Flow investigations in liquid metals for crystal growth under the influence of a travelling magnetic field using a dual-plane ultrasound measurement system

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The objective of the research project is a detailed investigation of complex flows occurring at the directed solidification of metal melts using the vertical gradient freeze method with a travelling magnetic field. For growing semiconductor crystals from a melt, the crystal quality can be improved by applying spatially and temporally tailored magnetic fields during the growth process. The occurring flow fields are characterized by complex three-dimensional and time-varying structures. The flow measurements have to be performed simultaneously in at least two planes. Up to now, such investigations failed because of the insufficient properties of measurement techniques applicable to liquid metals. To overcome this problem, a novel multi-component dual-plane ultrasound measurement system will be employed which uses ultrasound-arrays driven in a combined time and frequency division multiplexing operation. Due to the progress in available computing power the flow patterns and the temporal evolution from one to another can be recorded and studied in real-time and especially for long measurement time intervals.In this project, imaging investigations of the melt flow in two planes under thermal, geometric and magnetic conditions relevant for crystal growth will be accomplished for the first time. The investigation of transient and fluctuating flow situations in dependence of the melt height takes centre stage. From the fluid mechanical point of view a special interest lies in the point how the transition between two-dimensional and three-dimensional as well as between stationary and time-varying flow patterns takes place. The results gained by experiment will continuously be compared with those of numeric simulation. This benefits not only the development of the model configurations but also the verification of numeric models for flow simulation. The results will be transferred to real crystal growth processes by means of scaling laws which will be developed during the project. They also will be the basis for the development of future strategies for flow control using magnetic fields.