Shozo Ikeda

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.

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.

Uniformly layered mixtures of the Bi2Sr2Can−1CunOn+4 phases formed in artificially layered films—structural analysis and superconducting properties

Uniformly layered mixtures of the succeeding members of Bi2Sr2Can−1 CunO2n+4 series were found in the films synthesized by alternating sputter deposition of BiO, SrCu0.5O1.5, and CaCuO2 layers. The uniformly layered intergrowth was caused by the deviation of the CaCuO2 supply amount in each sequence, which is constant throughout the synthesis, from the stoichiometric amount. It is presented that the x‐ray diffraction peaks from the films with uniform intergrowth can be indexed by treating them as the satellites of superlattices. In this treatment, the number of CuO2 layers, which varies randomly but uniformly along the c axis, is expressed on average by noninteger n. The superconducting transition temperature (Tc) and concentration of holes in the CuO2 layers in those films decreased with increasing n (2.88≤n≤6) monotonically. From the continuity of the decrease in Tc with n, we conclude that the hole concentration is averaged between the two phases in the intergrowth by the diffusion of the holes.

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.

Enhancement of flux pinning in Bi2Sr2CaCu2O8 by 180 MeV Cu11+ irradiation

180 MeV Cu11+ irradiation was done on the c‐axis grain oriented Bi2Sr2CaCu2O8 tapes. Irradiation enhanced hysteresis in dc magnetization curves, especially in high magnetic fields. The irreversibility line can be moved to higher field by irradiation. Transport Jc is also enhanced by the irradiation of the small fluences. The irradiation produced large (∼100 nm) and small (∼5 nm) defects. They are effective for flux pinning in Bi2Sr2CaCu2O8.

Structure and Superconducting Properties of Y-Pd-B-C System.

A superconducting phase with transition temperature of 23 K found in the Y?Pd?B?C system was identified to have a body-centered tetragonal structure with the lattice parameters of a=0.38 nm and c=1.08 nm. The phase is of a platelike shape embedded in a matrix of other phases. The composition ratio of the superconducting phase determined using an electron probe X-ray microanalyzer is YPd2B2C1.5, although there may be uncertainty of the C concentration. The observation by high-resolution transmission electron microscopy indicates that the crystal structure is very similar to that reported recently for the superconducting LuNi2B2C intermetallic compound.

Continuous Observation of Cell-Formation in Iron Foils Extended in an Electron Microscope

Thin foils of iron single crystals which had the (001) or (10) foil surface were extended in the [110] axis on an extension device in a 500 kV electron microscope. The movements of individual dislocation and the process of the formation of elongated cell walls were continuously observed. The cell structure similar to that of bulk crystals was formed in the (10) foils thicker than 1.2 µm, but not in the (001) foils. The cell formation proceeded in the following way: (l) small tangles form randomly, (2) nearest tangles are interconnected with dislocations, (3) dislocation density between the tangles increases, (4) elongated zigzag cell walls are formed, and finally (5) the zigzag cell walls grow into sharp, narrow and straight cell walls.

High-resolution transmission electron microscopy of commensurate modulation in Bi2Sr2CoOy

The modulated structure of Bi2Sr2CoOy was directly observed by high-resolution electron microscopy. The modulation periodicity is four times commensurate with the tetragonal subcell, as was pointed out by Tarascon et al. (to be published in Phys. Rev. B), while the average structure is identical to that of compositionally analogous superconductor Bi2Sr2CuOy which has an incommensurate modulation. Some of the weak satellite reflections appearing in the a*-b* diffraction pattern are, however, incompatible with the body-centered orthorhombic symmetry proposed by Tarascon et al., suggesting that weak additional modifications are induced in the structure.

High Resolution Transmission Electron Microscopic Studies on a Superconductor YPd2B2Cx Compound

The structure of superconducting boride-carbide YPd2B2Cx has been investigated by electron microscopy. The 3D structure of the reciprocal lattice has been constructed from a series of electron diffraction patterns obtained by tilting. The symmetry of intensity distribution together with the reflection condition of h+k+l=2n shows that the crystal has body-centered tetragonal symmetry, I4/mmm. The lattice constants are a=0.38 and c=1.08 nm. The structural images of [100], [110] and [001] zones are observed. Computed images and diffraction patterns based on the same structure as LuNi2B2C can be fit with the observed HRTEM images and with intensity distribution of electron diffraction patterns, respectively. It is concluded that the structure of YPd2B2Cx is basically the same as that of LuNi2B2C.

Effects of Film Thickness and Annealing Conditions on 2223 Phase Growth in BSCCO Films Produced by Pb Vapor Doping

The conditions for increasing the fraction of the 2223 phase in BiSrCaCuO films in the Pb vapor doping method were investigated. The optimum range for the growth of the 2223 phase was determined as a function of annealing condition and film thickness. It is shown that the annealing at 835°C for 5 to 12 h after 850°C×3 h annealing in Pb atmosphere is most effective in obtaining the impurity-free 2223 phase. The fraction of the 2223 phase thus obtained exceeded 99%.