Growth morphology of isotactic polybutene-1

Isotactic polybutene-1 has been crystallized from solution by the film formation method. Well-defined single crystals have been grown from solutions of the polymer in amyl acetate and its mixtures with n-butanol in all the three modifications by varying the concentration and crystallization temperature. Multilayered orthorhombic, tetragonal and hexagonal crystals with spiral ramps and terraces have been obtained from concentrated solutions near the turbidity temperature. Rotation of the screw dislocation as well as interlacing multilayered growth have been observed in the tetragonal morphology. Different types of twinning are commonly observed in the orthorhombic and hexagonal modifications. It is suggested that (a) spiral ramps grow by the mechanism of screw dislocation, (b) terraced crystals grow by the mechanism of Frank-Read source or the basal lamellae being hairy on the molecular level nucleating the next lamella, (c) movement of screw dislocation is very common in the orthorhombic and hexagonal modifications, (d) the solution temperature does not affect the morphology but the solvent concentration and the crystallization temperature decide the growth habit of the single crystals and (e) the growth mechanism of multilayered crystals and twinning are similar to those of monomeric substances.     

AFM investigation of the {101} surface morphology of l-arginine trifluoroacetate (LATF) crystals

The {101} surface of l-arginine trifluoroacetate (LATF) crystals has been investigated by ex situ atomic force microscopy (AFM). The main step sources were observed in the form of growth spirals. The growth spirals are rounded and highly anisotropic. Microcrystals transferred from aggregates deposit at the step-edged on the facets. It is suggested that the microcrystals may lead to the formation of macrodefects during growth. Etching experiments reveal circular hollow cores, indicative of both screw dislocation growth, and negative crystals generated by further crystallization in the entrapped liquid inclusions are investigated.     

A source of screw dislocation in polyoxymethylene crystals

Polyoxymethylene crystallizes as hexagonal lamellar crystals accompanied by spiral growth from dilute bromobenzene solution. Spirals of this polymer are formed by several mechanisms. There is discussion of various origins of screw dislocation due to interlocking of two independently growing crystals with crevices. After entanglement, various growth stages of crystals have been observed.     

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.     

Effect of the surface energy state on the dynamics of dislocations in ionic crystals

The effect of the surface state and boundary physicochemical conditions on the starting stresses and dynamics of near-surface dislocations in lithium fluoride and sodium chloride crystals has been investigated by the method of prick dislocation rosettes arising in alkali halide single crystals upon microindentation. It has been shown that crystals with ionic bonds are characterized by a surface energy (potential) barrier that hinders the movement of near-surface dislocations and influences their starting stresses in crystal lattices.     

Step Morphology of Liquid Phase Epitaxial GaAs Determined by Atomic Force Microscopy

Single, double and triple growth spirals were observed on liquid phase epitaxy(LPE) grown layers using atomic force microscopy (AFM). Spiral growth arises from the motion of growth steps in a spiral fashion. The LPE grown GaAs Layers show quite a different step morphology as compared with cleaved GaAs crystals. The step treads are atomically flat planes in the cleaved crystals. but are generally not atomically flat planes and rather bow upwards in the LPE layers. The mechanism arousing this difference is unclear.     

AFM study of morphological development during the melt-crystallization of poly(ethylene oxide)

The atomic force microscope (AFM) has been used to investigate morphological development during the crystallization of poly(ethylene oxide) (PEO) from the melt. PEOs with molecular weights of 1 × 10⁵ and 7 × 10⁶ were used. Height and amplitude images were recorded, using the tapping mode. For both polymers, the mode of spherulite development varied with the velocity of the growth front. For slow growth velocities, the growth of the crystallites was linear, with growth initially occurring by single lamellae, later developing into growth arms by screw dislocation spawning of crystallites. At intermediate growth velocities, stacks of lamellae develop rapidly. The splaying apart of adjacent crystals and growth arms is abundant. The operation of growth spirals was observed directly in this growth velocity range. The crystals formed by the giant screw dislocations diverge immediately from the original growth direction, providing a source of interlamellar splaying. At low and intermediate velocities, the front propagates by the advance of primary growth arms, with the regions between the arms filled in by arms growing behind the primary front. At the highest velocity observed here, the formation of lamellar bundles and immediate splaying results in recognizable spherulites developing at the earliest stages of crystallization. The change from linear growth to splaying and nonlinear growth are qualitatively explained in terms of driving force, elastic resistance and the presence of compositional and/or elastic fields in the melt. © 1998 John Wiley & Sons, Inc. J. Polym. Sci. B Polym. Phys. 36: 2311–2325, 1998     

Atom movement on a dislocated surface

The standard picture of growth at a screw dislocation assumes that the movement of adatoms on a dislocation loop is the same as on an ideal plane. We have examined this proposition by investigating the movement of a single tungsten adatom on a W(110) plane intersected by a screw dislocation. Surprisingly enough, adatom movement was entirely different than on a normal (110) plane: the overall diffusivity was higher, and the mobility varied with the location of the adatom relative to the dislocation core. This study demonstrates that surface transport is strongly affected in the vicinity of dislocations.     

An in situ atomic force microscopy study of uric acid crystal growth

Kidney stones are heterogeneous polycrystalline aggregates that can consist of several different building blocks. A significant number of human stones contain uric acid crystals as a crystalline component, though the molecular-level growth of this important biomaterial has not been previously well-characterized. In the present study, in situ atomic force microscopy (AFM) is used to investigate the real-time growth on the (100) surface of uric acid (UA) single crystals as a function of fundamental solution parameters. Layer-by-layer growth on UA (100) was found to be initiated at screw dislocation sites and to proceed via highly anisotropic rates which depend on the crystallographic direction. The smallest b-steps exhibited minimum heights corresponding to two molecular layers, while fast-moving c-steps more commonly showed monolayer heights. Growth kinetics measured under a range of flow rates, supersaturation levels, and pH values reveal linear trends in the growth kinetics, with faster growth attained in solutions with higher supersaturation and/or pH. The calculated kinetic parameters for UA growth derived from these experiments are in good agreement with the values reported for other crystal systems.     

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.     

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.     

Molecular Dynamics Studies of Dislocations in CdTe Crystals from a New Bond Order Potential

Cd(1-x)Zn(x)Te (CZT) crystals are the leading semiconductors for radiation detection, but their application is limited by the high cost of detector-grade materials. High crystal costs primarily result from property nonuniformity that causes low manufacturing yield. Although tremendous efforts have been made in the past to reduce Te inclusions/precipitates in CZT, this has not resulted in an anticipated improvement in material property uniformity. Moreover, it is recognized that in addition to Te particles, dislocation cells can also cause electric field perturbations and the associated property nonuniformities. Further improvement of the material, therefore, requires that dislocations in CZT crystals be understood and controlled. Here, we use a recently developed CZT bond order potential to perform representative molecular dynamics simulations to study configurations, energies, and mobilities of 29 different types of possible dislocations in CdTe (i.e., x = 1) crystals. An efficient method to derive activation free energies and activation volumes of thermally activated dislocation motion will be explored. Our focus gives insight into understanding important dislocations in the material and gives guidance toward experimental efforts for improving dislocation network structures in CZT crystals.     

几种极性有机晶体的生长习性与形成机理 2: 分子堆积、界面结构与晶体的习性

极性有机晶体在不同的溶剂中具有明显不同的生长习性, 主要有两个方面的原因: 一是极性有机晶体属非中心对称性晶类, 晶体具有极轴, 极轴的存在对分子堆积和晶体生长具有重要影响; 另一是极性有机晶体的界面结构不同, 溶剂与晶体界面的相互作用不同, 使得晶体同一面族的生长速率不同, 从而导致了晶体习性的改变。本文从几种典型极性有机晶体的分子排列和结构特征出发, 着重探讨了极性有机晶体的界面结构的差异对晶体习性的影响; 结合晶体生长界面与溶剂分子的相互作用进一步理解了晶体生长的溶剂效应; 通过理解极性有机晶体的习性机制, 探讨了晶体实际形态的控制。     

Solution crystallization of nylon 6

Electron microscopy and x-ray diffraction data have been obtained on nylon 6 which has been crystallized from solutions in 1,6-hexanediol and 1,2,6-hexanetriol. Lamellar single crystals and spherulites of the γ form are obtained by crystallization from 1,2,6-hexanetriol. The morphology of the single crystals is different from that obtained from glycerine solutions. The spherulites of the γ form are composed of larger lamellae. Sheaflike crystals of the α form are obtained from both solvents. α-form and γ-form crystals both grow from 1,2,6-hexanetriol at appropriate crystallization temperatures. α-form crystals alone are obtained from 1,6-hexanediol solution at every crystallization temperature. The long periods measured by small-angle x-ray diffraction for the solution-grown crystals are in the range 56 to 66 Å. The melting behavior of the solution-grown crystals is examined and discussed. Effects of solvent on growth of the two crystalline forms from solution are investigated.     

重(三硝基乙基)乙二硝胺的热分解 Ⅱ. 晶体性质与热分解动力学参量的关系

我们曾发现,重(三硝基乙基)乙二硝胺(BTNE)晶体热分解反应速度常数k的数值与该晶体重结晶条件有关系,而不同重结晶过程有可能给出性质各异的品体。在本文中,作者们利用差示扫描量热(DSC)、X-光多晶衍射、电子扫描显微镜(SEM)技术研究了工作[1]中提到的十种BTNE晶体,BTNE的单晶(BT-M)和未经任何重结品处理的实验室合成的BTNE粗品(BT-U)的晶体性质。利用DSC和X-光多晶衍射技术研究了BTNE晶体的晶型。同一晶体的不同晶型具有各自的X-光衍射图,而不同晶型间的转换可在DSC曲线上表现出存在有相应的吸热峰。工作[1]中给出的DSC曲线表明,     

光子晶体的制备与应用进展

光子晶体概念的提出到现在已有20多年,由于其特有的带隙和缺陷结构可实现对光子传播的控制,与传统的基于半导体晶体的电子技术相比更具优势,是未来光子计算机和光子通讯技术研发的核心。近年来,基于新材料、新方法制备的具有复杂结构的光子晶体不断涌现,其应用范围也从较早的波导、光开关、反射镜拓展到传感器、超棱镜、微透镜、太阳能电池、增强LED发光、光存储、生物芯片、仿生等新兴领域。本文结合了最新的研究状况,探讨了光子晶体的制备与应用进展。     

Growth Defects in Cubic KTa1-xNbxO3 Crystal

Potassium tantalate niobate （KTa1-xNbxO3, KTN） crystals with different dimensions and quality situations were grown by Czochralski method. Crystal growth process and morphology properties of KTN are presented in this paper. It was found that some defects, such as bubble, inclusion, crack, dislocation etc., can all appear if the crystal is grown in an improper condition. The character and formation mechanism of such defects in macro growth are discussed. We consider that the CO2, which was not released absolutely during the sintering process and dissolved in the melt, led to bubbles. The composition of the inclusion caused by high pulling and rotation rates is KTN polycrystalline. The crack and dislocation in KTN crystal mainly come from improper temperature field. Etching and high-resolution X-ray diffraction （HRXRD） experiment results indicate that the central area is the defects concentrated.     

Preparation and crystallographic characterization of a new endohedral,Lu3N@C80.5 (o-xylene), and comparison with Sc3N@C80.5 (o-xylene)

The trimetallic nitride template (TNT) approach has been successfully utilized to prepare the new endohedral Lu(3)N@C(80). Well-ordered crystals of Lu(3)N@C(80).5 (o-xylene) and Sc(3)N@C(80).5 (o-xylene) form upon cooling of o-xylene solutions of these endohedrals and they are isomorphous. Although the positions of the fullerene cage (which is fully ordered and located at a crystallographic center of symmetry) and the o-xylene molecules are nearly identical in these two structures, the positioning of the metal ions in the two crystals differ in significant ways. However, the expected difference in sizes of lutetium and scandium does not affect the dimensions of the C(80) cage. Nevertheless, the positions of the metal atoms do produce a slight outward dislocation of the immediately adjacent carbon atoms.     

Helical structure of single-crystalline ZnGa2O4 nanowires

Two unique helical zinc gallate (ZnGa2O4) nanostructures were synthesized by thermal evaporation using the zinc selenide (ZnSe) nanowires; helical ZnGa2O4 nanowire rolls either on a straight ZnSe nanowire support or without any support. They all consist of single-crystalline cubic ZnGa2O4 crystals without any dislocation over the entire helical structure and have four equivalent growth directions of 011 with the axial direction of [001]. We suggest that the lattice matching with the ZnSe nanowires would be an important factor in determining the growth direction of the helical ZnGa2O4 nanowires.     

Morphological evolution of PbS crystals under the control of l-lysine at different pH values: The ionization effect of the amino acid

PbS crystals were synthesized by a simple hydrothermal process with l-lysine as the crystal growth modifier. Depending on the initial pH value of the reaction solution, the morphologies of the PbS crystals varied and the crystals with shapes of truncated nanocube, star-shaped dendrite, comb-like dendrite were obtained. The morphological evolution of the PbS crystals was explained by the different state of the ionization equilibrium of l-lysine in the solution at different pH values. The presence of differently charged l-lysine molecules would influence the crystal growth rates along different directions and result in the final morphologies of PbS crystals. It is proposed that when amino acids are used as crystal growth modifiers, besides the types of amino acids, which have been regarded to influence the crystals' growth, the ionized state of the amino acids at different pH values would play an important role in the morphological control, and this point may have been neglected before.