水平磁场下18英寸直拉硅单晶生长工艺的三维数值模拟

利用计算机模拟仿真技术对适用于生长18英寸直拉单晶硅的40英寸热场在水平磁场下的生长工艺进行三维模拟仿真计算,重点分析了不同强度水平磁场作用下熔体和晶体中的温度场分布、熔体中流场的变化及其对晶体生长固/液界面形状的影响及其变化规律,并进一步研究了不同水平磁场强度对熔体和晶体中氧杂质含量分布影响。数值模拟计算结果表明:由于外加水平磁场的引入,熔体的流动呈现完全的三维、非对称特点,从而导致熔体内的温度场、氧杂质的分布呈现明显的三维非对称特性;水平磁场强烈影响固/液生长界面下熔体的流动特性,从而显著影响熔体的固/液生长界面形状及氧杂质传输过程。     

VGF法生长6英寸GaAs单晶生长速率的优化

晶体的生长速率关系着晶体质量和生产效率,保证晶体质量的同时,实现较高的生产效率是晶体生长速率优化的主要目标.VGF技术生长晶体时,晶体的生长速率主要取决于控温点的温度及降温速率.在保持加热器控温点不变的情况下,利用数值模拟方法研究了0.9,1.8,3.6mm·h-13个速率下6英寸VGF GaAs单晶的生长,通过对比不同生长速率下温度梯度(均为界面附近晶体中的温度梯度)和固-液界面形状的变化及热应力的分布,得出以下结果:随着晶体生长速率的增加,轴向温度梯度增大的同时,沿径向增加也较快;但由于受氮化硼坩埚轴向较大热导率的影响,晶体边缘轴向温度梯度迅速减小;径向温度梯度在晶体半径70 mm处受埚壁的影响均变为负值,晶体中大量的热沿埚壁流失,导致生长边角上翘;生长速率的增加使得界面形状由凸变平转凹,"边界效应"逐渐增强,坩埚与固-液界面的夹角逐渐减小,孪晶和多晶产生的几率增加;通过对比,1.8 mm·h-1生长时晶体界面平坦、中心及边缘处热应力均较小、生长速率较大,确定为此时刻优化的生长速率.     

电流在电磁搅拌制备SiC/C19400复合材料中的影响

采用有限元软件对电磁搅拌过程进行数值模拟,研究电流对磁感应强度及洛伦兹力的影响规律。通过模拟计算结果显示,模型在通电线圈围合的范围内产生了感应磁场,且磁场强度呈现由模型中心向外缘线性上升的趋势,加载电流越大,磁场强度越大。在二维模型中距离熔体圆心30mm处取值,当电流为60A时,磁场强度为139.5mT,比电流为30A时提高了99%。此外,洛伦兹力的分布同样是从中心向熔体直径增大方向增加。表明了增加电流对电磁搅拌效果提高有积极的作用。     

VGF法生长4英寸GaAs单晶固液界面形状和热应力的数值模拟研究

采用专业晶体生长数值模拟软件CrysMas,模拟了垂直梯度凝固法(VGF)生长4英寸GaAs单晶过程中的固液界面形状,发现晶体在生长过程中固液界面形状经历了由凹到凸的变化.数值模拟结果表明熔体中和晶体中的轴向温度梯度之比小于0.4时,固液界面为凸向熔体,与理论推导结果一致.模拟了晶体生长过程中固液界面附近的晶体中的热应力值,发现固液界面为平界面时晶体中的热应力具有最小值.推导计算了VGF GaAs单晶生长过程中固液界面凹(凸)度的临界值,当固液界面凹(凸)度小于该值时,晶体中的热应力低于临界剪切应力.     

数字模拟法研究坩埚锥角对VGF法GaAs单晶生长的影响

坩埚锥角是诱发VGF法GaAs单晶出现孪晶与多晶的重要因素之一,应用数值模拟的方法对其进行了研究与探讨,研究发现,在晶体生长的放肩阶段,坩埚锥角为60°,90°,120° 3种情况下,晶体生长的固液界面均凹向熔体,随坩埚锥角α的增大,接触角θ变小,非均匀形核几率增加,易诱发孪晶和多晶.根据热量传输分析,在晶体生长的转肩阶段,随坩埚锥角α的增大,沿轴向传走的热量减小,径向传走的热量增加,增大了径向的温度梯度,造成凹的生长界面,导致三相点处过冷度的增加,也增大了孪晶及多晶产生的几率.而在等径生长部分由于远离了坩埚直径增加的区域,坩埚锥角对成晶率的影响较小.考虑到晶体生长的效率,为获得较长的等径部分,需要设计合适的坩埚锥角.选取了90°的坩埚锥角,模拟及实验均证实这一角度即能有效提高单晶成晶率,又能保证一定的晶体生长效率,并生长出直径4英寸的VGF GaAs单晶.     

6英寸砷化镓单晶VGF法生长中坩埚-埚托间隙对晶体生长过程影响

在VGF法生长6英寸GaAs单晶的实际生长过程中,坩埚与坩埚托之间难以实现完全的理想贴合.坩埚与埚托之间的空隙对晶体生长过程中的温场和固液界面形状影响较大.通过模拟计算5种不同的空隙形状时的温场与热应力分布,发现随着空隙面积的增大,固液界面在轴向上的位置逐渐下降.在锥形空隙情况下,得到一个平坦的固液界面.设计了两种不同的空隙填充方案,模拟计算的结果表明,液态Ga完全填充时,在晶体轴向上热应力的分布较为平缓,有利于生长低位错、高质量的GaAs单晶.     

VGF法GaAs单晶生长中热通量对固液界面形状的影响

采用专业的晶体生长模拟软件CGSim模拟了垂直梯度凝固法(VGF)GaAs单晶生长过程中固液界面形状及其变化;分析了生长过程中界面上不同位置的热通量及其变化,并利用能量守恒关系,分析了热通量对固液界面形状的影响,改进了前人在忽略凝固或熔化相变潜热的基础上推导出的固液界面形状和温度梯度之间的数学关系。结果表明:固液界面上各点热通量的不同导致各点生长速度的不同,从而形成偏离程度各异的固液界面形状。采用霍尔效应测量法检测了GaAs单晶中的载流子浓度分布,分析了固液界面形状对晶片电学均匀性的影响。结果表明:对于分凝系数k_01的溶质,平坦的固液界面,晶片中载流子浓度分布更为均匀;凸形界面,载流子浓度随晶片径向距离的增加而增加;凹形界面,载流子浓度随晶片径向距离的增加而降低;载流子浓度分布的不均匀性随固液界面非平坦性的增加而增加。     

水平超导磁场直拉法硅单晶熔体过热现象

基于水平超导磁场及特定热场结构，对直拉法硅单晶生长过程中的熔体过热现象进行了数值模拟和实验研究。在数值模拟中发现了熔体对流和温度变化的特殊趋势，随后在实验中发现了相应位置晶体直径突然收缩现象，依此推测拉晶过程中生长界面附近熔体温度及温度梯度的变化是引起晶体直径收缩的直接原因。此外发现改变熔体温度梯度将改变熔体过热现象发生的位置和幅度，模拟中对两种极端的功率条件进行了计算，并在实验中观察到了预期的差别。水平超导磁场对于熔体对流的抑制作用、特定的热场结构和工艺条件，都是引起熔体过热和晶体直径收缩现象发生的原因。     

300 mm 硅单晶生长过程中热弹性应力的数值分析

采用有限体积元法软件CrysVUn对直拉法生长300mm硅单晶热场和热应力分布进行了模拟，模拟考虑了热传导、辐射、气体和熔体对流、热弹性应力等物理现象。针对晶体生长过程中小形变量的塑性形变，以Cauchy第一和第二运动定律作为局部控制方程，考虑了硅单晶的各向异性，计算了〈100〉硅单晶生长过程中晶体内von Mises应力分布和变化规律，结果表明在等径生长阶段热应力上升最显著，界面上方晶体内热应力随晶体生长速率增大而升高。     

双流低频电磁连铸过程中磁场分布的数值模拟

采用数值模拟的方法研究了双流低频电磁连铸过程中磁场分布规律.建立了单、双流低频电磁连铸中包括铸锭、结晶器、线圈和屏蔽罩的二维轴对称有限元模型.利用ANSYS软件包进行划分网格及计算,模拟出了磁场在连铸结晶器区域的分布.研究了电流频率、电流强度、电流方向和线圈距离对金属熔体内磁感应强度的影响.计算结果表明:单流时,电流频率升高,磁感应强度降低;电流强度增大,磁感应强度升高.双流时,无论电流方向是同向还是反向,同一铸锭的远端磁场强度要大于其近端磁场强度,相邻线圈电流方向为反向时,铸锭内磁场强度高于相邻线圈电流方向为同向的情况;增大线圈之间的距离时,铸锭远端和近端的磁感应强度的差值降低.     

Electrochemical deoxidation of induction-stirred copper melts

Induction-stirred melts of copper contained in zirconia-base electrolyte crucibles were deoxidized by electrochemical pumping of oxygen. The rate-controlling step for the deoxidation reaction was found to be transport of oxygen in the electrolyte in the higher oxygen concentration ranges, and transport across a boundary layer in the melt at the melt/electrolyte interface for the lower concentration ranges. The mass transfer coefficient for transport of oxygen across the boundary layer at the melt/crucible interface was determined to be αco^u = 1.7 . exp (-12,900/äT) cm/s The optimization of the experimental parameters for this new deoxidation method are discussed.     

Computational modelling of heat⧹mass transfer near the LIQUID–SOLID interface during rapid solidification of binary metal alloys under laser treatment

This paper presents the results of an investigation into the problem of planar solid–liquid interface stability during rapid solidification of binary metal alloys under laser treatment. A new quantitative model is proposed. This model describes the self-organized development of stable spatially-periodic vortices in the melt near the solid–liquid interface due to concentration- (or thermal) capillary effectsfn2 together with effects due to the influence of normal concentration or temperature gradients directed from the interface towards the melt. These vortices give rise to a cellular structure at the solid–liquid interface of rapidly frozen melts.A computer code was developed to solve the set of second-order linear differential equations which describe heat and mass transfer at the liquid–solid interface. This model allows calculation of the liquid phase velocity field, the second component concentration field in the melt, as well as the temperature field in the liquid and solid phases near the solid–liquid interface at a given solidification rate.     

A pilot-scale trial of an improved galvanic deoxidation process for refining molten copper

A laboratory-scale galvanic deoxidation technology developed by earlier workers has been improved, with the aim of developing a prototype pilot-scale deoxidation unit. Each deoxidation cell consists of a one end-closed yttria-stabilized zirconia (YSZ) tube coated with a Ni-YSZ cermet anode on the inner walls. The YSZ tube is immersed, with its closed end in the metallic melt, and an oxygen-chemical-potential gradient across the tube is established by passing a reducing gas through the tube. The melt is then deoxidized by short circuiting it with the anode. Through laboratory experimentation, the nature of the anode/electrolyte interface adhesion was identified to be an important factor in obtaining enhanced deoxidation kinetics. The kinetics of oxygen removal from the melt was increased by an order of magnitude with an improved anode/electrolyte interface. A pilot-scale refining unit consisting of 53 cells with the improved anode/electrolyte interface was manufactured, and a field evaluation of the galvanic deoxidation of copper was conducted. The deoxidation-process model was modified to include multiple deoxidation cells, which were required for the pilot-scale trials, and to analyze the effect of electrolyte/electrode adhesion on deoxidation kinetics. Preliminary studies on process component lifetimes were conducted by investigating the thermal cycling, corrosion behavior of the electrolyte, and stability of the cermet anode structure. Based on the results of the field trial and the analyses of the process component lifetime, future work needed toward commercializing the technology is discussed.     

Pore nucleation in solidifying high-purity copper

High-purity copper (6 or 7 N) was melted and solidified unidirectionally in the atmosphere of a H₂-Ar gas mixture for the purpose of studying the mechanism of pore nucleation in solidifying metal. Hydrogen content in the melt was controlled by changing the partial pressure in the atmosphere. Pores were formed when the hydrogen partial pressure in the atmosphere was 0.3 atm or more. Oxides of aluminum and silicon were observed at the bottom of the pores and the pores were nucleated heterogeneously. Water vapor with a very low partial pressure existed in the furnace atmosphere, and the melt must have contained a small amount of oxygen in equilibrium with this water vapor. The solid/liquid (S/L) interface was planar and convection was eliminated. The redistribution of the solute during solidification can, therefore, be estimated. The concentration of oxygen in the liquid at the S/L interface is estimated to be much larger than its initial concentration, due to the very small equilibrium distribution coefficient of oxygen in copper, and aluminum and silicon were oxidized even though their concentrations were very low. The probability of homogeneous nucleation by α particles was very small in these experiments.     

Dynamics Study on Morphological Stabilities of Cellular Crystal Lateral Wall

The lateral wall stabilities during the growing process of cellular crystal in the melt were studied in this article. The dynamics equation of cylindrical solid-liquid interface morphological stabilities in melt was first derived, and then the expression of criterion for cylindrical solid-liquid interface morphological stabilities was defined. The effect of the shape factor, solid radius, and other relevant factors on the morphological stabilities was analyzed. Also, the critical shape factor and critical growth rate for keeping the stabilities of the interface were determined. The phenomenon during the lateral growth process of δ-phase cellular crystal in carbon steel was observed under a high-temperature confocal scanning laser microscope (HTCSLM), which was used to verify the theoretical analysis and calculated results. The results indicate that the shape factor is beneficial to improving the stabilities of the cellular crystal lateral wall. During the increase process of the cellular crystal radius, however, there is a certain value of the cellular crystal radius, which induces the shape factor to reduce stabilities of the cellular crystal lateral wall rapidly. Even if the other conditions are unchanged, the shape of the cellular crystal may also cause the cellular crystal lateral wall to lose its stabilities. There are two critical growth rates to keep the cellular crystal lateral wall growing stably under the conditions of this research. For the Fe-0.15 pct C-0.8 pct Mn alloy, these two critical growth rates are 10⁻⁴ and 10 cm/s orders of magnitude, respectively. The difference between them is more than 10⁵ times, so the slow critical growth rate conforms to the actual critical growth rate. The radius of the cellular crystal is the main influencing factor of lateral wall stabilities. The bigger the radius of the cellular crystal is, the worse the stabilities of the lateral wall are. That is also one of the reasons that the fine cellular crystal can survive during a certain period. The results of the theoretical analysis about stabilities of the cellular crystal lateral wall agree well with the lateral growth phenomena of δ phase cellular crystal in carbon steel observed by a HTCSLM. The theoretically calculated results of the radial critical growth rates are coincident with the experimental results.     

Absorption of gaseous oxygen by liquid iron

A constant volume technique was used to measure the rate of absorption of oxygen by liquid iron. The absorption process is found to proceed in two stages. When pure oxygen comes into contact with the surface of molten iron, an extremely rapid disappearance of oxygen from the gas phase accompanied by strong local super-heating of the melt at the gas/metal interface is observed. The rate of oxygen uptake during this stage is found to be dependent upon oxygen pressure and gas/metal interfacial area, but essentially independent of temperature, volume, and stirring conditions of the melt. The second phase commences after a few seconds with simultaneous cooling of the melt surface and the appearance at the gasmetal interface of a distinct third phase. The rate of absorption of oxygen during the second stage is markedly lower than the first stage. The presence of dissolved oxygen in the liquid iron has no influence on the absorption kinetics during either stage. A theoretical interpretation of the second stage is presented. A mechanism is proposed involving dissociative adsorption of oxygen molecules at the oxide/gas interface.     

高纯铝旋转偏析行为研究

采用自主研制的高纯铝的旋转偏析设备、 Element GD型辉光放电质谱仪（GDMS），结合界面稳定性理论，研究了偏析速度和旋转速度对高纯铝提纯效果的影响以及杂质元素在偏析锭上的分布特征。结果显示：晶体以平界面生长的临界速度为52 mm/h。偏析速度增加，导致杂质元素在晶体中的浓度明显增加，提纯率显著降低，偏析速度增加破坏原有稳定扩散状态，改变了杂质元素分凝能力。旋转速度增加，诱发杂质元素在晶体中的浓度明显降低，提纯率显著增加，旋转偏析有效提高熔体温度梯度，降低有效分配系数（Ke），改善晶体提纯效果。平衡分配系数（K0）<1的杂质元素含量从偏析锭心部到表面逐渐增加，K0>1的杂质元素正好相反，杂质元素浓度极差表明偏析锭底部均匀性最好。     

Behavior of carbon,oxygen, and sulfur during hydrogen blowing of liquid steel

The decarburization of liquid steel during hydrogen blowing has been studied. The decarburization is caused by the interaction of carbon with oxygen dissolved in the metal. As the melt is blown with hydrogen, the decarburization is enhanced owing to hydrogen bubbles, which increase the effect of mixing of a metal bath and substantially increase the melt-gas phase interface. As a result, the rate and completeness of decarburization increase significantly. It is experimentally shown that hydrogen blowing of a melt substantially decreases the sulfur concentration in the metal because of the interaction of hydrogen with sulfur.     

VGF法生长6英寸Ge单晶中热应力的数值模拟优化

VGF技术生长单晶时温度梯度较低,生长速率较小,目前已成为生长大直径、低位错密度晶体的主流技术之一。采用数值模拟研究了VGF法6英寸低位错Ge单晶的生长,结果表明在采用自主研发的VGF炉生长6英寸Ge单晶时,晶体生长过程中晶体与熔体中均具有较低的温度梯度(这里的温度梯度是指的界面附近的温度梯度),尤其当晶体生长进入等径生长阶段后,晶体中的轴向温度梯度在2～3 K.cm-1之间,熔体中的轴向温度梯度0.8～1.0 K.cm-1之间;晶体中的热应力除边缘外均在(2～9)×104 Pa之间,低于Ge单晶的临剪切应力,且晶体生长界面较平整;坩埚与坩埚托之间的间隙对于晶体生长中的边界效应影响显著,将8 mm间隙减小至2 mm后,埚壁外侧的径向热流增加,使得晶体边缘的最大热应力减小至0.21 MPa和Ge单晶的临剪切应力相当,实现了热场的优化。