The mass transfer process and the growth rate of NaCl crystal growth by evaporation based on temporal phase evaluation

The mass transfer process and the crystal growth rate have been proved to be very important in the study of crystal growth kinetics, which influence the crystal quality and morphological stability. In this paper, a new method based on temporal phase evaluation was presented to characterize the mass transfer process in situ and determine the crystal growth rate. The crystallization process of NaCl crystal growth by evaporation was monitored in situ by a Mach-Zehnder interferometer, and the absolute concentration evolution, the evaporation rate and the real-time supersaturation of solution were obtained using temporal phase analysis, which acted as a novel technique to extract phase variation along time axis recently. Based on the evaporation rate and the absolution concentration, a new method to calculate mass transfer flux during the crystal growth without the knowledge of the mass transfer coefficient was proposed, and then the crystal growth rate could also be retrieved under the hypothesis of cubic crystals. The results show that the crystal growth rate increases with the supersaturation linearly. It is in agreement with the diffusion theories, which presume that matter is deposited continuously on a crystal face at a rate proportional to the difference in concentration between the points of deposition and the bulk of solution. The method is applicable to the research of crystallization process based on evaporation or vapor diffusion of which the precise conditions of nucleation and supersaturation are usually unknown because of the complexity of the evaporation rate and crystal growth rate.     

Lysozyme crystallization in hydrogel media under ultrasound irradiation

Sonocrystallization implies the application of ultrasound radiation to control the nucleation and crystal growth depending on the actuation time and intensity. Its application allows to induce nucleation at lower supersaturations than required under standard conditions. Although extended in inorganic and organic crystallization, it has been scarcely explored in protein crystallization. Now, that industrial protein crystallization is gaining momentum, the interest on new ways to control protein nucleation and crystal growth is advancing. In this work we present the development of a novel ultrasound bioreactor to study its influence on protein crystallization in agarose gel. Gel media minimize convention currents and sedimentation, favoring a more homogeneous and stable conditions to study the effect of an externally generated low energy ultrasonic irradiation on protein crystallization avoiding other undesired effects such as temperature increase, introduction of surfaces which induce nucleation, destructive cavitation phenomena, etc. In-depth statistical analysis of the results has shown that the impact of ultrasound in gel media on crystal size populations are statistically significant and reproducible.     

Protein surface shielding agents in protein crystallization

The molecules adhering temporarily on the surface of protein molecules change the propensity of protein molecules to deposit on the crystal surface in a definite position and orientation. The concepts of competitive adhesion modes and protein surface shielding agents acting on the surface of molecules in a non-equilibrium process of protein crystallization provide a useful platform for the control of crystallization. The desirable goal, i.e. a transient preference of a single dominating adhesion mode between protein molecules during crystallization, leads to uniform deposition of proteins in a crystal. This condition is the most important factor for diffraction quality and thus also for the accuracy of protein structure determination. The presented hypothesis is a generalization of the experimentally well proven behaviour of hydrophilic polymers on the surface of protein molecules of other compounds.     

非晶一次晶化过程微观组织演化和生长动力学的相场法研究

在晶化物理模型中添加扩散系数对晶化过程的影响, 采用相场方法研究初始形核率和初始形核半径对一次晶化过程中微观组织和生长动力学的影响。结果表明: 随着初始形核率的增加, 相同时间内非晶一次晶化的晶粒数量逐渐增加, 晶粒尺寸逐渐减小。晶化分数随着演化时间和初始形核率的增加逐渐增大, 初始形核率越大, 相同演化时间内的晶化分数越高。不同初始形核半径情况下, 非晶一次晶化过程中的晶粒数量和尺寸随着演化时间的增加基本保持不变。晶化分数随着演化时间的增加而增大。不同初始形核率和初始形核半径情况下所对应的生长指数均小于1, 表明初始形核率和初始形核半径对晶化方式无影响, 均为一次晶化。改变初始形核率和初始形核半径可调控一次晶化微观组织结构, 而晶粒尺寸及晶化分数直接关系到合金性能。     

Characterization of crystal growth using a spiral nucleation model

Classical concepts of two-dimensional nucleation and spiral growth are used together with recent findings on the dynamics of dislocation spirals to derive a new crystal growth model. Initial growth nuclei result from the organization of adsorbed molecules in spirals around surface dislocations. The energetic barrier for the activation of the spiral nuclei is considerably lower than the admitted by classical two dimensional nucleation models. Stable nuclei evolve into bigger growth hillocks in supersaturated media through the incorporation of adsorbed units into their steps. The displacement velocity of steps during solution and vapour growth is calculated by different kinetic approaches, taking into consideration the distinct role of surface diffusion in each process, and avoiding known limitations of conventional theories. A generalized expression is obtained relating the crystal growth rate with main variables such as supersaturation, temperature, crystal size, surface topology and interfacial properties. At the end of the paper, the crystallization kinetics of sucrose measured at 40 °C is interpreted in the light of the new perspectives resulting from the proposed model. The application example illustrates how to estimate interfacial and topological properties from the experimental crystal growth results.     

The nature of oscillations of the crystallization front in a diluted binary melt during the initial transient process

The cause of oscillations of the crystallization rate of a binary melt that arise in the initial transient process and have been revealed earlier in numerical experiments is investigated theoretically. Within a simple time-dependent model of directed crystallization, the problem of impurity diffusion in the melt from which a crystal is drawn at constant rate is solved in a linear approximation. It is shown that, under certain conditions, the solution represents a superposition of two concentration waves, a traveling and a standing wave. The concept of a characteristic distance of directed crystallization of a binary melt is introduced. It is established that the nonmonotonic behavior of the crystallization rate during the initial transient process, in particular, its damped oscillations, is attributed to the accumulation of impurities in the melt that is excessive with respect to its distribution in the stationary regime. Impurities in the melt are accumulated excessively when the characteristic distance of directed crystallization starts to exceed the characteristic diffusion length of the impurity in the melt.     

Solid-phase crystallization of amorphous Si films on glass and Si wafer

When amorphous silicon films deposited on glass by physical or chemical vapor deposition are annealed, they undergo crystallization by nucleation and growth. The growth rate of Si crystallites is the highest in their 〈111〉 directions along or nearly along the film surface. The directed crystallization is likely to develop the 〈110〉//ND or 〈111〉//ND oriented Si crystallites. As the annealing temperature increases, the equiaxed crystallization increases, which in turn increases the random orientation. When amorphous Si is under a stress of the order of 0.1 GPa at about 540 °C, the tensile stress increases the growth rate of Si grains, whereas the compressive stress decreases the growth rate. However, the crystal growth rate increases with the increasing hydrostatic pressure, when the pressure is of the order of GPa at 530–540 °C. These phenomena have been discussed based on the directed crystallization model advanced before, which has been further elaborated.     

Effect of gravitation vector orientation relative to the solidification front on the micro- and macrohomogeneity of semiconductor crystals grown under terrestrial and space conditions

The influence of gravitation vector orientation relative to the solidification front on the dopant distribution micro- and macrohomogeneity in vertical oriented crystallization of highly Ga-doped Ge crystals has been investigated using computer simulation. The deviation of the axis for crystal growth relative to the g ₀ vector, when a free surface of the melt is present, has been found to provide the formation of striations with a reduced dopant concentration. The influence of the solidification rate, convective and diffusion mass flows on the dopant distribution macrohomogeneity has been investigated.     

AlN晶体物理气相传输法生长坩埚的热场分析

采用物理气相传输法在钨制坩埚上制备AlN单晶.通过采用COMSOL软件中的固体传热和磁场模块,对AlN晶体生长的坩埚的热场进行仿真,同时针对不同的线圈直径以及不同的线圈位置对坩埚热场的影响进行模拟,提出了相应的处理方式.结果表明:当线圈直径增大,坩埚结晶区和升华区的温度在相同的加热时间下会增加,并且增加的温度存在峰值.当线圈的垂直位置发生变化的时候,结晶区和升华区的温度场也会发生变化,从上向下移动的过程中仍然存在温度的峰值,并且结晶区和升华区的温度关系会发生翻转,导致温度梯度阻碍晶体生长.在晶体生长过程中升华区和结晶区的温度关系依旧会发生翻转.但是通过线圈跟随籽晶表面生长层的变厚而同步移动,可以保持相对稳定的温度关系,维持晶体正常持续生长.     

X-ray topography characterization of the structural response of Ge(Ga) crystals to variation in gravity force vector orientation during crystallization

The influence of crystallization process disturbances connected with the variation of the crystallization front orientation relative to the gravity force vector (characteristic for crystal growth under conditions of microgravity onboard space vehicles) on the real crystal structure has been investigated by plane-wave X-ray topography. It has been found that these disturbances can result in a local disorder in the impurity distribution in the form of microsegregation growth striations. Quantitative estimations of the amplitude for the composition variation based on the analysis of contrast in growth striation images obtained by the method of plane-wave X-ray topography have been carried out.     

微重力条件下晶体生长过程的实时观察

从材料科学的微重力效应和晶体生长基本过程等两个方面出发，阐明了一种新的生长技术（晶体生长过程的光学实时观察法）的必要性，并介绍了空间高温实时观察装置的基本特性．在我国的科学技术探测卫星上，该装置进行了搭载实验，并首次清楚地观察到了空间溶质扩散效应和高温溶液的表面张力对流图像．     

Instability of vicinal crystal surfaces with transparent steps: Transient kinetics and non-local electromigration

The steps at the crystal surfaces could be partially transparent in the sense that the migrating adatoms can cross the steps without visiting a kink position. In the case of significant transparency the velocity of a given step in a given moment is affected by atomic processes which took place at rather distant terraces in rather earlier moments. The reason is that an adatom needs time to cross many terraces before attaching to a kink position. That is why the theory of crystallization with non-local kinetics should account for the time dependence of the adatom concentrations on the terraces. Such a transient growth model (going beyond the quasi-static approximation of the BCF theory of crystal growth) is developed here for the limit of slow kinetics at the steps and fast surface diffusion. This model predicts instability of the vicinal surface (in agreement with the earlier studies) but rather different expression for the wavelength of the most unstable mode, which depends only on the ratio between the step transparency and the step kinetic coefficients.     

TGS晶体生长多形性和其表面扩散生长机制

选取了TGS晶体的(001),(110),(010)和(100)四个面为研究对象,分析讨论了TGS晶体的表面扩散螺位错生长机制,发现生长基元的脱水化进入表面层过程与表面扩散过程相比,前者在TGS晶体的生长过程中起着更重要的作用;同时,在较高的相对过饱和度下,观察到了TGS自结体的两种简单晶形. 关键词： TGS晶体 生长多形性 表面扩散     

Study of the Elementary Growth Processes During Vapor-Phase Epitaxy of Semiconducting III–V Compounds

A review of studies performed at the V. D. Kuznetsov Siberian Physical-Technical Institute at Tomsk State University and aimed at obtaining detailed information on the elementary growth processes proceeding at the crystallization front during vapor-phase epitaxy of semiconducting III–V compound films is presented in the paper. The general approach to the problem and methods of its solution are described. Results of investigations of the adsorption layer composition, surface diffusion processes, and incorporation of growth components into a crystal are presented. The mechanism of epitaxial layer growth in semiconducting III–V compounds is discussed.     

Kinetics of reversible surface crystallization and melting in poly(ethylene oxide): Effect of crystal thickness observed in the dynamic heat capacity

Poly(ethylene oxide) (PEO) in the semi-crystalline state shows a reversible surface crystallization and melting; a temperature decrease leads to a certain crystal thickening, a temperature increase reversely to an expansion of the amorphous intercrystallite layers. Dynamic calorimetry provides a means to investigate the kinetics of the process. The structural rearrangement in the region of the crystalline-amorphous interface can only be accomplished if the chains can slide through the crystallites. One therefore expects the associated time to change with the crystallite thickness. Variations of the crystal thickness of PEO can be achieved by choosing different crystallization temperatures. We studied the effect of the crystal thickness employing temperature-modulated differential scanning calorimetry and heat wave spectroscopy, and by carrying out small-angle X-ray scattering experiments for the structural characterization. The effect of the crystal thickness is clearly observed. Results indicate that the sliding diffusion through the crystallites takes place by helical jumps of whole stems. Data yield the activation energy per unit length of the stems. Received 20 April 2001 and Received in final form 13 August 2001     

The activation energy of crystallization of amorphous Fe40Ni40P14B6

The isothermal crystallization of Fe₄₀Ni₄₀P₁₄B₆ (Metglas 2826) has been studied by transmission electron microscopy, using static observations of partially crystallized ribbons at room temperature and in situ dynamic registration of the crystallization process at elevated temperatures. At all temperatures crystallization takes place by the nucleation and growth of individual crystals. Analysis of the transformation kinetics allowed to determine the nucleation rates and the activation energy for crystal growth. The growth velocity of the crystal phase was found to be controlled by the diffusion coefficient of phosphorus in this alloy withD ₀=2.5×10^10±1cm{swu2}/s andQ=(3.4±0.15)eV.     

Surface structure of polyethylene crystals as revealed by surface decoration. II. Electron-microscopic detection of mobilizable surfaces

The paper is the first step in the application of the gold decoration technique described in Part I to the electron-microscopic study of the nature of the fold surface of polyethylene single crystals. It was observed that different areas of the same crystal layer display different decoration densities. It appears that this is due to residual solvent which is retained to different extents by different crystal portions. This assumption is substantiated by the salient observation that the initially slight decoration differences are much accentuated by deliberate flooding of the decorated crystals with a swelling agent. The inference is that the swelling liquid mobilizes some loose elements along the crystal surface, and, consequently, the differences within the same crystal reveal a variable crystal surface as regards surface looseness. With the new technique we are now able to describe and follow these phenomena as a function of the relevant variables. Within the confines of the present work it has already become apparent by this test that, from the usual commercial polymer, surfaces with least looseness are formed toward the end of the crystallization.     

Molecular dynamics simulations of the formation for NaCl cluster at the interface between the supersaturated solution and the substrate

Molecular dynamics simulations of supersaturated aqueous NaCl solution including the Pt(100) or NaCl(100) crystal surfaces have been performed at an average temperature of 298 K. The behavior of the NaCl cluster produced in the solution have been studied through the consideration of the water dielectric property near the crystalline surfaces for understanding the role of crystal growth on the surface. The surfaces in the solutions greatly influence heterogeneous nucleation in crystallization process. Density profile of the supersaturated solution and polarization of water molecules was calculated in order to describe the effect of the surfaces on the solution structure at the solid–liquid interfaces. The formation levels of NaCl clusters heavily depended on the water orientation at the interfaces. NaCl clusters were easily formed near the Pt(100) surface compared with the NaCl(100) surface owing to a different construction of water molecules between the platinum and NaCl surface.