Perspectives of molecular and cellular electron tomography.

Nanotechnology Research Directions: IWGN Workshop Report

Symmetrically substituted octahydroxy porphyrins, tetrakis(3',5'-dihydroxyphenyl)porphyrin, H2T(3',5'- DHP)P, tetrakis(2',6'-dihydroxyphenyl)porphyrin, H2T(2',6'-DHP)P, and their Zn(II) and Mn(III) derivatives have been developed as building blocks for supramolecular hydrogen-bonded networks. The crystal structures of a series of these porphyrins exhibit unique structural features through assembly of porphyrin networks by means of directional hydrogen bonding. The position of the peripheral hydroxyl groups, the choice of metallo- or free base porphyrin, and the nature of the solvate (i.e., guest) dramatically influence structural features. A one-dimensional, columnar structure is found for H2T(3',5'-DHP)P‚5EtOAc. With benzonitrile as solvate, the structure of H2T(3',5'-DHP)P‚7C6H5- CN changes substantially to a three-dimensional corrugated-sheet structure in order to accommodate a larger pore size. When the hydroxyl substituents are simply changed from the m- to the o-phenyl positions, an essentially two-dimensional layered structure is formed for H2T(2',6'-DHP)P‚4EtOAc. Zn(T(2',6'-DHP)P)(EtOAc)2‚2EtOAc has a two-dimensional layered structure, similar to that of its free base H 2T(2',6'-DHP)P interaction between the aryl rings of the adjacent layers. The crystal structures of both Zn(T(3 ',5'-DHP)P) and Mn(T(3',5'-DHP)P)(Cl) exhibited three-dimensional hydrogen-bonding features. Zn(T(3',5'-DHP)P)(THF)2‚2THF‚3CH2Cl2 has a three-dimensional interconnected layered structure with metalloporphyrins arranged in a slipped stack orientation within the layers. In the structure of Mn(T(3',5'-DHP)P)(THF)2‚Cl‚2THF‚5C6H5CH3, a chloride anion dictates the three-dimensional packing by bridging four metalloporphyrin molecules through Cl‚‚‚HO bonding interactions. In all of these structures, large solvate-filled channels are present with cross-sections as large as 42 A 2 . The pore volumes of these channels are exceptionally large: as much as 67% of the unit cell volume.

Hydrogen-Bonded Porphyrinic Solids: Supramolecular Networks of Octahydroxy Porphyrins

Rapid grain growth during the early stage of sintering has been found in many nano material systems including cemented tungsten carbide WC–Co. To date, however, there have been few reported studies in the literature that deal directly with the kinetics or the mechanisms of this part of grain growth. In this work, the grain growth of nanosized WC during the early stages of sintering was studied as a function of temperature and time. The effects of other influencing factors, such as the initial grain size, cobalt content, and the grain growth inhibitor VC, were investigated. The kinetics of the grain growth process was analyzed and the evolution of the morphology of WC grains during heating-up was studied using high resolution scanning electron microscopy. The results showed that the grain growth process consists of an initial stage rapid growth process which typically takes place during heat-up and the normal grain growth during isothermal holding. The initial rapid grain growth is at least partially attributed to the process of coalescence of grains via elimination common grain boundary. The preferred orientation between WC grains within the aggregates is considered a favorable condition for coalescence of grains, hence rapid grain growth. The solution–reprecipitation process is considered a mechanism of coalescence.

Grain growth during the early stage of sintering of nanosized WC–Co powder

This paper describes recent progress on the research into improving the oxidation behavior of the bond coat using a HVOF nanostructured NiCrAlY coating. Commercially available NiCrAlY powder was mechanically cryomilled and HVOF sprayed onto Ni-based alloy to form a nanocrystalline bond coat. The powder and coating structure were characterized by XRD, SEM, and TEM. Oxidation experiments were performed on the coating to form the thermally grown oxide layer (TGO). After heat treatment at 1000 °C for 24 and 95 h, a homogeneous α-Al2O3 layer was formed on top of the bond coat. The oxide layer was analyzed and compared to the coating sprayed using the as-received powder. As shown in the results, the nanostructured characteristic of the coating and the presence of Al2O3 within the cryomilled powders (oxidation occurred during cryomilling process) seem to affect the nucleation of the alumina layer on the top of the coating. The formation of a continuous TGO layer protects the coating from further oxidation and avoids the formation of mixed oxide protrusions, such as those presented in the coating sprayed using the as-received powder.

Oxidation behavior of HVOF sprayed nanocrystalline NiCrAlY powder

The present paper describes the synthesis, characterization, and grain growth behavior of nanocrystalline Ni coatings generated using a novel synthesis approach, namely high velocity oxy-fuel (HVOF) thermal spraying. In the present investigation, the feedstock powders were prepared by mechanical milling in a methanol environment which yielded agglomerates with a flake-shaped geometry and an average grain size of less than 100 nm. The milled powders were then introduced into the HVOF spray system in order to investigate the feasibility of generating a coating with grain sizes in the nanocrystalline range (e.g., <100 nm). Scanning electron microscopy and transmission electron microscopy were used to study the morphology of the nanometric particles and the microstructure of the milled powders and the as-sprayed coatings. Transmission electron microscopy analysis performed on cross sections of the coating revealed a mixture of fine nanocrystalline grains and elongated coarse grains.

Thermal spraying of nanocrystalline Ni coatings

Quantitative characterization of point spread function and detection quantum efficiency for a YAG scintillator slow scan CCD camera

Convergent-beam electron diffraction is applied to measure the temperature factors of the intermetallic phase NiAl with high accuracy. The patterns are recorded in an energy-filtering transmission electron microscope at zero energy loss using a slow-scan CCD camera. The specimens were tilted in systematic row orientation. In this new approach, data are extracted from the bright-field disc as well as from several high-order dark-field discs along line scans. The temperature factors are determined by fitting Bloch-wave simulations to the intensity profiles. The harmonic approximation for temperature factors is used. For B2-phase NiAl, mean thermal displacements u(Ni) = 5.5 ± 0.1 and u(Al) = 5.7 ± 0.1 pm are obtained at 100 K. A very detailed error analysis is given, and stochastic and systematic errors are discussed and quantified.

High‐Precision Measurement of Temperature Factors for NiAl by Convergent‐Beam Electron Diffraction

An iterative method for the determination of structure-factor amplitudes and phases of crystals with unknown structure is proposed. The method is based on the quantitative evaluation of energy-filtered convergent-beam electron diffraction (CBED) patterns. It has been applied to the metastable phase AlmFe (m = 4.2–4.4), which was found as primary particles in commercial aluminium alloys. When the rocking curves of reflections along a (100) systematic row are fitted, a number of structure-factor amplitudes and signs of h00-type reflections of AlmFe have been determined successfully. The accuracy of the structure-factor amplitudes is better than 5% for the strong reflections and about 10% for the weak reflections. The aim of this method is to provide accurate structure-factor data, including both amplitudes and phases, for ab initio structure determination and refinement of unknown structures by using electron diffraction techniques.

Low-Order Structure-Factor Amplitude and Sign Determination of an Unknown Structure AlmFe by Quantitative Convergent-Beam Electron Diffraction

Quantitative convergent beam electron diffraction was applied to measure the twelve lowest indexed electron structure factors of NiAl. The experiments were carried out at 110 K. The electron structure factors were converted to X-ray structure factors which are the Fourier coefficients of the total electron charge density. For the five lowest coefficients the experimental error is 0.1 to 0.4% (after conversion). To obtain the bonding charge density, a reference was synthesized by superposing the charge densities of free, neutral, spherical atoms as computed by a relativistic Dirac-Fock program. The difference charge density exhibits mainly covalent bonding and a depletion of charge mainly in the Al core region. The difference charge density computed from the higher indexed coefficients cannot be explained by bonding effects but indicates that free neutral atoms might not be the best choice for the reference.

W. Nüchter

Papers

Thermal spraying of nanocrystalline Ni coatings

Quantitative characterization of point spread function and detection quantum efficiency for a YAG scintillator slow scan CCD camera

High‐Precision Measurement of Temperature Factors for NiAl by Convergent‐Beam Electron Diffraction

Low-Order Structure-Factor Amplitude and Sign Determination of an Unknown Structure AlmFe by Quantitative Convergent-Beam Electron Diffraction

Determination of Bonding Charge Density in NiAl by Quantitative Convergent Beam Electron Diffraction