Float-zone crystal growth with a novel melt flow control

We consider the growth of single crystals by the floating-zone method using a radio frequency (RF) heating magnetic field. The quality of the grown crystals, especially of complicated intermetallic compounds, strongly depends on the growth conditions, particularly on the shape of the solidification front. To obtain single crystals of high physical and chemical precision, a convex to the melt growth interface is desirable. The shape of the solid-liquid interface can strongly be influenced by the convective heat transport in the melt. The major mechanism driving the melt flow is the electromagnetic force due to the RF-heating. This force drives a melt flow directed radially inwards at mid-height of the floating zone. Favourable growth conditions, however, would require the flow to be directed away from the solidification front at the axis of symmetry and, respectively, to be directed against it at the free surface of the floating zone. This flow can be generated by an electromagnetic force directed along the free surface towards the solidification front.
We developed the solution of a two-phase stirrer which allows a flexible control of the melt motion. This stirrer basically consists in an additional coil superimposed to the primary induction coil. The additional coil is not connected to any power supply, but it is part of a secondary circuit with adjustable capacitor and resistance. The current in the secondary coil is solely induced by the primary coil. In that way, an electromagnetic pump is created. Its intensity and resulting flow direction can easily be adjusted to the process needs. The flexible system parameter are the location and distance of the secondary coil with respect to the primary one, and the capacitor and resistance of the secondary circuit.
We present numerical and experimental results for the melt flow and temperature fields resulting from the influence of such a two-phase stirrer. Ni-based single crystals as well as various intermetallic compounds have been grown, and the obtained results are discussed in relation to the provided flow control.