摘要:

Microcrystals of YBa2, Cu3, O7-δ, high-Tc, superconductors have been observed by high-resolution transmission electron microscopy (HREM) at 400 kV with a point-to-point resolution of 0·17 nm. Computer simulations of the HREM images from very thin crystals demonstrate that the oxygen columns of the structure can be imaged as black dots clearly visible between the darker metal dots at the Scherzer defocus condition. Under the same conditions the oxygen vacancy columns are imaged as bright spots: this allows a distinction to be made between the [100] and the [010] projections of the structure. Experimental images were obtained, which are consistent with those simulated images, along theaaxis and thus reveal the oxygen sublattice directly.

ISSN:0950-0839    

DOI:10.1080/09500839008215150

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The shapes of edge dislocation loops in β-NiAl have been studied. These loops have {001} habit planes, ⟨001⟩ Burgers vectors and exhibit two distinct forms. One of these is roughly square with segments nearly parallel to ⟨010⟩; this corresponds to the equilibrium shape calculated on the basis of elastic anisotropy. The other form consists of segments aligned accurately parallel to ⟨110⟩, which should be elastically unstable. It is proposed that this second shape is stabilized by dissociation of the perfect dislocation into three partial dislocations on the intersecting {110} planes for exampleThe consequences of such a dissociation for the plastic deformation of β-NiAl are discussed.

ISSN:0950-0839    

DOI:10.1080/09500839008215151

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The deformation recovery of Eu2O3crystal under electron irradiation has been investigated by using the profile-imaging method. It was found that the deformation on the {111} surfaces was by means of surface stacking fault or surface partial dislocations. The deformation recovery of the bulk was mainly through the moving of dislocations off the surfaces, which often left steps with an atomic layer spacing on the surfaces.

ISSN:0950-0839    

DOI:10.1080/09500839008215152

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

An atomic-resolution image of a symmetrical Σ99 {557} ⟨110⟩ tilt boundary in aluminium is compared with images simulated from models based on atomistic calculations using pair potentials and the embedded-atom method. The two methods for atomistic modelling result in very similar structures, and image simulations based on these structures closely match the experimental results. This study shows that high-resolution electron microscopy can now be used to assess quantitatively the degree of coincidence between experimental and theoretical atomic structures of high-Σ grain boundaries.

ISSN:0950-0839    

DOI:10.1080/09500839008215153

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

The quasicrystal-crystal transition in dodecagonal structures is studied in the case of a two-dimensional quasiperiodic tiling obtained by projecting a six-dimensional cubic lattice. By following the already proposed technique which consists of tilting the projection plane, we propose a decoration of the quasilattice which satisfies the hard-core condition and explains qualitatively some features of the electron diffraction patterns of the dodecagonal phase recently reported in a Bi-Mn alloy. It suggests that the quasicrystal-crystal transition in the dodecagonal phase occurs in a continuous fashion.

ISSN:0950-0839    

DOI:10.1080/09500839008215154

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

In this Letter we present a new geometric transformation to connect in a continuous way the octagonal phase with the related crystalline ones. This transformation avoids the tiles vanishing and hence it allows us to define a simple atomic decoration which preserves the ‘hard core’ condition. In the model presented here, the quasicrystal-related crystal density ratio is so close to 1 that it implies very short atomic displacementa and allows phase transitions to be physically possible in large enough clusters. A Shannon entropy is defined to estimate physically this evolutionary model. The strategy is extended to study the connection of the dodecagonal phase with two hexagonal phases rotated 30° from one to another.

ISSN:0950-0839    

DOI:10.1080/09500839008215155

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Millimetre-sized grains of F-type icosahedral phase are formed in ingots of Al—Cu—Fe alloy, for which the nominal composition is Al—20.41 at.% Cu—13.27 at.% Fe. These grains are located in the upper half part of the ingots. The lower half does not include the icosahedral phase but ordinary crystalline phases. In order to form these grains, the following heat treatment was applied The specimens were kept at 1143 K for 12 h and slowly cooled to 1095 K with the cooling rate 2 or 4 K h−1. They were successively annealed at this temperature for several tens of hours and then quenched in water and subsequently in liquid nitrogen. This quenching is essential in order to prevent structural transformation from the icosahedral phase to non-icosahedral approximate structure. Electron diffraction experiments as well as X-ray hue diffraction confirmed that the millimetre-size grains correspond to ‘single quasicrystals’, and that deviations from the perfect icosahedral symmetry are small.

ISSN:0950-0839    

DOI:10.1080/09500839008215156

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

X-ray microanalysis using an analytical electron microscope is applied to diffusivity measurements in an alloy system where the X-ray absorption correction is imperative. It is shown that the differential X-ray absorption (DXA) method is useful for the absorption correction in the determination of a concentration gradient across a diffusion couple interface.

ISSN:0950-0839    

DOI:10.1080/09500839008215157

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

In disordered systems, in which transport occurs by hopping between localized states, both Pollak and Efros and Shklovskii have pointed out that conduction could be greatly affected by the electron-electron repulsion, opening the so-called Coulomb gap in the one-electron density of states,N(E). The aim of this paper is to represent the effect of the Coulomb gap on the free energy,F. Fis expressed in terms of an unknown correlation length, r0. Minimization ofFwith respect to r0yieldsr0∝T−1, divergent at zero temperature,T. Owing to this singularity in the correlation length, models for transport must be based on finite temperature structure, as finite temperature conditions are not representable as small departures from those at zero temperature. The density of states at the Fermi level obtained by this treatment isin agreement with Efros and with recently published results of Mogilyanski.

ISSN:0950-0839    

DOI:10.1080/09500839008215158

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Combined techniques based on first-cycle xerographic residual potential, cycled-up saturated xerographic residual potential, and interrupted field time-offlight (IFTOF) measurements are used to determine the hole-drift-mobility-lifetime product μτ deep trapping time,τ, integrated deep-trap concentration,Nptrap capture coefficient and trap capture radius in a range of a-Se films. Very good agreement is observed between the μτ products determined via xerographic and IFTOF methods. Ballistic and diffusional deep-trapping models applied to this prototype amorphous semiconductor indicate that the capture radius is 2-3 Ă, indicative of neutral trapping centres with an integrated density of about 1013cm−3

ISSN:0950-0839    

DOI:10.1080/09500839008215159

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor