Effective Nucleation Rate in Size Dependent Growth Systems

Using of MSMPR technique for simultaneous obtaining crystal growth and nucleation data poses a problem in the case of size dependent growth systems. Namely, the necessary extrapolation of experimental population density plot down to zero size is dubious because of its very steep and varying slope. Thus only some effective nucleation rate can be determined accurately. The proposed method of calculating it comprises cutting population plot at some effective size L_e chosen as a fraction of dominant size of mass weighted CSD of product from MSMPR crystallizer. Such a point of reference enables to compare nucleation rates even of different systems. Correlations of experimental data for aluminium ammonium alum are presented.     

非配位性溶剂中CdS纳米颗粒的生长动力学及其机理研究

本文以氧化镉为镉源,硫单质为硫源,油酸为配体,在十八烯体系中合成单分散的CdS纳米颗粒.通过改变油酸的浓度,研究了油酸对颗粒的生长动力学、颗粒尺寸分布以及颗粒浓度的影响.所得到的CdS纳米颗粒直径从2.2～4.5nm变化,最大荧光发射波长为450nm.在油酸浓度为287M,反应时间为60min左右时,所得到的CdS颗粒直径为4.5nm,而颗粒浓度仅为10-5M;在油酸浓度为42mM时,实际可得到的CdS颗粒的直径可小至2.2nm,反应溶液中颗粒的平均浓度可达到8×10-5M左右.研究表明,油酸浓度增大,使初始成核速度加快,反应平衡时所得到的纳米颗粒直径较大,但是颗粒数目显著减少,并且尺寸分布也有所增宽.     

A Calorimetric Measurement of Phase Transformation Kinetics in Solid Octaphenylcyclotetrasiloxane: Evidence for Nucleation as the Rate Determining Process

The kinetics of transformation from the metastable (at high temperature) solid 3-phase of octaphenylcyclotetrasiloxane to the stable 2-phase have been measured by a calorimetric technique. The results of the kinetic study and several related experiments are consistent with the view that nucleation is the rate determining process. Transformation time constants for large and small crystallites were found; the temperature dependence of each is in accord with the Turnbull-Fisher nucleation theory. From that theory γ, the interfacial energy between the 3- and 2-phases, has been found to be -3 ergs/cm², an order of magnitude smaller than that for typical liquid-solid interfaces. A study of the dependence of the transformation kinetics on crystallite size suggests that the phase change is surface nucleated.     

A Study of the Nucleation Kinetics,Linear Growth and the Overall Crystallization Kinetics of Meltspun Pd80Si20 Amorphous alloy

Nucleation kinetics, linear growth rate and overall isothermal crystallization kinetics of Pd Si amorphous alloy are studied by means of DSC and optical microscopy. It is found that both the histograms of the crystallite sizes and the DSC isotherms (resp. the time dependences of the degree of transformation α(t)) are best described by the model comprosing the simultaneous occurrence of two reactions of crystallization: heterogeneous as well as homogeneous non-stationary nucleation with subsequent three-dimensional growth in both cases. From the fit of the experimentally measured and the calculated according the model thus proposed distribution curves N_i(Φ) the stationary rates of heterogeneous and homogeneous nucleation, the number of quenched-in nucleation sites and the transient time lags of heterogeneous and homogeneous nucleation have been estimated. Using the values of these parameters kintic curves degree of transformation vs. time have been generated, which are closed to the exerimental x(t) curves obtained by means of DSC at isothermal conditions.     

不同能量密度下脉冲激光烧蚀制备纳米Si晶粒成核生长动力学研究

在室温、10 Pa氩气环境下,采用脉冲激光烧蚀(PLA)技术,通过改变激光能量密度,在烧蚀点正下方、与烧蚀羽辉轴线平行放置的衬底上沉积制备了一系列纳米Si晶薄膜.采用SEM、Raman散射谱和XRD对纳米Si晶薄膜进行了表征.结果表明:沉积在衬底上的纳米Si晶粒分布在距靶一定的范围内,晶粒尺寸随与靶面距离的增加先增大后减小;随着激光能量密度的增加,晶粒在衬底上的沉积范围双向展宽,但沉积所得最大晶粒尺寸基本保持不变,只是沉积位置随激光能量密度的增加相应后移.结合流体力学模型、成核分区模型和热动力学方程,通过模拟激光烧蚀靶材的动力学过程,对纳米Si晶粒的成核生长动力学过程进行了研究.     

Nucleation and Growth of Crystallites from a Model Saliva Solution in the Presence of Amino Acids

Crystallography Reports - The nucleation and growth of crystallites from a model saliva solution in the presence of amino acids (in concentrations corresponding to the physiological level and...     

Characteristics of the Nucleation Process in a Model Solution of Synovial Fluid in the Presence of Organic Impurities

Crystallography Reports - The regularities of nucleation in a model solution of human synovia under conditions close to physiological have been investigated. The constant of the induction process...     

Physical Model of Paths of Microdefects Nucleation in Dislocation‐Free Single Crystals Float‐Zone Silicon

With the help of selective etching, transmission electron microscopy complex researches of non‐doped dislocation‐free single crystals of float‐zone silicon by a diameter of 30 mm were conducted. The crystals were obtained with various growth rates and were subjected to various kinds of technological effects. Is established that the process of microdefects formation in silicon proceeds simultaneously on two independent mechanisms:vacancy and interstitial. The physical model of formation of microdefects in dislocation‐free monocrystals of FZ silicon is offered.     

A New Structural Model for Lithium Niobate

A new model based on the crystal field theory is proposed for LiNbO₃. According to this the central position of the Nb ions in oxygen octahedra is energetically unstable leading to the splitting of the potential energy curve into the one having two equivalent off-centered minima. The phase transition temperature T_c is related to the average height of the barrier existing between these energy minima. The model explains satisfactorily some of the properties of LiNbO₃ in the paraelectric and ferroelectric phases, which were not fully understood by earlier models.     

Di-Ab-An、Q-Lc体系非平衡结晶及枝晶结晶学方向测试研究

本文对两个典型的硅酸盐体系:Di(透辉石)-Ab(钠长石)-An(钙长石)体系和Q(石英)-Lc(白榴石)体中结晶物相系进行了枝状形貌、枝体结晶学方向测试及结晶能力方面的研究.通过偏光显微镜(PL)、扫描电镜(SEM)观察发现:透辉石、低鳞石英、α-方石英都发育为枝晶,而斜长石发育成放射状球粒晶和针状晶体.通过电子背散射衍射(EBSD)测试确定出透辉石是沿<001>方向发育成枝晶的主干、低鳞石英是沿<100>和<010>方向发育成枝晶主干、α-方石英是沿两个<100>方向发育成枝晶的主干.并发现两个体系在相同降温速度下的结晶能力不同.     

A Test for Surface Energy Anisotropy Sign Determination

Creagh and Kmetz have assumed that the relative values of the nematic to the substrate surface tension permit an estimation of nematic orientation on that substrate. This much discussed assumption is studied in detail in this paper; a new rule will be proposed and a test for the sign of the surface energy anisotropy is applied.     

A Model for Soliton Excitations in (CH)x

We present a simple model for those properties of (CH)_x which are associated with the concept of solitions. We propose an “an initio” calculation of the wave function and energy of the soliton within an “Unrestricted Hartree-Fock” scheme. Electron-electron interactions are capable to originate an organization into domains and walls when described in terms of the so-called alterant orbitals of LÖWDIN. The essential features of the treatemnt are reviewed and numerical results are re ported. Then a connection is made between the model and some other formalisms. Finally we discuss a tent ative interpretation of electron spin resonance experiments. Specially emphasized are numerical simulations of spectra and analy sis of diffusive motions of solitons.     

Lattice Parameter Local Determination for Trigonal,Hexagonal, and Tetragonal Crystal Systems Using Several Coplanar X-Ray Reflections

The method for determining the local lattice parameters using quasi-multiple X-ray diffraction (which was proposed and used only for crystals of the cubic system) has been expanded to measure the local crystal lattice parameters for the trigonal, hexagonal, and tetragonal systems. Local variations in the lattice parameters in the tetragonal paratellurite (TeO₂) and trigonal lanthanum-gallium tantalate (La₃Ga_5.5Ta_0.5O₁₄) crystals have been investigated. Reflections necessary for implementing the quasi-multiple X-ray diffraction scheme are found for these crystals. The pairs of reflections that were found were used to complete quasi-multiple X-ray diffraction schemes on a laboratory X-ray source and determine the variation in the lattice parameter a along the surface of these crystals. The relative measurement error was 4 x 10^-6 at a spatial resolution of 140 μm. The accuracy of the method and the parameters determining this characteristic were analyzed based on the results. The locality of the method and the limits of the spatial resolution of an X-ray diffraction measurement of lattice parameters are estimated.     

A Model for Structures Growth by Sodium Electrodiffusion in Quartz Crystals

Dendritic structures were grown inside the quartz crystal by electrodiffusion of sodium ions from a NaCl layer at the anode. The electric field was applied along the optical axis Z and the structures were grown in the xy plane. An experimental arrangement using a pointed cathode and a graphite plate anode was employed. X ray diffraction analysis shows that the structures' chemical composition is a mixture of sodium silicates. To explain the structures' growth a model is proposed. The model is a bidimensional one. The transport of the particles along the Z axis structural channels by the electric field was taken into account by introducing a probability of generating the particles in the growth plane as a function of the distance to the cathode. The computer generated results and the experimental ones were compared and a good correlation was found.     

A Pragmatic Model for the Simulation of Self-induced Striations in Quartz Crystals

A mechanism is investigated which might lead to fluctuations in the growth of quartz crystals due to the periodic incorporation of impurities. The model ought to be applicable also to the incorporation of stoichiometric partners during the growth of complicated crystals from non-stoichiometric melts. The processes have been simulated with help of an equivalent circuit experimentally and by computation. Some typical cases are presented in the paper.     

A Direct Method for the Determination of Orientation Relationships Using TEM

A new method for the determination of orientation relationships has been developed. The method is based on a recently proposed direct method for orientation determination using a transmission electron microscope (TEM). The determination of the orientation relationship can be done by two different but generally applicable approaches. The first approach is especially applicable for crystals with orthogonal base vectors, the second approach is especially applicable for crystals with non-orthogonal base vectors and the general bicrystal.     

A Simple Model for Evaluation of Electronic Polarizabilities of Chalcopyrity Semiconductors

Simple correlations between the energy gap, optical electronegativity and the refractive index are given for some ternary chalcopyrite semiconductors. Refractive indices and electronic polarizabilities are evaluated. Results are in good agreement (±5.3%) with the experimental values, better than those of the valus (±19.5−28%) using bond charge model and plasma frequency formalism.     

Nucleation and growth kinetics for the crystallization of ice from dextrose solutions in a continuous stirred tank crystallizer with supercooled feed

The crystallization of ice in aqueous dextrose solutions is studied in an adiabatic continuous stirred tank crystallizer with a supercooled feed stream. The effective diameter of the ice crystals was determined for various values of mean crystal residence time, feed supercooling, magma density, stirring rate, and dextrose concentration. For all process conditions the supercooling was measured at 9-12 different locations in the crystallizer. These local supercoolings were averaged algebraically to yield the bulksupercooling. From the experimental results growth and nucleation rates have been calculated. By comparing the experimental growth rates to growth rates calculated by means of a mathematical model kinetics for the inbuilding of water molecules into the ice lattice have been determined. The growth rate appears to be directly proportional to the interface supercooling. The rate constant decreases exponentially with increasing weight percentage of dextrose in the solution. The nucleation rate was found to be directly proportional to total crystal surface per unit volume of suspension and proportional to the bulksupercooling to the power 2.1. Nucleation is believed to occur by breakage of dendrites from the surface of parent crystals.