The measurement of complex susceptibilities Xn = Xn' - ixn" in a weak ac magnetic field is one of the useful probes for the study of the magnetic response of type-II superconductors. The fundamental susceptibility X1 provides two different physical natures of the material. Namely, the real component is caused by the magnetic shielding, indicating the phase transition between the superconducting and the normal states. Meanwhile, the imaginary component is proportional to the energy dissipation in the specimen. Higher-harmonic susceptibilities Xn (n ≥ 2) give information on the magnetization which is closely related to the shielding current induced in the sample. We review in this article Xn observed for ceramic high-Tc superconductors and the theoretical analysis of the results within the construct of the critical-state model.
Microstructures of the high-Tc superconducting oxide found in the infinite-layer system ACuO2 (with A alkaline-earth elements) have been investigated by high-resolution electron microscopy. The superconducting samples always involve defect-layers inserted almost randomly into the parent infinite-layer structure, which we suggest are where the A cation vacancies are concentrated. Structure image observations reveal that the oxygen deficiency also occurs within a CuO2 plane in the middle of a defect layer in order to compensate for the chage imbalance due to the A vacancies. P-type superconductivity with maximum Tc of 110K is realized when the oxygen deficiency is filled in by the application of high oxygen pressure.
The preparation, structure and electrical properties of Ba1-xSrxPbO3-z are reported. These phases all have orthorhombic perovskite structure whose space group, however, changes at around x=0.3-0.4 from Inma (BaPbO3 side) to Pnma (SrPbO3 side). The transport property also changes from metallic (Inma) to semiconductive (Pnma) at this point. Wide range of oxygen deficiency was realized in this system by changing the synthetic oxygen pressure. With the increase of oxygen deficiency the metallic conductivity region expanded to the Sr-rich side. The doping of La3+ in A site was also effective to change the system to metallic.
T'-type 214 compounds Ln1.85Ce0.15CuOy with Ln=Pr. Nd, Sm or Eu were prepared. Lattice constants decreased with the substitution in a series from Pr to Eu. The compounds showed superconductivity after reduction. Their lattice constants and oxygen contents did not change so much with the reduction. The reduced Nd compound had a maximum oxygen content of y=3.93 in spite of its highest Tc=24K among the T' compounds. The oxygen contents were 3.79 for Pr, 3.90 for Sm and 3.75 for Eu compounds. Both Cu 2p1/2 and 2p3/2 peaks in XPS were most effectively shifted to lower binding energy in the Nd compound by the reduction process. Their satellite peak intensities were also most strongly reduced in the compound. Cu K-edge XANES spectrum showed 1s-4p*(π) transition for the 3d10 final state most clearly in the reduced Nd compound. These spectrasuggest that electron doping to the conduction plane most effectively occurs in the Nd compound.
High-purity ceramic sample of the superconducting Y2Ba4Cu7aO14.82 (247) phase has been prepared at 870°C by the polymerized complex method using neither high oxygen pressure nor the additive catalysts. The method is based on the formation of a polymer complex which is obtained through polyesterification between metal citrate complexes and ethylene glycol. Apart from not requiring high oxygen pressure, the present preparation technique offers more convenient fabrication of highly pure 247 compound compared with other wet chemical routes since it eliminates many steps such as aging and pH control. X-ray diffraction and Raman scattering analyses have shown that the material is virtually of single-phase without any indication of secondary phases. Zero resistance has been achieved at 88.0K with a transition width narrower than 4 K. Complex magnetic susceptibility measurements have confirmed the presence of a single bulk superconducting 247 phase with Tc(onset)=93.0 K and ΔTc(10-90%)=4.0 K.
This paper describes the condition needed for the occurrence of the superconductivity in one-unit-cell thick (1-UCT) YBa2Cu3O7 (YBCO) film. The 1-UCT YBCO is grown on non-superconducting PrBa2Cu3O7 buffer layer. The cross-sectional transmission electron microscopy (TEM) observation indicates that the terminating layer of YBCO is CuO chain layer. The 1-UCT YBCO layer shows a superconducting transition when it is covered with BaO layer. This means that CuO chain layer does not act as a hole donor until the charge reservoir layer of BaO-CuO-BaO is completely formed.
Y-Ba-Cu-O films were prepared at 700°C on Si(100) substrates and on (Y2O3)/(Y2O3 stabilized ZrO2) buffer layers by chemical vapor deposition using p-diketone metal chelates as sources. The films were mainly composed of c-axis oriented YBa2Cu3Ox. Auger electron spectroscopy revealed interdiffusion of elements between the substrates and the films deposited directly on the Si substrates. The interdiffusion was prevented using the buffer layers. Epitaxial growth of YBa2Cu3Ox was confiremed by X-ray pole figure analysis for the films deposited on the Si substrates with the buffer layers. The films on the buffer layers showed zero resistivity at 87 K. Its critical current density was 103A/cm2 at 77 K and 0 T.
The orientation control of Bi2Sr2CuOx film grown under various sputtering conditions has been investigated from view points of controlling "Self-texture" and epitaxy. The films were deposited on the SrTiO3(110) substrate heated between 150 and 300°C, and annealed at 820°C to promote solid phase epitaxy. The (115) orientated film with good crystallinity was obtained by optimizing partial pressure of oxygen, and the substrate temperature on sputtering. The (115) epitaxial film had anisotropy in orientation distribution. This anisotoropy was caused by an anisotropy in lattice misfit between the (115)Bi2Sr2CuOx and (110)SrTiO3. Furthermore, an asymmetry in the rocking curve was observed along the direction of the larger misfit. The asymmetric peak suggests the relaxation mechanism of the stress caused by the largely anisotoropic misfit at the interface between the film and substrate.
As an attempt to produce a thick superconductive film with a large area, an experimental study was made on the preparation of the film through laser post-treatment of thermal-sprayed oxide ceramic film. The ceramic powder of Bi-(Pb)-Sr-Ca-Cu-O system (2223-phase) was used as the raw material. The powder was sprayed onto a stainless steel substrate using conventional plasma spraying system and newly developed lower temperature hypersonic spraying system, followed by CO2 laser beam irradiation with oxygen gas blow to modify the superconductive crystals of the film. The laser post-treatment can produce the superconductive crystals of 2212 and 2201 phases in the plasma sprayed film and 2223, 2212 and 2201 phases in the hypersonic sprayed film. The latter film has higher value in Tc(end) than the plasma sprayed film.
Solid-solution range of the 30Å-phase in the Bi-Sr-Ca-Cu-O system has been studied mainly by powder X-ray diffraction. We have found that the 30Å-phase was formed topotactically on the 24Å-phase through a partial melting at around 730°C. Effects of grinding on the 30Å-phase were examined by powder X-ray diffraction. It was found that the 30Å-phase changed into three types of fine powders by mechanical grinding; i.e., fine crystals of the original 30Å-phase, fine powder of a new phase (not identified) and an amorphous phase. It was found that these powder produced a new unidentified phase and then partially melted on heating.
For the study of magnetic properties of sintered high-Tc ceramic superconductors, we measured harmonic ac susceptibilities Xn = Xn' - ixn" of Bi1.6Pb0.4Sr2Ca2Cu3Oy immersed in an external field H(t) = H'dc + Haccos(ωt), where Hdc (≥ 0) is a dc bias field and Hac (> 0) is an ac field amplitude. Measurements of Xn were carried out for n = 1, 2, 3, 5, 7, 9 and μOHac = 0.01-0.1 mT. The fundamental frequency used was 132 Hz. The observed results were analyzed within the construct of the Kim-Anderson model for critical-current density. Similar to our previous work for an YBa2Cu3Oy sample with Fe impurities, generation of the higherharmonic susceptibilities seems to arise from the supercurrent according to the same model. Details of the measurement and the analysis are presented.
The effect of Li-addition to the low-Tc phase in Bi-Sr-Ca-Cu-O system have been studied for the samples with nominal compositions of Bi:Sr:Ca:Cu:Li=2:1.5:1.5:2-x:x heated at 720 to 845°C for 12 to 24h in air (x=0 to 0.8). The Li-addition promotes the formation of the low-Tc phase. Single phase of the low-Tc phase was obtained for the sample with x=0.2 to 0.4 more easily than the sample with x=0. The lattice constant c of the low-Tc phase increases as the Li content increases. As the heating period becomes longer, the amount of Li content in the low-Tc phase decreases.
Li addition was found to raise Tc up to 116K in the high-Tc phase in the Bi-Pb-Sr-Ca-Cu-O system. But the role and behavior of Li has not yet been clarified. This study has been done for samples with starting compositions of Bi0.9Pb0.2Sr1.0Ca1.0Cu1.6LixOy with x=0.03 to 0.10 and heated at 825 to 840°C for 1 to 48h. ICP analysis showed that Li content decreased with increasing sintering time. The lattice constant a decreased at first and then gradually increased. On the other hand, the lattice constant c passed through a maximum around 4h.
The effects of Ba addition and grinding time on Tc of the high-Tc phase in the Bi-Pb-Sr-Ca-Cu-O system have been studied. Samples with compositions of Bi:Pb:Sr:Ba:Ca:Cu=0.9:0.2:1.0:X:1.0:1.6(X=0-0.05) were prepared by grinding the high-Tc phase and BaCO3 powders for various periods (20 to 120min). The high-Tc phase degraded with grinding time. In samples of a composition of X=0.05 heated at 845≥C for 60h in air, a small amount of the low-Tc phase and Bi3(Sr, Ba)4Ca3Oz besides the high-Tc phase were formed. The heating led to the grain growth of the high- Tc phase with increasing grinding time. However, Tc(R=0) was decreased due to the formation of the low-Tc phase and Bi3(Sr, Ba)4Ca3Oz. In a sample with X=0.05 heated at 850°C for 60h in air, only a solid solution of the high-Tc phase was formed. Tc(R=O)=125K was attained in this sample. The Meissner diamagnetic response at 125K was confirmed by SQUID, but the volume fraction of the 125K phase was estimated below 1%.
Highly-oriented sintered pellets of the high-Tc phase in the Bi-Pb-Sr-Ca-Cu-O system with high density have been prepared by the liquid-phase sintering method. A liquid phase was made from the mixture of the Sr-rich component of the 2201 solid solution, Bi2+xSr2-xCu1+yOz (0.125<x≤0.5, O<y<x/2), and Ca2PbO4. Samples with various compositions, (1-x) 2223 phase +x liquid phase with 0<x ?? 0.20, were heated in the temperature range from 820°C to 830°C for 16h. The maximum critical current density (J) of 700 A/cm2 at zero field and 77K was obtained for the sample with x=0.15, heat-treat at 825°C. It was found from X-ray diffraction measurement that Ca2CuO3 always exists in the sample showing a low Jc. On the other hand, a large Jc was attained for the sample where particles were densely packed to be tightly connected to each other as a result of the liquid-phase sintering.
Chemical and mechanical stability of the high-Tc phase has been studied for the samples of Bi0.9Pb0.2Sr1.0Ca1.0Cu1.6Ox. Mechanical grinding converts the high-Tc phase into a nonsuperconducting amorphous phase. The ground high-Tc phase is not stabilized on annealing at lower temperatures below 820°C. Annealing at lower temperature leads to precipitation of some impurity phases and the low-Tc phase which depends on annealing temperature and time. The high-Tc phase is recovered by annealing at 850°C. The lattice parameters a and c of the high-Tc phase annealed at 850°C are longer than those below 820°C.
Effect of annealing atmosphere and temperature on the superconducting properties Tc have been discussed in the high-Tc phase in the Bi-Pb-Sr-Ca-Cu-O system. Special attention has been given to oxygen sorption/desorption and formation of impurity phases. The bulk samples without presence of amorphous phase formed due to mechanical grinding were used for compositions of Bi0.9Pb0.2Sr1.0BaxCa1.0Cu1.6Oz (x=0 and 0.05). Annealing in O2 and air above 700°C leads to marked weight increase and formation of (Bi, Pb)3Sr2Ca2CuOz. In contrast, almost monophasic samples of the high-Tc phase, absent from any impurity phases, were obtained on annealing in Ar. Furthermore, the weight of samples with x=0 and 0.05 decreses by Ar-annealing at temperatures of 350°C and 800-830°C. The weight decrease due to oxygen-desorption results in an elongation of the c-axis length of the high-Tc phase, accompanying an increase in Tc.
Chemical and mechanical stability of the high-Tc phase (2223) was examined by XRD, SEM and TEM. An interesting segregation reaction occurred when annealed at low temperature (500-840°C) in air. The phase precipitated on the mother crystals has been identified as Sr2.5Bi0.5Pb3Ca2CuOz which is isostructural with Sr3Pb2.03Cu0.75O8. On the other hand, high resolution TEM showed that the high Tc phase changed into three types of fine powders by mechanical grinding; i.e., fine crystals of the original 37 Å-phase, fine powder of a new phase (thickness is less than 37Å) and an amorphous powder. It was found that heating of these fine powder provides another formation process of the 2223 phase compared with the conventional one.
A remarkable Tc enhancement of the Bi-based 2212 phase (Bi1.8Pb0. 2Sr2Ca1Cu2Oy) was observed by liquid phase treatments under mild conditions. Soaking treatments in the solutions of polyhydric phenols such as hydroquinone were specially effective, and in those cases, the Tc values were increased by more than 10K by the treatments at room temperature. The solutions of metal iodides such as LiI were also effective to increase the Tc. In both cases, the increases of the Tc were presumed to be caused by the reductive actions of the treating solutions, which decreased and optimized the hole concentrations of the superconductors. In the case of hydroquinone treatment, increase in the c-axis length of the 2212 phase, which was larger than that in the case of inert gas treatment, was observed. Therefore, it was suggested that the organic compounds derived from hydroquinone were intercalated, partially, between some layers of the 2212 phase, but no other evidence was obtained. In the case of metal iodide treatment, strong adsorption of metal cations into the superconductor was observed, but details of the adsorption state and effects on the superconductivity were not clarified.
We reported on the effects of water or organic solvents on the superconductor in the Bi-Pb-Sr-Ca-Cu-O system in the previous reports. In this study, We are concerned with their effects on the preparation processes such as mixing and milling in the solid-phase reaction method. The organic solvents used in the experiment were ethylalcohol, methylalcohol, acetone and toluene. Wet-mixing and wet-pulverizing with water or some organic solvents influenced the superconductor to some extent. Typically, wet-mixing with toluene made the Tc(end) lower by 3 K than the mixing without toluene. Wet-pulverizing with water after semi-sintering resulted in higher Tc.
Melt-solidification technique brings a textured microstructure and an excellent critical current density (Jc) value on Bi2Sr2CaCu2Ox(Bi2212)/Ag composite tape. However, the optimum condition of the heat treatment is tight, and the highest Jc value can not always be obtained so far. In order to improve the reproducibility of sample with excellent Jc value, we applied the melt-solidification treatment combined with a bismuth vapor atmosphere to Bi2212/Ag tape. In this paper, dependence of Jc value on the maximum temperature in the heat treatment and microstructural study on the tape surface are reported. Reproducibility in Jc value is extraordinarily improved by the bismuth vapor treatment in contrast with the case of the conventional one, although the highest Jc values are still comparable for both cases. Microstructural observation reveals that, in the case of the bismuth treatment, the Bi-free phase particles are much smaller than those in the usual air treatment, and that the flatness of the tape surface is improved. The grain size and distribution of Bi-free phase particles plays an important role in the formation of Bi2212 textured structure and controls its Jc value.
A Bi(Pb)-Sr-Ca-Cu-O superconducting tube has been developed for a magnetic shielding. The tube, which was composed of superconducting grains oriented parallel to the bottom surface, was made by laminating the oriented green sheets prepared by a doctor blade method. As the magnetic field was applied parallel and vertical to the axis of the tube, the center in the tube was shielded from the magnetic flux up to 77Gs and 28Gs, respectively. Compared with the magnetic shielding in a non-oriented tube, no effect of grain orientation was found.