Modelling and experiments for the convecto-diffusive removal of impurities from the solidification front

During crystal growth, the fluid flow plays an important role on the quantity and distribution of impurities in the crystal. This study examines with modelling and experiments the removal of impurities from the solidification front for the case where a turbulent fluid flow increases the effective diffusivity. The theoretical model is based on the properties of turbulent boundary layers. It predicts a transition between the diffusive regime (no segregation) and the convective regime (efficient segregation) when the stirring velocity is higher than a threshold depending on the solidification rate. Efficient segregation is generally obtained when the stirring velocity is 105 times higher than the solidification rate. Silicon solidification experiments in a Taylor-Couette flow showed that concentrations in the crystal are close to the thermodynamic minimum for stirring velocities above the threshold. This result shows that the turbulent fluid flow efficiently removes impurities from the solidification front and validates the theoretical model.