Directional solidification of gallium under time-dependent magnetic fields with in situ measurements of the melt flow and the solid-liquid interface

•Directional solidification model experiments with gallium are described.•A special seeding technique for a reproducible solidification process is presented.•Advanced Ultrasound techniques are used for in situ flow and interface measurements.•The changes of the flow structure and the interface shape during solidification are demonstrated.•The relevance of the model experiments for the growth of multi-crystalline silicon ingots is discussed. The results of solidification experiments with gallium in a square-based 220 × 220 mm2 container under the influence of alternating and travelling magnetic fields are presented. Such experiments are relevant for the growth of multi-crystalline silicon ingots for solar cells using directional solidification. The time-dependent magnetic fields are generated by a system of two inductors supplied with alternating current. The axial temperature field for solidification is established by thermostat-controlled heat exchangers at the top and the bottom of the container. A new approach for a reproducible directional solidification process using a special seeding technique is presented. The melt flow and interface positions are measured in-situ during solidification by using ultrasound sensor arrays for a 2D flow mapping, and single ultrasound transducers for interface detection. The experiments reveal significant changes of the flow structure and the interface shape with decreasing melt height. The results are compared with numerical simulation and a setup-independent categorization of the observed flow structures is proposed. Conclusions for silicon growth processes are discussed.