Thermodynamic and Kinetic Behavior of B and Na Through the Contact of B-Doped Silicon with Na 2 O-SiO 2 Slags
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BACKGROUND
ELEMENTAL silicon is produced through carbothermic reduction of quartz in the electric arc furnace, the product being metallurgical grade silicon (MG-Si), which is the basis of many metallurgical, chemical, and electrical applications. MG-Si contains about 99 wt pct Si with impurities such as Fe, Al, Ti, Ca, B, and P.[1] Solar Grade Silicon (SoG-Si) with more than 6 N (+99.9999) purity is produced from MG-Si and it is the feedstock for the production of silicon solar cells. Ultrahigh pure silicon with around 11 N purity, which is called Electronic Grade Silicon (EG-Si), is used for the fabrication of electronic devices.[1] A majority of SoG-Si and all the EG-Si existing in the market are currently produced from MG-Si through the Siemens process or the newly developed fluid bed reactor (FBR) technology. In these chemical processes, pure silicon is deposited on rods or seeds from a gas phase, which is produced from MG-Si and contains gaseous compounds of silicon such as SiHCl3 or SiH4. The Siemens process in particular is an expensive process with regard to high energy consumption.[2,3] The production of SoG-Si through metallurgical refining processes is more energy efficient and
JAFAR SAFARIAN, Researcher, is with the SINTEF Materials and Chemistry, Trondheim, Norway. Contact e-mail: Jafar.Safarian@ sintef.no GABRIELLA TRANELL, Associate Professor, and MERETE TANGSTAD, Professor, are with the Department of Materials Science and Engineering , Norwegian University of Science and Technology (NTNU), Trondheim, Norway. Manuscript submitted November 14, 2012. Article published online March 7, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS B
environmentally friendly than the chemical route which in turn may encourage a faster growth of the global PV market. This has been the motivation for the development of several refining processes such as the ELKEM Solar Silicon (ESS) process in Norway, the NS Solar process in Japan, Chinese metallurgical routes such as those of Shanghai Propower and Ningxia, and the Photosil process in France.[3] In all these processes, MG-Si is refined through the combination of a few subprocesses to produce SoG-Si. Almost all the present impurities in MG-Si except B and P can be removed by directional solidification, which is usually a final key process step in the metallurgical approach. Boron is the most difficult element to remove by directional solidification due to its large distribution coefficient between solid and liquid phases, which is KB = 0.8.[4] In order to remove this impurity, many potential processes have been studied such as slag refining, plasma refining, gas refining, solvent refining, leaching, etc.[3] The application of slag refining processes for B removal from silicon has been extensively studied and commercialized in the ELKEM Solar Silicon process. Slag refining is well established in many metallurgical processes and therefore its application is beneficial from a technical production point of view. Moreover, slag refining can be performed on a large scale, which makes it more econ
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