On the role of Bi2Sr2Ca1Cu2O8 and Bi2Sr2Cu1O6 on the weak links and critical currents in (Bi,Pb)2Sr2Ca2Cu2O10/Ag superconducting tapes

Silver-sheated (Bi,Pb)₂Sr₂Ca₂Cu₃O₁₀ (Bi2223) superconducting tapes with different Bi₂Sr₂CaCu₂O₈ (Bi2212) and Bi₂Sr₂Cu₁O₆ (Bi2201) concentrations, were prepared by using a two-step sintering processing and by varying cooling rates in the fabrication of the superconductors. The effect of residual Bi2212 and Bi2201 phases on weak links and critical currents of the Bi2223/Ag tapes was investigated. It was found that residual Bi2201 caused weak links at grain boundaries and limited the current-carrying capacity of the tapes. Comparatively, the residual Bi2212 phase had much less influence on both weak links and critical currents. Elimination of Bi2201 by sintering tapes at a low temperature in the final thermal cycle, or by cooling the tapes slowly, increased critical current by a factor of two. Flux pinning property was also improved by removing the residual Bi2201 phase.     

Microstructure and properties of Tl0.75Bi0.25Sr1.6Ba0.4Ca2Cu3Ox tapes

Ag-clad Tl_0.75Bi_0.25Sr_1.6Ba_0.4Ca₂Cu₃O_x tapes were fabricated by a powder-in-tube technique. The starting powders consisted of Bi₂Sr₂CaCu₂O_x plus simple oxides. The tapes were heated in flowing O₂ at 835–865 °C for 7.5–10 h. Room-temperature pressing at 1.0–1.5 GPa produced tapes with denser, more phase-pure cores, and transport critical current density J_c at 77 K in self-field that was increased by a factor of ≈2. The maximum J_c value of 6 × 10³ A/cm² was obtained with heating at 840 − 850 °C for ≈ 10 h, with three intermediate pressing steps. The cores of the best tapes were still rather porous and contained significant concentrations of nonsuperconducting phases.     

Unit cell parameter displacement with temperature and oxygen contents in a high-Tc phase Pb-Bi-Sr-Ca-Cu-O system superconductor

The ceramic superconductors were prepared by sintering a powder compact with a composition of Pb_0,4Bi₂Sr₂Ca₂Cu₃O_x at 1123 K. During the final firing process of this solid state reaction, the oxygen concentration of the atmosphere in the furnace was controlled at 0,10 and 20%. In this report, the expansion and contraction behaviors of the PBSCCO unit cell in the temperature range from 77 to 289 K, in connection with the O₂ partial pressures, are described. The powder X-ray diffraction patterns were carefully obtained under each experimental condition, and the diffracted intensity data were analyzed using the Rietveld method.     

Precipitation and pinning in Ca and Sr-Rich High-Tc superconducting “Bi2Sr2CaCu2O8” ceramics

From a consideration of the phase equilibrium diagram of the system Bi₂O₃-SrO-CaO-CuO, a simple annealing procedure was developed to precipitate Bi_2+xSr_2+xCuO_6+d, Sr₁₄Cu₂₄O_41−x, and Bi₂Sr₃O₆ in high-temperature superconducting Sr-rich “Bi₂Sr₂CaCu₂O₈” ceramics and Ca₂CuO₃ and a liquid in Ca-rich “Bi₂Sr₂CaCu₂O₈” ceramics. The transformation results in an increase of the critical current density of which is believed to express improved pinning properties of the superconducting crystals, in particular an increased pinning energy, which reduces the probability for thermally activated depinning. Possible pinning centers which were introduced during precipitation of the second phases are the surface of the precipitates.     

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.     

Processing and physical properties of single phase Tl2Ba2CaCu2O8 and (Tl0.5Pb0.5)Sr2−xBaxCa2Cu3O9 superconductors

Single phase Tl₂Ba₂CaCu₂O₈ (2212) superconductors with T_c of 97 K were obtained by a two-step synthesis. Prolonged annealing at 860°C in the second step of the synthesis resulted in a higher T_c, at the expense of the growth of an impurity trilayer 2223 phase in the form of stacking faults in the 2212 phase. Nearly single phase (Tl_0.5Pb_0.5)Sr_2−xBa_xCa₂Cu₃O₉) (1223) samples were obtained in a single air-sintering step. Replacement of Sr by Ba is necessary to grow the 1223 phase. Samples with x=0.75 displayed T_c as high as 117 K with a superconducting volume fraction over 50%.     

On the metal and oxygen content determination in Bi-2212 and Bi-2223 HTSCs

The oxygen content in Bi₂Sr₂Ca₁Cu₂O_8+δ and Bi₂Sr₂Cu₃O_10+δ superconductor samples is determined using a iodometric titration technique and using a thermogravimetric reduction technique. The reliability of the iodometric oxygen determination was exemplified by the reproducibility of at least 0.01 in δ and the excellent agreement between results obtained separately from iodometry and thermal analysis on a series of samples. Compared to similar determinations in YBa₂Cu₃O_7-δ superconductors, the determination of the metal content is an extra analytical requirement in the case of Bi and Pb containing ceramic materials. For the determination of the metal content, a reproducible and rapid method, based on differential pulse polarography, was optimized. Using EDTA-HCl as supporting electrolyte, we were able to determine the Pb, Bi and Cu content in a single experiment with a relative standard deviation (RSD) < 1 %. Using the techniques mentioned above, we were able to investigate the relationship between T_c and δ in a series of Bi-2212 and Bi-2223 HTSC's. There appears to be a maximum in the T_cvs δ curves for both phases.     

Increased pinning in “Bi2Sr2CaCu2O8” ceramics

From a consideration of the phase equilibrium diagram of the system Bi₂O₃-SrO-CaO-CuO, a simple annealing procedure was developed to transform single phase 2212 ceramics with compositions Bi_2.18Sr_1.75Ca_1.25Cu₂O_8+d and Bi_2.3Sr₂CaCu₂O_8+d into multi phase samples containing Ca₂CuO₃ and/or liquid. The transformations result in increases of the critical current densities at 1 T of 5–10 times, which is believed to reflect the increased pinning properties of these ceramics. The exact nature of the resulting pinning centres has not been determined yet.     

Phase relations and homogeneity region of the hightemperature superconducting phase (Bi,Pb)2Sr2Ca2Cu3O10+d

For the nominal composition of Bi_2.27−xPb_xSr₂Ca₂Cu₃O_10+d, the lead content was varied from x < 0.05 to 0.45. The compositions were examined between 800 and 890‡C which is supposed to be the temperatue range over which the so-called 2223 phase (Bi₂Sr₂Ca₂Cu₃O_10+d) is stable. Only compositions between x < 0.18 to 0.36 could be synthezised in a single phase state. For x <0.36, a lead-containing phase with a stoichiometry of Pb₄(Sr,Ca)₅CuO_d with a small solubiliy of Bi is formed, for x > 0.18 mainly Bi₂Sr₂CaCu₂O_8+d and cuprates are the equilibrium phases. The temperature range for the 2223 phase was found to be 800 to 890‡C but the 2223 phase has extremely varying cation ratios over this temperaure range. Former single phase 2223 samples turn to multiphase samples when annealed at slightly higher or lower temperaures. A decrease in the Pb solubility with increasing as well as decreasing temperature with a maximum at about 850‡C was found for the 2223 phase.     

Annealing effects on transition temperature of r.f. sputtered Bi-Sr-Ca-Cu-O thin films

High-T_c Bi-Sr-Ca-Cu-O thin films have been made on YSZ substrates by single-target r.f. sputtering. The target was prepared for a composition of Bi₄Sr₃Ca₃Cu₄O_x. The best film showed the onset of superconductivity at 83 K and zero resistance at 75 K. The dependence of T_c(o) of sputtered films on annealing conditions is studied in detail.     

Effect of Metal Substitution in BSCCO Ceramic Superconductors

The fabrication, electrical, and optical properties of ceramic Bi_2-xM_xSr₂Ca₂Cu₃O_y (where M=Al or Ni and x=0.3) superconductors are described. Resistivity measurements reveal that when Al is partially substituted at the bismuth site in the BiSrCaCuO compound, T _c decreases to 77 K and the nickel-substituted compound shows a T _c of 70 K, compared to the undoped ceramic BiSrCaCuO samples with T _c of 85 K. Infrared reflectance measurements, which cover the 50-4000 cm^?1 range, find that the reflectance from the ceramic samples decreases in the metal-doped samples. The observed phonon modes in the infrared conductivity spectra (obtained by Kramers-Kronig analysis) decrease in strength and some features are completely smeared out. Furthermore some of the modes are observed to shift slightly in the frequency on doping, the change in the T _c and vibrational modes is attributed to destabilization of 2223 phase.     

Engineered flux-pinning centers in Bi2Sr2CaCu2Ox and TIBa2Ca2Cu3Ox superconductors

Three methods were used to introduce flux-pinning centers into Bi₂Sr₂CaCu₂O_x (Bi-2212) and TlBa₂Ca₂Cu₃O_x (Tl-1223) samples. It was found that carbon induced local decomposition, that nanosized Al₂O₃ additions created stable reaction products, and that second phases could be isolated in Tl-1223 during synthesis. Each of these defects enhanced flux pinning and was of most benefit at temperatures ≤ 35K.     

Submillimeter transmission spectra of Sr2Ca2Cu4Oy

Transmission spectra of Sr₂Ca₂Cu₄O_y, which is a part of high T_c superconducting system Bi₂Sr₂Ca_n?1Cu_nO_z, and their temperature evolutions were investigated by monochromatic quasioptical submillimeter spectroscopy. Strong increase of high frequency conductivity in the range 80 to 350K was revealed. The obtained results were analyzed on basis of thermally activated carriers model.     

Role of silver addition on mechanical and superconducting properties of high-Tc superconductors

The effect of silver additions on the mechanical and superconducting properties of sintered bulk YBa₂Cu₃O_δ (YBCO), Bi₂Sr_1.7CaCu₂O_δ (BSCCO-2212), and Bi_1.8Pb_0.4Sr_2.2Ca₂Cu₃O_δ (BSCCO-2223) has been evaluated. Strength and fracture toughness of YBCO and BSCCO bars increased with increasing Ag content up to 30 vol.% Ag. Addition of 30 vol. % Ag to YBCO increased strength from 87 to 136 MPa and fracture toughness from 1.82 to 3.9 MPa√m. Addition of 30 vol.% Ag to 2212 and 2223 increased strength from 58 to 107 and 41 to 90 MPa, respectively. Corresponding increases in fracture toughness were from 1.89 to 2.79 and 1.09 to 1.94 MPa√m, respectively. These improvements in strength and fracture toughness are believed to be due to the presence of Ag particles that may induce compressive stresses in the superconducting matrix and resist crack propagation by pinning the propagating cracks. The values of strength and fracture toughness of BSCCO-30 vol.% Ag specimens are comparable to those of monolithic BSCCO obtained by sinter forging, hot pressing, and hot isostatic pressing. On the other hand, the hardness of YBCO and BSCCO decreased with increasing Ag contents because of the lower hardness of Ag. Addition of Ag showed no adverse effects on superconducting properties (J_c and T_c) of YBCO or BSCCO superconductors.     

High Oxide-Ion Conductivity through the Interstitial Oxygen Site in Sillén Oxychlorides

Oxide-ion conductors are gaining attention as future materials in energy applications, such as solid oxide fuel cells. Many Bi-containing compounds exhibit high oxide-ion conductivity via conventional vacancy mechanism. However, interstitial oxide-ion conduction is rare in Bi-containing materials. Herein, high oxide-ion conductivity is reported through interstitial oxygen sites in Sillén oxychlorides, LaBi_2−xTe_xO_4+x/2Cl (Bi₂LaO₄Cl-based oxychlorides). Oxide-ion conductivity of LaBi_1.9Te_0.1O_4.05Cl is 20 mS cm⁻¹ at 702 °C, and higher than best oxide-ion conductors as Bi₂V_0.9Cu_0.1O_5.35 below 201 °C. Despite of the presence of Bi and Te species, LaBi_1.9Te_0.1O_4.05Cl shows extremely high chemical and electrical stability at 400 °C from oxygen partial pressure 10⁻²⁵ to 0.2 atm and high chemical stability under CO₂ flow, wet 5% H₂ in N₂ flow, and air with natural humidity. Neutron scattering length density analysis, DFT calculations, and ab initio molecular dynamics simulations indicate that the extremely high oxide-ion conduction is attributed to cooperative diffusion through interstitial oxygen sites (interstitialcy diffusion mechanism) in triple fluorite-like layers. The present findings demonstrate the ability of LaBi_2−xTe_xO_4+x/2Cl as superior oxide-ion conductors, which can open new horizons for oxide-ion conductors.     

Effect of Hf and Hf-Ca substitution on the superconductivity of GdBa2Cu3O7 − δ

The structural and superconducting properties of (Gd_1−x−yCa_yHf_x)Ba₂Cu₃O_z samples are investigated using X-ray diffraction, resistivity, AC susceptibility and oxygen content measurements. The effect of increasing Hf concentration in (Gd_1−xHf_x)Ba₂Cu₃O_z lowers the oxygen content and decreases T_c which is attributed to hole filling by Hf. The substitution of Ca for Gd in (Gd_0.85−yCa_yHf_0.15)Ba₂Cu₃O_z provides proper matching between the ionic radius and valence of Gd³⁺ (0.94 Å) and the average ionic radius and valence of Hf⁴⁺ (0.78 Å) and Ca²⁺ (0.99 Å). As the Ca content increases, the T_c increases from 81 K (y = 0.05) to 86.5 K (y = 0.20, compensated oxide), closer to the value of 91 K for pure GdBa₂Cu₃O_7−δ due to the balance between the hole filling by Hf and hole doping by Ca. A comparative study of Hf doped samples of (R_1−xHf_x)Ba₂Cu₃O_z (R = Y, Er, Gd) indicates that the magnetic moment carried by R-ion plays an important role in the suppression of superconductivity and T_c.     

Crystal growth in Bi–Sr–Ca–Cu–O system

We report growth of Bi₂Sr₂CaCu₂O_8+δ single crystals with dimensions of 6×2×0.03 mm³ using a melt and growth technique. The oxygen content is determined to be δ≈0.13 by iodometric titration. The crystal is shown to be homogeneous and close to stoichiometric cation ratio. The superconducting temperature with a sharp transition width (10–90% level) of 6–8 K was determined to be T_c=92 K from resistivity and dc susceptibility measurements. The predominant impurity phase is a Cu-free crystal, whose composition is identified as Bi₁₀Sr₁₁Ca₅O_x. The crystal structure of Bi₁₀Sr₁₁Ca₅O_x is monoclinic with a=11.108(1) Å, b=5.9487(1) Å; c=19.838 (3) Åand β=101.5° (P2_1/c space group).     

The phase transformation of Bi-Pb-Sr-Ca-Cu-O bulk materials are rapidly melted and solidified by a CO2 laser

The phase transformation of Bi_1.7Pb_0.4Sr_1.6Ca_2.4Cu_3.6O_y bulk materials rapidly melted and solidified by a CO₂ laser with the scanning speed of 40 mm/s were investigated. Results of x-ray diffraction pattern, scanning electron microscopy and energy-dispersive x-ray analysis showed the decomposition of the high-T_c phase in the laser irradiated region. Nonsuperconducting phases such as CaO and (Sr_1−xCa_x)CuO_y were found to be in the melting zone. On the other hand, (Sr_1−xCa_x)CuO_y and 2212 phase were also found in the heat-affected zone. When the irradiated samples were treated with 835‡C for 72 h in air, the laser treated region changed into the high-T_c as a major phase, in addition to the low-T_c phase and nonsuperconducting phase. However, the high-T_c phases are piled up randomly. The transport critical-current density of the laser treated samples after annealing is lower than that of the original sintered one, i.e. at 77K and zero magnetic field.     

High Q-factor superconductor VHF antennae

Two versions of elementary antennae fabricated of high temperature superconductors (HTS) are presented. The ceramic YBa₂Cu₃O₇ strip as well as Bi₂Sr₂CaCu₂O₈ thick film have been used as radiators operating in the wavelength ranges of 5 cm and 8 mm respectively. Substantial gain and Q-factor enhancement if compared with similar metallic ones have been achieved for the HTS antennas.     

Rapid synthesis of Y- and Bi-based high-Tc cuprates by microwave irradiation

High-T_c superconducting samples of YBa₂Cu₃O_7−x with T_c∼90 K and Bi_2.2Sr_1.8Cu_1.05O_x with T_c∼9 K have been prepared for several ten min, using a domestic microwave oven operated at 2.45 GHz, without any post-heat-treatment. Post-heat-treatment is not necessary due to improvement of the sample environment during the microwave process. That is, a pellet of mixed powder of starting materials is surrounded by mixed powder of starting materials and, subsequently, wrapped in glass wool in order to suppress rapid dissipation of heat from the surface of the pellet. This leads to successful preparation of homogeneous and oxygenated samples. In addition, we have attempted to prepare Bi-2212 and Bi-2223 phases. A sample whose major phase was Bi-2212 was obtained. However, no sample with the Bi-2223 phase could be obtained.