**3. Buoyancy and Marangoni convection in oxide melt**

Buoyancy and thermal capillary convections (namely Marangoni convection) are the main styles of mass transfer in melt especially for high temperature condition. In this section, the typical steady buoyancy and Marangoni convections in the oxide melt/solution suspended on the loop heater will be shown. The unsteady convective flows will be illustrated in the section 6 of this chapter.

#### **3.1 Buoyancy driven convection**

Generally, buoyancy driven convection is in close correlation to the temperature distribution in the liquid phase. Fig. 4 shows the typical temperature distribution in the horizontal direction of the thin liquid film suspended by Pt loop. In the central portion of the loop, the melt temperature gradient can be negligible. This region is called pure diffusion region as indicated by sign A in Fig. 4. The situation has an advantage for studying interfacial kinetics process with pure diffusion transport. But in the marginal portion of the heater, the horizontal thermal gradient is significant, and this portion is called diffusion-convective region as hinted by the sign B in Fig. 4. In this region, the growth may be controlled by the buoyancy driven convection due to the higher temperature gradient. The width of diffusionconvective region depends on the loop diameter as well as on the thermophysical parameters of oxide melt, such as the density, thermal diffusivity and viscosity.

Fig. 4. Typical radial temperature distribution in the horizontal direction of the thin liquid film suspended by Pt loop. D is the distance from the loop margin. Here the temperature distribution profile is supplied for KNbO3 melt

Buoyancy driven convection can be visualized since Schlieren technique has been introduced. However, quantitative measurement of flow velocity needs the help of some tracing particles. Since some tiny crystals can be nucleated when the melt becomes undercooled, the buoyancy driven convection can be indicated by observing the movement of tiny crystals as described in the reference (W. Q. Jin, et al., 1993). As shown in Fig. 5, the tiny crystals move from the margin to the center of the heater. This is typical buoyancy driven flow caused by the melt rising along the hot wall and descending in the center of vessel which is heated from the side and cooled from the top surface. Fluid flow velocity measurement shows that the momentum profile in the melt is similar to the thermal one. In the central portion of the loop, the flow is steady because of low value of the applied temperature gradient.

Fig. 5. The buoyancy driven convection indicated by the movement of tiny KNbO3 crystals
