Study On The Segregation Behavior Of Boron During Si-Al(-Sn) Alloy Solidification Process

Solar grade silicon (SOG-Si) is the most important material for Si solar cells production, and makes up 50% of total costs. In order to promote the sustainable development of the Photovoltaic industry, strong efforts are invested in the improvement of purification techniques for the low cost production of SOG-Si. Recently, Si purification by alloy solidification refining method has become the focus of attention because of low cost and high refining efficiency.Due to the low vapor pressure and large segregation coefficient of B in Si, B is difficult to remove from Si by the conventional vacuum melting or directional solidification method, which remains a major challenge. Alloy solidification refining process consists of alloying Si with another metal to enable the solvation and recrystallization of Si from the solvent at a lower temperature than the melting point of Si, followed by separation of the crystallized Si. This method can obviously decrease the segregation coefficient of B and thus enhance its removal efficiency, which is regarded as a promising method for producing high purity Si. Among various alloy systems, the Si-Al alloy system exhibits outstanding potential in purifying Si effectively and economically, but fails to meet the requirement of SOG-Si limited by the segregation coefficient of B and the Si loss. Therefore, the segregation behavior of B should be confirmed and the recovery rate of Si should be improved during the alloy solidification refining process. In this thesis, the Si-Al(-Sn) alloys are used as solvents to purify Si, during which the segregation behavior of B is investigated to optimize the solidification process; Owing to the lower content of Si in Si-Sn alloy, the Si-Al-Sn alloy is proposed to increase the recovery rate of Si, and the effect of Sn addition on the thermodynamic properties and segregation behavior of B are studied; Furthermore, the directional solidification of Si-Al-Sn alloy is carried out to produce and separate bulk Si with high purity.The Si-Al alloy solidification process is investigated to determine the segregation behavior of B and the purity of Si. The results show that, at the Si precipitation stage, more B segregate and enrich in the Al phase, and thus high purity primary Si and eutectic Si are obtained; At the solid alloy cooling stage, the back-diffusion of B occurs, which is found to contaminate the Si. The Si-Al is solidified at various cooling rates at the final solidification process, which aims to investigate the effect of Si morphology on its purity. The results show that the Si obtained by quenching at 873 K has a planar interface, while the Si obtained by slow cooling rate has a coarse interface which can be contaminated by inclusions. According the above results, the optimized solidification process is proposed, namely, lower cooling rate should be applied in the initial solidification stage to produce high purity Si, followed by quenching at the temperature a little higher than the eutectic point. This aims to reduce the inner inclusions and inhibit the back-diffusion of impurity from the refined Si. Under the optimized solidification conditions, the B removal efficiency can reach to 90.5%.Si-Al-Sn alloy is proposed to purify MG-Si. Based on the phase diagram of Si-Al-Sn alloy, its solidification process is investigated. After the Si purification and precipitation process, primary Si, binary eutectic Si, and ternary eutectic Si are found in the Si-Al-Sn alloy, and the recovery rate of primary Si can be increased by 10% with the addition of Sn. In addition, the Si-Al-Sn alloy solidification process is applied to determine the effects of Sn addition on the segregation behavior of B and the morphology of Si. The results indicate that the B content in the primary Si phase is low, but increases in the aAl phase until reaching the highest level in the aSn phase; The plate-like Si crystals in Si-Al-Sn alloy show insensitive to the addition of Sn, but the eutectic Si distribute in the Si-Al-Sn alloy uniformly with a different morphologies, namely, Si has a relatively globular shape in aAl with modified by Sn, but convert into octahedral crystals in aSn.The phase equilibration method is used to study the thermodynamic properties of B in Al-Sn melts at 1273 K. The results show that the equilibrium between Al-Sn melts and solid pure AlB12 is attained after 22 hours. Under the equilibrium conditions, the solubility of B in the Al-Sn melts is determined, which is found to decrease with increasing Sn content. It is emphasized that the activity coefficient of B in the Sn-rich side is one-ninetieth of that in the Al-rich side. Accordingly, the activity coefficient of B is obtained, and there exists the following relationships which show that B becomes more unstable in the Sn-rich melts. RT lnr°B in molten Sn> RT lnr B in molten Al-Sn>RT lnr°B in molten Al Afterwards, the Si-Al, Si-Sn and Si-Al-Sn alloy solidification processes are respectively carried out to determine the B removal efficiency. In the Si-Al alloy system, the B removal efficiency increases with increasing the Al content, while it decreases with increasing the Sn content in the Si-Sn alloy. Due to the opposite effects of Al and Sn on B removal, the refining efficiency of B in Si-Al-Sn melt is placed between these two binary alloys.The directional growth of bulk Si from Si-Al-Sn melts is studied which has the potential to effectively separate high purity Si. The effects of cooling rate, alloy composition, G/R value and the component number are discussed. The results show that the bulk Si with a planar interface can be formed under proper solidification conditions, which can also favor the reduction of the Al and Sn. The Si growth is found to be controlled by the diffusion of Si in the melts, so the growth rate of bulk Si can be accelerated by increase the Si content in melts or the temperature gradient, which is ranged from 0.67×10-3 to 2.13×10-3 mm/min. In addition, comparing G/R values among Si-Al, Si-Sn and Si-Al-Sn alloy systems, it is found that the experimental value of Si-Al-Sn is larger than that of Si-Al and Si-Sn alloy, indicating that Si-Al-Sn alloy is less favorable for the stable growth of Si under the same solidification conditions. The bulk Si is treated by NaOH etching for Si crystal grains observation. It can be found that the bulk Si contains some large columnar grains with a size of 0.8 mm, whose boundaries extend approximately parallel to the movement direction.