Showing papers on "Electron backscatter diffraction published in 1996"

Characterization of dislocations in GaN by transmission electron diffraction and microscopy techniques

Abstract: A combination of transmission electron microscopy imaging and diffraction techniques is used to characterize crystal defects in homoepitaxial GaN thin films. The Burgers vectors of dislocations is established by combining large‐angle convergent beam electron diffraction and conventional diffraction contrast techniques. It is shown that dislocations with Burgers vectors c, a, and c+a are present. Evidence is presented that dislocation segments lying in the interfacial plane are dissociated on a fine scale. The significance of the observations for understanding homoepitaxial growth of GaN is discussed.

Phase Identification in a Scanning Electron Microscope Using Backscattered Electron Kikuchi Patterns.

Abstract: Backscattered electron Kikuchi patterns (BEKP) suitable for crystallographic phase analysis can be collected in the scanning electron microscope (SEM) with a newly developed charge coupled device (CCD) based detector. Crystallographic phase identification using BEKP in the SEM is unique in that it permits high magnification images and BEKPs to be collected from a bulk specimen. The combination of scanning electron microscope (SEM) imaging, BEKP, and energy dispersive x-ray spectrometry holds the promise of a powerful new tool for materials science.

Scattering and Diffraction

Abstract: The electron is a low mass, negatively charged particle. As such, it can easily be deflected by passing close to other electrons or the positive nucleus of an atom. These Coulomb (electrostatic) interactions cause the electron scattering which is the process that makes TEM feasible. We will also discuss how the wave nature of the electron gives rise to diffraction effects. What we can already say is that if the electrons didn’t scatter, then there would be no mechanism to create TEM images or diffraction patterns and no source of spectroscopic data. So it is essential to understand both the particle approach and the wave approach to electron scattering in order to be able to interpret all the information that comes from a TEM. Electron scattering from materials is a reasonably complex area of physics, but it isn’t necessary to develop a detailed comprehension of scattering theory to be a competent microscopist.

Atomic structure and faulted boundaries in the GaAs(001) beta (2 x 4) surface as derived from x-ray diffraction and line-shape analysis.

Abstract: The atomic structure of the 2\ifmmode\times\else\texttimes\fi{}4 reconstructed GaAs (001) surface, prepared in optimized molecular beam epitaxy growth conditions, has been fully determined by in situ grazing incidence x-ray diffraction measurements. The structure involves only two dimers in the As top layer, in agreement with recent scanning tunnel microscopy experiments by Avery et al. and Hashizume et al. and, in addition, the presence of an As dimer in the third layer as a consequence of a missing Ga row in the second layer is clearly established. Full agreement is obtained for the atomic displacements down to the fourth atomic layer below the surface with state-of-the-art first-principles total-energy calculations by Schmidt and Bechstedt. A quantitative line-shape analysis using diffuse scattering formalism demonstrates the influence on the intensity distribution of the most frequent faults in the reconstruction propagation and enables to understand the variations observed in the reflection high-energy electron-diffraction patterns with the surface preparation without need of a new surface structure.

Analysis of activated slip systems in fatigue nickel polycrystals using the EBSD-technique in the scanning electron microscope

Abstract: By comparison of the observed trace angles of active slip planes with the expected traces in plastically deformed metal polycrystals, conclusions for the local stress state within the grains of polycrystalline aggregates can be drawn. The expected slip systems can be calculated when the local stress tensor and the orientation of the crystallites in the specimen space are known. In fatigued nickel polycrystals, the crystal orientation was determined by the EBSD (electron backscattering diffraction) method in the scanning electron microscope. It was shown that at the relatively small plastic strains under fatigue conditions the crystalline interactions do not essentially influence the local stress state in the grains, but the external uniaxial stress tensor remains valid in good approximation.

Powder x-ray diffraction of turbostratically stacked layer systems

Abstract: A special form of the Debye formula for calculating the powder x-ray diffraction of a turbostratically stacked layer system is derived, and calculated diffraction patterns for turbostratically stacked graphite and MoS2 layers are presented. Single-molecular-layer MoS2, prepared by exfoliation of lithium-intercalated MoS2 in water or alcohols, has been deposited on various supports, and x-ray diffraction patterns show that the restacking of the MoS2 layers can be perfectly turbostratic. The restacked MoS2 may or may not have water or organic bilayers between them, depending on the deposition conditions.

High Resolution X-Ray Diffraction

Abstract: Conventional high resolution X-Ray diffraction has been developed into a powerful tool for the nondestructive ex-situ investigation of epitaxial layers, of heterostructures and superlattice systems: The information which is obtained from diffraction patterns concerns the composition and uniformity of epitaxial layers, their thicknesses, the built-in strain and strain relaxation, and the crystalline perfection related to their dislocation density. Furthermore information on interfaces like interdiffusion and intermixingis obtained under certain circumstances as well. For the analysis of the diffraction patterns from epilayers, heterostructures and multilayers, the kinematical diffraction theory, although being still useful for a quick inspection of the data, in general can no longer be used for the quantitative description of the experiments. Instead dynamical theory is applied which takes into account extinction, multiple scattering, and the slight deviation of the refractive index from one. The instrumentation has also been improved continuously and simple powder diffractometers using a focussing path for the X-Rays were replaced by double- and triple -axis spectrometers equipped with multiple crystal or channel cut monochromators and analyzers. Apart from investigations under normal Bragg conditions grazing angle incidence techniques both for the determination of layer thicknesses as well as for precise information on lattice constants of thin films have also been employed. X-Ray topography is used for imaging purposes of layers grown on large wafers.

Calculating the diffraction of electrons or X-rays by carbon nanotubes

Abstract: The theory developed by Cochran, Crick and Vand for the diffraction of X-rays by helical biological molecules has been used to compute the diffraction patterns of monolayer or multilayer carbon nanotubes. By describing a single-wall chiral tubule as a set of regular carbon helices, its exact, kinematical diffraction amplitude is obtained in closed form. The theory is illustrated by computer simulations which faithfully reproduce the observed intensity patterns of transmission electron diffraction from multilayer nanotubes.

Examination of fatigue crack plastic zones using scanning-electron-microscope-based electron diffraction techniques

Abstract: Two new scanning-electron-microscope-based electron diffraction techniques for examining strain fields in plastic zones associated with fatigue cracks are described. Electron channelling contrast imaging (ECCI) revealed the size and shape of the plastic zone much more swiftly and completely than an existing method which uses the degradation of electron channelling patterns (ECPs). Furthermore the sense of the lattice plane tilting can be deduced from the sense of the contrast seen in ECCI images. The shift in the position of zone axes in electron back-scatter diffraction (EBSD) patterns can be used to measure the extent of the lattice plane tilting and so provides quantitative information about the strains within the plastic zone. The advantages in the complementary ECCI and EBSD techniques over the ECP method are demonstrated through direct comparison of results obtained from studies of a fatigue-cracked superalloy.

Energy dispersive x-ray diffraction of micro-crystals at ultrahigh pressures

Abstract: Ultrahigh pressures and temperatures in diamond-anvil cells are achieved at the expense of reducing sample volume. The capability of x-ray diffraction with high spatial resolution is most fundamental for probing microscopic samples at the maximum P-T and for minimizing the effect of gradients. Polychromatic synchrotron radiation with energy dispersive x-ray diffraction is ideal for the development of new classes of structural microprobes. Primary x-ray beams down to 3 microns can be produced with microbeam slit systems and microfocusing optical devices. The microprobe can be routinely used for a variety of high-pressure experiments, including single-crystal x-ray diffraction above 50 GPa, polycrystal-line diffraction above 300 GPa, deviatoric strain measurements, and diffraction at simultaneous high pressure and temperature.

Combined in Situ Sem Annealing and Ebsd of Deformed Materials

Abstract: An SEM heating stage which enables microstructural and crystallographic information to be determined as a deformed specimen is annealed, is described and the application of the technique to recovery, recrystallization and grain growth in aluminium alloys is discussed. It is shown that the development of the recrystallized microstructure and the growth of grains during recrystallization are similar to those occurring in the interior of a specimen. However, the presence of a free surface influences several aspects of annealing, including recovery and grain growth. It is found that annealing twins are formed more frequently at a free surface than in the specimen interior, and the significance of this result is discussed.

Short-Range Order in KPO3 Glass Studied by Neutron and X-Ray Diffraction

Abstract: The short-range order of KPO3 glass has been studied by diffraction methods in order to make evident the different behaviour of the P-O bonds within the PO4 tetrahedron. The oxygen sites are devided into bridging and terminal (non-bridging) oxygen sites, corresponding to two P-O bond lengths, the difference of which amounts to 14.5 pm. Previous conclusions about the changes of the P-O bond lengths under the influence of modifier cations of different electric field strength are corroborated. The K-O environments reveal two apparently different distances rKO with equal contributions to the total K-O coordination number of about 6.7. To explain this phenomenon, the K+ cations are suggested to be located in non-spherical cavities

Real-time observations of the oxidation of mild steel at high temperature by neutron diffraction

Abstract: The in situ characterization of the phase composition of iron oxides, “scale,” that form on low carbon steel during oxidation at elevated temperatures was carried out using the neutron diffraction technique. Growths in the intensities of diffraction peaks from the crystal planes of the various oxides (Fe x O, Fe3O4, and Fe2O3) were monitored on-line. The volume fractions of the oxides in the scale were calculated on the basis of ideal structure factors and measured relative intensities of diffraction peaks. These were selected from a small region of the diffraction pattern. Calculated volume fractions of these oxides in the scale layer were in agreement with the area fractions obtained from scanning electron microscopy (SEM) analysis of the scale.

The low-temperature behaviour of analcime; 1, High-resolution neutron powder diffraction

Abstract: The structure of a synthetic sample of analcime has been determined as a function of temperature between 30–300 K by high-resolution neutron powder diffraction. Although there are some reports of samples of analcime having non-cubic structures, the sample in our experiments remained cubic (space group la3d), and hence disordered, down to low temperatures. The absence of phase transitions involving ordering of the orientations of the water molecules, ordering of the sodium positions, or a displacive instability as in leucite and related materials, is discussed. We speculate that part of the reason for the absence of ordering of the water molecules or sodium cations is associated with the Al/Si disorder, which cannot order at low temperatures. We also discuss the likely distribution of the orientations of the water molecules at low temperatures, and propose that the water diads lie close to any of the crystal diads with the H-H Vectors lying close to the triads.

A comparison of different texture analysis techniques

Abstract: With the advent of automated techniques for measuring individual crystallographic orientations using electron diffraction, there has been an increase in the use of local orientation measurements for measuring textures in polycrystalline materials. Several studies have focused on the number of single orientation measurements necessary to achieve the statistics of more conventional texture measurement, techniques such as pole figure measurement using x-ray and neutron diffraction. This investigation considers this question but also is extended to consider the nature of the differences between textures measured using individual orientation measurements and those measured using x-ray diffraction.

On the crystal structure of the “1201” superconductors Hg0.5Bi0.5Sr2−xLaxCuO5−δ

Abstract: Two compositions in the “1201” system Hg 0.5 Bi 0.5 Sr 2− x La x CuO 5−δ , x = 0 and x = 0.5, have been studied by neutron diffraction, electron microscopy and X-ray absorption spectroscopy. Neutron diffraction gives evidence for a high disordering of the oxygen located in the Hg/Bi plane which is fully oxygenated. For the lanthanum-free sample, a slight oxygen deficiency is observed with oxygen vacancies (0.08 per cell) located in the basal plane of the copper octahedra. This feature is connected to the appearance of local superstructures observed by electron diffraction and high-resolution microscopy which tends to disappear after oxygen annealing and are enlarged after argon annealing. XANES spectroscopy at Bi L 3 edge shows a valence state close to +5 for bismuth in the lanthanum-free sample which is slightly reduced when lanthanum substitutes for strontium. In the same way copper K edge spectra show that lanthanum substitution leads to a decrease of the copper valency. The connection with superconductivity is discussed.

Representation of electron backscatter diffraction data

Abstract: Current methods of data representation for electron backscatter diffraction (EBSD) measurements are reviewed. Obtaining diffraction data from microstructures using EBSD has become a relatively straightforward procedure, and EBSD software packages are used to represent these data as qualitative statistics in the form of ideal orientations, pole figures, inverse pole figures, Euler space, and Rodrigues–Frank space. Quantitative statistics in the form of secondary computations allow full microtextural analysis. Additionally, the power of EBSD is demonstrated through positional information representation. Through experimental examples, the conversion of EBSD data to statistical information to facilitate interpretation of results is demonstrated.MST/3678