Collaborative Research: Production of Solar Quality Silicon by Model-Driven Molten Salt Electrolysis

硅是主要的太阳能材料，因为它的丰富，低成本和太阳能效率高。但是，制造太阳能所需的高纯度硅非常复杂，难以规模化，而且不安全，因为它涉及处理有毒易燃气体。该奖项为基于熔盐电解的新型太阳能硅生产技术奠定了科学基础。该工艺以高纯度石英岩或二氧化硅为原料。通过这种方法，太阳能硅的生产成本有望从目前的每公斤10美元降至每公斤1-2美元，新的硅工艺比目前的做法消耗的能源要少得多，并且消除了所有直接的二氧化碳和相关排放。这些太阳能生产技术的进步有助于提高美国太阳能产业的经济竞争力。在这项研究中获得的基本知识可以用来设计类似的新型环保和高效的生产方法，用于高熔点难熔金属，以及碳化硅和其他半导体材料。通过公布的模型数据和代码、根据研究结果开发的教育模块以及在包括计算和实验工作在内的协作和多学科环境中对学生进行培训，该项目的影响也进一步扩大。本项目基于新的实验数据，建立了硅熔盐电解过程的四个综合数学模型，有助于理解硅熔盐电解及其规模化生产。第一个熔盐结构（MSS）模型将光谱分析与原子模型相结合，以理解五组分熔盐中络合离子的分子结构。第二个计算相图（CALPHAD）模型，以了解和预测熔盐的热力学和热物理性质，包括硅溶解度，硅化合物挥发性和离子迁移率。实验验证了这些性质，并将其应用于第三种输运模型中，用于研究硅电沉积在阴极-电解质界面的反应和扩散动力学，以预测沉积的结构和组成。第四相场模型利用这些熔盐性质来研究硅向阴极和氧向阳极的输运，以了解边界层结构并提高沉积速率。利用这些模型，开发了一种新的电流波形开关系统，以在高电流密度下保持无枝晶硅的稳定生长。新的低成本单操作硅熔盐电解取代了目前西门子工艺中使用的碳热硅生产、酸消化、三氯硅烷合成、蒸馏和化学气相沉积等多个高能耗单元操作。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Silicon is the dominant solar material because of its abundance, low cost, and high solar efficiency. But manufacturing high-purity silicon required for solar energy is very complex, hard to scale, and unsafe since it involves handling toxic flammable gases. This award lays the scientific foundations for a new solar silicon production technology based on molten salt electrolysis. This process uses high purity quartzite or silica as its raw material. By this method, the production cost of solar silicon is expected to fall from $10 per kg today to just $1-2 per kg. The new silicon process uses significantly less energy than current practice and eliminates all direct CO₂ and related emissions. These advances in solar energy production technology contribute to the economic competitiveness of the U.S. solar industry. The fundamental knowledge gained in this research can be used to devise similar novel environmentally sound and efficient production methods for high-melting refractory metals, as well as silicon carbide and other semiconducting materials. The project’s impact is also further broadened through published model data and code, educational modules developed from the research results, and student training in a collaborative and multidisciplinary environment which includes both computational and experimental work.This project establishes four integrated mathematical models of the process based on new experimental data which help in the understanding of silicon molten salt electrolysis and its scale-up. The first molten salt structure (MSS) model involves spectroscopic analysis coupled with atomistic models to understand the molecular structure of complex ions in the five-component molten salt. The second CALculation of PHAse Diagrams (CALPHAD) model to understand and predict thermodynamic and thermophysical properties of the molten salt including silica solubility, silicon compound volatility and ion mobility. These properties are experimentally validated and used in a third transport model to study the reaction and diffusion kinetics of silicon electrodeposition at the cathode-electrolyte interface to predict deposit structure and composition. A fourth phase field model uses these molten salt properties to study the transport of silicon to the cathode and oxygen to the anode to understand boundary layer structure and improve the deposition rate. Using these models, a new current wave-form switching system is developed to maintain stable dendrite-free silicon growth at high current density. The new low-cost single operation silicon molten salt electrolysis replaces multiple energy-intensive unit operations of carbothermic silicon production, acid digestion, trichlorosilane synthesis, distillation, and chemical vapor deposition used in the current Siemens process.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.