Marangoni convection and single crystal growth : effect of the sign of temperature coefficient of surface tension

为了生产高质量的氧化物和半导体单晶，有必要了解熔体对流现象的细节。在对流机制中，浮力流动机制已经被很好地理解，而热毛细流动（马兰戈尼对流）和浮力-马兰戈尼混合流动机制的基本原理还不太清楚。马兰戈尼流在微重力条件下的液体流动中也起着非常重要的作用。在本研究中，为了获得更多关于马兰戈尼对流的信息，我们通过实验和数值模拟研究了熔融硝酸钠（NaNO）和氢氧化钠（NaOH）的小液体桥中的马兰戈尼和马兰戈尼-浮力混合对流，其表面张力表现出非常特殊的温度依赖性。结果总结如下：1)在600 ~ 823k温度范围内，采用最大气泡压力法测量了NaOH熔液的表面张力（sigma）。它在T^*=725 K处显示出最大值。在熔融温度（约600K）和T^*之间，sigma_T （=*sigma/*T）为正，但在T^*以上的较高温度下，它变为负。2)温度低于T^*时，NaOH熔液桥中的Marangoni对流表现出从冷点到热点的表面运动。数值模拟很好地解释了观测到的速度分布。3) T^*以上的熔融NaOH的马兰戈尼对流从高温点流向低温点，这在正常液体中是常见的。4)温度在T^*附近的马兰戈尼对流表现出多（3^-4）卷胞模式。5)当液桥垂直放置时，浮力对对流的影响可以忽略不计，但当液桥水平放置时，浮力对对流的影响就变得重要起来。通过NaNO_3液桥实验和三维数值模拟证实了这一点。

For the sake of producing high quality single crystals of oxides and semi-conductors, it is necessary to understand the details of the melt convection phenomena. Among convection mechanisms, the buoyancy flow is rather well understood, but the fundamentals of the thermocapillary flow (the Marangoni convection) and the buoyancy-Marangoni mixed flow is not well understood. The Marangoni flow is also takes a very important role in liquid flows under micro-gravity conditions.In this research, in order to get more information on the Marangoni convections, we placed experimental and numerical simulations to investigate the Marangoni and Marangoni-buoyancy mixed convections in small liquid bridges of molten sodium nitrate (NaNO) and sodium hydroxide (NaOH), whose surface tension shows a very peculiar temperature dependency. The results are summarized as followings.1) Surface tension (sigma) of molten NaOH was measured by means of the maximum bubble pressure method in the temperature range 600-823K.It shows a maximum at T^*=725 K.Between the melting temperature (about 600K) and T^*, sigma_T (=*sigma/*T) is positive, but at higher temperatures above T^*, it becomes negative.2) The Marangoni convection in liquid bridge of molten NaOH at temperatures below T^* shows surface movement from cold point to hotter point. The observed velocity distributions are well explained by the numerical simulations.3) The Marangoni convection of molten NaOH above T^* flows from hot point to colder, as is commonly observed in the normal liquids.4) The Marangoni convections at temperatures around T^* exhibit multi(3^-4) roll cell patterns.5) The effect of buoyancy on convection is negligible if the liquid bridge is placed upright, but becomes important if the liquid bridge is placed horizontally. This was confirmed by an experiment with NaNO_3 liquid bridge and also by 3-dimensional numerical simulations.