Preparation, microstructure and dislocation of solar-grade multicrystalline silicon by directional solidification from metallurgical-grade silicon

A vacuum directional solidification with high temperature gradient was performed to prepare low cost solar-grade multicrystalline silicon （mc-Si） directly from metallurgical-grade mc-Si. The microstructure characteristic, grain size, boundary, solid-liquid growth interface, and dislocation structure under different growth conditions were studied. The results show that directionally solidified multicrystalline silicon rods with high density and orientation can be obtained when the solidification rate is below 60 μm/s. The grain size gradually decreases with increasing the solidification rate. The control of obtaining planar solid-liquid interface at high temperature gradient is effective to produce well-aligned columnar grains along the solidification direction. The growth step and twin boundaries are preferred to form in the microstructure due to the faceted growth characteristic of mc-Si. The dislocation distribution is inhomogeneous within crystals and the dislocation density increases with the increase of solidification rate. Furthermore, the crystal growth behavior and dislocation formation mechanism of mc-Si were discussed.

Redistribution of iron during directional solidification of metallurgical-grade silicon at low growth rate

Redistribution of iron during directional solidification of metallurgical-grade silicon (MG-Si) was conducted at low growth rate. Concentrations of iron were examined by ICP-MS and figured in solid and liquid phases, at grain boundary and in growth direction. Concentrations are significantly different between solid and liquid phases. The thickness of the solute boundary layer is about 4 mm verified by mass balance law, and the effective distribution coefficient is 2.98×10?4. Iron element easily segregates at grain boundary at low growth rate. In growth direction, concentrations are almost constant until 86% ingot height, and they do not meet the Scheil equation completely, which is caused by the low growth rate. The effect of convection on the redistribution of iron was discussed in detail. Especially, the “dead zone” of convection plays an important role in the iron redistribution.

Boron removal from metallurgical-grade silicon by CaO–SiO_2 slag refining

Boron removal from metallurgical-grade silicon(MG-Si) using CaO–SiO2 slag was studied by employing a medium-frequency electromagnetic induction furnace.The relationship between the optical basicity(K)of the CaO–SiO2 slag and the distribution coefficient of boron(LB) was investigated.Consequently, the local minimum and maximum LBvalues of 0.72 and 1.58 are obtained when K = 0.56 and K = 0.71, respectively.The boron content in MG-Si decreases gradually with refinement time increasing, down to a minimum value of4.73 9 10-6.The controlling step in the removal of boron from MG-Si is not the chemical reaction at the interface of the slag and silicon.Instead, the controlling step is a diffusion mass transfer, in which boron impurities diffuse from molten silicon to the interface of the slag and silicon,or B2O3 formed by the chemical reaction diffuses from the slag–silicon interface to molten slag.

Boron Removal From Metallurgical-Grade Silicon Using CaO-SiO_2 Slag

The removal of boron impurity from metallurgical-grade silicon for solar cell application in slag system of CaO-SiO_2 is investigated.The experiments are conducted in an electromagnetic induction furnace which is used to heat. The distribution coefficient of boron(L_B)between slag and silicon phase is particularly examined in terms of the optical basicity of slag.With the increase of optical basicity,L_B increases to a local maximum value of 1.58 when the optical basicity is 0.71 after getting to the minimum value of 0.72 when the optical basicity is 0.56.In that above optical basicity of 0.71,L_B decrease sharply which indicates that increasing the basicity of slag is not always effective in boron removal from silicon.

Numerical Simulation for Induction Refining Process of Metallurgical-Grade Silicon in Vacuum Furnace

The temperature and velocity distribution of melting pool fields is very important effect to the silicon purification in vacuum induction furnace.A numerical model for the electromagnetic-thermal-hydrodynamic coupling fields have been developed by using the finite element method(FEM)and a 2D numerical simulation for electromagnetic、 temperature and velocity fields of metallurgical-grade silicon melting in vacuum induction furnace were performed with a software Multi-physics Comsol 3.5a in this paper.The results showed that the temperature field was dependent observably on input power of coils and induction heating times and the maximum temperature gradient in melting pool was 215K in holding time.With the silicon molted gradually a clockwise vortex was come into being for electromagnetic stirring in the smelting poor.The variation of velocity field in melting silicon is mainly influenced with the change of the current intensity and power frequency.The numerical predications of temperature distribution are in good agreement with experiments.