Thermal Field Simulation And The Effect Of Static Magnetic Field For Directional Solidification Of Multi-crystalline Silicon

Solar energy as a great development prospects renewable energy, with unlimited and non-polluting characteristics, is an effective method to solve two major problems of energy and the environment. In the field of solar cells, multi-crystalline silicon solar cells occupy an important position for its advantages of low cost and high efficiency. The directional solidification casting technology of multi-crystalline silicon plays a vital important role in the process of production of multi-crystalline silicon solar cell.The transient global simulations of the directional solidification process for multi-crystalline silicon were carried out by COMSOL Multiphysics which is the software based on finite element method. The vertical distance between the top and side heater of the furnace for multi-crystalline silicon ingot was set to 60 mm, 150 mm and 300 mm to investigate its effect on multi-crystalline silicon melting and crystallization. It was found that the melting time is longest while the distance is 60 mm. When the distance is 150 mm and 300 mm, the melting time is respectively less than 34 min and 60 min compared with the situation that the distance is 60 mm. For the solid-liquid interface during crystallization, when the distance is 60 mm, the solid-liquid interface is similar to plane which is conducive to the growth of multi-crystalline silicon. While the distance is 150 mm, the central region of solid-liquid interface become more convex, but it is not very obvious. While the distance is 300 mm, the solid-liquid interface is obviously convex which is harmful to the quality of multi-crystalline silicon ingot.A series of two-dimensional numerical computations were conducted by numerical simulation technology to investigate the effects of three kinds of static magnetic field, including axial magnetic field, transverse magnetic field and cusp-shaped magnetic field, on the process of the directional solidification of multi-crystalline silicon. Numerical calculations were carried out when the current of coils were respectively set as: 0 A, 10 A, 20 A, 30 A and 40 A. The results showed that the three kinds of static magnetic field can effectively inhibit the melt convection,the axial magnetic field mainly inhibits the speed of melt in the horizontal direction,transverse magnetic field mainly inhibits the rate of melt in the vertical direction, cusp magnetic field can inhibit melt convection in both vertical direction and vertical direction. When the current increases from 0 A to 40 A, the axial magnetic field reduces the maximum flow velocity by 47.5%, the transverse magnetic field reduces the maximum flow velocity by 53.8%, while the cusp-shaped magnetic field reduces the maximum flow velocity decreases by 58.3%. It shows that cusp-shaped magnetic field has the biggest inhibitory effect on the melt convection among three kinds of static magnetic field. It is also found that cusp-shaped magnetic field can reduce the curvature of solid-liquid interface and improve the uniformity of radial distribution of impurities.