Dislocation network patterns in tungstenite single crystals

TEM studies of tungstenite crystals grown by sublimation method have been investigated. Different types of dislocation network patterns were observed. The observed network patterns in bright field are compared with weak beam dark field pictures and the results are discussed.     

一个特殊单滑移取向铜单晶疲劳位错结构研究

为了更细致地揭示面心立方金属单晶体的循环变形机制,利用扫描电镜电子通道衬度(SEM-ECC)技术观察研究了Schmid因子为0.5的[41841]单滑移取向铜单晶体的循环饱和位错结构.实验表明,在单滑移铜单晶体中,胞结构除了在高应变幅下的循环变形中出现外,还可能出现在循环应力-应变(CSS)曲线平台区的较低塑性应变幅下.驻留滑移带(PSBs)会随应变幅的增大而在试样表面聚集成内部含有位错胞的粗滑移带,带内的位错胞结构被认为是由于带内滑移阻力增大引起的应变集中所致形成的.此外,CSS曲线高应变幅区起始部分对应的循环饱和位错结构观察揭示出迷宫结构和胞结构是由PSBs逐渐演变而成的.     

Dislocation density in electrolytically-coloured KCl crystals

Dislocation densities have been investigated in potassium chloride crystals, electrolytically coloured in the temperature range of 550–710°C. The results show an increase in the dislocation density with coloration temperature upto 650°C and decrease thereafter. This is attributed to the movement of dislocations and interactions between them during electrolytic coloration of the crystals.     

用脉冲电流处理后疲劳铜单晶的位错行为

将[233]共面双滑移取向的铜单晶体在两种恒塑性应变幅下进行循环疲劳，形成密度不同的位错结构．用高密度脉冲电流对疲劳铜单晶体处理后，试祥中位错的结构由单纯的脉络结构转化成位错胞状结构．高密度脉冲电流处理引起的热压应力不但加强了主滑移系位错的运动，还使共面次滑移系开动，在主滑移系位错和共面次滑移系位错的共同作用下导致位错胞状结构的形成。     

Dislocation structures on the plane of shear in compressed double-notch copper single crystal specimens

Dislocation structures in the zone of shear of double-notch copper single crystals, generated by a shear stress 20 MPa acting in the specimen axis ([110] direction) at 773 K were investigated by means of transmission electron microscopy. Dislocation configurations in different crystal planes of shear, and (001), were compared. The role of the 110{111} slip systems in the process of shear is assessed and possible indications of a direct glide of dislocations with Burgers vector b=a/2[110] in the mentioned compact and non-compact planes are presented.     

The influence of defects on ferroelectric and pyroelectric properties of Pb(Mg1/3Nb2/3)O3-0.28PbTiO3 single crystals

Relaxor-based ferroelectric single crystals 0.72Pb(Mg_1/3Nb_2/3)O₃-0.28PbTiO₃ (PMN-0.28PT) were grown by a modified Bridgman technique. The direct current (dc) conductivity was investigated and corresponding conduction mechanisms were discussed. V^″Pb−VO defects are dominant from 245 °C to 650 °C. The ferroelectric properties of [1 1 1]-oriented PMN-0.28PT were systematically investigated, with the coercive field (E_c) of 5.2 kV cm⁻¹ and remnant polarization (P_r) of 37.8 μC cm⁻² at room temperature. Moreover, the dielectric and pyroelectric performances of PMN-0.28PT were measured and the integrated pyroelectric performances greatly enhanced after annealing in oxygen at 500 °C for 20 h. This is due to the decrease of oxygen vacancies in the single crystals when being annealed in the oxygen-rich atmosphere. These make [1 1 1]-oriented PMN-0.28PT crystals a promising candidate for infrared detectors and thermal imagers used at room temperature.     

The Influence of Elastoplastic Deformation on the Dislocation Structure of 15Kh2MFA Steel

The paper deals with dislocation structure of 15Kh2MFA steel upon static tension and elastoplastic strain cycling is studied. The dislocation density within small-angle boundaries is shown to grow with increasing plastic tensile strain and, in the case of elastoplastic cycling, with increasing strain amplitude and number of cycles. The growth of the dislocation density within small-angle boundaries upon elastoplastic cycling results in a smaller shear stress.     

Defects in liquid crystals (II)

Defects in the disordered (uniaxial) liquid crystals, nematic, smecticA and cholesteric, and the use of topological analysis in classifying them, are discussed. While the latter is very successful in classifying defects in nematics, it fails to do so in the case of smecticA and cholesteric liquid crystals because of geometrical constraints. However, topological arguments have been partially successful in predicting some of the defects in cholesterics. The known features of the isotropic (cubic and amorphous) cholesteric blue phases are summarised and the various theoretical models picturing them as defect lattices, are also discussed briefly.     

Electronic Doping Controlled Migration of Dislocations in Polycrystalline 2D WS2

Migration of dislocations not only determines the durability of large‐scale nanoelectronic and opto‐electronic devices based on polycrystalline 2D transition‐metal dichalcogenides (TMDCs), but also plays an important role in enhancing the performance of novel memristors. However, a fundamental question of the migration dependence on the electronic effects, which are inevitable in practical field‐effect transistors based on 2D TMDCs, and its interplay with different dislocations, remains unexplored. Here, taking WS₂ as an example, first‐principle calculations are used to show that the electronic contributions arising from defect states can greatly influence the migration barriers of dislocations. The barrier height can be reduced by as much as 50%, which is mainly attributed to the change in electronic occupation and the band energy of defect levels controlled by electronic chemical potential (Fermi level). The reduced barriers in turn lead to significantly enhanced migration, and thus the plasticity. Since defect levels from dislocations locate deep inside the bandgap, the doping‐induced tuning of barrier height can be achieved at relatively low doping concentration through either chemical doping or electrode gating. The effective electromechanical coupling in 2D TMDCs can provide new opportunities in material engineering for various potential applications.     

Studies on Ce2(C2O4)3·nH2O crystals grown in hydro-silica gel

Single crystals of cerium oxalate are grown by the gel method. Ce(NO₃)₃ is allowed to diffuse into a gel in which oxalic acid is incorporated. Cerium oxalate crystals are formed by chemical reaction and the growth process is observed. Crystals are yellow-white in colour. Morphology and size of the crystals are highly influenced by the acidity of the feed solution. With 30% HNO₃ long needle shaped, with 50% HNO₃ thin hexagonal and with 100% HNO₃, well developed crystals are obtained. Natural etch pits are observed on the grown crystals due to the presence of HNO₃. Appearance of a single, large interfacial crystal is observed for 0·5 M cerium nitrate. IR spectrum confirms the presence of water molecules and carboxylic group. X-ray diffractogram gives well defined peaks. Peaks are indexed. Unit cell dimensions are determined. Thermal analysis of the samples done using TGA and DTA show the release of water molecules (endothermic) and of CO and CO₂ (exothermic) with the rare earth oxide as stable residue.     

Low Energy Dislocation Structure in Deformed Metals

In this paper,the dislocation distribution struc-ture in deformed metal is discussed.The flow stressof material for the heterogeneous dislocation distri-bution which tends to the flow stress for the homo-geneous dislocation distribution in the limiting caseis derived.The causes and the effects of the longrange internal stresses are discussed.The totaldeformation energy of material system is obtainedand the trend of evolution of dislocation distribu-tion in deformed metals is discussed simultaneously.No micromechanisms of dislocations are involvedin the discussion,therefore the theory developed inthis paper is universal.     

二氧化硅胶体晶体制备方法进展

SiO2 胶体晶体制备过程的研究是许多新型功能材料发展的基础 ,对人们进一步了解物质在纳米、微米尺度的许多特殊性质也将具有指导意义。本文综述了SiO2 胶体晶体制备方法的进展。     

Photonic band gap in 1D multilayers made by alternating SiO2 or PMMA with MoS2 or WS2 monolayers

Atomically thin molybdenum disulfide (MoS₂) and tungsten disulfide (WS₂) are very interesting two dimensional materials for optics and electronics. In this work we show the possibility to obtain one-dimensional photonic crystals consisting of low-cost and easy processable materials, as silicon dioxide (SiO₂) or poly methyl methacrylate (PMMA), and of MoS₂ or WS₂ monolayers. We have simulated the transmission spectra of the photonic crystals using the transfer matrix method and employing the wavelength dependent refractive indexes of the materials. This study envisages the experimental fabrication of these new types of photonic crystals for photonic and light emission applications.     

The preparation and characteristics studies of microencapsulated cholesteric liquid crystals (I)

Abstract

This paper deals with the enmicrocapsulation of cholesteric liquid crystals by the colloid coaversation process. This incident light reflection study reveals that the encapsulated liquid crystal films sustain highly sensitive and precise temperature gradients with responding abilities determined by their iridescent coloration.     

X-ray topographic analysis of dislocation line defectsin calcium fluoride crystals

One-to-one correspondence of dislocation etch pits have been established on the matched cleavage faces and on the opposite sides of thin flakes of calcium fluoride crystals. By selecting 022 and 022 reflections and MoKα₁ radiation, stereopair projection x-rays topographs were studied and critically compared with optical micrographs. The dislocation etch pits and dislocation out crop images show a close resemblance. The orientation of the Burgers vectors of the dislocation lines has been identified and these lines lie parallel to the <110> directions. The growth history of the stratigraphical pattern has been studied using x-ray topographic technique.     

Magnetic AC susceptibility in Bi2Sr2CaCu2O8 single crystals

We measured the temperature dependence of the ac susceptibility in Bi₂Sr₂CaCu₂O₈ single crystals. The penetration fieldH* was determined, at different temperatures, from the field dependence ofX″ in the framework of the critical state model. Using a very simple formula, we estimated the values of the critical current density near T_c, which are consistent with the usually reported data for this material.     

NaCl改性SiO_2微球的制备及其自组装光子晶体研究

为提高SiO2微球的表面电荷密度,通过改进Stober法,引入电解质NaCl合成SiO2微球,并采用垂直沉积法制备出光子晶体.通过Zeta电位粒度仪、带EDS能谱仪的场发射扫描电子显微镜(SEM)和紫外-可见-近红外光谱仪对其电学性能、显微形貌和光学性能进行测试分析.Zeta电位测试结果显示改性SiO2:微球的Zeta电位平均提高11.39mV;EDS能谱分析表明微球中含有钠元素;SEM照片表明样品平均粒径为334 nm,平均标准偏差小于5%,所得光子晶体为面心立方密排结构;吸收光谱表明在725nm处具有光子晶体带隙.     

Polymorphism in molecular crystals

Reports concerning polymorphism in molecular crystals published in the period 2007–2008 are reviewed. A general survey is given of structural and crystallisation studies in polymorphic systems, and selected advances are described in more detail. These include application of contemporary crystallographic techniques to obtain comprehensive structural information for established polymorphic compounds, multi-disciplinary experimental and simulation studies to probe the mechanisms of polymorph crystallisation, exploration of crystallisation confined within nanopores and characterisation of single crystals containing distinct polymorphic domains.