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

该研究项目的目的是详细研究在金属熔体定向凝固过程中使用垂直梯度冻结法和行进磁场的复杂流动。为了从熔体中生长半导体晶体，可以在生长过程中施加空间和时间上定制的磁场来提高晶体质量。所产生的流场具有复杂的三维和时变结构。流量测量必须在至少两个平面上同时进行。到目前为止，由于适用于液态金属的测量技术的性能不足，这类调查失败了。为了克服这一问题，将采用一种新颖的多分量双平面超声测量系统，该系统使用时分和频分复合操作驱动的超声阵列。由于可用计算能力的进步，可以实时记录和研究熔体从一种流动模式到另一种流动模式的时间演变，特别是在较长的测量时间间隔内。在这个项目中，将首次完成与晶体生长相关的热、几何和磁场条件下两个平面内熔体流动的成像研究。与熔体高度相关的瞬变和波动流动情况的研究占据了中心位置。从流体力学的观点来看，人们特别感兴趣的是如何在二维和三维之间以及在静止和时变流型之间发生转变。实验结果将继续与数值模拟结果进行比较。这不仅有利于模型构型的发展，也有利于流动模拟数值模型的验证。结果将通过在项目期间开发的比例定律被转移到实际的晶体生长过程中。它们也将是未来开发利用磁场进行流量控制的策略的基础。

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.