Kinetic Modeling of a Silicon Refining Process in a Moist Hydrogen Atmosphere
- PDF / 653,361 Bytes
- 8 Pages / 593.972 x 792 pts Page_size
- 116 Downloads / 210 Views
TION
ONE of the major challenges in producing solar grade silicon from metallurgical grade silicon is the effective removal of boron impurities. The Tangstad group[1] has developed an efficient moist hydrogen treatment for refining silicon where the boron content in molten silicon was reduced from 130 ppmw to below 1 ppmw after 4.5 hours of treatment. The boron was removed as a volatile species in their experiments, and silicon was also lost (~ 10.7 pct of the initial sample weight). Many factors affecting the boron removal kinetics have been extensively studied. Theuerer[2] suggested that the reaction rate constant of boron removal in moist hydrogen is proportional to the square root of the partial pressure of the water vapor. Moreover, an increase in temperature resulted in a slower boron removal rate, possibly due to either the thermodynamics of the hydrogen-boron-oxygen (HBO) gas production[3] or the competitiveness of the vapor pressures of Si and B volatile species.[4] Considerable effort has been directed towards exploring the mechanism of this moist hydrogen
ZHIYUAN CHEN and KAZUKI MORITA are with the Department of Materials Engineering, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan. Contact e-mail: [email protected] Manuscript submitted December 2, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS B
treatment process. Tang et al.[5] suggest that H+, OH, or O2 interacts with the Si-B bonds without an obvious effect on Si-Si bonds, and thus boron volatile species are more easily generated than silicon volatile species. Khattak et al.[6] propose that HBO (g) is produced through the reaction of hydrogen gas, SiO gas, and boron at melt-gas interface. Safarian et al.[3] propose that the HBO (g) formation mechanism is the reaction between boron, the H species in silicon, and water vapor in the gas at the melt-gas interface. In this case, dissolution of hydrogen into silicon is a necessary pre-reaction. Recently, Safarian et al.[4] again confirmed in a new study that boron reacts with dissolved hydrogen and oxygen in molten silicon to produce HBO (g). In addition to studies exploring the mechanism for the successful removal of boron by the moist hydrogen treatment, the reaction kinetics of the process have also been investigated. According to several studies, the rate-determining step of this process is presumably the interfacial chemical reactions.[1,7] However, Sortland and Tangstad[8] indicated that the supply of steam in the bulk gas is the rate-determining factor in their experiments. Building on the commonly cited law of exponential decay,[3,4,7] Tang et al.[7] suggested an effective reaction zone model by simulating the boron removal using a ChemSheet add-in program with an MS Excel spreadsheet. However, the theoretical background of the kinetic model needs further investigation and verification. Furthermore, the obvious silicon loss during this reaction was not adequately quantified.
In the present work, we have developed a kinetic model which considers the mecha
Data Loading...
