Development of high-Tc superconducting Bi-2223 tape joints

High-T _c superconducting joints between Ag-clad Bi-2223 tapes have been developed for persistent current applications. Two presintered tapes with one side of the silver stripped were lapped and then wrapped by a silver foil. The complex was uniaxially pressed followed by appropriate sintering to form a high-T _c superconducting tape joint. It was found that the ratio of critical currents through the joint to that of the tape,I _cj/I_c, depended on the uniaxial pressure and the sintering conduction. At liquid-nitrogen temperature 77 K,I _cj/I_c=99% has been achieved. Persistent current loops formed by Bi-2223 tapes have also been fabricated and tested. Joint resistance of a loop was determined to be 4×10^–13 between the decay time of 120 and 3600 sec.     

Casting high-T c superconducting BiSCCO

Bi-Sr-Ca-Cu-O (BiSCCO) was melted and cast into molds in order to render the superconductor into fully dense and useful shapes. Special conditions for casting and subsequent heat treatments were required to ensure structural integrity and a high transition temperature,T _c .The history of the castings during heat treatments was studied by thermal analysis, X-ray diffraction, and microscopy.     

Bi系2223高温超导带材力学性质

对4种Bi系2223高温超导带材77 K下的力学性质进行了实验研究.通过选择合适的合金包套替代传统的银包套,带材的机械性能能有所提高,但电学性质有所降低,尤其是内包套采用合金、外包套采用银,其机械性能反而低于纯银包套带材.通过X射线衍射图谱发现,当包套采用合金材料,尤其是内包套采用合金、外包套仍采用纯银,会大大降低样品中2223相的体积含量.相比较临界电流密度,n值对材料的破坏更为敏感.利用X光同步辐射技术初步探讨了银包套带材在室温和77 K下的损伤破坏过程.     

Development of Bi-2223 HTS tape and its application to coil and current leads

We are developing Bi-2223/Ag tapes with a high engineering critical current density by optimizing the powder-in-tube process and are studying its application to coil and current leads. We have fabricated 250 m-long tape and investigated optimized processing conditions to enhance engineering critical current density. More bubbling was found when the tape was heat-treated with a higher heating rate. Different kinds of superconducting joints were fabricated with multi-filamentary Bi-2223/Ag tapes, and 58% of retained I_c was achieved using the insertion of Bi-2223 core between two exposed tapes. Current decay property of the persistent mode HTS coil was investigated. Rapid current decay was observed when the operating current is in a flux-flow range. We could successfully fabricate a low heat leak type HTS current lead with Bi-2223/Ag–Au tapes by employing a stepped geometry. Using this lead, safe operation of 2 kA current transport was confirmed.     

Long multifilament Bi-2223 Ag-sheathed superconducting tapes and solenoids

Multifilament Ag-sheathed BiPbSrCaCuO (2223) superconducting tapes containing 49 filaments were fabricated by the powder-in-tube route and the roll-anneal process. The transport critical current densityJ _c was 1.3×10⁴ A cm^–2 at 77 K and 7×10⁴ A cm^–2 at 4.2 K in self-field. A 12-m-long tape was used to construct superconducting solenoids (50, 28, and 14 mm internal diameters) generating dc fields 380–1070 G at 4.2 K. Measurements of the variation ofJ _c with field (0–1.6 T) and bend strain (0–5%) are used to explain the performance of the solenoids. The critical bend strain of tapes was about 1.5%.     

Phase-transition toughening of high-T c superconducting ceramics

The effect of the superconducting transition (SCT) on the plane-strain fracture toughness K _1c of YBCO and BSCCO is studied by the method of three-point bend loading at room temperature and at 77 K. Toughening is observed and related to variation of the Gibbs free energy through the SCT, and hence to the condensation energy of a Cooper-pair gas. This is explained by the additional energy needed to excite superconducting electrons to normal ones in crack surfaces owing to magnetic field penetration. The superconducting gap parameters of YBCO and BSCCO are estimated and the results are reasonable.     

Influence of preparation conditions on superconducting properties of Bi-2223 thin films

We report electrical transport properties of Bi₂Sr₂Ca₂Cu₃O_10+x (Bi-2223) superconducting thin films fabricated by pulsed-laser deposition on SrTiO₃ substrate. The aim of the study was to investigate the influence of preparation conditions such as deposition temperature (T _S), annealing time (t _A) and deposition rate (r). A critical temperature (T _c) as high as 110 K and critical current density (J _c) of 6·2 × 10⁶ A/cm² at 20 K were obtained for T _S = 760°C, t _A = 4 h and r = 1·5 Å/s. We also investigated the effect of Li doping on Bi-2223 thin films. Li intercalation results in high resistive onset transition temperature and the resistivity shows broadening in magnetic field that increases with field. The large broadening of resistivity curve in magnetic field suggests that this phenomenon is directly related to the intrinsic superconducting properties of the copper oxide superconductors. The sudden drop in J _c at relatively low magnetic field (H < 0·5 tesla) is due to the effect of Josephson weak-links at the grain boundaries.     

Fabrication of a working Bi-2223 superconducting magnet cooled by liquid nitrogen

A practical Bi-2223 superconducting magnet, working in liquid nitrogen (L.N₂), was designed and fabricated. Bi-2223 tape with a critical current of 147 A was prepared by a controlled overpressure (CT-OP) process at 77.3 K in self-field. Ten double-pancake coils were resistively connected by copper terminals. The bore diameter was 54 mm?, the magnet outer diameter was 122 mm?, the height of the magnet was 124 mm, and the weight of the magnet was about 3 kg. The maximum magnetic field at the center of the bore was 0.48 T with an operating current of 50 A. The experimental results agree well with design predictions calculated by finite element method. AC operation was also performed, and no distortion of the voltage waveform was observed. Therefore, this Bi-2223 superconducting magnet is a suitable replacement for copper magnets designed for applications in science and technology.     

The J c Limit of Multifilamentary Ag/Bi-2223 Tapes

To improve on present critical current (J _c) performance, multifilamentary Ag/Bi-2223 tapes with a large range of reduction rates were manufactured. The relative core mass density D was calculated, dependent on the measured geometric dimensions of the tapes. Experimental results, D vs. J _c, D vs. maximum pinning force density F _max , and D vs. irreversible magnetic field B _irr, are quantitatively formatted. In particular, the magnetic field dependence of J _c is critically dependent on its core density. If the core density increases by 10%, J _c of the tapes in this experiment is enhanced by as much as 100%. Therefore, in the present state of the technological process for manufacturing Ag/Bi-2223 tape, increasing the core density is clearly a significant strategy in improving the electronic and magnetic properties of the tapes and enhancing the capacity for carrying current at high magnetic fields. The limit of the bulk self-field-J _c can be calculated by the relationships of J _c vs. D. The limit is estimated to be on the order of 200 kA/cm² for multifilamentary Bi-2223 tapes, which was supported by magneto-optical (MO) magnetization measurements results. It is a hard task to approach this limit with the present state of the art in manufacturing Ag/Bi-2223 tape, and it is the time to suggest some new ideals for Bi-2223 tapes to promote large-scale applications.     

Nonlinear behaviour of high-T c superconducting bridges at microwaves

We have experimentally investigated the phenomenon of coherent hypervortices motion in highT _c superconducting Y-Ba-Cu-O thin film bridge-like structures. Narrow band internal radiation at frequencyf=21 GHz with intensityP=3×10⁻¹¹ W was detected. The radiation processes, occurring around the singularity of negative differential resistance at current-voltage characteristics of bridge-like structures allows one to consider their further application for sensitive microwave reception.     

Electrostatic bonding of high-T c superconducting films to any substrate

Complete integration of high-T _c thin-film superconducting technology with other electronic technologies requires hybrid structures with both technologies on the same substrate. This is difficult to do with direct growth of the superconductor on certain substrates (GaAs, InP) because of the high temperatures required for formation of the superconductor. A method is proposed to circumvent this problem by using electrostatic forces and appropriate thin-film materials to bond superconducting films to any substrate at 300°C. The same principle can be applied to the bonding of other devices on other substrates.     

Demagnetization of a Bi-2223 high-temperature superconducting coil in RT-1 through spontaneous temperature rise

The Ring Trap 1 (RT-1) device produces a magnetospheric configuration for the confinement of a high-β plasma with a Bi-2223 high-temperature superconducting magnet. Here we report the results of emergency demagnetization of the superconducting coil, where we could not connect current leads, temperature measurement connectors, and connectors for a persistent-current switch (PCS) heater to the coil. The spontaneous warming of the coil caused a rise in the flux-flow resistance of the superconducting coil, and the persistent current slowly decreased as coil resistance increased. Approximately 98% of the total stored magnetic energy was safely released before the quenching of the PCS, and there was no substantial damage to the superconducting coil.     

Ac-loss measurement of small superconducting Bi-2223 test coils wound with multifilament barrier type conductors

Internally assembled ring-bundle-barrier Bi-2223 tapes with a Rutherford type stranded structure were optimised with respect to low coupling losses. They had resistive barriers of a mixture of SrCO₃ and SrZrO₃ around the strands and around the central Ag core. From these tapes small test coils were wound and the ac losses were measured with a sinusoidal ac transport current at 77 K. The loss measurement was performed by the standard magnetization technique. A calorimetric calibration yielded the correct absolute values of the losses. The results show that the main loss component is hysteretic, and coupling losses do not contribute significantly to the total losses. The suppression of coupling losses due to the resistive barriers leads however to a small critical current of the tapes and further optimisation with respect to losses and critical current is necessary.     

Ic degradation behaviors of Bi-2223 superconducting tapes under axial fatigue loading

For the endurance evaluation of High Temperature Superconductors (HTS), the mechanical and transport properties of multifilamentary Bi₂Sr₂Ca₂Cu₃O_10+x (Bi-2223) superconducting tapes with different reinforcements subjected to high-cycle axial fatigue loading were investigated at 77 K in the self-field. The mechanical fatigue limits based on the relations between the applied stress amplitude and the numbers of cyclic steps to reach failure (S-N_f curves) were obtained. The transport properties were evaluated with the increase of repeated cycles, N, at different applied stress amplitudes which eventually leads to the electric fatigue limit. The influence of reinforcement on the mechanical and transport properties of Bi-2223 tapes were discussed. Fractographic observation was performed in order to understand the I_c degradation mechanism in fatigue tested Bi-2223 tapes.     

CO2 laser annealing of plasma-deposited high-T c superconducting thick films

Recent advances in the fabrication of high-T _c superconducting thick films demand processing techniques which can eliminate film/substrate interdifiusion that occurs during subsequent post-annealing heat treatment after the film is deposited, thereby limiting the application of the thick films for devices. The present study evaluates laser annealing techniques for plasma-deposited Y-Ba-Cu-O thick films using a high-energy CO₂ laser (10.6Μm) in a continuous wave mode. The results are compared with those obtained by conventional furnace annealing techniques necessary for post-heat treatment of as-deposited superconducting thick films. The high-T _c superconducting phase is recovered by cationic diffusion during subsequent post-annealing heat treatment. Crystallographic phases and microstructural characterization have been performed using XRD, SEM, and EPMA analytical techniques. The significance of the technology lies in the elimination of film/substrate interdiffusion problems, thereby resulting in high-quality superconducting thick films. The technology will significantly reduce the post-annealing times usually required by conventional furnace annealing techniques.     

1.5-GHz high-T c superconducting microstrip bandpass filter of miniaturized configuration

A 1.5-GHz four-stage microstrip bandpass filter was designed with the miniaturized hairpin resonator structure to reduce the configuration in size. The filter was fabricated using high-T _c superconducting films on a 12 × 12 mm² LaAlO₃ substrate. The passband insertion loss of the filter was measured to be -0.4 dB at 77 K, while it was estimated by a planar electromagnetic simulator to around -19 dB when gold films are used instead of the superconducting films.     

Increased homogenous clusters in superconducting paths with diffusion of optimum Ni impurities into Bi-2223 crystal

This study deals with variations of electrical and superconducting features of Bi-2223 superconducting materials exposed to Ni impurity diffusion at different annealing temperatures (650 °C?≤?T?≤?850 °C) by temperature-dependent resistivity measurements. It is found that the characteristic properties improve with annealing temperature up to 700 °C as a result of enhancement in the truly-metallic characteristics, interaction quality, formation of Cooper-pairs and overlapping of Cu-3d and O-2p wave functions. Similarly, the optimum annealing temperature of 700 °C diminishes the omnipresent flaws and structural defects. Additionally, we design a strong theory (Percolation) to discuss the role of nickel impurities on fundamental aspects of material science and physical quantities as regards stabilization of superconductivity in the homogeneous regions and formation of superconducting clusters in the paths for the first time. Further, we develop an empirical relationship between the structural problems and transition temperatures to obtain a superconductor exhibiting the highest electrical and superconducting features.