The effects of varying the pulse energy of a picosecond laser used in the pulsed-laser atom-probe (PLAP) tomography of an as-quenched Ni-6.5 Al-9.5 Cr at.% alloy are assessed based on the quality of the mass spectra and the compositional accuracy of the technique. Compared to pulsed-voltage atom-probe tomography, PLAP tomography improves mass resolving power, decreases noise levels, and improves compositional accuracy. Experimental evidence suggests that Ni2+, Al2+, and Cr2+ ions are formed primarily by a thermally activated evaporation process, and not by post-ionization of the ions in the 1+ charge state. An analysis of the detected noise levels reveals that for properly chosen instrument parameters, there is no significant steady-state heating of the Ni-6.5 Al-9.5 Cr at.% tips during PLAP tomography.

Following is a list of microscopy-related meetings and courses. The editors would greatly appreciate input to this list via the electronic submission form found in the MSA World-Wide Web page at http://www.msa.microscopy.com. We will gladly add hypertext links to the notice on the web and insert a listing of the meeting in the next issue of the Journal. Send comments and questions to Nan Yao, nyao@Princeton.edu.

The molecular distribution in nanocolloids of poly(dimethyl siloxane) (PDMS) and an organic copolymer (methyl acrylate co-methyl methacrylate co-vinyl acetate) preserved in a frozen aqueous solution was investigated using cryovalence electron energy-loss spectroscopy (EELS) coupled with a scanning transmission electron microscope. Low energy-loss spectra depend upon valence electron structure, and we show that they are substantially different for the PDMS, the copolymer, and the vitrified water studied here. Combining a high efficiency detection system and the use of high-signal low-loss spectra in EELS, we achieved a spatial resolution of 8 nm without serious beam-induced specimen damage in this radiation-sensitive soft-materials system. To obtain quantitative phase maps of silicone and copolymer composition within individual nanoparticles, spectrum datasets were processed via multiple least squares fitting. Quantitative line profiles from the resulting compositional maps indicate that the PDMS lobe of biphasic nanoparticles contained a significant amount of the copolymer and a diffuse interface was formed. Since the nanoparticle synthesis involves polymerization of acrylate monomer dissolved in PDMS nanoparticle precursors, these results suggest that the evolution of the nanocolloid morphology during synthesis is kinetically frozen as the acrylate copolymer achieves some critical molecular weight.

The movement of heavy atoms on a thin carbon substrate is readily observed using a sub-Ångstrom electron probe. The observed movement is consistent with an electron beam activation mechanism whereby atoms are occasionally detached from bonding sites, allowing rapid diffusion to new sites that may be quite far from the original. The bonding sites are most often observed to lie at defects, steps, and other asperities in the substrate. Formation of three-dimensional clusters can occur during diffusion of several isolated atoms. Coalescence and dissolution of larger clusters and islands both occur under varying observation conditions, but island coalescence appears most probable for islands that are greater than 2 nm in size.

Extended abstract of a paper presented at Microscopy and Microanalysis 2008 in Albuquerque, New Mexico, USA, August 3 – August 7, 2008

The interest in titanium based intermetallics, such as Ti-Al alloys, for high temperatures applications has been increased in recent years mainly due to their high plastic deformation resistance, chemistry stability (oxidation and corrosion resistance) and creep and fatigue resistance at high temperatures. However, the industrial application of Ti-Al intermetallics is very limited so far, due to the lack of ductility and fracture toughness at room temperature with associated processing difficulties. To overcome this problem and to improve mechanical properties of Ti-Al intermetallics, a powder mixture with atomic composition of Ti52Al48 was synthesized by mechanical alloying (MA) and subsequently coated with a ductile element (aluminium), by d.c. magnetron sputtering. Later, in order to obtain a compacted material, as final step, the coated MA'ed powders were submitted to hot isostatic pressing (HIP), giving rise to a bulk material. The aim of the current work is to understand the influence of coating layer in mechanical alloyed (MA'ed) powders and the influence in bulk material properties.

Observation of the internal ultrastructure of human chromosomes by transmission electron microscopy (TEM) has frequently been attempted in spite of the difficulties in detaching metaphase chromosome spreads from the glass slide for further processing. In this study we have used a method in which metaphase chromosome spreads were prepared on a flexible thermoplastic membrane (ACLAR) film. To assess chromosome identity, a diamidino-phenylindole staining and karyotying was first done using a conventional cytogenetic system. The chromosome spreads were then fixed with 1% osmium tetroxide, stained with freshly prepared 2% tannic acid, dehydrated, and flat-embedded in epoxy resin. The resin sheet was easily detachable and carried whole chromosome spreads. By this method, TEM observation of chromosomes from normal human lymphocytes allowed a thorough examination of the ultrastructure of centromeres, telomeres, fragile sites, and other chromosomal regions. Various ultrastructural patterns including thick electron dense boundaries, less dense internal regions, and extended chromatin loops at the periphery of the chromosomes were discernible. Application of the present method to chromosome research is expected to provide comprehensive information on the internal ultrastructure of different chromosomal regions in relation to function.

The ability of electron microscopes to analyze all the atoms in individual nanostructures is limited by lens aberrations. However, recent advances in aberration-correcting electron optics have led to greatly enhanced instrument performance and new techniques of electron microscopy. The development of an ultrastable electron microscope with aberration-correcting optics and a monochromated high-brightness source has significantly improved instrument resolution and contrast. In the present work, we report information transfer beyond 50 pm and show images of single gold atoms with a signal-to-noise ratio as large as 10. The instrument's new capabilities were exploited to detect a buried Σ3 {112} grain boundary and observe the dynamic arrangements of single atoms and atom pairs with sub-angstrom resolution. These results mark an important step toward meeting the challenge of determining the three-dimensional atomic-scale structure of nanomaterials.

Ag on γ-alumina is a promising catalyst for hydrocarbon selective catalytic reduction in lean-burn gasoline and diesel engines for transportation applications. Although much is known about the mechanism of NOx reduction and the various intermediates, little agreement exists on the nature of the active silver species. In the present work, aberration-corrected STEM has provided new information about the nature of Ag on alumina both as impregnated and following treatments at various temperatures with exposure to simulated exhaust gas. Ex situ techniques have provided new insights into the evolution of Ag on alumina following exposure to temperature and simulated exhaust gas.

Digital imaging has been applied to structure analysis of biological macromolecules in combination with electron energy filtering. Energy filtering can improve the image contrast of frozen-hydrated specimens, but needs a high-sensitivity imaging device instead of photographic film, because of a decrease in electrons after filtration. Here, a lens-coupled slow-scan charge-coupled device (SSCCD) camera with a post-column-type energy filter were examined to image bacterial flagellar filaments embedded in ice. We first measured the modulation transfer function of this camera and showed the remarkable improvement, compared to other fiber-coupled SSCCD cameras. The 3D structure calculated at ∼7-Å resolution clearly resolves α-helices. Furthermore, filtered datasets recorded on the SSCCD camera with liquid-nitrogen and liquid-helium cooling were compared with the previous unfiltered one on film with liquid-helium cooling. This report describes the suitability of digital imaging with energy filtering for higher-resolution structure studies from its practical application.

When compared with conventional bare metal stents, such as 316L stainless steel, the introduction of drug-eluting stents can promote reduction in the incidence of in-stent restenosis. However, the chemical discrepancy between the metallic stent and the polymeric material that acts as the reservoir for the drug is responsible for some problems during the cardiovascular surgery. Besides the research work aiming at the development of new bulk alloys for stent production, focus as been also directed to the surface modification of these devices. However, the use of functional graded coatings (FGC), i.e., coatings with a gradient of chemical composition between the substrate and the outmost layer, has never been reported in devices for cardiovascular surgery.

Extended abstract of a paper presented at Microscopy and Microanalysis 2008 in Albuquerque, New Mexico, USA, August 3 – August 7, 2008

Bioactive materials for potential medical application have inspired and stimulated new searches. It has been confirmed that bioactive glasses bond to living bone through an apatite layer that precipitates on their surface in physiological media. These glasses normally constituted by silica, where the silicon is the network former, induce apatite nucleation by the formation of Si-OH groups. The presence of OH groups seems to be of utmost importance in the bioactive behaviour of materials. It was revealed that even metals, such as pure titania gel can bond to bone, if previously subjected to alkali and heat treatments. In the present work a new composite with a Ca-P-Ti glass was synthesized and its in vitro bioactivity was studied in Kokubo's simulated body fluid. It is believed that the formation of Ti-OH groups on the glass can induce apatite precipitation on the composites surface.

Extended abstract of a paper presented at Microscopy and Microanalysis 2008 in Albuquerque, New Mexico, USA, August 3 – August 7, 2008