ISSN:0741-0581    

DOI:10.1002/jemt.1060080302

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1988

数据来源: WILEY

摘要:

AbstractThis introductory article reviews the recent developments in high Tcsuperconducting ceramics. Particular emphasis is given to the structural and chemical aspects of these compounds that may be of interest to the electron microscopist. Included are some of the contributions made by electron microscopy techniques over the last 10 months. The applications of electron microscopy to solving practical problems associated with these exciting new materials are also discussed.

ISSN:0741-0581    

DOI:10.1002/jemt.1060080303

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1988

数据来源: WILEY

摘要:

AbstractWe show that high‐resolution transmission electron microscopy can be used directly to reveal the structure of the metal framework and its defects in Ba2YCu3O7. The use of dynamical effects under carefully selected and controlled conditions allows the detection of long‐range order in the disposition of the oxygen vacancies, but inevitable misalignments make the quantification of the order parameter difficult. Specimen preparation methods are compared, and a wide‐angle convergent‐beam pattern is demon

ISSN:0741-0581    

DOI:10.1002/jemt.1060080304

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1988

数据来源: WILEY

摘要:

AbstractThin films of the superconductive oxide YBa2Cu3O7−xhave been made by electron‐beam coevaporation of the metals in an oxygen atmosphere onto single‐crystal {001}‐oriented SrTiO3and yttria‐stabilized zirconia (YSZ) substrates. The oxide films were superconducting in the as‐deposited state (Tc= 81–83K, Jc= 106A/cm2at 4.2K). Bright‐field imaging, selectedarea diffraction (SAD), and high‐resolution imaging in the transmission electron microscope were used to characterize the microstructure of these films. All of the films were polycrystalline. On SrTiO3the films were oriented, for the most part, with {110} parallel to the substrate surface. On YSZ, two microstructures were observed: one with smaller rectangular grains oriented with (100) or (010) parallel to the substrate surface and the other with (001) parallel to the surface

ISSN:0741-0581    

DOI:10.1002/jemt.1060080305

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1988

数据来源: WILEY

摘要:

AbstractHigh‐resolution transmission electron microscopes operating at 300 and 400 kV were used to investigate the crystallography and microstructure of the perovskitelike YBa2Cu3O7−x. In this paper, we evaluate the performance attainable with these microscopes both empirically and by computer modelling. Based upon the assumption that oxygen may be a key to superconductivity properties, we have also investigated the visibility of the oxygen sites as well as the heavier yttrium and barium ion positions and the lighter Cu atom positions. We propose a scheme for observing different twin orientations in these structures and hence the oxygen atom positions seen in projection for the [100] and [010].Our observations of both thick and thin regions of Y‐Ba‐Cu‐O materials are reported as well as the problems of adjusting microscope parameters and specimen alignment to obtain interpretable images. We also give a preliminary report on the effects of heat treatment as seen in high‐resolution micrographs to assess disorder of the heavy atoms and oxyge

ISSN:0741-0581    

DOI:10.1002/jemt.1060080306

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1988

数据来源: WILEY

摘要:

AbstractThe superconducting material Y1Ba2Cu3O7−δhas been investigated by electron microscopy. Special attention has been paid to the defects occurring in the material. Twinning on (110) or (110) planes is intrinsically related to the orthorhombicity, and when cooled slowly the twin bands are pseudoperiodic with an average width of ˜ 50 nm. The orthorhombic‐tetragonal transition is reversible and diffusion controlled. Under particular conditions and in slightly reduced material a 2a0X b0superstructure resulting from vacancy ordering is formed. When kept in air under powder form the material seems to be unstable. Planar defects along (001) accompany the degeneration of the mat

ISSN:0741-0581    

DOI:10.1002/jemt.1060080307

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1988

数据来源: WILEY

摘要:

AbstractWe review recent results obtained at Northwestern using high‐resolution electron microscopy to study high‐temperature superconductors. While in general these materials form large, very perfect single crystal grains which display very few imperfections, there is also evidence of slip defects, amorphous regions, and order‐disorder transformations. We also report that the gadolinium‐based superconductors and in one case yttrium‐based superconductors show evidence for some copper solid solubility in the form of copperrich planar defects. The structure of a metastable trigonal polytype is also reported, as are the effects of electron beam and water vapor damage to the

ISSN:0741-0581    

DOI:10.1002/jemt.1060080308

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1988

数据来源: WILEY

摘要:

AbstractThe high Tcsuperconducting material YBa2Cu3O7shows a complex relationship between microstructure and oxygen content, which are controlled by length of heat treatment, atmosphere, and quench rate. An AEM investigation studying changes in the oxygen near edge features was undertaken. Electron energy loss spectroscopy (EELS) measurements have the important advantage that they can be made on single crystal grains, allowing orentation‐dependent studies. Both ion‐milled and crushed samples with varying O2content were analyzed. The structure of YBaCu3O7was determined by neutron diffraction to be orthorhombic with distinct Cu‐O chains along the b‐axis as well as Cu‐O planes in the a—b plane. Therefore, by looking for a crystallographic dependence of the oxygen K‐edge one might be able to distinguish inequivalent oxygen atoms by their core level binding energy and correlate site occupancy with varying O2content. The EELS results on the oxygen K‐edge are strongly dependent on oxygen content, most noticeably when the c‐axis is parallel to

ISSN:0741-0581    

DOI:10.1002/jemt.1060080309

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1988

数据来源: WILEY

摘要:

AbstractWe have obtained the electron‐energy‐loss spectra below 1,000 eV for YBa2Cu3O7−δ. The spectra show contributions from all elemental constituents, although the Y‐signal is very weak owing to its small concentration. The low‐loss region and the Ba‐core losses are very similar to those obtained for BaO. The Cu absorption is similar to that seen in CuO with minor differences relating to Cu concentration and anisotropy. The oxygen 1s absorption has a shape that is quite different from that obtained in CuO and shows striking orientational anisotropy. Radiation damage in the microscope is a problem and leads to changes in the shape of the

ISSN:0741-0581    

DOI:10.1002/jemt.1060080310

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1988

数据来源: WILEY

摘要:

AbstractTwins are ubiquitous in the perovskite superconductors. They form during cooling from high temperatures through the tetragonal‐to‐orthorhombic phase transformation. Their behavior has been studied by transmission electron microscopy using a liquid‐nitrogen‐cooled specimen holder. It is observed that extra twins sometimes form in coarse‐twinned regions to give a “refined” twin structure; this is thought to be induced by thermal stresses from the electron beam. Prolonged electron irradiation (with little beam heating) results in transformation to tetragonal with a loss of the twin structure; this is probably due to disordering of the oxygen sublattice by knock‐on displacement. The same transformations can be induced by deliberate electron beam heating. In both cases the twinned orthorhombic structure can be restored by re‐ordering of the oxygen ions, in the first case by removing the beam and in the second case by allowing the

ISSN:0741-0581    

DOI:10.1002/jemt.1060080311

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1988

数据来源: WILEY