Crystallization and stability of the Bi2Sr2CaCu2Ox superconductor

A basic study of the stability of the Bi₂Sr₂CaCu₂O_x phase has been carried out in order to identify the composition and processing conditions for optimum superconducting properties. Analytical electron microscopy has been used to follow the crystallization of this phase during annealing in the solid state as well as from the melt. The crystal chemistry and phase purity is found to depend strongly on the processing conditions. Significant differences in crystallization behavior may be related to kinetic limitations of oxygen transport. The Bi₂Sr₂CaCu₂O_x phase has also been subjected to thermomechanical processing in order to improve our understanding of the deformation processes involved during fabrication of wires and tapes. It has been found that mechanical deformation can have significant effects on the microstructure of the material which in turn result in changes in superconducting properties.     

Reactive sintering of Bi2Sr2CaCu2Ox superconductors

Three phase assemblages were used to produce Bi₂Sr₂CaCu₂O_x (2212) during sintering: a mixture of 20% Ca-rich 2212+80%2212, partially synthesized 2212, and Bi₂Sr₂CuO_x (2201)+(1/2 Ca₂ CuO₃+1/2 CuO) (denoted 0011). The mixture of 2201+0011 produced highly pure 2212 within 50 h of heating in air at ≈850°C. Ag tubes were filled with a mixture of 2201+0011 and worked into tapes by a powder-in-tube process. Heat treatments produced microstructures consisting of small, highly textured 2212 grains. T_c values were ≈70K. Transport J_c values at 4.2K were ≈10⁴ A/cm².     

Pure and lithium-doped Bi2Sr2CaCu2Ox silver-clad tapes

Silver-clad tapes with transport J_c values to 10⁵ A/cm² at T-4.2K were produced by a powder-in-tube process. Moderate reductions per pass and proper tape thicknesses were essential for producing good conductors. Partial-melt sintering, followed by a solid-state heat treatment, produced microstructures consisting of large, well-aligned 2212 grains and alkaline-earth cuprate phases. The deleterious effects of these second phases were minimized by proper heat treatments, but further reductions in the sizes and concentrations of these phases are needed to improve J_c values substantially. Lithium additions raised T_c, improved flux pinning, and lowered melting points. They offer hope that the presence of alkaline-earth cuprates can be limited through use of low-temperature heat treatments.     

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.     

Microstructure and properties of melt-processed Bi-2212 (Bi2Sr2CaCu2Ox)

Microstructural development and properties during melt processing of Bi-2212 were investigated with regard to the production of superconducting oxides in bulk shape for the application in electrical engineering. Oxygen loss during heating and melting leads to incongruent solidification on cooling and therefore multiphase microstructures. Phase compositions depend on oxygen stoichiometry, which is determined by oxygen partial pressure, maximum sintering temperature as well as cooling rate. During annealing, solid/liquid and subsequent solid/solid reactions yield high volume fractions of 2212. The oxygen absorption and the 2212 formation mechanism and its kinetics are strongly correlated. The11905→2212 transformation proceeds via intermediate states of high planar defect density and is promoted by frequent stacking faults, that allow diffusion of Ca- and Cu-atoms over short distance. Microstructures of the 2212 phase were also controlled by variations of the cation stoichiometry leading to an improvement of the superconducting properties.     

Far infrared response of thin film Bi2Sr2CaCu2O8 using a free electron laser

The far infrared response of granular thin-film Bi₂Sr₂CaCu₂O₈ superconductor has been investigated using long (≈5 μs) but sharply truncated free electron laser pulses in the frequency range between 50 cm^?1 and 125 cm^?1. Under constant current bias, a fast response and a slow bolometric signal component could be identified in this energy range, which is below the BCS energy gap (≈ 200 cm^?1). Measurements of the power dependences of the signal voltages showed that both the fast and the thermal responses are consistent with the predictions of the resistively shunted Josephson junction model.     

高温超导体Bi2Sr2CaCu2O8+δ单晶解理面的AFM研究

用ＡＦＭ在室温和大气下对高温超导体Ｂｉ２Ｓｒ２ＣａＣｕ２Ｏ８＋δ（Ｂｉ２２１２）单晶解理面的形貌和结构进行了系统的研究。观测到解理面绝大部分是原子级平滑的晶面。在晶面的边缘观测到高度为结构单元（ｃ／２）整倍数的台阶，这与该晶体是以层状模式生长的机制相一致的。进而证明了解理位置和解理分布。在解理面的局部区域，观测到高度为１．２５ｎｍ，面积约为０．０５μｍ２的平台。Ｘ－射线微区分析证明这些小平台正是在Ｂｉ２２１２单晶生长中常存在的少量Ｂｉ２２０１相。我们的研究表明ＡＦＭ不但在研究高温超导体单晶的形貌特征方面有重要用途，而且在对晶体的质量进行分析和结构特征研究中也有重要应用。     

Development of needle-shaped grains from Bi2Sr2CaCu2O8+x with high critical current density suitable for making superconducting wires

We present a grain microstructure for Bi(2212) consisting of only giant needle-shaped grains of around 1.5 mm length and 100 μm diameter. We study the structural and chemical changes suffered by a conventional ceramic Bi(2212) sample in the course of the thermal treatment used to obtain those giant needle-shaped grains. For that, different samples of the same batch were treated with incomplete thermal treatments, and the resulting samples were analysed by using scanning electron microscopy (SEM), optical microscopy, energy dispersed spectroscopy (EDS), inductively coupled plasma (ICP) and X-ray diffraction (XRD). To verify the superconducting nature of the needle-shaped grains, we have performed magnetization, resistivity, and critical current measurements on the original ceramic sample, and on that formed as giant needle-like grains. The critical temperature of these last grains is nearly the same as that of the ceramic sample (T_c∼90 K), which is a high value for the Bi(2212) compound. The critical current density (J_c) of the needle-shaped grains is around 2500 A/cm² at 77 K and in absence of applied magnetic field, a value comparable with that presented for the best wires and thick films. Not only are the shape and the size of these grains very suitable for making superconducting wires, but also the superconducting properties, T_c and J_c, are both high enough to be confident about the possibility of improving the actual Bi(2212) superconducting wires for high current applications.     

Visualization of Dopant Oxygen Atoms in a Bi2Sr2CaCu2O8+δ Superconductor

A proper amount of excess oxygen plays a significant role in hole‐doped cuprate high‐T_c superconductivity. Here, the dopant oxygen in Bi₂Sr₂CaCu₂O_8+δ is directly imaged via integrated differential phase contrast combined with state‐of‐the‐art scanning transmission electron microscopy. The location of dopant oxygen is observed to be consistent with the position inferred from local strain analysis of the incommensurate structure. The influence of dopant oxygen on the local atomic lattice and electronic structure is further explored using first‐principle calculations. The dopant oxygen atoms not only aggravate the distortions of the local atomic arrangement but also alter the electronic states by transferring charge from the BiO planes to the CuO₂ planes. The underlying mechanism of charge transfer is resolved. The results may also be applicable to other oxygen‐doped cuprates with high‐T_c superconductivity.     

Fabrication of Bi2Sr2CaCu2O x and Bi2Sr2Ca2Cu3O x Superconductor Thick Films by Using Cu-Free Precursors on Ni Substrates

We studied the formation of Bi₂Sr₂CaCu₂O _x (Bi-2212) and Bi₂Sr₂Ca₂Cu₃O _x (Bi-2223) thick films in a heat treatment process of the Ni-sheathed Bi-Sr-Ca-Cu-O (BSCCO) system. Cu was electrodeposited initially on the Ni substrates (Cu/Ni). Well-oriented Bi-2212 superconductor thick films were formed successfully on Ni tapes by liquid reaction between Cu-free precursors and Cu/Ni tapes. However, only a small amount of Bi-2223 was formed. Thick films were prepared by screen-printing with Bi₂O₃, SrCO₃, and CaCO₃ powders on Cu/Ni tapes and heat treating them. Heat treatment was performed in the temperature range of 750–850°C in a tube furnace for several minutes to hours. The phases and the microstructures of the high temperature superconductor thick films were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. Electrical properties were examined by the standard four-probe method. At the heat treatment temperature, the specimens were in a partially molten state during reaction between the oxidized copper layer and the screen-printed precursors on the Cu/Ni tapes.     

Josephson vortex motion in stacks of intrinsic Josephson junctions in Bi2Sr2CaCu2O8+x

We have investigated the motion of Josephson vortices in stacks of intrinsic Josephson junctions in Bi₂Sr₂CaCu₂O_8+x (BSCCO). The stacks were realized as mesa structures on top of BSCCO single crystals. In external magnetic fields up to 5 T, oriented parallel to the layers, the current–voltage (I–V) characteristics exhibit a field dependent branch. By microwave emission measurements we show that this branch is due to a collective motion of Josephson vortices. For magnetic fields larger than 17 kOe we find a pronounced flux-flow step in the I–V characteristic that corresponds to a Josephson frequency of 1.5 THz. In this regime, we observe an intensive non–Josephson emission signal. We propose that this signal is due to Cherenkov radiation emitted from Josephson vortices moving faster than the lowest mode velocity of the stack. Numerical simulations using the coupled sine-Gordon model demonstrate downconversion of Cherenkov radiation due to the mixing of different modes.     

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.     

Investigation of superconductivity effects in single-phase and multiphase Bi2Sr2CaCu2O8 single-crystals at 2-MM wavelengths

Experimental investigations of superconductivity effects in single-phase and multiphase Bi₂Sr₂CaCu₂O₈ single-crystals have been carried out at 142 GHz frequency by means of the standing wave profile method [1]. Josephson harmonic generation has been observed to be responsible for the appearence of additional peaks on the standing wave profile of the open dielectric resonator loaded with a properly orientated multiphase high-T_c superconductor specimen. This leads to the conclusion that most of the Josephson junctions in multiphase crystals are located in certain crystallographic planes. The investigations of temperature dependencies showed that sharp resonant peaks of conductivity observed earlier [2] at 60 GHz could also be observed at 142 GHz.     

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.     

A new process with a potential to rapidly texture bulk YBa2Cu3Ox superconductor

A new process has been developed to texture bulk YBa₂Cu₃O_x superconductors at temperatures about 100°C below the peritectic temperature. The texture is achieved in this process by directional recrystallization of compacts fabricated from quenched YBCO powders. Isothermal recrystallization of quenched precursors at 890°C for 3 min is found to result in the formation of more than 75% of YBa₂Cu₃O_x phase without any Y₂BaCuO₅. Recrystallization at higher temperatures leads to rapid formation of fine Y₂BaCuO₅ precipitates in addition to YBa₂Cu₃O_x. The formation of YBa₂Cu₃O_x increases with increasing heating rates. Directional recrystallization at 10 mm/h results in a well-textured microstructure with the YBa₂Cu₃Ox grains oriented parallel to the sample length. The magnetic field dependence of the critical current density at 77K of the directionally recrystallized material compares well with that of melt-textured YBCO and is superior to that of magnetically aligned and sintered YBCO.     

Infrared absorption and far-infrared diffuse reflection spectra in Pb-doped Bi−Sr−Ca−Cu−O high-temperature superconductor

In this paper we study the room-temperature infrared transmission spectra (400–1600cm^?1) and far-infrared diffuse reflection spectra (50–450cm^?1) in Pb-doped Bi?Sr?Ca?Cu?O (2223) single phase (Tc=107k, sp1), multiphase (Tc=110k, sp2) and nonsuperconducting samples (sp3). The spectral features in superconductor are totally different from those in nonsuperconductor, which show the different crystal structure. The correlation existing between a factor group analysis of the phonons in (2223) and (2212) compounds affords a tentative assignment of ir-active modes in Pb-doped (2223) single phase by comparison with reported data in (2212) materials. The Cu?O stretching E_u vibration (605cm^?1) of CuO₂ layers is the characteristic vibrational mode related perovskitelike crystal structures. Two phonon coupling effect emerges in the infrared transmission spectra in Pb-doped superconductor. The Ca?O vibration A_2u (254cm^?1) might be related to superconductivity of Bi-based family.     

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 of high Jc NdBa2Cu3Ox/Nd4Ba2Cu2O10 composite superconductor

Single crystalline NdBa₂Cu₃O_x (Nd123) superconductors with dispersed Nd₄Ba₂Cu₂O₁₀ (Nd422) particles were produced by the floating zone partial melting and solidification (FZPMS) method. The initial composition of the precursor material was Nd_1.8Ba_2.4Cu_3.4O_x with or without 0.1 wt% Pt addition. FZPMS was carried out in a low oxygen partial pressure atmosphere. Microstructure of quenched samples were investigated by optical microscopy, scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). Furthermore, superconductive properties were measured by superconducting quantum interference device (SQUID). The results of SQUID measurements indicate that the critical temperature (T_c) of the samples with Pt addition so produced with the oxygen heat treatment (623 K for 300 h) was 94 K and the critical current density (J_c) was 2.3×10⁴ A/cm² at 77 K, 0.2 T.