P.S. Schabes-Retchkiman

Decagonal and hexagonal structures in small gold particles

Abstract In the present work we report the study of gold particles using HREM and image processing. Particles which are not conventional fcc are discussed as well as penta-twinned particles which show twin boundaries that do not join along a common point. It is shown that there are particles with an hcp structure. This structure is probably due to repeated faulting and excess vacancies.

Microdiffraction and lattice resolution studies of fivefold symmetry gold particles

The structure of fivefold particles is studied using single particle diffraction in a STEM microscope and lattice images. The results show excellent agreement with the rhombohedral and body‐centered orthohombic models for the particle. Theoretical calculations were carried out to study the influence of the number of atoms in the diffraction patterns.

Phase identification during aging of some nitrided ferrous materials

Abstract A characterization study of the phases formed during aging of ferrous materials nitrided in a microwave post-discharge was performed by microhardness, electron microscopy (SEM, TEM and HREM) and X-ray diffraction. Iron and AISI-1010 steel samples were microwave nitrided for 10 min at 840 K and then quenched in Ar at 273 K. The quenched samples were then annealed for treatment times ranging from 0.5 h to 8 h at 673 K. The results from the microstructural characterization of steel show the presence of a compact surface layer composed of epsilon nitride (Fe 2 – 3 N). For iron, Fe 4 N needles appeared underneath the surface after nitriding. During annealing, the amount, size and morphology of the needles change as the treatment time progresses. The presence of Fe 16 N 2 was also detected after aging. The amount of this phase increases with aging time. The microhardness measurements correlate with the microstructural changes.

On the structure of small palladium particles

The study of small noble metal particles is becoming increasingly important in many fields in physics (1). The advent of high-resolution electron microscopy (HREM) has allowed a deeper understanding of structural aspects of small particles. This work reports the study of particles of palladium with a diameter less than 3 nm. Specimens were prepared by in-situ deposition of Pd onto thin carbon films under near-UHV conditions in the specimen preparation chamber. Faulted decahedral MTP was grown using a recursive (R) growth model which generates infinite, space-filling structures reproducing the structure of crystals, twinned particles and quasicrystals. R growth consists of the formation of a cluster by iterative addition of points (atoms) from a given star vector. The method presented sheds some light on a point that has been controversial in the past about the nature of MTPs. Some authors have claimed that these structures can be considered as FCC twins with a disclination to close the resulting gap. The fact that they can be obtained quite simply from stable smaller units appears to make the disclination unnecessary.

On the long period superstructure of the superconducting phase Bi2Sr2CaCu2O8+x

Abstract We have analyzed the structure of the superconducting phase Bi 2 Sr 2 CaCu 2 O 8+ x from the long-period superstructure point of view in the a - b plane. An incommensurate modulation along the b -axis has been found with mean size for the anti-phase boundaries of M= 5 2 . Simulated electron diffraction patterns and images from these compounds are presented. It is concluded that the superstructure is consistent with a long period structure based on planar defects such as antiphase boundaries. Calculated diffraction patterns and images using this model are in full agreement with the experimental results obtained by several groups. We therefore conclude that the incommensurate structure along the b -axis observed in these materials can be explained in terms of a simple defect structure.

Structure and shape of epitaxial metal clusters

Abstract The structure of small gold particles produced by evaporation on alkali-halide surfaces is studied using ultra-high resolution electron microscopy. During growth two main types of particles are observed; five-fold (icosahedral and decahedral) and regular fcc structures. Weak beam and ultra-high resolution projected potential images are obtained. The different shapes of the particles are thus disclosed. For fcc triangular, rhombic, pyramidal, and irregular shapes are found. Evidence towards the transition from icosahedron to fcc cubo-octahedron occurring possibly through an intermediate spherical state is presented. Ultra-high resolution projected potential images of icosahedrons show that the interatomic distance along a radial direction does not change substantially. Particle distortions predicted by energy calculations based on a pair-wise Lennard-Jones potential are not observed. Also, no dislocations were found in the tetrahedra interface, for particle sizes below ∼300 A.

The effect of crystal geometry on the formation and crack development in molybdenum single crystals

The systematic study of the defect structure evolution in single crystal specimens deformed in situ in the high voltage transmission electron microscope (HVTEM) was started around 1977 in the Baikov Metallurgy Institute of the USSR Academy of Sciences. All the experience gathered for the past 30 years has set up the basis for the determination of the way that metals adapt to the different physical factors inside the HVTEM column. In the present work the detailed phenomenology of the generation and propagation process of cracks in single crystals of Mo with different crystallographic orientations as well as the kinetics of opening of primary and secondary cracks and the tip evolution for cracks with different dislocation generation sources is presented.

Stress State Localized Analysis on the Tip of the Crack

In this paper is presented a study of the analysis of the stress state of micro specimens exposed to a load inside the column of a High Voltage Electron Microscope (HVTEM), in particular for a Mo single crystal. The experimental tangential stresses distribution images were obtained and compared with theoretical calculations using Naiber approach, obtaining an excellent match.

Structure of Bi2.1(Ca,Sr)n+1CunO2n+4+δ superconductors by electron diffraction

The discovery of a new class of Bi-based oxides with high Tc has given fresh impetus to the research in this area. This family of oxides can be described by the more general formula Bi2.1(Sr,Ca)CunO2n+4+ delta . Electron diffraction has shown that these compounds have a very clear superstructure along the b axis. Although several models have been proposed, no agreement has been reached on the type of superstructure. In the present work the authors report a theoretical and experimental study of the electron diffraction patterns from these compounds. It is concluded that the superstructure is consistent with a long period structure based on planar defects such as antiphase boundaries. Calculated diffraction patterns using this model are in full agreement with the experimental results. They therefore conclude that the incommensurate structure along the b axis observed in these materials can be explained in terms of a simple defect structure.

(1 × 2) Surface reconstruction in {110} gold particles

Abstract The surface reconstruction in {110} oriented small gold particles is studied by STEM microdiffraction and high resolution TEM images. The well known (1 × 2) surface net is found in the diffraction patterns. Projected potential images show that the surface is composed of (1 × 2) and (1 × 1) regions. The fine structure in the diffraction patterns seems to give some support to the Bonzel and Ferrer model for the (1 × 2) reconstruction.