bluuebunny/crossref_metadata_2025 huggingface dataset:
A study of oxygen and vacancy ordering in YBa2Cu3O7−x
Authors: Lin Y. P. , O’Reilly A. H. , Greedan J. E. , Post M.
Published: June 2020

In the basal planes of the orthorhombic YBa2Cu3O7-X compound with x=0.07, which has a Tc of around 90K, chains of copper-oxygen are formed along the [010] direction. Previous investigations on the variation of Tc with oxygen content have shown the existence of a plateau at Tc = 60K for x=0.3 to 0.4, suggesting the presence of a separate phase. This phase has also been identified to be orthorhombic, but with a 2x superlattice along [100] of the parent structure, and the superlattice has been attributed to the formation of alternating copper-oxygen and copper-vacancy chains. In our work, we have studied the chain ordering phenomenon by electron microscopy and neutron diffraction on samples with different oxygen contents. We report here some of our electron microscopy findings for samples with x=0.4.

Powder samples of YBa2Cu3O7-X were prepared by controlled re-oxidation of previously reduced material. For electron microscopy, the sample was dry ground using a mortar and pestle in a dry nitrogen atmosphere without the use of any solvent and transferred dry onto holey carbon film for examination in a Philips CM12 microscope.
High-resolution STEM study of grain boundaries in high-Tcsuperconducting YBa2Cu3O7-xthin films
Authors: Shin D. H. , Silcox J. , Lathrop D. K. , Russek S. E. , Buhrman R. A.
Published: June 2020

The grain boundary structures of high Tcsuperconducting YBa2Cu3O7-xthin films grown on yttrium-stabilized zirconia and MgO have been studied using a VG-HB501A STEM operating at 100 keV (bright field, Cs=1.l mm, αobj=12 mrad,θcol=4 mrad). In Fig. 1, lattice images of various types of grain boundaries of a superconducting film grown on MgO by low-temperature (660 °C) in situ sputtering are shown. This sample has a critical current density of ∼5 × 106amp/cm2at 4 K and a somewhat depressed transition temperature near 70 K possibly due to ion bombardment during the sputtering process and oxygen deficiencies. Beyond a few atomic distances most of the grain boundaries are clean and coherent.

Since each grain in the film can take any of the possible orientations satisfying substrate-film lattice matching (along [100], [110], and [120] of the substrate, with either c-axis parallel or c-axis perpendicular, in the case of (100) MgO substrate), several different types of grain boundaries occur. Fig. 1(a) shows a grain boundary between two grains with c-axes in the plane of the substrate. Fig. 1(b), (c), and upper left area of (d) show grain boundaries between two grains, one with c-axis in the plane, and the other with c-axis perpendicular to the plane. At these grain boundaries c-axes are rotated through 90°.
Interfaces in high-Tcsuperconducting oxides
Authors: Carter C. Barry , Tietz Lisa A.
Published: June 2020

Interfaces in high-Tc superconducting oxides are influential during both the processing of bulk materials and the growth of thin epitactically aligned layers. In the first case, the formation of the superconducting phase involves the movement of phase boundaries during the solid-state reaction, while in the second, the phase boundary is formed as the superconducting material grows on the single-crystal substrate. Having formed the superconducting material, the superconducting phase will, in general, contain a large number of grain boundaries varying from the simple twin boundaries which can be produced during the cubic-to-tetragonal transformation, to low-angle grain boundaries, special high-angle grain boundaries, other high-angle grain boundaries and phase boundaries due to incomplete or on-going solid-state reactions. During the course of this presentation, recent results on these topics will be reviewed, paying particular attention to the more widely studied material, YBa2Cu3O6+x.

The importance of grain boundaries in high-Tc superconducting oxides has been firmly established by the systematic analysis of Dimos et al who have shown that the misorientation of the grains in layers of YBa2Cu3O6+xwhich had been grown on polycrystalline SrTiO3substrate varies with the relative misorientation between the grains.
Energy-loss studies of carbon content in yttrium barium cuprate
Authors: Batson P.E. , Chisholm M.E. , Clarke D.R. , Dimos D. , Shaw T.
Published: June 2020

In spite of early reports of the simplicity of preparation of the high temperature superconducting cuprates, it has been found that reproducibly good mechanical and bulk superconducting properties are difficult to obtain. This occurs partly because the densification mechanisms in this system are poorly understood. Since the good superconducting properties are dependent on proper oxygen content, and since oxygen movement appears to occur mainly by surface and grain boundary diffusion, the final density, and the rapidity of densification, are important. In practice, it has been found that rapid densification occurs in the presence of a couple of conditions — 1) the initial mixing is done in air, or 2) that BaCO3 is in the initial mix. In both cases, carbon is found in the final material, and it has been suggested that carbon may aid densification by participating in a low melting temperature peritectic material, promoting liquid phase sintering. In some cases, carbon can be picked up from carbon dioxide in the air.
Phase transformations and kinetics in small particles
Authors: Ajayan P. M.
Published: June 2020

Small particles of metals supported on various substrates are important heterogeneous catalysts. The rate of catalysis depends critically on the surface morphology of these catalysts and can vary significantly due to strong metal substrate interactions. On the other hand the structure of an isolated small particle is ill-defined and has been shown to fluctuate between various forms, existing in a quasi-molten state. We present here results which shows the transformation of a small particle sitting in a deep potential well on a substrate with a stable shape, into a free floating state which is structurally unstable.

The experimental procedure consisted of dispersing gold clusters produced by the reduction of organometallic compounds, onto magnesium oxide smoke particles which were collected onto a microscope grid placed over burning magnesium in air. The specimens were then imaged in a H- 9000 high resolution electron microscope operating at an accelerating voltage of 300 KV. The distribution of small gold panicles on the low energy (100) face of the MgO cube particles is shown in Fig.1
Electron Microscopy study of metal-support interaction in Rh/CeO2 catalysts
Authors: Pan M. , Garcia R. , Smith David J. , Cowley J.M. , Cifredo G.A.
Published: June 2020

In the characterization of supported metal catalysts, it is important to understand the influence of the support on the metal particles since many catalytic properties are controlled by their interaction. Previous studies of supported Rh catalysts have shown that the oxide support plays an Important role in the activity and stability of the catalysts: the morphology and dispersion as well as the activity of the supported Rh particles can be affected significantly. High resolution electron microscopy (HREM), scanning transmission electron microscopy (STEM) and microdiffraction represent powerful techniques for studying the microstructure of supported small metal particles (<50 Å) and their structural relationship with the support.

In this study, the techniques of HREM and STEM/microdiffraction have been combined to characterize the structures of the Rh particles in Rh/CeO2 and their structural relationship with the cerium oxide support. The microscopes used were JEM-200CX, JEM-4000EX and VG HB-5 STEM. The following catalyst samples with various treatments have been studied:
A high resolution and Analytical Electron Microscopy study of novel solid state hydrodesulfurization catalysts
Authors: Lindner J. , Sachdev A. , Villa-Garcia M.A. , Schwank J.
Published: June 2020

The removal of sulfur from petroleum feedstocks is of great importance to the oil industry. The process, known as hydrodesulfurization (HDS), is typically catalyzed by Group VIB metal oxides. The workhorse of the industry today is an alumina supported CoO-MoO3 catalyst. Recently, several models have been proposed for the active site responsible for HDS activity, but despite extensive research efforts there is still no clear relationship between structure and activity. We have prepared promoted and non-stoichiometric catalyst samples via a novel solid state synthesis route. These catalysts are not only quite active in the HDS of thiophene, but are also more thermally stable and consequently easier to characterize than the standard HDS materials prepared by wet chemistry methods. Most studies on HDS catalysts rely on bulk techniques for characterization analysis, however, these do not provide any information at the microscopic level where catalysis occurs. For that reason we have used analytical and high resolution electron microscopy to obtain information at the atomic level, coupled with bulk techniques such as x-ray diffraction and surface area measurements. The objective was to develop a link between the microstructure of our solid state catalysts and their HDS activity.
The structure of rapidly solidified Fe-Ni-Mo-Co-B ribbons
Authors: Vanderwalker D. M.
Published: June 2020

Fundamental aspects of solidification can be examined by experimentation in rapid solidification processing. The structure produced depends on parameters such as cooling rate, degree of undercooling, heat flow, and growth rate. Rapidly solidified iron base alloys are being developed for structural applications.

RSR I Fe-19.7Mo-14.4Ni-7.3Co-1.9Bwt % and RSR II Fe-15.0Ni-11.1Mo-7.4Co-0.84B wt% ribbons were melt spun by a jet casting technique. RSR I ribbons were annealed for one hour at 816°C.Specimens were prepared for transmission electron microscopy by punching 3 mm discs from ribbons and electropolishing in a methanol 5% perchloric acid solution.The TEM was performed on the JEM 200CX electron microscope.

As solidified RSR I was found to be canposed of fine (7nm) polycrystalline α-Fe. There is evidence for the presence of Ni Mo and FeB (Fig.1). On annealing, the α-Fe transforms to γ-Fe and FeB2Mo2, with significant grain growth (Fig.2). The as-solidified RSR II contains cellular γ-Fe with fcc-Fe2 3B6 of lattice parameter a=l.067nm at the cell walls (Fig. 3).
TEM images of overlapped precipitates
Authors: Hu C.T.
Published: June 2020

The Ni3Al precipitates with a cubic shape and densely aligned along the [100] direction are always observed in Ni-Al alloys and nickel base superalloys. It is quite frequently observe the overlap of two or more separate Ni3Al particles, which locate at different depths in a thin foil specimen during a TEM examination. However, no one has paied much attention to the variations of intensity and contrast on the overlapped area of two separated particles in TEM dark field image. Hence, the analyzed results of dark field microscopy may give an unfaithful determination of particle size as well as a miscount of the total particle number. This investigation has been concentrated on the image forming mechanism and the explanation of intensity change on the overlap area of two Ni3Al particles.

Fig.1 indicates a unit cell with Ll2 structure in Ni3Al precipitate A, and an identical one in precipitate B.
Combined low-voltage and field-emission SEM of polymers
Authors: Sawyer Linda C. , Jamieson Marjorie
Published: June 2020

Scanning electron microscopy (SEM) of polymers has been a challenge during the past two decades as the conditions required for maximum resolution in the SEM are coincident with conditions which result in maximum beam damage of the specimens. The accelerating voltage requirement for best detail using a tungsten thermionic source is typically about 20 kV, a voltage at which polymers exhibit mass loss, shrinkage and other changes while only providing minimal surface detail. Lower voltages, which produce better surface detail do not provide enough brightness to reveal the details required. The last decade has seen the use of lanthanum hexaboride thermionic sources which have higher brightness and smaller interaction volumes, permitting operation at lower voltages with minimal beam damage and better surface detail due to decreased depth penetration with increased signal to noise. Thus low voltage SEM (LVSEM) is a method used for structure-property studies of polymers.
Morphology of High-Performance Rigid-Rod Polymer Fibers and Molecular Composites
Authors: Adams W.W. , Krause S. J.
Published: June 2020

Rigid-rod polymers such as PBO, poly(paraphenylene benzobisoxazole), Figure 1a, are now in commercial development for use as high-performance fibers and for reinforcement at the molecular level in molecular composites. Spinning of liquid crystalline polyphosphoric acid solutions of PBO, followed by washing, drying, and tension heat treatment produces fibers which have the following properties: density of 1.59 g/cm3; tensile strength of 820 kpsi; tensile modulus of 52 Mpsi; compressive strength of 50 kpsi; they are electrically insulating; they do not absorb moisture; and they are insensitive to radiation, including ultraviolet. Since the chain modulus of PBO is estimated to be 730 GPa, the high stiffness also affords the opportunity to reinforce a flexible coil polymer at the molecular level, in analogy to a chopped fiber reinforced composite. The objectives of the molecular composite concept are to eliminate the thermal expansion coefficient mismatch between the fiber and the matrix, as occurs in conventional composites, to eliminate the interface between the fiber and the matrix, and, hopefully, to obtain synergistic effects from the exceptional stiffness of the rigid-rod molecule. These expectations have been confirmed in the case of blending rigid-rod PBZT, poly(paraphenylene benzobisthiazole), Figure 1b, with stiff-chain ABPBI, poly 2,5(6) benzimidazole, Fig. 1c A film with 30% PBZT/70% ABPBI had tensile strength 190 kpsi and tensile modulus of 13 Mpsi when solution spun from a 3% methane sulfonic acid solution into a film. The modulus, as predicted by rule of mixtures, for a film with this composition and with planar isotropic orientation, should be 16 Mpsi. The experimental value is 80% of the theoretical value indicating that the concept of a molecular composite is valid.
Apparatus for “on-the-fly” sample preparation of aerosol-grown block copolymer microdroplets
Authors: Bellare Jayesh R. , Thomas Edwin L.
Published: June 2020

The morphology of phase separated amorphous A/B block copolymers depends not only on the relative volume fraction of the A and B components but also on external shape constraints imposed on the sample. Whereas it is well established that thin planar of microstructured copolymers differ considerably from the bulk materials, the influence of roughness and curvature, which are of practical importance in coating technology, are presently unknown. Because the simplest curved surface is a sphere, the effect of surface curvature on block copolymer microdomains can be conveniently studied by TEM observations of spherical microdroplets prepared with the apparatus described here.

Samples for microscopy are prepared by the “gunshot” technique: an atomized solution of the copolymer in a homogeneous solvent is allowed to fall onto carbon coated grids. During fall the solvent evaporates, leaving small spheres of the copolymer that continue falling. By means of a special set of tubes along the fall path, the spheres are annealed (by heating in a nitrogen atmosphere) and stained/fixed (by exposure to osmium tetroxide vapors).
Electron microscopic investigation of the structure and morphology of syndiotactic polypropylene
Authors: Lovinger Andrew J. , Lotz Bernard , Davis Don D.
Published: June 2020

In contrast to its isotactic isomer, syndiotactic polypropylene has received only little attention. Our main source of understanding of its structure is the X-ray study by Conradini et al., who found the chains to have a (t2g2)2 conformation (corresponding to a 4∗2/1 helix with molecular repeat 0.74 nm), and to be packed in a C-centered unit cell as shown in the left side of Fig. 1. We have recently begun a study of the structure, crystallization, and morphology of syndiotactic polypropylene using electron microscopy and diffraction. Here we concentrate specifically on the electron-diffraction evidence as a function of temperature, in order to obtain an understanding of the evolution and variation of structure in this polymer.

Thin films of syndiotactic polypropylene (synthesized by Dr. R. E. Cais as reported previously) were prepared by casting from dilute solution in xylenes at ca. 140°c onto freshly cleaved mica substrates. Following evaporation of the solvent, they were melted and then isothermally crystallized at a variety of temperatures. After shadowing with Pt/C and coating with carbon, they were floated off their substrates for examination by transmission electron microscopy (bright- and dark-field) and selected-area electron diffraction at 100-200 keV.
Analysis of interference patterns in acoustic photomicrographs
Authors: Downs Donna A. , El-Shiekh Aly , Tucker Paul A. , Russ John C.
Published: June 2020

Images from the Scanning Acoustic Microscope (SAM) have been used previously to study fiber orientations in opaque matrices. SAM images are superficially similar to reflected light images. They also frequently exhibit interference patterns which can be used for quantitative analysis. These patterns arise at discontinuities near or intersecting the specimen surface, and result from interference between acoustic waves reflecting from subsurface boundaries and from the specimen surface. For the case of fibers intersecting a planar surface, the position of the interference fringes gives the orientation of the fiber with respect to the surface, and the shape of the fringes indicates any surface irregularities. Thus, if the fiber shape and orientation are known and if the waves are determined to be longitudinal, then fringe spacings can be used to calculate the velocity and attenuation of the longitudinal acoustic wave in the matrix surrounding the fiber.

This technique was applied to graphite-epoxy composites, imaged in the SAM at a frequency of 1.2 GHz. First it was determined that other waves, such as shear waves, surface-skimming longitudinal waves, and surface acoustic waves (Rayleigh waves) were not present in the composite. Their addition to the interference pattern would have disrupted the interpretation of the object’s shape and surface features from the longitudinal fringes and complicated the solutions for wave reflections.
Fiber polymer structures in thermally adaptable fabrics
Authors: Goynes W. R. , Vigo T. L. , Bruno J. S.
Published: June 2020

Textile fabrics can be given temperature-adjusting properties by finishing them with formulation of polyethylene glycols (PEGs) and dimethyloldihydroxyethylene urea (DMDHEU) applied in a pad-dry-cure process. This treated fabric increases wearer comfort by lowering or raising its temperature as it is heated or cooled. In addition to thermal properties, these finishes produce improved resistance to abrasion, pilling, static charges, and oily soil deposition. Great increases in water absorbency and moisture regain are also achieved.

Property changes in chemically finished fibers and fabrics generally correlate with changes in morphological structures related to the finish. Optical, and transmission and scanning electron microscopy were used to study structural changes in the thermally-adaptable fabrics to better understand property changes, and to modify finishing procedures in order to maximize property improvements. Finished cotton, cotton/polyester, and wool fabrics were studied.
Valence electron energy loss spectroscopy in reflection geometry
Authors: Liu J. , Cowley J. M.
Published: June 2020

The low energy loss region of a EELS spectrum carries information about the valence electron excitation processes (e.g., collective excitations for free electron like materials and interband transitions for insulators). The relative intensities and the positions of the interband transition energy loss peaks observed in EELS spectra are determined by the joint density of states (DOS) of the initial and final states of the excitation processes. Thus it is expected that EELS in reflection mode could yield information about the perturbation of the DOS of the conduction and valence bands of the bulk crystals caused by the termination of the three dimensional periodicity at the crystal surfaces. The experiments were performed in a Philipps 400T transmission electron microscope operated at 120 kV. The reflection EELS spectra were obtained by a Gatan 607 EELS spectrometer together with a Tracor data acquisition system and the resolution of the spectrometer was about 0.8 eV. All the reflection spectra are obtained from the specular reflection spots satisfying surface resonance conditions.
Measurements of white line intensities in 4d transition metals using Electron Energy Loss Spectrometry (EELS)
Authors: Pearson Douglas H.
Published: June 2020

A feature of considerable interest in the electron energy loss spectra of transition metals is the L23 edge which is composed of two sharp “white lines” superimposed on a broad edge that makes up the trailing background. The white lines are due to the excitations of electrons from filled 2p1/2 and 2p3/2 states to unoccupied 3d or 4d states, and the background intensity is due to continuum excitations. Recent work on the white lines of the 3d transition metals has shown that when normalized to the continuum excitations, the total white line intensity decreases nearly linearly with atomic number (or d band occupancy) across the 3d series. Furthermore, this linear relationship may be used to measure the changes in 3d state occupancy local to specific atoms during alloying and during solid state phase transformations. In the present paper the experimental analysis of the white lines is extended to the 4d transition metals and is then compared to experimental results from the 3d metals.

Electron-transparent specimens of the pure 4d metals were prepared by standard electropolishing and evaporation techniques. To prevent oxidation of the Yttrium specimen, however, a 40 nm film of Yttrium was deposited between 15 nm films of Vanadium by direct current ion sputtering in a chamber with a base pressure of < 1.0 × 10−7 Torr. Energy loss spectra were obtained using a Gatan 607 electron energy loss spectrometer attached to a Philips EM 430 electron microscope operating in image mode at 200 kV.
Planar defects in AIN
Authors: McKernan Stuart , Carter C. Barry
Published: June 2020

Aluminum nitride has recently become the subject of much interest as a technologically useful ceramic. The mechanical strength, high thermal conductivity and large electrical resistivity and a relatively small thermal expansion coefficient, make this material extremely well suited as a semiconductor substrate material. AlN has the hexagonal, wurtzite structure rather than the cubic structure of the more common semiconductors. It is also a polar material. The characterization of microstructural defects in this material is obviously necessary to the understanding of the materials properties.

In sintered AlN material, several different planar defects have previously been examined. Anti-phase boundaries (which produce the same configuration as basal twins in this structure) and stacking-faults have been identified in this material. A defect described as a “dome-like defect” has also been reported. The association of oxygen impurities with these extended defects has also been proposed. In this paper we report the observation of a defect (which may be the same as the “dome-like defect”) consisting of a flat, planar, basal fault and a curved, planar fault (rather than a spherical one) which join together to enclose a region of AlN, and separate it from the rest of the grain.
Structural development of solution-derived, monodisperse powders studied using cryo-TEM.
Authors: Bailey J.K.
Published: June 2020

High quality advanced ceramic materials can be produced by using unagglomerated monodisperse powders for preparation of ceramic pre-forms. Stöber et al. first showed that monodisperse silica particles could be derived from silicon alkoxides, and Bugosh et al. demonstrated the control over particle size and mass fraction that could be obtained. Similarly, Barringer and Bowen produced titania colloids but without using a catalyst. In these preparation procedures, a solution of metal alkoxide monomers undergoes hydrolysis and condensation to form the particles. By the chemical processing of monodisperse powders, one can control the particle size and stabilization. In order to understand and control the influence of the processing conditions on the morphology of these particles, the growth mechanism needs to be understood. As yet, there are no widely accepted growth models for the various materials.

Using cryo-microscopy, one can directly observe the development of these particles by freezing the reaction at various stages and observing the products at that time. A thin liquid film of solution is fast frozen, vitrifying the solvent and trapping the particles suspended in solution, thus avoiding potential artifacts, such as agglomeration, that can be caused by drying the sample.
Metal/GaAs interfaces
Authors: Carter C. Barry , Zhu Jane G. , Palmstrøm C. J.
Published: June 2020

Reactions between III-V compound semiconductors and metals occur during the formation of Ohmic or Schottky contacts for semiconductor devices. Since the products of such reactions usually have crystal structures different from the zinc-blende structure of GaAs, it is necessary to understand the structure of the metal/GaAs interface in order to optimize the quality of the contact. At low temperatures, ternary phases given of the form, MxGaAs, grow during the metal-GaAs reactions. Well-documented examples are given by Ni-GaAs and Co-GaAs reactions. These ternary phases are thermodynamically unstable and decompose to MyGa and MzAs at higher temperatures. For contacts involving rare-earth metals, such as Er, Tb and Dy, on GaAs, a substantial interface chemical reaction has been found in which GaAs is dissociated and a more than 12Å-thick rare-earth arsenide interfacial layer is formed. In order to improve the reliability of these contacts and to control the reactions which take place at the GaAs interface, intermetallic compounds, NiAl and CoGa, and rare-earth metal arsenides, YbAs and ErAs, have recently been grown epitactically on GaAs. The work described here has concentrated on the characterization of the compound/GaAs interfaces formed during this process, paying particular attention to the structure of the misfit dislocations present at the interface. An additional advantage of this approach is that, if the quality of the contact layer is sufficiently high, it might be possible to grow a second high-quality layer of GaAs on top and thus begin to generate a 3-dimensional structure.
Direct STEM ADF imaging of gold on silicon (111)
Authors: Xu P. , Kirkland E. J. , Silcox J.
Published: June 2020

Many studies of thin metal film growth and the formation of metal-semiconductor contacts have been performed using a wide range of experimental methods. STEM annular dark field imaging could be an important complement since it may allow direct imaging of a single heavy atom on a thin silicon substrate. This would enable studies of the local atomic arrangements and defects in the initial stage of metal silicide formation.

Preliminary experiments were performed in an ultra-high vacuum VG HB501A STEM with a base pressure of 1 × 10-10 mbar. An antechamber directly attached to the microscope for specimen preparation has a base pressure of 2×l0-10 mbar. A thin single crystal membrane was fabricated by anodic etching and subsequent reactive etching. The specimen was cleaned by the Shiraki method and had a very thin oxide layer left on the surface. 5 Å of gold was deposited on the specimen at room temperature from a tungsten filament coil monitored by a quartz crystal monitor.
Microstructure of chromium films grown on glass and silicon substrates
Authors: Duan J.Z.
Published: June 2020

Thin metal films are widely used in microelectronics and this has stimulated a great deal of research about their microstructures and related properties. During the past two years, chromium microstructures and internal stresses generated during e-beam evaporation Cr film growth have been researched by us. Many relationships between Cr columnar microstructures and internal stresses have been discovered. In this paper, the results of studies of the dependance of Cr columnar microstructures on Si and glass substrates are reported. They have been found to be very useful for analyzing the internal stresses in Cr films and understanding the atomistic mechanism of their growth.

It is well known that microstructures and properties are closely related to the processing conditions, so in our experiments, all experimental parameters are kept constant except for factor whose influence on on microstructure and internal stress is under study.
A new method of computing the phase-grating function in multislice calculation
Authors: Pan M. , Rez P. , Cowley J.M.
Published: June 2020

In multislice calculations of electron diffraction from crystal surfaces and convergent beam electron microdiffraction as well as STEM images, it is essential to construct a supercell containing the 'crystal defect' and to avoid any interference from two adjacent cells. The computing time for the phase-grating function of such a supercell is no longer negligible and sometimes could become dominant in a complete calculation. This is especially true for systems with an array processor where the multislice calculation proceeds rapidly due to the fast implementation of multiplication and the Fourier transform. The current multislice programs are not suitable for this kind of phase-grating calculation since the projected potential is calculated from the Fourier transform of the structure factors. With an increasing number of atoms in the cell, calculating structure factors becomes a time-consuming procedure.
Geometrical explanation of parabolas and resonance in electron diffraction
Authors: Gajdardziska-Josifovska M. , Cowley J. M.
Published: June 2020

Reflection electron microscopy (REM) relies on the surface resonance (channeling) conditions for enhancement of the intensity of the specular reflection from a flat surface of a single crystal. The two most frequently cited geometries for attaining surface resonance conditions are: i) tilting the incident beam such that the specular beam in the RHEED pattern falls on an intersection of a K-line parallel to the surface with some oblique K-line; ii) positioning the specular beam on an intersection of a K-Iine parallel to the surface with some of the surface resonance regions bound by parabolas. Parabolas are also observed in the transmission diffraction patterns and have been explained as Kikuchi envelopes. Recent studies indicated a similarity between the CBED transmission and reflection patterns. We will describe a simple geometry which can be used to interpret the above observations.

A parabola is by definition a curve of equal distance from a point (called focus) and a line (called directrix; see Fig.1 ).Simple previously unnoticed facs are that the zone axis is a focal point of all the parabolas belonging to a given zone, and that the directrix of each parabola corresponds to a K-line.
Radial distribution functions from diffracted electron intensities
Authors: Bentley J. , Angelini P. , Sklad P. S. , Fisher A. T.
Published: June 2020

Many previous studies have shown the benefits of electronically recorded intensity profiles of electron diffraction patterns obtained with a transmission electron microscope (TEM). The technique, which is based on the scanning diffraction method developed by Grigson et al., avoids the complex procedures involved in making densitometer traces from film, greatly expands the dynamic range, and allows energy filtering to remove inelastically scattered electrons that have lost more than a few eV. Early applications to amorphous materials employed TEMs fitted with scanning systems and electrostatic filters below the projector lens. The main emphasis of the work of Graczyk et al. was on structural models for amorphous semiconductors such as silicon and germanium. However, a treatment for binary materials was developed and measurements were made for SiO2 and Ge-Te alloys. Cockayne et al. have recently extended these early techniques to modern 100 and 300 kV analytical electron microscopes, which when equipped with energy loss spectrometers and energy-dispersive x-ray analysis systems, do not require further major modification. Applications for which radial distribution functions have been determined from online measurements of energy-filtered selected area electron diffraction pattern intensity profiles have included amorphous thin films of carbon (a-C), germanium (a-Ge), boron nitride (a-BN), hydrogenated silicon (a-Si:H), silicon-carbon (a-Si1-xCx:H), and phosphorus- and boron-doped hydrogenated silicon.
Using crystallographic symmetry in diffraction and contrast analysis
Authors: Dahmen U. , Westmacott K.H.
Published: June 2020

Despite the increased use of convergent beam diffraction, symmetry concepts in their more general form are not commonly applied as a practical tool in electron microscopy. Crystal symmetry provides an abundance of information that can be used to facilitate and improve the TEM analysis of crystalline solids. This paper draws attention to some aspects of symmetry that can be put to practical use in the analysis of structures and morphologies of two-phase materials.

It has been shown that the symmetry of the matrix that relates different variants of a precipitate can be used to determine the axis of needle- or lath-shaped precipitates or the habit plane of plate-shaped precipitates. By tilting to a special high symmetry orientation of the matrix and by measuring angles between symmetry-related variants of the precipitate it is possible to find their habit from a single micrograph.
Some practical aspects of CBED applications in materials science
Authors: Liu D. R. , Williams D. B.
Published: June 2020

The paper by Buxton et al. has firmly laid down the theoretical foundation for the point group determination of a crystal structure with convergent beam electron diffraction (CBED). Numerous examples of successful applications of this theory to materials science problems have been published. However, it has also been observed that, for an unambiguous determination of some crystal structures, more zone axis CBED patterns are needed than it is indicated by the tables in the paper. Yet sometimes ambiguity can still exist as in the case of the point group determination of the MgAl2O4 spinel, where CBED is unable to detect an alleged Al3+ ion displacement of Δb=0.0002∼0.0006 nm along a <111> direction and thus unable to determine whether its point group should be m3m or m. This difficulty might be attributed to the fact that, as for microanalysis, such as x-ray energy dispersive spectroscopy and electron energy-loss spectroscopy, the CBED technique must have its detection limit beyond which the detail in an acquired CBED pattern is not adequate enough to permit an unambiguous determination of its diffraction group.
AEM study of perovskite Pb(Zn1/3Nb2/3)O3 ferroelectric relaxor
Authors: Sung C.M. , Gorton A.J. , Chen J. , Harmer M.P. , Smyth D.M.
Published: June 2020

Lead zinc niobate (Pb(Zn1/3Nb2/3)O3 : PZN) is a ferroelectric relaxor with perovskite-related stmcture exhibiting a diffuse phase transition at ˜140°C. Single crystals of PZN have shown excellent dielectric, electro-optic and electrostrictive properties. However, attempts to achieve these properties in polycrystalline ceramic PZN samples have failed. Using conventional powder processing it has not been possible to synthesize a monophase perovskite stmcture, due to the pervasion of lead niobate pyrochlore phases. It is thought that the pyrochlore phases are the detrimental factor of most importance. However, lack of information on pyrochlore structures and on the PZN perovskite phase prevents full characterization of the microstructures of ceramic samples. To understand more completely the composition and stmcture of the various phases present in PZN ceramics, an AEM study of polycrystalline samples has been undertaken. Furthermore a clarification of the microchemical ordering situation in PZN has been achieved.
The limits of strain and lattice parameter measurements by CBED
Authors: Fraser Hamish L.
Published: June 2020

The topic of strain and lattice parameter measurements using CBED is discussed by reference to several examples. In this paper, only one of these examples is referenced because of the limitation of length. In this technique, scattering in the higher order Laue zones is used to determine local lattice parameters. Work (e.g. 1) has concentrated on a model strained-layer superlattice, namely Si/Gex-Si1-x. In bulk samples, the strain is expected to be tetragonal in nature with the unique axis parallel to [100], the growth direction. When CBED patterns are recorded from the alloy epi-layers, the symmetries exhibited by the patterns are not tetragonal, but are in fact distorted from this to lower symmetries. The spatial variation of the distortion close to a strained-layer interface has been assessed. This is most readily noted by consideration of Fig. 1(a-c), which show enlargements of CBED patterns for various locations and compositions of Ge. Thus, Fig. 1(a) was obtained with the electron beam positioned in the center of a 5Ge epilayer and the distortion is consistent with an orthorhombic distortion. When the beam is situated at about 150 nm from the interface, the same part of the CBED pattern is shown in Fig. 1(b); clearly, the symmetry exhibited by the mirror planes in Fig. 1 is broken. Finally, when the electron beam is positioned in the center of a 10Ge epilayer, the CBED pattern yields the result shown in Fig. 1(c). In this case, the break in the mirror symmetry is independent of distance form the heterointerface, as might be expected from the increase in the mismatch between 5 and 10%Ge, i.e. 0.2 to 0.4%, respectively. From computer simulation, Fig.2, the apparent monocline distortion corresponding to the 5Ge epilayer is quantified as a100 = 0.5443 nm, a010 = 0.5429 nm and a001 = 0.5440 nm (all ± 0.0001 nm), and α = β = 90°, γ = 89.96 ± 0.02°. These local symmetry changes are most likely due to surface relaxation phenomena.
Acceleration voltage effect on electron surface channeling
Authors: Yao Nan , Cowley J. M.
Published: June 2020

In transmission electron microscopy (TEM), the usable thickness of specimens increases with the incident electron energy. Some Bloch wave calculations of electron channelling along the zone axis in the transmission case show that the energy level for the channelled electrons increases (negatively) with the incident electron energy due to the correction for relativistic effect. Similarly, for reflection electron diffraction geometry, the faster the incident electrons, the deeper they penetrate into the surface. The diffraction patterns are expected to approach steadily to that for a three-dimensional lattice similar to the case of an increase in the angle of the incident electron beam. The potential distribution seen by incident electrons in the reflection geometry for an ideally perfect crystal surface should be similar to that for planes or rows in the transmission of electrons through thin crystal samples although some modifications due to the surface potential barriers have to be taken into account.
Scanning reflection electron imaging of crystal surfaces
Authors: Liu J. , Cowley J. M.
Published: June 2020

Scanning reflection electron imaging has been developed in recent years as a potential surface probing technique. The small beam size (0.5 nm) used in the SREM work make it possible to perform nano-scale analysis of crystal surfaces. Microdiffraction RHEED patterns can be obtained to identify surface reaction products. The problems of reductions on metal oxide crystal surfaces, metal surface oxidation processes and metal-substrate interactions have been investigated by SREM technique. Surface channelling effect have also been studied in SREM mode by taking advantage of the small incident beam size. In principle SREM images are analogous to, due to the reciprocity theorem, the REM images. In practice, in order to get reasonable signal large objective apertures are generally used whereas small detector apertures have to be used to obtain adequate image contrast. The main contrast mechanism in SREM images of atomic steps and surface defects may be due to phase contrast since most of the diffraction contrast will be washed out because of the large beam convergence. For example the RHEED pattern in figure 1 is effectively a convergent beam pattern and was obtained from a cleaved MgO (001) surface. These patterns are useful for determining the crystal orientation and for the inspection of diffraction conditions such as surface resonance condition. By employing small detector apertures and moderate objective apertures single atomic steps can be imaged with high contrast as shown in figure 2 which is a SREM image of platinum (111) surface obtained with (555) specular beam excited. The characteristic black and white contrast of the down and up steps is revealed clearly (arrowed in fig. 2). The similarity between the SREM image and the corresponding REM images suggests that the two imaging modes are equivalent.
Surface studies of single copper crystals by REM
Authors: Yao Nan
Published: June 2020

In the last decade, the use of transmission electron microscopes for surface imaging in the reflection mode has been shown to have considerable potential as a means for studying surfaces. Many REM (Reflection Electron Microscopy) studies have been done as reviewed recently by Cowley and Yagi. However, due to the lack of success with clean and flat single-crystal surface preparation technique, the surface studies of less inert metals, such as copper single crystal, have not been reported. The normal method of forming spherical platinum or gold single crystal by simply melting the end of platinum or gold thin wire with an acetylene-oxygen flame in air for REM observation is difficult here. The formation of flat copper surfaces occurs during the well-controlled crystallization process and is sensitively influenced by its physical and chemical properties. This paper reports a new method of overcoming these difficulties in the copper single crystal preparation. Observations of the samples have been made by REM and RHEED (Reflection High Energy Elecrton Diffraction).
Annealing effects on the sapphire (0001) surface
Authors: Ndubuisi G. C. , Liu J. , Cowley J. M.
Published: June 2020

The techniques of RHEED, REM and SREM have proven to be important in the study of surface structures. Since these techniques are sensitive to surface perturbations at nanometer scale they can be used to study surface structures as well as surface interactions such as metal surface oxidation processes, metal substrate interactions, etc. The study of the as-grown and the cleaved sapphire surfaces have benn reported. This paper intends to reports some preliminary results on the study of annealed sapphire (0001) surface structures and possible interactions with deposited small metal particles.

The sapphire rod, with c-axis parallel to the rod, was purchased from Saphikon, Inc. A 2.0 mm disc was cut, thinned, oriented to the (0001) face and polished to 2.8х 1.0х0.2mm strips. Some of the strips were examined by Laue back reflection to ensure preservation of the [0001] orientation, then cleaned and transported to the furnace. The specimens were annealed in pure oxygen at high temperatures with a vacuum pressure a little above atmospheric. The experiments were performed both in a Philips 400T TEM with a vacuum pressure around 10-7 to 10-6 Torr. and in a VG HB5 STEM microscope with a vacuum pressure of about 5×10-9.
Electron Microscopy studies of spinel interphase formation at the Ni/Al2O3 interface
Authors: Trumble K. P.
Published: June 2020

Many metal-ceramic combinations considered for structural components and composites react at high temperatures to form intermediate phases, usually in the form of thin interfacial layers. Interphase formation generally leads to marked changes in interface fracture resistance. A fundamental understanding of metal-ceramic interphase reactions is thus key to optimizing the mechanical behavior of such systems. In the system Ni/Al2O3, nickel aluminate spinel (NiAl2O4) reaction layers have been found to form in diffusion bonding studies. However, the underlying reaction process by which spinel formation occurs is not well understood. It has been suggested that the reaction requires conditions sufficiently oxidizing to first produce NiO which then reacts with the Al2O3. In the present study controlled oxygen diffusion bonding and electron microscopy were used to further explore the spinel formation reaction at the Ni/Al2O3 interface.
Seashells as a natural model to study ceramic-polymer composites
Authors: Sarikaya Mehmet , Gunnison Katie L. , Aksay Ilhan A.
Published: June 2020

Interests in ceramic/polymer composites are increasing mainly because of their superior physical properties for structural and electronic applications. Naturally produced composites, such as seashells, are ideal to study the formation, microstructure, and physical properties of these composites as these materials have far superior properties with well-defined microstructures than man-made ones which require complex fabrication techniques. In this paper, a summary of a recent study on the microstructure of abalone shell (Haliotis refuscens) will be described in conjunction with its mechanical properties.

A longitudinal cross-section of abalone shell displays two types of microstructures: outer prismatic layer and inner nacreous layer. Two forms of CaCO3, calcite (rhombohedral,
) and aragonite (orthorhombic, Pmcn) constitute the inorganic component of the organic/ceramic composite in the prismatic and nacreous layers, respectively. The structure and properties of the nacreous will be described here as this is the part which provides a good combination of mechanical properties for the seashell.
Interface reactions in a high-temperature metal-matrix composite
Authors: Allard L. F. , Coffey D. W. , Ohriner E. K. , Sikka V. K.
Published: June 2020

Metal-matrix composites are currently of interest for application in high-temperature, elevated-stress environments. Of a number of such materials currently being investigated in our laboratory, nickel aluminides reinforced with ceramic whiskers or fibers have shown promise in that the maximum operating temperature of the alloy has been extended from 800°C to 1100°C. The microstructure of the fiber/matrix interface in a nickel aluminide strengthened with ceramic fibers is detailed in this paper.

The composite material was fabricated by mixing gas-atomized −100 mesh IC-50 powder (11.7 Al, 1.76 Hf, 0.025 B wt %, bal. Ni) with Nextel 440 fiber (3M Co., nominally 70 A12O3, 28 SiO2 and 2 B2O3 wt %) to give a composite with about 20 vol % fiber. The material was placed in a heavy-walled stainless-steel can which was evacuated while being heated to 600°C to remove fiber sizing. The can was subsequently sealed and preheated at 1100° C for 2 h, then hot consolidated at high pressure for 30 s to produce a fully dense material.