Substrate-induced crystal plastic phase of a discotic liquid crystal

A new phase of a known discotic liquid crystal is observed at the interface with a rigid substrate. The structure of the substrate-induced phase has been characterized by atomic force microscopy, specular X-ray diffraction, and small-angle and wide-angle grazing incidence X-ray diffraction. The substrate-induced phase, which has a thickness of ～30 nm and a tetragonal symmetry, differs notably from the bulk phase. The occurrence of such phase casts a new light on alignment of discotic liquid crystals.     

Transient photoconductivity in amorphous thin films of Ge22Se78

Transient photoconductivity measurements on amorphous thin films of Ge₂₂Se₇₈ are reported in the present paper. The measurements were made in red light (660 nm) in the temperature range 302–352 K at various levels of illumination. The transient photocurrent was also studied for various illumination times. At all temperatures and intensities and for all illumination times the decay in the photocurrent was found to be non-exponential. The decay was found to be independent of temperature in the temperature range 302–322 K. However, it became faster at higher temperatures (above 322 K). The decay became slower as the intensity or illumination time was increased. The results are explained in terms of recombination within localized states in the mobility gap of this material.     

Numerical simulations of roughening temperatures in a hexagonal van-der-Waals4He crystal

We have carried out numerical simulations to investigate roughening on⁴He crystal surfaces. Algorithms were constructed for an h.c.p. crystal, incorporating van-der-Waals interatomic interactions. The Wulff plot was calculated at 0 K. Also, by sectioning the crystal in several symmetry directions, roughening temperatures, T_R, have been calculated for these orientations using a lattice-gas model. In particular, these have been determined for thec, a ands facets, as well as for lower-symmetry facets which involve interactions between higher-than-nearest-neighbours. On one of these facets, we have observed the formation of anisotropic clusters below a certain critical temperature. The values for the high-symmetry facets are more precise but in broad agreement with those calculated by Touzani and Wortis¹ and do not agree particularly well with the observed values. We are therefore investigating quantum corrections to the calculations.     

Modelling twinning evolution during plastic deformation in hexagonal close-packed metals

A new model describing the twin size and volume fraction evolution at the grain level is proposed. An evolution equation for the mean twin length on individual grains is expressed in terms of the local grain structure. This includes characterising the grain size and orientation distributions in the deformed specimen. Additionally, the twin volume fraction is predicted by computing the collective twin volume increments on each grain, if the grain structure is known. A twin nucleation-rate equation is proposed; it depends on dislocation activity, and the local twin and grain orientations. The model is applied to describe twinning behaviour in Be, Hf, Mg, Ti and Zr for various loading and texture conditions, including Y addition effects in MgY alloys. Twinning evolution is compared in Mg holding unimodal and bimodal grain size distributions.     

Traveling waves in 2D hexagonal granular crystal lattices

This study describes the dynamic response of a two-dimensional hexagonal packing of uncompressed stainless steel spheres excited by localized impulsive loadings. The dynamics of the system are modeled using the Hertzian normal contact law. After the initial impact strikes the system, a characteristic wave structure emerges and continuously decays as it propagates through the lattice. Using an extension of the binary collision approximation for one-dimensional chains, we predict its decay rate, which compares well with numerical simulations and experimental data. While the hexagonal lattice does not support constant speed traveling waves, we provide scaling relations that characterize the directional power law decay of the wave velocity for various angles of impact. Lastly, we discuss the effects of weak disorder on the directional amplitude decay rates.     

Transient phenomena in crystal growth systems

Summary In a model of the Czochralski crystal growth system the crystal is represented by a solid rotating plane adjacent to a semi-infinite region occupied by the melt from which the crystal is grown. The melt itself may be subjected to rotation at large distances from the interface. In a co-ordinate system advancing with the interface the crystal growth is modelled by a suction velocity at the interface. A transient analysis is performed following a discontinuous change of solute concentration in the bulk of the melt. Two cases are considered, namely (i) solid body rotation of crystal and melt and (ii) no melt rotation at large distances from the interface. The paper represents a re-examination and re-interpretation of earlier work by Burton, Prim and Slichter [1].     

Transient thermal lens in a ZnGeP(2) crystal

The first observations, to the authors' knowledge, of transient thermal lensing in a ZnGeP(2) crystal achieved with 2.09-μm laser excitation of 800-μs, 70-mJ pulses at 30 Hz and by burst-mode Q-switched pumping are presented. The laser power transmitted through an aperture was approximated by an adiabatic model with currently accepted values for the thermal properties of ZnGeP(2) and corresponded to focal-length changes from infinity to 10 cm during each 800-μs pulse. Similar results were seen when the crystal was operated as an optical parametric oscillator. This transient thermal lens severely limits ZnGeP(2) as a material for use in optical parametric oscillators for these modulated, high-power operating conditions.     

Tetragonal → twinned hexagonal crystal phase transformation in polybutene-1

The spontaneous transformation of the metastable (Form II) tetragonal crystals into the stable twinned hexagonal (Form I) crystals in polybutene-1 was studied using several techniques. The mechanical properties of the heat moulded material undergo significant changes in the process and the crystalline melting point increases from 112 to 128‡ C. Results from an Avrami analysis suggest the nucleation of the stable crystalline phase occurs shortly after crystallization from the melt and the subsequent growth of the nuclei follows a rod-like geometry. Transmission electron microscopy of melt-grown thin films shows that nucleation occurs at random positions within a spherulite and growth propagates along the radially oriented fibrillar crystals. As a result of multiple nucleation, each tetragonal crystal generates several twinned hexagonal crystallites with different crystallographic orientations. Besides exhibiting multiple nucleation within each individual tetragonal crystal, solution-grown single crystals also reveal twisting of the crystal lattice about thec-axis. The information obtained shows that residual stresses present in the material do not appear essential for the nucleation of the stable phase. Current concepts of the transformation mechanism are examined.     

Evolution of the spread of crystal orientation with plastic deformation in a cold-rolled Cu single crystal

Using scanning electron microscopy, we investigated how the microstructure of a Cu single crystal with a {15 12 9}〈9 10 3〉 orientation evolved from cold rolling. The first 50 % rolling caused its crystal orientation to rotate to {211}〈111〉. Although orientation splitting occurred near the surface of the single crystal, band-like regions with near-{211}〈111〉 orientations were still present after the fourth 50 % rolling. We measured the spread of crystal orientation in the near-{211}〈111〉 regions as a function of the total plastic equivalent strain induced by all rolling steps. When 50 % rolling was performed less than twice, the spread of crystal orientation was proportional to the square root of the plastic equivalent strain. Based on our results, we discussed the relationship between the spread of crystal orientation and the plastic equivalent strain generated by rolling.     

Analysis of cladded acoustic fibers of hexagonal crystal symmetry

A cladded acoustic fiber consisting of a core and an infinite thick cladding both having a hexagonal crystal symmetry is analyzed. The crystalline Z axes of both core and cladding are parallel to the fiber axis. Dispersion equations and field distributions for all modes are derived, and previously reported results for the isotropic case are retrieved. Numerical results for a few lowest-order pure guided torsional, radial-axial and flexural modes are shown. Material considerations are discussed.     

Nanoscale plastic deformation mechanism in single crystal aragonite

Molecular dynamics simulations have been performed to study the dynamic behaviors of single crystal aragonite under indentation, tension, and compression. The elastic modulus and hardness of single crystalline aragonite measured in our simulations are found in good agreement with experimentally measured values. Our simulation results show that the mechanical properties of aragonite crystal, including the elastic modulus, hardness, strength, and toughness, strongly depend on the crystallographic orientations and loading conditions. We have identified that this dependence is resulted from different deformation mechanisms, i.e., phase transformation, amorphous phase formation, dislocation, and twining. This work is an attempt to identify the deformation mechanisms in aragonite and to establish the relationship between the dominant deformation mechanisms and its crystallographic orientations and loading conditions.     

Nanostructuring with a crosslinkable discotic material

A high-yielding synthesis afforded a hexa-peri-hexabenzocoronene carrying acrylate units at the end of six attached alkyl spacers. The polymerization of these acrylate moieties could be initiated with thermal energy and through direct photoactivation without the addition of a photoinitiator. This allowed the organization of the liquid-crystalline material to be fixed in either the crystalline state or the mesophase, which preserved the organization in the respective phase. The use of a focused synchrotron beam permitted selected regions of a thin film to be rendered insoluble. After "developing" the film in this lithographic process by dissolving the soluble, unpolymerized material, defined nano-objects remained on the substrate. In addition, the pronounced aromatic pi stacking of the novel material allows an organization in mesoporous membranes that could be fixed by thermal crosslinking. After the removal of the inorganic template, mechanically stable nanotubes were obtained, which were characterized by different microscopy techniques.     

以苯并菲为基元的盘状液晶载流子传输材料的合成与性能研究

采用邻苯二酚、溴代正戊烷和乙酰氯为原料合成了以苯并菲为基元的盘状液晶化合物2，6，10-三乙酯基-3，7，11-三戊氧基苯并菲，化合物的结构采用核磁手段^1HNMR进行鉴定：采用偏光显微镜POM和DSC手段对该种化合物的相行为进行了研究；采用时间飞程谱（Time-of-flight）对该种材料的载流子迁移率进行了测试，发现载流子迁移率可达到10^-4cm^2·V^-1·s^-1数量级。     

Determination of the structural properties of a columnar hexagonal liquid crystal by light scattering

In this study we examine a new computer model of light scattering. Light propagation through a columnar hexagonal liquid crystal has been represented by using a cylindrical model. Numerical aspects of the light scattering process, which are based on numerically solving Maxwell's equations, have been calculated for a liquid crystal. We describe in detail the circular cylindrical model for computing light scattering from a columnar hexagonal liquid crystal and present the results of benchmark computations. We report results of extensive calculation for oriented columnar molecular systems. Our results are compared with previous studies on light scattering by other materials.     

Size-dependent nonlocal strain gradient modeling of hexagonal beryllium crystal nanoplates

International Journal of Mechanics and Materials in Design - In this paper, a size dependent numerical model for free vibration and bending analyses of hexagonal beryllium crystal (HBC) nanoplates...     

Analysis of weakly guiding cladded acoustic fibers of hexagonal crystal symmetry

An approximate analysis of the weakly guiding cladded acoustic fiber consisting of a core and infinite thick cladding both with hexagonal crystal symmetry is presented. The crystalline Z axis is parallel to the fiber axis. Weak guidance conditions require that the stiffness constants of the core are slightly less than those of the cladding, and the density of the core and cladding are almost the same. Approximate dispersion and cutoff equations are derived for all pure guided modes. Dispersion curves of several lower-order guided torsional, radial-axial and flexural modes are evaluated using both exact and approximate formulas and they are in good agreement. A simple approximate dispersion equation for leaky (longitudinal-type) modes is also obtained.