Crystalline quality of the trigonal piezoelectric materials and effects of the extended defects

Many frequently used or promising piezoelectric materials belong to crystal classes 32 or 3m. Among them are α quartz and its crystallographic analogs (AlPO(4), GaPO(4), α-GeO(2), etc.), the numerous materials of the langasite (La-(3)Ga(5)SiO,sub>14) family and also lithium tantalate (LiTaO,sub>3) and lithium niobate (LiNbO 3). In this paper we study the present state of the art for these materials, indicate their principal point and extended defects, and present methods to reduce the dislocation density. Large concentrations of intrinsic point defects often exist in crystal grown at very high temperatures. The point defects (intrinsic or related to impurities) modify the constants and can increase the acoustic losses. This is the case for the alkali ions and the OH that induce severe losses in different temperature intervals. The extended defects also affect the performances of the piezoelectric devices. Some, such as twins, ferroelectric domains, or large solid or liquid inclusions, have very detrimental effects. Dislocations, growth bands, and planar defects are more difficult to avoid and affect the devices in a more subtle manner. In quartz and its analogs, dislocations seem to increase the nonlinear elastic effects and have a collective effect on the vibration modes, particularly in energy trapping resonators. Growth bands and stacking faults also produce similar effects.     

Characterization of defects in gallium arsenide

It is well-known that the properties of semiconductor materials including gallium arsenide are controlled by defects and impurities. The characterization of these defects is important not only for better understanding of the solid state phenomena but also for improved reliability and performance of electronic devices. We have been investigating the defects in gallium arsenide for several years using deep level transient spectroscopy, photoconductivity, transient photoconductivity, photoluminescence etc. Results drawn from our recent studies are presented here to illustrate some of the problems concerning transition metal impurities, process-induced defects, occurrence of intracentre transitions and metastability of deep levels in gallium arsenide.     

A new indirect method of measuring the contents of solid inclusions in liquid aluminium alloys

The introduction of derivative methods and the classification of three flow states (i.e. initial transient, steady and terminal transient stages) provide new approaches to understand the filtration behaviors of liquid aluminum alloys during the Prefil Footprinter tests. The effective-ness of the filtration equations of incompressible cake mode can be well identified over some steady stages during the course of the filtration tests. Based on these new findings a new indirect method of measuring the contents of solid inclusions in liquid aluminum alloys is devel-oped using the Prefil Footprinter tests. However, the benchmarks of specific cake resistance should be made for commercial aluminum alloy melts before this method becomes feasible in laboratories and industries. It is found that for a given volume concentration of solid inclusions, lighter inclusions usually lead to higher specific cake resistance, and the heavier solid inclusions reduce the specific cake resistance. For higher quality liquid metal, usually with lower lumped parameter (σ α), the types of solid inclusions have less influence on specific cake resistance.     

KDP晶体缺陷形成机理的研究

采用激光散射层析术及 X 射线形貌术研究了 KDP(KH_2PO_4)晶体中散射颗粒及位错等缺陷的形成机理。作者采用微分干涉衬差显微镜(DICM)装置、通过录相放大系统观察研究了晶体生长过程中有机和无机固态包杂物的成因。     

Distribution of Te inclusions in a CdZnTe wafer and their effects on the electrical properties of fabricated devices

We quantified the size and concentration of Te inclusions along the lateral- and the growth-directions of a ∼6 mm-thick wafer cut axially along the center of a CdZnTe ingot. We fabricated devices, selecting samples from the center slice outward in both directions, and then tested their response to incident X-rays. We employed, in concert, an automated IR transmission microscopic system and a highly collimated synchrotron X-ray source that allowed us to acquire and correlate comprehensive information on Te inclusions and other defects to assess the material factors limiting the performance of CdZnTe detectors.     

Investigation on defects in HPHT-grown diamond single crystals

In the diamond single crystals synthesized at high temperature and high pressure using FeNi as catalyst, there are usually supersaturated vacancies and inclusions formed during the diamond crystal growth and rapid cooling from high temperature. Some defects such as prismatic dislocation loops, stacking faults and array of dislocations are closely related to such supersaturated vacancies and inclusions. The supersaturated vacancies agglomerate into discs on the (111) close-packed planes, subsequent collapse of the discs forms the dislocation loops and stacking faults. The thermal internal stresses, which are caused by the difference of thermal contraction between the diamond and the inclusions due to the difference of thermal coefficients between them as the diamond is cooled from high temperature, may be relieved by the formation of array of dislocations. In the present paper, these defects in the diamond single crystals were directly examined by transmission electron microscopy (TEM). The characteristics and formation process of these defects were analyzed briefly.     

Effect of defects on electrical properties of 4H-SiC Schottky diodes

Most of power electronic circuits use power semiconductor switching devices which ideally present infinite resistance when off, zero resistance when on, and switch instantaneously between those two states. Switches and rectifiers are key components in power electronic systems, which cover a wide range of applications, from power transmission to control electronics and power supplies.

Typical power switching devices such as diodes, thyristors, and transistors are based on a monocrystalline silicon semiconductor or silicon carbide. Silicon is less expensive, more widely used, and a more versatile processing material than silicon carbide. The silicon carbide (SiC) has properties that allow devices with high power voltage rating and high operating temperatures. The technology overcomes some crystal growth obstacles, by using the hydrogen in the fabrication of 4H-SiC wafers.

The presence of structural defects on 4H-SiC wafers was shown by different techniques such as optical microscopy and scanning electron microscopy. The presence of different SiC polytypes inclusions was found by Raman spectroscopy. Schottky diodes were realized on investigated wafers in order to obtain information about the correlation between those defects and electrical properties of the devices. The diodes with voltage breakdown as 600 V and ideality factor as 1.05 were obtained and characterized after packaging.     

Cavity growth and grain boundary sliding in polycrystalline solids

A basic method is presented for the estimate of the overall mechanical response of solids which contain periodically distributed defects (inhomogeneities, regions undergoing inelastic flow, voids, and inclusions). This method is then applied to estimate the shape and growth pattern of voids that are periodically distributed over the grain boundaries in a viscous matrix. The interaction effects are fully accounted for, and the results are compared with calculations for a single void in an infinitely extended viscous solid, by Budiansky, Hutchinson, and Slutsky. Then, for a polycrystalline solid that undergoes relaxation by grain boundary sliding, the relaxed moduli are obtained, again fully accounting for the interaction effects. Finally, the overall inelastic nonlinear response at elevated temperatures is discussed in terms of a model which considers nonlinear power law creep within the grains, and linear viscous flow in the grain boundaries.     

Cupric oxide inclusions in cuprous oxide crystals grown by the floating zone method

Abstract

Phase-pure cuprous oxide (Cu₂O) crystals are difficult to grow since cupric oxide can form within the crystal as the crystal is cooled to ambient conditions. Vacancies are the solute which causes precipitation of macroscopic defects. Therefore, even when a mostly phase-pure single crystal is used as a feed rod, cupric oxide inclusions persist in the recrystallized solid. Control of the thermal profile during crystal growth, however, can improve phase-purity; a slow counter-rotation rate of the feed and seed rods results in fewer inclusions. Cupric oxide can be removed by annealing, which produces a factor of 540 ± 70 increase in phase-purity.     

Material defects as the basis of fatigue design

Microstructural inhomogeneities can exist in metals, such as precipitates and inclusions. These can be introduced purposely to strengthen the metal matrix (e.g.: CuAl₂ in α-phase aluminium) or by faulty manufacture (e.g.: large inclusions). A major part of the history of metal fatigue has been to eliminate, or at least reduce, detrimental precipitates, inclusions and manufacture defects such as scratches, surface roughness and shrinkages in cast irons since these can initiate fatigue cracks. The deleterious nature of foreign bodies or other forms of inhomogeneities, e.g.: pores or de-bonded zones within a microstructure are related to their size, position, shape, orientation and physical properties.Small sizes of precipitates and inclusions are to be preferred to large ones; such defects being less detrimental inside a grain rather than at grain boundaries where they can simultaneously affect two or three near-neighbour grains. The orientation of a defect is dangerous should it be inclined to the future direction of Stage I and/or Stage II crack growth planes. An important duty of a metallurgist is to reduce the effectiveness of these different forms of micro-defects produced during manufacture whilst a mechanical engineer is required to derive a suitable form of fracture mechanics in order to account for their behaviour in a quantitative fatigue fracture analysis.In this paper, some important types of microstructural defects will be illustrated and discussed in terms of their size, position, shape, orientation and properties, together with their effect on the fatigue resistance of a material. This will be coupled with a fracture mechanics (FMs) approach that quantifies their behaviour in terms of their relationship to fatigue crack propagation. In this approach, the boundary condition between failure and non-failure is explored using FM as a threshold condition for a small crack coupled with the Vickers hardness HV of the material that represents the condition (ΔK_th) for the onset of micro-plasticity (yielding) required for the growth of a crack from the defect and the non-propagation behaviour of the crack.Statistical scatters of microstructures, defects and inclusions are the major factors of statistical scatters of fatigue strength and fatigue life. Directions for optimizing microstructure to improve fatigue strength are explained from the viewpoint of equality control based on the statistics of extremes of defects and inclusions.A new efficient and reliable inclusion rating method for high strength steels based on the statistics of extremes using the phenomenon of hydrogen embrittlement is proposed.     

Dynamic behavior of a finite-sized elastic solid with multiple cavities and inclusions using BIEM

The 2D elastodynamic problem is solved for a finite-size solid containing multiple cavities and/or elastic inclusions of any shape that are arranged in an arbitrary geometrical configuration. The dynamic load is a tensile traction field imposed along the sides of the finite-size solid matrix and under time-harmonic conditions. Furthermore, the cavity surfaces are either traction-free or internally pressurized, while the inclusions have elastic properties ranging from very weak to nearly rigid. The presence of all these heterogeneities within the elastic matrix gives rise to both wave scattering and stress concentration phenomena. Computation of the underlying kinematic and stress fields is carried out using the boundary integral equation method built on the frequency-dependent fundamental solutions of elastodynamics for a point load in an unbounded continuum. As a first step, a detailed validation study is performed by comparing the present results with existing analytical solutions and with numerical results reported in the literature. Following this, extensive numerical simulations reveal the dependence of the scattered wave fields and of the resulting dynamic stress concentration factors (SCF) on the shape, size, number and geometrical configuration of multiple cavities and/or inclusions in the finite elastic solid. The pronounced SCF values invariably (but not always) observed are attributed to multiple dynamic interactions between these heterogeneities that may either weaken or strengthen the background elastic matrix.     

FGH96合金反复镦拔与挤压缺陷对比分析

对FGH96合金反复镦拔变形与挤压变形两种制坯工艺的夹杂缺陷水平进行了比较,并分析了不同变形状态下非金属夹杂物的形变特性。结果表明:反复镦拔变形后,锻件内检测到的非金属夹杂物数量有不同程度的增多,超标缺陷成倍增长;而FGH96合金挤压+等温锻造变形后,锻件内检测到的非金属夹杂物数量明显减少,纯净度水平有大幅提高。从微观夹杂物形貌的变形特性来看,等温锻造变形时在垂直于变形方向合金中夹杂物缺陷的尺寸增大;反复镦拔变形时,合金中原有非金属夹杂物缺陷聚集使可检测到的缺陷面积增大;而在挤压变形过程中,夹杂物缺陷在挤出方向被拉长成不连续的线状,每个方向上夹杂物的面积均减小;FGH96合金挤压+等温锻造变形后夹杂缺陷的大小主要由挤压变形后夹杂缺陷破碎情况决定;因此大挤压比变形可有效破碎合金中非金属夹杂物,改善锻件质量。     

钛合金冶金缺陷实例分析

对在工程实践中遇到的部分钛合金冶金缺陷实例进行了总结分析,并提出了相应的预防解决措施。实例分析结果表明：钛合金冶金缺陷形式主要有硬a缺陷、夹杂物、偏析、孔洞等。这些冶金缺陷主要是在钛合金的真空自耗熔炼过程中形成并遗传下的,且基本上可以通过提高原材料品质和改进熔炼工艺加以控制或消除。     

The role of defect size on the fracture toughness of powder processed Ti-10V-2Fe-3Al

In powder-processed Ti-10V-2Fe-3Al, the inherent microstructural defects of large pores and small chloride inclusions strongly affected fracture toughness. However, comparison of the fracture surface and sample inclusion size distributions showed that the smaller inclusions did not participate in the fracture process. In addition, the average size of the inclusions that did participate increased as the volume fraction of defects increased. When only the inclusions and pores that participated in the fracture process were combined into one defect distribution, ductile fracture toughness theory indicated that all theses defects were primary void nucleation sites. Fracture toughness was controlled by the spacing between the defects.     

The effect of substrate defects on epitaxial magnetic garnet films

The stress patterns associated with various defects in Czochralski-grown gadolinium gallium garnet (GGG) crystals have been observed using a polarizing microscope. The effect of these defects on both the surface topography and the magnetic behaviour of epitaxial magnetic garnet films grown on GGG substrates is reported. In particular, iridium inclusions and defects of a filamentary nature affect the surface topography. Other types of defect influence the magnetic behaviour of the films in a manner attributable to a change in the lattice parameter.     

Thermocapillary phoresis of inclusions with interphase surfactant

The motion of a spherical inclusion (gaseous, liquid or solid) submerged in an infinite (continuous) viscous media with a constant temperature gradient is considered. The analysis is carried out by considering negligible values of the Reynolds and Marangoni (Peclet) numbers and some other certain assumptions. The formulae for the thermocapillary phoresis velocity of the inclusions with the surfactant films are derived and can be applied for emulsions containing spherical drops, systems of liquids with solid or gaseous inclusions, e.g. at flotation, metal foaming, steel refining from the inclusions.     

液相夹杂复合软材料的设计、制备与力学性能研究进展

液相夹杂复合软材料是一类由功能液体或相变材料作为夹杂物的智能材料,由于其具备优异的变形特性和功能可设计性,近年来在柔性电子器件、可穿戴设备、软体机器人等领域得到广泛研究和应用。本文从以下几个方面回顾液相夹杂复合软材料的最新研究进展:首先,重点介绍非相变夹杂和相变夹杂复合软材料的功能设计及制备方法;然后,详细阐述液相夹杂复合软材料等效力学性能研究及尺寸效应;最后,简要探讨液相夹杂复合软材料研究所面临的挑战及值得关注的研究方向。     

新型高效非线性光学晶体ZCTC宏观缺陷的研究

研究了采用蒸发法生长的新型高效紫外非线性光学晶体ZnCd(SCN)4（简称ZCTC）中的溶液包裹物、负晶、开裂、生长条纹、扇形界及直线管道等宏观缺陷的形成。利用光学显微镜观察和分析了这些缺陷的形态、分布规律和形成原因，讨论了消除或抑制ZCTC晶体中缺陷产生的措施。     

Einflüsse unterschiedlicher Reinheitsgrade auf die Dauerschwingfestigkeit des Federstahles 55 Cr 3

Influence of Different Degrees of Purity on the Fatigue Strength of Spring Steel 55 Cr 3 One of the most important factors for construction and development of high strength springs is the material selection. In this connection the opinion is often represented in bibliography that material defects are the main reason for fractures. Besides surface defects (surface decarburization or rolling defects) essentially nonmetallic inclusions are described as causes of fractures. Therefore high degrees of purity are demanded especially for severe strained springs, which only can be obtained by very expensive special melting processes. By testing specimens of spring steel 55 Cr 3 with different degrees of purity in the present work the quantitative influence of nonmetallic inclusions on the fatigue life was to be examined. About 400 repeated torsion tests with annealed as well as annealed and shotpeened specimens were carried out accompanied by investigations to determine inclusions causing fracture and their size.     

Brownian motion of liquid lead inclusions along dislocations in aluminum

Nanosized solid Pb inclusions in Al have fcc structure and perfect cuboctahedral shape. When the inclusions melt they attain a rounded equilibrium shape and due to the low miscibility between the two elements, the liquid inclusions are retained as individual nanosized droplets located inside the solid Al matrix. As the temperature is increased the smaller liquid Pb inclusions are observed to move in a three-dimensional random walk over distances that can be orders of magnitude larger than their size. Inclusions attached to dislocations exhibit a similar type of random walk confined to long-distance one-dimensional movement along the dislocation lines and short-range two-dimensional vibrations perpendicular to the dislocation lines. The diffusion coefficients of the moving inclusions can be obtained from an Einstein-Smoluchowski analysis of the inclusion traces providing input for information on the mechanisms responsible for the movements.