The alignment of Bi(Pb)-Sr-Ca-Cu-O near the Ag-(Bi(Pb)-Sr-Ca-Cu-O) interface in silver-sheathed multifilamentary Bi(Pb)2Sr2Ca2Cu3O

The Ag-(Bi(Pb)-Sr-Ca-Cu-O (BSCCO)) interface region has been studied by scanning electron microscopy and transmission electron microscopy to a distance of 2µm away from the silver in the Ag-sheathed multifilamentary Bi(Pb)₂Sr₂Ca₂Cu₃O (Bi(Pb)-2223) tapes. Owing to undulation of the silver surface, silver was found to play an important but not always positive role in the alignment of BSCCO grains. Based on the kink angle variation, the effective thickness of multifilamentary Bi(Pb)-2223 tapes capable of carrying a large current has been defined. We believe that the difference in effective thickness is the key reason leading to different critical current densities J c for these tapes.     

Crystallographic orientation dependence of nanoindentation hardness in austenitic phase of stainless steel

ABSTRACT

This paper presents the indentation hardness evolution in different in-plane directions of austenite grains whose {111} planes are parallel to the sample surface determined by nanoindentation tests and electron back scattering diffraction (EBSD) analysis. A scanning electron microscopy (SEM) image of the indentation surface around one of the indents indicated the activation of two sets of slip planes with respect to each of the three indenter surfaces for a Berkovich tip. The identification of the slip traces by EBSD data analysis is in accord with Schmid`s law. We therefore proposed a new approach for defining the orientation parameter to interpret the indentation hardness. The orientation parameter was shown to be the minimum value of the three maximum Schmid factors on the secondarily activated slip planes in three directions for a Berkovich tip. Indentation hardness increased with the decrease in the orientation parameter and was dependent on in-plane orientation.     

Stability in ordered and icosahedral aluminium–transition metal alloys

Abstract

We present calculated electron state densities for A1T (T is a transition metal) compounds and icosahedral Al₈₆T₁₄ alloys which suggest that the relative stability of different short-range atomic arrangements varies inversely with the electron state density at the Fermi level. Using this argument, we show that the absence ofstable highly coordinated configurations is a necessary, but not sufficient, condition for the existence of icosahedral Al₈₆T₁₄ alloys.     

Hexagonal surface structure during the first stages of niobium growth on sapphire (1120)

A detailed reflection high-energy electron diffraction study of the first stages of the niobium growth on (1120)s sapphire is presented for several substrate temperatures. It is shown that the niobium film exhibits an hexagonal surface structure when the deposited thickness is smaller than a critical value, which, depending on the substrate temperature, varies between 5 and 15 A. For thicknesses larger than this critical thickness, the surface hexagonal structure relaxes to the (110) bcc niobium structure. The hexagonal surface structure is observed for high substrate temperatures (820-410oC) but does not appear when the substrate temperature is 270oC. The epitaxial relationships between the substrate, the surface hexagonal structure of niobium and the cubic niobium phase are presented.     

High-resolution in situ electron microscopy of a silicon surface modification by molten aluminium at high temperatures

High-resolution in situ heating experiments in a transmission electron microscope have been applied to a study of the surface modification of Si by molten Al in a vacuum of 1 10^-5 Pa. Transformation from an atomically rough to an atomically flat surface was induced by wetting of molten Al atoms. Si {110}, {112}, {115} and {773} surfaces show sawtooth-like structures composed of nanofacets. It is concluded that molten Al atoms remove a native amorphous oxide layer which existed on the original Si surface, making the surface 'clean' even in a non-ultra-high vacuum such as 10^-5Pa.     

Pattern evolution of crystalline Ge aggregates during annealing of an Al/Ge bilayer film deposited on a SiO2 substrate

When an Al/Ge bilayer film deposited on a SiO2 substrate is annealed at 373- 398 K, Ge atoms diffuse out from the inner amorphous Ge layer and spread over the free surface of the outer Al layer to form crystalline Ge aggregates exhibiting complex substructures. Scanning electron microscopy observations indicate that the activation energy for the pattern evolution of Ge aggregates on the free surface because of annealing is 1.56 eV which is about half the activation energy for crystallization of amorphous Ge.     

Grain-boundary sliding and accommodation mechanism during creep of yttria-partially-stabilized zirconia

The role of grain-boundary sliding in deformation of ceramic materials is analysed from an atomic force microscopy investigation of the change in surface topography during high-temperature deformation of yttria-partially-stabilized zirconia polycrystals. From transmission electron microscopy observations of strained specimens, we attempt to model the accommodation mechanism of grain-boundary sliding.     

Screw misfit dislocations perpendicular to one or two free surfaces

ABSTRACT

When a screw dislocation pierces one or two free surface(s), noticeable elastic relaxation takes place nearby these surfaces. To refine the theoretical interpretation of an electron microscopy image this relaxation should be included in a precise imaging model. In the present work, the relaxation field is evaluated for a periodic set of misfit screw dislocations normal to the free surface of a semi-infinite or plate-like heterogeneous bicrystal. The proposed approach uses an appropriate combination of known biperiodic elastic fields. For a homogeneous medium and when the period increases, the results around a misfit screw dislocation converge to those of Eshelby and Stroh (1951) who considered an isolated translation screw dislocation.     

The surface topography of cracks in strained In0.72Ga0.28P films

Cracks in a 2% tensile strained In_0.72Ga_0.28P film grown on an InP substrate by molecular-beam epitaxy have been studied by cross-section transmission electron microscopy and scanning probe microscopy. A dislocation analogue (i.e. replacing the crack by an array of equivalent infinitesimal edge dislocations) is employed to account for the ratio of the crack-opening displacement to the normal surface displacement associated with the crack.     

Recent advances in electron-beam-induced damage models in yttria fully stabilized zirconia single crystals

Dislocation loop formation in yttria fully stabilized zirconia single crystals observed in a transmission electron microscope has been reported in the literature. To account for the loop formation, two different explanations have been proposed. Here we present original experiments focusing on a comparison of the two models. We conclude that the origin of the loops is precipitation of a second phase, enhanced by heating in the electron beam.     

A new modulated structure in GaN nanoparticles

A new modulated structure with a superlattice having parameters a = 2.209nm, b = 3.826nm, c = 1.037nm and f = g = n = 90 has been found in GaN nanoparticles synthesized by a dc arc plasma method. The nanoparticles transformed further into particles with holes at their centres under electron-beam irradiation during high-resolution electron microscopy observations. At the same time, Ga atoms were extruded on to the surface of the nanoparticles and formed an amorphous layer. A series of simulations of high-resolution images and electron diffraction patterns revealed that the modulation could be attributed to aggregations of N vacancies founded during the electron bombardment. Molecular mechanics calculations show that the aggregation of N vacancies is far more energetically favourable than that of Ga vacancies. The stability of the GaN particles is discussed.     

Reactive solid-state dewetting

High-temperature annealing in air of thick crystalline silver films deposited on high-purity nickel foils promotes solid-state dewetting, whereas no hole through the film is produced when annealing under high purity argon. Scanning electron microscopy observations of the film surface and of cross-sections reveal that dewetting occurs only if a nickel oxide layer forms at the silver-nickel interface, as a result of oxygen diffusion through the silver film. The main dewetting mechanism over short times (1h 1120K in air) is observed to be the condensation, at the silver-nickel oxide interface, of vacancies into voids which grow towards the free surface of the silver film.     

Information search activity: An overview

IntroductionThis article exposes a brief history of information search models, from general and behavioural models to cognitive ones and presents recent theoretical advances in this research field.Literature findingsSince the beginning of 1990s, many theoretical models have tried to describe and predict the human behaviours involved in the information search tasks, and specifically when this activity is performed in computer systems. Some of these models elaborated by psychologists have attempted to infer the cognitive processes involved.DiscussionInformation search can be considered as a complex cognitive activity that involves various individual (cognitive, affective), contextual and social factors.ConclusionWe also suggest research issues to be developed in order to increase our knowledge in this area and we plead for further research on specific concepts depending on future and/or recent areas of interest.     

Experimental and simulated grain boundary groove profiles in tungsten

Grain-boundary grooving has been studied on polished surfaces of polycrystalline tungsten annealed at 1350°C. Atomic force microscopy images were taken in the same area for each groove after different annealing times. Secondary oscillations next to the main groove maxima (predicted for grooving by surface diffusion) were observed, to our knowledge for the first time. The agreement between experimental and calculated groove profiles (using the surface diffusion model of Mullins (1957, J. appl. Phys., 28, 333)) improved when grain-boundary fluxes were introduced.     

On the dislocation mechanism of amorphization of Si by indentation

The formation of an amorphous phase underneath a Vickers indentation produced on a Si(001) surface at room temperature has been observed by cross-sectional transmission electron microscopy. Two types of location are observed for the amorphous phase. One is formed just underneath the image of the indentation and the other is parallel to the slip planes of Si. It is concluded that the latter type, at least, is formed as a result of activation of dislocations which is induced by an external shear stress combined with a hydrostatic pressure.     

Interactions of persistent slip bands with a grain boundary on the common primary slip plane in a copper bicrystal

In this letter, the dislocation patterns on the common primary slip plane in a fatigued \[1-34]-\[182-7] copper bicrystal with a Sigma=19b grain boundary (GB) have been investigated using the electron channelling contrast technique in a scanning electron microscope. The results show that the two-phase dislocation structure, such as veins and persistent slip band (PSB) walls, embedded within veins, can be clearly seen on the common primary slip plane. In particular, the interactions of PSBs with the GB are clearly revealed. It is found that there are three kinds of interaction mode between the GB and the dislocations during cyclic deformation, and those are discussed. It is suggested that the dislocations carried by PSBs cannot transfer through the GB continuously even though the bicrystal has a common primary slip plane and its surface slip bands are continuous across the GB.     

Structural characteristics of a high-quality Al?Ni?Ru decagonal quasicrystal with 1.6nm periodicity,studied by atomic-scale electron microscopy observations

We report the structural characteristics of a high-quality stable decagonal quasicrystal (D phase) with 1.6nm periodicity, formed in Al₇₅Ni₁₅Ru₁₀ alloy annealed at 890°C for 24h. The tiling structure and the arrangement of transition-metal atoms (Ru and Ni) in this Al-Ni-Ru D phase have been clearly revealed by high-resolution electron microscopy (HREM) and by highangle annular detector dark-field (HAADF) scanning transmission electron microscopy (STM) respectively. On the basis of the HREM and HAADF STM observations, the relationship between the arrangement of local structural units and the formation of the long-range quasiperiodic tiling structure is discussed.     

Determination of the Burgers vector of dislocations in icosahedral quasicrystals by a high-resolution lattice-fringe technique

A technique for the determination of the full six-dimensional Burgers vector characterizing a dislocation in an icosahedral quasicrystal is presented. It is based on the lattice-fringe analysis of two high-resolution transmission electron microscopy images taken at two different sample orientations. As an example we present the analysis of a dislocation in a bent icosahedral Al-Pd-Mn quasicrystal. We obtained a Burgers vector B = A₀[-2,0,3,-2,3,0] where A₀ = 0.645nm is the six-dimensional hyperlattice constant. This result is consistent with previous results obtained by diffraction contrast analysis and convergent-beam electron diffraction techniques.     

Real-space first-principles electronic structure of edge dislocations: NiAl

The electronic structure of the 100 {010} edge dislocation in NiAl has been calculated using the real-space tight-binding linear muffin-tin orbital recursion method with a self-consistent treatment of electron density redistribution effects in the dislocation core. We demonstrate that quasilocalized states may exist in this metallic system as a result of specific lattice distortions in the dislocation core with 'broken bonds'.     

Evidence for structural disorder in the icosahedral phase

Abstract

We report transmission electron microscopy results on quasicrystalline samples of Al–25 wt% Mn and Al-38 wt% Mn-5 wt% Si melt-spun flakes. The selected-area diffraction patterns (SADPs) from ‘off-axis’ orientations show not only sharp diffraction maxima but also a diffuse ring, invariant of sample thickness, indicating that it does not arise from amorphous surface oxide. Over-exposed SADPs taken from axial orientations also showed the weak diffuse ring. We suggest that our results indicate the presence of disordered material within the icosahedral phase. Models for the icosahedral phase which involve an assembly of icosahedral clusters inherently contain interstitial voids, which may contain disordered material. Thus our results support these models rather than those derived from space-filling filings, or multiple twinning.