High Tc superconducting oxides discovered so far are classified into several systems, that is, 40 K class (La1-xMx) 2CuO4-δ (M : Sr, Ba, Ca), 90K class Ba2LnCu3O7-δ (Ln : rare earth elements excepting Sc, Ce, Pr and Tb), 110K class Bi-Sr-Ca-Cu-O, and 130K class Tl-Ba-Ca-Cu-O systems. These oxides have the perovskite related crystal structures with copper ions on B sites. Mutual relations among these structures are briefly reviewed. The 3d electron holes responsible for the electrical conduction are considered to be Cu3+ from the chemical picture. Therefore the oxygen nonstoichiometry and average copper valence were determined. The relation between critical temperature and average copper valence are shown. The superconducting oxides exhibit normal conductivities of 102 to 103 Ω-1 cm-1 and the temperature coefficienty of nearly zero. This fact would provide a useful guiding principle for finding new superconductors.
A study has been made to utilize the oxygen vacancies of perovskite-like Y-Ba-Cu-O compounds as the catalytically active sites. Y-Ba-Cu-O compounds were found to have catalytic activity for NO decomposition, but they were deactivated at 800°C by sintering. Subsequently, MgO supported Y-Ba-Cu-O catalysts were prepared in order to increase the catalytic activity and to suppress sintering. The Y-Ba-Cu-O supported catalysts have a higher activity for NO decomposition than a commercial Pt supported catalyst at 800°C. XPS analysis suggests that the decomposition of NO is facilitated by the redox cycle of Cu (Cu+ Cu2+). It is also reported that a large quantity (2.53 in molar ratio) of NO was taken up at 573K and desorbed at 773K by YBa2Cu3O7-y. The uptake ratio of NO increased with increasing number of oxygen vacancies which were introduced by annealing in N2. The characteristic features of Y-Ba-Cu-O compounds shown above are ascribed to two-dimensional Cu-O plane with oxygen deficiency. The oxygen deficient Cu-O plane is stabilized by the lattice framework containing Ba2+ and Y3+.
Unexpected low Jc of bulk-YBa2Cu3O6.8 measured by the direct current method, suggests that the grain boundaries might possibly act as an unfavorable region for super conducting. Grain boundaries calcined and sintered YBa2Cu3O6.8 were therefore examined by transmission electron microscopy. Three types of boundaries : (i) clean boundaries, (ii) boundaries with amorophous layer and (iii) boundaries with precipitates were observed with an increase in aging time and temperature, ratio of the above third type boundary increased. Most of the precipitation bands were composed of complex Ba-Cu oxides. They were polycrystalline and composed of grains which were smaller than 100nm in diameter and often included minor phases rich in Y. These oxide bands grew by partial melting at grain boundaries at 930°C. Some stoichiometric oxides such as BaCuO2, CuO and BaO were also observed along grain boundaries. Results of preliminary observation on the structure of BiSrCaCu2Ox are also reported.
The microstructures of high-Tc superconducting oxides such as Ba2LnCu3Oy (Ln : Y and lanthanoides) and Bi2 (Sr, Ca) n+1CunOy (n=2 or 3) are examined by a high resolution transmission electron microscope (HRTEM). Arrangements of metal atoms in these compounds are directly observed in HRTEM images. Existences of oxygen vacancies, planar defects, twinnings, dislocations, 90-degree domains etc. are found in Ba2LnCu3Oy. On the other hand, intergrowth structures, twinnings and incommensurate modulated structures are found in Bi2 (Sr, Ca) n+1CunOy.
A review is given on applications of scanning tunneling microscopy to the study of various superconductor surfaces. Measurements on surfaces of Nb3Sn films, NbN films, and oxide superconductors are described following a brief introduction to the characteristics of scanning tunneling microscopy and spectroscopy.
Recent studies on high-Tc oxide superconductors by using various electron spectroscopies have been reviewed. Main novel results obtained for the past half a year can be summarized as follows; (1) Unoccupied O 2p holes above the Fermi level have been observed with YBa2Cu3O7-x. (2) A photon-energy-dependent emission from the states near the Fermi level was observed with a single crystal of YBa2Cu3O7-x (001). The emission was apparently of angular dispersion and of photon-energy dependence. (3) Combined XPS and TD studies suggest that the appearance of the superconductivity strongly related to the Cu-O bonds along a one-dimensional chain.
Paramagnetic species in the normal conducting state of various high Tc superconducting oxides such as the La-Ba (or Sr) -Cu-O, RE-Ba-Cu-O (RE=Y, Gd, Dy, Ho, Er and Tm) and Bi-Ca-Ba-Cu-O systems were characterized by ESR spectroscopy. Low-field nonresonant microwave absorption spectra were observed for the superconducting state of the above high Tc superconductors. The nonresonant absorption is caused to the reduction of the shielding effect by the supercurrent and intergrain Josephson current on the surface of above samples. Magnetic hysteresis and transient response observed in the nonresonant absorption arise from frozen-in magnetic fluxes, which are in a metastable state. The critical field obtained from the flux trapping allows one to estimate the average size of the superconducting clusters with a uniform phase in the spin-glass-like high Tc superconductors. It is suggested that the low-field nonresonant microwave absorption method is applicable to the characterization of thin-layer high Tc superconductors as well as novel composite types of superconductors.
A technique for preparing high Tc superconducting oxide films from liquid organic precursors, and the influences of the preparation conditions on the superconducting properties of the derived films are reviewed. The preparation process basically consists of three steps; elimination of solvents, pylorysis and formation of high Tc crystal phases. Superconducting transition temperature significantly depends on the preparation conditions. A large amount of oxygen is necessary for the pylorysis of organics, and the selection of substrates is likewise one of the quite important factors for high Tc superconduction. In particular, mismatch in thermal expansion or interaction at the interface cause cracks or impurity phases in the films, resulting in low transition temperature. From our experiments, yttria stabilized zirconia can be considered as a suitable material for the substrate, and zirconia is even quite useful material as a buffer layer on Si wafer to deposits high Tc films, especially for Ba-Y-Cu-O system.
Y-Ba-Cu-O films were deposited on polycrystalline YSZ substrates by rf sputtering. Two types of targets were employed for the deposition. The as-sputtered films were insulators, but they became superconductive after being annealed in oxygen at 900°C for 3h. The plasma oxidation was performed in a mixture gas of Ar-50% O2 at 15mTorr by using the anti-sputtering in the same vessel as used for deposition; the rf power was 25W. The oxidation raises Tc and lowers the resistance ratio R (100K) /R (300K) of the films. The best films obtained so far exhibited a superconducting onset at 94K and zero resistance at 82K. Aging effect of the films at room temperature was also investigated. The higher Tc and the lower ratio R (100K) /R (300K) were maintained over a period of more than 30 days in dry air. This indicates that the impovement by the oxidation was not temporary.
We have established a low-temperature process for the preparation of high Tc superconducting films by rf-magnetron sputtering. The films were deposited under sufficiently crystallizing and oxidizing conditions at (650°C), that is lower than the tetragonal-orthorhombic transition point. The as-grown Er-Ba-Cu-O film on a MgO substrate exhibited a sharp superconductive transition with zero resistance at 86K. The zero resistance temperature of the as-grown film on a SrTiO3 substrate was 80K. This process prevented the diffusion at the interface between films and the substrates, and thus reduced the porous structure in the films. Thin films of Bi-Sr-Ca-Cu-O and Tl-Ba-Ca-Cu-O systems have been prepared by rf-magnetron sputtering and subsequent annealing. These thin films showed a highly oriented structure with c axis perpendicular to the substrates. The Bi-Sr-Ca-Cu-O thin films on MgO exhibited a superconducting transition with a zero resistance temperature (Tc=104K) and the Tl-Ba-Ca-Cu-O thin films on MgO also exhibited the transition at Tc=102K.
This article introduces a couple of actual superconductor applications such as the magnet systems for magnetically levitated trains. The method for the persistent current switching, which is pertinent in the practice of the persistent current mode operation of superconducting magnets, is explained. Finally the requirements for oxide superconductors from the users are described.