Numerical Analysis of Phosphorus Concentration Distribution in a Silicon Crystal during Directional Solidification Process

For bulk doping, boron and phosphorus are usually used as p-type and n-type dopants, respectively. The distribution of these dopant concentrations in a silicon crystal along the vertical direction is governed by the segregation phenomena. As the segregation coefficient of phosphorus is small, phosphorus concentration distribution in a silicon crystal becomes inhomogeneous; inhomogeneous phosphorus concentration distribution affects the distribution of resistivity in the crystal. Therefore, it is important to control the phosphorus concentration distribution in a silicon crystal and make it uniform. In this study, by numerical analysis, we investigated the effect of the evaporation flux at the melt surface on phosphorus concentration distribution during the directional solidification process. To obtain a homogeneous phosphorus concentration distribution in the silicon crystal, we had to control the phosphorous evaporation flux at the melt surface and maintain approximately the same phosphorus concentration in the melt during the entire solidification process even though the growth rate was always changing.

Study on the Directional Solidification Process of an Aluminum Alloy Bar in Multishell Mold Being Gradually Immersed in Water

A multishell mold structure and water-immersion cooling method (MSMWI) is proposed for the directional solidification of castings. A four-layer-shell sand mold was designed for a bar with diameter of 40 mm. As the aluminum melt was poured, the multishell mold was gradually immersed in water, and the water level drove the advancement of the solidification front from bottom to top. The multishell mold was helpful for the heat insulation of its upper part, and its bottom was chilled by the water. Therefore, directional solidification of the bar was vertically realized. The water-cooled solidification process of the bar was 5.8 times faster than that by air natural cooling (MSMNC), and the temperature gradient was increased by 78 times. The secondary dendrite arm spacing (SDAS) and eutectic silicon were significantly refined. Its tensile strength, elongation, and hardness were increased by 56%, 185%, and 62.6%, respectively.