Increase in the diamagnetic response from low-density Bi1.8Pb0.3Sr1.9Ca2Cu3Ox high-temperature superconductors and Bi1.8Pb0.3Sr1.9Ca2Cu3Ox + Ag composites

Low-density polycrystalline Bi_1.8Pb_0.3Sr_1.9Ca₂Cu₃O _x high-temperature superconductors with a foamlike microstructure and composites consisting of this superconductor and silver in an amount of 20, 25, and 30 vol % are synthesized. The microstructure, as well as the temperature and field dependences of the magnetization, M(T) and M(H), are studied. It is found that, in Bi_1.8Pb_0.3Sr_1.9Ca₂Cu₃O _x high-temperature superconductors and Bi_1.8Pb_0.3Sr_1.9Ca₂Cu₃O _x + Ag composites, the diamagnetic response is enhanced and the screening properties are improved compared with high-temperature polycrystalline superconductors with the same composition that are prepared by the standard technology. The observed effect is explained by the features of magnetic flux penetration into a porous medium.     

Specific heat anomaly in Bi1.6Pb0.4Sr2Ca2Cu3Oy superconductor

The specific heat anomaly ΔC of Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O_y was observed in the temperature range 80–100 K. It is estimated from ΔC that γ15 mJ/mol·K² and H_{c2(T=0)100 T} in this sample. The specific heat anomaly confirmed the occurrence of bulk superconductivity of the high-T_c phase in Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O_y superconductor.     

Harmonic generation in TlBa2Ca3Cu4Oy superconductor

We have studied harmonic generation in the magnetic response of bulk samples of the ceramic superconductor TlBa₂Ca₃Cu₄O_y. Dependence of the odd and even harmonics of magnetization on the DC magnetic field, amplitude of the AC magnetic field and temperature was investigate experimentally. We have used a critical state model recently developed by Müller et al. to calculate the full harmonic response. At fixed temperature, the dependence of the odd order harmonic response on the amplitude of the AC magnetic field H_m shows an unambiguous crossover from an H²_m behaviour (Bean-like) at small amplitudes to an H³_m behaviour for larger H_m, in quantitative agreement with the modified critical state model.     

Intergrain and intragrain superconductivity in Sn- and Sb-doped Bi1.7Pb0.3Sr2Cu3Oy

Measurements of the resistivity and current dependent resistivity for small current densities ranging from 0.03 A/cm² to 3.0 A/cm² were performed with two Sn-doped and two Sb-doped polycrystalline Bi_1.7Pb_0.3Sr₂Ca₂Cu₃O_y samples in magnetic fields up to 1.7 T. Features in the temperature derivative of the resistivity curves were associated with the presence of a superconducting transition between superconducting grains, coupled by weak links with a distribution of critical currents and critical temperatures, and the superconducting transition within grains. The transition between grains was more strongly suppressed in temperature with the application of a magnetic field in samples with weaker coupling between grains. The presence of a transition in a magnetic field due to weak links between grains was verified at 77 K by the observation of a current dependent resistivity in a magnetic field.     

Magnetic and transport properties of YBa2Cu3Oy superconductor foams

The recently reported superconducting YBa₂Cu₃O_y (Y123) foams are highly interesting and promising for variety of applications. In this report we present first magneto-transport measurements of the superconducting properties of these foams. The investigations reveal the superconducting properties being similar to those of bulk melt processed materials. The 123 foams reveal a T_c of 92 K and have a magnetization J_c of 40,000 A/cm² at 77 K and 0 T. The measurements of magnetic hysteresis versus field show a high anisotropy of the critical current density up to J_c^ab/J_c^c7.     

Anomalies of ultrasonic velocity in high-Tc ceramics YBa2Cu3Oy and BiSrCaCu2Oy

Ultrasonic longitudinal velocity and adsorption have been measured in ceramic superconductors YBa₂Cu₃O_y with various porosity and also in BiSrCaCu₂O_y. A velocity drop of about 400 ppm was found at T_c only in the measurements on cooling. The magnitude of the velocity drop is anomalously large compared with the value expected from the thermodynamics. A hysteresis of velocity with respect to temperature was observed in all the samples studied. It is suggested that some structural change at pore size level is responsible for this phenomenon.     

The formation mechanism of Bi2Sr2Ca2Cu3Ox superconducting phase

The mechanism and dynamic model of the formation of 2223 phase are presented and verified in the paper. It reveals that with the solubilization of part of the 2212 phase, 2223 phase is formed by the reaction being carried out according to a nucleation and growth mechanism in the presence of liquid phase, which conforms to the JMA dynamics equation. The addition of a little 2223 phase as nucleator, regrinding and resintering can promote the formation of 2223 phase. But the quick formation rate of 2223 phase may lead to irregularity of the 2223 phase structure, which degrades its electric and magnetic properties, while annealing treatment can improve its superconductivity.     

Effects on structure and superconductivity for La and Ca codoping in YBa2Cu3Oy cuprates

The nominal composition of Y_0.8Ca_0.2Ba_2−xLa_xCu₃O_y (YBLCO) cuprates with x≤0.50 has been synthesized by the standard solid state reaction technique. X-ray diffraction and the resistivity measurements are used to characterize the structure and the superconductivity of YBLCO cuprates. There is no structural phase transition in the whole doping range. The dependencies of the lattice constants and some other structural parameters on the content of La for the samples YBLCO with x≤0.20 are different than those for the samples with x≥0.25. The zero resistance temperature T_c0 increases with the increase of the content of La in YBLCO as x≤0.20, and decreases as x≥0.25. We compared these results with those of Nd-doped Y_0.8Ca_0.2Ba_2−zNd_zCu₃O_y cuprates. It seems that T_c0 is related to the structural parameters due to Ca and La codoping in YBLCO.     

Preparation of melt-textured NdBa2Cu3Oy bulk with Nd4Ba2Cu2O10 addition

The NdBa₂Cu₃O_y (Nd123) bulk superconductor, to which Nd₄Ba₂Cu₂O₁₀ (Nd422) particles were intentionally added, was prepared through the so-called MMTG process in Ar (99% purity) flowing atmosphere at an ambient pressure. The quasi-single crystal thus grown was about 1 cm × 1 cm × 1 cm in dimension. It turned out that the Nd422 particles were uniformly distributed in the Nd123-phase matrix in a fashion similar to the distribution of the intentionally added Y211 particles in the Y123 phase matrix. The superconducting transition temperature T_c for the sample subjected to post-annealing in oxygen atmosphere was 94 K. The critical current density J_c was determined to be 45 000 A/cm² at 77 K and 1 T, when the field was applied parallel to the c-axis of the sample. To the best of our knowledge, the J_c value is the highest and the size of the quasi-single crystal is the largest in the melt-textured Nd123 bulk superconductors so far reported.     

Magnetic levitation of YBa2Cu3Oy single crystals

By using an ordinary electric balance, we are able to measure the magnetic levitation forces acting on a superconducting YBa₂Cu₃O_y (YBCO) single crystal. It is found that when the external magnetic field is parallel to the c-axis of the single crystal, the hysteretic levitation curve is similar to that of a melt-processed YBCO superconducting sample. However, when the external magnetic field is perpendicular to the c-axis of the YBCO single crystal, the levitation forces are too small to be measured by our equipment. Also, we have introduced a simple model with the Bean's approximations to explain the levitation forces. The critical current density derived from this model by fitting with experimental data is quite close to the value obtained from magnetization measurements.     

Resistivity and thermoelectric power of Bi2Sr2Ca−xPrxCu2Oy system

Samples of Bi₂Sr₂Ca_1−xPr_xCu₂O_y have been characterized by resistivity and thermoelectric power measurements. All metallic samples show superconductivity with a maximum T_c = 90 K at X = 0.2. The sample of x = 0.6 shows a crossover from hopping conduction at low temperature above T_c to metallic conduction at high temperature. For the metallic samples below x = 0.6, the results of thermoelectric power are well fitted by both of a phenomenological band spectrum model and the Nagaosa and Lee model.     

Zone melting of suspension spun Bi2Sr2Ca1Cu2Ox filament

Zone melting was used to enhance the J_c of suspension spun Bi₂Sr₂Ca₁Cu₂O_x filaments. By controlling the directional solidification of the molten zone, highly orientated textures were produced and a transport critical current density of more than 2 × 10⁴ A/cm² at 77 K, 0 T was achieved.     

Electrical transport behavior of La0.67Ca0.33MnO3/Fe3O4 composites

The influence of Fe₃O₄ contents on the electrical transport properties (resistivity and ac susceptibility) of a series of composite samples of La_0.67Ca_0.33MnO₃/Fe₃O₄ is studied. Results show that the Fe₃O₄ phase not only shifts the intrinsic insulator-metal (I-M) transition temperature T_P1 to a lower temperature, but also causes a new I-M transition at a lower temperature T_P2 (T_P2<T_P1). On the basis of an analysis by scanning electron microscopy and X-ray diffraction, we suggest that the decrease of the I-M transition temperature and the formation of the new I-M transition are caused by the segregation of a new phases related to the Fe₃O₄ at grain boundaries or surfaces of the La_0.67Ca_0.33MnO₃ grains.     

Surface versus bulk pinning in a HgBa2Ca2Cu3O8+x ceramics

Flux creep measurements on a HgBa₂Ca₂Cu₃O_8+x ceramics are reported. The results of the magnetic relaxation measurements are analyzed both by assuming that the pinning is due to the existence of a surface barrier or exclusively caused by bulk pinning. The action of both the surface and the bulk barriers is evidenced. At 70 K, a very high critical current density of the surface currents is determined, which is higher than the critical current density of the bulk. The field and temperature dependence of the pinning behaviour reflects mainly bulk pinning in 2D. The measurements were repeated after 4 and 12 months to investigate the influence of aging. A destruction of the superconducting properties of the grain boundaries accompanied by a degradation of the surfaces of the grains with time is proved.     

Phase composition and properties of superconducting ceramics based on Bi1.7Pb0.3Sr2Ca2Cu3Oy precursors fabricated by melt quenching in a solar furnace

Production of superconducting ceramics based on precursors with rated composition Bi_1.7Pb_0.3Sr₂Ca₂Cu₃O _y is studied. The precursors are synthesized in a solar furnace by melt rapid quenching. The phase composition of the samples is examined by microstructural and X-ray analyses. The temperature dependences of the resistance and magnetic susceptibility are measured. The influence of the composition and crystal structure of the substrate on texturing in the Bi-Sr-Ca-Cu-O system is studied. It is found that the type of quenching plays a significant role, while the type of substrate is of minor significance.     

Electrochemical reduction of Bi2Sr2Ca1Cu2O8 superconductor single crystals

Single crystals and polycrystalline pellets of the high-temperature cuprate superconductor Bi₂Sr₂Ca₁Cu₂O₈ were doped at room temperature by electrochemical reduction at > 95% Coulombic efficiency using lithium dopant ions in propylene carbonate electrolyte. Cyclic voltammetry and potential step measurements on single crystals suggest an unusual reduction mechanism, with a diffusion coefficient for Li⁺ in the c-axis direction of bulk superconductor of ca. 3 × 10⁻¹¹ cm²s⁻¹. Sintered pellets of polycrystalline powder could be doped more rapidly, with an apparent diffusion coefficient of 7 × 10⁻⁸ cm²s⁻¹. X-ray susceptibility analysis show extensive disordering occurs on heavy Li doping, with a first-order transition from a crystalline/superconducting to an amorphous/non-superconducting phase. Single, crystals of Bi₂Sr₂Ca₁Cu₂O₈ exhibited a color change on reduction from metallic gray to golden bronze. The reduced material was highly air-sensitive, forming a hydroxide surface film on exposure to ambient atmosphere.     

EDXRF measurements on gold diffusion-doped Bi1.8Pb0.35Sr1.9Ca2.1Cu3Oy

Gold (Au) diffusion in superconducting Bi_1.8Pb_0.35Sr_1.9Ca_2.1Cu₃O_y was investigated over the temperature range 500-800 °C by the energy dispersive X-ray fluorescence (EDXRF) technique. It is found that the Au diffusion coefficient decreases as the diffusion-annealing temperature decreases. The temperature dependences of Au diffusion coefficient in grains and over grain boundaries are described by the relations D₁=6.7×10⁻⁵exp(−1.19 eV/k_BT) and D₂=9.7×10⁻⁴exp(−1.09 eV/k_BT), respectively. The diffusion doping of Bi-2223 by Au causes a significant increase of the lattice parameter c by about 0.19%. For the Au-diffused samples, dc electrical resistivity and transport critical current density measurements indicated the critical transition temperature increased from 100 to 104 K and the critical current density increased from 40 to 125 A cm⁻², in comparison with those of undoped samples. From scanning electron microscope (SEM) and X-ray diffraction (XRD) measurements it is observed that Au doping of the sample also improved the surface morphology and increased the ratio of the high-T_c phase to the low-T_c phase. The possible reasons for the observed improvement in microstructure and superconducting properties of the samples due to Au diffusion are also discussed.     

Tunneling studies on single crystals of superconducting Bi2Ca1−xYxSr2Cu2O8+δ

Tunneling studies have been carried out on single crystals of Bi₂Ca_1−xSr_xCu₂O_8+δ over a wide range of compositions wherein the hole concentration varies by a factor of 2.5. The 2Δ value varies between 25 meV and 75 meV over the composition range studied, but scales with 2Δ/k_BT_c≈9.5 throughout.     

Electrical transport and magnetic properties of La2/3Ca1/3MnO3/SiO2 composites

The influence of SiO₂ on the electrical transport properties of LCMO/SiO₂ composites with different SiO₂ contents x is investigated, where LCMO represents La_2/3Ca_1/3MnO₃. Results show that the SiO₂ phase not only shifts the metal–insulator transition temperature (T_p) to a high temperature range, but also has an effect on the magnetoresistance (MR) of the composites. The temperature dependence of resistivity indicates that the T_p of the composites is obviously higher than that of pure LCMO, and that the peak resistivity ρ_max of the composites is lower than that of pure LCMO. In the SiO₂ content x∼0.02, the T_P is the highest and ρ_max becomes the lowest. The experimental observation is discussed on the basis of the analysis of scanning electron microscopy (SEM) images and X-ray diffraction (XRD) patterns. Compared with pure LCMO, a possible interpretation is presented by considering the influence of SiO₂ on the coupling between ferromagnetic (FM) domains of LCMO.     

Electrical and infrared study of Bi2Sr2Ca1Cu2O8 in semiconducting, superconducting ceramic and superconducting glass ceramic state

We have studied the electrical and infrared properties of Bi₂Sr₂Ca₁Cu₂O₈ compound in three states. Electrical and IR measurements show that the pure powder state sample is a semiconductor, the ceramic Bi₂Sr₂Ca₁Cu₂O₈ sample prepared after annealing at 820°C for 240 hours shows a T_c of 85 K, whereas Bi₂Sr₂Ca₁Cu₂O₈ sample prepared through glassy route, i.e. melting at 1250°C and annealing at 820°C for 240 hours shows a drop of T_c by 5 K. The infrared spectra of superconducting ceramic and glass ceramic states in the available frequency range of measurement reveals the presence of three phonons. Since the vibrational mode around 595 cm^–1 is due to CuO₂ layers and as the CuO₂ layers are responsible for T_c in the ceramic superconductors, any change in these layers will affect the T_c. The shifting of the 595 cm^–1 mode towards lower frequencies in the glass ceramic due to different preparation process, indicates that there is a change in CuO₂ layers resulting in a change of T_c, which is confirmed by Four probe dc measurements.