Gupta Nivedita R, Haj-Hariri Hossein, Borhan Ali
Department of Chemical Engineering, University of New Hampshire, Durham, New Hampshire 03824, USA.
Ann N Y Acad Sci. 2006 Sep;1077:395-414. doi: 10.1196/annals.1362.006.
Many materials-processing applications such as crystal growth from the melt involve thermocapillary flows that can affect the quality of the final product, particularly under microgravity conditions where the influence of buoyancy-driven convection is minimized. When the melt contains volatile components, as in the production of III-V semiconductor crystals, it is often encapsulated in a low-melting point amorphous molten glass phase such as boron oxide or pyrolytic boron nitride in order to prevent evaporation of the volatile components. The addition of the encapsulant layer and the melt-encapsulant interface in such cases can alter the thermocapillary flow in the melt. In this study, thermocapillary convection within a differentially heated rectangular cavity containing two immiscible liquid layers is considered in the absence of gravity. Domain mapping is used in conjunction with a finite difference scheme on a staggered grid to solve for the temperature and flow fields. The melt-encapsulant and the air-encapsulant interfaces are allowed to deform, with the contact lines pinned on the solid boundaries. The computed flow fields are compared to the corresponding results for a cavity with a rigid top surface. The presence of a free surface at the top leads to increased convection in the encapsulant phase while suppressing the thermocapillary flow in the melt phase. The flow pattern in the encapsulated layer is strongly dependent on the viscosity of the encapsulant layer. The intensity of the thermocapillary flow within the melt is significantly reduced as the viscosity of the encapsulant layer is increased. However, for a higher encapsulant viscosity, the retarding effect of the free top surface on thermocapillary convection in the melt is weakened.
许多材料加工应用，如从熔体中生长晶体，都涉及热毛细流动，这种流动会影响最终产品的质量，特别是在微重力条件下，此时浮力驱动对流的影响最小化。当熔体含有挥发性成分时，如在III-V族半导体晶体的生产中，通常将其封装在低熔点非晶态熔融玻璃相中，如氧化硼或热解氮化硼，以防止挥发性成分蒸发。在这种情况下，添加封装层和熔体-封装层界面会改变熔体中的热毛细流动。在本研究中，考虑了在无重力情况下，包含两个不混溶液体层的差动加热矩形腔内的热毛细对流。区域映射与交错网格上的有限差分格式结合使用，以求解温度和流场。熔体-封装层和空气-封装层界面允许变形，接触线固定在固体边界上。将计算得到的流场与具有刚性顶面的腔体的相应结果进行比较。顶部自由表面的存在导致封装层中的对流增加，同时抑制了熔体相中的热毛细流动。封装层中的流动模式强烈依赖于封装层的粘度。随着封装层粘度的增加，熔体中热毛细流动的强度显著降低。然而，对于较高的封装层粘度，自由顶面熔体中热毛细对流的阻碍作用减弱。
