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

马兰戈尼对流和单晶生长：表面张力温度系数符号的影响

基本信息

批准号：
06452341
负责人：
IMAISHI Nobuyuki
金额：
$ 4.16万
依托单位：
KYUSHU UNIVERSITY
依托单位国家：
日本
项目类别：
Grant-in-Aid for General Scientific Research (B)
财政年份：
1994
资助国家：
日本
起止时间：
1994 至 1995
项目状态：
已结题

项目摘要

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.

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，浮力流是相当好理解的，但热毛细流（马兰戈尼对流）和浮力-马兰戈尼混合流的基本原理还没有得到很好的理解。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，为了获得更多关于Marangoni对流的信息，在本研究中，我们采用实验和数值模拟的方法，研究了表面张力具有非常特殊的温度依赖性的熔融硝酸钠（NaNO）和氢氧化钠（NaOH）的小液桥中的Marangoni和Marangoni浮力混合对流。结果表明：（1）用最大气泡压力法测定了600- 823 K范围内NaOH熔融液的表面张力σ，在T^*=725 K时有一个极大值，在熔融温度T^*=725 K时，表面张力σ与熔融温度T^*=725 K时有一个极大值（约600 K）和T^*，sigma_T（=*sigma/*T）为正，但在T^* 以上的较高温度下，2）在低于T^* 的温度下，NaOH液桥中的Marangoni对流表现为从冷点到热点的表面运动。数值模拟结果很好地解释了所观察到的速度分布; 3）温度高于T^* 时，熔融NaOH的Marangoni对流是从热点向冷点流动的，这与在普通液体中观察到的现象相似; 4）温度接近T^* 时，Marangoni对流呈现出多（3^-4）个滚胞模式; 5）如果液桥直立放置，浮力对对流的影响可以忽略不计，但是如果液桥水平放置则变得重要。这一点通过NaNO_3液桥实验和三维数值模拟得到了证实。