bluuebunny/crossref_metadata_2025 huggingface dataset:
The role of EM studies on the development of ceramic composites
Authors: Alexander K.B. , Lin H. T. , Becher P. F.
Published: June 2020

Advances in the development of ceramics and ceramic composites rely on an understanding of the mechanisms which contribute to the formation of desired microstructural features or control the mechanical response of the material. For example, in silicon nitride, careful control of the liquid phase composition and growth conditions permits the development of elongated grains which provide in-situ reinforcement of the material. Electron microscopy studies have aided in understanding the solution-reprecipitation process which controls the growth of silicon nitride as well as the role of the liquid phase on this process. The composition and thickness of the residual glass phase at the grain boundaries and triple points are important for both the toughening and creep response of the material and can only be realistically analyzed by high resolution analytical and electron imaging techniques.
Phase identification in SiC whisker reinforced Al2O3-R2O3-based composites
Authors: Sung C.M. , Ostreicher K.J. , Huckabee M.L. , Buljan S.T.
Published: June 2020

A series of binary oxides and SiC whisker reinforced composites both having a matrix composed of an α-(Al, R)2O3 solid solution (R: rare earth) have been studied by analytical electron microscopy (AEM). The mechanical properties of the composites as well as crystal structure, composition, and defects of both second phases and the matrix were investigated. The formation of various second phases, e.g. garnet, β-Alumina, or perovskite structures in the binary Al2O3-R2O3 and the ternary Al2O3-R2O3-SiC(w) systems are discussed.

Sections of the materials having thicknesses of 100 μm - 300 μm were first diamond core drilled. The discs were then polished and dimpled. The final step was ion milling with Ar+ until breakthrough occurred. Samples prepared in this manner were then analyzed using the Philips EM400T AEM. The low-Z energy dispersive X-ray spectroscopy (EDXS) data were obtained and correlated with convergent beam electron diffraction (CBED) patterns to identify phase compositions and structures. The following EDXS parameters were maintained in the analyzed areas: accelerating voltage of 120 keV, sample tilt of 12° and 20% dead time.
Deformation at interfaces of Ti-6Al-4V/SiC fiber composites
Authors: Moberly Warren J. , Miracle Daniel B. , Krishnamurthy S.
Published: June 2020

Titanium-aluminum alloy metal matrix composites (MMC) and Ti-Al intermetallic matrix composites (IMC), reinforced with continuous SCS6 SiC fibers are leading candidates for high temperature aerospace applications such as the National Aerospace Plane (NASP). The nature of deformation at fiber / matrix interfaces is characterized in this ongoing research. One major concern is the mismatch in coefficient of thermal expansion (CTE) between the Ti-based matrix and the SiC fiber. This can lead to thermal stresses upon cooling down from the temperature incurred during hot isostatic pressing (HIP), which are sufficient to cause yielding in the matrix, and/or lead to fatigue from the thermal cycling that will be incurred during application, A second concern is the load transfer, from fiber to matrix, that is required if/when fiber fracture occurs. In both cases the stresses in the matrix are most severe at the interlace.
The role of microscopy in the development of metal matrix composites
Authors: L'Espérance Gilles , Lloyd David J.
Published: June 2020

From the very beginning of the development of metal matrix composites, (MMC's), electron microscopy has played a major role in their development. Thus, analytical transmission electron microscopy, (ATEM), has been used to characterize and study: the reinforcements in MMC's, interfacial reactions and products that can occur at the interface between the matrix and the reinforcement and the detailed matrix microstructure, particularly the dislocation and grain structure and the precipitation/constituent phases. In this presentation, we will review and discuss the contribution of ATEM to each of these points and describe how it provided necessary information in the design and use of these materials. The presentation will mainly discuss Al-based composites although work from Ti and Mg-based composites will also be presented.
The role of microscopy in the development of Ti-based intermetallic alloys
Authors: Hall Ernest L.
Published: June 2020

For the past two decades there has been interest in the development of Ti-based intermetallic alloys for aircraft structural and engine applications, culminating in intense activity over the past five years. The primary driver for this interest is the increase of the strength/weight ratio for aircraft materials, and the low density, high modulus, and high melting temperature of some Ti alloys make them promising candidates. Most of the work has centered on the Ti-Al system, particularly Ti3Al (ordered DO19 structure, space group P63/mmc), TiAl (ordered Ll0, space group P4/mmm), and, more recently, an new ordered orthorhombic alloy (O-phase) based on Ti2AlNb (space group Cmcm). The primary limitation of these alloys is low ductility at room temperature, although improvements in ductility, strength, and creep resistance at low and elevated temperatures are also continuing goals. Transmission electron microscopy has played a key role in the understanding of the behavior of these alloys and has defined possible pathways for property improvement. Microscopy studies have focused on many issues, including: deformation mechanisms at low and elevated temperatures, and their relation to ductility, strength, creep, and fracture; the role of interstital elements; the role of ternary and quaternary additions; phase equilibria and stability; and the microstructure of alloys containing dispersoids or reinforcing fibers. Some highlights of these studies will be discussed briefly below.
Defects in molybdenum disilicide
Authors: Mitchell T.E. , Srinivasan S. R. , Schwarz R. B.
Published: June 2020

INTRODUCTION. Molybdenum disilicide(MoSi2) is used mainly as an electrical conductor in integrated circuits and as a heating element in furnaces. It is now being studied as a potential high temperature structural material because of its good strength and oxidation resistance. MoSi2 has the tetragonal Cllb crystal structure, space group 14/mmm, with a=3.202Å and c=7.85lÅ. At temperatures >1900°C, MoSi2 has the hexagonal C40 structure where the basal planes have ABC stacking. This compares with the AB stacking of the pseudo-hexagonal {110} planes of the tetragonal structure. In the present paper we will describe and discuss TEM observations of defects in MoSi2 prepared variously by hot pressing, plasma spraying and mechanical alloying.
A TEM characterization of precipitates evolution and their effect on the grain-growth behavior of gas atomized superalloy A-286
Authors: Shin Keesam
Published: June 2020

It is generally accepted that the refined grain size and stable grain boundaries observed in rapid-solidification-processed (RSP) materials are due to the fine precipitates produced during and after the processing. This is based on the observation of the materials after consolidation of the powders. Though there is little doubt that the as-atomized powder structure is crucial for the understanding of the consolidated sample, there are few studies of the powder. This is mainly because of the difficulty in preparation of thin electron-transparent samples.

The most common method of powder sample preparation uses two or three different embedding materials whose polishing conditions are usually not identical. In this study, a monolayer of powder was gold coated, electroplated with nickel, and then electropolished. Electropolishing provided a nice and smooth surface without any surface artifact. In this study, as-atomized and heat treated powders were studied by TEM (JEOL JEM 200CX) and STEM (VG HB501). Because of the fineness of the defects involved, TEM observation is crucial.
Room temperature deformation and stress-induced phase transformation in Laves phase Fe2Zr
Authors: Liu Yaping , Allen Samuel M. , Livingston James D.
Published: June 2020

Laves phases have three structures: Cubic C15 (MgCu2), hexagonal C14 (MgZn2) and dihexagonal C36 (MgNi2). Their high-temperature deformability has been studied by many investigators. But the study of their room-temperature deformation is very limited. By such studies, it may be possible to enhance the room-temperature deformability of Laves phases.

In the present study, arc-melted samples of Fe-10 at% Zr, containing α-Fe and Fe2Zr Laves phase, were annealed at 1190°C for 48 hr and then compressed at 0.001 inch/min to strains of 46∼48% at room temperature. The 0.2% offset yield stress is about 75 kg/mm2. Micrographs revealed dislocation structures in α-Fe and cracks in proeutectic Fe2Zr after deformation. X-ray diffraction patterns show that the crystal structure of Fe2Zr is mainly C36 before deformation. After compression, the C15 peaks, which overlap other peaks in the pattern, increase in intensity, as seen in Fig. 1. This demonstrates that a phase transformation occurred as a result of deformation. The detail and mechanism of the transformation were studied by TEM. A characteristic banded structure which was rarely found in undeformed sample was frequently observed in the compressed Fe2Zr particles, as shown in Fig. 2. Electron diffraction patterns and high-resolution TEM showed that these bands have a C15 structure with a {111} plane parallel to the basal plane of C36 structure in the adjacent parts of a particle. Fig. 3 is a high-resolution TEM picture showing the stacking sequence of the close-packed planes of the particle containing the C15 band in Fig. 2. It has a ABCABC stacking sequence in the central part (C15) and ABAC on the two sides (C36). Inside the C15 band there are many stacking faults on {111} planes. A twinned region (C15T) is also seen to the right of the C15 region.
Precipitation in Al-Li-Zr alloys
Authors: Vanderwalker D.M.
Published: June 2020

Aluminum-lithium alloys have a low density and high strength to weight ratio. They are being developed for the aerospace industry.The high strength of Al-Li can be attributed to precipitation hardening. Unfortunately when aged, Al-Li aquires a low ductility and fracture toughness. The precipitate in Al-Li is part of a sequence SSSS → Al3Li → AlLi A description of the phases may be found in reference 1 . This paper is primarily concerned with the Al3Li phase. The addition of Zr to Al-Li is being explored to find the optimum in properties. Zirconium improves fracture toughness and inhibits recrystallization. This study is a comparision between two Al-Li-Zr alloys differing in Zr concentration.

Al-2.99Li-0.17Zr(alloy A) and Al-2.99Li-0.67Zr (alloy B) were solutionized for one hour at 500oc followed by a water quench. The specimens were then aged at 150°C for 16 or 40 hours. The foils were punched into 3mm discs. The specimens were electropolished with a 1/3 nitric acid 2/3 methanol solution. The transmission electron microscopy was conducted on the JEM 200CX microscope.
Investigation of the visibility of early-stage Ω precipitates in an Al-Cu-Mg-Ag alloy
Authors: Chang Y.C.
Published: June 2020

Nucleation is the process through which the smallest stable particle of a new phase of critical size r* is formed. During precipitation, growth of such a particle takes place solely by statistical diffusional fluctuations. Typical examples of this occur in the Al-Cu system, where precipitation involves the nucleation and growth of several metastable phases according to the sequence: Al (solid solution) → G.P. zones →θ' → θ (AI2Cu). In the case of Al-Cu-Mg-Ag alloys, nucleation of the Ω phase on the {111} Al planes is thought to be fundamentally different from nucleation of the other θ', θ or S' phases. In order to understand this difference, the nucleation behavior of Ω phase in an Al-3.9Cu-0.5Mg-0.5Ag (wt. pet.) alloy was examined by high-resolution transmission electron microscopy (HRTEM). The visibility of small precipitates embedded in a matrix is an important question that arises in such an investigation and that is the subject of this paper.
The structure and chemistry of polymer metal interfaces: A combined EM and XPS investigation
Authors: Leong D. B. , Helfand M. A. , McConville R. L. , Mercer F. W.
Published: June 2020

Polymer to metal relationships for applications from coating finishes to electronic devices require interfacial synergy. Polymer application requirements range in temperature, thermal stability, thermal expansion, moisture absorption, oxidative stability, etc. and choice of metal and deposition technique must take into account these characteristics. While the ability to deposit metal in a controllable fashion is well established the present study focuses on what takes place at the polymer metal interface. Polymer/metal interactions on polyimide systems have been studied in detail for the past decade by a variety of analytical techniques. For the first time this is being investigated in the FPAE polymers combining EM study for structural analysis and XPS for chemical analysis. Fluorinated poly (arly ethers) FPAE, are a newly developed class of polymeric dielectric materials for electronic packaging. These new materials exhibit excellent oxidative stability, low moisture absorption and low dielectric constant. While these parameters are critical for electronic device performance and longevity, device performance is also highly dependent on metallization of the polymer.
High-resolution TEM of Cr-Gd and Co-Gd multilayers
Authors: Bertero Gerardo A.
Published: June 2020

Thin film multilayered structures have assumed significant technological importance. In particular, several have proven adequate for magneto-optic recording applications in which vertical magnetization, large coercivities and a large Kerr rotation effect are needed. In this regard, Co-Gd and Cr-Gd, among others, are candidate systems and specifically Gd-Co has already been used with some success.

In the present work, HRTEM was performed on sputter-deposited Cr-Gd and Co-Gd multilayers. These two systems are expected to show contrasting behavior which may affect the long term magneto-optic performance of the multilayer. Cr and Gd are almost totally immiscible, suggesting a large positive heat of mixing. Conversely, Co and Gd form a large number of intermetallic phases and show a deep eutectic at 60 at.% Gd indicating a negative, and possibly large, heat of mixing. Thus, a tendency for intermixing and solid state amorphization is expected for the Co-Gd system and not so for Cr-Gd. These considerations may be of importance since thermal stability is required for magneto-optical applications where the magnetic media is subjected to local thermal cycling during the magnetic writing process.
The structure of a near Σ=27 <110> tilt grain boundary in Ge
Authors: McKernan Stuart , Carter C. Barry
Published: June 2020

The modeling of tilt grain boundaries in terms of repeating structural units of varying separation is now a well established concept. High-resolution electron microscope (HREM) images of different tilt grain boundaries in many materials display a qualitative similarity of atomic configurations of the grain boundary structure. These boundaries are frequently described in terms of characteristic structural units, which may be separated from each other by regions of ‘perfect’ crystal (as, for example, in low-angle grain boundaries), or may be contiguous, forming ordered arrays of the structural units along the boundary. In general there will be a different arrangement of the structural units or an arrangement of different structural units, according to the precise geometry of the particular grain boundary. The structure of some special grain boundaries has been examined and these are found to exist in several different configurations, depending on the orientation of the grain boundary plane among other parameters. Symmetry-related symmetric tilt grain boundaries and asymmetric tilt grain boundaries with one grain having a prominent, low-index facet, are commonly observed, low-energy configurations. Structural multiplicity of these configurations along the same grain boundary has been observed in some systems. Defects in the perfect ordering of the structural units may be caused by deviations of the grain boundary plane away from the perfect tilt orientation. Deviations of grain boundary structure away from the exact orientation will also produce defects in the repeating structural unit configuration. These deviations may have a regular and well-defined structure, producing a more complex structural unit.
In situ hot-stage TEM study of a diffusion-bonded Al/SiC interface
Authors: Ratnaparkhi P. L.
Published: June 2020

Metal/ceramic interfaces have been of great scientific and technological interest for the past two decades. Most studies have been performed on commercially important ceramic-fibre and particulate-reinforced metal-matrix composites. In this investigation, the interfacial structure and chemistry at a diffusion bonded Al/SiC interface was examined at the atomic level in order to understand the fundamental principles that govern bond formation in the solid state between a low temperature non-reactive metal and a high-temperature covalently bonded ceramic.
A HREM study of the interfaces between TiO2 precipitates and the Al2O3 matrix in star sapphire
Authors: Xiao S.Q. , Heuer A. H. , Pirouz P.
Published: June 2020

The asterism, or star effect, present in star sapphire (Ti-doped A12O3) single crystals is known to arise from the needle-like rutile(r) (TiO2) precipitates in the sapphire(s) matrix. The specific orientation relationship between the precipitates and the matrix is {100}r//{0003}s and <011>r//<1010>s . In this work we report a HREM study of the rutile/sapphire interface.

The cross-section perpendicular to the needle axis of a very small precipitate, which is coherent, or has just one misfit dislocation at its interface, is a rhombus; the precipitate/matrix interface (habit plane) is {111}r{1123}s. As the coherency break down and misfit dislocations are introduced into the interface, the shape of the cross-section becomes nearly square; the two orthogonal interfaces are {100}r//0003}s and {011}r//{1120}s. Larger precipitates show rectangular interfaces elongated along the {100}r//{0003}s interface (Fig.l). A regular array of misfit dislocations with Burger vector b = 1/3<0001>S are present at the {011}r//{1120}s interface, the mean distance between every two adjacent 1/3<0001>S misfit dislocations being 8.7 nm which compensates exactly the 5.9% lattice mismatch estimated from the lattice constants of these two structures. Similarly, a regular array of misfit dislocations with Burgers vector b = 1/3<1010>S at the {100}r//{0003}s interface compensates the 4.5% lattice mismatch at that interface.
Diffraction from interfaces
Authors: Eaglesham D.J.
Published: June 2020

A wide variety of approaches have been used to study diffraction from interfaces and thereby gain information on the structure or chemistry at the interface. In this paper the various techniques will be reviewed and a few examples will be used to illustrate the methods.

Techniques for studying interfaces can be broadly divided into cross-section and plan-view approaches, and further classified according to whether the information to be extracted lies on the coincident site lattice or at some intermediate position (i.e. whether diffraction will determine a rigid body shift or a full reconstructed interface structure). While cross-section diffraction studies have been shown to be extremely powerful in elucidating a number of problems, we will concentrate here on plan-view methods. The reasoning for this is that cross-section diffraction retains many of the drawbacks to cross-sectional HREM (small field of view, indeterminate averaging and above all increased specimen preparation damage) and has substantially reduced signal, whereas plan-view diffraction can give us access to unique information on both local and long-range variations in the interface.
Morphology and orientation of ultrafine Y2O3 precipitates in YBa2Cu3O7-x thin films
Authors: Lu P. , Zhao J. , Chern C.S. , Li Y.Q. , Gallois B. , Norris P. , Kear B. , Cosandey F.
Published: June 2020

Yttrium-rich YBa2Cu3O7-x (YBCO) thin films grown by plasma-enhanced metalorganic chemical vapor deposition (PE-MOCVD), have been reported to have high critical current densities (Jc) of > l×106A/cm2 at 77K. Electron microscopy has been used to understand how excess Y is incorporated into thin film microstructures without affecting their superconducting properties. In this work, we report identification of small Y2O3 precipitates, their morphologies and orientation relationships in YBCO thin films.

Details of thin film growth process have been reported elsewhere. The films were examined both in planar and in cross-sectional view by electron microscopy. TEM samples were prepared by conventional mechanical polishing and ion milling techniques. An ISI-002B high resolution electron microscope with a resolution limit of ∼0.18 nm at 200keV was used for observations.
Imaging of ferroelectric domain walls by electron holography
Authors: Zhang Xiao
Published: June 2020

Electron holography has recently been available to modern electron microscopy labs with the development of field emission electron microscopes. The unique advantage of recording both amplitude and phase of the object wave makes electron holography a effective tool to study electron optical phase objects. The visibility of the phase shifts of the object wave makes it possible to directly image the distributions of an electric or a magnetic field at high resolution. This work presents preliminary results of first high resolution imaging of ferroelectric domain walls by electron holography in BaTiO3 and quantitative measurements of electrostatic field distribution across domain walls.
Morphology and atomic structure of segregated grain boundaries in Cu-Sb
Authors: Fonda R. W. , Luzzi D. E.
Published: June 2020

The properties of polycrystalline materials are strongly dependant upon the strength of internal boundaries. Segregation of solute to the grain boundaries can adversely affect this strength. In copper alloys, segregation of either bismuth or antimony to the grain boundary will embrittle the alloy by facilitating intergranular fracture. Very small quantities of bismuth in copper have long been known to cause severe grain boundary embrittlement of the alloy. The effect of antimony is much less pronounced and is observed primarily at lower temperatures. Even though moderate amounts of antimony are fully soluble in copper, concentrations down to 0.14% can cause grain boundary embrittlement.
Freeze-fracture TEM of the helical smectic A* phase
Authors: Ihn K.J. , Pindak R. , Zasadzinski J. A. N.
Published: June 2020

A new liquid crystal (called the smectic-A* phase) that combines cholesteric twist and smectic layering was a surprise as smectic phases preclude twist distortions. However, the twist grain boundary (TGB) model of Renn and Lubensky predicted a defect-mediated smectic phase that incorporates cholesteric twist by a lattice of screw dislocations. The TGB model for the liquid crystal analog of the Abrikosov phase of superconductors consists of regularly spaced grain boundaries of screw dislocations, parallel to each other within the grain boundary, but rotated by a fixed angle with respect to adjacent grain boundaries. The dislocations divide the layers into blocks which rotate by a discrete amount, Δθ, given by the ratio of the layer spacing, d, to the distance between grain boundaries, lb; Δθ ≈ d/lb (Fig. 1).
Characterization of clean gallium phosphide (110) surfaces by reflection electron microscopy
Authors: Gajdardziska-Josifovska M. , McCartney M.R. , Smith David J.
Published: June 2020

The clean (110) surface of GaP has been studied in UHV systems with diffraction and spectroscopy techniques and found to have a 1×1 reconstruction. However, no imaging studies have been reported characterizing the topography of the reconstructed surface. In this work we use reflection electron microscopy (REM) to determine the effects of ion-milling and annealing treatments used to clean the surface. The annealing temperatures for the compound semiconductors are limited by preferential sublimation of one component so we have undertaken heating to higher temperatures to study the surface dissociation and melting processes.
EM studies of reduction and oxidation behavior of model heterogeneous catalysts
Authors: Yao Nan , Meitzner G. D. , Fung S. C. , Disko M. M. , Behal S. K. , McHenry J. , Sinfelt J. H.
Published: June 2020

Supported metal heterogeneous catalysts, such as Pt/γ-Al2O3, are used for reforming reactions in which saturated hydrocarbons, in petroleum naphtha, are converted to aromatics with a much higher anti-knock quality for automotive fuel. Reforming is now one of the most important industrial applications of catalysts. The platinum on alumina catalyst was developed in the 1950s, and remains in use around the world.

The mechanisms of deactivation of alumina-based catalysts are still not well understood, since their deactivation on oil is accompanied, or rapidly followed, by massive carbon deposition which prevents inspection by chemisorption. In order to fully understand the general loss of activity for supported metal catalysts during operation, investigations of the atomic structure and the chemical composition for both working and model catalyst systems have been performed using high resolution transmission electron microscopy (HRTEM). Morphological and chemical changes of small catalytic particles distributed on the top surface of a porous alumina and impregnated within a porous alumina have been compared.
Atomic scale imaging of surfaces in UHV
Authors: Marks L.D.
Published: June 2020

Electron Microscopy of surfaces can in principle provide information that is not available by other techniques. Arguably the most significant is information about the surface structure simultaneously with the bulk atomic structure. Whereas bulk atomic scale information is now routinely available using standard high resolution imaging techniques, imaging surface with similar resolution has proved far harder. To date the only extensively used technique is profile imaging, but this has an inherent scientific problem; the imaged area is so thin that there is great uncertainty about how representative the results are of a bulk surface. For instance, one can conclude almost without ambiguity that any results on large unit cell surface structures will be erroneous since the long range elastic strains will be truncated by the specimen thickness.
Objective-lens design for high resolution ultra high vacuum EM
Authors: Bleeker Arno J. , Gibson J. Murray
Published: June 2020

Although the main use for Transmission electron microscopy is to study bulk phenomena it is also possible to do surface sensitive experiments with this type of instrument. In order to do reliable surface physical experiments it is necessary to improve the vacuum within the vicinity of the specimen to the Ultra High Vacuum (UHV) level. A number of authors report on such improvements. In most designs the experiments with the sample such as deposition and oxidation are done outside the main microscope column. This means that it is not possible to observe the sample under high resolution conditions during these experiments. The importance of the electron microscope as a surface sensitive instrument can be greatly enhanced if it would be possible to do surface physical experiments in-situ. In that way it would become possible to observe the specimen with high resolution during all kinds of surface processes. In order to be able to do these experiments there must exist a large free space around the sample. In this free space auxiliary equipment such as ion guns and MBE cells can be placed. To further enhance the capabilities of the instrument, analyzing tools such as an Auger spectrometer and SIMS equipment can be attached to the microscope. At the University of Illinois an electron microscope capable of imaging the sample during surface physical experiments is presently under construction. In this machine the objective lens section has been replaced by a large (800 mm diameter and 400 mm high) UHV chamber. The specimen is outside the magnetic field of the objective lens in order to obtain as much free space around the sample as possible thus sacrificing resolution.
High-resolution TEM of fullerenes of different sizes
Authors: Seraphin Supapan , Zhou Dan , Jiao Jun , Lamb Lowell D. , Huffman Donald R.
Published: June 2020

The extraction of macroscopic quantities of the solid phase of the carbon cluster C60 by the technique of Krätschmer, Huffman (KH) et.al. has stimulated a large number of investigations into their physical properties. Recently, further developments have led to the extraction from KH-carbon and subsequent mass-spectroscopic characterization of larger clusters Cn, n = 100 to 330, termed giant fullerenes. Of particular interest are studies into the structure of these giant fullerenes. The paper presented here applies the technique of High-Resolution Transmission Electron Microscopy (HRTEM) to an investigation of the structure of the C60, C70, and giant fullerenes, including their transformation under electron beam irradiation. The results show significant differences in the structural response to electron beam irradiation among fullerenes of different sizes.
A study of the crystal nucleation mechanism in submicron droplets of pure elements
Authors: Holzman Louis M.
Published: June 2020

Liquid-to-crystal nucleation is an important fundamental process to understand as many materials go through the liquid-to-crystal phase transformation in processing. Unfortunately, most previous nucleation experiments have been unable to demonstrate that homogeneous nucleation was the active mechanism and the distribution of heterogeneous nucleants in these systems was unknown, so few concrete conclusions could be reached. In this research, homogeneous nucleation is shown to be operative for the case of submicron droplets of pure silicon and germanium quenched from the melt.
Imaging small metal particles with Auger electrons
Authors: Liu J. , Hembree G. G. , Spinnler G. E. , Venables J. A.
Published: June 2020

High spatial resolution Auger electron spectroscopy (AES) and scanning Auger microscopy (SAM) have been developed in a UHV scanning transmission electron microscopy (STEM) instrument. A resolution < 3 nm has been achieved in SAM images. The application of high resolution AES and SAM to the study of supported catalysts has proved very powerful for extracting chemical information of the surface species. In this paper we report further study of supported metal particles by using high resolution AES and SAM. These experiments were conducted in a VG HB-501S UHV STEM codenamed “MIDAS”. Auger electrons were collected from the entrance surface of the sample using a 100 keV probe.
Investigation of volume fraction of omega particles and hardness value of aged Ti-Mo alloy using dark-field TEM imaging
Authors: Sukedai E. , Hashimoto H.
Published: June 2020

Investigation about omega phase has been carried out in many alloy systems using X-ray diffraction, electron microscope methods and so on. Volume fraction of aged omega particles seems to be the important factors on the view point of industry, because it influences mechanical properties of alloys. The volume fraction has been investigated using X-ray diffraction and small angle X-ray scattering methods. Except one alloy system, it is very hard to distinguish omega particles using bright field TEM imaging because of strain field in the matrix of alloys and/or around omega particles. However, it is known that using dark field TEM imaging, omega particles in Ti alloys can be seen. This method has other advantageous point; particles of individual omega variants can be seen using isolated diffraction spots from variants. In the present paper, volume fractions of omega variants in Ti-20mass%Mo alloy aged at 623K for 12.6ks, 28.8ks and 100.8ks were estimated using this method and the values were evaluated by considering the strength of alloys aged for these periods.
HREM investigation of mesopore formation, aluminum migration in USY catalysts
Authors: Choi-Feng C. , Hall J. B. , Huggins B. J. , Beyerlein R. A.
Published: June 2020

The catalytic properties of ultrastable Y (USY) zeolite are directly influenced by the zeolite destruction which occurs during formation of USY and during subsequent hydrothermal treatment Previous transmission electron microscope (TEM) studies of hydrothermal aging of neat USY materials and also of USY cracking catalysts have shown 5-50nm defect domains, which were attributed to mesopores. Such features are characteristic of extended hydrothermal treatment. It has been suggested that these regions of zeolite destruction comprise the silica source for “healing” the tetrahedral site vacancies left by hydrothermal dealumination. The observations of both homogeneous and inhomogeneous distribution of these mesopores have been reported.
Surface studies with a UHV-TEM
Authors: Smith David J. , Gajdardziska-Josifovska M. , McCartney M.R.
Published: June 2020

The provision of ultrahigh vacuum capabilities, as well as in situ specimen treatment and annealing facilities, makes the transmission electron microscope into a potentially powerful instrument for the characterization of surfaces. Several operating modes are available, including surface profile imaging, reflection electron microscopy (REM), and reflection high energy electron diffraction (RHEED), as well as conventional transmission imaging and diffraction. All of these techniques have been utilized in our recent studies of surface structures and reactions for various metals, oxides and semiconductors with our modified Philips-Gatan 430ST high-resolution electron microscope.
UHV-TEM imaging and diffraction study of Cu-Au on Si(111)
Authors: Xu P. , Marks L. D.
Published: June 2020

It has been demonstrated that ultra-high vacuum transmission electron microscopy is a powerful technique in solving surface atomic structures. During some recent work while we were testing surface imaging modes using the Si(111)-7×7 surface, we accidentally contaminated the surface by sputtering copper and some gold from the specimen holder onto the silicon. This paper presents the results of transmission electron diffraction and imaging studies of this surface.

Experiments were performed in a Hitachi UHV H-9000 300 keV electron microscope with a stable operation pressure of 1x10-10 Torr. Attached to the microscope is a UHV surface science chamber for in situ sample preparation. A thin sample of silicon (111) (P doped to 80 ohm-cm) was mechanically polished, dimpled, and ion-beam thinned before being transferred into the surface science chamber. The sample was then ion beam sputter cleaned using 3-4 Kv argon ions and annealed to about 600°C using an electron gun (4-5 Kv, 2-3 Ma). Later tests indicated that the ion gun was not centered around the 3 mm disk and a part of the sample surface was covered by the sputtered materials from the sample holder. EDX results from a Hitachi HF-2000 analytical microscope showed that the deposited layer consisted of about 70% Cu and 30% Au.
Imaging micro-twin distributions in as-grown CVD diamond films with TEM
Authors: Wang Z.L. , Horton L.L. , Clausing R.E. , Heatherly L. , Bentley J.
Published: June 2020

Fractured edges of diamond films grown by chemical vapor deposition (CVD) have been examined directly in a conventional transmission electron microscope (TEM) without thinning, An important advantage of the fracture specimen preparation technique is that the microstructures in the diamond grains at the growth face can be characterized directly by bright-field (BF) and dark-field (DF) TEM imaging and diffraction. Additionally, the topography of the same region can be directly determined from secondary electron (SE) images available in the same TEM.
HREM study of heteroepitaxial interfaces in the Ni.50Co.50O/Al2MgO4(100) system
Authors: Cao W. , Thomas G.
Published: June 2020

Epitaxial CoO/NiO multilayers and alloys have been produced which show interesting structural and magnetic properties. Typically, epitaxial films are grown either by chemical vapor deposition or evaporation. However, sputtering can also yield high quality epitaxial films. The structure of the substrate and the interface of the film/substrate are critical factors to determine the quality of the epitaxial film. The CoO/NiO epitaxial films on the α-Al2O3 had been studied extensively in our previous work. In this paper, the heteroepitaxial interface in the Ni.50Co.50O/Al2MgO4 is studied by using high resolution electron microscopy. Transmission electron microscopy was performed using the JEOL-200CX and JEOL-ARM1000 microscopes at the National Center for Electron Microscopy, Berkeley.
Direct observation of monomer image of poly(p-xylylene) crystal
Authors: Zhang Weiping , Thomas Edwin L.
Published: June 2020

Poly(p-xylylene) (PPX) single crystals have been studied recently with high-resolution electron microscopy (HREM) by two groups. The resolution achieved to date is 0.26 nm for the α-form and 0.25 nm for the β-form. Polymer crystals are generally very sensitive to electron irradiation since the molecular chains become crosslinked or are decomposed (chain scission) in a short time upon electron irradiation. Individual chain stems in a polymer crystal can be resolved in micrographs if the electron beam is introduced into die crystal along the chain axis. Obtaining structural images from different crystal orientations is very useful in three dimensional reconstruction. Most HREM studies on polymer crystals have focused on taking images along the polymer chain direction. The present work demonstrates that monomer images can be obtained from other directions, and a 0.18 nm resolution is achieved for α-form PPX crystals at room temperature.
The dynamics of local chemical changes underlying white-cell chemotaxis measured with digital imaging microscopy
Authors: Fay F. S. , Brundage R. , Perry K. , Gilbert S.H.
Published: June 2020

The ability of cells to polarize and move towards or away from a chemical stimulus is a fundamental property of virtually all cell types at some stage in their development. This process is important for biological processes ranging from morphogenesis, to wound healing, to the attack by leukocytes of invading organisms in our body. Despite the importance of this phenomenon, surprisingly little is known regarding the mechanism whereby an external chemical gradient is converted into an intracellular gradient of 2° messengers, how the local chemistry of the cell is compared and how the result of that comparison gives rise to the polarized organization of the cell and the resulting directional migration of the cells. In order to obtain insights into this fundamental cellular process, we have been investigating the local chemical changes underlying the response of newt eosinophils to chemoattractants. This cell system is particularly well suited for asking such questions as the cells are large (30 microns wide, 70 microns long), thereby facilitating micro-injection of various fluorescent compounds as well as the imaging of local differences in the concentration of important species involved in responses to chemoattractants. Furthermore, these cells respond in seconds to gradients of serum chemoattractants, thereby facilitating an analysis of the sequence of events linking the gradient of chemoattractant to the generation of local chemical signals ultimately leading to the polarization and directed migration of these cells.
Lamellipodia, stress fibers and the actin cortex of motile cells: Their structure and dynamic interaction studied by video DIC microscopy and correlated Electron Microscopy
Authors: Heath Julian P. , Holifield Bruce F.
Published: June 2020

We have used high resolution video differential interference contrast (VDIC) light microscopy and correlated electron microscopy to examine the spatial and functional relationships between actin structures in the lamellipodia, adhesion plaques, stress fibers and the dorsal cortical actin microfilament sheath (DCMS) of cultured motile fibroblasts. Fibroblasts were examined on a Zeiss Axiophot using a 1.3NA 100x plan objective and 1.4 NA condenser and imaged with a Dage CCD camera. Contrast-enhanced images were recorded on a Panasonic S-VHS time-lapse recorder.
Two-dimensional porin OMPF-lipid crystals: A comparison of EM and x-ray data
Authors: Henn C. , Hönger A. , Cowan S. , Rosenbusch J.P. , Engel A.
Published: June 2020

InEscherichia coli (E.coli)cells porins represent a major component of the outer membrane. Embedded in the lipid bilayer, these channel forming proteins facilitate the diffusion of solutes up to a mass of 600 Da. Porins OmpC, OmpF and PhoE exhibit a high sequence homology, whereas the sequence of LamB, the maltose channel, is different. Nevertheless, allE.coliporins have pronounced common features: (i) the wall of the cylindrical channel is in a β-sheet conformation with the antiparallel β-strands running at approximately 45 degrees;(ii) functional porins exist as trimers, and (iii) voltage-induced conductance steps of OmpC, OmpF and PhoE are triplets of the monomeric channel closure step.In addition,E.coliporins share many features with porins from other species such asRhodobacter capsulatus,whose porin structure has been solved to atomic resolution.
Structure of crystalline VDAC, the voltage-gated channel in the mitochondrial outer membrane
Authors: Guo X.W. , Smith P.R. , Radermacher M. , Mannella C.A.
Published: June 2020

The voltage-gated, mitochondrial channel, VDAC, is formed by a 31-kDa outer-membrane polypeptide. Crystalline arrays of this channel, produced by phospholipase A2 treatment of Neurospora crassa mitochondrial outer membranes, consist of groups of 6 channels repeated on a parallelogram (“oblique”) lattice (a=13.3nm, b=11.5nm, γ = 109°). These membrane crystals are polymorphic, i.e. lateral contraction is triggered by a polyanion which also decreases VDAC’s gating potential. Projection images of unstained, frozen-hydrated VDAC arrays indicate that lattice contraction is accompanied by changes in the distribution of protein away from the hexameric repeat unit.
Transient DNA-protein complexes trapped and studied by cryo-electron microscopy
Authors: Gogol Edward P. , Young Mark C. , von Hippel Peter H.
Published: June 2020

Replication of an organism’s genome is a complex process which involves the coordination of multiple enzymatic activities to perform both the work of template-directed DNA synthesis as well as numerous necessary associated duties. Even the central function of DNA synthesis requires the interaction of multiple protein subunits or subassemblies to achieve the rate and fidelity observed in vivo. The enzymatic activity of the DNA polymerase, which in simple organisms is a single polypeptide, is enhanced by a set of accessory proteins which dramatically increases its processivity on the DNA template. As a first step in obtaining a structural understanding of the assembly and function of the multiple protein subunits required for DNA replication, we have directly visualized the complexes formed by the accessory proteins with DNA cofactors, and explored the conditions required for their formation.
DNA complexed with RECA protein
Authors: Egelman Edward H. , Yu Xiong
Published: June 2020

We have been using electron microscopy and computed image analysis to understand the structure of the helical polymer that the RecA protein from E. coli forms on DNA. Recent work has adressed the following points:

1) RecA binds an ATP analog, ATP-γ-S, and hydrolyzes this analog several thousand times more slowly than ATP is hydrolyzed by RecA. We have shown that structural transitions may be seen within RecA filaments on the same time scale (several hours) on which ATP-γ-S is being hydrolyzed, and several lines of evidence suggest that these conformational changes are due to the RecA ATPase. We have therefore been able to directly visualize these motions, using image analysis of bundles of RecA filaments.
Transcription factor/DNA interactions visualized by electron spectroscopic imaging
Authors: Bazett-Jones David P. , Brown Mark L.
Published: June 2020

A multisubunit RNA polymerase enzyme is ultimately responsible for transcription initiation and elongation of RNA, but recognition of the proper start site by the enzyme is regulated by general, temporal and gene-specific trans-factors interacting at promoter and enhancer DNA sequences. To understand the molecular mechanisms which precisely regulate the transcription initiation event, it is crucial to elucidate the structure of the transcription factor/DNA complexes involved. Electron spectroscopic imaging (ESI) provides the opportunity to visualize individual DNA molecules. Enhancement of DNA contrast with ESI is accomplished by imaging with electrons that have interacted with inner shell electrons of phosphorus in the DNA backbone. Phosphorus detection at this intermediately high level of resolution (≈lnm) permits selective imaging of the DNA, to determine whether the protein factors compact, bend or wrap the DNA. Simultaneously, mass analysis and phosphorus content can be measured quantitatively, using adjacent DNA or tobacco mosaic virus (TMV) as mass and phosphorus standards. These two parameters provide stoichiometric information relating the ratios of protein:DNA content.
Organization of dsDNA in icosahedral virus capsids
Authors: Booy Frank P. , Trus Benes L. , Newcomb William W. , Brown Jay C. , Serwer Philip , Steven Alasdair C.
Published: June 2020

Condensation/decondensation reactions which effect the packaging of viral genomes into their capsids and their subsequent release constitute one of the basic phenomena which structural biology seeks to explain. The appropriate nucleic acid molecule(s) must be selected; then packaged at a high density to allow efficient utilization of the available space; the mutual electrostatic repulsion of DNA strands must somehow be overcome; and the arrangement of the DNA must allow for its ready release upon initiation of infection. Cryo-electron microscopy has emerged as an incisive tool for visualizing the internal organization of packaged DNA inside viral capsids. We illustrate its effectiveness with observations relating to bacteriophages T4, T7, and herpes simplex virus, type 1 (HSV-1).