Influence of Impurities and Testing Atmosphere on High-temperature Deformation of CsI Crystals

The influence of anion (bromine) and cation (cadmium and barium) impurities, as well as testing atmosphere (air, argon, vacuum) on high temperature deformation of CsI crystals is studied. This deformation in CsI has some peculiarities, which are the superplasticity signs and evidencing about instability of a lattice state. All the impurities studied caused the hardening, their effect in superplasticity depends on its solubility in the CsI lattice: the less the solubility, the stronger the effect. With testing in vacuum and argon, when the formation of point defect clusters occurs, enhanced the superplasticity evidence compared to that by testing in air, where clustering is prevented. These facts, as well as a quality resemblance of stress-strain curves of single- and polycrystalline specimens give the basis for the conclusion, that superplasticity is the manifestation of a plastic instability of the crystal, the instability centers may be precipitates and point defect clusters.     

Field-Induced Orientation Transition in Ferroelectric Liquid Crystals

Static and dynamic effects of a fast bistable device containing a ferroelectric liquid crystal are investigated theoretically. The switching mechanisms are different at low and high electric fields. There is a critical voltage which separates a bulk switching regime from the regime of domain wall motion. This threshold is determined as a function of physical device parameters.     

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.     

Shape Transformation of Plastic Crystals under the Effect of Surface Energy. Adamantane in Temperature Gradient Conditions

The spontaneous transformation of a polycrystal into a single crystal at high temperatures is observed with adamantane samples. Since the plastic crystal phase at such temperatures is not elastically strained it is concluded that the process is due to the surplus of surface energy in the system. The spontaneous transformation to the equilibrium form of crystals under temperature gradient conditions is investigated. The effect is established both of the surface energy and the shape of the isothermic surfaces.     

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.     

Dynamic Aspects of Dislocation Behaviour Near Low-Angle Boundary in Molybdenum Single Crystals

The dislocation behaviour was inventigated in specimens of monocrystals of molybdenium during in — situ stretching in the direction [001] near low — angle twist boundary which was close to be parallel to the direction of the external force. Low-angle boundary is easily penetrable for the fast moving nonscrew dislocations and acts as an effective stopper for screw dislocations which are quite parallel to the forming boundary dislocations. The mixed tipe dislocations emission by the low-angle boundary was noticed. On the stage when plastic deformation is performed mainly by the motion of screw dislocations the “relay-race”-like transmission of the dislocations motion through the boundary was observed.     

Ce:YVO4晶体的生长及其电荷迁移发光

为探索新型的白光LED荧光粉材料,采用提拉法生长了Ce2(CO3)3掺杂的Ce3+∶YVO4晶体,并对生长晶体的结构和光谱性能进行了表征。通过XRD测试,确定了由提拉法生长的Ce3+∶YVO4晶体的晶相没有发生变化。Ce3+∶YVO4晶体在450 nm处有一个较宽的发射带,在620 nm处有一个明显的发射峰。分析表明,在Ce3+掺杂的YVO4晶体中,铈离子主要以三价离子的形式存在,但在激发光照射下出现的620 nm发射表明有Ce4+存在,并且Ce4+与配位O2-形成了电荷迁移态(CTS)。     

Photo Controlled Surfaces in Rheology of Liquid Crystals

The modified method for shear viscosity measurements of nematic liquid crystals (NLC) oriented by surfaces is described. The main modification is concerned with usage of photoalignment technique for preparation of mono-domain samples with an orientation controlled by a polarization state of the secondary light irradiation. It provides the azimuthal rotation of a sample at any desirable angle. Thus different geometries of shear flows can be realized after filling the measuring cell with a liquid crystal. The method is experimentally approved at investigation of Poiseuille decay flows of 4-pentyl-4-cyanobiphenyl (5CB) through the channels with a rectangular cross section. The proposed construction of the cell with different surface treatment provides measurements of the three principal viscosities coefficients (Miesowicz’ viscosities) with additional usage of electric field. A small amount (less than 0.2 ml) of a liquid crystal needed for measurements and simple measuring procedure makes the method to be useful for rheological studies of newly synthesised liquid crystal materials.     

Switching Behaviour of Pb5Ge3O11 Single Crystals

Single crystals of lead germanate have been grown by Czochralski method. Hysteresis and switching pulses could only be measured after treating the crystal samples at elevated temperature and electrical a.c. field. Hysteresis is strongly frequency dependent. We observed the motion of the domain walls in polarized light simultaneously. The hysteresis loop is distorted at elevated temperature because of electric conductivity of the crystals. Temperature dependence of electric conductivity has been measured.     

InSeI单晶的制备及其结构与性能研究

利用化学气相传输法(CVT)制备了InSeI单晶。该晶体为黄色的针状物,晶体较脆。在室温下进行X射线衍射分析发现,其属于四方晶系,晶胞参数为a=b=1.864 3(5) nm,c=1.012 0(3) nm,V=3.517 2 nm³,空间群为I4₁/a。紫外可见光吸收光谱、光致发光光谱等结果显示该晶体的禁带宽度是2.48 eV,在一定波段光的激发下,InSeI单晶在600 nm左右有较宽的发射峰,表明该晶体的发光方式为缺陷态发光。介电温谱表明InSeI单晶在440 K时其四方相的结构发生了相变。     

Temperature and Strain Rate Effects on Work-Hardening of KCl Single Crystals

The influence of temperature and strain rate on work hardening of a KCl crystal has been investigated. Using the experimental data on flow stress relaxation, the activation “volume” for dislocation motion and internal counteracting stresses have been calculated in the temperature range of 300 to 40 K. Estimation has been carried out for the variation of a number of mobile dislocations ϱ₂/ϱ₁, when the strain rate is changed by a factor of four.     

Intramolecular Photochemistry and Energy Transfer in Molecular Crystals of [2.2] metacyclophane-1-ene and 1,2-diphenylcyclopentene

The intramolecular photocyclisation reactions of the stilbene analogues [2.2] metacyclophane-1-ene (MCPE) and 1,2-diphenylcyclopentene (DPCP) have been investigated in the solid state. In MCPE there is little or no energy transfer through the crystal prior to photoreaction, which occurs in regions of otherwise perfect crystal. This may in large part be due to a slow energy transfer rate because excited molecules relax to a distorted state which is only weakly coupled to its neighbours. In DPCP in the solid state photocyclisation will not occur, presumably for steric reasons. Energy transfer in DPCP is measurably slower than that expected for a rigid lattice.     

Light Scattering from Nematic Liquid Crystals in a Nonuniform Velocity Field

Fluctuations about a nonequilibrium steady state of a nematic liquid crystal subject to small, linear shear, are studied by means of a generalized Langevin method. We find a symmetric broadening of the director power spectrum and estimate the numerical size of the effect for a typical material.     

The Influence of High Temperature Predeformation on Ductility of LiF and BaF2 Single Crystals

The effect of high temperature predeformation on deformation characteristics of ductile LiF and brittle BaF₂ crystals was investigated. The same predeformation, as has been shown previously, leads to the increasing of the limited plastic deformation of MgO crystals by one order of magnitude. It was found that in both investigated crystals, as in the case of MgO crystals, at the test temperature T₂ ≅ 0.1 T_m limited plastic deformation before fracture obviously increases if the specimen was prestrained up to ε_p ≦ 1% at T₁ = 0.5 T_m. This increase is as higher as the plasticity of crystal is lower. In BaF₂ crystals it reaches a factor of 20 and more. The effect is connected with the creation of mobile dislocations during the high temperature predeformation and dislocaton sources for subsequent deformation at lower temperatures and with the homogeneity of the process of plastic deformation. These conditions prevent the appearance of dangerous places in which the fracture can begin.     

Ferroelectric Liquid Crystals

A review of the ferroelectricity in liquid crystals is given, beginning with a discussion of the symmetry properties allowing a macroscopic polarization in some of the more ordered liquid crystal phases. The fundamental mechanisms behind the dipolar ordering are discussed in some detail. The two linear electric effects, ferro- and flexo-electricity, give rise to completely different phenomena. In the smectic C phase there is one ferroelectric coefficient and nine flexoelectric ones, giving independent contributions to the polarization of the medium. We further discuss helical and non-helical structures, doped and intrinsic infinite-pitch ferroelectrics and, finally, the rich potential of liquid crystal ferroelectrics aligned in the »book-shelfgeometry» for fast electro-optic devices.     

Plateau Behaviour of Fatigued FCC Single Crystals

The mechanical behaviour of fatigued pure nickel monocrystals oriented for single slip was studied in push-pull tests at room temperature. Especially the cyclic hardening curves, the cyclic stress-strain curve and the shape changes of the hysteresis loops were investigated in the range of the plastic resolved shear strain amplitude, γ_ap, between 10^-10 and 10^-2. In this range the cyclic stress-strain curve exhibits a plateau which is related to plastic strain localization in persistent slip bands (PSBs) developing within the residual “matrix” volume. Using a two-phase model the cyclic saturation mechanical behaviour of the PSB and the matrix volumes has been determined. An explanation has been given of the constant plateau stress τ_B of the cyclic stress-strain curve by taking into account the nucleation stress for PSBs depending on the plastic resolved shear strain amplitude of the matrix volume. Further, the propagation rate of PSBs after strain amplitude increase in the plateau range has been calculated by applying the two-phase model.     

Liquid Crystals in Education

Abstract

The advancement of liquid crystal science and technology has been a truly interdisciplinary effort which combines basic principles of physics, chemistry, and engineering. The success of liquid crystal materials in flat panel displays and electro-optic applications can be attributed to the collaborations of scientists in various fields and the unique interface between academia and industry. Many scientists and engineers engaged in liquid crystal research are also educators in their respective disciplines andlor area of expertise. Those in academia routinely teach undergraduate and graduate courses in physics, chemistry, and engineering, and those in industry often find themselves teaching technical short courses and seminars. Many of us have visited elementary and high schools in the USA to speak to students about careers in science and engineering, and to present some eye-catching demonstrations that excite students about science and technology.     

高功率激光与“超热导”激光晶体

高功率固体激光技术的发展史就是一部与“废热”的斗争史,为抑制热效应对光束质量的不利影响,先后出现了热容激光器、薄片激光器、板条激光器以及光纤激光器,新的增益介质形态结合先进的散热技术将激光输出功率提升至百千瓦量级。固体激光增益介质的热学性能是限制激光功率进一步取得突破的重要瓶颈。因此,寻找具备超高热导率的激光晶体材料意义重大。本文介绍了上述四种激光器的基本原理及其在高功率激光方面取得的研究进展,从提高增益介质材料热导率的角度出发,对目前已有的方法和研究成果进行了分析与总结,对超热导激光晶体研究和高功率激光技术的发展进行了展望。     

对三联苯有机晶体的生长及性能

对三联苯是一种常见的有机闪烁剂,通常用作闪烁计数器的发光材料。对三联苯闪烁晶体具有对中子探测效率高以及不易潮解等特性,这使其在实际应用中具有广阔的前景。本文采用坩埚下降法,使用单层安瓿成功生长出φ12 mm×30 mm对三联苯晶体。在生长开始前通过差热分析,确定晶体的生长温度。生长完成后测试了晶体粉末的X射线衍射谱、摇摆曲线、红外光谱、荧光光谱和拉曼光谱。X射线衍射结果表明,生长的晶体为纯对三联苯相。从摇摆曲线结果可以看出,生长晶体质量良好。红外和拉曼分析结果显示,峰位并没有出现明显的偏移,表明晶体中杂质含量较少并未引起晶体分子化学结构的变化。荧光光谱没有杂质峰的出现也说明对三联苯晶体存在较少杂质或晶格缺陷。     

钙钛矿型闪烁晶体的研究进展

闪烁晶体材料一般可用于X射线、γ射线、中子及其他高能粒子的探测。经过100多年的发展,以闪烁晶体为核心的探测和成像技术已经在核医学、高能物理、安全检查、工业无损探伤、空间物理及核探矿等方面得到了广泛的应用。随着人们对闪烁晶体材料进一步深入的研究和科技的发展,现今市面上较好的LaBr₃∶Ce等卤化物闪烁晶体由于生产成本过高、各向异性及脆性等缺点逐渐不能满足发展的需要,而钙钛矿型闪烁晶体材料由于其容易被改善的潮解性、低的生产成本、易于调整的生长条件以及良好的闪烁性能,逐步进入人们的视野。本文从晶体结构、性能、生长方法、发展趋势和应用前景等方面,着重介绍了ABX₃(A⁺为Cs⁺,B²⁺为部分碱土金属离子,X^-为非氟卤族元素离子)钙钛矿型闪烁晶体材料和K基钙钛矿结构闪烁晶体材料。最终,通过掺杂部分稀土元素和改善生长工艺等方法,即可得到光输出高、能量分辨率好,且成本较低、可广泛应用于市场的钙钛矿型闪烁晶体。