bluuebunny/crossref_metadata_2025 huggingface dataset:
Electron optical improvements for field emission SEMS
Authors: Moll S.H.
Published: June 2020

The superiority of field emission electron sources for high resolution electron microscopy, as originally developed by Crewe et al, is well understood. A number of commercially available SEMs of satisfactory quality have taken advantage of the small energy spread and high relative brightness of the FE source. More recently, AMRAY has introduced high resolution field emission SEMs which take advantage of alternative configurations for the electron source and the electron optics of the column.

The cold (room temperature) FE source has been replaced with ZrO/W FE sources operating in the Schottky mode. These sources have proven to be a practical alternative to cold FE for both high resolution and high incident current applications. The zirconiated source requires no periodic "flashing" to clean the emitter tip since it operates continuously at a temperature of the order of 1800° K. The ZrO/W gun current is an order of magnitude higher than that of the cold source, it exhibits even higher angular brightness, and operates stably at a vacuum of 10-9 torr, an order of magnitude poorer in pressure than that required for cold FE sources.
HREM of twin intersections in Silicon
Authors: Hetherington C.J.D. , Dahmen U. , Pirouz P. , Westmacott K.H.
Published: June 2020

A twin band is an effective barrier to the propagation of another, noncoplanar twin band. However, under conditions of sufficiently high local stress, twin intersections are possible. The geometric configuration of intersecting twin bands has been considered in the past in the context of dislocation mechanisms and deformation behavior. Observations on plane-strain deformed Co-8wt%Fe single crystals showed that intersecting twins gave rise to highly complex local deformation structures and led to the conclusion that the strain in the region of intersection was accommodated either by slip or by (secondary) twinning. The recent discovery of hexagonal ribbons of Si in hot-indentation experiments on silicon has renewed the interest in twin intersections as an integral part of the deformation microstructure, and prompted their study by high resolution microscopy.

It is frequently necessary to record high resolution images under less than optimum conditions, e.g. because of radiation damage or local distortions. It then becomes desirable to devise methods for extracting the maximum information from such imperfect images.
Structure and defect structure of recent high Tc superconductors
Authors: Tendeloo G. Van , Zandbergen H. W. , Landuyt J. Van , Amelinckx S.
Published: June 2020

Defects in the new superconducting materials play an important role and the superconducting properties are often influenced by their presence. In this review we will not consider the “classical” 1-2-3 compounds since they will be treated in a separate contribution.

The Bi2Sr2CanCun+1Ox compounds show a strong intergrowth of lamellae of each of the phases corresponding with different n-values. An experimental relationship has been established between the n-values and the critical temperature for superconductivity; n=0,1,2 corresponding respectively with Tc values of 20K, 80K and 110K. Each of these phases is moreover incommensurately modulated. The origin of this modulation is still not definitively elucidated but we have performed a number of experiments and substitutions in order to get more information on the nature of this distortion. Several models are compared; the most likely one being the presence of extra oxygen in the BiO layers and the subsequent relaxation of the atom positions.
Crystal structure and microstructure of high-Tc superconducting Bi2Sr2CanCun+1O6+2n+δ
Authors: Eibl O.
Published: June 2020

Results of the analyses of the crystal structure and crystal defects of high-Tc Bi2Sr2CanCun+1Oz ceramics are presented. Specimen were prepared in a conventional mixed oxide process. In series of electron diffraction patterns of the (n=1) phase(Tc≈ 80 K) reflections appear which are not consistent with the Bbmb space group symmetry suggested in the litterature. Extinction conditions obtained from electron diffraction patterns are compared to extinction conditions of space groups (e.g. Pbmb and Pbnb) which have a primitive Bravais lattice and are “klassengleiche” subgroups of Bbmb.

Planar crystal defects on (001) planes were observed in Bi2Sr2CanCun+1Oz phases. These defects are (i) intrinsic and extrinsic (001) stacking faults forming lamellae of {n=0,1,2,3,4) phases and (ii) (001) twin boundaries. An extrinsic (001) stacking fault in the {n=1) phase is shown in fig.1. The image was obtained with a 400 kV electron beam in a microscope equippped with a pole piece with small spherical aberation (Cs= 1mm).
Twin boundary structure of a Y-Ba-Cu-O superconductor
Authors: Zhu Yimei , Suenaga Masaki , Sabatini R. L. , Xu Youwen
Published: June 2020

The (110) twin structure of YBa2Cu3O7 superconductor oxide, which is formed to reduce the strain energy of the tetragonal to orthorhombic phase transformation by alternating the a-b crystallographic axis across the boundary, was extensively investigated. Up to now the structure of the twin boundary still remained unclear. In order to gain insight into the nature of the twin boundary in Y-Ba-Cu-O system, a study using electron diffraction techniques including optical and computed diffractograms, as well as high resolution structure imaging techniques with corresponding computer simulation and processing was initiated.

Bulk samples of Y-Ba-Cu-O oxide were prepared as described elsewhere. TEM specimens were produced by crushing bulk samples into a fine powder, dispersing the powder in acetone, and suspending the fine particles on a holey carbon grid. The electron microscopy during this study was performed on both a JEOL 2000EX and 2000FX electron microscopes operated at 200 kV.
High-resolution analytical microscopy of YBa2Cu3O7-δ grain boundaries
Authors: Babcock S. E. , Kelly T. F. , Larbalestier D. C
Published: June 2020

Grain boundaries are generally considered to be the microstructural entity which is responsible for the weak-link behavior that limits the transport critical current densities (Jct) of the high temperature oxide superconductors to values which are unacceptable for most applications. However, the mechanism by which grain boundaries suppress Jct has yet to be identified. This issue is particularly challenging because the microstructural features which affect Jct may be as small as the superconducting coherence length (< 1 nm for the high-temperature superconductors). Electron microscopy studies, nevertheless, have suggested a number of possible microstructural origins for the weak-link behavior. Among the candidates are: the strains associated with grain boundary dislocations; segregated impurity atoms layers of second phase; and local (within ∼ 5 nm) composition changes at grain boundaries which involve a reduction in the oxygen atom fraction.
The effects of probe characteristics and specimen thickness on the spatial resolution in the AEM
Authors: Michael J. R.
Published: June 2020

In order to interpret the high-resolution x-ray microanalytical data generated by modern Analytical Electron Microscopes (AEM), it is necessary to understand the effects of the electron probe characteristics (size, current distribution) and the specimen thickness and composition on the spatial resolution. The spatial resolution obtained in a VG HB-501 dedicated Scanning Transmission Electron Microscope (STEM) has been determined over a range of specimen thicknesses and for different probe sizes. The experimentally determined spatial resolutions are compared with calculations based on the Gaussian beam broadening technique.

The spatial resolutions were measured from composition profiles obtained from specimens which had abrupt changes in composition. Suitably heat treated 40wt%Ni- 30wt%Cr-Fe specimens consisted of precipitates of 80-90wt%Cr in a matrix of 25- 29wt%Cr. Composition profiles were obtained by stepping the electron probe across a precipitate/matrix interface. Accurate probe placement was achieved by computer control of the analysis positions in conjunction with software which corrected for specimen drift. Beam broadening in the specimen degrades the abrupt step change in composition into a continuous one with a finite width.
High-spatial-resolution secondary and Auger imaging in a STEM
Authors: Drucker J.S. , Krishnamurthy M. , Hembree G.G. , Hong Luo Chuan , Venables J.A.
Published: June 2020

Secondary electrons form the main signal in a standard SEM, and machines incorporating Auger electron spectroscopy and imaging have become widely commercialized. However, these approaches to low energy (0-2000eV) electron spectroscopy and imaging do not work at the highest spatial resolution, since there are geometrical and electromagnetic conflicts as the focal length of the probe forming lens is reduced. As discussed elsewhere in more detail, the solution is to make the magnetic probe forming lens of the SEM/STEM also function as the first stage of the electron collection and analysis system.

A new lOOkV field emission STEM has been constructed for the NSF HREM facility, which incorporates provision for using these low energy electrons from both sides of a thin sample. The outline design has been described previously. The microscope, codenamed MIDAS, is of UHV construction throughout with ∽10−10 mbar at the sample position, and extensive surface preparation facilities. The region of the column concerned with secondary and Auger electrons is shown diagrammatically, but to scale, in fig. 1. This region consists of the objective lens, O, bounded by analyser chambers AC1 and AC2, onto which the electron detectors are mounted.
High-angle annular dark-field imaging on a TEM/STEM system
Authors: Otten Max T.
Published: June 2020

High-Angle Annular Dark-Field (HAADF) imaging is a technique that brings out compositional contrasts with a sensitivity about 106 times higher than X-ray images, thereby allowing image recording in normal STEM exposure times (a matter of minutes) instead of the time required for X-ray images (usually several hours). The technique uses those electrons that have undergone high-angle scattering. These have Z dependence and are therefore very effective in bringing out compositional contrasts.

Much of the HAADF work to date has been performed on dedicated STEMs. However, the technique can be performed as well with the annular Dark-Field detector on several types of TEM/STEM systems. Such systems have the added advantage of camera-length flexibility, providing a range of images with different kinds of information from the same detector.
High-spatial-resolution x-ray microanalysis of Al-2wt.% Cu aluminum thin films
Authors: Romig A. D. , Frear D. R. , Headley T. J.
Published: June 2020

Aluminum - 2 wt.% copper alloys are commonly used in thin film form as interconnect metallization lines for integrated circuits. Experience has shown that the addition of the Cu to the Al, albeit at a decrease in conductivity, makes the metallizations more resistant to failure by electromigration. However, the mechanism by which Cu increases the resistance to electromigration has never been positively identified. One theory proposes that Cu coats the Al grain boundaries (boundaries are enriched in Cu) and retards grain boundary diffusion thereby reducing electromigration. Another theory suggests that a continuous thin layer of CuAl2 forms along the boundaries also reducing grain boundary transport and therefore the tendency to electromigrate. Recently, Frear et al. have reported on a detailed set of experiments to examine these theories from a microstructural viewpoint. Here, the details of the high spatial resolution microanalysis done to support the study of Fear, et al. are reported.

Al- 2wt.% Cu was magnetron sputtered onto a borosilicate glass (BSG) coated (100) Si wafer. The Al-Cu films were sputtered at room temperature from a single source under an argon atmosphere at a deposition rate of 100 nm/min. Films 400 and 800 nm thick were prepared. The films were annealed under a 15% hydrogen forming gas (reducing) at 425°C for 35 min.
The role of secondary hardening dispersions in the ductile failure of an ultrahigh-strength steel
Authors: Hayzelden Clive
Published: June 2020

Ultrahigh strength (UHS) steel is often achieved only at the expense of toughness. High hardness martensitic steels used for structural applications involving significant tensile loading and therefore requiring a KIC fracture toughness of 50 MPa., are limited to a Rockwell C hardness (Rc) of 50-55. In UHS steels the failure mechanism is predominantly that of plastic shear instability in which flow localization leads to the formation of intense shear bands. The underlying cause of the flow localization is investigated here in an UHS (Rc 55-56) 4340 steel.

Azrin et al. have recently investigated the critical strain for shear localization in UHS 4340 steel under conditions of isothermal and adiabatic shear. The air melt 4340 steel of composition (wt.%): Fe-1.71Ni-0.75Mn-0.4lC-0.79Cr-0.25Si-0.24Mo-0.13Cu-0.011S-0.005P, was normalized at 910°C for one hour then austenitized at either 840°C or 1100°C for one further hour. Following austenitization, the specimens were stage I tempered at l60°C for one hour to form epsilon carbide and produce a peak hardness in the range Rc 55-56. The fracture surfaces of the double linear shear test specimens were examined in a JEOL 35U SEM and the alloy microstructures were investigated using a combination of CBED, EDS and EELS on both jet electropolished thin foils and carbon extraction replicas in a Philips EM 420T S(T)EM.
Microanalysis of a thin native oxide layer on silicon surfaces by parallel EELS
Authors: Kim M.J. , Carpenter R.W.
Published: June 2020

It is known that a thin layer of so-called “native oxide” is formed immediately on clean silicon surfaces upon exposure to air at room temperature. The composition of native oxide, SiOx, is uncertain. Various experimental methods have yielded values less than 2 for x. Parallel energy loss spectrometers fitted to analytical electron microscopes with field emission sources provide sufficient spatial resolution (≤4nm) for direct quantitative analysis of native oxide films. In this note we report the first EELS analysis of native oxide on silicon.

CZ Si {001} wafers, HF etched at room temperature and rinsed in doubly deionized water, were exposed to air until placed in an UHV chamber. A thin layer of Au wasthen deposited on Si surfaces, so that the native oxide layer was preserved between Si substrate and deposited Au film. Cross section TEM specimens were made in the usual way.
Sublattice occupancies of Pd in Cu0.5Au0.5−xPdx alloys by ALCHEMI
Authors: Bentley J. , Hisatsune K.
Published: June 2020

The composition dependence of the lattice parameters of Cu0.5Au0.5−xPdx (0<x<0.25) alloys with the face-centered tetragonal ordered L10 structure has been measured by X-ray diffraction. The results confirm the behavior reported earlier by Nakahigashi et al. Whereas the unit cell volume decreases monotonically with increasing palladium content, the axial ratio c/a exhibits a maximum at about 4 at. % Pd, corresponding to local minimum and maximum values for a and c, respectively. In theoretical modelling of this behavior it was assumed that Pd atoms were randomly interchangeable with only Au atoms. Several techniques, including the ALCHEMI method, are being used to measure directly the sublattice occupancy of palladium.

Electropolishing with various solutions failed to produce good quality specimens for analytical electron microscopy (AEM). Ion milling of specimens previously electropolished with an electrolyte of 10% perchloric acid in acetic acid was more successful. Analyses were performed with Philips EM400T/FEG and CM12/STEM instruments and EDAX 9100/70 and 9900 energy dispersive X-ray microanalysis systems.
Materials science of cells: Electron probe x-ray microanalysis and x-ray mapping in biology
Authors: Somlyo Andrew P.
Published: June 2020

The general aims of Electron Probe X-ray Microanalysis (EPMA) in Biology is similar to that in Materials Science: the determination of composition at sub-micron resolution. Special requirements include stringent precautions for specimen preparation, high sensitivity for detecting low concentrations of elements avoidance, and if not possible, quantitation and control of radiation damage, and spatial resolution of at least tens of nanometers.

Special precautions for specimen preparation are dictated by the fact that biological materials exist in an aqueous milieu, and one of the most common objectives of biological EPMA is the localization and quantitation of diffusible elements. Therefore, specimen preparatory techniques must include handling of live tissues in a manner that maintains normal physiological states, and rapid freezing to trap diffusible elements in their physiological compartments. In order to obtain high spatial resolution, ultrathin cryosections have to be obtained, freeze dried and transferred to the microscope under conditions that prevent elemental translocations. Specimen temperatures during cryo-sectioning are usually at about -100°centigrade.
Fibrillin molecules aggregate to form an extendible beaded structure which is a major component of “Elastin Associated” microfibrils
Authors: Keene Douglas R. , Maddox B. Kerry , Spurgin Marie B. , Sakai Lynn Y. , Glanville Robert W.
Published: June 2020

A mouse monoclonal antibody was used to identify beaded aggregates found in guanidine extracts of human amnion as assemblies of fibrillin molecules. These aggregates were also shown to be a major component of extracellular matrix microfibrils. We further demonstrated that the periodicity of these aggregates can be increased when subjected to mechanical stress.

Human amnion was extracted with guanidine and the extracted material purified using ion exchange and molecular sieve chromatography. A high molecular weight fraction was precipitated by dialyzing against dilute acetic acid. Part of the precipitate was suspended in 0.2 M ammonium bicarbonate buffer and rotary shadowed. A second portion was resuspended in culture medium containing antibody which recognizes matrix microfibrils, diluted 1:5 in ammonium bicarbonate and reacted for 120 minutes at room temperature. Antibody labeled precipitate was washed by repeated pelleting and resuspension in buffer and then incubated in Janssen GAM 5 nm gold conjugate for 60 minutes at room temperature.
Higher-order assembly of gastropodan hemocyanins as revealed by Scanning Transmission Electron Microscopy (STEM)
Authors: Hamilton M.G. , Rodriguez R.R. , Herskovits T.T. , Wall J.S.
Published: June 2020

The hemocyanins of gastropods consist of aggregates of a cylindrical decameric subparticle that assembles into di-, tri-, tetra-, penta-, and larger multi-decameric particles with sedimentation coefficients of ca. 105 S, 130 S, 150 S, 170 S, and higher values. We are using STEM to measure the masses of individual particles and analytical ultracentrifugation to determine the distribution of sedimenting components.

Hemocyanins were isolated from freshly collected hemolymph by gel filtration on BioGel A-5m columns. Samples were analyzed with schlieren optics in a Beckman Madel E ultracentrifuge. Specimens were diluted into 0.1 M HEPES, pH 8.0, 0.01 M MgC12 to a final concentration of 100 ug/mL and freeze-dried for STEM analysis. The STEM instrument was operated at 40 kV using a -140 °C cold stage. The elastically scattered electron signal from the STEM large angle annular detector was used to form the images. The specimens were imaged with 10 A pixels at a dose of 6-10 e/A2. Molecular weights of individual particles were measured as previously described.
Crystal structure-size relationship in ultrafine metallic particles
Authors: Liu Jun , Sarikaya Mehmet , Aksay Ilhan A.
Published: June 2020

Ultrafine particles usually have unique physical properties. This study illustrates how the lattice defects and interfacial structures between particles are related to the size of ultrafine crystalline gold particles.

Colloidal gold particles were produced by reducing gold chloride with sodium citrate at 100°C. In this process, particle size can be controlled by changing the concentration of the reactant. TEM samples are prepared by transferring a small amount of solution onto a thin (5 nm) carbon film which is suspended on a copper grid. In this work, all experiments were performed with Philips 430T at 300 kV.

With controlled seeded growth, particles of different sizes are produced, as shown in Figure 1. By a careful examination, it can be resolved that very small particles have lattice defects with complex interfaces. Some typical particle structures include multiple twins, resulting in a five-fold symmetry bicrystals, and highly disordered regions. Many particles are too complex to be described by simple models.
EPMA/TEM characterization of a Pt/Al2O3 catalyst with a nonuniform internal distribution
Authors: Hepburn J. S.
Published: June 2020

The internal distribution of catalytically active metal within a supported metal catalyst can determine its activity, selectivity, and durability. Since the early 1950s, electron probe microanalysis has grown into a mature and widely available technique. By providing quantitative elemental analysis from volumes on the order of 1μm3, EPMA is uniquely suited for the study of metal concentration profiles in supported metal catalysts. However, independent knowledge of metal particle size or dispersion is needed to identify the catalytically active fraction of deposited metal. Most available techniques for metal particle size determination are indirect methods such as selective chemisorption. The transmission electron microscope is the only available device which is capable of resolving metal particles on the order of lnm in diameter. For years, the EPMA has been used in determining internal distributions of deposited metal within supported catalysts, while the TEM has been used to observe supported metal catalyst particles on powdered supports. However, there has been no study that combines both EPMA and TEM which attempts to furnish the information about a supported metal catalyst which is truly desired (its internal distribution of catalytically active metal). In the present work, a Pt/Al2O3 catalyst with a nonuniform internal distribution of deposited Pt was analyzed by incorporating EPMA and TEM.
High resolution microscopy of a Σ = 17 grain boundary in gold
Authors: Krakow William
Published: June 2020

Because of the resolution limits of most microscopes, the imaging of the atomic structure of tilt grain boundaries in metals has been restricted to the most open structures with relatively large projected lattice parameters. In the case of Au the boundaries have been observed along a common [110] axis to both grains of bicrystalline specimens where coincidence cases have been studied such as the: Σ= 11, (113); Σ= 3, (111) and (112) and the ∑ = 19, (331) as well as other low and high angle boundary structures. Here the lattice periodicities visualized correspond to the {111} planes of 2.3Å. For the case of the b.c.c. lattice a Mo Σ = 41, (910) boundary was viewed along a common [001] were the {110} lattice planes of 2.2Å were visualized in a 200kV microscope. Even more recently the same author studied a boundary between the ∑ = 41 and ∑ = 25 reference orientations in Mo in a 400kV microscope, but apparently showed the same resolution as the previously mentioned study. Consequently, only a few studies of boundaries in metals have approached the 2Å resolution limit and recently images were obtained from a ∑ = 5, (310) symmetrical boundary in Au. In this latter study a relatively noisy image was obtained due to the reduced contrast transfer function of a 300kV microscope in the region of the second phase contrast zone. In the case of a 400kV microscope it is expected that a considerable improvement in information transfer would occur. It is therefore the intent of this study to present high resolution results for a tilt boundary in Au near a ∑ = 17, (530) orientation taken at 400kV which has been image processed to enhance the visualization of structural detail.
Solidification of alternative phases in submicron spheres of nickel-chromium
Authors: Lin Hong-Ming , Kelly Thomas F.
Published: June 2020

Kelly et. al. have processed submicron spherical particles of iron-nickel alloys and pure elements by using electrohydrodynamic (EHD) atomization. The liquid droplets solidify in free-flight through a vacuum chamber operated at about 10-5 Pa. Each droplet is a separate nucleation experiment in which the droplet diameter is readily determined. Both high cooling rates and extreme isolation of heterogeneous catalytic sites are realized in the EHD process. Thus, rapid quenching of submicron spherical particles by using EHD process in Ni-Cr alloy may provide an opportunity to produce segregation-free alternative crystal phases (ACP) with low chromium contents.

In this study, Ni-10, 20 and 30 at%Cr alloys were atomized and several samples of each composition were prepared for scanning transmission electron microscopy (VG HB501 STEM) analysis. EDS spectra from about ten different sphere sizes were recorded for each sample. Table I shows the average composition and standard deviation of the EDS measurements. The results indicate that each sphere composition deviates less than two percent. The X-ray counting times of chromium peak are greater than 1000 times in each sampling.
SEM study of humid air effect on fatigue of aluminum alloy 2024-T351
Authors: Jahn M. T. , Voris H. C.
Published: June 2020

There is general agreement that the fatigue life of high strength aluminum alloys is reduced in humid environment. However, there are also data supporting the theory that humidity plays an insignificant role in the reduction of the fatigue life of aluminum alloy 2024-T351. In this study we examined the effects of stress level and water vapor density on the fatigue life of aluminum alloy 2024-T351 using scanning electron microscope (SEM). SEM evidence of the deleterious effect of humid air on the fatigue life of specimens cycled at intermediate stress level was presented. Discrepancies between this study and others were explained.

Commercial aluminum alloy 2024-T35l (4.40Cu-1.45Mg-0.70Mn-0.23 Fe-0.15Si-0.13Zn) extruded bars were fatigue tested in reversed bending. The cycling was conducted in an environmentally controlled chamber. Ten specimens were machined for each fatigue stress level of 248, 276, 290, 317 and 359 MPa. Five specimens fran each stress level were cycled in desicated air at a relative humidity less than 45%.
Dislocation fine structure and mechanical properties of intermetallic alloys
Authors: Veyssière P.
Published: June 2020

Weak-beam (WB) TEM was introduced in the late sixties and flourished during the following decade. It revealed the most powerful all-materials technique for the imaging of very localized strain gradients as for instance in dislocation studies. Although some of its first achievements dealt with the resolution of subdissociation in ordered materials, WB happened to be mostly utilized to answer questions raised along 25 years of active speculation on dissociation in fcc, bcc and hcp metals and alloys; in addition, it helped exploring dislocation properties in ceramic materials (oxides, semiconductors, carbides). With the loss of general interest in core-related dislocation phenomena, the WB technique then became much less popular (excepted for semiconductors) until the new impulse given by the materials community in the field of intermetallic alloys; this effort is the object of the present review. In the meantime, the ultimate resolution obtainable from WB conditions got constantly improved: a separation between dislocation images lesser than 1.5nm is now accessible on modem microscopes. Due to the failure of the column approximation, new precautions then have to be taken at analyzing images (especially in the case of fourfold dissociation in Ll2alloys). It should be added that whatever the technique employed in order to analyze dislocation cores (in-situstraining as opposed to static WB or HREM), it is often difficult to assess that TEM observations represent the very configurations that operate in the bulk materials and control its mechanical behaviour; this point has been addressed elsewhere.
Dislocation structures in TiB2/NiAl composites deformed at high temperatures
Authors: Wang L. , Arsenault R.J.
Published: June 2020

An investigation was undertaken of deformed TiB2/NiAl composites to determine the effect of TiB2 particles on the dislocation density and morphology. Also, the purpose of this investigation was to compare the dislocation structures of the composites with the dislocation structures which had been reported previously in NiAl. The material used in this investigation was produced by the XD process of Martin Marietta and contained 0 and 20V% TiB2 particles in NiAl. The TiB2 particles were about one micron in size, and the grain size of the 0 and 20V% samples is 5 and 1 μm, respectively. In addition to the TiB2 particles, there was also about 10V% aluminum-oxide (α-Al2O3).

The samples were tested in compression to 30% deformation at 870° C at various strain rates ranging from 10-4 per sec. to 2×10-2 per sec. The TEM foils were prepared from the deformed and annealed samples and examined in a 100 KV microscope. Also, a 1 MeV HVEM microscope was used.
Shot-noise simulations of HREM images of polymer crystals
Authors: Pradère Philippe , Thomas Edwin L.
Published: June 2020

High Resolution Electron Microscopy (HREM) is a very powerful technique for the study of crystal defects at the molecular level. Unfortunately polymer crystals are beam sensitive and are destroyed almost instantly under the typical HREM imaging conditions used for inorganic materials. Recent developments of low dose imaging at low magnification have nevertheless permitted the attainment of lattice images of very radiation sensitive polymers such as poly-4-methylpentene-1 and enabled molecular level studies of crystal defects in somewhat more resistant ones such as polyparaxylylene (PPX) [2].

With low dose conditions the images obtained are very noisy. Noise arises from the support film, photographic emulsion granularity and in particular, the statistical distribution of electrons at the typical doses of only few electrons per unit resolution area. Figure 1 shows the shapes of electron distribution, according to the Poisson formula :
Electron spectroscopic diffraction at (111) silicon foils
Authors: Reimer L. , Fromm I.
Published: June 2020

An electron diffraction pattern (EDP) consists of an overlap of patterns of all energy losses in the electron energy-loss spectrum (EELS). Electron spectroscopic diffraction (ESD) in an energy filtering electron microscope (EFEM) allows to separate the contributions of different energy losses to the unfiltered diagram observed in conventional TEM. We report about diffraction experiments with a Zeiss EM902 on (111) silicon foils which show how the EDP of single-crystal foils changes with increasing energy loss and foil thickness. An EDP normally contains the Bragg spots, diffuse streaks by electron-phonon scattering, excess and defect Kikuchi lines when the number of electrons striking the lattice planes is different from opposite sites, a system of excess (bright) Kikuchi bands with an intensity proportional to the probability ψψ⋆ of the Bloch wave field at the nuclei, and defect Ki-kuchi bands when the number of diffusely scattered electrons is equal on both sides of the lattice plane and the intensity becomes proportional to ΣIg.

EDPs of thin foils show an increase of contrast of the Bragg spots and the thermal diffuse streaks when comparing an unfiltered (Fig.1a) and zero-loss filtered EDP (Fig.1b). Because the streaks are caused by elastic scattering, they can not be ob served with the plasmon loss (Fig.1c). Bragg spots are also observed at higher energy losses because all delocalized inelastic scattering processes with energy losses less a few hundred eV show intraband transitions which preserve the type of excited Bloch waves.
Electron energy loss near edge structure
Authors: Weng Xudong , Rez Peter
Published: June 2020

In inner shell absorption spectroscopy, the near edge structure (NES) up to 30 eV above threshold is a prominent solid state effect. It is well known that electron energy loss spectroscopy is advantageous for the study of light elements, like carbon, nitrogen, oxygen, and fluorine. Due to the complexity of the near edge region, it is only recently that a significant amount of research have been carried out to extract information on the solid state effects.

The theory to interpret the near edge structure is based on Fermi’s Golden rule. Two methods have been derived which give different and complimentary physical pictures about the inner shell excitation process. Single electron band structure techniques have been successfully applied to the interpretation of x-ray near edge structure. using the projected densities of states (DOS) of L±1 characterpermitted by the dipole selection rule. The multiple scattering method views the near edge structure (XANES) [2.3] as coming from the interference between the outgoing ejected electron wave and renections from neighboring atoms.
New instrumentation for time-resolved microanalysis using parallel EELS
Authors: Weiss J. K. , Rez Peter , Higgs A. A. , Das Chowdhury K. , McCartney M. R.
Published: June 2020

The new parallel detection EELS systems have opened up the possibility of performing time-resolved studies of compositional changes and the evolution of near-edge structure caused by radiation damage or positional changes across heterogeneous interfaces. Zaluzec has described a system using a 2-D CCD array in which one axis corresponds to time. Such a system can acquire 100 512-channel spectra in a time limited by the CCD readout and digitization time. Our system is an extension of an existing data acquisition system using modified software and a Gatan 666 parallel EELS equipped with a 1024-element 1-D photodiode array. We have taken advantage of the computer’s memory architecture to stack a time series of multiple spectra (or any portion of these spectra) in the extended memory space above 256 K-bytes. The 4 M-bytes of system memory enables storage of up to 1920 spectra 1024 channels long. For inner-shell edges, a 100-500 msec signal integration time per spectrum is typical. After acquisition the series of spectra are recovered from extended memory and are stored on disk. They can then be retrieved for post processing or can be played back through the display, from which VCR recordings can be directly made.
Conditions for atomic imaging of mullite
Authors: Epicier T. A. , Thomas G.
Published: June 2020

Mullite is an aluminium-silicate mineral of current interest since it is a potential candidate for high temperature applications in the ceramic materials field.

In the present work, conditions under which the structure of mullite can be optimally imaged by means of High Resolution Electron Microscopy (HREM) have been investigated. Special reference is made to the Atomic Resolution Microscope at Berkeley which allows real space information up to ≈ 0.17 nm to be directly transferred; numerous multislice calculations (conducted with the CEMPAS programs) as well as extensive experimental through-focus series taken from a commercial “3:2” mullite at 800 kV clearly show that a resolution of at least 0.19 nm is required if one wants to get a straightforward confirmation of atomic models of mullite, which is known to undergo non-stoichiometry associated with the presence of oxygen vacancies.

Indeed the composition of mullite ranges from approximatively 3Al2O3-2SiO2 (referred here as 3:2-mullite) to 2Al2O3-1SiO2, and its structure is still the subject of refinements (see, for example, refs. 4, 5, 6).
Dislocation analysis in Al2TiO5
Authors: Epicier T. A. , Wohlfromm H. , Thomas G.
Published: June 2020

Aluminium titanate (Al2TiO5, space group Cmcm, a=0.359 nm, b=0.943 nm and c=0.964 nm) is a ceramic compound which is considered as a material of high technological interest, due to its elevated melting point and its low thermal expansion. However, its noticeable thermal expansion anisotropy and its decomposition into Al2O3 + TiO2 below 1300°C seriously reduce the range of industrial applications of this material. Therefore, efforts are currently made in order to improve both mechanical strength and thermal properties of Al2TiO5, especially by sintering the product with various additives, such as MgO (which lowers the decomposition temperature) or ZrO2 (which might have a toughening effect). In the present work dislocations have been studied in several samples as given in Table 1. Both Conventional Transmission Electron Microscopy (CTEM) and High Resolution EM (HREM) analyses were undertaken on specimens prepared in the usual manner viz ion-beam thinning. Structure imaging was done at 800 kV using the ARM microscope, and CTEM was conducted on a Jeol 200 CX instrument.
Characterization of thermal barrier coatings formed by electon-beam physical vapor deposition (EB-PVD)
Authors: Unal Ozer
Published: June 2020

Interest in ceramics as thermal barrier coatings for hot components of turbine engines has increased rapidly over the last decade. The primary reason for this is the significant reduction in heat load and increased chemical inertness against corrosive species with the ceramic coating materials. Among other candidates, partially-stabilized zirconia is the focus of attention mainly because ot its low thermal conductivity and high thermal expansion coefficient.

The coatings were made by Garrett Turbine Engine Company. Ni-base super-alloy was used as the substrate and later a bond-coating with high Al activity was formed over it. The ceramic coatings, with a thickness of about 50 μm, were formed by EB-PVD in a high-vacuum chamber by heating the target material (ZrO2-20 w/0 Y2O3) above its evaporation temperaturef >3500 °C) with a high-energy beam and condensing the resulting vapor onto a rotating heated substrate. A heat treatment in an oxidizing environment was performed later on to form a protective oxide layer to improve the adhesion between the ceramic coating and substrate. Bulk samples were studied by utilizing a Scintag diffractometer and a JEOL JXA-840 SEM; examinations of cross-sectional thin-films of the interface region were performed in a Philips CM 30 TEM operating at 300 kV and for chemical analysis a KEVEX X-ray spectrometer (EDS) was used.
Radiation damage characteristics of silicon oxynitride ceramic
Authors: Das Chowdhury K. , Carpenter R.W. , Weiss J.K.
Published: June 2020

Silicon Oxynitride is a ceramic that often forms during sintering of Si3N4, as a result of reactions with the sintering aid or from oxygen adsorbed on the surface of the powder particles. It is an orthorhombic crystal in which Si is tetrahedrally coordinated, and it is quite susceptible to electron irradiation effects. We present here the first quantitative observations of radiation effects.

Time-resolved Electron Energy Loss Spectroscopy (EELS) was used to follow the changes in elemental concentration with irradiation time in crystals of Si2N2O during electron-irradiation at 100kV. The experiments were performed in a Philips EM400ST microscope fitted with a field emission source using a focussed probe and a Gatan 666 Parallel EELS spectrometer. An LN2 cooled specimen holder was used to minimize contamination. Specimens were made from a ceramic containing plate-type Si2N2O microcrystals with long edges parallel to [010], described earlier. The special software for time resolved spectrum acquisition was developed in this laboratory. The Si-L, O-K and N-K edges were acquired as a function of irradiation time at known current density and then individually processed and numerically integrated over a 100eV interval, beginning at edge onset.
Crystallization behavior of Li2O-Al2O3-6SiO2glasses containing Ta2O5
Authors: Hsu J.Y. , Berta Y. , Speyer R.F.
Published: June 2020

TiO2and ZrO2have been successful nucleating agents in lithium aluminosilicate glass-ceramics, which have applications including cooking ware, heat exchangers, telescope mirrors, and the matrices of fiber reinforced composites. The major crystalline phases, stuffedβquartz ss (solid solution) andβspodumene ss, have very small volume coefficients of thermal expansion, and thus these glass-ceramics have excellent thermal shock resistance. Ta2O5has recently been established as an effective (phase separation inducing) nucleation catalyst in glasses of this system. The effect of varying (re-heating) thermal treatment on microstructure formation in these glasses with Ta2O5additives is discussed herein.

Glass samples were prepared in 100 gram batches using SiO2, Li2CO3, Al2O3, and Ta2O5powders as raw materials. The mixture, 1:1:6 in molar ratio of Li2O:Al2O3:SiO2with 4 mol% Ta2O5, was calcined in a Pt - 3.5% Rh crucible at 1350°C for 2 hours, held for another 12 hours at 1650°C, and then quenched onto a copper plate. The glass and devitrified samples were then mechanically thinned and ion-milled using a liquid nitrogen cold stage.
Nonstoichiometry effects on microstructure in amorphous CdGeAs2
Authors: Hong K.S. , Berta Y. , Speyer R.F.
Published: June 2020

The mechanism of nucleation and growth in amorphous semiconducting CdGeAs2has been demonstrated with the aid of thermal analysis, TEM, and XRD to be: 1) germanium precursor formation, 2) nonstoichiometric chalcopyrite structure crystal growth from the germanium precursor, and 3) solid state transformation of the precursor to the chalcopyrite structure. This crystallization sequence does not follow the pseudo-binary phase diagram which indicates that only in the case of germanium content in excess of stoichiometric CdGeAs2shouldαsolid solution form in equilibrium with chalcopyrite structured CdGeAs2(see Fig. 1). To establish the extent of the germanium precursor nucleation mechanism, samples of deficient (CdGe0.8As2), stoichiometric, and excess (CdGe1.2As2) germanium content were prepared in the bulk amorphous form.

Samples were fabricated by water-quenching the molten compound, vacuum sealed in carbon coated (by pyrolysis) fused silica tubes. The amorphous ampoules were broken out of the tubes, sliced to fit the hermetically sealing sample capsule for the differential scanning calorimeter (DSC) and heat treated at 0.5°C/min to various points along the exotherms. The partially devitrified samples were mechanically thinned and ion-milled using a liquid nitrogen cold stage, for examination in the TEM.
TEM microstructure of laser-trimmed Si-Cr devices
Authors: Anderson Ron , Benedict John , Klepeis Stanley J.
Published: June 2020

Laser ablation adjustment of resistance values of resistors Incorporated within integrated circuits is common practice. We analyzed cross-section and plan-view Si-Cr laser ablated resistors in actual devices. Resistors were prepared as follows: nominally 500Å Si-Cr films were deposited on 2nd level quartz, patterned and then covered with 3rd level quartz. Oxygen was bled into the sputtering chamber during the Si-Cr deposition. No special heat-treatment was given to the Si-Cr resistors, but they did see 5-hours at 450C during the 3rd level quartz sputter. The resistors were contacted by 2-micron thick 2nd level Al-Cu metal stripes. The resistors were trimmed by focussed laser light impinging on the Si-Cr layer buried in SiO2. Trimming was accomplished, step-wise, using a YAG laser operating at different power settings. Plan-view samples were made by mechanically polishing to just above the resistor, inverting the sample, and mechanically polishing from the substrate side to just below the resistor. A copper microscope grid was epoxied on the thin specimen. After the two mechanical polishing operations, the sample was about one micron thick. Final thinning of the sample to about 0.5 μm was accomplished via a short ion-milling operation. Cross-sectioned samples are prepared the same way, taking care to have the final mechanical polishing operation terminate in the laser ablated area. No acids are used in either case. The samples see only acetone and water.
Symmetry breaking in convergent-beam diffraction
Authors: Eaglesham D.J.
Published: June 2020

Convergent Beam Electron Diffraction is now almost routinely used in the determination of the point- and space-groups of crystalline samples. In addition to its small-probe capability, CBED is also postulated to be more sensitive than X-ray diffraction in determining crystal symmetries. Multiple diffraction is phase-sensitive, so that the distinction between centro- and non-centro-symmetric space groups should be trivial in CBED: in addition, the stronger scattering of electrons may give a general increase in sensitivity to small atomic displacements. However, the sensitivity of CBED symmetry to the crystal point group has rarely been quantified, and CBED is also subject to symmetry-breaking due to local strains and inhomogeneities. The purpose of this paper is to classify the various types of symmetry-breaking, present calculations of the sensitivity, and illustrate symmetry-breaking by surface strains.

CBED symmetry determinations usually proceed by determining the diffraction group along various zone axes, and hence finding the point group. The diffraction group can be found using either the intensity distribution in the discs
On the accurate measurement of low-order structure factor amplitudes and phases by quantitative CBED
Authors: Spence J.C.H. , Zuo J.M.
Published: June 2020

Over the past four years we have developed the software and computer control of electron microscope data acquisition needed for quantitative electron crystallography. CBED patterns in the systematics orientation (see Fig. 1) are scanned over the slit of a Gatan Model 607 ELS unit, and the energy filtered elastic scattering rocking curves compared with the results of theoretical calculations (see Figs. 2 and 3). The calculations are based on the Bloch wave method, and treat three-dimensional dynamical diffraction from a non-centrosymmetric crystal with absorption and inclined boundary conditions. The Fortran source code has been published, and is available by Bitnet from ZALUZEC @ANLMST. High voltage, sample thickness, absorption coefficients Vg and selected low order structure factors Vg are adjusted(using the most sensitive portion of the pattern for each)for best fit. The resulting Fourier coefficients of crystal potential may be used (via the Mott Formula) to obtain a charge density difference map, which reveals the distribution of valence electrons involved in bonding, as shown for GaAs in figure 4.The partially ionic character of the bond is seen. We find V(111)1/V(111) = 0.05, V1 (220)/V(220) = 0.035, V1 (400)/V (400) = 0.07 and V1 (333)/V(333)=0.08. Results for work on MgO in progress are shown in figs. 2 and 3.
Observation of cerium-oxides in rapidly solidified Ti-4wt% Ce
Authors: Wittig J.E. , Kegley D.R. , Hofmeister W.H. , Bayuzick R.J.
Published: June 2020

Rapid solidification of Ti rare earth alloys can produce a fine rare earth oxide dispersion. A stable distribution of rare earth oxides in a hexagonal Ti matrix has been shown to increase the flow stress at both room and elevated temperatures. Previous studies have investigated the precipitation phenomena in Ti-erbium Ti-Ianthanum, and Ti-Al-gadolinium alloys. The present research addresses oxide precipitation in a Ti-cerium alloy as a result of rapid solidification processing.

Samples of Ti-4 wt% Ce were rapidly solidified by splat quenching 300 mg droplets, after free fall in a 100 m drop tube, onto a copper block. Pyrometer measurements and heat flow calculations estimate that the liquid drop was undercooled approximately 200 K below its equilibrium melting temperature (Tm=1920 K) at impact. Thin foils from the splats were prepared by punching 3 mm diameter discs and twin-jet electropolishing in a solution of 3% perchloric acid, 37% n-butylalcohol, and 60% methanol at 240 K and 34 V. The TEM investigations used a Philips CM12, a CM30, and an EM420T.