摘要:

AbstractAfter reviewing microstructural studies on superconducting materials showingT,T′, andT* structural types, results are presented on the microstructure of some n‐type superconductors and related materials prepared with accurate control of the oxygen stoichiometry. Electron microscopy is used to describe the ordering of interstitial oxygen defects inT‐type La2NiO4+δleading to the formation of the n = 2 term of a homologous series with the general formula La8nNi4nO16n+1. Structural transitions and superstructure formation in the Pr2−x−yCexSryCuO4‐δsystem are reported, where T, T′;, and T* phases are isolated as a function of both Ce and Sr content. © 1995

ISSN:1059-910X    

DOI:10.1002/jemt.1070300302

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

年代:1995

数据来源: WILEY

摘要:

AbstractElectron energy loss spectrometry (EELS) with a cold field emission gun (cFEG) transmission electron microscope (TEM) is implemented to analyze the evolution of the electronic structure and dielectric function of oxide superconductors. The O‐K core loss spectra ofp‐type doped oxide superconductors are analyzed in terms of holes formation on oxygen sites, while low loss spectra are analyzed for free carrier plasmas, other spectral excitations, and their crystallographic confinement.It is illustrated that the transmission EELS with a cFEG TEM very much complement soft X‐ray absorption spectroscopy and optical spectroscopy, with the added advantages of high spatial resolution (∼1–100 nm), and is compatible with other analytical, diffraction, and imaging techniques, which are readily available in a cFEG TEM. © 1995 Wiley

ISSN:1059-910X    

DOI:10.1002/jemt.1070300303

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

年代:1995

数据来源: WILEY

摘要:

AbstractThis paper summarizes results obtained by high‐resolution transmission electron microscopy on the crystal structure and microstructure of the (Bi,Pb)2Sr2Can−1CunO4+2n+δhigh‐Tc superconducting oxides. The experimental basis for the work presented here are high‐resolution structure images obtained at ultra‐thin (3 nm) areas of carefully prepared transmission electron microscope (TEM) samples. The analysis was carried out on a 400 kV TEM equipped with a pole piece yielding 0.17 nm point‐to‐point resolution. From the images obtained the projected crystal potential of the cations can be extracted directly, as confirmed by detailed image simulation. Structural analysis of the oxygen sublattice remains an unsolved problem by high‐resolution TEM (HRTEM), mainly because of the small scattering factors, and thus the contribution of the oxygen sublattice to the image contrast is small. The (Bi,Pb)2Sr2Can−1CunO4+2n+δphases are modulated structures that can be understood as an average structure plus a superimposed displacement field. The crystal structure consists of BiO double layers and perovskite‐type cuboids (containing Sr, Ca, Cu, and O), which are sandwiched between the BiO double layers. The displacement field can be directly analyzed by HRTEM, and the largest displacement amplitudes of 70 pm were determined for the Bi atoms in the n = 1 compound. The chemical composition of the n = 2 and n = 3 compounds was determined by EDX in the TEM for the cation sublattice. A significant (Ca + Sr) deficiency (approximately 10%) with respect to Cu was found. The (Sr + Ca)/Cu mole fraction ratio was 1.31 for the Bi‐2212 phase and 1.14 for the Bi(Pb)‐2223 phase. The oxygen content cannot be determined by EDX in the TEM with the accuracy necessary for a correlation with electrical and superconducting properties. The defect structure present in these materials, that is, intergrown lamellae with different crystal structures and equal or different chemical compositions, stacking faults, and grain boundaries, is summarized. The importance of grain boundaries for understanding and improving superconducting properties is emphasize

ISSN:1059-910X    

DOI:10.1002/jemt.1070300304

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

年代:1995

数据来源: WILEY

摘要:

AbstractFreeze‐fracture technique was used to analyse the structure of conventionally fixed and quickly frozenBodo sp., a free‐living kinetoplastid. In the former method, chemically fixed and cryopreserved cells presented a corrugated membrane pattern in the flagella and cell body surfaces. In the latter, however, replicas from quickly frozen unfixed flagellates showed membranes with a smoother aspect, allowing the observation of intramembranous particles (IMPs) on the fracture faces, hardly detectable in previously fixed samples. The IMPs were randomly distributed throughout the cell surface, except in the sparsely seen short IMP rows. © 1995 Wiley‐Lis

ISSN:1059-910X    

DOI:10.1002/jemt.1070300305

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

年代:1995

数据来源: WILEY

摘要:

AbstractBovine aortic endothelial cells (BAEC) were exposed to glucosylated albumin‐gold complexes (GgA), and the distribution of the tracers was compared after cryofixation and after glutaraldehyde fixation. Morphometric analysis revealed differences in the GgA distribution depending upon the method of fixation used. In BAEC monolayers cryofixed after 3 min of incubation with GgA, tracer was observed in predominately apically located vesicular elements. After 16 min of incubation, all vesicular elements were labelled, and multivesicular bodies were the prominent labelled structure. In contrast, chemically fixed monolayers exhibited a heterogeneous distribution of GgA within vesicular profiles after 3 min and 16 min of GgA incubation. The differences in tracer distribution depending upon the fixation method must be resolved before the mechanism of vesiclemediated endothelial cell transport function is defined and universally accepted. © 1995 Wiley‐Liss,

ISSN:1059-910X    

DOI:10.1002/jemt.1070300306

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

年代:1995

数据来源: WILEY

摘要:

AbstractMicrostructures of Bi(Pb)‐family 2212 superconductors, which were prepared by a sol‐gel method with three different compositions, were examined mainly by scanning and transmission electron microscopy. The magnetization of the specimens strongly depends on the ratio between Bi and Pb content, while Tc is almost constant. In specimen 1, prepared with the nominal composition of Bi/Pb = 9/1, small grains of 2212 phase are formed with a minor fraction of some impurity phases. In specimen 2, with Bi/Pb = 17/3, which is optimum from the viewpoint of magnetization, large grains of the 2212 phase are formed during heating at 800°C, also with the impurity phases. In specimen 3, with Bi/Pb = 8/2, the 2212 grains are divided by layers of (Bi0.86, Pb0.14) (Ca0.7, Sr0.3)Ox. Moreover, plate‐like 2212 crystals are severely bent so that small cracks appear often with an inclusion of amorphous substance being rich in Ca and Pb. These layers and cracks must degrade the magnetization. A modulated structure of Bi‐type is formed in the 2212 grains of specimens 1 and 2, while not only Bi‐type but also Pb‐type are formed in specimen 3. The wavelength of Bi‐type is different for each specimen. © 1995

ISSN:1059-910X    

DOI:10.1002/jemt.1070300307

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

年代:1995

数据来源: WILEY

ISSN:1059-910X    

DOI:10.1002/jemt.1070300308

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

年代:1995

数据来源: WILEY

ISSN:1059-910X    

DOI:10.1002/jemt.1070300301

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

年代:1995

数据来源: WILEY