过冷熔体中枝晶生长的相场法数值模拟

利用相场法模拟了过冷纯金属熔体中的枝晶生长过程,研究了各向导性、界面动力学、热扩散和界面能对枝晶生长的影响.结果表明,热噪声可以促发侧向分支的形成,但不影响枝晶尖端的稳态行为;随着各向异性的增加,枝晶尖端生长速度增加,尖端半径减小;当界面动力学系数减小及在界面动力学系数小于1的条件下热扩散系数减小时,枝晶尖端生长速度随之减小,而尖端半径相应增大;界面能趋于增大枝晶尺度并保持界面在扰动下的稳定,界面能越大,形成侧向分支的趋势越小 关键词： 过冷 枝晶生长 相场法 数值模拟     

相场模型模拟液固界面各向异性作用下自由枝晶生长

采用自适应有限元方法求解相场模型,分别对界面能各向异性和界面动力学各向异性条件下自由枝晶生长过程进行了模拟.计算表明两种各向异性均显著影响枝晶的生长,随着各向异性的增大枝晶尖端生长速度增大,尖端半径降低. 两种各向异性对自由枝晶生长有着不同形式的影响,在界面能各向异性条件下,枝晶生长稳定性系数与各向异性系数成幂函数关系;而在动力学各向异性条件下,稳定性系数与各向异性系数成线性关系. 关键词： 界面能各向异性 动力学各向异性 自由枝晶生长 相场模型     

微重力条件下Ni-Cu合金的快速枝晶生长研究

采用落管方法实现了Ni-50%Cu过冷熔体在微重力和无容器条件下的快速枝晶生长.对微重力条件下的晶体形核和快速生长进行了研究，发现随着过冷度的增大，晶体生长形态由粗大枝晶向规划均匀的等轴晶转变.实验中最大冷却速率达到8×103K/s，获得了218K(014TL)的最大过冷度.理论分析表明，过冷熔体中优先发生异质形核，形核率可达1012m-3s-1以上；Ni-50%Cu过冷熔体中的枝晶生长随过冷度的增大发生由溶质扩散控制向热扩散控制的生长动力学机理转变.在68K过冷度条件下，生长界面前沿的偏析程序最大. 关键词： 落管 微重力 深过冷 枝晶 熔体     

远场来流条件下过冷熔体球晶生长的稳定性

建立了远场来流条件下过冷熔体球晶生长的温度场和浓度场稳态模型,分析了对流对球晶周围温度场和浓度场的影响,并以Trivedi的纯扩散球晶稳定性判据为基础,推导出远场来流条件下过冷熔体球晶生长的临界稳定性判据. 研究表明:远场来流条件下,迎流面的扰动振幅增加速率明显大于背流面的扰动振幅增加速率. 振幅增加速率最大值对应的扰动阶次从迎流面到背流面逐渐减小,随着球晶半径增加而增大. 对流使迎流面的稳定性降低,背流面的稳定性增大. 随着流速的增加,球晶的临界稳定半径减小. 关键词： 球晶 远场来流 界面形态稳定性 Trivedi判据     

简单金属固液界面固化过程生长机制的分子动力学研究

采用分子动力学方法,研究两种简单金属Ni、Al固液界面的动力学过程.结果表明：两种金属表现出相同的特征,即界面温度存在某个特征值（T^*）,生长速度在这个特征温度附近达到最大值.高于这个温度时,随着过冷温度（熔点温度与界面温度差）的增加生长速度单调增加,低于这个温度时,Ni的生长速度几乎不变,而Al的生长速度随过冷温度的增加而快速减小到零.在此基础上,基于高温BGJ碰撞约束模型和低温W-F扩散模型分析界面的生长机制,发现在小过冷温区和深过冷温区存在碰撞机制和扩散机制的渐变过程,不同温区二者所起的主导作用不同,生长机制的转变是T^*存在的原因.     

弱熔体对流对定向凝固中棒状共晶生长的影响

利用渐近方法求出在弱对流熔体中定向凝固棒状共晶生长的浓度场的渐近解,研究了弱熔体对流对定向凝固中棒状共晶生长的影响.结果表明,弱熔体对流对定向凝固中棒状共晶生长有显著的作用;平均界面过冷度不仅与棒状共晶的棒间距、生长速度有关,还与流动强度有关;当生长速度一定时,随着流动强度增大,棒状共晶的平均界面过冷度减小.利用最小过冷原则,获得棒间距与生长速度和流动强度的关系.结果表明,当生长速度比较小时,随着流动强度增大,棒状共晶的棒间距增大;当生长速度比较大时,随着流动强度增大,棒状共晶的棒间距变化减弱;棒状共晶的生长速度越小,流动对棒状共晶生长的影响越大.利用本文的解析结果计算在对流条件下Al-Cu共晶的棒间距,结果显示随着转速增大或径向距离增大,共晶的间距增大,这与Junze等的实验结果相符合.     

三维多相场数值模拟共晶CBr4-C2Cl6合金在不同抽拉速度下的形态选择

利用KKSO多相场模型对定向凝固共晶CBr4-C2Cl6合金的三维恒速及变速生长过程进行了研究,再现了不同抽拉速度下共晶形态演化及选择过程,建立了形态选择图,研究了变速过程的界面平均生长速度及界面平均过冷度的变化.结果表明,变速前后的形态选择与恒速下的形态选择一致;变速过程的形态演变、界面平均生长速度和界面平均过冷度的变化均产生滞后效应;界面平均生长速度和界面平均过冷度之间的关系与理论结果符合较好.     

Straining流对柱状晶体在三元过冷熔体中生长的影响

研究了三元过冷熔体中柱状晶体在非等温条件下受straining流作用的生长问题,给出了柱状晶体生长形态的近似解析表达式.发现流入的straining流加快了界面的生长速度,而流出的straining流减缓了界面的生长速度,即straining流使得柱状晶体的界面发生变形.同时发现,随着流动速度的增大,界面变形也更为显著.通过比较straining流对纯熔体、二元熔体、三元熔体中柱状晶体界面的影响,发现相比于纯熔体,柱状晶体在稀合金熔体中的界面形态受straining流的影响更大.     

三维多相场数值模拟共晶CBr4-C2Cl6合金在不同抽拉速度下的形态选择

利用KKSO多相场模型对定向凝固共晶CBr₄-C₂Cl₆合金的三维恒速及变速生长过程进行了研究,再现了不同抽拉速度下共晶形态演化及选择过程,建立了形态选择图,研究了变速过程的界面平均生长速度及界面平均过冷度的变化.结果表明,变速前后的形态选择与恒速下的形态选择一致;变速过程的形态演变、界面平均生长速度和界面平均过冷度的变化均产生滞后效应;界面平均生长速度和界面平均过冷度之间的关系与理论结果符合较好. 关键词： 多相场模型 共晶生长 抽拉速度     

远场来流对过冷熔体中球状晶体生长的影响

利用渐近分析方法研究了远场来流引起的对流对过冷熔体中球状晶体的生长形态的影响.结果表明,由远场来流导致的对流使得正在生长的球状晶体的界面在向着来流的前部朝来流方向相反的方向生长, 并且提高了朝来流的相反方向的生长速度, 在背风方向衰减；正在衰减的球状晶体的界面在向着来流的前部加速衰减, 在背风方向减缓衰减. 关键词： 球状晶体 远场来流 对流 界画形态     

Linear stability analysis on a spherical particle growing from a binary melt under the far-field flow

The solutions of temperature and solute fields around a spherical crystal growing from a binary melt under the far-field flow are obtained.Based on the results,a linear stability analysis on the spherical interface growing from the binary melt under the far-field flow is performed.It is found that the constitutional supercooling effect ahead of the spherical crystal interface under the far-field flow is enhanced compared with that without the flow.The growth rate of the perturbation amplitude at the up-wind side of the spherical crystal interface is larger than that at the down-wind side.The critical stability radius of the crystal interface decreases with the increasing far-field flow velocity.Under the far-field flow,the whole spherical interface becomes more unstable compared with that without the flow.     

About the kinetics of normal crystal growth

Usually, the kinetic coefficient of crystal growth, i.e. the coefficient of proportionality of the crystal growth velocity to the undercooled temperature, is supposed to be a constant parameter. In the present study we show that this assumption is valid only for low undercooling temperatures, and in general it is a function of temperature. The kinetic coefficient was found to be a linear function of the temperature of the crystal/melt interface, which becomes zero for the glass transition temperature. PACS 81.10.AJ     

The effect of two-dimensional shear flow on the stability of a crystal interface in the supercooled melt

A model is developed based on the time-related thermal diffusion equations to investigate the effects of two-dimensional shear flow on the stability of a crystal interface in the supercooled melt of pure substance. Similar to the three-dimensional shear flow as described in our previous paper, the two-dimensional shear flow can also be found to reduce the growth rate of perturbation amplitude. However, compared with the case of Laplace equation for steady state thermal diffusion field, due to the existence of time partial derivatives of the temperature fields in diffusion equation the absolute value of the gradients of the temperature fields increases, therefore destabilizing the interface. The circular interface is more unstable than in the case of Laplace equation without time partial derivatives. The critical stability radius of the crystal interface increases with shearing rate increasing. The stability effect of shear flow decreases remarkably with the increase of melt undercooling.     

The effect of two-dimensional shear flow on the stability of a crystal interface in a supercooled melt

A model is developed based on the time-related thermal diffusion equations to investigate the effects of twodimensional shear flow on the stability of a crystal interface in the supercooled melt of a pure substance.Similar to the three-dimensional shear flow as described in our previous paper,the two-dimensional shear flow can also be found to reduce the growth rate of perturbation amplitude.However,compared with the case of the Laplace equation for a steady-state thermal diffusion field,due to the existence of time partial derivatives of the temperature fields in the diffusion equation the absolute value of the gradients of the temperature fields increases,therefore destabilizing the interface.The circular interface is more unstable than in the case of Laplace equation without time partial derivatives.The critical stability radius of the crystal interface increases with shearing rate increasing.The stability effect of shear flow decreases remarkably with the increase of melt undercooling.     

直拉硅单晶生长过程中工艺参数对相变界面形态的影响

为改善晶体相变界面形态,提高晶体品质,提出了一种融合浸入边界法（immersed boundary method,IBM）和格子Boltzmann法（lattice Boltzmann method,LBM）的二维轴对称浸入边界热格子Boltzmann模型来研究直拉法硅单晶生长中的相变问题.将相变界面视为浸没边界,用拉格朗日节点显式追踪相变界面;用LBM求解熔体中的流场和温度分布;用有限差分法求解晶体中的温度分布.实现了基于IB-LBM的动边界晶体生长过程研究.得到了不同晶体生长工艺参数作用下的相变界面,并用相变界面位置偏差绝对值的均值和偏差的标准差来衡量界面的平坦度,得到平坦相变界面对应工艺参数的调整方法.研究表明,相变过程与晶体提拉速度、晶体旋转参数和坩埚旋转参数的相互作用有关,合理地配置晶体旋转参数和坩埚旋转参数的比值,能够得到平坦的相变界面.     

定向结晶条件下聚乙二醇6000的强动力学效应

在单向温度场条件下, 采用不同抽拉速度实现了聚乙二醇6000的定向生长、界面形貌的实时观测及界面温度的测量, 进而揭示了其生长机制. 实验结果表明, 随着抽拉速度的增大, 界面的温度逐渐减小, 过冷度逐渐增大. 运用高聚物结晶的次级形核理论模型, 对实验数据进行了计算, 得到在界面过冷度为13.5 K左右时, 生长机制发生了由区域Ⅱ向区域Ⅲ的转变. 实验数据与等温结晶数据的比较发现等温结晶方法中获得过冷度相对较大, 是因为其包含了热过冷. 聚乙二醇6000定向结晶过程中需要的最大动力学过冷度为20 K, 说明由于高聚物的二维形核, 其生长主要由界面动力学控制, 具有较强的动力学效应.     

Liquid/solid interface profile of melt grown oxide crystals II. Crystal quality

Temperature gradients and rotation rates influence the quality of Czochralski grown oxide crystals. Decreasing the heat transfer from the melt level and increasing the rotation rate increases the optical homogeneity and structural perfection of the crystal, due to convex interface becoming flattened. Growth striations due to temperature fluctuations in the melt during growth are affected by the heat losses from the melt level but are practically unaffected by the rotation rate as long as a narrow ring of centrifugally streaming melt in the neighbourhood of the growing crystal is not formed. The parts of the crystals grown in the presence of this ring contain no striations.     

The kinetics of the anomalous state formation and the growth of helium crystals at high supersaturation

The kinetics of the formation of an anomalous state of a helium crystal with a fast-growing surface are investigated. It is demonstrated experimentally that it is external supersaturation that is the determining factor of formation of an anomalous state. The dependence of the time of formation of anomalous state on temperature and initial supersaturation is measured. The problem of crystal growth with the excitation of the first-sound wave in the container is solved. This solution is used to determine the dependence of the kinetic coefficient of growth of anomalous facets on temperature and initial supersaturation. It is demonstrated that the kinetic coefficient of facet growth decreases on approaching the boundaries of the region of the existence of an anomalous state. The kinetic coefficient of growth of atomically rough surfaces in an anomalous state is determined by the damping of pressure oscillation. It is found that the value of the latter coefficient is three-four times that of the respective value for the facets but is considerably, by an order of magnitude, less than the value of the coefficient of growth of such surfaces in the normal state. Phenomena are treated which accompany the spiral growth of facets, namely, the excitation of oscillations of a screw dislocation during spiral rotation and the emergence of vortex rings in superfluid helium. The effect of these phenomena on the kinetics of facet growth and on the formation of an anomalous state is discussed.