The Free Surface Effect on the Dislocation Mobility in Molybdenum Single Crystals

The in-situ high voltage electron microscopic method has been used to study the phenomenon of spontaneous screw dislocation accelerations near the wedge-like edges of molybdenum single crystal specimens. The effects of either of the free wegde surfaces on dislocation mobility have been included in the discussion of this phenomenon. The experimental data were used to evaluate the surface image forces.     

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.     

半导体单晶生长过程中的位错研究

阐述了现有的半导体单晶位错模型,即临界切应力模型和粘塑性模型的基本理论及应用状况.分析了熔体法单晶生长过程中影响位错产生、增殖的各种因素,以及抑制位错增殖的措施.与熔体不润湿、与晶体热膨胀系数相近的坩埚材料,低位错密度的籽晶可有效地抑制生长晶体的位错密度;固液界面的形状及晶体内的温度梯度是降低位错密度的关键控制因素,而两因素又受到炉膛温度梯度、长晶速率、气体和熔体对流等晶体生长工艺参数的影响.最后,对熔体单晶生长过程的位错研究进行了展望.     

Dislocation Etch Studies on Diammonium Hydrogen Citrate Single Crystals

Diammonium Hydrogen Citrate single crystals have been grown from aqueous solution. The etch technique for defect studies is applied to these crystals. Propionic acid is shown to be a reliable etchant capable of bringing out dislocations intersecting the (001) face. Various other interesting features observed during etching are also presented and discussed.     

Dislocation Etching Behaviour of Hydrothermal and Organic Acid Etchants on Natrolite Crystals

Studies on etching of (110) cleavages of natrolite crystals in organic acids (viz., succinic acid, formic acid, and phthalic acid) and stream under high hydrostatic pressures have been made. It is observed that on successive etching of matched cleavages in any of these organic etchants, the pits get bigger both in regard to depth as well as lateral dimensions. Study of matched cleavages etched in any of these etchants indicates 1:1 correspondence of point bottomed pits. Similar results are obtained when matched cleavages are etched by the hydrothermal method. In contrast to such results, it is observed that the etch pit density on a cleavage etched by the hydrothermal method is always higher when compared with its matched cleavage etched in any of the given organic acids. Through a series of experiments, it is found that while corresponding to every etch pit due to any of these organic acids there is a pit due to hydrothermal etching, the converse is not true. It is suggested that the hydrothermal etching is more reliable so far as dislocation density estimation on (101) cleavages of natrolite is concerned.     

Dynamic Mechanical Behaviour of Some Cholesteryl Liquid Crystals

Dynamic mechanical studies of cholesteric liquid crystalline substances in the solid state have been reported using Du Pont's Dynamic Mechanical Analyser, DMA-981. All the substances showed multiple transitions. There is no common transition though the parent structure was similar in the compounds. These weak transitions are not detectable by DSC and hence appear to be second order transitions possibly arising due to rotation of the molecules around the long axis. Alternatively the lattice defects may also be responsible for such transitions.     

Saturation Dislocation Structures in Cyclically Deformed Nickel Single Crystals of Different Orientations

The major aim of this paper is to describe and characterize qualitatively the saturation dislocation structures in fatigued nickel single crystals with different orientations and to relate them to the mechanical properties. Within the investigated plastic strain amplitude region all measured cyclic stress strain curves exhibit two ranges with a sharp transition. In the first range of the cyclic stress strain curve the dislocation arrangement of the nickel single crystals oriented for single slip is significantly dependent on the plastic strain amplitude, that is the volume of the dislocation-poor regions decreases at increasing amplitude. In the second range ladder-like structures of the persistent slip bands and vein-like structures of the matrix are observed in agreement with the results of copper single crystals. Beside, at great amplitude, but within the second range, there is a steady increase of parquet-like matrix structures. On [001] single crystals the spatial dislocation structure is characterized by condensed and uncondensed {100} dislocation walls. The limited regions with condensed walls are supposed to be regions of high plastic strain amplitude (“mesoscopic” strain localizations). Amplitude changing tests are suitable to obtain results on the stability of the dislocation structures to increasing amplitude and decreasing amplitude, respectively.     

Effect of Crystallization Front Shape on the Dislocation Density in Germanium Single Crystals

Crystallography Reports - The effect of crystallization front shape on the dislocation density in Ge single crystals with a diameter of 100 mm, grown by the Czochralski method, has been studied....     

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.     

Dislocation Structure and Strain Localisation in Nickel Single Crystals Cyclically Deformed at 77 K

An experimentally based investigation is presented of the dislocation structure and of glide effects occurring in single slip oriented nickel crystals cyclically deformed at 77 K until saturation of the stress amplitude. Special attention is paid to a comparison of slip and structure phenomena observed in fatigue tests at 77 K and those found after cycling at room temperature (RT) and elevated temperatures. At strain amplitudes within the plateau region of the cyclic stress‐strain curve, where at higher temperatures in the crystal two structure types co‐exist, at 77 K nearly the entire specimen volume is occupied by one structure “phase”, a dislocation‐“condensed” wall configuration. On different scale levels the main characteristics of this extended wall structure were found to be independent of the imposed amplitude, and they turned out to fit in the temperature dependence of the structure features of the ladder‐like wall phase characterising the zones of intense slip (persistent slip bands) at RT and elevated temperatures. At 77 K the strain is localised in narrow slip bands (SBs) in the same way as at higher temperatures, although there is no indication of a “two‐phase” structure. From the experimental findings it is concluded that WINTERs “two‐phase” model remains valid, when averaging the plastic strain values over all SBs and over a sufficient number of cycles.     

Dislocation Photoconduction and Related Dislocation Memory Effects in KCl Crystals

Two types of memory phenomena are discussed, both related to dislocation photoconduction of highly pure and coloured KCl crystals, using the combination of Lyman- and F-illuminations. A very simplified explanation of the dislocation memory effects is given taking into consideration possible changes in the direction of the photocurrents. Some considerations may be of interest for the explanation of photoplastic effects.     

4 inch低位错密度InP单晶的VGF生长及性质研究

采用高压垂直温度梯度凝固法(VGF)生长了非掺、掺硫和掺铁的4 inch直径(100)InP单晶,获得的单晶的平均位错密度均小于5000 cm-2.对4 inch InP晶片上进行多点X-射线双晶衍射测试, 其(004)X-射线双晶衍射峰的半峰宽约为30弧秒且分布均匀.与液封直拉法(LEC)相比, VGF-InP单晶生长过程的温度梯度很低,导致其孪晶出现的几率显著增加.然而大量晶体生长结果表明VGF-InP晶锭上出现孪晶后,通常晶体的生长方向仍为(100)方向,这确保从生长的4 inchVGF-InP(100)晶锭上仍能获得相当数量的2~4 inch(100)晶片.由于铁在InP中的分凝系数很小,掺Fe-InP单晶VGF生长过程中容易出现组份过冷,导致多晶生长.通过控制生长温度梯度及掺铁量,可获得较高的掺铁InP单晶成晶率.对VGF-InP单晶的电学性质、位错密度及位错的分布特点、晶体完整性等进行了研究.     

Effect of Stressed State Type on Work Hardening and Evolution of Dislocation Structure in Magnesium Single Crystals

The type of stressed state (compression, simple shear) is shown to effect the parameters of a work hardening curve for Mg single crystals under basic slip. The electron-microscopic results for the dislocation structure are indicative of the fact that work hardening at stage A of the work hardening curve is connected with the basis-basis dislocation interaction, while the to-B-stage transition and a rapid growth of strain stress are due to the interaction between basic dislocations and “forest” dislocations in the {101 0} {112 0} systems, the latter dislocations arising from the stress relaxation in the primary slip system.     

Lifshitz-Like Behaviour in Ferroelectric Liquid Crystals

In order to explain the anomalous behaviour of the critical magnetic field and the helical pitch in the vicinity of the Λ-line and the Lifshitz point in the ferroelectric liquid crystal DOBAMBC a model involving not only chiral but also non-chiral biquadratic coupling between the polarization and the tilt has been proposed. The model predicts an “S - like” dependence of the modulation wave vector close to the smectic C* - smectic A transition. High temperature resolution measurements of the helical pitch and the intensity of the diffraction sattelites indeed support the proposed model and seem to show that the pitch is finite at T_c. In such a case the Lifshitz field H_L is finite too.     

Atomic Force Microscopy Determination of the Direction of Dislocation Lines in Single Crystals of Bismuth and Its Alloys

Crystallography Reports - A technique for determining the directions of dislocation lines using an atomic force microscope after selective etching of a single crystal is discussed. The technique...     

CMTD晶体的螺位错生长机理

用原子力显微镜在有机非线性光学晶体CdHg(SCN)4(H6C2OS)2(CMTD)的(001)面上观察到了许多奇特的心形螺旋生长丘,这种心形线不同于由反向双螺位错发展而成的瑞德环(Frank-Read).我们认为这种心形螺旋生长丘的相邻层交替生长除了与21螺旋轴有关外,还与在台阶源附近形成的结构畴有关.在由心形缺口推展出去的直线"划痕"两侧的台阶流上观察到的相互垂直分布的二维核,使我们认为直线"划痕"可能是90°的结构畴.实验结果说明晶体的结构及台阶源附近的分子键合方向及强度使不同晶体的生长具有独特的规律性.     

Connection of the Thermocapillary Flow Characteristics and the Impurity Distribution Pattern in Floating Zone Molten Molybdenum Single Crystals

The results of the impurity distribution in W-doped Molybdenum single crystals grown by electron beam floating zone melting are related to the characteristics of computed steady thermocapillary flow and impurity distribution within the molten zone. Particularly the influence of the number and the strength of eddies in the molten zone on the impurity distribution pattern in the grown crystal for different aspect ratios is considered.