Bi-system Superconductors Research Articles

Ｘ帯におけるＢｉ系超電導体を用いた電磁波吸収体の反射損失特性

The fundamental properties of a single layer type microwave absorber using the Bi-system superconductors for the X band (8-12.5 GHz) have been examined. The samples were prepared by crushing the Bi(Pb)-2223 superconductors exhibiting Tc(end)=93.4 K into powders ( < 50 μm) and mixing with the epoxy resin at different ratios from 0 to 50 vol%. The complex reflective intensities of samples were measured by using a vector network analyzer in frequency range from 8.8 to 10.4 GHz at room temperature (300 K) and at liquid nitrogen temperature (77 K). The difference of the complex reflective intensity value between the sample and a metal plate was defined as our reflection loss (R.L.). The electromagnetic wave absorption properties were determined from the frequency dependence of the reflection losses. From the measured results, it was found that the electromagnetic wave absorption properties are influenced by the superconductor content of samples and the temperature. At a room temperature, the electromagnetic wave absorption increased with increasing the Bi (Pb)-2223 superconductor system content, which was due to the increase of the complex permittivity. At a liquid nitrogen temperature, the electromagnetic wave absorption decreased because the superconductor system showed superconductivity.

Study of the interaction between two structural blocks in high Tc superconductors by a block model

All the high T c superconductors are structurally composed of two different blocks, the perovskite, and the rock salt. The perovskite block is the active block where the Cu(2)–O planes are located and the carriers concentrate. The rock salt block corresponds to the charge-reservoir block, which supplies the carriers to the Cu(2)–O planes. Based on this fact, we developed a model to calculate the interaction between the two blocks (marked by combinative energy) and to study its relationship with the value of T c. Bi-system superconductors, Bi 2Sr 2CuO y , Bi 2Sr 2CaCu 2O y , and the Bi 2Sr 2Ca 2Cu 3O y , Hg-system superconductors, HgBa 2CuO y , HgBa 2CaCu 2O y , HgBa 2Ca 2Cu 3O y , HgBa 2Ca 3Cu 4O y , and HgBa 2Ca 4Cu 5O y , and Y-system superconductors, with different oxygen deficiencies, were studied with our model, respectively. A close relationship between the superconducting transition temperature and the combinative energy in these superconducting systems is established. The results show that the interaction between the two blocks is of great importance for high T c superconductivity.

Interaction of two different structural blocks in Bi-system superconductors

The influence of the interaction between two structural blocks on superconductivity was studied by calculating the cohesive energy in 3 Bi-system superconducting phases [Bi 2Sr 2CuO y (2201), Bi 2Sr 2CaCu 2O y (2212), and Bi 2Sr 2Ca 2Cu 3O y (2223)] and the 2223 phase with different oxygen deficiency. The result indicates that if we consider the cell as the two blocks combined together, a close relationship among the combinative energy between the two blocks(calculated from the cohesive energy of the unit cell), the value of T c, and the number of the CuO planes in the Bi-system superconductors is established. It suggests that the interaction between the two structural blocks, perovskite and rock salt, has a close relationship with the superconductivity.

Radiation effects on the Tc of bi-system superconductors by proton and gamma irradiation

Bi-system superconductors, prepared by the solid state reaction method and the nitrate method, were irradiated by 20 MeV protons and gamma-rays produced by nuclear reaction. The T c0 and gamma-ray intensity were measured with the time after irradiation. The T c0 increases on the gamma-ray irradiation, then the T c0 decreases gradually with the gamma decay. The results indicate that there are two competitive effects on the specimen's T c0 during gamma irradiation. One is the coupling of excited electrons and holes to the superconductive carriers (such as Cooper-pairs) in which the energy gap is higher than normal one and induces the T c0 to increase. The other is the defects produced by the irradiation, which induced the T c0 to decrease. The proton irradiation causes the T c0 to decrease monotonously.

Development of a large current capacity conductor using Bi-system superconductors

We developed a new strong Rutherford cable using Bi 2Sr 2CaCu 2O y (Bi-2212) high temperature superconducting (HTS) wire, which consisted of 427 superconducting filaments and AgMgSb alloy outer sheath. This wire exhibited excellent critical current ( I c) and critical current density ( J c) values at 4.2 K in self-field (285 A and 340 000 A/cm 2, respectively) and a room temperature tensile strength of 180 MPa. The Rutherford cable was prepared with a NiCr alloy strip as a cable core. The degradation of full cable I c induced by Ni diffusion was reduced by introducing a ceramic barrier layer between the Bi-2212 superconducting wires and the NiCr cable core. A full cable I c value of 3500 A at 4.2 K in self-field was obtained in short sample fabricated by using a cable core with a ZrO 2 film barrier layer. Finally, we fabricated an 80-m-long Rutherford cable by using a cable core with a MgO tape as a barrier layer and evaluated its full cable I c. I c values of 17-m-long sections cut from a continuous 80-m-long cable, were greater than 2700 A at 4.2 K in self-field.

Ｐｂ‐ｆｒｅｅ Ｂｉ系バルク電流リードの作製とその超伝導特性 冷凍機冷却型超伝導マグネットへの適用の可能性

The cylindrical bulk current lead was fabricated using Pb-free Bi system superconductors in a solid-state reaction. The current lead attained a high transport critical-current density (Jc) of about 320A/cm2 at 77K, 0T and more than 1, 200A/cm2 at 40K, 2T. The Pb-free current lead was composed of both Bi-2223 and Bi-2212 phases judging from the AC susceptibility curve and X-ray diffraction pattern. The heat leakage and mechanical strength of the Pb-free cylindrical bulk current lead were approximately similar to those of a Pb-dope current lead consisting of the Bi-2223 phase only. The Pb-free bulk lead particularly showed a high Jc property at low temperatures and high magnetic field. The Pb-free current lead had the characteristics of both Bi-2212 and Bi-2223 phases. The application of the bulk current lead for a cryocooled superconducting magnet was investigated.

圧縮成型試料を詰めたＢｉ‐２２２３テープの超電導特性

We prepared Ag-sheathed Bi system tape by the stuff with compression molding powder-in-tube method. Ag-sheathed tape of Bi system superconductors were studi-ed for sample with nominal composition of Bi1.8Pb0.4Sr2Ca2.2Cu3Oy. The critical current density of sample apparently depended on the thickness of the tape and increased markedly with decreasing tape thickness. The maximum value of the critical current density of sample 6.1×105A/cm2 at 6 K in zero magnetic field was achieved in a tape with 0.3 mm thickness and at 77K were 1.4×104A/cm2 in zero magnetic field.

Dynamic study of oxygen in Cu [sbnd]O plane in Bi-system superconductors

Superconducting transition temperature (T c), hole concentration (n c), and crystal structure for Bi 1.7Pb 0.3Sr 2Ca 2Cu 3O y (2223) polycrystals and Bi 2Sr 2CaCu 2O y (2212) single crystals annealed in different atmospheres have been measured. It is found that the consequence of annealing in O 2 is mainly the modification of the oxygen in Cu O planes, which is different from that in N 2 where the change is in the Bi-O planes. The result provides a good explanation for the complicated changes of the T c with the annealing conditions.

Study of mismatch between two different blocks in Bi-system superconductors

Samples of Bi 1.7Pb 0.3Sr2Ca 2Cu 3O 10+y (2223) annealed in different atmospheres and having different value of T c and oxygen contents were obtained and examined by a wide range of measurements as well as theoretical calculation. A roughly quantitative relationship among T c, oxygen content, c direction thermal expansion coefficient, and mismatch factors are established. It is revealed that the mismatch is important for superconductivity.

Anomalous behavior of Bi-system superconductors in the normal state and their correlation with superconductivity

Our experiments show that Bi-system samples exhibit two parameter anomalies and two heat and weight anomalies in normal state. For the samples of (Bi,Pb) 2Sr 2CaCu 2O y, as the superconducting volume is over 65%, two anomalies of heat and weight at about 400 °C and 750 °C, respectively, are significantly observed; as the superconducting volume of the samples is under 50%, or the samples are non-superconducting, the anomaly at about 400 °C is hardly observed. For the samples of (Bi,Pb) 2Sr 2Ca 2Cu 3O y, as the superconducting volume is over 65%, they exhibit two anomalies at about 300 °C and 750 °C, respectively; as the superconducting volume is under 50%, the anomaly at about 300 °C disappears, and as the samples are non-superconducting, it shows a quite opposite behavior to the superconducting samples at about 300 °C. We develop a computer program based on calculation of lattice energy to simulate the experimental results, it is found that the change of the lattice energy can well explain the anomalous phenomena in the samples.

A quantitative x-ray photoelectron spectroscopy study of the site-selection behaviours of cations in the crystal lattice of bismuth system superconductors

A new method is applied to determine the cation distribution state in the crystal lattice of Bi system superconductors by x-ray photoelectron spectroscopy (XPS) analysis. The results indicate that the XPS analysis, an effective `chemical state' analytic method, can detect the various site-selection states of cations through the analysis of the `chemical shift' of the XPS peaks corresponding to different lattice sites which the cations occupy in the crystal structure. Furthermore, a quantitative description of this site selection state of cations is achieved reliably with the quantitative XPS analysis process discussed in this paper. We find that not only does the composition of the solid solution depend on the element substitution according to the different Sr:Ca ratios but also the site selection in the solid solution depends on the variable site-selection state in the crystal structure. The site selection in the solid solution is dependent on the Sr:Ca ratio of the raw mixed powders, the Sr:Ca ratio of the superconductor phase, and the synthesis process.

Cu 2p core level spectra in Bi system superconductors

The XPS spectra of Bi 2Sr 1.8La 0.2Cu 1− x Co x O y taken with Mg Kα revealed a new discovery in Cu 2p spectrum, which is very important to the calculation of the ratio of the satellite versus main line intensities ( I s/ I m) and the BE separation W. Bi 4s core level spectrum whose peak position is 942 eV coincides with the satellite peak of Cu 2p spectrum affects not only I s/ I m and W values, but also consequently calculated charge transfer energy ɛ and hybridization energy T that are crucial parameters in the electronic correlation mechanism in HTSCs.

Effect of F doping in the (Bi, Pb)-Sr-Ca-Cu-O system

The effect of F doping in the (Bi, Pb)SrCCuO system has been investigated by resistance and AC susceptibility measureme The influence of fluorine on the forming process of superconducting phases and the superconducting properties are shown, and the role of fluorine in the Bi system superconductors is discussed. The effect of the heat treatment of the F doped samples has also been studied. The results indicate that F doping promotes the forming of 2223 phase and increases the bulk T c.

Substitution effect on superconductivity in Fe-doped Bi-system superconductors

The electronic structures of FeO 4Ca 8 and FeO(1) 4O(2)Ca 4Sr 4 clusters in the Fe-doped Bi-systemhigh- T c phase are calculated using the SCF-Xα-SW method. From the calculated result, we find that the Fe atom does not contribute to the density of states near the Fermi energy in both clusters. Fe substitution for Cu sites of Bi-2233 phase is highly deleterious to superconducting behavior. The energy gap of FeO(1) 4O(2)Ca 4Sr 4 on the Fermi energy is about seven times larger than that of FeO 4Ca 8. Moreover, for the FeO(1) 4O(2)Ca 4Sr 4 cluster, the peak of the density of states (DOS) in the high energy range shifts to the low energy range and forms new DOS peaks, in contrast to the density of states in the FeO 4Ca 8 cluster. The bonding of the hybridized orbital in FeO(1) 4O(2)Ca 4Sr 4 is stronger than in FeO 4Ca 8, namely the FeO(1) 4O(2)Ca 4Sr 4 is even stabler than the FeO 4Ca 8 cluster. Such calculated result provides satisfactory agreement with Fe-doped experiments. It is observed that T c decreases with increasing iron content and Fe dopant gives rise to enhancement of the 2212 phase at expense of the 2223 in Bi-system superconductors. The calculated results can help us to understand the nature of Fe-doping for Cu on quantum mechanics background. Furthermore the theoretical value of the electric field gradient given by the SCF-Xα-SW method is agreement with the value deduced from quadrupole splittings by Mössbauer spectroscopy. This calculation can provide theoretical support for assignment of Fe species to Cu sites in high- T c superconductors.

Inductance characteristics of pipe-type oxide superconductor for various configurations

The authors present the magnetic inteeeons of pipe-type superconductors which may be used as the current lead in the cryogenically cooled power cable and magnet. Bi-system superconductors were successfully fabricated by a glass-ceramic process. Inductance characteristics for a simple configuration of a single bar-type superconductor are discussed. The inductance was evaluated from the measurements of reactive voltage and alternating current. The magnetic interactions for coaxial and parallel configurations of the pipe-type superconductor are considered. >

The investigation of bi-system cuprate superconductors

Abstract In this report, the Bi-system cuprate superconductors will be discussed in the following aspects: phase formation, especially for the 2223 phase; stoichiometrical deviation; structural details and instability at temperatures near T c . In principle, the Bi-2223 phase, even for the Pb-doped one, exists in a multi-phase equilibrium at optimum temperature. For the Bi-2212 phase, the stoichiometric deviation influences the structural parameters and critical temperature, which leads to inhomogeneity in superconductivity. The incommensurate structure both in occupational and positional modulation will be discussed, which could be controlled in some ways. Then the quality of the sample could be improved. The instability of the structure at temperatures near and above T c will be discussed based on studies of X-ray diffraction, Mossbauer effect, internal friction, and positron annihilation. It seems that charge transfer appears at temperatures above T c .

Electronic and silver ionic conductions in perovskite and related oxides

Electronic and silver ionic conductivities in some silver-containing oxides with perovskite and related structures, such as AgNbO 3 (perovskite), AgSbO 3 (pyrochlore-related structure), and Ag-doped Bi (Pb) Sr Ca Cu O-system superconductor, were measured by four-probe techniques using Pt electrodes and Ag-β″-Al 2O 3 electrodes with silver ionic conduction, respectively. Both AgNbO 3 and AgSbO 3 were n-type semiconductors and their electronic conductivities increased with an increase in the silver content. The electronic conductivity of AgSbO 3 was higher than that of AgNbO 3, due to the presence of a large amount of oxygen vacancies. Sb 4+ and/or Sb 3+ in AgSbO 3, and Nb 4+ in AgNbO 3 will act as a donor. Silver ionic conductivities were always lower than the electronic conductivities brought about by the above donors for both AgNbO 3 and AgSbO 3 ceramics. The activation energies of silver ionic conductivity were in the ranges of 1.01–1.58, 0.78–0.90, and 0.94–1.12 eV for AgNbO 3, AgSbO 3, and Ag-doped Bi-system superconductors, respectively. The silver ion was assumed to move via the Ag sites in the bulk for AgNbO 3, but to move in the grain boundaries for AgSbO 3 and the Bisystem superconductor. The mechanisms of the electronic and silver ionic conductions are discussed for the present oxide ceramics.

Copper valency of superconducting phases in the Pb-doped Bi-Sr-Ca-Cu-O system

Pb-doped Bi-Sr-Ca-Cu high- T c superconducting oxides have been prepared by a modified sol-gel process via the EDTA route. Upon decomposing the X-ray photoemission spectrum of the Cu 2p 3 2 core level of the superconducting oxide into three components Cu 1+, Cu 2+ and Cu 3+, we have found an obvious trend: the corresponding [Cu 1+]/[Cu 2+] ratio increased as more 110 K phase was formed in the sample. According to this trend, a defect model was proposed; it can reasonably explain some previous experimental results, such as a reduced oxygen atmosphere sintering, which effectively facilitates the formation of the 110 K phase, and the coexistence of copper and oxygen vacancies in Bi-system superconductors, observed by high-resolution transmission electron microscopy. On the other hand, the dimerization of holes in oxygen sites was also suggested to be a possible mechanism responsible for the increase in the ratio of [Cu 1+]/[Cu 2+].

STRUCTURES AND PROPERTIES OF F-DOPED BI-SYSTEM SUPERCONDUCTORS

In this paper, the possibility of F entering the superconducting phase, its position and the structure properties of the superconductors, and the mechanism of superconductivity in F-doped Bi-systems are studied based on experimental and theoretical results. It has been found that, because the ΔT c is narrowed and [Formula: see text] raised, the T co of F-doped Bi-system superconductors increases obviously, with the highest T co at 118 K, that F has entered into the superconducting phase, that generally F can reduce the potential barrier of forming high-T c superconducting phase and improve the superconductivity of Bi-systems, but only if there exists certain amount of Pb in the system, and that with F-doping the weak linkage of superconductor is decreased, the flux pinning is enhanced, the carrier-hole concentration is increased, and the critical current is improved.

Study of nuclear electric quadrupole splittings in the high-T c phase of Bi-system superconductors

57Fe Mossbauer measurement have been performed in the Fe-doped high-Tc phase of Bi-system superconductor. Isomer shifts display presence of Fe3+ ions in square and pyramidal coordinations. Regarding the high-Tc superconductors as primarily ionic crystals and using effective valence and point charge model, we have calculated the electric field gradients for Fe as a replacement for Cu in the high-Tc phase. Good agreement between theory and experiment is obtained.