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，浮力流动是相当清楚的，但热毛细管流(Marangoni对流)和浮力-Marangoni混合流动的基本原理还不是很清楚。Marangoni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Marangoni对流的信息，我们用实验和数值模拟的方法研究了熔融的硝酸钠和氢氧化钠的小液桥中的Marangoni和Marangoni浮力混合对流，其表面张力表现出非常特殊的温度依赖性。结果表明：1)在600-823K温度范围内，用最大气泡压力法测定了NaOH熔体的表面张力(Sigma)，在T^*=725K时有一极大值，熔化温度(约600K)到T^*之间，Sigma_T(=*Sigma/*T)为正值，高于T^*时为负值；数值模拟很好地解释了观测到的速度分布。3)熔融NaOH在T^*以上的Marangoni对流是从热到冷，这是在正常液体中常见的现象。4)在T^*附近温度下的Marangoni对流呈现出多(3^-4)个滚动室模式。5)如果液桥垂直放置，浮力对对流的影响可以忽略不计，但如果液桥水平放置，浮力对对流的影响变得重要。用NaNO_3液桥的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.