【作者】 罗大伟；

【导师】 李廷举； 张国梁；

【作者基本信息】 大连理工大学 ， 材料加工工程， 2011， 博士

【摘要】 太阳能发电具有安全可靠、无污染、分布广泛和无机械转动部件等独特优点,受到了世界各国的高度重视。同时,在目前可再生能源中光伏发电是比较成熟的技术,世界各国尤其美、日、德等发达国家先后启动了大规模的国家光伏发展计划,刺激光伏产业迅速发展。光伏产业的高速增长催生了对多晶硅的大量需求。目前,光伏产业应用过程中面临的主要障碍为成本过高以及硅材料的短缺。在不影响效率的前提下,降低硅的成本是降低硅太阳能电池成本的关键。目前,太阳能级硅生产所需的原料主要来自于微电子工业中的边角料,考虑到应用于微电子工业和光伏领域中硅材料的规模和纯度以及包含在生产工艺过程中成本的不同,为了满足光伏产业对多晶硅的迫切需求以及摆脱太阳能级硅对电子级硅的依赖,世界各国积极开展了低成本太阳能级硅生产工艺的研究。本文通过对冶金级硅中不同杂质存在形式与分布特点的分析,提出了一种新的冶金提纯工艺路线,具体提纯工艺路线如下：①冶金级硅的酸洗预处理→②电磁感应造渣精炼→③电子束精炼一④定向凝固。试验结果表明上述四种工艺针对硅中不同类型的杂质具有较好的去除效果。最终成功制备出纯度为5N-6N的太阳能级多晶硅铸锭。各个精炼工艺的研究结果如下：选用盐酸与氢氟酸进行了对比试验,结果表明：盐酸的酸洗效果要优于氢氟酸。由于氢氟酸在酸洗过程中会产生难溶性的物质,且易挥发和产生爆炸,因此本试验最终选择盐酸作为酸洗溶液。盐酸酸洗时的最优化工艺参数如下：固液比1：20,15wt%,80℃,10h,100um。酸洗后硅中杂质铝、铁和钙的去除效率分别达到75.6wt%,79.3wt%和61.7wt%。酸洗处理后硅的纯度由99%提高到99.9%左右。同时在酸洗过程中施加超声震荡,声流和声空化作用能够去除硅粒表面晶界狭缝处的杂质,因此可以提高酸洗提纯效果。但酸洗工艺对冶金级硅中的B和P等非金属杂质没有任何去除效果,而且存在酸洗极限即酸洗并不能将硅中的金属杂质全部去除。酸洗虽然仅仅作为冶金工艺中的预处理环节,但它也是一个非常重要而且是不能缺少的环节。重点对SiO2-CaO-Na2O和SiO2-CaO-Al2O3造渣系在不同的精炼温度,精炼时间和造渣剂成分条件下对杂质B的去除规律进行了热力学与动力学研究。研究结果表明：SiO2-CaO-Al2O3造渣系除杂效果明显好于SiO2-CaO-Na2O造渣系。在SiO2-CaO-Al2O3造渣系下,当造渣剂比重为10%时,精炼温度为1823K,精炼时间为2h,精炼后杂质B的含量由原来的15 ppmw成功地降低到2 ppmw。同时硅中金属杂质元素Al、Ca和Mg也得到了很好的去除,去除率分别达到了85.0%、50.2%和66.7%。电磁感应的搅拌作用可以对熔体产生强烈的搅拌作用,通过提高杂质在边界层内的传质系数,增强杂质在边界层内的扩散,从而可以改善和加速B的去除过程。采用自行设计的电子束熔炼炉对冶金级硅进行了电子束精炼研究,并对杂质的去除过程进行了热力学与动力学研究。研究结果表明：硅中金属杂质铝、钙与非金属杂质磷的挥发反应均为一阶反应,即单原子挥发反应。因此杂质元素通过气相边界层扩散到气相中的自由挥发过程为整个去除过程的控制环节。当电子束功率为20kW,时间为30min,精炼硅中主要杂质除了铁、铝和钛的含量仍大于1ppmw,其它金属杂质的含量都降到了0.1ppmw以下,尤其是杂质磷的含量降低到了0.16ppmw。本文推导出了硅中各种杂质含量与电子束功率和精炼时间的关系表达式如下：试验验证表明理论计算值与实验值基本一致。由于硅中的杂质含量与精炼时间成负指数关系,因此硅中的杂质含量与硅料的进料速度也成反比。因此为了提高生产效率,降低成本,本文选择电子束功率为30kW进行连续精炼。当出口硅中杂质磷的含量为1ppmw时,要求硅料的进料速度大约在O.1kg/min。采用自行设计的电子束定向凝固炉对硅的定向凝固工艺进行了详细地研究。研究结果表明：当硅熔体过热度为130K、凝固速度为3.0×10-5m/s时,采用涂有氮化硅涂层的石英陶瓷坩埚进行定向凝固得到了表面质量非常光滑,完整的多晶硅铸锭。硅锭尺寸为Φ70 mm×210 mm,且硅锭中部柱状晶晶粒尺寸宽为3～4mm,长为10-30mm。硅锭的临界提纯高度约为140mm左右,约占整个铸锭高度的66.7%。铸锭中铝、铁和钛等金属杂质的含量都控制在0.1 ppmw以下,非金属杂质硼的含量控制在0.6ppmw左右,磷含量控制在0.1 ppmw以下,碳、氧和氮的含量基本控制在10ppmw左右。多晶硅铸锭纯度达到了5N以上,铸锭中各种杂质的含量都达到了太阳能级硅对杂质所要求的范围之内。更多还原

【Abstract】 Among renewable energy, solar power has many advantages, such as safe, reliable, clean, widely distributed, no mechanical moving parts and so on, it has been attracted the world’s attention. At the same time, photovoltaic power generation is the more mature technology in renewable energy, Many countries in the world, especially the United States, Japan, Germany and other developed countries have launched a large scale photovoltaic development plan to stimulate the rapid development of photovoltaic industry. The rapid growth of photovoltaic industry gave birth to a large demand for silicon. Currently, the major faced obstacles are the high cost and the shortage of silicon in the photovoltaic industry application. Reduce the cost of silicon is the key to reduce the cost of silicon solar cells without affecting the efficiency of solar cells.Currently, raw materials used for the production of SoG-Si are mainly from the scraps of microelectronics industry. Taking into account the size, purity and the cost in production process of silicon used in microelectronics industry and photovoltaic field are different, meantime, in order to meet the urgent demand of PV industry for silicon and get rid of the dependence of SoG-Si on electronic grade silicon (EG-Si). Therefore, many countries begin to carry out the low cost production technology research for solar grade silicon (SoG-Si).A new metallurgical purification route is proposed and the specific purification process route is as follows:①Acid pretreatment of metallurgical grade silicon (MG-Si),②Electromagnetic induction slag refining,③Electron beam refining and④Directional solidification based on the analysis for the presence forms and distribution of different impurities in MG-Si. The results show that the above four refining process have better removal effect for different types of impurities in silicon. SoG-Si ingot with the final purity of 5N-6N has been prepared successfully. The results of each refining process are as follows:Hydrochloric acid and hydrofluoric acid were used in this paper. Contrast test results showed that acid effect under hydrochloric acid was better than that of hydrofluoric acid. Not only pickling process would produce insoluble material under hydrofluoric acid but also hydrofluoric acid was easy to volatile and exploded. So hydrochloric acid was selected as the final pickling solution. Optimize process parameters under hydrochloric acid were as follows: solid-liquid ratio 1:20,15wt%,80,10h, 100um.Under above conditions, the main impurities removal efficiency of aluminum, iron and calcium in silicon reached 75.6wt%,79.3wt% and 61.7wt%, respectively. The purity of silicon increased from 99% to 99.9% after acid pickling. Ultrasound vibration was used during pickling process and acoustic streaming and acoustic cavitation could remove impurities which were not fully exposed at the slit of grain boundary, so it could increase the effect of acid pickling. But there was no removal effect for B, P and other non-metallic impurities in MG-Si with pickling process and pickling could not remove all metal impurities in silicon because pickling limit. Although pickling only just as a pretreatment process, it was a very important and indispensable link.The effect of different refining temperature, time and slag agent components on the removal rules of boron was mainly studied under SiO2-CaO-Na2O and SiO2-CaO-Al2O3 systems. Meantime, the thermodynamics and kinetics of the removal rules of boron was also discussed. The results showed that impurities removal effect under SiO2-CaO-Al2O3 slag system was better than that of SiO2-CaO-Na2O slag system. The content of boron elements in silicon was successfully reduced from the original 15 ppmw to 2 ppmw under SiO2-CaO-Al2O3 slag system. Refining process parameters were as follows:slag dosage was 10wt%, temperature was 1823K and time was 2h. At the same time, metal impurities Al, Ca and Mg were also well removed and the removal efficiency of them reached 85.0%,50.2%, and 66.7%, respectively. Electromagnetic induction slag refining could improve and accelerate the removal process of boron because the mass transfer coefficient of impurity elements in the boundary layer was strengthened by electromagnetic stirring.In this paper, MG-Si was refined by electron beam melting furnace which designed by us and the thermodynamics and kinetics of impurities removal were also discussed. The results showed that the volatile reactions of metal impurities aluminum and calcium and nonmetallic impurities phosphorus were all first order reaction (that is a single atom of volatile reaction). Therefore, the free volatile process of impurity elements spread from the gas boundary layer to gas phase was the control link of whole volatilization process. The content of main metallic impurities besides iron, aluminum and titanium in silicon all reduced to below 0.1 ppmw, especially the content of phosphorus reduced to 0.16 ppmw when the electron beam power was 20kW and refining time was 30 minutes. The expression of impurity content in silicon as electron beam power and refinement time was derived as follows: The calculated value was consistent with the experimental value. Impurity content in silicon was inversely proportional to the feed speed because the relationship between impurity content and refining time was a negative exponential. Therefore, in order to increase productivity and reduced costs, electron beam power used in this paper was 30kW for continuous refined. The required speed of the feed silicon was about 0.1kg/min when the phosphorus content in export molten silicon was lppmw.Silicon directional solidification process was studied in detail in this paper via utilizing an electron beam directional solidification furnace which designed by ourselves. The results showed that a silicon ingot with very smooth surface quality and no defect was obtained by the use of quartz ceramic crucible coated with silicon nitride coating when the superheat of silicon melt was 130K and the directional solidification rate was 3.0×10-5m/s. Ingot with the diameter of 70 mm and the length of 210mm and the grain size of columnar crystal at the central of silicon ingot with the width of 3～4mm and the length of 10～30mm was obtained. Critical purification height of silicon ingot was about 140mm which was approximately 66.7% of the total ingot height. The contents of aluminum, iron and titanium and other metal impurities in ingot were controlled below 0.1ppmw, the content of boron and phosphorus was below 0.6ppmw and 0.1ppmw, the content of carbon, oxygen and nitrogen were basic controlled below 10ppmw. The purity of silicon ingot had reached more than 5N and various impurities content in silicon ingot had reached the required impurity range of solar grade silicon.更多还原