Revisiting the twinning mechanism in directional solidification of multi-crystalline silicon sheet

The formation of the twin nucleus is an important phenomenon during the solidification of multi-crystalline silicon. However, the available model by Duffar and Nadri [Scripta Mater. 62(12) (2010) 955] was unable to predict a critical undercooling for twinning that was comparable with the experiments. Through the detailed analysis of the grain structures in the multi-crystalline silicon sheets grown by directional solidification, we find that the grain boundary plays a crucial role. By taking this into account, we revise the previous model. The free energy change (per unit length) when one or two facets anchor to the groove tri-junction and the critical undercooling for twinning are derived and compared with experiments. The predicted undercooling and the probability of forming a twinned nucleus at the groove based on the revised model are consistent with the experimental observations.

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