Electromagnetic control of local temperature gradients in a Czochralski crystal growth model

This experimental work is concerned with optimisation of the Czochralski crystal growth process. With respect to the shape of the solidification front and the related mono-crystalline growth, the ratio of the horizontal and the vertical temperature gradient r* = DTh /DTv at the triple point liquid-solid-atmosphere is thought of being a crucial magnitude, which desirably should be in the order of unity. A liquid metal model experiment was therefore built that allows studying this ratio under the influence of magnetic fields applied to the melt. The cylindrical liquid metal column was homogeneously heated from below, whereas on top the heat was extracted in a concentrical region covering only about one third of the surface in order to simulate the growing crystal. It was shown that the native, i. e. without flow control, r* ≈ 3 is far removed from unity. In a first series of measurements, it was then possible to reach the target value r* = 1 for any temperature difference between the bottom and the top at a moderate field strength while applying a rotating magnetic field.