Three-Dimensional Flow Studies in Cylindrical Magnetohydrodynamic Experiments Using Ultrasound Array Velocimetry.

Crystal growth processes can profit from an electromagnetically driven melt flow since controlling them allows optimizing the mass and heat transfers in the melt and, thereby, improves the structural and electrical properties of the grown crystals. This process optimization requires a precise understanding of magnetohydrodynamics (MHD) phenomena in crystal growth. Studying time-dependent MHD demands for a high temporal resolution combined with a long measurement duration to analyze the transitional flow behavior. Furthermore, a spatially resolved measurement of the global flow structure is desired to capture the complex 3-D flow structures. We present an ultrasound array Doppler velocimeter (UADV) for time-resolved flow imaging in MHD model experiments with low-melting metals. Flow imaging at frame rates of several Hertz is achieved by using a combined spatial and temporal multiplexing scheme. Long-running measurements are enabled by a field-programmable gate array (FPGA)-based signal processing that reduces the measurement data rate by a factor of 5. A reconstruction of the 3-D flow structure in cylindrical containers with a rotation-symmetric flow is proposed. The UADV is demonstrated at an MHD experiment with a melt flow in a cylindrical container driven by a traveling magnetic field. The transition from a laminar to a time-dependent flow is studied, revealing an oscillating flow. The demonstration of the 3-D reconstruction gives comprehensive insight into the global flow structure. Hence, the UADV method is shown to be a valuable tool for measuring complex, time-dependent melt flows, which can contribute to a better understanding of the flow phenomena during crystal growth.

Effect of screw threading dislocations and inverse domain boundaries in GaN on the shape of reciprocal-space maps.

The microstructure of polar GaN layers, grown by upgraded high-temperature vapour phase epitaxy on [001]-oriented sapphire substrates, was studied by means of high-resolution X-ray diffraction and transmission electron microscopy. Systematic differences between reciprocal-space maps measured by X-ray diffraction and those which were simulated for different densities of threading dislocations revealed that threading dislocations are not the only microstructure defect in these GaN layers. Conventional dark-field transmission electron microscopy and convergent-beam electron diffraction detected vertical inversion domains as an additional microstructure feature. On a series of polar GaN layers with different proportions of threading dislocations and inversion domain boundaries, this contribution illustrates the capability and limitations of coplanar reciprocal-space mapping by X-ray diffraction to distinguish between these microstructure features.

Ultrasound Flow Mapping for the Investigation of Crystal Growth.

A high energy conversion and cost efficiency are keys for the transition to renewable energy sources, e.g., solar cells. The efficiency of multicrystalline solar cells can be improved by enhancing the understanding of its crystallization process, especially the directional solidification. In this paper, a novel measurement system for the characterization of flow phenomena and solidification processes in low-temperature model experiments on the basis of ultrasound (US) Doppler velocimetry is described. It captures turbulent flow phenomena in two planes with a frame rate of 3.5 Hz and tracks the shape of the solid-liquid interface during multihour experiments. Time-resolved flow mapping is performed using four linear US arrays with a total of 168 transducer elements. Long duration measurements are enabled through an online, field-programmable gate array (FPGA)-based signal processing. Nine single US transducers allow for in situ tracking of a solid-liquid interface. Results of flow and solidification experiments in the model experiment are presented and compared with numerical simulation. The potential of the developed US system for measuring turbulent flows and for tracking the solidification front during a directional crystallization process is demonstrated. The results of the model experiments are in good agreement with numerical calculations and can be used for the validation of numerical models, especially the selection of the turbulence model.