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调查复杂的流动发生在金属熔体的定向凝固使用垂直梯度凝固方法与移动磁场。为了从熔体生长半导体晶体，可以通过在生长过程期间施加空间和时间定制的磁场来改善晶体质量。发生的流场的特点是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，这种调查失败，因为适用于液态金属的测量技术的性能不足。为了克服这个问题，将采用一种新的多分量双平面超声测量系统，该系统使用在组合的时分和频分复用操作中驱动的超声阵列。由于现有计算能力的进步，可以实时记录和研究流动模式和从一个到另一个的时间演变，特别是长测量时间间隔。在该项目中，将首次完成在与晶体生长相关的热、几何和磁条件下两个平面中熔体流动的成像研究。对熔体高度相关的瞬态和波动流动情况的研究占据了中心位置。从流体力学的角度来看，一个特别感兴趣的是如何在二维和三维之间的过渡，以及固定和随时间变化的流动模式之间发生。将实验结果与数值模拟结果进行比较。这不仅有利于模型结构的发展，也有利于流动模拟数值模型的验证。通过在项目期间开发的标度律，将结果转化为真实的晶体生长过程。它们也将成为未来利用磁场进行流量控制的策略发展的基础。

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.