Siemens Process Research Articles

Kinetic Studies on Direct Electrolytic Reduction of SiO2 Granules in Molten CaCl2

1. Introduction The photovoltaic (PV) industry develops rapidly in recent years. As the main raw material of solar cells, solar-grade silicon (SOG-Si; 6N purity) is dominantly produced by the Siemens process which is based on hydrogen reduction and thermal decomposition of trichlorosilane (SiHCl3). The disadvantages of the Siemens process such as low productivity, low reaction efficiency and high energy consumption could seriously restrict the mass dissemination of solar cells in the future. Therefore, the development of a new production process of SOG-Si is urgently needed. Our group has been investigating a new process to produce SOG-Si via the combination of molten salt electrolysis and directional solidification in recent years.[1,2] Solid SiO2 granules can be directly reduced to Si by electrolysis in molten CaCl2. In this paper, the kinetics of direct electrolytic reduction of SiO2 granules in molten CaCl2 was investigated. The effects of the SiO2 granule size and cathodic potential on the reaction rate were clarified. The improvement of kinetics by using SiO2-Si mixtures as the raw material was confirmed. 2. Experimental The experiment was conducted in molten CaCl2 at 1123 K in an Al2O3 crucible. Before melting, the CaCl2 was dried well under vacuum to remove the residual moisture. The working electrode was an Al2O3 tube with SiO2 granules inside and with a graphite plate at the bottom. The counter electrode was a glassy carbon rod. The reference electrode was an Ag+/Ag electrode prepared by immersing a silver wire into molten CaCl2containing 0.5 mol% AgCl in a mullite tube. Approximately 0.1 g of SiO2 granules with four size ranges (<0.1 mm, 0.10-0.25 mm, 0.5-1.0 mm, and 1.0-2.0 mm) were electrolyzed at different cathodic potentials (0.6 to 1.2 V vs. Ca2+/Ca). In some cases, SiO2-Si mixtures with different composition ratios were used as the raw material. The reduction kinetics was evaluated on the basis of the growth of the reduced Si layer and the current behavior during electrolysis. The post-electrolysis samples were observed by SEM and analyzed by EDX. 3. Results and Discussion The observation of the cross-sections of the post-electrolysis samples indicates that the reduced Si layer grows from the bottom to the top. The reduction occurs via two routes: along the surface of SiO2 granules and from the surface to the core, to form a core (SiO2)-shell (Si) structure.[3] Smaller SiO2 granules are favorable for faster reduction because the contact resistance between the bottom graphite plate and the reduced Si particles is small and the diffusion of O2- ions in CaCl2inside the porous Si shells is easy.[4] A more negative cathodic potential is also favorable for faster reduction due to the fact that electron transfer is involved in the rate-determining step of the overall process. Figure 1 shows the photographs of the cross-sections of the working electrodes using SiO2-Si mixtures with different composition ratios as the raw material after electrolysis at 0.6 V for 20 min. The thickness of the reduced layer apparently increases with the increase of Si amount in the raw material. Since Si has a high electrical conductivity at 1123 K, new electron transfer paths are created and thus the reaction area is larger by using SiO2-Si mixtures compared with using only SiO2granules as the raw material. Therefore, the reduction kinetics is improved by the addition of Si. Acknowledgement This study was partly supported by JST-CREST and Grants-in-Aid for Scientific Research A from the Japan Society for the Promotion of Science (JSPS).

In Search of an Alternative Solar Grade Silicon Production By Electrochemical Reduction of SiO2

Silicon plays an important role in fabrication of solar cells and solar chips. Metallurgical grade silicon (98 to 99 % Si) demands further purification such as zone refining [1, 2], metallothermic reduction [3] and Siemens process [4] to convert it to solar grade. A promising method, based on direct electrochemical reduction of oxides by FFC Cambridge Process [5], was adopted to form silicon from porous SiO2 pellets in molten CaCl2 and CaCl2–NaCl salt mixture [6]. The contamination of silicon powder by iron and nickel originating from stainless steel cathode was reported to disqualify the product from solar applications. SiO2 pellets, sintered at 1300oC for 4 hours, were sandwiched between pure silicon wafer plates to minimize contamination of silicon. The promising results indicated a potential alternative method of direct solar grade silicon production for expanding solar energy industry. [1] P.R. Mei, S.P.Moreira, E.Cardoso, A.D.S.Cortes, F.C.Marques, “Purification of metallurgical silicon by horizontal zone melting”, Sol. Energ. Mat. Sol., 98 (2012), 233-239. [2] P. Siffert, E.F. Krimmel, eds. Silicon: Evolution and future of a technology. Berlin,Springer (2004). [3] K. Yasuda, T.H. Okabe, “Solar-grade silicon production by metallothermic reduction”, JOM,62 (12) (2010), 94-101. [4] Siemens & Halske: BRD Patents 1.102.117 and 1.140.1594 filed 1954, issued 1956 [5] G.Z. Chen, D. J. Fray, T. W. Farthing, “Direct Electrochemical Reduction of Titanium Dioxide to Titanium in Molten Calcium Chloride”, Nature, 407 (2000), 361-364. [6] E. Ergül, İ. Karakaya, M. Erdoğan, “Electrochemical decomposition of SiO2 pellets to form silicon in molten salts” Journal of Alloys and Compounds, 509 (2011), 899-903.

Thermodynamic Estimation of Silicon Tetrachloride to Trichlorosilane by a Low Temperature Hydrogenation Technique

The chemical reactions in the SiCl4-Si-H2 system using a low temperature hydrogenation technique related to the Siemens process were studied based on thermodynamics. The diagrams of standard Gibbs free energy of formation and equilibrium constants for seven reactions used as a function of temperature in this system were calculated and plotted for a temperature range of 473 K to 1073 K. It showed that the lower the temperature, the larger the conversion ratio of SiCl4. The equilibrium composition of gaseous species in the SiCl4-Si-H2 system with different initial SiCl4/H2 ratio and systematic pressure was calculated and the corresponding conversion ratio of SiCl4 was obtained. The conversion ratio was improved by increasing the initial ratio of H2 in raw materials and the systematic pressure but was reduced with the increase of temperature. The conversion ratio of SiCl4 reached 0.41 with an initial SiCl4/H2 ratio of 1/5 and a systematic pressure of 5 MPa at 473 K.

Analysis of radiative energy loss in a polysilicon CVD reactor using Monte Carlo ray tracing method

In this study, the model basing on Monte Carlo ray tracing method is built to analyze the radiative energy loss in the polysilicon CVD reactor. It can be conveniently applied to arbitrary distribution with any number of silicon rods and solve the complex problem of radiative blockage between the rods effectively. The effects of the rod's number, thermal shield and surface emissivity on the radiative energy loss of CVD reactor were investigated in detail. Results show that the average saving energy rate is 39% when the rod number changes from 36 to 72, and it can reach 58% after using thermal shield in 72 rods reactor. Otherwise, results show that the computing time using the Monte Carlo ray tracing method only increased by 66.70% when the number of rod increased from 36 to 72, while the computing counts of cross-string method increased by 711.43%. Therefore, the model based on Monte Carlo ray tracing method is an efficient and fast way to design and optimize any complex CVD reactor in Siemens process and achieve the goal of energy conservation.

Fermilab Muon Campus g-2 Cryogenic Distribution Remote Control System

The Muon Campus (MC) is able to measure Muon g-2 with high precision and comparing its value to the theoretical prediction. The MC has four 300 KW screw compressors and four liquid helium refrigerators. The centerpiece of the Muon g-2 experiment at Fermilab is a large, 50-foot-diameter superconducting muon storage ring. This one-of-a-kind ring, made of steel, aluminum and superconducting wire, was built for the previous g-2 experiment at Brookhaven. Because each subsystem has to be far away from each other and be placed in the distant location, Siemens Process Control System PCS7-400, Automation Direct DL205 & DL05 PLC, Synoptic and Fermilab ACNET HMI are the ideal choices as the MC g-2 cryogenic distribution real-time and on-Line remote control system.This paper presents a method which has been successfully used by many Fermilab distribution cryogenic real-time and On-Line remote control systems.

Deposition reactors for solar grade silicon: A comparative thermal analysis of a Siemens reactor and a fluidized bed reactor

Polysilicon production costs contribute approximately to 25–33% of the overall cost of the solar panels and a similar fraction of the total energy invested in their fabrication. Understanding the energy losses and the behaviour of process temperature is an essential requirement as one moves forward to design and build large scale polysilicon manufacturing plants. In this paper we present thermal models for two processes for poly production, viz., the Siemens process using trichlorosilane (TCS) as precursor and the fluid bed process using silane (monosilane, MS). We validate the models with some experimental measurements on prototype laboratory reactors relating the temperature profiles to product quality. A model sensitivity analysis is also performed, and the effects of some key parameters such as reactor wall emissivity and gas distributor temperature, on temperature distribution and product quality are examined. The information presented in this paper is useful for further understanding of the strengths and weaknesses of both deposition technologies, and will help in optimal temperature profiling of these systems aiming at lowering production costs without compromising the solar cell quality.

Computational Results Show Gas Phase Reactions Have Great Impact on the Deposition Rate of Silicon in Siemens CVD Reactors

Over 80% of solar grade silicon is produced by using Siemens process. The chemical and physical phenomenon involved in the Siemens CVD reactors is very complex. For the purpose of finding the effect of gas phase reactions on the deposition rate of silicon in Siemens CVD reactors, four different gas phase reaction routes were applied in computational simulations using a commercial software Ansys Fluent. The simulation results show that the gas phase reaction mechanisms have great impact on the predicted Si growth rates. Specifically, the silicon growth rates decrease with an increase in HCl concentration on the rods’ surfaces with a fixed surface temperature. The formation of SiCl4, however, shows insignificant impacts on the growth rate of Si, and the surface concentration of SiCl4 is not directly associated with that of HCl.

Changes in microstructure of metallurgical-grade silicon during heating for fractional melting

Fractional melting is a good method of refining metallurgical-grade silicon. It can be used to replace the traditional Siemens process for that purpose. It is important to observe the changes in the microstructure of silicon during heating to increase the efficiency of fractional melting. The microstructures and the chemical compositions of impurity phases in heated samples were observed by using scanning electron microscopy and wavelength dispersive spectroscopy. The affinities among the various impurities were investigated using the Miedema model. Ti and Al had strong affinities with V and Ca, respectively. It is possible to use elements that have strong affinities with B and P to effectively remove B and P from the silicon matrix, which are otherwise difficult to remove because of their high segregation coefficients.

Silicon Electrodeposition on Glassy Carbon, Silver, and Tungsten Substrates

Increasing concerns on global warming and current environmental issues have directed research attention to the problems of energy saving, alternative energy sources, as well as to the improvement of the efficiency of existing chemical current sources. A particular focus is currently concentrated on solar energy use. High-purity silicon plates are employed as details in solar cells. Over the past ten years researchers have sought to create silicon nanomaterials able to significantly improve the efficiency of lithium-ion electrochemical batteries and photovoltaic cells. It is a common knowledge that the principle industrial method for producing high-purity silicon is based on vapor deposition; for example Siemens process [1]. However, it has a number of drawbacks in terms of power consumption, costly reagents, and sophisticated equipment. The development of a low-cost production process for solar and nanocrystalline silicon seems therefore an interesting research task. An alternative approach to the production of silicon is electrodeposition from molten salts containing silicon ions [2-7]. This method can be applied to obtain both coherent covers and Si-nanostructures, such as nanopowders and nanofibres.

Doping Optimization of Solar Grade (SOG) Silicon Ingots for Increasing Ingot Yield and Cell Efficiency

In the close future, use of SoG should become prominent for photovoltaic ingot production as it requires much less energy for purification compared to Silicon grades using gas transformation and purification (usually Siemens process or equivalent also used for electronic−grade preparation). During this study, several kinds of silicon were compared with different rates of dopant content (mainly boron and phosphorus). Ingot yield and cell efficiency were optimized for each source of silicon at a production level (450 kg ingots) using boron or gallium doping. Starting from the resistivity specification given by the cell process, the doping level was adjusted in order to maximize the ingot silicon yield (weight of silicon bricks used for wafer cutting/ weight of Silicon ingot). After doping adjustment, ingot quality was checked: brick resistivity, lifetime of minority carriers and wafers were processed into solar cells. Optimizing of doping led to get comparable ingot yields and cell efficiencies using SoG and silicon purified by Siemens process or equivalent. The study was implemented at Kazakhstan Solar Silicon plant in Ust−Kamenogorsk using Kazakhstan SoG, SoG from a European manufacturer and polycrystalline Silicon purified by Siemens process. Directional solidification furnaces were manufactured by the French company ECM Technologies.

Thermodynamic behaviors of SiCl2 in silicon deposition by gas phase zinc reduction of silicon tetrachloride

The modified Siemens process, which is the major process of producing polycrystalline silicon through current technologies, is a high temperature, slow, semi-batch process and the product is expensive primarily due to the large energy consumption. Therefore, the zinc reduction process, which can produce solar-grade silicon in a cost effective manner, should be redeveloped for these conditions. The SiCl2 generation ratio, which stands for the degree of the side reactions, can be decomposed to SiCl4 and ZnCl2 in gas phase zinc atmosphere in the exit where the temperature is very low. Therefore, the lower SiCl2 generation ratio is profitable with lower power consumption. Based on the thermodynamic data for the related pure substances, the relations of the SiCl2 generation ratio and pressure, temperature and the feed molar ratio nZn/nSiCl4 are investigated and the graphs thereof are plotted. And the diagrams of KpΘ–T at standard atmosphere pressure have been plotted to account for the influence of temperature on the SiCl2 generation ratio. Furthermore, the diagram of KpΘ–T at different pressures have also been plotted to give an interpretation of the influence of pressure on the SiCl2 generation ratio. The results show that SiCl2 generation ratio increases with increasing temperature, and the higher pressure and excess gas phase zinc can restrict SiCl2 generation ratio. Finally, suitable operational conditions in the practical process of polycrystalline silicon manufacture by gas phase zinc reduction of SiCl4 have been established with 1200K, 0.2MPa and the feed molar ratio nZn/nSiCl4 of 4 at the entrance. Under these conditions, SiCl2 generation ratio is very low, which indicates that the side reactions can be restricted and the energy consumption is reasonable.

Heat losses in a CVD reactor for polysilicon production: Comprehensive model and experimental validation

This work addresses heat losses in a CVD reactor for polysilicon production. Contributions to the energy consumption of the so-called Siemens process are evaluated, and a comprehensive model for heat loss is presented. A previously-developed model for radiative heat loss is combined with conductive heat loss theory and a new model for convective heat loss. Theoretical calculations are developed and theoretical energy consumption of the polysilicon deposition process is obtained. The model is validated by comparison with experimental results obtained using a laboratory-scale CVD reactor. Finally, the model is used to calculate heat consumption in a 36-rod industrial reactor; the energy consumption due to convective heat loss per kilogram of polysilicon produced is calculated to be 22–30kWh/kg along a deposition process.

Experimental verification of upgraded metallurgical silicon photovoltaic power plant

The upgraded metallurgical silicon (UMG-Si) purified by a metallurgical process route directly is more energy efficient than the conventional Siemens process, but high metallic impurities are the cause of a large fraction of the total recombination events in solar cells made from UMG-Si. The efficiency of crystalline silicon solar cells made by such materials is lower than that from a chemical route, and UMG solar cells have big light-induced degradation. So, there always exist debates about using the upgraded metallurgical silicon for photovoltaic. In this paper, the impurity contents of silicon samples obtained in different stages were investigated, and a four-year verification of the largest UMG photovoltaic power plant in the world was conducted for the first time. The reliability, operating performance, and defects of UMG photovoltaic power plant were analyzed.

Processes for Production of Solar‐Grade Silicon Using Hydrogen Reduction and/or Thermal Decomposition

AbstractHigh‐purity Si used for photovoltaic applications, namely, solar‐grade Si (SOG‐Si), is commercially manufactured using the Siemens process. Although the present levels of supply satisfy the demand, there can potentially be a shortage of SOG‐Si in the long term. To overcome the low productivity of the Siemens process, various types of SOG‐Si production/purification processes have been developed as post‐Siemens processes. Some processes are under development as new commercial processes. These processes can be classified into the following three categories: (1) hydrogen reduction and/or thermal decomposition of silane‐based gases in improved Siemens‐based processes, (2) metallothermic reduction of silicon halides by Zn or Al, and (3) upgrading metallurgical‐grade Si by employing metallurgical purification methods. This paper presents a review of various types of SOG‐Si production processes, particularly those based on the hydrogen reduction and/or thermal decomposition of halides and silane‐based gases. These processes are classified on the basis of Si compounds used and the reaction types; further, the features of these processes are also analyzed. Future prospects for the development of new high‐purity Si production process are also presented.

Influence of Slim Rod Material Properties to the Siemens Feed Rod and the Float Zone Process

The identification and understanding of material properties influencing the float zone process is important to crystallize high purity silicon for high efficiency solar cells. Also the knowledge of minimal requirements to crystallize monocrystalline silicon with the float zone process is of interest from an economic point of view. In the present study, feed rods for the float zone process composed of a central slim rod and the deposited silicon from the Siemens process are investigated. Previous studies have shown that the slim rod has a significant impact on the purity and suitability for further crystallization processes. In particular, contaminations like substitutional carbon and the presence of precipitates as well as the formation of oxide layers play an important role and are investigated in detail. For this purpose different slim rod materials were used in deposition and float zone crystallization experiments. Samples were prepared by cross sectioning and core drilling of Siemens rods, which were recrystallized with the float zone process. Recrystallized drilled cores are analyzed with FT-IR spectrometry concerning the carbon and oxygen content. To estimate the grain growth behavior on the slim rod surface in dependence of the used slim rod material, EBSD mappings inside a SEM are performed on squared and circular slim rods. TEM analysis was used to investigate the presence of an oxide layer at the interface between slim rod and deposited polycrystalline silicon. Additionally the influence of a nitrogen-containing gas atmosphere during the slim rod pulling is investigated by IR microscopy and ToF-SIMS regarding Si3N4 precipitation.

Irradiance Dependent Temperature Coefficients for MC Solar Cells from Elkem Solar Grade Silicon in Comparison with Reference Polysilicon

An increase in sun intensity enhances the output power of the solar cells, but at the same time the rise of cell temperature affects its electrical performance negatively. The present authors have made a comparative study on temperature coefficients at various irradiances of multicrystalline solar cells made from the standard Siemens process and from material produced by a metallurgical route – the Elkem Solar Silicon (ESS®) process. Such temperature coefficients measured while exposed to various irradiance levels are essential to build a better understanding and prediction of field performance of solar cells manufactured from various feedstock types. In the current experiment, the current-voltages characteristics are measured for all solar cells by exposing them to an AM 1.5 spectrum from a AAA sun simulator in order to calculate the temperature coefficients. Electroluminescence (EL) images in forward bias mode are also provided. It was observed that ESS™ solar cells show significantly better temperature coefficients for current, efficiency and fill factor compared to similar cells from polysilicon at all incident irradiances. Such better temperature coefficients have also been reported previously from field measurements [1].

Direct Current Heating Model for the Siemens Reactor

The modified Siemens process is the primary technology of polycrystalline production at present. The Siemens reactor, which is the main equipment in the modified Siemens process, consists of a chamber where several high purity silicon slim rods are heated by an electric current flowing through them. The temperature on the rod centre must be under melting temperature of silicon (1687K) in order to avoid its breaking-down because of an uneven temperature profile of the silicon rod. Therefore the temperature profile of the silicon rod heated by direct current (DC) has been investigated by molding. The current density profile of silicon rod has also been studied to investigate the interaction of current density and temperature. The results show that the temperature is not homogeneous in the rod and the temperature in the center of the polysilicon rod is 1750K which is much higher than the melting temperature (1687K) when the temperature is 1423K on the surface of polysilicon and the radius of rod is 5cm. Therefore, the maximum growth radius of the polysilicon rod in Siemens reactor should be less than 5cm when the joule heating generated by DC. The current density increases from the center to the surface of the polysilicon rod.

Thermodynamic Behavior of SiH&lt;sub&gt;2&lt;/sub&gt;Cl&lt;sub&gt;2&lt;/sub&gt; in Polysilicon Production by Siemens Process

The present of SiH2Cl2 (DCS) is a big problem because of its instability. The content of SiH2Cl2 should be controlled below somewhat concentration. Therefore, the present study is aimed at investigating the thermodynamic behavior of SiH2Cl2 in polysilicon production by Siemens process in order to provide effective theory for the production process. The effect of pressure (1-5atm), feeding ratio of H2 to SiHCl3 (2-50) and temperature (1000-1500K) have been studied in the paper. The results show that the productivity ratio of DCS increases with increasing pressure and increases firstly and then decreases with increasing feeding ratio or temperature. In a word, the pressure have biggest effect on productivity ratio of DCS compared to temperature and feeding ratio.

Effect of hydrogen radical on decomposition of chlorosilane source gases

The effect of hydrogen radical on production of Si from chlorosilane sources has been studied. We used hydrogen radical generated from pulsed thermal plasma to decompose SiHCl3 and SiCl4. Hydrogen radical was effective for lowering the temperature to produce Si from SiHCl3. SiCl4 source, which was chemically stable and by-product in Siemens process, was decomposed effectively by hydrogen radical. The decomposition of SiCl4 was consistent with the thermo-dynamical calculation predicting that the use of hydrogen radical could drastically enhance the yield of Si production rather than case of H2 gas.

Radiation heat savings in polysilicon production: Validation of results through a CVD laboratory prototype

This work aims at a deeper understanding of the energy loss phenomenon in polysilicon production reactors by the so-called Siemens process. Contributions to the energy consumption of the polysilicon deposition step are studied in this paper, focusing on the radiation heat loss phenomenon. A theoretical model for radiation heat loss calculations is experimentally validated with the help of a laboratory CVD prototype. Following the results of the model, relevant parameters that directly affect the amount of radiation heat losses are put forward. Numerical results of the model applied to a state-of-the-art industrial reactor show the influence of these parameters on energy consumption due to radiation per kilogram of silicon produced; the radiation heat loss can be reduced by 3.8% when the reactor inner wall radius is reduced from 0.78 to 0.70m, by 25% when the wall emissivity is reduced from 0.5 to 0.3, and by 12% when the final rod diameter is increased from 12 to 15cm.