Pt在多晶Au微盘电极上电沉积成核与晶体生长

于不同H2PtCl6浓度和超电势下,应用Fleischmann的电结晶成核及晶体生长模型和Barradas Bosco的电化学成相吸附成核模型,拟合多晶Au微盘电极( =30μm)上电沉积Pt的恒电势阶跃电流暂态曲线.得出:在稀H2PtCl6溶液中,上述沉积过程初期,发生H2PtCl6吸附并遵循二维瞬时成核与圆柱形生长模型.二维生长速率常数随超电势线性增加.跟随其后的是核的层状生长,其速率常数随超电势呈非线性变化.而在高浓H2PtCl溶液中,沉积机理转变为H2PtCl6的吸附、瞬时成核及三维正圆锥形的生长模式.其晶核的垂直生长速率常数kPERP比水平生长速率常数kPARA大两个数量级以上.况且,logkPERP随超电势线性增加,而logkPARA则随超电势呈反S形变化的关系.相同超电势下,无论kPERP还是kPARA,均比稀H2PtCl6溶液中的二维层状生长速率常数大几个数量级.     

Effects of SiO2 overlayer at initial growth stage of epitaxial Y2O3 film growth

We investigated the dependence of the Y₂O₃ film growth on Si surface at initial growth stage. The reflection high-energy electron diffraction, X-ray scattering, and atomic force microscopy showed that the film crystallinity and morphology strongly depended on whether Si surface contained O or not. In particular, the films grown on oxidized surfaces revealed significant improvement in crystallinity and surface smoothness. A well-ordered atomic structure of Y₂O₃ film was formed on 1.5 nm thick SiO₂ layer with the surface and interfacial roughness markedly enhanced, compared with the film grown on the clean Si surfaces. The epitaxial film on the oxidized Si surface exhibited extremely small mosaic structures at interface, while the film on the clean Si surface displayed an island-like growth with large mosaic structures. The nucleation sites for Y₂O₃ were provided by the reaction between SiO₂ and Y at the initial growth stage. The SiO₂ layer known to hinder crystal growth is found to enhance the nucleation of Y₂O₃, and provides a stable buffer layer against the silicide formation. Thus, the formation of the initial SiO₂ layer is the key to the high-quality epitaxial growth of Y₂O₃ on Si.     

Effect of growth conditions on the structure of two-dimensional latex crystals: modeling

Essential experimental features of the nucleation and growth of a 2D colloidal crystal on a solid substrate are modeled. The crystal, composed of sub-micron-sized latex spheres, is grown by the evaporation of water from the particle suspension in a circular cell. The calculation of the meniscus profile in the cell allows the prediction of the particle volume fraction in the suspension surrounding the crystal as a function of time. This quantity enters into a convective-diffusion model for the crystal growth which calculates the crystal radius as a function of time. Comparison with experimental data for 2D latex particle crystals shows predominant convective growth over a wide range of evaporation rates set by varying the humidity of the air. Microscopic parameters of the particle assembly can also be estimated such as the particle velocity, diffusivity, characteristic time constants, Peclet number, etc. The nucleation is simulated by simultaneously solving the equations of motion for the ensemble of particles trapped in a thin liquid film using the discrete-element method. These equations account for the forces which are physically important in the system: contact particle–particle friction, increased viscous resistance during the particle motion in a wetting film, long-range capillary attraction between two particles screened by the rest of particles. The final result of the simulation is a particle cluster of hexagonal packing, whose structure resembles very much the monolayer nucleus of latex particles observed experimentally. The models proposed by us could also be implemented for the aggregation of species in a variety of practical processes such as coating, texturing, crystal growth from a melt or liquid solution, or a biological array. Received: 10 May 1999 Accepted in revised form: 6 July 1999     

Gas cluster growth by solute diffusion in porous media. Experiments and automaton simulation on pore network

The study presented in this paper deals with the liquid–gas phase change by pressure decline of supersaturated CO₂ solutions in 2D porous media. The growth of the gas phase is studied experimentally and numerically as a function of supersaturation, wettability and gravity. Experiments are performed on a transparent etched network (micromodel) and simulations with a specific numerical automaton.In the experiments, the nucleation process, i.e. the occurrence of the gas bubbles, as well as the growth of these bubbles are visualised and analysed by means of a micro video camera and an image processing apparatus. The observations confirm the heterogeneous nature of nucleation and the disordered growth pattern of the gas phase. The analysis of the growth rate of a single gas cluster shows that this phenomenon is different from the compact growth of an isolated single bubble in the bulk. As previously predicted, the bubble growth by mass transfer and volume expansion in porous media is characterised by a pattern of the invasion percolation type under normal laboratory conditions.Numerical simulations of the growth pattern and the growth rate of a single gas cluster are performed with a numerical automaton. Based on a pore network modelling technique and a set of hypotheses derived from the observations, this automaton is first validated by comparing the numerical results with the experiments. Then, the automaton is used to conduct a sensitivity study. In particular, the influences of the Jakob number, pressure decline rate, Bond number, wettability and characteristics of the microstructure are investigated.     

Enhanced protein crystallization around the metastable critical point

We report on a computer-simulation study of homogeneous crystal nucleation in a model for globular proteins. We find that the presence of a metastable vapour-liquid critical point drastically changes the pathway for the formation of a critical nucleus. But what is more important, the large density fluctuations near the critical point also lowers the free-energy barrier to nucleation and hence increases the nucleation rate. As␣the location of the vapour-liquid critical point can be controlled by changing the solvent conditions, our simulation results suggest a guided approach to protein crystallization. Received: 4 June 1998/Accepted: 3 September 1998 / Published online: 10 December 1998     

Crystallization of proteins under an external electric field

An external electric field affects the crystallization of proteins when applied under some conditions of temperature, pH, and precipitating agent composition. As suggested in the theoretical part of this paper, it produces large protein concentration gradients inside the mother liquor leading to a local supersaturation area in the crystallization solution. Such an experiment has been used for the first time on the crystallization of a protein. The effects of an external electric field on the crystallization of hen egg-white lysozyme at 293 K, pH 4.5, and two NaCl concentrations (0.6–0.7 M) have been investigated using the vapor diffusion method. The application of electric field results in a smaller number of crystals with larger size. The crystals grew at the droplet surface, near the cathode. The nucleation rate is drastically reduced and this experimental method could be used to control the number of crystals in the droplet.     

Power law of nucleation rate of folded-chain single crystals of polyethylene

The number-average molecular weight (M _n) dependence of the primary nucleation rate (I) of polyethylene (PE) folded-chain single crystals was studied in the ordered phase. We observed that the M _n dependence of I is mainly controlled by the diffusion process of polymer chains within the interface between a nucleus and the melt and/or within the nucleus. The results show that I decreases with increasing M _n and follows a power law I∝M _n ^−2.3 for the ordered phase. It is named the power law of the nucleation rate. In a previous article we showed that for the disordered phase I∝M _n ⁻¹. In this article, we concluded that I decreases with increasing M _n and follows a universal power law, I∝M _n ^−H for both ordered and disordered phases. The power H depends on the degree of order of the crystalline phase, which is related to the morphology. Received: 13 September 2000 Accepted: 15 November 2000     

甲基丙烯酸甲酯微乳液聚合中粒子成长过程的探讨

以γ射线、过硫酸钾（ＫＰＳ）、过氧化苯甲酰（ＢＰＯ）和偶氮二异丁腈（ＡＩＢＮ）引发高单体含量的甲基丙烯酸甲酯（ＭＭＡ）微乳液聚合，观测了聚合过程中聚合物粒子大小及其分布随转化率的变化．水溶性与油溶性引发剂引发聚合有许多相似之处．聚合初期，体系内很快生成大聚合物粒子；随聚合的进行，体系中大聚合物粒子与小聚合物粒子共存；在更高的转化率下，微液滴成核都成为唯一的成核聚合方式，体系内只留有小粒子．但是两种类型的引发剂引发聚合时，也表现出明显的差别。水溶性引发剂引发聚合时，存在由均相成核到微液滴成核的转变；而油溶性引发剂引发聚合时，在较低的转化率下，聚合主要是在大聚合物粒子内进行的．     

Influence of EDTA on the hydrothermal synthesis of CdTe nanocrystallites

Transformation from Te nanorods to CdTe nanoparticles was achieved with the assistance of EDTA as a ligand under hydrothermal conditions. Experimental results showed that at the beginning of reaction Te nucleated and grew into nanorods. With the proceeding of reaction, CdTe nucleus began to emerge on the surface, especially on the tips of Te nanorods. Finally, nearly monodispersed hexagonal CdTe nanoparticles with diameters of about 200 nm were obtained. The effects of EDTA on the morphology and formation of CdTe nanoparticles were discussed in consideration of the strong ligand-effect of EDTA, which greatly decreased the concentration of Cd²⁺. Furthermore, the possible formation process of CdTe nanoparticles from Te nanorods was further proposed. The crystal structure and morphology of the products were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM).