直拉单晶硅生长过程中固液界面形状研究

本文采用晶体生长软件CGSim对直拉单晶硅生长过程中固液界面进行了研究。讨论了不同晶体等径长度下固液界面形状的特点,模拟了晶体生长界面的变化过程;与此同时对不同晶体拉速、晶转、埚转对固液界面形状的影响进行研究,根据模拟结果确定影响固液界面的最优拉速、晶转及埚转。     

真空坩埚下降法晶体生长中传热的有限差分数值分析

本文针对目前实用的三种典型的坩埚下降法晶体生长温场形式,采用有限差分数值方法,计算出生长NaCl、CaF_2和YAG等晶体时物料中的温场分布,全面地讨论了各种参数对固液界面形状、位置及轴向温度梯度的影响,指出了在各种温场情况下为得到较为理想的界面形状和轴向温度梯度分布时可供选择的生长条件。     

薄液膜蒸发传热影响因素分析

用数值方法求解了蒸发薄液膜区域的控制方程,得到了不同过热度下界面的形状和液膜内部的压力分布,计算结果显示:在蒸发薄液膜区域存在着很大的热流密度,脱离压力起到了液体输运的主导作用,表面张力系数的改变对本区域换热影响很小.毛细微槽宽度较大时(μm量级),其改变对薄液膜区域的影响也很小.选取汽化潜热较大的工质和采用降低液体黏...     

甘油水溶液中气液传质过程的研究

对二氧化碳气泡群在不同质量分数的甘油水溶液中的气液传质过程进行了研究。利用二氧化碳探针测定了不同操作条件下二氧化碳的体积传质系数,通过图像处理技术得到了气泡群的界面面积,考察了液相质量分数,气体流量对体积传质系数、界面面积和传质系数的影响。结果表明:体积传质系数均随气体流量的增大而增大,随溶液质量分数的增大而减小。气体流量通过影响界面面积对传质速率产生作用,而甘油水溶液的质量分数对传质速率的影响则归因于黏度对传质系数的影响。     

壁面滑移对两种聚合物熔体共挤出影响的数值研究

在壁面滑移的边界条件下,利用聚合物流体计算软件包POLYFLOW对两种熔体的二维等温共挤出进行了数值模拟.在两侧壁面滑移系数相同和两侧壁面滑移系数不同这两种情形下分别计算了共挤出流动的速度场、压力场、黏度场及剪切速率场,讨论了壁面滑移对共挤出流场、界面形状和挤出胀大的影响.模拟结果表明：当两侧壁面滑移情况相同时,滑移系数越大,界面偏移越大,熔体胀大率增大;当两侧壁面滑移情况不同时,滑移系数相差越大,界面偏移越大,滑移系数小的一侧熔体挤出胀大显著.     

Characteristics of vertical sharp-edged orifice discharge（Ⅲ）Effect of geometry on orifice discharge coefficient

通过实验,分别考察了相同流通截面积下,圆形、椭圆形、正方形、矩形和三角形等不同几何形状的垂直锐边典型“大孔”和“小孔”的自由出流特性。所得孔流系数曲线形态基本相同,孔流系数值略有差异,圆孔最高,三角形最低,说明孔几何形状对孔流能量损失有一定影响,但不是关键结构参数,不会从根本上改变孔流机理。其能量损失差异可根据孔口的水力半径、锐角界面张力以及非圆形孔射流的穿透现象加以解释,藉此对前期的圆孔流动机理进行了补充。此后,为了详细考察孔形状对孔流速度分布和能量损失的影响,采用计算流体力学软件Fluent 6.2对其进行了模拟,模拟流场说明孔形状对孔前流动影响区的主体范围和速度分布基本没有影响,孔前流动的机械能损失仍可采用半球形模型研究,进一步说明不同孔形状的孔流机械能损失差异是入孔以后造成的。     

坩埚下降法生长时体系参数对界面形状影响的计算机研究

本文用有限元分析法研究了坩埚下降法(Bridgman法)晶体生长时界面的位置、温度梯度、生长速率和潜热对界面形状的影响。由计算发现,界面的形状和温度梯度的突变点到界面的距离有密切的依赖关系;由坩埚的移动而引起的热传输,正如预期的那样,有使界面变得更凹的倾向。     

垂直锐边孔口的自由出流特性（Ⅲ）孔几何形状对孔流系数的影响

通过实验,分别考察了相同流通截面积下,圆形、椭圆形、正方形、矩形和三角形等不同几何形状的垂直锐边典型“大孔”和“小孔”的自由出流特性。所得孔流系数曲线形态基本相同,孔流系数值略有差异,圆孔最高,三角形最低,说明孔几何形状对孔流能量损失有一定影响,但不是关键结构参数,不会从根本上改变孔流机理。其能量损失差异可根据孔口的水力半径、锐角界面张力以及非圆形孔射流的穿透现象加以解释,藉此对前期的圆孔流动机理进行了补充。此后,为了详细考察孔形状对孔流速度分布和能量损失的影响,采用计算流体力学软件Fluent 6.2对其进行了模拟,模拟流场说明孔形状对孔前流动影响区的主体范围和速度分布基本没有影响,孔前流动的机械能损失仍可采用半球形模型研究,进一步说明不同孔形状的孔流机械能损失差异是入孔以后造成的。     

界面湍动对气液传质的影响

气液传质过程中经常伴有界面湍动。界面湍动由传质的不均匀性引起,反过来又极大地促进传质。介绍了界面湍动的产生机理和形成条件,对4种不同Ra和Ma准数的情况分别进行了分析。讨论了界面湍动强度对气液传质系数的影响关系。传质系数与Man成正比,其中n随着Marangoni对流胞类型的不同而在1/3和1之间变化。在气液系统中,液相与气相阻力比越大,由界面湍动引起的传质的增强效应越显著。     

基于LBM-CA耦合模型的定向凝固多晶硅小平面枝晶生长数值模拟

为了更好的研究高各向异性小平面枝晶生长。提出了一种综合考虑动力学各向异性和界面能各向异性的小平面枝晶生长的格子Boltzmann方法-元胞自动机(LBM-CA)耦合模型,对硅小平面枝晶的生长过程及生长形状进行模拟;验证所采用的新动力学各向异性方程的正确性;研究了界面能各向异性、动力学各向异性和过冷度对小平面枝晶生长的影响。结果表明:小平面枝晶较非小平面枝晶表现出更强的各向异性;界面能各向异性系数的增大,小平面枝晶生长的各向异性显著增大,生长速率变化、部分取向缺失;过冷度的增大,硅晶体由小平面晶粒生长变为小平面枝晶,其各向异性也显著增强,取向缺失增加,棱角更加明显,二次枝晶臂生长速率加快;动力学各向异性增大,二次枝晶臂生长速率、长度和数量均有增加。     

A STUDY ON THE INTERFACE CONTROL OF VERTICAL BRIDGMAN CRYSTAL GROWTH USING A TRANSPARENT MULTIZONE FURNACE

A transparent computer-controlled multizone furnace is used to study the interface control of the vertical Bridgman crystal growth. Sodium nitrate is chosen as a model material. The effects of heating profiles and growth speeds on the interface shape during steady-state growth are illustrated. It is observed that the growth front becomes more concave with the increasing growth speed and with decreasing axial temperature gradients near the growth front. When a suitable combination of the growth speed and the heating profile is used, a flat or slightly convex interface can be easily obtained. Numerical simulation is also conducted, and the calculated interface shape and position under various growth conditions are in good agreement with the observed ones.     

Investigation on hydrate growth at the oil–water interface: In the presence of asphaltene

Natural gas hydrates can readily form in deep-water oil production processes and pose a great threat to the oil industry. Moreover, the coexistence of hydrate and asphaltene can result in more severe challenges to subsea flow assurance. In order to study the effects of asphaltene on hydrate growth at the oil–water interface, a series of micro-experiments were conducted in a self-made reactor, where hydrates nucleated and grew on the surface of a water droplet immersed in asphaltene-containing oil. Based on the micro-observations, the shape and growth rate of the hydrate shell formed at the oil–water interface were mainly investigated and the effects of asphaltene on hydrate growth were analyzed. According to the experimental results, the shape of the water droplet and the interfacial area changed significantly after the formation of the hydrate shell when the asphaltene concentration was higher than a certain value. A mechanism related to the reduction of the interfacial tension caused by the absorption of asphaltenes on the interface was proposed for illustration. Moreover, the growth rate of the hydrate shell decreased significantly with the increasing asphaltene concentration under experimental conditions. The conclusions of this paper could provide preliminary insight how asphaltene affect hydrate growth at the oil–water interface.     

Effect of Grain Shape on Abnormal Grain Growth in Liquid-Phase-Sintered Nb1−xTixC–Co Alloys

The effect of titanium substitution for niobium on the grain shape change, grain growth inhibition, and abnormal grain growth during liquid-phase sintering of Nb_{1− x} Ti_xC–Co alloy was studied. With increased titanium substitution, the shape of the Nb_{1− x} Ti _x C grains in the liquid cobalt matrix was changed from a cube with round corners to a cube with angular corners, which implied increased edge energy. As the grain corners became more angular, the grain growth became markedly inhibited, and abnormal grain growth occurred. The results could be best explained by the increased edge energy of the interface of the Nb_{1− x} Ti _x C grains, which increased the barrier for the growth by two-dimensional nucleation.     

自然对流对于熔体生长中小面化的影响

应用基于边界保角变换技术的Ｇａｌｅｒｋｉｎ有限元方法，研究熔体生长中动力学效应和自然对流的耦合作用探究了对流对生长系统中的温场分布、相界面２以及界面过冷度的影响。研究结果表明：自然对不充可使相界面的弯曲度减小，界面的相对位置降低，同时，对流使得小面端点处的夹角变得圆滑，小面域和粗糙面域的界线变得不明显，相应地，小面尺寸有所减小。     

How the shape of catalyst nanoparticles determines their crystallographic orientation during carbon nanofiber growth

A theoretical model is presented that explains spontaneous changes in the crystalline orientation of nanoparticles. The spontaneous changes in crystalline orientation are attributed to the crystal anisotropy of the surface energy of nanocrystalline particles. We consider an important specific case of the chemical vapor deposition growth of carbon nanofibers, where previous studies have shown that both the catalyst nanoparticle shape and the nanofiber growth rate change with changes in the chemical potential of diluted carbon. Energetic considerations of the nanoparticle’s free surface and its interfacial energy with the nanofiber during these shape changes are shown to force a reorientation of the nanoparticle crystallographic axes at a critical growth rate. The model therefore reveals the mechanism by which the shape and crystallographic orientation of the catalyst nanoparticle are linked to the nanofiber growth rate. The model suggests a new way, based upon measurable geometry of nanoparticles during in situ growth experiments, to estimate the role of chemisorption in the attraction of the graphene film to the curved catalyst surface and the anisotropy energy of this interface.     

Bubble growth mechanism in carbon foams

The present work is a numerical study to predict the growth mechanism of a non-spherical bubble assisted for a carbon foam fabrication process. An approach for two dimensional non-spherical mass-diffusion controlled bubble growth in an isothermal Newtonian liquid of infinite extent is considered. Using the two dimensional unsteady form of the equations governing the conservation of mass and momentum, bubble growth is solved as a function of time using a fixed-grid sharp interface finite volume method. A comparative study is performed by considering previous cases of study and shows good agreement, which reflects the validity of the present model. A parametric study highlighting the effects of the non-spherical growth of the bubble is performed in order to emphasize how controlled bubble growth can be achieved. In each case a change in a particular parameter resulted in a distinct change of the bubble shape.     

Analysis of moving boundary problem of growth of Bismuth germanate crystal by heat exchanger method

Transient two-dimensional model of the growth of BGO crystal by heat exchanger method has been developed. A finite element method with nonorthogonal mapping technique for the solution of the moving boundary problem is developed where the melt/solid interface shape changes from hemispherical to planar. The moving boundary problems for the melt/solid interface location and the temperature field were solved by two mapping rule method which enables the computation of interface shape changing from hemispherical to planar. The maximum deflection of interface is shown when the melt/solid interface meets the corner of crucible. As the excess heating temperature and the heat exchanger temperature were increased, more growth time for whole process is required but the quality of BGO crystal may be improved. The ratio of the height to the radius of crucible hardly affects the deflection of BGO melt/solid interface when it is greater than 1.5. As the cooling zone radius is decreased, maximum deflection is decreased. The heat transfer between the crucible and the heating element should be suppressed to maximize planarity of the interface shape.     

Ammonia absorption from a bubble expanding at a submerged orifice into water

To investigate the mechanism of gas absorption from a bubble containing soluble and insoluble components, a gaseous mixture of ammonia and nitrogen was bubbled into water. The growth curve, volume, surface area and shape of the growing bubbles were measured with parameters such as inlet gas composition, gas flow rate and gas chamber volume. The bubble volume decreased with the increasing composition of ammonia in a bubble, decreasing gas chamber volume and decreasing gas flow rate.To reasonably express the mass transfer from the bulk of a gas in a bubble to the bulk of a liquid, the overall mass transfer resistance was evaluated by the mass transfers in the gas phase, interface and liquid phase.The non-spherical bubble formation model combined with the overall mass transfer resistance estimated well experimental bubble shape, bubble volume at its detachment from an orifice, growth rate and mass transfer rate.Moreover, the change of concentration with bubble growth time and the fractional absorption during bubble formation were simulated.     

Investigation of Nucleation Mechanism and Tapering Observed in ZnO Nanowire Growth by Carbothermal Reduction Technique

ZnO nanowire nucleation mechanism and initial stages of nanowire growth using the carbothermal reduction technique are studied confirming the involvement of the catalyst at the tip in the growth process. Role of the Au catalyst is further confirmed when the tapering observed in the nanowires can be explained by the change in the shape of the catalyst causing a variation of the contact area at the liquid–solid interface of the nanowires. The rate of decrease in nanowire diameter with length on the average is found to be 0.36 nm/s and this rate is larger near the base. Variation in the ZnO nanowire diameter with length is further explained on the basis of the rate at which Zn atoms are supplied as well as the droplet stability at the high flow rates and temperature. Further, saw-tooth faceting is noticed in tapered nanowires, and the formation is analyzed crystallographically.     

Effect of surface pressurization on the growth of α-Fe₂O₃ nanostructures

By suitably pressurizing iron substrates under different conditions, the resulting α-Fe(2)O(3) nanostructures, formed by its direct thermal oxidation, can gradually change in succession from nanowires to nanoleaves and to micropillars as the pressure is increased. The inter-relation between the pressure conditions and the resulting nanostructure is studied by density functional calculations using ultrasoft pseudopotentials with a plane-wave basis method and with the generalized gradient approximation (GGA). It is shown that the shape of the formed nanostructures is primarily determined by the anisotropic activation energy and, as the latter is lowered, there is a shape change from wire to pillar. A simulation model of diffusion using the Monte Carlo method is applied in the 3-D (dimensional) case to show how the anisotropic activation energy influences the growth process of the α-Fe(2)O(3) nanostructure. The present study provides a way to control the shape of the nanostructures grown by the thermal-oxidation method.