types. This constitutes rmal conduction in solids, convective heat transport in gaseous media, and electromagnetic radiation in form of phons in both transparentt and opaque media. The CFD module solves Navier-Skes equations, obtaining gas velocity profile, will be used by HT module for calculation of convective rmal losses. 4.1 Material properties Input Port Figure 2. Main components of microwave system. Selection of proper materials is an important part of design. An important featuree of design of a microwave oven is that structural materials should be of high chemical stability in order prevent unwanted reactions with reaction mixture. Moreover, effective insulation materials should be used in order enable an energy efficient process by minimizing heat losses. The dimensions of insulation depend on geometry of reaction zone and of resonar. Basically, selection of right and optimal combination of materials has enormous effects on design of microwave furnace. In order realize se effects, costly and time-consuming tests should be performed.

4 However, with help of numerical simulations, several material combinations can be put testt for identifying optimum ones. 6. Results 6.1 Frequency Dependent Electromagnetic Wave Study The electromagnetic field intensity and distribution inside resonar chamber and sample holder are shown in cross section in Figure 3. The wave-like spread of electric field through waveguide is clearly visible. At height of waveguide ports, electric field is increased relative rest of resonar. Moreover, a field enhancement at core of crucible is also observed. This is ideal height, where crucible can be located be heated. However, electric field strength alone cannot be used as a measure of heat input in system. Therefore, a furrr heat study is required. was expected be at core of crucible, is tally in agreement with result from temperature study. Besides, adjacent insulation layers around sample were expected heat up less, due comparatively low rmal conductivity, is also compatible with result. Figure 4. Heat distribution in resonar. 6.3 Stationary Laminar Gas Flow Study Ideally, resonar should be filled with an inert gas in order prevent unwanted reactions like combustion of sample in presence of Oxygen. In addition, products of gaseous reaction should be eliminated from resonar avoid electrical arcs and generation of plasmas, because this affects process stability and efficiency. As shown in Figure 5 no homogeneous velocity profile is made in resonar chamber. The inert gas flows substantially across surface of sample, later willl be deflectedd by resonar chamber wall botm. Moreover, only a slight gas flow through p and botm areas of chamber and areas close waveguides ports can be observed. Figure 3. Electric field distribution in resonar and waveguide. 6.2 Stationary Heat Transfer Study Figure 4 showss resulting temperature distribution throughout resonar chamber. From electromagnetic study, hottest point Figure 5. Gas velocity distribution in resonar.

5 6.4 Optimum Position of Crucible The material be heated in reaction chamber is located on an insulating plate. In order investigate influence of height of insulation plate on electric field distribution, a parametric study of its height is performed. As shown in Figure 6 height of insulation plate varies from 30 mm 40 mm and 50 mm from p botm. The results indicate that at height of 40 mm, electric field is mainly concentrated in middle of crucible, contains sample be heated. However, at higher (50 mm) and lower (30 mm) heights a less strong concentration can be Figure 6. Study of observed. This means, electric field distribution ideal height of for different heights of insulation plate for this insulation plate. geometry is 40 mm. 7. Conclusions COMSOL Multiphysics made it possible simulate internal processes within microwave furnace. This includes calculation of electromagnetic field intensity and distribution throughout furnace and resulting heat inside reaction mixture through conduction, convection and radiation, followed by calculation of heat loss of system. The simulation results could mainly help identify optimum design and appropriate materials in orderr reach a high-efficiency system. Table 1: List of abbreviations. Symbol Description Unit μ σ Magnetic Field 4πe-7 Constant Magnetic Flux Wb Electrical Flux Density Electric Field Magnetic Field Tesla Current Density Temperature of External Flow Far K Away from Body Temperature of Body in Flow K Heat Flow Rate Permittivity of Vacuum 8.854e-12 Temperature Gradient Stefan Boltzmann 5. 67e-8 Constant. Heat Transfer. Coefficient Material s Conductivity. Charge Density 6. References 1. S. Pielsticker, Simulationen zum Temperaturprofil im Reaktionsgemisch der carbormischen Reduktion von Quarz im Mikrowellenofen, JPM Silicon GmbH, Germany (2014) 2. H. Ziebold, Wärmetechnische Auslegung eines Hochtemperatur-Mikrowellenreakrs, JPM Silicon GmbH, Germany (2012)