Diffusion of self-point defects in body-centered cubic iron crystal containing dislocations

The energetic, crystallographic, and diffusion characteristics of self-point defects (SPDs) (vacancies and self-interstitial atoms (SIAs)) in body-centered cubic (bcc) iron crystal in the absence of stress fields have been obtained by the molecular statics and molecular dynamics methods. The effect of elastic stress fields of dislocations on the characteristics of SPDs (elastic dipoles) has been calculated by the methods of the anisotropic linear theory of elasticity. The SPD diffusion in the elastic fields of edge and screw dislocations (with Burgers vectors 1/2 〈111〉 and 〈100〉) at 293 K has been studied by the kinetic Monte Carlo method. The values of the SPD sink strength of dislocations of different types are obtained. Dislocations are more effective sinks for SIAs than for vacancies. The difference in the sink strengths for SIAs and vacancies in the case of edge dislocations is larger than the screw dislocations.     

Debye characteristic temperature of some silver-base binary alloys

The Debye characteristic temperature (θ) of eight silver base binary alloys in the cubic phase having various compositions have been calculated making use of four different averaging methods from the data on elastic constants. The values of θ are also evaluated from series expansion method. The results obtained were examined in terms of solute concentration, reciprocal of the lattice parameter, mass, electron to atom ratio, and elastic anisotropy. Important conclusions are made.     

Zur Integration der Grundgleichungen für bewegte Eigenspannungsquellen in kubischen Kristallen

Solutions of the equations of dislocation theory are given being based on a mathematical method which decouples the system of differential equations for the displacement vector. The solutions regard elastic anisotropy of crystalline matter not only in statics but also in dynamics. — The method succeeds in the case of plane strain in cubic or hexagonal crystals, allowing most general plastic distorsions as well as processes of arbitrary timedependence. Decoupling is the basis for using complex variable methods which may be applied when dislocations move in a uniform way.     

Acoustic study of elastic properties,dislocation damping and plasticity of crystals

The results of acoustic studies of elasticity and dislocation inelasticity of crystals are presented. Regularities of elastic anisotropy variation with ionicity of bonds in A_NB_8-N crystals have been considered. Amplitude dependences of ultrasound damping supply the information on dislocation structure and dislocation — to point defect interaction. Oscillation of dislocations during internal friction measurements are compared to their translation motion at the onset of plastic flow.     

Electron Beam Induced Changes of the Real Structure of Semiconductors

The formation of irradiation defects produced in a high voltage electron microscope (HVEM) has been studied in Ge and other semiconductor crystals in dependence on various thermal treatments and on covering of the specimen surface. It has been concluded that the defect formation depends — besides general features known for pure metals — also on the state of electrically neutral impurities. Besides, various interactions between dislocations and irradiation defects were observed.     

Possible dynamical mechanism of dislocation drag in metals in the easy slip stage

The slip of an edge dislocation through a system of parallel immobile dislocation dipoles oriented parallel to it has been investigated. A new mechanism of dislocation drag (irreversible transformation of the kinetic energy of a moving dislocation into the energy of natural vibrations of a pair of edge dislocations (forming a dipole), excited by the elastic field of the moving dislocation) is proposed and analyzed. The dynamic drag force of moving dislocation caused by this mechanism is calculated. It is shown that this force is inversely proportional to the slip velocity of mobile dislocations.     

Electro—Optic and Dielectric Studies in Cholesteric Liquid Crystal Mixtures

In binary mixtures of cholesteryl chloride and cholesteryl alkanoates, the threshold field for the cholesteric to nematic transformation depends on the length of the alkanoate ester chain. The threshold field also depends on the pitch, the dielectric anisotropy and the twist elastic constant of the mixtures. This paper describes the experiments performed to establish the role of these three parameters in the observed variation of threshold field with alkanoate ester chain length in such binary mixtures.

The temperature dependence of threshold field, dielectric anisotropy and pitch has been studied. From these experiments the relative response of the various mixtures to applied electric fields has been established. In most of the mixtures, temperature ranges over which pretransitional effects do not predominate, have been identified. Twist elastic constants of the mixtures in these temperature ranges have been calculated. It has been shown that both the intrinsic response of the mixtures to applied electric fields, and the twist elastic constants do not show a regular variation with chain length of the alkanoate ester tail. Some observations suggest that mixtures containing non-smectogenic alkanoates show different temperature dependence of threshold field as compared to those containing smectogenic alkanoates.     

Energy bands of the BCC metals rubidium and cesium

Energy bands of BCC rubidium and cesium are calculated for the first time using the full non-local but energy-independent Shaw's optimized model potential. The energy dependence of the potential is then included as first order perturbation. The effects of changing the model potential and of reversing the sign of the depletion charge d are reported. The Fermi energy E_F is calculated in all cases. Our results are compared with those obtained by other authors. The band gap at N is very small for both metals, except when all model potential parameters are increased considerably (50% to 100%).     

Calculation of displacement fields and simulation of HRTEM images of dislocations in sphalerite type A(III)B(V) compound semiconductors

The displacement fields of different kinds of both perfect and dissociated dislocations have been calculated for an isotropic continuum, and by means of linear elasticity. Additionally, the corresponding HRTEM images have been simulated by the well-established EMS program package in order to predetermine the structural aspects of dislocations, and then to compare it with experimental HRTEM micrographs. The latter ones resulted from plastically deformed GaP single crystals and InAs/(001)GaAs single epitaxial layers. It could be established that using the simple approach of linear elasticity and isotropy results can be obtained which correspond well to the experimental images. So, the structure of various Shockley partial dislocations bounding a stacking fault can be detected unambiguously. The splitting behaviour of perfect 30° dislocations (separation into a 0° and 60° partial) and 90° dislocations (separation into two 60° partials) both with line direction along 〈112〉, 60° dislocations (separation into 30°/90° and 90°/30° configuration) and screw dislocations (separation into two 30° partials) along 〈110〉 are discussed in the more detail. Moreover, the undissociated sessile Lomer dislocation, glissile 60° dislocation and edge dislocation have been considered too.     

Time-dependent trench profile formation during RIE-processing of silicon with CCl2F2-gas mixture

The formation of the three main types of trench profiles (step-like profile and trapez-like profile the small side down or up, respectively) and their time-dependent development during trench etching have been explained by taking into consideration not only the vertical etch rate and the factor of etching anisotropy given by mask undercutting but also the additional factor of etching anisotropy given by applying the definition to the bottom rim of the trench. Concidering both time-and depth-dependence of etching anisotropy the formation of projecting brims within a trench can be interpreted, too.     

Analysis of synthetic diamond single crystals by X-ray topography and double-crystal diffractometry

Structural features of diamond single crystals synthesized under high pressure and homoepitaxial films grown by chemical vapor deposition (CVD) have been analyzed by double-crystal X-ray diffractometry and topography. The conditions of a diffraction analysis of diamond crystals using Ge monochromators have been optimized. The main structural defects (dislocations, stacking faults, growth striations, second-phase inclusions, etc.) formed during crystal growth have been revealed. The nitrogen concentration in high-pressure/high-temperature (HPHT) diamond substrates is estimated based on X-ray diffraction data. The formation of dislocation bundles at the film-substrate interface in the epitaxial structures has been revealed by plane-wave topography; these dislocations are likely due to the relaxation of elastic macroscopic stresses caused by the lattice mismatch between the substrate and film. The critical thicknesses of plastic relaxation onset in CVD diamond films are calculated. The experimental techniques for studying the real diamond structure in optimizing crystal-growth technology are proven to be highly efficient.     

Influence of a.c. Electric Field on Infrared Absorption Spectra of Liquid Crystals and Determination of Orientational Order Parameter by Infrared Dichroism. II. Molecules with Weak Negative Dielectric Anisotropy

The influence of a.c. electric field (10 kHz) on the IR spectra of the nematic phase of six p-alkoxybenzylidene-p,n-butylanilines with weak negative dielectric anisotropy was studied and the temperature dependence of the orientational order parameter and threshold voltage have been investigated. The values of band elastic constants K₃₃ were calculated for MBBA and EBBA.     

Misfit Strain Relaxation by Dislocations in InAs Islands and Layers Epitaxially Grown on (001)GaAs Substrates by MOVPE

The misfit dislocation configurations in InAs islands as well as in more or less continuous layers grown on (001) oriented GaAs substrates were studied by weak-beam and high-resolution electron microscopy. The islands are confined by {101} and {111} facets where the aspect ratio (height/lateral extension) can be affected by the growth conditions. It is possible to grow well-defined islands as well as relatively continuous layers by MOVPE under As-stabilized conditions. At constant deposition parameters the growth is characterized by islands of different sizes (but with constant aspect ratio) in various strain states depending on their dislocation content. Coherently strained islands without any dislocation can be observed for heights up to 23 ML InAs, or otherwise, up to a maximal island extension of about 12 nm (for the particular aspect ratio ≈︂0.585). With further increase of island height and lateral extension, the introduction of dislocations becomes favourable. Independent of the island size, the layer thickness and the dislocation density, a residual elastic strain of about ε^r = —0.8% remains after relaxation. This means, about 88% of the total misfit strain of ε = —6.686 × 10^—2 were compensated by Lomer dislocations. These sessile Lomer dislocations lie in the island interior only, where single 60° dislocations were observed exclusively in their near-edge regions. With increasing island size and/or layer thickness some close-spaced 60° dislocations occur additionally within the interfacial region. The Lomer dislocations that are always located 4 monolayers (ML) above the InAs/GaAs interfacial plane result from the well-known fusion of two 60° slip dislocations. These 60° dislocations have been nucleated 7 … 8 ML above the interface at surface steps on the {111} facets confining the islands. Based on our experimental observations a new mechanism is proposed that explains the origin of these 60° dislocations. Their further fusion to sessile Lomer dislocations that compensate the misfit strain most efficiently occurs in the way as commonly accepted.     

The influence of laser annealing in the presence of longitudinal weak magnetic field on asymmetrical magnetoimpedance response of CoFeSiB amorphous ribbons

Asymmetrical magnetoimpedance (AMI) is very important to further improve micromagnetic sensor performance in terms of linearity and sensitivity. This behavior was investigated for Co_68.15Fe_4.35Si_12.5B₁₅ amorphous ribbons irradiated by a 1064 nm Nd:YAG pulsed laser in air and in the presence of 3 Oe longitudinal magnetic field with changing pulse repetition rate. Results indicate that for different pulse repetition rates, various types of AMI profiles appear. For the samples annealed in the presence of longitudinal field, because of induced anisotropy, rising in asymmetry factor takes place and another peak in the magnetoimpedance (MI) profile transpires.     

X-Ray Diffuse Scattering Study of Anisotropy of Elastic Properties in Silver Thiogallate

Measurements of the diffuse X-ray scattering intensity are performed from the (100) and (001) planes in the AgGaS₂ compound with tetragonal chalcopyrite-type structure. From these data the phase velocities of elastic waves along the principal symmetry directions for longitudinal and transverse polarizations as well as all six elastic moduli C_ik are found. The comparative analysis of both the anisotropy of the thermal diffuse scattering intensity distribution and the character of the elastic anisotropy for the ternary compound AgGaS₂ with the chalcopyrite structure and the binary analogs CdS and ZnS with the sphalerite structure is carried out.     

Efficiency of dislocations as sinks of radiation defects in fcc copper crystal

The sink efficiency of perfect dislocations for self-point defects (interstitials and vacancies) in fcc copper crystal has been calculated by the kinetic Monte Carlo method in a temperature range of 293–1000 K and a range of dislocation densities from 1.3 × 10¹² to 3.0 × 10¹⁴ m^?2. Screw, mixed, and edge dislocations with a Burgers vector 1/2<110> in different slip systems are analyzed. The interaction energies of self-point defects with dislocations are calculated using the anisotropic theory of elasticity. Analytical expressions are proposed for the dependences of the calculated values of dislocation sink efficiency on temperature and dislocation density.     

Discontinuous Freedericksz transition in the smectic C phase

The threshold field of the Freedericksz transition in planar oriented S_C monocrystals possessing positive dielectric anisotropy is calculated for two boundary conditions. It is shown that the threshold field is relatively low when a discontinuous transition is taken into consideration. The effective elastic constant is determined from the experimental data.     

On the Ability of Homogeneously Aligned Nematic Mesophases with a Positive Dielectric Anisotropy to Exhibit Williams Domains as a Threshold Effect

The ability of homogeneously aligned nematics with a positive dielectric anisotropy to exhibit Williams Domain as a threshold effect is numerically investigated. A simplified two-dimensional model which has, already, given satisfactory results in the negative dielectric anisotropy case is used. The frequency dependence of the threshold fields obtained for materials with small dielectric anisotropies is in satisfactory qualitative agreement with the known experiments. The frequency dependence of the threshold spatial wavenumbers is calculated. In accordance with earlier work, a positive dielectric anisotropy nematic which normally does not exhibit Williams Domains, does so under the action of a stabilizing magnetic field. This effect is confirmed and the frequency dependence of the threshold fields as well as the assoiated threshold spatial wavenumbers, for various values of the stabilizing field, are calculated. In addition, the model predicts the existence of a class of positive dielectric anisotropy nematics which do not form Williams Domains whatever the applied magnetic field is. This suggests a distinction between three classes of positive dielectric anisotropy nematics.     

On microcrack nucleation at twin vertices and boundaries in bcc and fcc metals

Nucleation of microcracks at the vertex and boundary of a decelerated twin is studied for a number of bcc and fcc metals. A twin and its boundary are represented as stepped pileups of twinning dislocations located in the neighboring glide planes. The formation of microcracks through the merging of head dislocations by the force and thermally-activated mechanisms is analyzed. Analytical expressions are obtained that describe the conditions necessary for microcrack nucleation at the vertices of stepped dislocation pileups. It is established that with an increase in the shear-modulus value, the critical parameters of microcrack nucleation by the two above mechanisms become closer in all the metals considered.     

高压下ReB2的结构特性及弹性性质

本文采用密度泛函理论中的赝势平面波法计算了ReB₂的P6₃/mmc晶体结构(即hP6-ReB₂)的结构特性及弹性性质。在计算了hP6-ReB₂的平衡结构参数后,从热力学、动力学及机械力学三方面验证了其结构稳定性。研究发现,hP6-ReB₂在高压下的弹性系数、各个弹性模量均随压强的增加而增大。泊松比显示hP6-ReB₂表现为脆性。三种类型的弹性波随压强的变化趋势显示hP6-ReB₂为弹性各向异性晶体。经估算,hP6-ReB₂结构的维氏硬度约为38.2 GPa。电子态密度揭示了hP6-ReB₂的Re—B和B—B之间存在着强共价键,并且随着压强的增加共价键逐渐增强。