共查询到19条相似文献,搜索用时 968 毫秒
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《中国材料进展》2017,(12)
在合金凝固过程中施加时变电磁场,导电熔体在洛仑兹力的驱动下产生的强迫对流可有效控制合金溶质的再分配过程。在材料凝固与晶体生长过程中施加电磁场有助于深入研究其热物理性质、相平衡、亚稳态、组织形成、成分过冷和形核。人们对重力环境下合金的凝固过程已进行了大量深入详尽的实验探究,利用电磁场手段对凝固前沿进行有效干预并在晶体组织结构、成分偏析等方面取得了一定的进展。而微重力环境下浮力对流减少,为在宏观和微观尺度上获得成分更为均匀的半导体或合金材料提供了一个独特的平台,目前的研究表明在微重力环境下合金的凝固特征与在重力环境下的有所不同。从理论与实验的角度阐释了在微重力及重力环境下电磁场引起的强迫对流强度及方向对晶体生长过程中的宏观偏析、微观偏析、晶体形态以及金属间化合物的生长模式及空间分布的影响规律。 相似文献
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一种新型的超导磁场单晶炉的研制 总被引:1,自引:0,他引:1
磁场对于半导体晶体生长过程中的熔体流动模式有着明显的影响,因而可以改善晶体的组分和杂质的分布。在半导体单昌生长过程当中利用超导磁场来抑制熔体的对流,采用这种方式既可以生长出优质的单昌又可以研究熔体的对流与扩散对生长半导体单晶的质量影响,本文报道了新近自行设计,制造的一种新型超导磁场直拉单晶炉。 相似文献
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空间微重力环境下几乎无对流和沉降,可为晶体生长提供一个相对稳定和均一的理想环境,易于得到尺寸较大的高质量单晶。但是,空间结晶实验成功率低,费用昂贵,实验机会受限。因此,研发各种空间微重力环境地基模拟技术具有重要意义。目前可用于晶体生长的地基无容器悬浮技术主要有空气动力悬浮、静电悬浮、电磁悬浮、液体界面悬浮、超声悬浮和磁场悬浮技术等。这些地基模拟技术可实现晶体的无容器悬浮生长,避免器壁对晶体生长的不良影响,提高晶体质量,为解决X射线单晶衍射技术中的瓶颈问题提供新途径,还可为在地基进行结晶动力学和机理研究提供简单易行的方法。从技术原理、优势、缺陷及在结晶(特别是蛋白质结晶)中的应用4个方面对这些技术逐一进行了介绍和评述。重点介绍了液体界面悬浮、超声悬浮和磁场悬浮技术这3种用于蛋白质晶体生长的较为成熟的地基无容器悬浮技术。 相似文献
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直径300mm硅片的生产技术是当今硅材料生产研究的重要方向之一,而晶体生长界面的形状、温度分布、晶体中氧的浓度和均匀性等对熔体流动状态十分敏感,采用实验的方法来测量熔体的流动、温度场分布是很困难的,因此很难通过实验的方法获得熔体的流动是如何影响晶体生长的质量的,而数值模拟能提供熔体流动、温度分布等详细内容,为单晶硅的生长提供有利的指导.本文采用低雷诺数的K—ε紊流模型,对直径300mm的大直径单晶硅生长进行了数值模拟,通过熔体在有、无勾形磁场作用时的流场、温度场的分析,阐明了勾形磁场影响熔体流动的机理. 相似文献
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目前在KDP/KD*P晶体的实际生长过程中,仍以传统降温法为主.在传统降温速度的基础上适当提高降温速度,可以加快KDP/KD*P晶体的生长速度,但与此同时有可能产生柱面扩展.为此,我们对不同生长环境下的KDP/KD*P晶体生长过程中柱面扩展进行了一系列研究.实验中所用KDP原料和去离子水均与生长大口径KDP晶体相同,其它各项条件也尽量模拟大口径KDP晶体生长过程中的实际情况.在晶体生长实验过程中通过研究不同条件下KDP/KD*P晶体的柱面扩展情况来研究柱面扩展对KDP/KD*P晶体光学质量影响的共同特点.通过分析和研究实验数据及晶体生长过程,我们认为在正常生长条件下引起柱面扩展的主要因素有两个溶液的过饱和度和籽晶柱面存在的缺陷.扩展部分的晶体的光学质量与本体部分差别较大,扩展部分对光的透过率在紫外部分下降很快,明显低于本体部分在这一波段的透过率.本体部分和扩展部分对光的透过率在其它波段差别不十分突出. 相似文献
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Numerical simulations are carried out to investigate the crystallization process of a protein macromolecular substance under
two different conditions: pure diffusive regime and microgravity conditions present on space laboratories. The configuration
under investigation consists of a protein reactor and a salt chamber separated by an “interface”. The interface is strictly
related to the presence of agarose gel in one of the two chambers. Sedimentation and convection under normal gravity conditions
are prevented by the use of gel in the protein chamber (pure diffusive regime). Under microgravity conditions periodic time-dependent
accelerations (g-jitter) are taken into account. Novel mathematical models are introduced to simulate the complex phenomena
related to protein nucleation and further precipitation (or resolution) according to the concentration distribution and in
particular to simulate the motion of the crystals due to g-jitter in the microgravity environment. The numerical results show
that gellified lysozyme (crystals “locked” on the matrix of agarose gel) precipitates to produce “spaced deposits”. The crystal
formation results modulated in time and in space (Liesegang patterns), due to the non-linear interplay among transport, crystal
nucleation and growth. The propagation of the nucleation front is characterized by a wavelike behaviour. In microgravity conditions
(without gel), g-jitter effects act modifying the phenomena with respect to the on ground gellified configuration. The role
played by the direction of the applied sinusoidal acceleration with respect to the imposed concentration gradient (parallel
or perpendicular) is investigated. It has a strong influence on the dynamic behaviour of the depletion zones and on the spatial
distribution of the crystals. Accordingly the possibility to obtain better crystals for diffraction analyses is discussed. 相似文献
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Kyoichi Kinoshita Yasuyuki Ogata Satoshi Adachi Naokiyo Koshikawa Shinichi Yoda 《Microgravity science and technology》2005,16(1-4):71-73
The influence of convection in a melt on the compositional homogeneity of the TLZ-grown In0.3Ga0.7As crystals has been investigated by growing crystals with various dimensions on the ground. Excellent compositional homogeneity such as 0.3 plus or minus 0.01 in InAs mole fraction for a distance of 25 mm was obtained when the melt diameter was limited to 2 mm and convective flow in the melt was suppressed. On the other hand, when the crystal diameter was increased to 10 mm, both axial and radial compositional homogeneity was deteriorated due to convection in the melt. Comparing with the numerical simulation, convective flow velocity less than 1.4 mm/h may be sufficient for growing homogeneous crystals and it is not so difficult to suppress convective flow velocity below 1.4 mm/h for 10 mm diameter crystals in microgravity. Therefore, larger homogeneous In0.3Ga0.7As crystals are expected to be grown by the TLZ method on board the International Space Station. 相似文献
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《Materials Science & Technology》2013,29(11):1266-1272
AbstractThe influence of convection on the morphology of primary dendrites and on the distribution of germanium in primary crystals of hypoeutectic Ag–Ge alloy was studied. The experiment in microgravity provided conditions for the suppression of melt flow. Both the morphology of the primary dendrites and their chemical composition profiles were found to vary considerably depending on the intensity of convection. Primary crystals in specimens solidified in the space laboratory exhibited strong supersaturation with germanium. This supersaturation is associated with a decrease of solute in the eutectic. The overall segregation of germanium between crystals and the melt during the growth of primary dendrites is strongly promoted by convection. The influence of convection on the non-equilibrium primary crystallisation, particularly on the solute trapping process, the occurrence of which is likely in the initial stage of primary crystallisation (solute rich core), is discussed. A higher solute gradient close to the core in the space sample implies a higher melt stability below the liquidus when convection is suppressed. 相似文献
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Carotenuto L Cartwright JH Castagnolo D Garcia Ruiz JM Otalora F 《Microgravity science and technology》2002,13(3):14-21
We present an order-of-magnitude analysis of the Navier-Stokes equations in a time-dependent, incompressible and Boussinesq formulation. The hypothesis employed of two different length scales allows one to determine the different flow regimes on the basis of the geometrical and thermodynamical parameters alone, without solving the Navier-Stokes equations. The order-of-magnitude analysis is then applied to the field of protein crystallization, and to the flow field around a crystal, where the driving forces are solutal buoyancy-driven convection, from density dependence on species concentration, and sedimentation caused by the different densities of the crystal and the protein solution. The main result of this paper is to provide predictions of the conditions in which a crystal is growing in a convective regime, rather than in the ideal diffusive state, even under the typical microgravity conditions of space platforms. 相似文献
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L. Carotenuto J. H. E. Cartwright D. Castagnolo J. M. García Ruiz F. Otálora 《Microgravity science and technology》2001,13(3):14-21
We present an order-of-magnitude analysis of the Navier-Stokes equations in a time-dependent, incompressible and Boussinesq
formulation. The hypothesis employed of two different length scales allows one to determine the different flow regimes on
the basis of the geometrical and thermodynamical parameters alone, without solving the Navier-Stokes equations. The order-of-magnitude
analysis is then applied to the field of protein crystallization, and to the flow field around a crystal, where the driving
forces are solutal buoyancy-driven convection, from density dependence on species concentration, and sedimentation caused
by the different densities of the crystal and the protein solution. The main result of this paper is to provide predictions
of the conditions in which a crystal is growing in a convective regime, rather than in the ideal diffusive state, even under
the typical microgravity conditions of space platforms. 相似文献
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Fluid Flow and Solidification Under Combined Action of Magnetic Fields and Microgravity 总被引:1,自引:0,他引:1
Mathematical models, both 2-D and 3-D, are developed to represent g-jitter induced fluid flows and their effects on solidification under combined action of magnetic fields and microgravity. The numerical model development is based on the finite element solution of governing equations describing the transient g-jitter driven fluid flows, heat transfer, and solutal transport during crystal growth with and without an applied magnetic field in space vehicles. To validate the model predictions, a ground-based g-jitter simulator is developed using the oscillating wall temperatures where timely oscillating fluid flows are measured using a laser PIV (Particle Image Velocimetry) system. The measurements are compared well with numerical results obtained from the numerical models. Results show that a combined action derived from magnetic damping and microgravity can be an effective means to control the melt flow and solutal transport of single crystal growth in space environment. 相似文献
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Heat and mass transfer during crystal growth 总被引:2,自引:0,他引:2
Quality of semiconductor and oxide crystals which are grown from the melts plays an important role for electronic and/or optical devices. The crystal quality is significantly affected by the heat and mass transfer in the melts during crystal growth in a growth furnace such as Czochralski or horizontal Bridgman methods. This paper reviews the present understanding of phenomena of the heat and mass transfer of the melts, especially instability of melt convection from the detailed numerical calculation, which helps to understand the melt convection visualized using X-ray radiography. Large scale simulation of melt convection during crystal growth is also reviewed.
Characteristics of flow instabilities of melt convection with a low Prandtl number (ratio between momentum and thermal diffusivities) are also reviewed by focusing on the instabilities of baroclinic, the Rayleigh-Benard and the Marangoni-Benard, from the points of view of temperature, rotating and/or magnetic field effects during crystal growth. Oxygen concentration in grown crystals is also discussed how melt convection affects. 相似文献
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Torbjörn Carlberg 《Microgravity science and technology》2005,16(1-4):35-39
Under conditions of weak convection in the liquid during metal solidification and crystal growth, large chemical inhomogeneities can occur due to insufficient mixing of the melt. In this project a series of experiments with increasing melt flow, driven by thermo-capillary forces, have been performed in a Get Away Special flight. The level of convection was evaluated quantitatively from analysis of axial segregation profiles. As expected a significantly stronger radial segregation has occurred in the space grown material compared to ground based reference experiments. It was also found that there was a shift of the position of maximum concentration when the flow was increased. Unexpected axial solute maxima were also revealed in the space samples. A comparison was made to numerical simulations, and good correlations were found to first order effects. 相似文献
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MADHUKAR M. RAO WEI SHYY 《International journal for numerical methods in engineering》1997,40(7):1231-1261
A computational capability has been developed to predict the free surface shape, heat transfer and melt–crystal interface shapes in float-zone processing. A moving boundary, second order, finite volume, incompressible Navier–Stokes solver has been developed for the fluid flow and heat transfer calculations. The salient features of the approach include solving the dynamic form of the Young–Laplace equation for the free surface shape, dynamic remeshing to fit the free boundary, a flexible, multi–block, grid generation procedure and the enthalpy method to capture the melt–crystal and the melt–feed interfaces without the need for explicit interface tracking. Important convective heat transfer modes; natural convection and thermocapillary convection have been computed. It is shown that, whereas the overall heat transfer is not substantially affected by convection, the melt–crystal interface shape acquires significant distortion due to the redistribution of the temperature field by the thermocapillary and buoyancy-induced convective mechanisms. It is also demonstrated that the interaction of natural and thermocapillary convection can reduce the melt–crystal interface distortion if they act in opposing directions. It is found that the meniscus deformation can cause the height of the zone to increase but the qualitative nature of the melt–solid interface shapes are not significantly affected. Results are compared with literature to validate the predictive capability developed in this work. © 1997 by John Wiley & Sons, Ltd. 相似文献