晶体生长的缺陷机制

晶体生长是一个复杂的相变过程。自80年代以来,闵乃本及其研究组系统地研究了晶体生长的缺陷机制。在理论分析和实验观察的基础上,他们发展了晶体生长的位错机制（包括刃位错和混合位错机制）,层错机制,孪晶机制、重入角机制以及重入角生长和粗糙界面生长的协同机制。根据这些机制可以得出结论：任何可以在晶体生长表面提供台阶源的缺陷都能为晶体生长作出贡献,这些台阶源包括完全台阶和不完全台阶（亚台阶）,近年来,P.Bennema及其合作者系统地研究了在照相工业中广泛应用的卤化银和金属银的晶体生长机理,在大量实验事实和理论分析的基础上,他们认为亚台阶理论（称作闵氏理论）不仅可适用于溶液生长,也适用于气相生长的机理研究;不仅适于作理论分析,而且可用于寻求最佳生产条件的指导,亚台阶理论是晶体生长的一个普适理论,文章介绍了闵乃本及其研究组提出理论及P.Bennema研究组近年来在这方面的工作进展。     

微重力下溶液法晶体生长模型中晶体生长界面稳定性的研究

研究在微重力条件下溶液法晶体生长的一个二维数学模型中晶体生长界面的稳定性问题.对晶体生长界面为不同倾斜角度的平面的情况作了数值计算,计算结果表明有可能存在一个稳定的晶体生长界面,它应该是一个在Y=λ这一端向后倾的适当形状的曲面 关键词： 微重力 溶液法晶体生长 二维模型 生长界面的稳定性     

浮区区熔生长的单晶光纤形状稳定问题

本文用晶体区熔生长理论对激光加热基座法生长的单晶光纤形状稳定问题作了研究.说明了稳定生长的熔区长度与晶纤直径、源棒直径的关系.讨论了影响晶纤直径波动的干扰因素.指出了干扰因素中激光功率的稳定和源棒直径的均匀是主要的.理论推出的结论得到实验证实.     

Ib型金刚石大单晶的限形生长

金刚石的限形生长有利于其后续加工.对于磨料级金刚石限形生长的研究已经比较透彻,但金刚石大单晶的限形生长尚缺乏全面系统的研究.本文以Fe Ni(64wt%:36wt%)合金为触媒,利用高温高压下的温度梯度法在5.6 GPa时对不同温度下分别沿(100)面和(111)面生长的Ib型金刚石大单晶的晶形进行了研究.研究表明:随着温度的升高,沿(100)晶面生长的金刚石大单晶的晶形分别为板状、塔状直至尖塔状,而沿(111)面生长的金刚石大单晶的晶形则分别为塔状和板状;分析了不同温度下分别沿(100)面和(111)面生长金刚石大单晶不同晶形高径比的变化情况.利用不同压力和温度下的金刚石大单晶合成实验绘制了沿(100)和(111)面生长金刚石大单晶的晶形在V形生长区域内的分布示意图,表明沿(111)面生长的金刚石大单晶V形区温度下限明显比以(100)面生长的高,而沿这两面生长金刚石大单晶的V形区温度上限差别并不明显.对不同生长面V形区温度上下限的差别进行了解释,据此实现了Ib型金刚石大单晶的限形生长.     

分子晶体萘的生长技术

本文描述了一种将区域融熔法与Bridgman方法相结合的单晶生长炉,用它可生长直径为1—1.3cm、长度可达15cm且适用于超声研究用的萘(Naphthalenc)单晶.该晶体生长炉具有结构简单、操作方便以及随时可以观察炉内单晶生长情况等优点.文中讨论了晶体容器的形状对单晶生长的影响,也简单介绍了汽相生长萘单晶薄片的实验.萘是较软的单晶体,所以本文提及的研磨和抛光方法对制备其它软的单晶体(同样适用于声学测量)也许具有参考价值.     

SiC晶体生长中流场的优化设计

物理气相输运法(PVT)是实验室最为常见的碳化硅(SiC)大块单晶生长方法.本文在碳化硅晶体生长模型化研究中,针对碳化硅单晶PVT生长过程中的传热传质等现象引入了对流传热中的场协同原理,利用这一原理对生长室内的流场温度场进行了优化,并对改良前后分别进行了数值模拟,研究了该原理对晶体生长的影响.实验室碳化硅单晶的生长成功...     

微重力下温差对液桥自由界面变形的影响

浮区法是一种无坩埚容器接触、生长高质量单晶的重要晶体生长技术。微重力环境下,热毛细对流成为熔体中的主要对流,因此,研究浮区法晶体生长中热毛细对流的不稳定性具有重要意义。本文利用VOF自由表面追踪模型来捕捉自由界面的运动,数值研究了微重力下动态自由界面液桥内热毛细对流特性及温差(△T)对自由界面变形的影响。结果显示,在较强的温差作用下,整个熔区出现三维不稳性的热毛细对流振荡,并且随着温差的增加,振荡强度增强,振荡波数也随之增加;自由界面呈现出冷、热壁面膨胀而中间收缩的"瓶颈"状分布。自由界面变形率随温差先增长较快,后增长较慢。     

Czochralski晶体生长中非定常热对流过程的数值模拟

本文应用有限控制体算法,采用中心差分格式处理具有Boussinesq近似的N-S方程的对流和扩散通量项,计算了无坩埚和籽晶旋转情况下CZ晶体生长中熔体的非定常自然对流。数值计算的结果得到了某些典型时刻的流场结构和温度分布情况。结合计算得到的CZ熔体总动能和总热能随时间变化的规律,分析了流场和温度场的不稳定变化过程。此外,计算结果还给出了在非稳定热对流影响下的晶-液界面热通量的变化。非定常计算的结果对于研究自然对流和CZ单晶生长,具有理论和实际意义。 关键词：     

籽晶尺寸对宝石级金刚石单晶生长的影响

本文在国产六面顶压机上,在5.6 GPa, 1250—1450℃的高压高温条件下,分别选用边长0.8, 1.5和2.2 mm三种尺寸的籽晶,系统开展了Ib型宝石级金刚石单晶的生长研究.文中系统考察了籽晶尺寸对宝石级金刚石单晶生长的影响.首先,考察了籽晶尺寸变化对宝石级金刚石单晶裂晶问题带来的影响.研究得到了籽晶尺寸变大,裂晶出现概率增加的晶体生长规律.其次,在25 h的生长时间内,考察了上述三种尺寸籽晶生长金刚石单晶时,生长时间与单晶极限生长速度的关系.得到了选用大尺寸籽晶,可以提高优质单晶合成效率、降低合成成本的研究结论.借助扫描电子显微镜和光学显微镜,对三种尺寸籽晶生长金刚石单晶的表面形貌进行了标定.最后,傅里叶微区红外测试,对三种尺寸籽晶生长宝石级金刚石单晶的N杂质含量进行了表征.研究得到了选用大尺寸籽晶实现快速生长金刚石的同时,晶体的N杂质含量会随之升高的晶体生长规律.     

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：     

Phase field simulation of the columnar dendritic growth and microsegregation in a binary alloy

This paper applies a phase field model for polycrystalline solidification in binary alloys to simulate the formation and growth of the columnar dendritic array under the isothermal and constant cooling conditions. The solidification process and microsegregation in the mushy zone are analysed in detail. It is shown that under the isothermal condition solidification will stop after the formation of the mushy zone, but dendritic coarsening will progress continuously, which results in the decrease of the total interface area. Under the constant cooling condition the mushy zone will solidify and coarsen simultaneously. For the constant cooling solidification, microsegregation predicted by a modified Brody- Flemings model is compared with the simulation results. It is found that the Fourier number which characterizes microsegregation is different for regions with different microstructures. Dendritic coarsening and the larger area of interface should account for the enhanced Fourier number in the region with well developed second dendritic arms.     

Osmium has the lowest experimentally determined compressibility

On the basis of high pressure diamond-anvil compression studies for the precious metals Ru, Ir, and Os we report the surprising discovery that metallic osmium has a lower compressibility than covalently bonded diamond. We also find that Ir and Ru are as incompressible as Re. In addition, we have performed first principles calculations that confirm the trend in the measured transition metal compressibilities.     

各向异性表面张力对定向凝固中共晶生长形态稳定性的影响

研究各向异性表面张力对定向凝固中共晶生长形态稳定性的影响.应用多重变量展开法导出了共晶界面表达式和扰动振幅的变化率满足的色散关系.结果表明,共晶生长系统有两种整体不稳定性机理:由非震荡导致的"交换稳定性"机理和由震荡导致的"整体波动不稳定性"机理.震荡有四种典型模式,即:反对称-反对称(AA-),对称-反对称(SA-)、反对称-对称(AS-)和对称-对称(SS-)模式.稳定性分析表明:共晶界面形态稳定性取决于Peclet数ε的某一个临界值ε_*,当ε大于临界值ε_*时,共晶界面形态不稳定;当ε小于临界值ε_*时,共晶界面形态稳定.随着各向异性表面张力增大,对应于AA-,SA-和SS-模式的临界值ε_(aa*),ε_(sa*)和ε_(ss*)随之减小,表明各向异性表面张力减小这三种模式的稳定性区域;然而,随着各向异性表面张力增大,对应于AS-模式的临界值ε_(as*)随之增大,表明各向异性表面张力增大AS-模式的稳定性区域.     

Numerical modelling of three-dimensional thermo- and solutocapillary-induced flows in a floating zone during crystal growth

Numerical modeling of temperature distribution, velocity and dopant concentration in crystal growth by the floating zone method in microgravity conditions is carried out taking into account thermocapillary and solutocapillary effects. The deformations of free surface and the curvature of crystallization and melting fronts are neglected. Non-stationary three-dimensional problem is solved by finite differences method. The establishing of stationary axisymmetrical regimes as well as the development of non-stationary three-dimensional regimes are observed.     

Fabrication, properties, and applications of porous metals with directional pores

Lotus-type porous metals with aligned long cylindrical pores are fabricated by unidirectional solidification from the melt with a dissolved gas such as hydrogen, nitrogen, or oxygen. The gas atoms can be dissolved into the melt via a pressurized gas atmosphere or thermal decomposition of gaseous compounds. Three types of solidification techniques have been developed: mold casting, continuous zone melting, and continuous casting techniques. The last method is superior from the viewpoint of mass production of lotus metals. The observed anisotropic behaviors of the mechanical properties, sound absorption, and thermal conductivity are inherent to the anisotropic porous structure. In particular, the remarkable anisotropy in the mechanical strength is attributed to the stress concentration around the pores aligned perpendicular to the loading direction. Heat sinks are a promising application of lotus metals due to the high cooling performance with a large heat transfer.     

Magnetic field controlled floating-zone single crystal growth of intermetallic compounds

Radio-frequency (RF) floating zone single crystal growth is an important technique for the preparation of single bulk crystals. The advantage of the floating-zone method is the crucible-free growth of single crystals of reactive materials with high melting points. The strong heat diffusion on the surface, as well as the melt convection in the molten zone due to induction heating, often leads to an undesired solid-liquid interface geometry with a concave (towards the solid phase) outer rim. These concave parts aggravate the single crystal growth over the full cross-section. A two-phase stirrer was developed at IFW Dresden in order to avoid the problems connected with these concave parts. It acts as a magnetic field pump and changes the typical double vortex structure to a single roll structure, thus pushing hot melt into the regions where the concave parts may arise. The current in the secondary coil is induced by the primary coil, and the capacitor and the resistance of the secondary circuit are adjusted to get a stable 90 degree phase-shift between the coil currents. Single crystal growth of industrial relevant RuAl and TiAl intermetallic compounds was performed based on the material parameters and using the adjusted two-phase stirrer. Very recently, the magnetic system was applied to the crystal growth of biocompatible TiNb alloys and antiferromagnetic Heusler MnSi compounds.     

我国首次空间生长GaAs单晶实验的结果分析

在空间微重力环境中,用重熔再结晶法从悬浮熔体生长了掺杂Te-GsAs单晶,晶体从中间断开表明长悬浮熔体的不稳定性,晶体中部未见杂质条纹说明浮力对流已消失,而外层有杂质条纹表明存在Marangoni对流,晶体中杂质含量减少和宏观分布不均匀,是短熔区杂质分凝机制控制和杂质Te从熔体挥发的结果,晶体中的高位错缺陷是快速生长和退火时产生的热应力以及界面籽晶侧的空位团崩塌造成的。 关键词：     

Calculation of the surface energy of fcc metals with modified embedded-atom method

The surface energies for 38 surfaces of fcc metals Cu, Ag, Au, Ni, Pd, Pt, A1, Pb, Rh and Ir have been calculated by using the modified embedded-atom method. The results show that, for Cu, Ag, Ni, A1, Pb and Ir, the average values of the surface energies are very close to the polycrystalline experimental data. For all fcc metals, as predicted, the close-packed (111) surface has the lowest surface energy. The surface energies for the other surfaces increase linearly with increasing angle between the surfaces (hkl) and (111). This can be used to estimate the relative values of the surface energy.     

Anisotropic growth of multigrain in equiaxial solidification simulated with the phase field method

The phase field method has been mainly used to simulate the growth of a single crystal in the past. But polycrystalline materials predominate in engineering. In this work, a phase field model for multigrain solidification is developed, which takes into account the random crystallographic orientations of crystallites and preserves the rotational invariance of the free energy. The morphological evolution of equiaxial multigrain solidification is predicted and the effect of composition on transformation kinetics is studied. The numerical results indicate that due to the soft impingement of grains the Avrami exponent varies with the initial melt composition and the solidification fraction.     

Four-zone solidification microstructure formed by laser melting of copper thin films

Polycrystalline copper film microstructures produced by laser melting and rapid lateral solidification are analyzed using transmission electron microscopy. The microstructure is predominantly composed of directionally solidified grains up to 22 μm long and 1 μm wide lying in the plane of the film. We identify four morphologically solidification zones, corresponding to occlusion, steady lateral growth, defective growth, and nucleation. Electron diffraction analysis indicates clustering of 1 0 0 orientations around the direction of solidification for the grains in the steady lateral growth zone. Simple estimates of solidification times based on heat flow modeling and a critical nucleation temperature suggest that interface velocities of several hundred m/s may be attained during solidification.