TEM investigation of the microporous compound VSB-1: Building units and crystal growth mechanisms

Surface fine structure and structural defects in the open framework material VSB-1 have been investigated by electron microscopy. Crystal growth phenomena are proposed by a building unit model: (i) a unit is formed by two building units; (ii) they are linked to form first channels; and (iii) the whole network is grown via a layer-by-layer growth mechanism. A planar defect was observed in high-resolution transmission electron microscope (HRTEM) image taken with the [0001] incidence, and diffuse streaks related to the presence of defects were observed in a series of electron diffraction (ED) patterns. The microstructure model derived from the defect structure gives information on crystal growth. These defects highlight an open site that could be the pillar of a new crystal growth process. The study of defects and crystal growth is important in understanding physical properties such as catalytic or magnetic properties, and in synthesising a new open framework structure.     

Structural and optical properties of KTa0.77Nb0.23O3 nanoplates synthesized by hydrothermal method

Two-dimensional KTa(0.77)Nb(0.23)O(3) (KTN) nanoplates with edge sizes of about 100 nm and thickness of about 10 nm have been fabricated by hydrothermal method at 200 degrees C using Ta(2)O(5), Nb(2)O(5), and KOH served as the precursors. Detailed structural studies indicate that the synthesized products are made up of large quantity single crystalline nanoplates with quadrilateral shape, which have a cubic perovskite structure without any other impurity phase and (011) growth direction. The stacking faults may be the key in the formation and growth of the (011) plate. Room temperature photoluminescence spectra excited at different wavelengths exhibit a strong emission band centered at 470 nm (2.63 eV) as well as two weak emission peaking at 423 nm (3.06 eV) and 505 nm (2.46 eV), respectively. The electron-hole recombination of localized excitons should responsible for the light emissions at 423 and 470 nm. Room temperature Raman spectrum of KTN nanoplates reveal that the frequencies of vibration mode are lower slightly than that of KTN bulk materials, and the scattering profile is to be more diffused and enlarged, which may be induced by the crystal structure defects, such as stacking faults.     

Hydrothermal crystal growth of the potassium niobate and potassium tantalate family of crystals

Single crystals of KNbO₃ (KN), KTaO₃ (KT), and KTa_1−xNb_xO₃ (x=0.44, KTN) have been prepared by hydrothermal synthesis in highly concentrated KOH mineralizer solutions. The traditional problems of inhomogeneity, non-stoichiometry, crystal striations and crystal cracking resulting from phase transitions associated with this family compounds are minimized by the hydrothermal crystal growth technique. Crystals of good optical quality with only minor amounts of metal ion reduction can be grown this way. Reactions were also designed to provide homogeneous distribution of tantalum and niobium metal centers throughout the KTN crystal lattice to maximize its electro-optic properties. Synthesis was performed at relatively low (500-660 °C) temperatures in comparison to the flux and Czochralski techniques. This work represents the largest crystals of this family of compounds grown by hydrothermal methods to date.     

透明KTN薄膜的PLD法制备及其铁电性能研究

Using the Sol-Gel method for producing the KTN ultrafine powder and the sintering technique with K2O atmosphere to prepare KTN ceramics as the targets instead of the KTN single crystal, highly oriented KTN thin films were produced on the transparent single crystal quartz (100) by the pulsed laser deposition (PLD). The XRD analysis indicates that the perovskite structure with (h00) orientation is the major phase (>98%) in KTN. The Current-Voltage characteristics were found to be Ohmic at low fields and space-charge-limited at higher fields. This phenomenon is reasonably explained by SCLC theory. The leakage current was lower than 250 μA/mm2 at 0-5 V, which shows the film has good ferroelectric performances. The frequency dependence of dielectric constant results demonstrates that the dispersion of electric capacity is large at low frequency but small at high frequency. The dielectric constant was 12600 at frequency of 10 kHz. The P~E hysteresis loop shows that the remanent polarization and the coercive field are 9.25 μC/cm2 and 7.32 kV/cm, respectively. The SEM results shows that the surfaces are homogeneous, smooth, crack-free and dense. The refractive index was 1.776 at incident wave length 1.2 μm, the thickness 968 nm, and the growth rate 0.027 nm per pulse.     

Dislocation Analysis for Large-sized Sapphire Single Crystal Grown by SAPMAC Method

In this paper,large-sized sapphire (230×210 mm,27.5 kg) was grown by SAPMAC method (sapphire growth technique with micro-pulling and shoulder-expanding at the cooled center). Dislocation peculiarity in large sapphire boule (0001) basal plane was investigated by chemical etching,scanning electron microscopy and X-ray topography method. The triangular dislocation etch pit measured is 7.6×101～8.0×102 cm-2,in which relative high-density dislocations were generated at both initial and final stages of crystal growth. The analysis of single-crystal X-ray topography shows that there are no apparent sub-grain boundaries; the dislocation lines are isolated and straight. Finally,the origins of low-density dislocation in sapphire crystal are discussed by numerical analysis method.     

Effect of growth conditions on the structure of two-dimensional latex crystals: experiment

The growth kinetics and structure of two-dimensional crystals of fine latex particles on solid substrates have been studied using a variety of microscopic techniques: optical microscopy, surface plasmon resonance microscopy, transmission electron microscopy, scanning electron microscopy and atomic force microscopy. A circular-shaped crystal is grown from a thin layer of a latex suspension by a two-step mechanism: nucleation and crystal growth. Here we report an experimental study of the factors influencing the crystallization process, especially focusing on the water evaporation rate, the liquid meniscus at the crystal boundary, the particle size and concentration, the substrate, etc. Crystals of good quality and structure are grown at a high evaporation rate (low humidity) favoring a convection-dominated influx of particles from the suspension. The particle diffusion plays a role at suppressed evaporation thus causing an increase in the number of crystal defects. The dynamics of the meniscus slope leads to growth instability resulting in a sequence of multilayer rings. A hexagonal lattice prevails in the final crystal whereas a square lattice is observed in the transition regions between two different hexagonal multilayers. These general trends of the crystallization process are the same for different particle diameters (19 nm, 55 nm, 144 nm and 1.696 μm), volume fractions (0.001–0.01) and substrates (bare and metal-coated glass and mica). Received: 8 February 1999 Accepted in revised form: 12 May 1999     

Thermal conditions of growth and the necking evolution of Si,GaSb AND GaAs

The configuration of thermal gradient is illustrated for various types of crucible rotation, which is important for the creation of dislocations, which decreases along the grown axis of crystal. A new mechanism for dislocation elimination during the growth is proposed to explain this phenomenon, which provides a good agreement with the experimental results. The concentration of etch pits rapidly decreased from the beginning to the end of the crystals and the dislocation densities in the middle portion of all investigated crystals were found less than 102 cm^-2. The shallow vertical temperature gradients and virtually flat solidification interface prevented thermal stress from their building up in the crystals. As a result, the dislocation formation had random distribution. Using good necking procedures and choosing an appropriately oriented starting crystal with the shoulder angle <38.94° (assuming growth in <111> direction) it is possible to produce almost dislocation-free crystals without resorting to additional doping normally employed to reduce dislocation formation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effect of Cationic Valence State on Formation of Defects inCrystal of Thallium Hydrogen Phthalate（TAP）

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KDP晶体台阶生长动力学的激光干涉实验研究

采用迈克尔逊干涉技术,通过测量KDP晶体生长的法向速率和台阶斜率来研究其台阶生长的动力学系数、台阶自由能、溶质在边界层内的扩散特征以及激发晶体生长台阶的位错活性.实验表明, KDP中不同活性位错的台阶动力学系数差异较大,例如高活性和低活性台阶动力学系数分别为10.3×10-2和5.21×10-2 cm/s,位错源在晶体表面的形状、面积的变化,以及Burgers矢量的变化是造成晶体生长动力学测量数据重复性差的主要原因.     

Influence of SiO_3~（2-）Impurity on the Optical Properties of KDP Crystal

In this paper,potassium dihydrogen phosphate(KDP) crystals were grown in the presence of a series of silicate(SiO32-) impurity concentrations via conventional temperature cooling and rapid growth methods,respectively.It revealed that the SiO32-impurity could lead to the decrease of transmittance at the region of ultraviolet band for pyramidal sectors and slightly increased the transmittance for prismatic sectors.SiO32-could enter into the crystal lattice and create lattice defects,which consequently increased the density of light scatter.The decrease of laser damage threshold was attributed to the lattice defects and the redundant electrons brought by the replacement of SiO32-at the PO43-position.     

Radiation-induced solid-state polymerization in binary systems. II. Relationship between polymerization rate and physical structure of binary systems

The radiation-induced solid-state polymerization of binary systems consisting of acrylic monomer (acrylamide, acrylic acid) and organic compounds was investigated. In the previous paper on binary systems the authors reported that the rate of polymerization increased in the solid state (eutectic mixture systems). The mechanism of rate increase has been investigated by examination of phase diagrams, viscosities, and surface tension of the binary systems. Viscosity and surface tension are the measure of the molecular interaction of the two-component systems. In addition, the effect of linear crystal growth rate and half maximum width of the x-ray diffraction diagram of the crystallization process were determined. The larger the molecular interaction between the two components, the slower the linear crystal growth rate of monomer. The size of the monomer crystal decreases and the dislocation density of the monomer crystals increases in systems with large molecular interaction. Consequently it can be concluded that the physical structure of a binary solid system is the most important parameter determining the rate increase of solid-state polymerization. Dislocation on the grain boundary is more important than defects inside of the crystal lattice. It was found that the acceleration of polymerization rate is large in binary systems with larger molecular interaction. In some systems such as organic acid—amide systems with strong hydrogen bonds, glassy phases may be formed in which monomer may readily polymerize at very low temperatures.     

铅基复合钙钛矿型弛豫铁电单晶的生长基元与生长机理Ⅰ.PMNT单晶的表面形貌、负晶结构及生长基元的组装与拆分

铅基复合钙钛矿型弛豫铁电单晶体PMNT的生长基元为多种[BO_6]配位八面体,晶体生长过程可视为多种八面体基元与Pb~(2 )的组装过程.这些生长基元向{111}面叠合时易采取法向生长机制,向{001}面叠合时易采取层状生长机制,由此决定了晶体生长速度的各向异性与晶体的形貌.Bridgman法生长的PMNT晶体在生长过程中由内向生长机制形成规则的负晶结构;在晶体生长过程中,在其自然表面上可形成正形与负形两种形貌;在高温退火过程中,由于PbO的分解,晶体表面上可形成类似“蚀象”的构型,这些可从[BO_6]八面体生长基元的组装或拆分方面获得解释.     

Dynamic crack growth characteristics of half-circular disk PMMA specimens with prefabricated penetration defects under impact loading

In this paper, the dynamic fracture experiment of half-circular disk PMMA specimens with prefabricated penetration defects under impact loading was carried out on the digital laser dynamic caustics experimental system (DLDC), the law of crack initiation, growth and traversing under different tilt angles (30°, 45°, 60°) was examined, and the variations of crack growth trajectory, velocity and DSIF at crack tip were compared for analysis. In addition, the discrete lattice spring method (DLSM) as a new development was employed to simulate how the dynamic cracks grew in PMMA specimens with defects, it was found that the numerical model can reproduce the experiment phenomenon. On the basis, the effect of the elastic modulus of the penetration defect medium on crack growth was analyzed. Referring to the experiment and numerical results, it was thus concluded that the angle and the elastic modulus of penetration defect would dramatically influence the crack growth characteristics.     

Physical vapor transport and crystal growth of GeSexTe1−x solid solutions

Physical vapor transport studies of GeSe_xTe_1–x (x = 0.1, 0.2, 0.3, and 0.4) solid solutions demonstrated, that individual, large single crystals of these materials can be grown in closed ampoules. A compositional analysis of the grown crystals revealed, that the mass transport (crystal growth) process under steady-state conditions is pseudo-congruent and controlled by diffusion processes in the source material. From these experiments, the degree of nonstoichiometry (Ge-vacancy concentrations) of GeSe_xTe_1–x single crystals could be estimated. The effects of the cubic to rhombohedral phase transformation during cooling on the microstructure and morphology of the grown mixed crystals are observed. This work provides the basis for subsequent defect studies and electrical measurements on these crystals.     

Czochralski's creative mistake: a milestone on the way to the Gigabit Era

Much of the rapid change in industry, science, and society is brought about by the meteoric development of the microelectronics industry. Daily life is affected by this development; one has only to think of mobile telephones and the chips on modern credit cards. The raw material for microelectronics is the single crystal of silicon, with very high purity and almost perfect crystal structure. About 95% of the world's current production of silicon single crystals is achieved using the process that Jan Czochralski discovered in 1916. Today, single crystals of silicon can be grown that are up to 2 m long, 300 mm in diameter, and weigh up to 265 kg. The use of magnetic fields has led to significant advances in crystal-drawing technology. Intensive research and development reveals that in addition to the technology, which provides crystals of ever-increasing diameter, defect engineering, and the control of the numerous temperature-dependent reactions of crystal defects, are of paramount importance.     

Investigation of the crystallization kinetics of cyclotetramethylenetetranitramine from nitric acid by microcalorimetry

The total heat produced and the rate of heat production during the crystallization of cyclotetramethylenetetranitramine (HMX) from nitric acid are measured using a conduction calorimeter. The data of thermograms of HMX are treated based on the dislocation theory model. The results show that the crystal growth process of HMX accords with the dislocation theory.     

Dislocation approach to the plastic deformation of semicrystalline polymers: Kinetic aspects for polyethylene and polypropylene*

The plasticity of semicrystalline polymers is analyzed in the framework of Young's dislocation model under the assumption of nucleation of screw dislocations from the lateral surface of the crystalline lamellae. It is proposed that the driving force for the nucleation and propagation across the crystal width of these screw dislocations relies on chain twist defects that migrate along the chains stems and allow a step‐by‐step translation of the stems through the crystal thickness. Such defects are identified as thermally activated conformational defects responsible for the so‐called crystalline relaxation. Dislocation kinetic equations are derived. Plastic flow rates attainable by dislocation motion in polyethylene and polypropylene are assessed with frequency–temperature data of the crystalline relaxation. Comparisons are made with experimental strain rates that enable homogeneous plastic deformation. In addition to temperature, the crystal lamellar thickness, which is a basic factor of the plastic flow stress in Young's dislocation model, is a major factor in dislocation kinetics through its influence on chain twist activation. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 593–601, 2002; DOI 10.1002/polb.10118     

片晶折叠表面成核机制与结晶温度的相关性研究

利用自晶种方法,在邻二氯苯稀溶液中培养聚丁二酸丁二醇酯(PBS)晶体,系统研究了结晶温度对其晶体形貌的影响.使用PBS单晶作为研究对象,有效避免小尺寸观察不具有统计意义的缺点.在结晶过程中,通过改变结晶温度和自晶种温度,可有效调控稀溶液中生长的PBS晶体尺寸大小和晶体中缺陷的数量,得到了单层无缺陷的单晶、双层晶体和多层晶体等一系列PBS片晶.基于对不同实验条件下得到片晶的形貌和表面粗糙度的统计结果,提出晶体中可容忍的缺陷数量与结晶温度和晶种温度密切相关这一结论,通过建立热力学模型,定性分析了晶体中缺陷数量和结晶温度的依赖关系,从片晶表面粗糙度统计结果出发,提出高分子片晶折叠表面成核机制,较好地解释了实验中观察到的不同PBS晶体的形貌.     

Heterogeneous crystal growth of methane hydrate on its sII [001] crystallographic face

This paper presents a systematic molecular simulation study of the heterogeneous crystal growth of methane hydrate sII from supersaturated aqueous methane solutions. The growth of sII hydrate on the [001] crystallographic face is achieved through utilization of a recently proposed methodology, and rates of crystal growth of 1 A/ns were sustained for the molecular models and specific conditions employed in this work. Characteristics of the crystals grown as well as properties and structure of the interface are examined. Water cages with a 5(12)6(3) arrangement, which are improper to both sI and sII structures, are identified during the heterogeneous growth of sII methane hydrate. We show that the growth of a [001] face of sII hydrate can produce an sI crystalline structure, confirming that cross-nucleation of methane hydrate structures is possible. Defects consisting of two methane molecules trapped in large 5(12)6(4) cages and water molecules trapped in small and large cages are observed, where in one instance we have found a large 5(12)6(4) cage containing three water molecules.     

Studies on the growth, optical, electrical and spectral properties of potassium pentaborate (KB5) single crystals

The bulk single crystals of nonlinear optical material potassium pentaborate (KB5) have been grown by slow evaporation solution growth technique using water as solvent. The grown crystal was confirmed by single crystal X-ray diffraction studies. The structural perfection of the grown crystals has been analyzed by high resolution X-ray diffraction (HRXRD) studies by recording rocking curve. The photoluminescence (PL), UV-vis spectral studies were performed and the optical bandgap of the material was calculated. FTIR and FT-Raman measurements enumerate the functional groups present in the compound. The factor group analysis on KB5 reveals that there are 117 vibrational optical modes. The dielectric behavior of KB5 was investigated with different frequencies and temperatures.