Numerical investigation of the effect of a crucible cover on crystal growth in the industrial directional solidification process for silicon ingots

• We study the crucible cover effects on heat transfer through the DS process.
• Cover increases the melting time, but affects little on the melting sequence.
• Cover decreases the growing time and affects the melt flow at free surface center.
• Cover has little effect on the ingot isotherm shape when annealing and cooling.

We carried out transient global simulations of heating, melting, growing, annealing and cooling stages for the industrial directional solidification (DS) process. Two separate DS processes were performed with a covered crucible and an uncovered crucible to investigate the effect of a crucible cover on global heat transfer and silicon crystal growth. It is found that the cover blocks heat transfer from the heaters to the silicon domain significantly, and therefore influences the crystal growth through the entire DS process. For the melting of silicon feedstock, covering the crucible can lead to a significant increase in melting time, whereas it has little effect on the melting sequence. During the growing stage, the solidification time for the process with a crucible cover is much shorter than the process without a cover. The cover can also influence the melt convection near the free surface center, but it hardly changes the melt flow pattern deep in the crucible. For the annealing and cooling processes, heat transfer is not significantly affected by covering the crucible and the isotherm shapes in the silicon ingots are similar with and without a cover.