过饱和温度和降温速率对BBO晶体浓度边界层厚度的影响

浓度边界层是晶体生长过程中分析界面输运现象的重要元素.我们从实验角度研究了扩散机制下的BaB2O4单晶生长中过饱和温度和降温速率对浓度边界层厚度的影响.过饱和温度和浓度边界层厚度之间为抛物线关系,降温速率和浓度边界层厚度变化率之间为线性关系.     

Thermalcapillary Convection in NaBi（WO4）2 Melt

Surface-tension drive flow in NaBi(WO4)2 melt is visualized by means of the already proved method of high temperature in situ optical observation.The transition phenomena of the melt from steady spatial to oscillatory behaviour in melt are observed.The oscillatory thermocapillary convection is accompanied by synchronous temperature oscillations with the frequency 10Hz.The onset temperature of thermocapillary convection varies with the change of meniscus curvature of the melt free surface,which probably due to the liquid pressure at the surface associated with the curvature of meniscus.The oscillatory frequency is proportional to the temperature.Finally another kind of oscillatory thermocapillary convection with the frequency about 5Hz dominating in the undercooled melt is also observed.     

In Situ Observation of Cell-to-Dendrite Transition

The cell-to-dendrite transition of succinonitrile melt suspended on a loop-shaped Pt heater is observed in real time by a differential interference microscope coupled with Schlieren technique. The transition is divided into two parts： a dendrite coalition process and a subsequent dendrite elimination process. Firstly the dendrites from the same cell are united into a single dendrite. Secondly the competitive growth of dendrites from different cells leads to the elimination of dendrites. The two processes can be understood when involving crystallographic orientation. In addition, the tip velocity and primary spacing of a cell/dendrite are also measured. It turns out that the primary spacing has a significant jump, whereas the growth velocity has no abrupt change during the cell-to-dendrite transition.     

Solute distribution in KNbO3 melt-solution and its effect on dendrite growth during rapid solidification

This paper reports that the rapid solidification of mixed Li 2 B 4 O 7 and KNbO 3 melted in a Pt loop heater has been performed experimentally by the method of quenching, and various morphologies of KNbO 3 crystals have been observed in different regions of the quenched melt-solution. Dendrites were formed in the central region where mass transfer is performed by diffusion, whereas polygonal crystals with smooth surface grew in the marginal region where convection dominates mass transport. Based on measurement of KNbO 3 concentration along crystal interface by electronic probe analysis, it finds the variety of crystal morphologies, which is the result of different solute distributions: in the central region the inhomogeneity of solute concentration is much sharper and morphological instability is easier to take place; nevertheless in the marginal region the concentration homogeneity has been greatly enhanced by convection which prevents the occurrence of morphological instability. Additional solute distribution in the melt along the primary dendrite trunk axis as well as that in mushy zones has also been determined. Results show that the solute concentration in the liquid increases linearly with distance from the trunk tip and more solutes were found to be concentrated in mushy zones. The closer the mushy zone is to trunk tip, the lower the solute concentration will be there.     

空间高温氧化物晶体生长过程中流体效应的数字模拟研究

数字模拟方法可以模拟实际晶体生长过程,了解晶体生长参数的变化对晶体生长的影响.本文利用数字模拟的方法,在本实验室建立的空间二维实时观察装置中,对高温Li2B4O7熔液中生长KNbO3晶体的流体效应及温场进行了数字模拟研究,结果表明,在地面生长,熔液内部存在复杂的双涡流动模式,重力对熔液中的速度场和温场的分布产生强烈的作用,而在空间,当重力水平达到一定程度时,可以使熔液中的流动模式简单化,从而降低流动效应对传热、传质造成的不稳定性和不均匀性,有利于提高晶体生长的质量.同时通过对空间搭载实验与地面实验结果进行对比分析,与数字模拟分析的结果一致,随着重力水平的下降,熔液中的速度场,温场分布朝着稳态生长的方向发展,表面张力对流是形成均匀胞状晶体结构的主要因素.     

熔体温度对负离子配位多面体生长基元聚集体结构的影响

据Na2SiO3与BaB2O4晶体和熔体的高温拉曼光谱测试结果综合分析后提出:熔体的温度对负离子配位多面体生长基元相互联结的聚集体结构有影响.在高温熔体中,高电价、离子半径小的阳离子配位多面体往往孤立存在.当熔体过冷度大时,孤立的负离子配位多面体顶角上的氧与低电价、离子半径大的阳离子配位构成大维度的聚集体.同质异构晶体的熔体拉曼谱都显示出相同的孤立负离子配位多面体振动峰.     

熔体生长水杨酸苯脂晶体的形貌

在实验上用实时观察法和Kyropoulos技术生长水杨酸苯脂单晶,研究晶体形貌随过冷度的变化关系。在理论上采用Hartman-Perdok的PBC方法,分析水杨酸苯脂单晶形貌和晶体结构的关系。应用木原的核心势能模型,扩展了PBC理论。解析的晶体形貌与实时观察法得到的晶体形貌相当一致。 关键词：