Dislocation Structure Of Crystals Research Articles

Fractal MTW Zeolite Crystals: Hidden Dimensions in Nanoporous Materials

Screw dislocation structures in crystals are an origin of symmetry breaking in a wide range of dense-phase crystals. Preparation of such analogous structures in framework-phase crystals is of great importance in zeolites but is still a challenge. On the basis of crystal-structure solving and model building, it was found that the two specific intergrowths in MTW zeolite produce this complex fractal and spiral structure. With the structurally determined parameters (spiral pitch h, screw angle θ, and spatial angle ψ) of Burgers circuit, the screw dislocation structure can be constructed by two different dimensional intergrowth sections. Thus the reported complexity of various dimensions in diverse crystals can be unified.

Concentration and structural inhomogeneities in highly doped GaSb(Si) single crystals

Concentration and structural inhomogeneities in highly doped GaSb(Si) single crystals grown under various conditions of heat and mass transfer are studied by methods of X-ray topography, high-resolution X-ray diffractometry, and digital image processing. It is established that the inhomogeneity of crystals is determined by specific features of impurity microsegregation during growth under conditions of nonstationary convection in a melt and by peculiarities of the dislocation structure of crystals. The processes related to the initial stage of the decay of the Si supersaturated solid solution in GaSb contribute considerably to the inhomogeneity of crystals on the micro- and macrolevels.

Comparison between superheating of crystals in melting and supercooling of melts in crystallization

The influence of surface tension forces and the dislocation structure of crystals on the formation of new phase nuclei in melting and crystallization is considered. It is shown that the main cause of the dramatic difference between superheating of the solid phase in melting and supercooling of the liquid phase in crystallization is surface defects.

Structural features of Ge(Ga) single crystals grown under conditions simulating the microgravity perturbation factors

Peculiarities of the real structure of Ge(Ga) single crystals grown under external mechanical perturbations of the crystallization process simulating actual microgravity environment aboard spacecrafts were investigated by X-ray topography and diffractometry methods, etching analysis and spreading resistance measurements. The applied perturbations included vibrations and variations of growth facility orientation with respect to the direction of gravity force. It has been shown that microinhomogeneity of the crystals grown is defined not only by formation of dopant growth striations at peculiar vibrational perturbations of a melt, but also by specific features of dislocation structure of crystals related with the formation of small angle boundaries, slip bands and other inhomogeneities in dislocation distribution. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The mechanics of deformation–induced subgrain–dislocation structures in metallic crystals at large strains

We present a streamlined limiting case of the theory of Oritz & Repetto for crystals with microstructure in which the crystals are assumed to exhibit infinitely strong latent hardening. We take this property to signify that the crystal must necessarily deform in single slip at all material points. This requirement introduces a non–convex constraint that renders the incremental problem non–convex. We have assessed the ability of the theory to predict salient aspects of the body of experimental data compiled by Hansen et al. regarding lamellar dislocation structures in crystals deformed to large strains. Although the comparisons with experiment are somewhat indirect, the theory appears to correctly predict salient aspects of the statistics of misorientation angles and lamellar–boundary spacings, and the scaling of the average misorientation and spacing with increasing macroscopic strain.

Modelling macro- and microstresses in fluorite crystals

An original statistical model based on the Monte Carlo method is proposed and tested that makes it possible to simulate dislocation structures in crystals and calculate the stresses that they create. By applying this model to fluorite crystals, it is shown that the contribution of the dislocations to the residual stresses is one of the most important such contributions. © 2003 Optical Society of America

Image Forces on 3D Dislocation Structures in Crystals of Finite Volume

AbstractThe present work aims at studying the image stress and image Peach-Koehler force fields for three-dimensional dislocation configurations in a single crystal of finite volume. It is shown that the image stress field is significant within the entire crystal volume, and that the image Peach-Koehler force can be of the same order of magnitude as the direct interaction force calculated from the infinite domain solution. The results demonstrate that image stress gives rise to long-range interaction forces that are important in meso-scale dynamics of dislocation structures.

Acidic dissolution mechanism of natural fluorapatite. I. Milli- and microlevels of investigations

Two different levels of acidic dissolution of natural fluorapatite crystals were investigated. The millilevel (i.e. for the whole crystals) was studied by a flow installation and optical microscopy. Dissolution process at this level was found to be determined by the dislocation structure of crystals, resulting in permanent increasing of dissolution rates for the whole crystals. The microlevel (i.e. for single etch pits) was studied by scanning electron microscopy. Dissolution process at this level was found to have random irregularities equal to 1.15 ± 0.05 μm, which appeared to be close to dimensions of the dislocation blocks. Both levels of acidic dissolution of natural fluorapatite were found to correspond with diffusion-controlled, polynuclear and etch-pit formation mechanisms, proposed in literature for dissolution of chemically pure apatites.

Dislocation structures in fatigued copper single crystals oriented for double-slip

Abstract It is known that two distinctive phases of dislocation structures, namely the wall-structure of the persistent slip bands (PSBs) and the vein-structure of the matrix, form in a fatigued single crystal. This letter presents TEM observations on copper crystals oriented for double-slip, [122], [012] and [1l2], which reveal that the dislocation structures in these crystals have a great variety and are different from those observed in crystals oriented for single-slip.

Effect of doping on formation of dislocation structure in semiconductor crystals

Numerous experimental data on the effects of dopants on the properties of dislocations and on the formation of dislocation structure in semiconductor single crystals grown from the melt are analyzed on the basis of model concepts derived from the results obtained in recent years (primarily by the authors of the present communication). Effects of dopants on generation, motion, and multiplication of dislocations are discussed in the high temperature range ( T ≳ 0.7 T melt), for the strress level typical for crystal growth τ/ G ≲ (10 -5−10 -4), dislocation density not exceeding 10 4−10 5 cm -2, and dopant concentrations far from the solubility limit. Changes in the dislocation structure of crystals caused by the effects of dopants on the generation, motion, and multiplication of dislocations are discussed. It is demonstrated that the changes caused by doping in the dislocation structure of semiconductor crystals can be explained on the basis of the relationship obtained between the doping impurities and the dynamic properties of dislocations.