On splitting of dislocations at a reverse motion in germanium single crystals

The dislocation dissociation after the reverse motion along a slip plane in Ge single crystals have been investigated. It has been established that the dissociation width as well as the constriction density were independent on the motion direction. The results obtained have been used to clarify the role of the dislocation splitting in the formation of the phenomenon of the dislocation mobility asymmetry.     

Investigation of the structure and mechanisms of thermal motion in nanostructured undecylenic acid

Abstract

We have compared the structure, phase transitions and the thermal motion mechanisms of the undecylenic acid in bulk and inside a porous matrix using powder X-ray diffraction, calorimetry and IR-spectroscopy methods. The matrix used was a porous silicon with cylindrical pores (D?=?20?nm). The interaction between the walls of the porous silicon and the nanocrystals of the undecylenic acid results in a crystalline structure, which is significantly different from the structure of bulk undecylenic acid. A phase transition in the wall-adjacent layer of the undecylenic acid nanocrystals was shown to take place. The solid-state phase transition temperatures, as well as the melting point, were shown to be lower for nanocrystals. Topological soliton motion mechanism was suggested to explain the thermal motion of the undecylenic acid molecules in the rotator phase.     

Experimental and numerical investigation of buoyancy driven convection during PDAMNA thin film growth

This paper presents results from numerical simulations as well as laboratory experiments of buoyancy driven convection in an ampoule under varying heating and gravitational acceleration loadings. The modeling effort in this work resolves the large scale natural convective motion that occurs in the fluid during photodeposition of polydiacetelene films which is due to energy absorbed by the growth solution from a UV source. Consequently, the growth kinetics of the film are ignored in the model discussed here, and also a much simplified ampoule geometry is considered. The objective of this work is to validate the numerical prediction on the strength and structure of buoyancy driven convection that could occur under terrestrial conditions during nonlinear optical film growth. The validation is used to enable a reliable predictive capability on the nature and strength of the convective motion under low gravity conditions. The ampoule geometry is in the form of a parallelepiped with rectangular faces. The numerical results obtained from the solution to the Boussinesq equations show that natural convection will occur regardless of the orientation of the UV source with respect to the gravity vector. The least strong convective motion occurred with the UV beam directed at the top face of the parallelepiped. The strength of the convective motion was found to be almost linearly proportional to the total power of the UV source. Also, it was found that the strength of the convective motion decreased linearly with the gravity due to acceleration. The pattern of the convection flow on the other hand, depended on the source location.     

Measurements of Soliton Trapping and Motion in Trans-Polyacetylene Using Dynamic Nuclear Polarization

Abstract

The motion of solitons in undoped trans-(CH)_x is studied over the temperature range 1.5–300 K using dynamic nuclear polarization. The results of a theoretical model for the combined solid state and Overhauser effects in terms of the time dependence of the electron nuclear interaction due to motion of electrons is presented. Its application to the experiment shows that most of the solitons are trapped below 30 K and that the mobile solitons above 30 K have a diffusion coefficient that is approximately proportional to the square root of the temperature. At 300 K the time for a soliton to diffuse its length is greater than or equal to 1.7 × 10^?11 seconds.     

Friction memory in the stick-slip of a sheared granular bed

We investigate friction memory effects in the chaotic stick-slip motion of a plate shearing a granular bed. By analyzing separately trajectories' portions having increasing or decreasing instantaneous velocity, it is found that there are two quantitatively distinct granular friction-velocity curves for positive or negative acceleration, which cross one another in the weakening region. Interpreting acceleration dependence as an indirect consequence of contact ageing, we try to explain these effects by including rate-and-state (RS) friction equations in the stochastic model describing the plate motion. Preliminary results on a study case show that the main experimental features can be reproduced in this way, although quantitative agreement is partial. From the value obtained for the RS parameters we conclude that sliding friction decorrelation takes place at the length-scale of the solid-on-solid micro-contacts between grains and plate. In addition, the contemporary presence of noise and RS effects influences the average friction curve at large shear rate.     

Experimental evidence for extended hydrogen diffusion in silicon thin films during light-soaking

We have used mass spectrometry to detect hydrogen effusing from silicon thin films exposed to light. Our results indicate a long range diffusion of hydrogen through the whole film, which ends with its release into the vacuum system. The changes in the film properties are characterized by dark and photoconductivity and hydrogen exodiffusion measurements. From the evolution of dark conductivity measurements after turning off the light, we show that this long range motion of hydrogen is not due to the heating of the sample. A comparison of hydrogen exodiffusion spectra of as-deposited and light-soaked samples shows that the weakly-bonded hydrogen content decreases by 30% for a-Si:H films and that the tightly-bonded hydrogen migrates to grain boundaries of crystalline regions in the case of pm-Si:H films. These results clearly demonstrate the long range motion of hydrogen during light soaking.     

The calculation of critical size of carbon clusters in ferrite

The formation of carbon clusters in ferrite is investigated by the simulation of random motion of carbon particles in a cubic primitive model lattice at different annealing temperatures. All computer runs are repeated to average the results. An interaction potential is assumed between the particles precipitated in the formed cluster and the particles in the matrix. Additionally a decay of the cluster is allowed. A cluster of critical size is formed if the probability of decay is lower than the precipitation rate. The critical size calculated depends on the annealing temperature as the results show unambigously.     

Analysis of GaAs Growth Rate Anisotropy in Some CVD Systems

Growth rate anisotropy in the vicinity of singular faces is being analized for GaAs layers deposited in some chemical deposition systems. Classical crystal growth theory conceptions was being used. In accordance with them a linear connection between growth rate component in the direction of the normal to the singular face and the growth step density is shown to accur in the range of angle deviation 20, 10 and 6° from the faces (111)B, (001) and (111)A correspondingly. Step motion velocity has been calculated on the assumption that the atoms reaching the surface may be crystallized either by joining the step in echelon or by joining the interstep surface due to nucleation or normal growth mechanism. The discussion shows the model which takes into consideration the growth owing to nucleation and step echelon motion gives more selfconsistent results.     

In-situ HVEM Observations of Dislocation Processes during High Temperature Deformation of Silicon Crystals

The characteristics in the plastic deformation of silicon crystals are first reviewed. Such characteristics have been interpreted quantitatively on the basis of some models on the velocity and the multiplication of dislocations during deformation. The results of the in-situ observations of silicon crystals deformed at elevated temperatures in a HVEM are presented. The slowness and the smoothness in the dislocation motion, the dynamic pile-up as a general mode of the collective motion of dislocations, the formation processes of multiplication centers of dislocations observed during the deformation all support the validity of the models adopted. Dislocation dipoles and Lomer-Cottrell sessiles are observed not to act as strong obstacles which play important roles in the work hardening of the crystals.     

KDP晶体中包裹体形成机制的探讨

本文介绍了包裹体对KDP晶体质量的影响,并从两个方面探讨了KDP晶体生长过程中包裹体的形成机制.通过分析KDP晶体表面原子结构研究了不同杂质的吸附情况以及杂质对生长台阶的阻碍作用,通过分析晶体生长过程中流体动力学和质量输运条件的变化研究了旋转晶体的流体切应力和表面过饱和度,结果表明吸附杂质对生长台阶的阻碍和表面过饱和度的不均匀造成了生长台阶的弯曲和宏观台阶的形成,导致生长台阶形貌的不稳定是包裹体形成的重要原因.     

NMR and X-ray Investigations of a Plastic Crystalline Phase in MBBA

Abstract

Wideline and high resolution NMR studies have been carried out on MBBA in its isotropic, nematic and solid phases. Isotropic and nematic phase spectra correspond to what has been reported earlier. In the solid phase, contrary to expectations, very intense narrow signals similar to signals of the isotropic phase have been observed for the first time at temperatures close to the solid ? nematic phase transition temperature. This indicates rapid reorientational or translational motion in the system. The X-ray results however confirm the existence of translational order. The results are interpreted as indicative of the existence of a plastic crystalline phase in MBBA.     

Anisotropic Atomic Displacement Parameters and Molecular Motions in Mesogenic Crystals

Abstract

In this paper we describe the results of vibrational analysis of molecules in the crystalline phase of three mesogenic compounds of the series n-alkyl cyclohexyl cyanophenyl pyrimidine from experimental atomic displacement parameters using the rigid body model of Cruickshank. The rigid body model has been applied so far for estimation of corrections in bond lengths arising from the librational modes of motion. We have undertaken this vibrational analysis to find out whether we can get any idea about the transition from crystalline to liquid crystalline phase. Two tensors T (translation) and L (libration) are used to describe the motion. These tensors have been diagonalized to obtain the principal axes and amplitudes of translational and librational motions.     

Electrons,dislocations, and low-temperature plastic deformation

The influence of electron drag processes on the plasticity of normal state metals is demonstrated in the case of zinc crystals. These results show that dislocations move as underdamped oscillators at low temperature and that dislocations flutters as they move. This flutter motion, in contrast to kink motion which is expected to have no long range stress, is the dominant drag process at temperatures below ≈ 4.2 K.     

Random motion of IV-dipoles in alkali halide crystals forming stable aggregates

The formation of higher aggregates of IV-dipoles consisting of a divalent impurity ion and a cation vacancy is investigated by the simulation of their random motion in a model lattice at different annealing temperatures. An interaction potential between the sink for the dipoles and the dipole in the lattice is assumed. Additionally a decay of the sink is allowed. The results show, that already two dipoles (i.e. a dimer) from a stable aggregate. This is in agreement with theoretical calculations of the equilibrium concentrations of higher aggregates and the results of kinetic measurements of dipole aggregation. The reason is the relative high binding energy of dipoles in alkali halide crystals.     

Enzyme hydration,activity and flexibility: A neutron scattering approach

Recent measurements have demonstrated enzyme activity at hydrations as low as 3%. The question of whether the hydration-induced enzyme flexibility is important for activity is addressed by performing picosecond dynamic neutron scattering experiments on pig liver esterase powders at various temperatures as well as solutions. At all temperatures and hydrations investigated here, significant quasielastic scattering intensity is found in the protein, indicating the presence of anharmonic, diffusive motion. As the hydration increases a temperature-dependent dynamical transition appears and strengthens involving additional diffusive motion. At low temperature, increasing hydration resulted in lower flexibility of the enzyme. At higher temperatures, systems containing sufficient number of water molecules interacting with the protein exhibit increased flexibility. The implication of these results is that, although the additional hydration-induced diffusive motion and flexibility at high temperatures in the enzyme detected here may be related to increased activity, they are not required for the enzyme to function.     

The Observation of Hydrodynamic Instability in a Cholesteric Liquid Crystal Layer Having a Free Surface

It was for the first time that a hydrodynamic instability has been experimentally observed in a layer of a cholesteric liquid crystal (CLC) having a free surface and being locally heated from below. Steady structures of CLC motion have been obtained in the form of rotating (in the place) spirals consisting of distinct rolls with sharp boundaries; motion in every such roll was note also. It is supposed that the convective instability observed is due to Marangoni effect.     

Kinetics of state switching in a disordered linear system

The Kolmogorov approach is used to describe the transition from the metastable to the stable state of a linear system, as a statistical process of multiple formations of nuclei of a new state and their merging. Within the framework of the model of a random-force field decelerating the motion of the boundaries between various states, it is shown that the kinetics of state switching undergoes a drastic change in the vicinity of a certain critical driving-force F = F _c . At F < F _c the switching process is considerably decelerated because of the anomalous boundary motion. The theory can also be used to calculate the influence of defects on the interphase domain boundaries on substrates, crystal growth by step mechanism, plastic flow of materials due to dislocation motion in the Peierls-Nabarro relief, and kinetics of their physical processes.     

On the delay of thermally activated motion of pyramidal dislocations in zinc

The effect of shear stress and temperature on the delay time, t_i, of thermally activated motion of dislocations in the {112 2} 〈112 3〉 slip system in Zn single crystals is studied. The delay is associated with a higher density of crystal defects at the initial path of dislocation motion.     

Effect of devitrification on ion motion in lithium-disilicate glass

Li-disilicate glass-ceramics consist of microcrystallites imbedded in the glassy Li₂O · 2SiO₂ matrix where the number and size of the crystallites depend on the devitrification heat treatment. To assess ion motion in these model glass-ceramics, we have measured the temperature dependence of the dc conductivity, σ_dc, and the ⁷Li nuclear spin relaxation (NSR) rate, 1/T₁, in samples with various crystalline fraction, c, ranging from c = 0 (pure glass) to c = 1 (fully devitrified polycrystalline ceramic). The Cole–Cole presentation of the complex impedance shows two separate arcs caused by the remarkable difference of the ionic motion in the glassy and crystalline phase. These two arcs correspond to a bi-exponential decay of the ⁷Li nuclear spin magnetization where the resulting two NSR rates are induced by the ionic motion in the two phases. Thus the NSR and σ_dc data provide a comprehensive picture of the ionic motion in the glassy and crystalline phases. In particular, the ionic motion is the fastest in the glass; then at lower values of c we observe a metastable crystalline phase with ionic motion much greater than in the stable (LS₂) crystalline phase existing at large c-values.     

粒子束注入单晶硅尺寸效应的机理研究

使用分子动力学方法研究硅粒子注入技术.系统比较分析了团簇粒子的包含反射,扩散和植入基底在内的全部运动过程,同时使用可视化方法观测记录基底表面形貌演化过程.所建立模型直观地显示了低注入能量域内的新特征.注入过程中,团簇粒子由不同粒径(数量)的硅原子组成.通过对粒径变化在注入过程的影响研究揭示了注入技术机理.仿真结果表明提出方法可用于定量预测注入粒子表面分布.本文工作可作为原子尺度下生成基底表面特征或设计图案的参考,并对可控表面沉积技术提供理论指导.