Growth of NaBi（WO4）2 Dendrite and Mechanism

The solid-liquid interface motion of NaBi（WO4）2 （NBWO） melt crystal growth is observed in an in situ system, in which the whole processes of interface transition from fiat interface and cellular to dendrite are visualized. The spacing of the dendrite under smaller temperature gradient turns out to be larger than that under larger temperature gradient, which is found to be sensitive to the temperature distribution. The mechanism of dendrite growth of NBWO is studied based on the model of the growth units of anion coordination polyhedra. The { 001} face has two apex links, so it shows higher stability and has high growth rate and forms the arm of dendrite, whereas the {010} face has only one apex link, and thus shows relative slower growth rate and firstly forms the branches.     

In Situ Observation of Skeletal Shape Transition during BaB2O4 Crystal Growth in High-Temperature Solution

The transition from a fiat solid-liquid interface to a skeletal shape during BaB2O4 （BBO） single crystal growth in Li2B4O7 flux is observed in real time by an optical high-temperature in-situ observation system. The movement of crystal step is also investigated. The observation results demonstrate that the steps propagate along and parallel to the fiat interface when the crystal size is small. Nevertheless, they will ‘bend＇ close to the face centre if the crystal size becomes greater. Atomic force microscopy reveals that more deposition places near the face centre give rise to the bending of advancing steps and thus the formation of a vicinal interface structure. Measurements of step velocity show that the velocity keeps nearly constant at different moments for one specific step, whereas the step on a newly formed layer advanced faster than that on a previously formed one when the crystal size is larger than 210μm or so. Thus interracial morphological instability occurs and a skeletal interface is obtained.     

Optical and atomic force microscopic study on step bunching in BaB2O4 crystal growth

The formation of macrostep during high-temperature phase of barium meta-borate (α-BaB 2 O 4) single crystal growth has been investigated by both optical in-situ observation system and atomic force microscopy (AFM).The insitu observation results demonstrate that the critical linear size of growing facet exceeding the size that the macrostep generates is significantly anisotropic.The critical linear sizes are around 280 μm and 620 μm for {1010} and {1010} planes,respectively.AFM study illustrates that macrostep train with a height of 150 nm～200 nm is one typical morphological feature of the as-grown crystal surface.The riser of each macrostep consists of several straight and parallel sub-steps,indicating the occurrence of step bunching.Additionally,triangular sub-steps with heights of several nanometers on the treads of the macrosteps are found to be another typical feature of surface morphology,which implies a microscopically competitive bunching of sub-steps between various crystallographic orientations.     

微重力条件下晶体生长过程的实时观察

从材料科学的微重力效应和晶体生长基本过程等两个方面出发，阐明了一种新的生长技术（晶体生长过程的光学实时观察法）的必要性，并介绍了空间高温实时观察装置的基本特性．在我国的科学技术探测卫星上，该装置进行了搭载实验，并首次清楚地观察到了空间溶质扩散效应和高温溶液的表面张力对流图像．     

轴向振动对圆柱形液体自由表面流的影响

通过光学实时观察法研究了轴向振动对恒温圆柱形容器内液体自由表面流动行为的影响。该地面实验是在一个频率为100赫兹的电磁振动台上进行的，水－甘油的混合溶液作为模型体系。结果发现，小于100mm的振动振幅能够使自由表面产生稳定的流动。这种流动主要是由沿自由表面传播的毛细波与斯托克斯边界层效应的共同作用驱动的。随着振动振幅的变化，稳态流会呈现不同的流动花样。高振幅作用下，稳态流动趋向于周期性流甚至成为湍流，这个过程可以通过临界无量纲雷诺数(Nre)c来表征，实验中得到的(Nre)c值在10-2 ～ 10-1范围内。此外，利用示踪粒子散射技术对自由表面的流动速度进行了测量，结果显示流速随粘度的增大而减小，并且随振幅的增加呈抛物线形增长。     

高温氧化物晶体生长过程中流体效应的实验研究及理论分析

流体效应是影响晶体生长传热、传质过程和晶体完整性的主要因素,了解其机理将有利于生长优质晶体.我们在空间晶体生长实时观察装置上,利用20μm的碳酸钠作示踪粒子,首次在高温氧化物熔液内同时观察到了表面张力和重力对流效应,并利用测量的结果计算了熔液内表面张力对流和重力对流的速度场.实验结果表明:近坩埚壁处,主要以表面张力对流起主导作用,而在熔液区域内,重力对流效应显著.     

锗酸铅(Pb5Ge3O11)铁电单晶的生长与缺陷研究

采用坩埚下降法成功生长了铁电锗酸铅(Pb5Ce3O11)单晶.所用Pt坩埚尺寸为φ25mm × 200mm和φ10mm×60mi,炉温控制在高于熔点50～80℃,固液界面温度梯度小于25℃/cm,生长速率小于0.5mm/h.所得晶体呈浅棕色,最大尺寸达φ25mm×60mm.采用光学显微镜(OM)及电子探针(EPMA)研究了所得晶体的生长缺陷(气泡、包裹体等),讨论了产生这些缺陷的原因,提出了控制及减少此类缺陷的方法.     

Numerical Modelling of Velocity and Temperature Distributions of the Buoyancy Convection Effect in KNbO3 Melt

Numerical Modelling of velocity and temperature fields in high-temperature KNbO3 melt of a loop-shaped Pt wire heater is carried out by using the commercial computational code ANSYS for the mathematical solution of the governing equations.Based on the experimental boundary conditions and the Boussinesq approximation,the numerical modelling of a steady and two-dimensional model is applied to study the process under consideration of the buoyancy-driven convection condition.The result is compared with the previous experimental and theoretical data obtained in our laboratory,and the former is in agreement with the latter.Thus a theoretical guide for reasonable growth conditions is provided by studying in depth the real fluid flow effects in the crystal growth from the melt.     

固液界面温度的一种测量方法

固液界面处温度的精密测量是晶体生长动力学研究的一个重要实验数据。本实验设计了一种适合于偏光显微镜用的温度调节试样台,用直接动态观察法,借助于直径25微米的铜-康铜热电偶,测量了水杨酸苯酯晶体的固液界面附近的温度分布。在△T<3℃的低过冷度的生长条件下,铜制试样台的热量输运条件稳定。固液界面处的温度增加值约为熔液过冷度的1％。这种温度调节试样台适合于生长机理的高精度定量实验研究。 关键词：