A SIMPLE METHOD FOR IMPROVING THE STABILITY OF FLOAT ZONES UNDER NORMAL GRAVITY

Stability of the molten zone is the most critical issue in float zone crystal growth and refining. Under normal gravity, a long zone is unstable and tends to collapse during crystal growth or zone refining. A simple method for shortening and hence significantly improving the stability of float zones was discovered. This method is unique in that it is not required that the material be electrically conducting. To demonstrate this uniqueness, a nonconducting material NaNO3 was used. By counterrotating the upper and lower rods at sufficiently high speeds, it was possible to reduce the heat input and hence shorten the molten zone, without causing its centerline to freeze. Sufficiently fast single rotation of either the upper or lower rod produced similar results. Without this method, the zone was stable in an NaNO3 rod of 4 mm diameter. However, in 6 and 10 mm diameter rods the zone was too long and collapsed. By applying this new method, a short and very stable zone was maintained even in the larger diameter rods. This improved zone stability was explained qualitatively based on melt convection and the resultant heat transfer in the zone. Since the ring heaters were stationary, these molten zones can be considered equilibrium zones.