Interfacial anisotropy complicates the prediction of microstructural evolution, particularly ir extreme cases for which the presence of facets and corners prevents the application of classical notions of grain-boundary curvature. Although there has been much effort at incorporating anisotropic grain-boundary properties, including faceted geometries, into computational approaches for microstructural evolution, at present our mechanistic understanding of the behavior of facets anc their junctions remains limited. In this presentation, we investigate the development of faceted boundaries between Σ=3 <111> oriented grains in epitaxially deposited gold thin films. This system is well suited tc experimental studies of facet evolution since the crystallography and structure of the boundaries is already well understood. It is well known that “double-positioning” of epitaxially aligned <111> grains on a surface of three-fold or six-fold symmetry results in a microstructure composed of grains in two twin-related (Σ=3) variants that are separated by facets running vertically through the film and forming 120 degree corners [1,2].

Grain boundary migration (GBM) during the sintering and densification of a polycrystalline ceramic material occurs as a result of mass transport across an interface. When there is a liquid film present, either due to additives used for liquid-phase sintering or unavoidable impurities in the material, the mass transport can be visualized in terms of dissolving material from one grain and precipitating it on another. in order to study the effects of crystallography on GBM in the presence of a liquid phase, alumina bicrystals have been fabricated with anorthite (CaA12Si208) glass films at the interface. The alumina-anorthite system in a bicrystal geometry is used because the pseudo-binary phase diagram of the system is well known, the bicrystal geometry allows for control over the original interface misorientation, and the glassy phase of anorthite is the most commonly occurring glass in commercially used alumina.

Fig. 1 is a secondary-electron SEM image of an alumina bicrystal recorded using a field-emission SEM (Hitachi S900) operating at 5 kV.

We have used High Velocity Oxy-Fuel (HVOF) thermal spray coating process to obtain tungsten carbide coatings on steel. It is important to control the powder quality and surface roughness of the substrate in the thermal spray coating process to obtain good adherence. The adhesive strength is greatly effected by the interfacial impurities. Most commonly used in the thermal spray industry is Al2O3 grit abrasive for good surface roughness of steel substrate to produce anchor patterns for the coating. The previous studies indicated that some fine Al2O3 particles are trapped in the rough surface grooves during this process. The ring shear test method, described previously, appears to give the most accurate results on shear adhesive strength. The ring shear strength test was comprised of a 0.02” thick and 0.25” wide ring coating on a 1” diameter cylindrical substrate rod. This rod is placed in a disk with a cylindrical hole of the rod size and an incremental pressure is applied on the rod.

Interest in wide band gap III-V nitride semiconductor devices is increasing for optoelectronic and microelectronic device applications. to ensure the highest quality, TEM analysis can characterize the substrate and buffer layer interface. Measurements taken by TEM reveal the density of dislocations/cm2 and the orientation of Burger's vectors. This information allows for changes to be made in deposition rates, temperatures, gas flow rates, and other parameters during the processing.

The GaN/sapphire samples grown at AFRL were produced in two consecutive steps, first to provide a thin buffer layer, and the other to grow a lum thick epitaxial film. Both growth steps were prepared using metallic organic chemical vapor deposition (MOCVD) in a vertical reactor. Buffer layers were prepared using a range of temperatures from 525 to 535°C and with a range of flow rates and pressures in order to optimize the nucleation conditions for the epitaxial films.

Nonorthogonal twinings have commonly been observed in perovskite oxides such as SrTi03 and BaTi03. Among them, the ﹛111﹜ Σ3 type twining exists with a relative large amount of population and has been extensively studied. By combining quantitative high resolution transmission electron microscopy (HRTEM) and spatially resolved electron energy loss spectroscopy (EELS), one was able to determine the atomic structure of the ﹛111﹜Σ3 twin boundary in these oxides.[l] On the other hand, nonorthogonal twinings in SrRuO3 have been much less studied. SrRu03, a ternary ruthenium metal oxide, has a perovskite-compatible structure and exhibits low electrical resistivity (10-4 Ω•cm), showing an unparallel technique importance in microelectronic applications. Since the properties of material strongly depend on the microstructure and defect configurations, it is important to study the twining structures and their formation mechanisms in SrRuO3 thin films.

Transmission electron microscopy (TEM) was used to study the SrRuO3 thin films grown on (001) LaAlO3 by 90° off-axis rf sputtering.

Multilayers, heterostructures, nanostructures and composites are of great interest to the materials scientists, and frequently we encounter crystals lacking centrosymmetry. Thus crystal polarity determination on a microscopic scale is becoming increasingly important in describing interface structures and the internal defects in small crystals. in many cases the polarity of a crystallite can be determined by convergent beam electron diffraction, CBED. Powerful alternatives are to monitor the electron induced x-ray emission, EDS, or electron energy losses, EELS, under channeling conditions. While the determination of the phase of the structure factors, and thus the determination of the crystal polarity, relies on many beam diffraction effects when the CBED technique is used, two-beam experiments provide information about the phase of the structure factor when localized EDS or EELS signals are detected under channeling conditions.

The experimental conditions used to determine the polarity and absolute orientation from electron channeling are similar to those used in ALCHEMI experiments to locate small amounts of atoms by electron channeling.

Transparent conductive oxide materials (TCO) are currently under development for use as electrodes in devices such as photovoltaics and flat panel displays. As electrode area increases in large-area applications (e.g. photovoltaics), the resistivity of the TCO must decrease in order to maintain efficiency. The ideal next generation TCO will be a low cost material with a substantially higher conductivity than ITO obtained by improving mobility. Two promising TCO's are CdIn2O2 and Cd2SnO4 which both exhibit high conductivities (4300 S/cm and 8300 S/cm, respectively) and high mobilities (44 and 60 cm2/Vs, respectively).

A bulk investigation of the system CdIn2O4 - Cd2SnO4 reveals a large spinel solid solution, Cd1+xIn2.2xSnxO4 (0<x<0.75) at 1175°C exhibiting a sharp decrease in measured optical gap (from 3.0 to 2.8 eV) at x=0.2 and an increase in conductivity (from 2200 S/cm to 3500 S/cm) for reduced specimens between x=0.4 and x=0.60) with increasing x. It is suspected that the cation distribution (e.g. normal, random, or inverse) on the spinel lattice may be important in understanding these shifts in electrical and optical properties.

Atom location by channeling-enhanced microanalysis (ALCHEMI) promises a straightforward measurement of ordering in materials. On tilting the electron beam in a transmission electron microscope (TEM) about a superlattice Bragg position the electron channeling changes, weighting X-ray production from each of the sublattices differently. The overall X-ray signal, converted to an apparent composition via a kinematical non-channeling standardization, moves between the composition of the constitute sublattices. However, the accuracy of measurement always bothers many researchers. Results may vary from time to time, and in the worse cases, the composition outputs are nonsense. Table 1 gives an example of a series of ALCHEMIC measurements in a B2 Ti50Al42Mo8 intermetallic alloy.

In general, two major sources cause errors in ALCHEMI: weak channeling and delocalization effects. The later only results in “systematic errors”, which can always be corrected or reduced by theoretical calculations. However, it is difficult to improve the error due to weak channeling.

Quantitative analysis in the TEM by EELS or EDXS depends on the K-factor method in which uniform ionization, independent of specimen orientation and thickness, is assumed. This assumption is of limited validity for crystals, where channeling of the electron wave ψ affects the ionization rate as observed in both energy loss and X-ray signals. Both EELS and EDXS are sensitive to changes in ψψ*near the atomic sites, and this variation as a function of crystal orientation forms the basis for ALCHEMI. Simultaneously recorded EELS and EDXS spectra were used to monitor changes in Cu/Ba ratio from YBaCu-oxides using L2,3/M4,5 ionization edges or K/L X-rays respectively. Although the acceptance aperture for EELS (11 mrad at 300 keV) may not be sufficiently large to mask double-channeling effects, it is small enough that momentum transfer is sufficiently limited to enhance derealization. Thus it is expected that the EELS signal should be less sensitive to crystal orientation than EDXS (an estimate of impact parameters yields 0.73 and 0.61 Å for the Ba and Cu energy loss signals, reducing to 0.06 and 0.04 Å respectively for X-ray emissions).

Introduction:Atomically resolved chemical analysis using techniques such as electron energy loss spectroscopy and annular dark field imaging relies on the ability to form a well-characterised sub-nm electron beam in a FEGTEM/STEM [1-2]. to understand EELS+EDX-signal formation upon propagation of a sub-nm beam through materials we first have to assess precisely the beam intensity distribution in vacuum and find conditions for the best obtainable resolution.

Experimental Details:Modern TEM/STEM instruments combine features of both imaging and scanning technology. The beam forming capability approaches closely that for dedicated STEMs, while CCD recording devices allow us to measure the beam profile by direct imaging at magnifications up to 1.5 M. The recording of a “z-section” series through the 3D intensity distribution of the cross-over can therefore be realised by recording of a “condenser focal series”.

Nickel-base superalloys (NBSAs) have remarkable mechanical properties (and corrosion resistance) at elevated temperatures. Single crystal NBSAs have found major use as turbine blades in aeroengines, where there is a large world market because the turbine blades have to be replaced at regular intervals. in order to develop engines with higher efficiency, considerable efforts are being devoted to enhance the temperature capabilities of superalloys. The two phase γ-γ’ structure that is crucial to NBSA characteristics is primarily produced by the presence of Al in combination with Ni: above a critical concentration the Ll2 γ’ phase based on Ni3Al precipitates out of the γ Ni-base solution. However, superalloys usually contain many other alloying elements such as Cr, Co, Mo, W and Ta to enhance strength and impart higher temperature performance. Among these alloying additions, much attention has recently been paid to Re because of its contribution to the improvement of both creep and corrosion properties.

Precisely known atomic scattering factors are essential for accurate atom location by channelling enhanced microanalysis (ALCHEMI) based on inner-shell ionization.1 For ALCHEMI using energy dispersive x-ray analysis (EDX), first principles calculations of ionization cross sections, realistically modeling the “delocalization” of the ionization interaction, give excellent agreement with experiment.2 Such calculations are complex and computationally intensive. Hence, simple analytic forms are often assumed to describe the ionization potential. However such analytic forms require prior knowledge of the “delocalization” of the effective ionization interaction. Such an approach assumes that the precise shape of the ionization potential is not important but that at least the half width at half maximum (HWHM) should be accurately estimated, for example using estimates of the HWHM from root-mean-square impact parameters for ionization. However this is generally not a good approximation3 and we have provided more realistic estimates (Fig. 1).

Two-dimensional Atom Location by Chanelling Enhanced Microanalysis ( Alchemi ) maps of characteristic Xray emission as function of the diffraction conditions of a 200kV electron beam hav been obtained from thin crystals by several workers . Reciprocity shows that these are equivalent to the diffraction patterns produced at infinity by 200kV point sources of electrons on atom sites, sue! as D in figure 1, for CsCl. Here ray SCD at left has been time-reversed at right. Using multislici superlattice simulations which launch a spherical wave from an atom site, we find that the resultinj Kikuchi pattern breaks up into separated blobs along the K-lines at small thickness, with one blol generated at infinity along each atomic string direction . Each blob (e.g. ID) is a Gabor in-lim hologram (e.g of atom C, formed by source S at right). Each source atom on equivalent sites (E, D produces identical holograms, whose intensities add.

Contrast in zone axis channelling patterns, formed by variations in characteristic X-ray emission rates as an electron beam is scanned in angle, is related to projected sublattice site symmetries of ionized atoms within the unit cell. Whereas overall Brillouin zone geometry is identical to that observed in large angle convergent beam patterns, this incoherent X-ray channelling contrast is related to ADF or BSE contrast by integration over thickness of the signal generated within the specimen. Contrast is thus relatively stable and easily interpretable.1 This is useful in separating the response of dilute atoms from abundant atomic species, since the specific channelling pattern of each atom has its own overall fingerprint. Correlation of patterns formed from Ta-doped TiAl with bright field LACBED is shown in Fig. 1 (clearly Ta occupies Ti sublattice sites). No simulation of channelling contrast is necessary if minority atomic species are distributed over sublattice sites in a way that can be reconstructed from a linear superposition of majority (or host) atom sites that form part of the regular lattice structure. The distribution over sublattice sites may be determined by any one of a variety of analysis methods.2,3

It has been long known that orientation effects in crystalline materials can influence characteristic x-ray emission and microanalysis1-7. High Angular Resolution Electron Channeling X-ray Spectroscopy (HARECXS)6-7. a variation of ALCHEMI4-5, has been used at ANL for the last few years to investigate the effects of channeling on quantitative XEDS analysis of materials. More recently we have also been using HARECXS to carefully measure elemental disordering in a number of systems and have found that it can be used very successfully to elucidate the various stages of disorder.

Perovskite (nominally CaTiO3) is a host phase for actinides in various wasteforms for the immobilization of high level radioactive nuclear waste. Over geologic time, alpha decay damage of the actinides in perovskite will cause displacive effects that influence the dimensional and chemical stability of the wasteform. in the past, the progression of damage has been studied by monitoring changes in selected area electron diffraction (SAED) patterns with increasing dose (e.g. 11).

The mineral spinel (MgAl2O4) is a candidate material for proposed fusion reactors because of its resistance to structural damage during irradiation. When irradiated to high damage levels, spinel is eventually driven amorphous. However, at lower fluences the nearly cubic close packed arrangement of the oxygen anions remains intact, as indicated by electron diffraction patterns consistent with a face centered cubic Bravais lattice. in this regime, the damage to the material is manifested primarily in the redistribution of the magnesium and aluminum cations among the interstices of the anion sublattice. For ion irradiation, the study of these early stages of damage is well suited to electron beam characterization methods, because the damage is spatially localized at the nearsurface region. Electron diffraction methods have been used for the characterization of cation disorder, but are notoriously unreliable because the similarity in the elastic scattering cross sections offers little discrimination between the magnesium and aluminum cations.

Garnets were one of the first materials in which an occupation of separate lattice sites by different atomic species was determined with an ALCHEMI technique proposed by Spence and Tafto in l982. The reason of so much interest in this material was twofold, i.e. first its known high sensitivity of X-ray generation depending on orientation especially in the axial orientation and second its complicated crystal structure allowing different atomic arrangements in the unit cell depending on its chemical composition. The dodecahedral (X), octahedral (Y) and tetrahedral (Z) sites between the relatively large oxygen atoms can be filled with a variety of small cations in accordance with the formula X3Y2Z3O12. Partial substitution of one cation with another is common in this structure. The results presented in the previous literature indicated that ALCHEMI can only separate the Y-sites from the sum of the other two (X+Z), while the latter has to remain unresolved.

Everyone presently answers “no” to this question, since this methodology for specimen preparation and preservation commonly is believed to be inherently limited to 20-25Å (2.0-2.5nm) resolution. Achievement of resolution levels smaller than this figure have been published only rarely. This report presents experimental evidence from electron diffraction showing that negative staining can preserve periodic protein structure to the level of at least 4Å.

A suspension of bovine liver catalase crystals (orthorhombic: a= 69Å, b= 174Å, c= 206 Å [1]) is deposited on a hydrophilic carbon support and negatively stained using an on-grid protocol [2,3]. For low-dose electron diffraction with a JEOL 100CX transmission electron microscope (l00kV), the set-up for routine selected area diffraction is modified to keep the second condenser lens maximally overfocused; by a defocusing of the diffraction pattern, the grid can be surveyed and suitable thin single crystals centered in the diffraction aperture with only negligible irradiation.

EBSD analyses are routine for understanding and quantifying the orientations of materials. Analyses of single-phase cubic materials are by far the most prevalent reported in the literature. However, the understanding of the orientations of multi-phase materials are just as important from a technological standpoint. The analysis of non-cubic and non-hep materials is also of interest to investigators. However, analyses of these materials provide some unique challenges for accurate and precise orientation and phase determination compared to cubic materials. The ORKID analysis system was designed from the beginning to address these materials and analysis concerns.

Single-phase materials, especially with cubic symmetry, do not require any special treatment for indexing of diffraction patterns. Because only one crystal system is under investigation, the algorithm for matching the measured bands and the expected bands can have some relaxation in error limits. Furthermore, d-spacing measurements of the bands are not strictly required because the bands angles are sufficiently unique.

The Hough Transform is widely used to detect linear features in image processing techniques. However, the features sought in the EBSD patterns are typically Kikuchi bands from low-index crystal planes, which are not lines but are bands of above average intensity bordered by dark bands. This gives rise to a characteristic shaped peak in the Hough transform that has been called the “butterfly” shape. in most cases, to reduce noise, the Hough Transform is convoluted with a mask having a matching “butterfly” shape. Unfortunately, this method sacrifices resolution through averaging the intensity of neighboring pixels. We have been concerned to use the Hough transform to locate the position of the Kikuchi bands with the highest possible precision. in order to achieve this goal we have looked into the way the Hough transform works on Kikuchi bands in detail, and found some anomalies that must be considered if accuracy is to be achieved.