Control of floating-zone single crystal growth by magnetic fields

The aim of the investigation is the growth of high-precision single crystals of intermetallic compounds using well defined magnetic fields. The fundamental problem is conditional on the fact that especially complicated intermetallic compounds are not producible as single crystals over the whole cross-section by the inductive floating-zone melting technique since the geometry of the zone at the solid-liquid-interface is unfavourable. The controlled adjustment of a solid-liquid-interface of desired curvature and the knowledge of the melt convection in liquids with high conductivity can be arranged by the contact-less influence of electromagnetic fields. Numerical simulations for the determination of the electromagnetic field configuration induced by the RF heater coil and the solution of the coupled heat and hydrodynamic equations were done for the model substance Ni [1]. The significant influence of the electromagnetically driven flow, which acts opposite to the Marangoni convection, makes the solid-liquid interface concave towards the melt.
Comprehensive investigations of the influence of growth parameters such as zone length, power influence and heated region on the solid-liquid interface geometry during RF floating zone crystal growth were carried out experimentally. It could be shown that undesired concave (towards the melt) interface regions cannot be avoided even at optimised parameter configurations [2].
As a result, a two-phase magnetic field pump, which allows a strong manipulation of the convection and therefore of the solid-liquid interface, was designed. It allows to change the flow structure from the typical double vortex to a single one with a strong influence on the phase boundary shape (Fig. 1). The adjustment of a complete convex solid-liquid interface shape in RF-floating zone crystal growth was tackled with this additional melt stirring resulting in a definite improvement of the solid-liquid-interface.