固液界面形貌及其转变

凝固时的固液界面通常为平面、胞状、枝晶状。在某些条件下结晶,会形成双胞结构、三胞结构、倾斜枝晶、密集分枝和退化枝晶等不规则界面。本文介绍了不规则界面的形貌特征,讨论了界面能各向异性、压力和温度对界面形貌的影响以及规则界面与不规则界面的相互转变等问题。当晶体沿着某些特定位向生长时,界面能接近于各向同性,晶体以密集分枝方式生长;当界面能各向异性时,大多数晶体以规则枝晶方式生长。在VF工艺中,低压下得到密集分枝界面;压力增加,界面以枝晶方式生长;压力进一步增加,重新得到密集分枝界面。     

凝固速度对金属基复合材料液-固界面形态选择的影响

液－固界面形态的转变决定着材料内部显微组织的变化,本文采用定向凝固垢方法,研究了凝固速度对金属基复合材料液－固界面形态选择的影响。随着凝固冷却速度的提高,Ａｌ２Ｏ３／Ａｌ－４．５Ｃｕ复合材料液０固界面形态将发生由胞状向树枝状的转变、其胞－枝转变的临界速率实验值是４９．８μｍ／ｓ。     

改进布里奇曼法生长HgCdTe晶体的生长速度对固液界面形态的影响

为选择合理的晶体生长速度,在用改进Bridgman法生长直径为φ19mm的HgCdTe(x=0.21)晶体过程中,对正在生长的单晶体及熔体进行淬火,以观察其固液界面形态.初步的实验结果表明:在2mm/d及9mm/d的两种生长速度条件下,石英安瓶中的固液界面形态均为凹形抛物面,但其凹陷深度分别为10mm和14mm.较低的晶体生长速度条件下,凹陷深度较小,固液界面形态较平.由实验和讨论得知,宜选择较低的晶体生长速度用改进Bridgman法生长HgCdTe晶体.     

泡生法大尺寸蓝宝石晶体的生长速率对固液界面的影响

采用有限元法,对泡生法生长蓝宝石晶体不同生长阶段固液界面的形状和温度梯度进行模拟计算,探讨分析了生长速率对放肩、等径阶段蓝宝石生长的影响.结果表明:固液界面凸出度在放肩阶段较大,在等径阶段凸出度相对较小,固液界面温度梯度随着晶体生长不断减小.在合理速率范围内,放肩阶段0～2 mm/h,速率对固液界面的影响很小,等径阶段2～5 mm/h,速率对固液界面的影响越来越大,固液界面温度梯度和形变均随速率的增大而减小.利用模拟结果,调节实际晶体生长工艺参数,成功长出80 kg的大尺寸高质量蓝宝石晶体.     

泡生法蓝宝石单晶不同生长阶段的数值模拟研究

针对泡生法蓝宝石单晶生长的不同生长阶段的温场、流场和固液界面形状进行数值模拟研究.并分析了加热器相对坩埚的轴向位置和不同生长速率对蓝宝石单晶生长的影响.结果表明:在蓝宝石单晶生长中,在靠近坩埚壁面和固液界面的熔体内,等温线密,温度梯度较大;在靠近坩埚底部的熔体内,等温线稀疏,温度梯度较小.随着晶体高度的增加,熔体对流由放肩阶段的两个涡胞变成等径阶段的一个涡胞,熔体平均温度有小幅度下降;加热器相对坩埚的轴向位置对晶体生长炉内温场和固液界面形状影响很大,随着加热器位置上移,晶体内平均温度升高,温度梯度减小;熔体内平均温度降低,温度梯度增大.同时固液界面凸度增大.随着晶体生长速率增大,固液界面凸度增大,界面更加凸向熔体.     

氩气流速对400 mm大直径磁场直拉单晶硅固液界面、热应力及氧含量的影响

大直径化是太阳能光伏用单晶硅发展的趋势之一.由于炉体结构的增大,炉内气体流场的变化对晶硅生长过程产生了一定的影响.本文采用CGSim晶体生长软件,系统分析了氩气进口流速对固液界面,热应力和晶体氧含量的影响.结果表明,随氩气流速的增加,固液界面高度逐渐下降,当氩气流速为中等范围时,固液界面波动最低,有利于提高拉晶过程的稳定性;另一方面,三相交界处热应力最大值随氩气流速的增加而降低,固液界面热应力波动幅度随氩气流速的增加而增加,综合两方面考虑,确定采用中等氩气流速(0.9～1.5 m·s-1)工艺可有效避免断晶等缺陷的发生.同时,在中等氩气流速范围内,晶体中心处的氧含量下降至6.55×1017 atm/cm3(氩气流速为1.5m·s-1时),与低氩气流速时相比,氧含量降低了18;.     

热交换法蓝宝石晶体生长的数值模拟研究

针对热交换法蓝宝石晶体各生长阶段的温场、流场和热应力进行数值模拟研究,并讨论了上部保温层结构、热交换器内管高度对晶体生长的影响.结果表明:长晶初期,固液界面呈椭球形;等径阶段,固液界面平坦,晶体与坩埚壁不接触;长晶后期,中心轴向晶体生长速率增加,晶体中心首先冒出熔体液面.随晶体高度增加,熔体对流由初期的两个涡胞变为等径阶段的一个涡胞,最大对流速度量级为10-3 m/s.晶体中最大热应力分布在晶体底部,热应力分布呈W型.增加炉体上部保温层,长晶后期固液界面变得平坦;降低热交换器内管高度,有利于降低晶体底部热应力.     

碲锌镉垂直布里奇曼法晶体生长过程固液界面的演化

计算模拟了半导体材料碲锌镉垂直布里奇曼法单晶体生长过程,以等温线图展示了固液界面形状的演化,分析了温度梯度和坩埚移动速率对固液界面形状以及晶体内组分偏析的影响.计算结果表明在凝固的初始段,固液界面的凹陷深度较大,随后有较大幅度的减小.整个凝固过程中固液界面的凹陷深度值有一定的波动性.提高温度梯度、降低坩埚移动速率均能有效地减小固液界面的凹陷,改善晶体的径向组分偏析.     

大直径硅单晶生长过程中固/液界面形状及熔体流动的数值分析

数值模拟技术是提升大直径硅单晶质量、降低晶体制备成本的有效工具.利用切克劳斯基法生长硅单晶时,固/液界面的形变程度是衡量晶体质量的关键参数.由于单晶炉体内的高温环境导致对界面的直接观察极为困难,因此本文采用有限元法对生长16英寸直径硅单晶过程中,不同生长阶段的固/液界面形状及熔体流动情况进行计算.数值计算结果表明:在本文所用的热场及工艺参数条件下,随着晶体长度的不断增加,固/液界面的形变量增加同时晶体内部的热应力加大;通过对晶体提拉速率及晶体转速-坩埚转速的比值的调整,我们发现,降低晶体的提拉速率以及精确的控制转速比可以使晶体各个阶段都获得比较理想的界面形状.     

声光晶体TeO2的生长及缺陷研究

本文研究了直接ＴeO２晶体中的主要晶体缺陷形成机理,讨论分析了Ｔ eO2单晶生长的工艺参数对晶体缺陷的影响,结果表明：晶体裂缝的主要与温度梯度有关,温度梯度大于２０－２５℃/cm及出现界面翻转时,易造成晶全的开裂,位错密度增加,晶体中的包裹体主要为气态包裹全,它的形成主要与籽晶的转速和晶体的提拉速率有关,转速１５－１８r/min,拉速０.55mm/h,固液界面微凹,可以减少晶体中的气态包裹体,晶体台阶由晶体生长过程中温度和生长速度的引起伏引起,当台阶间距较宽时,易形成包裹体。     

High-velocity banding structure in the laser-resolidified hypoperitectic Ti47Al53 alloy

Stability analysis of a growing solid/liquid interface is the fundamental concept of modern solidification theory. Here, serial laser rapid solidification experiments were performed on a hypoperitectic Ti₄₇Al₅₃ alloy to explore the dendritic growth behavior near the limit of high-velocity absolute stability. SEM and TEM techniques were carried out to investigate the microstructure and identify the phase composition. By adopting an improved sampling method of TEM, the growth morphology evolution of the laser-resolidified layer was observed directly and high-velocity banding structure was firstly detected in Ti–Al peritectic alloys. The high-velocity banding structures are parallel to the solid/liquid interface (normal to the growth direction) and made of the oscillation structures grown alternatively in modes of cell and plane morphologies. In light bands with cellular growth mode, all dislocation assembles are parallel to the growth direction and forms the cell boundaries, while all dislocation distributes randomly in dark bands. The determined growth velocity range for the appearance of high-velocity banding structures is about 0.51.1 m s⁻¹ according to the rapid solidification experiments, and the origin of the banding agrees well with the prediction of the CGZK phenomenological model (Acta Metal. Mater. 40 (1992) 983).     

Directional Solidification and Melting of Eutectic GaIn

Eutectic gallium-indium is studied in a horizontal Bridgman furnace geometry. Differential temperature gradients are applied to solidify and melt the alloy while observing in-situ the interface morphology and the chemical segregation in the melt and in the solid as well. Upon cooling, a wedge-type indium-rich mushy zone develops at the cold wall. The melt is initially stirred by convective flow. After solidification starts the roll cell recedes to be replaced by a chemically layered conductive melt that eventually solidifies with rather uniform eutectic structure. Upon re-melting, the morphology of the interface adopts a profile that is predetermined by the original solid structure. Those patterns, as well as the flow, are different from single element solid melting experiments and have yet to be modeled. Under high thermal gradient the convective flow mixes the binary melt and the visualized density pattern eventually becomes that of a homogeneous melt.     

Development of dendrite array growth during alternately changing solidification condition

To investigate competitive growth in a dendrite array a directional solidification study was carried out on a succinonitrile–acetone alloy. In the experiment the temperature gradient G applied is alternately altered over a wide range, increased from a lower to a higher limit, and then decreased back to the lower one. It was found that source dendrites, i.e. parent dendrites, are not superior to derived dendrites in terms of growth competition. The orientation deviation between neighbouring dendrites impacts both local dendrite creation and elimination. At lower gradients, the new dendrites grow out from the ternary arms, while at the higher gradients new dendrites originate directly from the secondary arms. During increasing and decreasing G, average primary dendrite spacing λ measured traces out different paths, revealing an unclosed hysteresis loop in the λ–G-diagram.     

Bubbles engulfment and entrapment by cellular and dendritic interfaces during directional solidification

The purpose of this work was to investigate bubbles engulfment and entrapment by cellular and dendritic interfaces during directional solidification. The experiments were performed in succinonitrile-based transparent alloys (SCN-1.5 wt%ACE). While the solid–liquid interface is cellular, the solid–liquid interface is separated into two layers by the bubble. Experimental results show that a cellular–planar–cellular transition for solid–liquid interface occurs on the lower layer and the stability of the tubular bubble is determined by local microstructure on the upper layer. When the interface is dendritic, morphological instability occurs on the solid thin film attached to the bubble after the solid–liquid interface hits the bubble. We analyzed the evolution of such cells (some cells become dendrites with time) as a function of the angle between the opposite growth direction of dendritic array and the small cell growth direction. It is demonstrated that the relative position between the existing bubble and the dendritic tip influences on the local growth pattern of dendritic array.     

Simulation of the cell to plane front transition during directional solidification at high velocity

Using the alloy phase-field method with a frozen temperature approximation, interface morphology and solute segregation patterns during directional solidification are examined near the high velocity (absolute stability) condition for planar growth. The dynamics of the breakdown of initially planar interfaces into cellular structures are shown. At sufficiently high solidification speed, a planar interface is reestablished after breakdown during the initial transient. The cell spacings, depths, tip temperatures, tip radii, and concentration patterns are determined as a function of solidification velocity. The presence of solute trapping is manifest in the variation of the degree of solute partitioning across the interfacial region with interface speed.     

On the engulfment of spherical particles by a moving ice-liquid interface

Second phase particles suspended in a liquid undergoing solidification are either repelled and swept along by the advancing solid-liquid interface or, beyond a critical velocity of the solidification front, engulfed and encapsulated into the solid phase. In this study critical encapsulation velocities were determined for spherical latex particles at an advancing ice-water interface by means of a gradient freezing stage attached to a light microscope. The influence of the particle radius, the temperature gradient at the planar solidification interface, and the viscosity of the melt has been investigated. The experimental data confirm the theoretically predicted 1/R relationship between the particle radius and the critical growth velocity V_c. The effect of the temperature gradient can be described by a power law (V_c ∝ G^1/4), but its influence on V_c may be even weaker. The inverse proportionality between V_c and the dynamic viscosity of the melt may also be weaker than it would result from Stokes' law. The critical distance between particle and solidification interface was calculated to be in the range of 0.76 to 16.3 nm depending on the theoretical model applied. The resulting Hamaker constant of the system ice/polystyrene/water is -1.7 x 10^-21 J.     

Control of the phase composition and morphology of a Cu‐Sb eutectic alloy via liquid‐liquid structure transition

The current paper focuses on the solidification characteristics of a Cu‐Sb eutectic alloy in its different liquid states. Liquid alloy resistivity‐temperature patterns suggest an irreversible temperature‐induced liquid‐liquid structure transition (TI‐LLST), and a reversible TI‐LLST occurred during the heating‐cooling runs. A set of solidification experiments was conducted based on the results. The irreversible TI‐LLST caused an enhanced solidification undercooling, increased solidification rate, refined regular eutectic morphologies, and absence of a pre‐eutectic Cu₂Sb phase. The reversible TI‐LLST resulted in different phase compositions and eutectic structures. The mechanisms behind these transitions are also briefly discussed.     

Effects of surface tension anisotropy on interfacial instability in directional solidification

Effects of surface tension anisotropy on the planar interfacial stability are studied with asymptotic analysis method in both the solidification from undercooled pure melts and the unidirectional solidification of binary alloys. The asymptotic approach developed by Xu is adopted to study the interfacial stability here, which is different from that used by other investigators previously in their works. A simple linear analysis result is obtained, i.e., the surface tension anisotropy may compete to determine interfacial stability near some critical conditions in unidirectional solidification of binary alloys. The exsitstence of the surface tension anisotropy enlarges the instability region of disturbed wave number. And the threshold of instability is strongly affected by surface tension anisotropy, especially at high pulling velocity or high temperature gradient. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A new thermal assembly design for the directional solidification of transparent alloys

A modified design of the thermal assembly is presented for the directional solidification of transparent alloys that eliminates the radial temperature gradient and minimizes the curvature of the interface. An additional booster heater is designed, and the position of the heater is shown to be critical in obtaining a flat interface. A full-scale numerical calculation, carried out for succinonitrile-0.5 wt% Salol, shows that the interface concavity can be reduced gradually by placing the booster heater just above the cold end and by adjusting the temperature of the booster heater while keeping the hot and cold zone temperatures fixed. Experimental measurements of temperatures at the wall and at the center have been carried out systematically by using two calibrated thermocouples, and the observed thermal profiles have been shown to strongly support the numerical prediction. When a macroscopically flat interface is obtained, it is shown that columnar growth away from the ampoule wall can be observed and photographed. The effects of thermal gradient and the temperature setting of the booster heater on the planarity of the interface are discussed.