The origin of nucleation peak in transformational plasticity

A typical stress-strain relation for martensitic materials exhibits a mismatch between the nucleation and propagation thresholds leading to the formation of the nucleation peak. We develop an analytical model of this phenomenon and obtain specific relations between the macroscopic parameters of the peak and the microscopic characteristics of the material. Although the nucleation peak appears in the model as an interplay between discreteness and nonlocality, it does not disappear in the continuum limit. We verify the quantitative predictions of the model by comparison with experimental data for cubic to monoclinic phase transformation in NiTi.     

Controlling of Solution Concentration on Nucleation During Hydrothermal Synthesis of Analcime

Hydrothermal experiments of analcime nucleation and glass dissolution were carried out in autoclaves under the conditions of autogeneous pressures, temperatures of 150 °C ～ 210 °C, and 0.25 M ～ 0.75 M NaOH solutions. The curves of the glass dissolution and analcime formation are S‐shaped. Yet, the two curves are essentially parallel to each other. The time required for a complete dissolution of glass or the commencement of analcime formation is shorter when the concentration of NaOH solution or the temperature is higher. The amount of the weight percent of analcime synthesized is obviously less than the glass dissolved at a given time of an experiment. The weight difference between glass dissolved and analcime (dehydrated) formed is calculated to be represented as the solution concentration. During an experiment, the solution concentration increases gradually to a maximum, then decreases slowly to a constant value with time. On the other hand, nucleation is also gradually increased to a maximum, then is decreased finally to zero with time in a bell‐shaped curve. It is clear that nucleation of analcime is mainly controlled by the solution concentration.     

由成核率数据拟合饱和蒸汽压力

从成核率的实验数据出发,对饱和蒸汽压力进行计算拟合并应用于经典成核理论．在计算过程中,采用了最新发展的成核率模型,其中考虑了真实气体效应和核子曲率对表面张力的影响．作为比较,还从热力学平衡条件和经验方程估算了几种物质的饱和蒸汽压力．结果表明,从成核率实验数据计算拟合的饱和蒸汽压力曲线与经验方程曲线有微小的差别;在经典成核理论的应用中,基于该拟合的饱和压力方程计算的成核率更接近于实验结果．     

溶液间歇结晶过程成核与生长阶段的定量识别

为定量识别溶液间歇结晶过程中的成核和生长阶段,基于晶粒数目和粒度的变化对粒度分布(CSD)的二阶和三阶矩量影响程度的不同,定义并关联了无因次变量K和K^*.添加晶种KNO₃-H₂O溶液结晶过程模拟计算的结果表明,K和K^*值均呈先降后升的变化趋势,成核时单调下降,生长过程中单调上升;且K与K^*值较接近.测定了KNO₃-H₂O溶液自发成核结晶过程中溶液浓度和透光率的变化,用K^*判据定量识别出成核阶段和生长阶段,并与晶体线性生长速率模型检验的结果相吻合.K值的计算依赖于CSD和结晶动力学参数,而K^*作为成核和生长阶段的模型判据,由实验测定的溶液浓度和透光率计算得到.     

Measurement, modelling and simulation of defects in as-grown Czochralski silicon

Defects in as-grown and heat-treated 150 and 200 mm Czochralski silicon crystals are investigated for different crystal pulling conditions and thermal treatments. First results are presented using noncontact carrier recombination imaging for detection of electrically active defects. The defect densities and sizes are compared with the results of computer modelling, suggesting that the observed defects in as-grown material are most likely large voids, possibly partially filled with silicon oxide. In contrast, the defects observed after prolonged heat treatments are silicon oxide precipitates with a density which is several orders of magnitude larger. The voids nucleate at temperatures above 1100°C by a homogeneous nucleation process and grow further to the observed size during further cooling of the crystal. They are responsible for the midfield breakdown of 10–50 nm gate oxides.     

Complex pole approach in thermodynamic description of fluid mixtures with small number of molecules

The subject matter of classical thermodynamics is the asymptotic behavior of equilibrium systems in thermodynamic limit, for small molecular systems, when transition to thermodynamic limit is impossible, the extension of thermodynamics is required. This work studies novel approach for the evaluation of partition functions of small systems by complex pole analysis. Several cases for molecular systems in small cavities are studied numerically. In particular size-dependent additional pressure for small systems is evaluated analytically and numerically. Similar approach was developed earlier in nuclear physics for finite systems of nucleons. The obtained results correspond to published experimental data and molecular dynamics simulations.     

Inhomogeneous phase separation kinetics in liquid binary mixtures: Sensitivity to initial local composition 2

The kinetics of phase separation subsequent to a finite temperature quench is assumed to be driven by diffusion on the altered free energy surface and is generally assumed to be slow. The situation can be different in phase separating liquid binary mixtures, especially for systems characterized by the large difference in mutual interactions between solute and solvent molecules. In such cases, the phase separation kinetics could be fast and may get completed within a short time (ns) scale. As a result, in these systems, one may observe diverse dynamical features arising out of local heterogeneity leading to the onset of phase separation through pattern formation, spinodal decomposition, nucleation, and growth. By using a coarse-grained analysis, we examine phase separation kinetics in each spatial grid and indeed observe important effects of initial heterogeneity on the subsequent evolution. Interestingly, we observe slower separation kinetics for those regions that correspond to the composition at the minimum of the high-temperature surface. The heterogeneous dynamics has been captured here through the non-linear susceptibility function, which shows a pattern similar to what is observed in the supercooled liquid. Each grid shows somewhat different dynamics in the three-stage (exponential, power-law, and logarithmic regime) phase separation dynamics. The late stage of phase separation kinetics is usually attributed to the coarsening of the phase-separated domains. However, in a liquid binary mixture, the late-stage power-law decay undergoes a further change. A new dynamical regime arises characterized by a logarithmic time dependence, which is due to the “smoothening” of the rough interface of already well-separated phases. This can also be described as opposite to the roughening transition described by Chui and Weeks [Phys. Rev. Lett. 40, 733 (1978)]. This reverse roughening transition can explain the logarithmic time dependence observed in the simulation.     

Hexagonal Defect Array Formed under Steady Shear Flow in Water-in-Cholesteric Liquid Crystal Emulsions

The relation between the liquid crystalline textures and the steady shear viscosity has been investigated in cholesteric emulsions, composed of water, surfactants, and cholesteric liquid crystals. Both at the substrate surface and at the surfactant-coated droplet surface, a homeotropic anchoring is enforced to the adjacent liquid crystal. Under a steady shear flow with the shear rate below 100 s^–1, we observed that point defects spontaneously appear, and tend to adopt a regular hexagonal arrangement in the low shear-rate range of 1–20 s^–1. In viscosity measurements, the shear-thinning behavior was found, showing a clear correspondence to the texture change. We estimated the height of point defects in the hexagonal array from the viscosity increase in the shear-thinning, assuming that the shear-thinning behavior is caused by the reduction of the effective gap between cone and plate, owing to the presence of the hexagonal array anchored on the plate. The relation between the estimated height and the measured lateral size of point defects agreed with that of the quadrilateral array, which is formed after termination of the shear. This agreement informs that the shape of a point defect is preserved in both defect arrays, independently of shearing conditions.     

Tip-growth mode and base-growth mode of Au-catalyzed zinc oxide nanowires using chemical vapor deposition technique

Tip-growth and base-growth modes of Au-catalyzed zinc oxide nanowires (ZnO NWs) were synthesized on Au-film pre-deposited silicon substrates using Chemical Vapor Deposition (CVD) technique. The diameter of tip-growth Au-catalyzed ZnO NWs was proportional to the Au film thickness, whereas the areal density of these NWs was inversely proportional to the Au film thickness. It would be more appropriate to explain the growth of Au-catalyzed ZnO NWs by a combination of Vapor–Liquid–Solid and Vapor–Solid (VLS–VS) mechanisms instead of the conventional VLS mechanism, regardless of tip-growth or base-growth mode of Au-catalyzed ZnO NWs. The competition between the VLS and VS mechanism in the effectiveness of capturing the adsorbed Zn and O atoms would determine the final morphology of ZnO NWs. In addition, Au catalyst promoted the growth rate of NWs as compared to the self-catalyzed ZnO NWs.