Microstructure Of Single Crystals Research Articles

Ambient-condition growth of high-pressure phase centrosymmetric crystalline KDP microstructures for optical second harmonic generation.

Noncentrosymmetric potassium dihydrogen phosphate (KH2PO4 or KDP) in the tetragonal crystal phase is arguably the most extensively studied nonlinear optical crystal in history. It has prolific applications ranging from simple laser pointers to laser inertial confinement fusion systems. Recently, type IV high-pressure KDP crystal sheets with a monoclinic crystal phase having centrosymmetric properties have been observed. However, it was found that this new crystal phase is highly unstable under ambient conditions. We report ambient-condition growth of one-dimensional, self-assembled, single-crystalline KDP hexagonal hollow/solid-core microstructures that have a molecular structure and symmetry identical to the type IV KDP monoclinic crystal that was previously found to exist only at extremely high pressures (>1.6 GPa). Furthermore, we report highly efficient bulk optical second harmonic generation (SHG) from these ambient condition-grown single-crystalline microstructures, even though they have a highly centrosymmetric crystal phase. However, fundamental physics dictates that a bulk optical medium with a significant second-order nonlinear susceptibility supporting SHG must have noncentrosymmetric properties. Laue diffraction analysis reveals a weak symmetry-breaking twin-crystal lattice that, in conjunction with tight confinement of the light field by the tubular structure, is attributed to the significant SHG even with sample volumes <0.001 mm(3). A robust polarization-preserving effect is also observed, raising the possibility of advanced optical technological applications.

On the computation of diffraction peaks from discrete defects in continuous media: comparison of displacement and strain-based methods

This study introduces a numerical tool to generate virtual diffraction peaks from known elastic displacement or strain fields arising in the presence of discrete straight or curved dislocations in continuous media. The tool allows for the generation of diffraction peaks according to three methods: the displacement-based Fourier method of Warren, the Stokes–Wilson approximate method and a new average-strain-based Fourier method. The trade-off between the accuracy and the demand for computational power of the three methods is discussed. The work is applied to the cases of single-crystal microstructures containing (i) straight dislocations, (ii) low-angle symmetric tilt grain boundaries, (iii) a restrictedly random distribution of dislocations and (iv) complex microstructures generated by discrete dislocation dynamics, to illustrate the differences and domains of validity of the aforementioned methods. Dissimilar diffraction profiles reveal that peak broadening from dislocated crystals has additional contributions coming from strain gradients – a feature that is rejected in the Stokes–Wilson approximation. The problem of dealing with multi-valued displacement fields faced in the displacement-based Fourier method is overcome by the new average-strain-based Fourier method.

Synthesis of Nano-/Microsized KHCO3 Fibers via Quick Thermal Process and Its Toughness and Electron-Irradiating Degradation

High-density nano-/microsized KHCO3 fibers were first synthesized by using a quick thermal process. It was proposed that the thermal-treatment process, heat-treatment temperature, and stearate precursor played key roles for growing the KHCO3 fiber. It was found that the KHCO3 fiber exhibited sizable toughness due to its perfect single-crystal microstructure during bending experiment by using a nanomanipulator + SEM system. The electron-irradiating studies indicated that the monocrystalline microstructure of the KHCO3 fiber could be transformed into a polycrystalline K2CO3 phase under electron-beam illumination within TEM and SEM.

The principles of grain refinement in equal-channel angular pressing

Equal-channel angular pressing (ECAP) is a convenient processing tool for introducing very significant grain refinement, typically to the submicrometer level, in a wide range of metals. It is shown by experiment that processing by ECAP produces very similar microstructures in single crystals and in polycrystalline materials. Thus, after a single ECAP pass, aluminum single crystals and polycrystalline high-purity aluminum both exhibit microstructures consisting of bands of elongated subgrains and the experiments on single crystals have established unambiguously that these bands lie with their longer axes oriented parallel to the primary slip system. A model for grain refinement is developed incorporating the major experimental observations. Calculations of the shearing patterns for different processing routes lead to the conclusion that an equiaxed microstructure is achieved most rapidly in ECAP when slip occurs on three orthogonal planes over a wide range of angles: an example is route B C where the sample is rotated by 90° in the same sense about the longitudinal axis after every pass through the ECAP die.

TEM study of defects generated in 4H-SiC by microindentations on the prismatic plane

The deformation microstructure of single crystals of 4H-SiC resulting from microindentations on a prismatic surface was investigated by TEM. Indentations were performed at 400 and 675°C, i.e. below the brittle to ductile transition temperature of 4H-SiC (temperature close to 1100°C). TEM analysis reveals dissociated dislocations as well as extended stacking faults in the basal plane. In addition, perfect edge dislocations are observed on prismatic planes. From the observations, it is assumed that perfect dislocations are nucleated in the prismatic plane and cross-slip on the basal one where they dissociate.

Microscale simulation of martensitic microstructure evolution.

A new model for the evolution of multivariant martensitic microstructure in single crystals and polycrystals is developed. In contrast with Landau-Ginzburg models, which are limited in practice to nanoscale specimens, this new scale-free model is valid for length scales greater than 100 nm and without an upper bound. It is based on a thermodynamic potential in the volume fractions of the martensitic variants that exhibits an instability resulting in microstructure formation. Simulated microstructures in elastic single crystals and polycrystals under uniaxial loading are in qualitative agreement with those observed experimentally.

Kinetics, thermodynamics and microstructure of tungsten rods grown by thermal laser CVD

Single crystal and polycrystalline tungsten rods have been grown by thermal laser assisted chemical vapour deposition. The rods were grown from a reaction gas mixture of WF 6 and H 2 by focusing an Ar + laser on a tungsten wire substrate. The kinetics for the single crystal growth was investigated for different partial pressures. The growth rate was monitored in situ by inspection with a microscope and the deposition temperature on the tip of the rod was determined by a photoelectric pyrometric set-up. The activation energy was determined to 77 kJ mol −1 for a H 2/WF 6 ratio of 3 and to 50 kJ mol −1 for a H 2/WF 6 ratio of 5. The obtained reaction order of 3/2 with respect to the H 2 partial pressure is higher than the usual reported value of 1/2 for this process. The deposition rate showed no dependence on the WF 6 partial pressure i.e. the reaction order was 0. Thermodynamic calculations were performed for the experimental conditions to confirm the etching behaviour of non-reduced WF 6. Morphology and microstructure of single crystal and polycrystalline rods were investigated by scanning electron microscope and transmission electron microscope. The preferred growth direction for the single crystal rods was 〈1 0 0〉 while the polycrystalline rods showed a random distribution of directions.

Evolution of the Microstructure of Dynamically Loaded Materials

The paper deals with the evolution of the microstructure in materials after explosive loading by the method of a hollow thick-walled cylinder. The materials considered differ in the type of crystal lattice and initial state (grain size and initial defect density). The role of crystal structure in the formation of the microstructure of single crystals and coarse-grain copper specimens formed under explosive deformation is investigated. The microstructures formed are compared with the corresponding strains. It is shown that during high-rate deformation, fragmentation of the structural elements occurs at all scale levels. The fragmentation mechanism and the associated properties depend on the initial structure and state of the material. The special features of the microstructure evolution in materials revealed in this work are taken into account in producing new materials by dynamic and quasidynamic methods.

Effect of heavy ion irradiation on near-surface microstructure in single crystals of rutile TiO 2

Effect of heavy ion irradiation on near-surface microstructure in single crystals of rutile TiO 2

Effect of heavy ion irradiation on near-surface microstructure in single crystals of rutile TiO2

Abstract Radiation damage near the surface of rutile (TiO2) single crystals irradiated with 360 keV Xe2+ ions has been studied by combining Rutherford back-scattering spectroscopy and ion channelling (RBS/C) analyses with direct observations using transmission electron microscopy (TEM). lrradiations were performed at ambient temperature on TiO2 single crystal surfaces with (110) and (100) orientations, using ion fluences of up to 7 × 1019 Xe ions m−2. The RBS/C spectra revealed two damage peaks: one peak due to a surface damage layer and a second peak due to a buried damage layer near the mean projected range of the implanted ions. Cross-sectional TEM and high-resolution electron microscopy revealed that the surface damage layer consists of TiO2 crystallites with different orientations compared with the original single crystal.

Effect of Capacitor Discharge Welding on Single-Crystal Metals

The objective of this study was to investigate the use of capacitor discharge welding (CDW) as a suitable technique for joining metal single crystals by studying the effect of CDW on the microstructure of single-crystal copper. According to solidification theory, single-crystal formation requires high thermal gradients and moderate solidification rates. Therefore, an initial analysis was performed to determine if single-crystal CDW would result in a single-crystal microstructure. Joints were made using samples with joining surfaces at 0°, 15°, 30°, and 45° to the (111) crystal plane. Results show that single-crystal copper can be joined using CDW without the formation of voids and grain boundaries at the weld centerline. Average etch-pit densities after CDW are lower than those reported in the literature for single-crystal copper, suggesting minimal microstructural disruption. Crystal orientation does not seem to have any effect on fusion zone thickness or etch-pit density. Further work is needed to evaluate the effect of crystal orientation on void content in single-crystal copper welds made by CDW.

Mesa-supported, Single-crystal Microstructures Fabricated by the Surface/Bulk Micromachining Process

In fabricating microelectromechanical systems (MEMS), bulk micromachining using (100) and (110) single crystal silicon and surface micromachining using polycrystalline silicon are most commonly used. However, both micromachining methods have drawbacks, and fabricating actuating or sensing microdevices using single crystal silicon has been an active research topic in recent years. The surface/bulk micromachining (SBM) process using (111) silicon, developed by us previously, allows fabricating released structures in single crystal silicon. This paper extends the SBM process to fabricate released structures that are anchored to the substrate via mesa islands. This allows fabricating folded-type comb drive resonators. With the extension of the SBM process developed in this paper, any microstructure fabricated by the one structural polysilicon surface micromachining technique can also be fabricated in single crystal silicon.

Materials chemistry and thermodynamics of REBa2Cu3O7−x

AbstractThis review focuses on the materials chemistry and thermodynamics of the REBa2Cu3O7−x (RE = rare earth) series of superconductors. The importance of this basic information for predicting and controlling structure and microstructure of single crystals, thin films, and bulk materials is discussed. Examples of where an understanding of the thermodynamics has led to advances in processing are presented, and future areas of research that are key to the second decade of superconductor processing are highlighted.

Nuclear microscopy of Fe substituted 2212 BiSrCaCuO superconducting crystals

Crystals of the new generation BiSrCaCuO superconductors represent challenging materials to grow and fabricate into useful devices. Analysis of the material is also challenging. However the nuclear microprobe offers a possible solution to the problem of measurement of the stoichiometry, including oxygen, and sensitive mapping of micro-structures in single crystals. In the present paper we describe the results of nuclear microscopy of 2212 crystals, both as pure material and with up to 0.02 Fe substituted for Cu. We show evidence that the Fe is fully substituted into the crystal lattice.

Relationship between microstructure and optical properties in high Tc superconductors

Spectroscopic ellipsometry investigations on several groups of high Tc superconductors have been performed in the region of 1–5 eV. The optical data have been compared with results of structural studies made by transmission electron microscopy and X-ray diffraction. The dependence of the optical features in the dielectric function on the microstructure of single crystals has been determined. The microstructure in a Cu(1)-O layer has been shown to be closely related to the optical features in the region of 4 eV for (123)-type compounds. The characteristics features in the dielectric function spectra have been determined for single crystals of isostructural (2212)-type phases. In Pb-based (1212)-type single crystals a strong optical anisotropy in the a–b plane has been predicted to be related to an ordering of oxygen vacancies in the Pb-O plane.

Fundamental Research in Structural Ceramics for Service Near 2000°C

Structural ceramics for high‐temperature applications should embody the following properties: oxidation resistance, chemical stability, low volatility, resistance to creep deformation, resistance to creep cavitation at interfaces, sufficient toughness at ambient temperature, and thermalshock resistance. These criteria lead to five themes for fundamental research and design of high‐temperature structural ceramics: chemical and environmental stability, grain‐boundary sliding and cavitation, single‐crystal microstructure design, room‐temperature mechanical properties, and thermal shock. It is recommended that research that is confined to any one of these five areas takes into consideration the broader implications of research results. For example, microstructure designs that require weak interfaces for obtaining toughness at room temperature directly or indirectly conflict with creep and cavitation resistance needed for long‐term service at high temperatures. New research should be directed at mechanisms that can simultaneously achieve good mechanical properties over a wide range of temperatures. This paper addresses the following recommendations: (i) although non‐oxide systems can be viable for structural applications below 1500°C, oxidebased ceramics are necessary for service above 1500°C; (ii) microstructure designs based on acicular grain morphologies and/or single‐crystal fiber reinforcements have the potential for meeting the mechanical property requirements from room temperature up to very high temperatures; (iii) for fundamental studies of mechanical properties at high temperatures, simple uniaxial tension experiments should be used in tandem with four‐point bending and uniaxial compression experiments; (iv) the study of the reinforcement phase should center on very pure, highly stoichiometric materials in the case of non‐oxides, and on mixed and alloyed single crystals of cubic symmetry, or crystals having isotropic properties, and large unit cells in the case of oxides; (v) the study of interfaces in non‐oxides should focus on the chemistry of the intergranular glass phase, particularly the control of the oxygen content and the crystallization of this phase for improvement of high‐temperature properties; (vi) the study of interfaces in oxides is best directed at the relationship between interface structure, defect chemistry, and interfacial mechanical properties over a wide range of temperature; (vii) the understanding of the micromechanisms of thermal‐shock failure and the application of this understanding for designing graded interfaces that may be able to cope with thermal‐expansion stresses without leading to microfracture and cavitation is important in all classes of ceramic materials, and is of critical importance in the development of oxides for very‐high‐temperature applications; and (viii) research in processing science should emphasize the study of basic mechanisms that lead to in‐situ growth of acicular and fibrous microstructures.

Microstructural characterization using x-ray diffraction imaging

X-ray diffraction imaging, also known as x-ray topography, is a powerful tool to study the defect microstructure of single crystals. As the name implies, this technique is based on recording an image of the diffracted x-ray beam from a crystal. Contrast in the image results from point-to-point variation in the diffracted intensity through the crystal. An example of a diffraction image is shown in figure 1. That this image is in some way a topographic representation of the sample can be seen in the impression of differing elevations and textures in different parts of the image. However, since this image is a result of diffraction from the sample the interpretation of the image is much more complex.Diffraction contrast is usually separated into two types: mosaic contrast and extinction contrast. Mosaic contrast occurs for crystals considered to be formed from a collection of small perfect crystal blocks. These blocks have a well defined rocking curve width, the angular range over which they will diffract, and may be slightly misoriented with respect to each other and/or may have different lattice spacing.

Microstructure of single crystals of austenitic stainless steel

Auger spectroscopy and x-ray structure analysis have been used to study the chemical and phase inhomogeneity in single crystals of the austenitic nitrided stainless steel Cr22Ni17Mo3 with various morphologies of the dendritic structure. In single crystals grown in the 〈100〉 direction with a classical dendritic structure the interaxial regions are highly enriched with S, N, and C and weakly enriched with Mo, V, and Mn. At the same time these regions have a high density of particles of the type Me23C6, Me7C3, and vanadium carbonitrides, oriented in the growth direction. The axial regions have a subcellular structure, with oxygen-rich subcell boundaries. In single crystals with growth direction close to 〈110〉, in which only secondary dendrite axes are formed, the alloying elements, impurities, and second-phase particles are distributed relatively uniformly throughout the ingot.

Microstructure of superconductor bulk materials and thin films

The microstructure of single crystals of Y-Ba-Cu-O, Bi-Sr-Ca-Cu-O and Tl-Ba-Ca-Cu-O prepared by different methods were studied by various physical techniques, in particular by Raman microspectroscopy.Correlations have been established between Raman spectra and structure, allowing the determination of crystalline geometry of the material versus the composition and the preparation parameters.For YBa2Cu3Ox, a relationship: x = 0.025 VH - 5.57 is proposed and permits to evaluate the oxygen content by measuring simply in ZZ scattering geometry the Cu-O stretching mode V'H along the c axis of the crystal. This frequency varies from 465 cm-1 in YBa2Cu3O6 to 503 cm-l in YBa2Cu3O7 (Fig.l).This technique also offers the possibility to follow the structural conversion Y124 into Y123 and TI2212 into TI1212 (Fig.2) by appropriate thermal treatment. The preferentiel departure of the “CuO” or “TIO” chain/layer (Fig.3) is explained by weaker interaction forces between these layers and the adjacent ones, compared to those between the remaining layers. This helps to predict the localization of an eventual intercation in Y123 and TI1212.

Microhardness and microstructure in single crystals of mercuric iodide

Experimental studies were made of microhardness and microstructure on the (001) planes of vapor grown single crystals of HgI 2 using optical microscopy at room temperature and secondary electron emission and cathodoluminescence (CL) at 80 K in a SEM operated at 20 kV. Optical microscopy and etching techniques were used to identify three characteristic microstructures, each of which responded differently to microhardness indentation. These were regions (1) relatively free of etch pits, (2) containing a high density of relatively large, randomly distributed etch pits, and (3) containing orthogonal arrays of relatively small etch pits aligned with 〈100〉 crystallographic axes. The regions with random distributions of etch pits were softer than those without etch pits; the indentations in regions without etch pits exhibited brittle fracture; the regions containing orthogonal arrays of etch pits were particularly hard. Evidently the “orthogonal arrays” are much more detrimental to radiation detector performance than the “random arrangements”, since the very high Knoop hardnesses associated with the orthogonal arrays have been observed only in the worst detectors. The orthogonal arrays are apparently the traces of nested dislocation loops surrounding inclusions. In the SEM studies, it was determined that HgI 2 does exhibit strong CL image contrast at 80 K and does not pose problems of dissociation in the electron beam. Both ingrown electrically active defects (i.e. the orthogonal arrays) and electrically active defects produced by microhardness indentations were clearly resolved in the CL images. The indented samples exhibited well defined dark bands associated with dislocations on slip bands radiating from the indentations. Comparisons of microhardness measurements and microstructural observations suggest that ingrown microstructural defects that quench luminescence in HgI 2 are particularly likely to affect detector quality adversely.