Control of Thermocapillary Convection and Homogeneity of Floating-Zone Grown Crystals by Vibration.

In this novel study, the controlled surface streaming flow (CSSF) was generated by end-wall vibration and applied to offset TC flow in a float-zone. Evidence gathered through flow visualization, temperature measurements, and observation of a flat solid-liquid interface proved the validity of controlling thermocapillary convection in the float-zone by this active-flow-balancing technique. In our experimental range, the streaming velocity (U) depends linearly on vibration amplitude (A), the square of vibration frequency ( rm f^2), the zone geometric dimensions (LcdotR), and it is inversely proportional to the fluid kinematic viscosity (nu). The physics of this vibration induced internal flow is a new streaming involving not only a solid wall, as in acoustic streaming, but also a free surface connected to the wall at the solid-liquid-air contact line, which is pinned. To future verify the effect of flow balancing on crystal growth, mechanical vibration of amplitude in the range of 10~20mu m and frequency in the range of 1.0~ 1.5 KHz was employed during floating-zone processing to improve the homogeneity of NaNO_3-18 % wt pct Ba(NO_3)_2 eutectic crystals. On superimposing CSSF to the float-zone, TCs in the lower region of the melt zone was balanced and a quasi-stagnant liquid layer was observed to form in front of the growth interface. Temperature measurements in the zone verifies that vibration essentially eliminated radial temperature gradients ahead of the growth interface. Accordingly, the microstructural segregation was also eliminated. The radial uniformity of solidification structures and the observation of a flat interface shape during processing provide clear evidence for suppressing thermocapillary convection through vibration. By examining and comparing crystal structures obtained from different growth conditions, it is further established that it is the temperature gradient change that promotes structural uniformity in the crystals.