Purification of metallurgical silicon using iron as impurity getter,part II: Extent of silicon purification

Purification of metallurgical grade silicon (MG-Si), using iron as the impurity getter has been investigated. The technique involves growing Si dendrites from an alloy of MG-Si with iron, followed by their separation using a gravity based technique and acid leaching. The effects of cooling rate of the alloy and the subsequent quenching temperature on the segregation of the impurities were studied. It was found that slow cooling of the alloy below the eutectic temperature causes an increase in the Si impurity concentration due to diffusion of the impurities from the alloy to the Si. Quenching the alloy from temperatures above the eutectic eliminated this effect, increasing the purity of the Si product. A significant reduction in the concentration of the major impurities was achieved, making the Si product a suitable feedstock for solar grade silicon generation. The concentrations, in ppmw, of some elements in the Si product are Al: 10, B: 2, Mn: 3, Ni: 3, Cr: 1, Fe: 1, P: 29. Other impurities including V, Ba, Li, Be, and Mg were all below 0.5 ppmw.     

冶金级硅吹氧精炼过程中杂质的热力学行为和赋存状态

吹气精炼是一种从冶金级硅熔体中直接去除杂质的有效方法.研究了冶金级硅中杂质在吹氧过程中的热力学行为,通过与吹氧精炼前的比较,得到了精炼后硅中杂质的去除效率.研究发现,Ca、Al的去除率高于90％,B、Yi的去除率接近50％.通过XRD手段研究了冶金级硅中杂质的赋存状态,并确定了Al3Ni、NiSi2和Al3Ni等金属间化合物的存在.同时实验发现,除了Liquid、Diamond-Si、SiB3、SiB6、SiBn和β-B六个物相外,Si-B二元系中还存在SiB4相.通过SEM-EDS分析了吹O2精炼前、后冶金级硅中夹杂物的化学组成及形态变化.由于精炼过程中大多数Ca、Al以熔渣形式氧化而被去除,硅中白色夹杂的数量大幅度减少,而Fe、Ni、Mn等由于不能氧化而去除,在夹杂物中含量升高.     

采用高频磁场去除太阳能级硅熔体中的SiC粒子(英文)

高质量硅材料在光伏太阳能和电子设备中具有重要应用,然而原料中的非金属颗粒和金属杂质严重影响其电学性能和力学性能。由于SiC粒子会降低光伏电池的力学性能并导致分流问题,因此在制备太阳能电池之前必须将这些杂质从硅材料中去除。利用磁场去除液态金属中的非金属杂质是制备高纯金属的一项尖端技术。利用该方法去除冶金级硅材料中的SiC粒子,并结合杂质去除经典模型和计算流体力学对熔体中粒子浓度和分离效率进行计算。为检验该方法的有效性,采用感应炉进行多次实验。结果表明:该方法能有效去除非金属杂质,提纯硅熔体,且实验结果与模型的预测结果相符。     

Hydrometallurgical purification of metallurgical grade silicon

The effects of the particle size of ground metallurgical grade silicon (MG-Si), the sort of acids, and the type of stirring on the purified efficiency of MG-Si were investigated. It was found that a particle size less than 0.1 mn was most effective for acid leaching; the extraction yield of impurities was increased by 9% with I-IF leaching compared with HCl leaching and HNO3 leaching, and increased by 7% with ultrasonic stirring compared with mechanical stilting. The principle of hydrometallurgical purification of metallurgical grade silicon under ultrasonic fields was also discussed.     

High-Temperature Refining of Metallurgical-Grade Silicon: A Review

Among currently known alternatives for renewable energy sources, solar power is generally regarded as having the most potential to satisfy the ever-growing demand. While solar photovoltaic power is a well-established technology, its widespread uptake has been hindered by the prohibitively high price of units and thus electricity. This is due mainly to the high cost of the silicon used to fabricate the devices. This article presents a review of the development of established pyrometallurgical techniques as applied to refining metallurgical silicon to solar grade for the purposes of reducing reliance on expensive traditional silicon feedstock. Four basic high-temperature methods??solvent refining, vaporization, electrorefining, and slag treatment??are described, and the limitations and advantages of each method are presented. It is apparent that these techniques are very useful for removing impurities from silicon, but are often selective and not able to remove all problematic elements. Therefore, refining may need to be as a sequence of steps, targeting specific elements each time, or as novel methods combining multiple techniques simultaneously. Ultimately, the successful approach will have to achieve large-scale production by cost-effective means to replace current methods.     

冶金法制备太阳能级多晶硅的热力学研究进展

冶金法制备太阳能级硅是目前多晶硅材料的研究热点。为更深入理解冶金法制备太阳能级硅的原理,本文综述了几种常见的冶金法提纯多晶硅的热力学研究进展,重点对定向凝固过程中杂质的分凝系数问题、真空精炼中杂质蒸气压与温度的关系、以及合金造渣和氧化造渣中杂质的分配规律,以及渣相和硅相体系中各组元的活度测定方法进行了分析,提出了多晶硅热力学研究中尚存在的问题。     

低温净化冶金硅工艺(英文)

硼、磷杂质的去除在冶金净化法生产太阳能级多晶硅工艺中耗能最大。金属熔析净化法可以实现冶金硅在金属液中低温下熔化,而后再结晶净化,是一种可行的低能耗硼磷去除方法。对熔析体系的选择原则进行总结,筛选出铝、锡和铟金属作为合适的熔析介质。对于Sn-Si体系,1500K时硼的分凝系数为0.038,远小于纯硅熔点的对应值0.8。冶金硅二次熔析净化处理可使硼的质量分数由15×10-6降至0.1×10-6,而多数金属杂质可一次性去除至0.1×10-6以下。在熔析过程中,杂质和硅生成化合物是主要的杂质去除方式。提出一种以金属熔析法为基础的低温冶金硅净化工艺。     

Element Distribution in Silicon Refining: Thermodynamic Model and Industrial Measurements

To establish an overview of impurity elemental distribution among silicon, slag, and gas/fume in the refining process of metallurgical grade silicon (MG-Si), an industrial measurement campaign was performed at the Elkem Salten MG-Si plant in Norway. Samples of in- and outgoing mass streams, i.e., tapped Si, flux and cooling materials, refined Si, slag, and fume, were analyzed by high-resolution inductively coupled plasma mass spectrometry (HR-ICP-MS), with respect to 62 elements. The elemental distributions were calculated and the experimental data compared with equilibrium estimations based on commercial and proprietary, published databases and carried out using the ChemSheet software. The results are discussed in terms of boiling temperatures, vapor pressures, redox potentials, and activities of the elements. These model calculations indicate a need for expanded databases with more and reliable thermodynamic data for trace elements in general and fume constituents in particular.     

High-Purity Composite Briquette for Direct UMG-Si Production in Arc Furnaces

In metallurgical grade Si (MG-Si), the coal (B) and charcoal (P) contents are on average above 30 ppm as the carbon reduction materials used in the arc furnace are either rich in B or in P. A decrease of both impurities by a factor of 3 using purer raw materials would allow for the direct production of the upgraded metallurgical grade (UMG).This would significantly improve the efficiency of the resulting photovoltaic (PV) cells made with the refined solar grade silicon (SoG-Si) or massively decrease the costs of Si purification by shortening the number of steps needed for reaching B and P contents below 1 ppm requested for the SoG-Si used for the PV cells. A composite C/SiO₂ briquette fulfilling the purity targets for the direct production of UMG-Si in the arc furnace was developed. The composite contains several carbon materials with different levels of reactivities and quartz sand. The raw materials aspects, the paste and briquette preparation, as well as the final carbonization step are discussed. The finished briquettes are free of volatiles and are mechanically and thermally very stable, thus, ensuring stable arc furnace charges with minimum losses of dust and SiO gas. Semi-industrial trials including the downstream purification steps for the production of SoG-Si by a metallurgical low-cost route are contemplated.     

A New Method for Estimation of Emissions and Sources of Measurements Error in the Silicon Refining Process

In the production of metallurgical grade silicon (MG-Si), fugitive emissions are a serious concern due to the health risks associated with the fumes formed in different parts of the production. The fumes are also a potential environmental hazard. Yet, the chemical composition of the fumes from most process steps in the silicon plant, such as oxidative refining ladle, remains unknown. This in turn constitutes a problem with respect to the correct assessment of the environmental impact and working conditions. A comprehensive industrial measurement campaign was performed at the Elkem Salten MG-Si production plant in Norway. Samples of the ingoing and outgoing mass flows were analyzed by high-resolution inductively coupled plasma mass spectrometry, with respect to 62 elements. In every step of the sampling and sample treatment processes, possible sources of error have been identified and quantified, including process variation, mass measurement accuracy, and contamination risk. Total measurement errors for all elements in all phases are established. The method is applied to estimate the order of magnitude of the elemental emissions via the fumes from the tapping and refining processes, with respect to production mass and year. The elements with higher concentrations in the fume than slag and refined silicon include Ag, Bi, Cd, Cu, In, K, Mg, Na, Pb, Rb, Se, Sn, Tl, and Zn: all being present in the ppm range. This work constitutes new and vital information to enable the correct assessment of the environmental impact and working conditions at an MG-Si plant.     

点杂多晶太阳能硅片电解磨削多线切割试验研究

提出了一种电解磨削多线切割点杂多晶太阳能硅片的新方法。电源的正极接硅锭,负极接切割线网,电解过程中硅锭发生微区钝化反应,形成硬度较小的钝化膜,使点杂多晶太阳能硅片更易切割。试验结果表明:该技术具有切割效率高、切片合格率高等优点。进一步检测发现,硅片宏观表面线痕浅、隐裂少,微观表面平整性好。该技术的应用降低了硅片加工的成本,提高了硅材料的利用率,也为太阳能电池的运用拓宽了空间。     

Effect of porous silicon layer on the performance of Si/oxide photovoltaic and photoelectrochemical cells

Photovoltaic and photoelectrochemical systems were prepared by the formation of a thin porous film on silicon. The porous silicon layer was formed on the top of a clean oxide free silicon wafer surface by anodic etching in HF/H₂O/C₂H₅OH mixture (2:1:1). The silicon was then covered by an oxide film (tin oxide, ITO or titanium oxide). The oxide films were prepared by the spray/pyrolysis technique which enables doping of the oxide film by different atoms like In, Ru or Sb during the spray process. Doping of SnO₂ or TiO₂ films with Ru atoms improves the surface characteristics of the oxide film which improves the solar conversion efficiency.The prepared solar cells are stable against environmental attack due to the presence of the stable oxide film. It gives relatively high short circuit currents (I_sc), due to the presence of the porous silicon layer, which leads to the recorded high conversion efficiency. Although the open-circuit potential (V_oc) and fill factor (FF) were not affected by the thickness of the porous silicon film, the short circuit current was found to be sensitive to this thickness. An optimum thickness of the porous film and also the oxide layer is required to optimize the solar cell efficiency. The results represent a promising system for the application of porous silicon layers in solar energy converters. The use of porous silicon instead of silicon single crystals in solar cell fabrication and the optimization of the solar conversion efficiency will lead to the reduction of the cost as an important factor and also the increase of the solar cell efficiency making use of the large area of the porous structures.     

CaO-SiO_2熔渣精炼去除冶金级硅中杂质硼(英文)

利用CaO-SiO2熔渣去除冶金级硅(MG-Si)中的杂质硼。热力学分析和实验结果表明:纯SiO2基本上不能去除冶金级硅中的杂质硼。通过建立硼的分配系数与熔渣中SiO2和CaO活度之间的关系,从热力学上对CaO-SiO2熔渣的除硼能力进行表征。结果表明:随着渣中CaO配比的升高,硼的分配系数和去除效率大大提高。当熔渣组成为60%CaO-40%SiO2(质量分数)时,硼的分配系数达到最大值1.57。当渣硅比为2.5,精炼温度为1600°C以及精炼时间为3 h时,利用60%CaO-40%SiO2熔渣可以将冶金级硅中的硼含量从18×10-6降低至1.8×10-6,去除效率达到90%。     

Effect of Thermal Pretreatment and Acid Leaching on the Removal of Boron from Metallurgical Grade Silicon

The present work aims at investigating the impact of thermal pretreatment and acid leaching (HCl–HF) on the boron removal efficiency of metallurgical grade silicon (MG-Si). The impact of various parameters, involving oxidation temperature (700–1200 °C), oxidation time (1–5 h) and acid leaching (4 mol L^?1 HCl–3 mol L^?1 HF), on the removal of boron from MG-Si was thoroughly explored. It was found that thermal oxidation resulted in an enhanced removal efficiency of boron from MG-Si. By employing MG-Si particles in the range of 75–106 μm in conjunction with acid leaching at 65 °C for 6 h, the boron content was decreased from 19.60 to 14.10 ppmw, offering a removal efficiency of ca. 28%. When the MG-Si powder was subjected to thermal oxidation at 1100 °C for 5 h before leaching, the boron concentration in the purified Si was reduced from 19.60 to 8.90 ppmw, giving an extraction efficiency of 54.59%. An extended characterization study, regarding the microstructure, morphology and chemical composition of both un-treated and treated samples, was conducted to gain insight into the underlying mechanism of boron removal under different conditions.

     

Purification of metallurgical silicon using iron as an impurity getter part I: Growth and separation of Si

The purification of metallurgical grade silicon (MG-Si) using a combination of solvent refining and physical separation is studied. MG-Si was alloyed with iron and solidified under different cooling rates in order to grow pure Si dendrites from the alloy. The Si dendrites were then separated using a gravity-based method. The separation method relies on the significantly different densities of Si and FeSi₂, and it uses a heavy liquid with specific gravity between the two phases to float the light Si particles to the surface of the liquid, while the heavy iron silicide sinks. The effects of the particle size and cooling rate on the yield and separation efficiency of the Si phase were investigated by quantifying the fraction of Si in the sinks and floats. The results demonstrate that the crushing size of the particles prior to separation should be approximately the same as the width of the dendrites in order to maximize the separation efficiency while simultaneously lowering the grinding cost.     

Critical assessment of the impurity diffusivities in solid and liquid silicon

The diffusion of impurities in solid and liquid silicon is critically reviewed and assessed in this paper. The activation energies and pre-exponential factors in theArrhenius equation have been evaluated using the least-squares analysis and semi-empirical correlations. Impurity diffusion coefficients for Ag, Al, As, Au, B, Bi, C, Co, Cr, Cu, Fe, Ga, In, Li, Mn, N, Ni, 0, P, S, Sb, Te, Ti, and Zn in both solid and liquid silicon have been obtained. The current assessed impurity diffusivities can be coupled with the assessed thermochemical properties for the simulation of diffusion phenomena in the production of solar grade cell silicon feedstock. The assessed diffusivities have been applied to simulate the impurity diffusion profiles and the denuded zone in the intrinsic gettering annealing.     

冶金级硅吹Ar-H2O混合气精炼除

通过吹高纯氩气和水蒸汽混合气体的方法除去冶金级硅中的杂质磷。采用自行设计的吹气精炼装置,研究喷嘴类型、精炼时间、精炼温度、精炼气温度、精炼气流速等因素对除磷效果的影响。研究表明：使用侧壁和底部多孔型喷嘴,精炼时间3 h,精炼温度1793 K,精炼气温度373 K,精炼气流速2 L/min作为最优吹气精炼条件时,冶金级硅熔体中的磷元素质量分数由94×10-6降低到11×10-6。表明吹气精炼是一种有效的去除冶金级硅中磷的方法。     

Siemens and siemens-like processes for producing photovoltaics: Energy payback time and lifetime carbon emissions

Polysilicon photovoltaics will play a significant role in meeting the world’s shortfall in electrical energy this century. The photovoltaic industry relies on high-purity silicon produced in the Siemens process. New Siemens-like processes (which convert metallurgical silicon to trichlorosilane and deposit purified silicon through the decomposition of silane) and metallurgical processes for producing solar silicon are under development. Their energy payback time and lifetime carbon emissions are reviewed. The history of development of Siemens and Siemens-like processes is summarized.     

冶金法制备太阳能级硅的原理及研究进展

随着光伏市场需求不断增加,满足光伏电池技术经济指标要求的硅材料出现严重短缺,低成本提纯冶金硅至太阳能级硅工艺技术越来越受到广泛重视,成为研究开发热点.本文分析了全球光伏产业的发展现状和趋势,对目前获得太阳能级多晶硅的化学路径和冶金路径进行了对比分析;重点介绍了冶金法制备太阳能级硅的工艺原理,以及目前常用的提纯技术;同时,简单介绍低成本生产太阳能级多晶硅的新工艺,并指出了冶金法可能是今后提纯多晶硅的主要研究方向.     

Solidification of silicon for electronic and solar applications

Silicon is widely used for its semiconducting properties in electronic and photovoltaic devices. These applications require strict control of the structure and impurity levels. Silicon must, therefore, be solidified using processes that give a minimum of impurity contamination and microstructural defects, as well as a planar solidification front to avoid microsegregation. Czochralski growth, and to some extent float zone crystallization, are processes used to obtain defect-free single crystals for integrated circuits and advanced solar cell wafers, whereas the Bridgman process can be used for production of multicrystalline silicon for standard solar cells. Direct solar cell wafer solidification processes have also been developed and reached limited commercial use. The paper will review these silicon crystallization processes and discuss recent developments and trends.