bluuebunny/crossref_metadata_2025_split huggingface dataset:
HREM study of M, Y and W hexagonal type ferrites
Authors: Vallet-Regí M. , Nicolopoulos S. , González-Calbet J.M. 
Published: June 2020

Hexagonal ferrites have similar structural symmetries and are built by the same structural units. HREM has proved to be an excellent tool to identify individual stacking bloks, in M, Y and W hexaferrites.M, Y and W hexagonal type ferrites are well-known magnetic materials for applications. Some previous electron microscopy studies on these ferrites, reveal a variety of new phases coming from the microsyntactic intergrowth of M, Y and W structures. We report in this paper some microstructural features of these ferrites.BaFe12O19 (M) hexagonal ferrite was synthetized by using the standard ceramic technique. Crystals of Y type ferrite (nominal composition: BaLi0.5Fe0.5ZnFe16027) were prepared from stoichiometric amounts of BaCO3, ZnO and LiCO3. The synthesis of W-type ferrite (formula: Ba2Zn2Fe16027) is described elsewhere.Samples for observation were prepared by grinding powders of the above ferrites under acetone. HREM study was performed at the "Centro de Microscopía Electrónica, Univ. Complutense" using both 400 Kv and 200 Kv electron microscopes (JEM-400EX and JEM-200FX)
HREM observations of microstructrues of silicon carbide whiskers
Authors: Ning X.G. , Ye H.Q. , Cao W. 
Published: June 2020

As we know, SiC whiskers take an important role in lightweight composite materials such as Al-Li-SiC material and etc. The study on microstructures of SiC whiskers is needed for well understanding the properties of the composite materials reinforced by SiC whiskers. Microstructures such as multi-twins and polytypes of bulk crystal of SiC have been studied in detail. Information of microstructures of SiC whiskers in atomic resolution, however, has not been reported yet.Defects in SiC whiskers made from rice hulls were identified and analyzed using TEM. The whiskers were characterized by a high density of planar faults lying on closed planes perpendicular to the whisker axis. Defects in SiC whiskers here (made by solid reaction method, IMR, Academia Sinica) were different. The basic structure of SiC whiskers here is β-type (f.c.c.) (Fig.1a). There were many twins and some polytypes, too. Fig.1a shows normal structure with many twins formed on the (111) planes perpendicular to the growth direction of the whiskers. There were two kinds of microtwins. Multiple twins, which may occurred on four sets of the {111} twin planes ( Fig.1b gives its diffraction patterns). Normally, only multiple twins on two sets of {111} twin planes, which were parallel to the direction of electron beams (viewed along a [110] axis), could be seen (Fig.1c). Thin lamellar twins of predominant β phase interspersed with stacking faults (Fig.1a). Within the twinned lamellae, microtwins sometimes terminated in the matrix (B in Fig.1a). Fig.1d gives clear image of this kind of termination in another area. The interfacial structures between such twins and the matrix were twinning dislocations and Shockley partials. It will be discussed in detailed elsewhere. It can be seen in Fig. 2 that there was a intrinsic stacking fault interrupted by a intersected twin and closed by two twins. Generally, there were few intrinsic stacking faults but many extrinsic ones as indicated by the arrows in Fig.1a. This is because that the energy of intrinsic stacking fault is higher than that of extrinsic one. Polytypes of 2H (Fig. 3a), 4H (Fig.b), 6H (Fig.c), 9R (Fig.d) and 15R (Fig.e) were observed in SiC whiskers by HREM. It is interesting that Fig.3c was taken from a bar of the whisker with strange [1010] growth direction, because the normal growth axis should be [0001] in α-SiC.
EM of hot-dip zinc-iron (galvannealed) coatings
Authors: 'Ecuyer J.D.L , Gagné M. , Cheng C.C. , 'Espérance G.L. 
Published: June 2020

Galvannealed coatings have superior weldability and paintability characteristics than conventional zinc coatings. As a result, the market for galvannealed steel sheets is steadily increasing especially in the automotive industry. The formation of the brittle intermetallic Zn-Fe phases, Γ, Γ1, δ, and Z also affects the for- mability properties of the coated steel product. In order to minimize powdering and flaking, the microstructure of the coating must be closely controlled.The characterization of the galvannealed coating microstructure is essential but has proven to be quite difficult. The individual phases cannot be resolved using optical microscopy. X-ray diffraction appears promising, however some of the intermetallic phases are too thin to produce a detectable signal. Electrochemical stripping has been used, but phase identification is ambiguous. Direct observation of the phases using electron microscopy appears to be the best method for characterizing these coatings.
Morphological and structural studies on ATS terpolymers
Authors: Braglia R. , Ayroldi G. , Fumagalli M. 
Published: June 2020

The outdoor resistance of ABS terpolymers (acrylonitrile-co-styrene-graft-polybutadiene) can be greatly improved by substitution of diene rubber with a nearly saturated ethylene-co-propylene-co-diene (EPDM) elastomer terpolymer. The grafting reaction of styrene and acrylo= nitrile onto EPDM (ATS terpolymer) can be carried out, for example, by suspension polymerization using coarse milled rubber!Transmission electron microscopy (TEM) and dynamic-mechanical spectro= scopy (DMS) at about I Hz are used to understand the grafting reaction and the morphological changes induced by processing the graft or by dilution of the graft with styrene-acrylonitrile (SAN) copolymer. TEM investigation is carried out either on grafted material, embedded in a hard matrix to facilitate sectioning operation, or on pelletized diluted or undiluted materia 1.0smium tetroxide is used as a staining agent.
Crystal chemistry of high-Tc superconductors
Authors: Hervieu Maryvonne 
Published: June 2020

Four years after the discovery of superconductivity at high temperature in the Ba-La-Cu-O system, more than thirty new compounds have been synthesized, which can be classified in six series of copper oxides: La2CuO4 - type oxides, bismuth cuprates, YBa2Cu3O7 family, thallium cuprates, lead cuprates and Nd2CuO4 - type oxides. Despite their quite different specific natures, close relationships allow their structures to be simply described through a single mechanism. The fifth first families can indeed be described as intergrowths of multiple oxygen deficient perovskite slabs with multiple rock salt-type slabs, according to the representation [ACuO3-x]m [AO]n.The n and m values are integer in the parent structures, n varying from 0 to 3 and m from 1 to 4; every member of this large family can thus be symbolized by [m,n]. The oxygen deficient character of the perovskite slabs involves the existence or the co-existence of several types of copper environment: octahedral, pyramidal and square planar.Both mechanisms, oxygen deficiency and intergrowth, are well known to give rise easily to nonstoichiometry phenomena. Numerous and various phenomena have actually been characterized in these cuprates, strongly depending on the thermal history of the samples.
Imaging of Silicon (111) at 1.92Å Resolution using a 100keV STEM
Authors: Xu Peirong 
Published: June 2020

Atomic structure imaging using bright field phase contrast at less than 2Å resolution has become routinely possible in medium and high voltage microscopes (>200 keV). Radiation damage at these elevated voltages can be serious and this limits the length of useful observation time. For example, the knock-on threshold energy for silicon is 120-190keV. Recently, a VG HB501A STEM equipped with a newly developed ultra-high resolution pole piece (Cs=0.7mm) has demonstrated the capability of achieving sub-2Å resolution in imaging the (111) silicon latticer using both bright field (BF) and annular dark field (ADF) modes at an operating voltage of l00keV (Fig.1).A thin silicon specimen was prepared through successive steps of chemical etching, anodic etching and reactive ion etching. Large flat thin areas about 100Å thick were produced in the specimen. Since there is no tilting mechanism for the stage used with this ultra-high resolution pole piece, the specimen was not oriented exactly along the (111) zone axis as indicated by CBED but was less than 1-2° off.
Identification of {110} Inversion Boundaries in GaAs Layers Grown on Si by High Resolution Electron Microscopy
Authors: Mazur J.H. 
Published: June 2020

Epitaxical growth of GaAs on (001)Si substrates can result in formation of inversion boundaries resulting from the presence of monatomic height (a/4[001]Si) steps on the Si substrate surface or nucleation of GaAs initiated with As or Ga in two areas in the same plane. The lack of TEM evidence for the presence of the inversion boundaries in the GaAs film which characterized earlier research in this area has been a source of speculation about the mechanism of inversion boundary elimination. Recently Liliental-Weber et al. have shown, both experimentally and by computer simulation, that the energetically most favorable (110) inversion boundaries could be imaged edge-on in an atomic resolution microscope for thicknesses in the narrow range of 21-26 nm. Earlier multislice calculations indicated no contrast changes across the boundary for imaging at 200 kV for samples thinner than 18 nm.In the present work image simulations were performed for the (110) inversion boundary in GaAs. The aim was to interpret the high resolution image contrast observed in the GaAs thin film grown on Si on JEOL 200CX microscope operating at 200 kV. Other microscope parameters used for computation were: Cs=1.2 mm, DEL=8 nm (half width of a Gaussian spread of focus due to chromatic aberration), ALPHA=0.9 mrd (semi-angle of beam convergence), R=7 nm-1 (objective aperture radius). Simulation were performed for a wide range of thicknesses (up to 31 nm) and focus (-17 to -127 nm) and the results are shown on the simulated image map in Fig. 1. The simulated image map demonstrates that the contrast change ({111} fringe shift) indicating the (110) inversion boundary in the GaAs thin film imaged along <110> direction can be observed for specimens 11 nm thick for focus -127 nm, for 15 nm and above 23 nm thick specimens for a wide range of calculated focus values.
Slow-Scan CCD Camera for Transmission Electron Microscopy
Authors: Mooney P.E. , Fan G.Y. , Meyer C.E. , Truong K.V. , Bui D.B. , Krivanek O.L. 
Published: June 2020

Transmission electron microscopes aretraditionally equipped with three imaging devices: 1) the main viewing screen, which shows a large portion of the specimen in real time, 2) the focussing screen, which is viewed through binoculars and shows a small area of the specimen at a high magnification also in real time, and 3) a film camera, which permits time-integrated images to be recorded with a large number of pixels and a good detective quantum efficiency (DQE). In many laboratories, TV cameras have replaced the real-time viewing devices 1 and 2. The slow-scan CCD technology now promises to replace the photographic film.Slow-scan charge coupled device (CCD) sensors differ from the CCDs found in home video cameras in several aspects: their pixels are typically 10x larger in area, there is no frame-transfer buffer which would enable the CCD to function at TV rate, and the CCDs are optimized for operation at low temperature and slow readout speed. The increased size permits more electrons to be stored per pixel, which increases the dynamic range of the CCD. The elimination of the frame-transfer buffer allows the whole area of the CCD to be used for image detection. The low temperature and slow readout speed minimize the CCD dark current and readout noise.
Surface Magnetic Microstructural Analysis using Scanning Electron Microscopy with Polarization Analysis (SEMPA)
Authors: Scheinfein Michael R. , Unguris John , Pierce Daniel T. , Celotta Robert J. 
Published: June 2020

High resolution imaging of magnetic microstructure has important ramifications for both fundamental studies of magnetism and the technology surrounding the magnetic recording industry. In SEMPA, a focused beam of electrons excites secondary electrons on a ferromagnet's surface. The secondaries leave the solid with an electron spin polarization which is characteristic of the net spin density in the ferromagnet. This is related directly to the sample magnetization. By scanning the beam and analyzing the secondary electron spin polarization at each point, a magnetization map of the ferromagnet's surface is generated.As SEMPA is a surface sensitive, magnetic microstructural analysis technique, the environment local to the specimen must be ultra-high vacuum. A schematic of our SEMPA instrument is shown in figure 1. The probe forming electron optical column of a SEMPA system must produce small probes with high currents (> 1 nA) at long working distances ( > 10 mm). The SEMPA system may be equipped with a single, or multiple spin detectors as in figure 1. Two detectors are used for the acquisition of all three orthogonal components of the polarization vector signal. The inefficiency of all polarimeters makes SEMPA time consuming when compared to conventional SEM. The polarized secondary electrons must be extracted efficiently without introducing any deleterious effects into the focused incident electron beam. This imposes limits on incident beam energies and extraction voltages for the transport optics. Additionally, the transport optics must map the scanned spot of the secondary electrons produced under the incident rastered beam, to a (stationary) position on the spin detector such that undesirable instrumental asymmetries are not introduced.
Hepatitis A Virus Aggregation in Suspensions of Purified Virus
Authors: Humphrey Charles D. , Robertson Betty H. , Khanna B. 
Published: June 2020

Virus capsid structure as determined by x-ray crystallography or by elec imaging of virus particles in thefrozen hydrated state has received con interest during the past decade. Specific functional proteins were localized recently on the Rotavirus capsid by using cryo-electron microscopy, indicating the potential for morphological studies that rel directly to capsid protein function.Most cryo-electron microscopy studies have involved investigation of vir were 40 nm or greater in diameter. Smaller viruses may provide a greate practical difficulty in their preparation and imaging.Highly purified virus for study by x-ray crystallographic studies or ele microscopic evaluation of frozen hydrated particles requires stringent c particle aggregation. Preparations for x-ray crystallography should agg an orderly manner to form arrays and ordered crystals of sufficient size Preparations for cryo-electron microscopy are more desirable when the pa numerous but not over-lapping (Fig. 1). We have experienced uncontrolla aggregation including considerable over-lapping of hepatitis A virus (HA particles when purified preparations were stored in phosphate or tris buffers (Fig. 2).
The Panoramic Rheed Patterns
Authors: Kim Yootaek , Hsu Tung 
Published: June 2020

When applying the reflection high energy electron diffraction (RHEED) and reflection electron microscopy (REM) methods[1] on the study of crystal surfaces it is necessary to index the RHEED spots and recognize the azimuth of the electron beam direction. This can be difficult because the RHEED pattern, unlike the transmission electron diffraction (TED) pattern, is distorted by the inner potential of the specimen and only one half of the pattern is shown. We found that it is useful, at the beginning of working on a certain surface of a certain crystal, to record a panoramic RHEED pattern by rotating the crystal through a large azimuth angle. This produces a map which is similar to the Kikuchi maps[2] used in transmission electron microscopy (TEM).Two examples of these panoramic RHEED patterns, one from the Pt(111) [3] and the other from α-Al203 (0001) [4,5,6), are shown in Figs. 1 and 2.The transmission Kikuchi maps are recorded using a specimen of suitable thickness such that the Kikuchi lines are strong and the diffraction spots are practically invisible. On the contrary, in making the panoramic RHEED patterns (or RHEED maps) we have no control over the thickness of the specimen. The electron beam enters and exits the same surface of the crystal; therefore, the relative intensities of the Bragg diffracted spots and the Kikuchi lines are not adjustable. The only adjustment lies in choosing the accelerating voltage and the incidence angle of the electrons such that the RHEED pattern has relatively low diffuse scattering.
The Characterization of the Low-Index α-Al203 Surfaces by REM
Authors: Kim Yootaek 
Published: June 2020

Four low-index surfaces; (A-surface), (0001) (basal surface), (R-surface), and of the α-Al203 single crystals were investigated by reflection electron microscopy (REM) and reflection high energy electron diffraction (RHEED) techniques[1] and the characteristics of these surfaces are reported. Specimens were prepared by polishing and annealing[2, 3], Orientation of each surface was confirmed with x-ray diffraction using a Laue camera. Microscopy observation was carried out on a JEOL JEM-200CX TEM operated at 100 KV.To facilitate a fair comparison, the surface areas which were obviously affected by a macroscopic tilt, localized contamination, etc., were avoided. Only the flat, clean, and smooth areas were compared. The surface (Fig. 1. The scale in this figure also applies to all other figures.) shows large atomically flat areas divided by steps running in <000 1> and directions. The steps in this image are not of the same height. Some are one or two atoms high and others are much higher. Fig. 2 shows the
surface which is periodically facetted. One type of the facets is flat and atomically smooth while the other type is rough. The (0001) planes display curved and/or straight steps[4]. An example of curved step configuration is shown in Fig. 3. Most of the steps in this image have the same height. This step configuration is similar to the surface shown in Fig. 4. However, the contrast of the steps are very different; on the (0001) surfaces the steps are much darker and well-defined. A summary of these four surfaces is presented in the table below.
REM Observation of the Prismatic Faces of Sapphire
Authors: Ndubuisi G. C. , Liu J. , Cowley J. M. 
Published: June 2020

The structures and properties of the prismatic faces and
of as-grown single-crystal sapphire are still not well understood. Known information have been limited to calculated surface energies and TEM characterization . The TEM characterization has been limited to identification of the orientation of the structures. For better understanding of the behavior of these surfaces, the use of the RHEED and REM techniques, which have proved their value for purely surface analysis, is necessary. In this paper we report the characterization of these surfaces by the RHEED and REM techniques.The sapphire ribbons were purchased, as-grown, from Saphikon, Inc.. The method of preparation of the REM samples has been reported. The samples were oxygen-annealed for 3 6h at 1500°C. The experiments were performed in a Philips 400T TEM with a vacuum pressure of 10-6 Torr.Fig. 1(a) is a REM image showing the structural features that characterize the
surface in the
beam direction. The surface can be seen to exhibit periodic bright and dark domains, with parallel small steps (S) in average direction oriented 7° from the
(after de-foreshortening). The appearence of two domains may be due to a difference in termination of the lattice at the surface. The THREED pattern (insert) shows the specular reflection used for imaging. Fig. 1(b) shows the structural features on the
surface in the
beam direction. Two domains (grey, G and white, W) can also be seen here, and may be attributed to the same effect. A well defined step can be seen at the boundary of the two domains. The faceted steps in the white domain can be seen to adopt average direction oriented 5° from the
. Strain field of dislocation (D) can be seen to run across the steps, and is displaced at the steps. The RHEED pattern (insert) shows the specular spot used for imaging. Fig. 2 shows the structural features on the
surface in the
beam direction, with the spot (insert) used for imaging. Emerging dislocation (D) can be seen terminating at kink of the large step.
Detection by Induction in the Environmental SEM
Authors: Danilatos Gerasimos D. 
Published: June 2020

The advent of environmental SEM (ESEM) has revealed the true mechanism of detection, when the current mode is used. The ESEM allows the introduction of gas in the specimen chamber with a pressure at least sufficient to maintain specimens in their wet state. The gas is also used to suppress charge accumulation on insulating specimens and, furthermore, it can be used as a detection and amplification medium. The resolving power of the instrument is not impaired by the presence of gas, whereas new contrast mechanisms are now possible. The foundations of this technology have been outlined in an extended survey previously.It has been shown that the products of interaction between various signals and gas can be used for imaging. Ionization, in particular, was first used for imaging by Danilatos. A comprehensive theoretical survey of the gaseous detector device can be found elsewhere. Here, the principle of pulse induction by moving charge carriers between electrodes is highlighted. Any charge moving between two electrodes induces a charge of opposite sign on each electrode with a concomitant current flow in the external circuit. The current flow stops when the charge carriers terminate their motion. Therefore, pulses are generated by each electron in the beam and in the signals emerging from the beam-specimen interaction. The sum of all pulses corresponding to a given pixel element results in the total signal pulse carrying information from the specimen. This way of looking at signal formation would be superfluous, if one could always arrive at the same result by simply counting the number of charge carriers that arrive at a particular electrode. However, the creation of ions in the gas of an ESEM and the possibility of imaging non-conducting surfaces has compelled us to resort to first principles in order to properly understand, explain and evaluate new phenomena. First, the ions are more than 1000 times slower than electrons, and they may limit the frequency response of the system; the simple counting of electrons that arrive at the anode, while the corresponding ions have not yet reached the cathode, provides an incorrect signal intensity. Second, microscopists would not use the “specimen absorbed current mode” with insulating specimens in vacuum SEM, whereas a new situation exists in ESEM.