Oscillation phase relations in a Bridgman system

The thermal conditions under which convection in a tin melt undergoes the transition from steady flow to time-dependent flow are investigated. Previously, it has been shown that crystals that are directionally solidified from a time-dependent melt will have a larger defect density and increased chance of compositional striations. Thus, it is important for this transition to be well characterized. The experimental results to be presented were obtained from a two-zone Bridgman furnace with a middle insulation zone. Thermocouples were placed on the axis and on the outer wall of a cylindrical vitreous silica tube which contained molten tin. The phase relations between temperature oscillations measured at different positions in the cell are discussed. Fourier transforms are used to investigate the increasing complexity of convection as the temperature gradient is increased.