Keiichi Ogawa

Growth of the 2223 Phase in Leaded Bi–Sr–Ca–Cu–O System

Growth of the 2223 phase has been studied in the leaded Bi-Sr-Ca-Cu-O system. Differential thermal analysis showed that the enhanced formation of the 2223 phase occurs in the temperature range between 835°C and 869°C where partial melting occurs. The density of the pellet was found to increase by 30% at one hour and then to decrease by 40% from the highest density attained. The former is attributed to the growth of the 2223 phase with the aid of the partially melted liquid phase and the latter to the flaky shape of the resultant 2223 crystals. The effect of Pb addition for enhanced 2223 phase formation will be discussed.

Growth of the 2223 Phase in Leaded Bi-Sr-Ca-Cu Oxide under Reduced Oxygen Partial Pressure

The growth process of 2223 phase in a leaded Bi-Sr-Ca-Cu oxide system has been investigated under various oxygen partial pressures using differential thermal analysis, combined with XRD of the quenched specimen. It is shown that under a slow heating rate, the DTA curves exhibit two or three endothermic peaks between 820 to 900°C, and the peak positions are lowered with reduction in the O2 partial pressure. The 2223 phase is found to grow most efficiently in the partial melting state sandwiched between the two endothermic peaks, and the temperature range between the two peaks becomes maximum around 0.1-atm O2 partial pressure.

A new mode of modulation observed in the Bi-Pb-Sr-Ca-Cu-O system

With the aid of transmission electron microscopy, the Bi-Pb-Sr-Ca-Cu-O system has been found to have a new mode of modulation in addition to the conventional one reported earlier in the Pb-free Bi-Sr-Ca-Cu-O system. The new modulation is observed in the perovskite {110} plane, as is the conventional one. The wave vector of the new modulation, however, is parallel to and the observed wavelength is about 50 A. Periodicities of the new as well as conventional modes of modulation become less regular in the Pb-added system. The disturbed modulation is argued in light of the strain energy relaxation due to the Pb addition.

New high-Tc superconductors without rare earth element

Abstract We have discovered new high-Tc oxide superconductors in the Bi Sr Ca Cu O system with no rare earth elements. The high-Tc phase, with Tc of about 105K, greater than that of YBa 2 Cu 3 O 7 by more than 10K, appears to have the approximate composition of Bi 1 Sr 1 Ca 1 Cu 2 O x . As the composition deviates from this value, a low-Tc phase with Tc of 75–85K tends to appear. In this oxide system, the coexistence of Sr and Ca is essential to realize the high Tc. The chemical compositions of the high and low- Tc phases are estimated to be Bi 4 Sr 3 Ca 3 Cu 6 O x and Bi 4 Sr 3 Ca 2 Cu 4 O y , respectively. The crystal structure of both phases is body-centered orthorhombic with a=0.54,b=5a=2.7and c=3.08nm. This structure is layered structure with such that alternating Bi 2 O 2 and perovskite layers occur along the c-axis. A modulated structure with highly strained a-c lattice planes is observed by high resolution TEM when the electron beam is incident along the a-axis direction.

Superconducting and Transport Properties of B-Y-Cu-O Compounds –Orthorhombic and Tetragonal Phases

The superconducting properties and crystal structures of the Ba-Y-Cu-O compounds have been studied. Annealing and quenching experiments have been carried out to study the phase transformation between the high temperature tetragonal phase (oxygen deficient) and low temperature orthorhombic phase (oxygen rich). The former phase is semiconductor-like down to 4.2 K, whereas the latter is metallic and superconducting with Tc as high as 94 K. The conspicuous difference in the transport and superconducting properties between the two phases has been attributed to the loss or formation of oxygen and Cu atom chain in the crystal structure.

Synthesis of Artificially Layered Bi–Sr–Ca–Cu Oxide Films and Their Thermal Stability

Bi-O/Sr-Ca-Cu-O multilayered films have been synthesized using two target reactive magnetron sputtering. It is shown that the modulation wavelength \varLambda (c/2 for the crystals) can be simply controlled by varying the deposition time for a one layer unit of Sr-Ca-Cu-O while keeping that for Bi-O constant. The films thus obtained have smooth surfaces and have \varLambdas ranging from 12 to 26 A, i.e., almost equal to those of the crystalline or intergrowth phases o bserved in the Bi-Sr-Ca-Cu-O system. The thermal stability of these artificially layered films is also reported.

Oxygen Ordering in YBa2Cu3O6+x

The orthorhombic and tetragonal phase relation, i.e. the order-disorder phase transformation in YBa2Cu3O6+x, has been studied by employing statistical thermodynamics, and the equilibrium value of order parameter -1s1 has been determined as a function of the temperature and oxygen concentration. In the above-mentioned treatment, plus and minus signs of s in the ordered state (|s|≠0) are found to describe the degree of order in the twin related crystals. The calculated properties are compared with the experimental data, i.e. the temperature dependence of oxygen occupancy at a constant oxygen pressure and the concentration dependence of axial lengths, a and b, at room temperature. The agreement is excellent for both cases when the axial lengths at room temperature are measured for the samples with oxygen removed or doped at low temperatures. The statistical thermodynamical expression for further ordering has also been derived.

Low Temperature Specific Heat and Electrical Resistivity in Orthorhombic YBa2Cu3O6.8 and Tetragonal YBa2Cu3O6.0

The low temperature specific heat and electrical resistivity of YBa2Cu3O6.8 and YBa2Cu3O6.0 have been measured. The specific heat data were taken at a temperature range from 2 K to 40 K. Abnormal upturns at the lower temperature end and a large electron contribution were observed in the C/T vs T2 curve. The resistivity ρ was measured from 20 K to 270 K for YBa2Cu3O6.0. A strong current density dependence was observed. Log ρ measured at low current density was found to show T-1/3 dependence below 170 K.

Structure of High-Tc Superconductor YBa2Cu3O6.6 at Low Temperatures

A single-phase YBa2Cu3O6.6 shows a sharp superconductive transition at 94 K. The crystal structure of this compound has been determined by the Rietveld profile analysis of neutron powder diffraction data measured at 10 K and 120 K. The space group Pmmm is confirmed, and the positions and occupations of oxygen atoms are established. No phase transitions or other structural anomalies were observed in relation to the superconductive transition.

Transmission Electron Microscope Studies on High-Tc Superconductor Ba0.7Y0.3Cu1Ox

Suscessful electron microscope observation was made for samples prepared by ion milling. Energy dispersive X-ray analysis showed that the composition of dominant phase was 18% Y, 29% Ba and 53% Cu in metal atoms. Plate like twins were observed in this phase; the twinning planes are {110} and the shape of twins is lenticular. This observation suggests that twins were induced mechanically to save the elastic energy produced during the phase transformation. Dislocations were also frequently observed in the phase. The directions of Burgers vectors are most likely to be 100 and 010.