Q.Y. Hu

Formation mechanism of high-Tc and critical current in (Bi,Pb)2Sr2Ca2Cu3O10/Ag tape

Abstract X-ray diffraction method was used to characterise the phase composition and investigate the formation mechanism of the (Bi,Pb)2Sr2Ca2Cu3O10 phase from the precursor with (Bi,Pb)2Sr2CaCu2O8 as the main phase. The reaction is found to be a two-dimensional nucleation (random)-growth type, −[ ln (1 − F)] 1 2 = kt , where F is the conversional fraction of (Bi,Pb)2Sr2CaCu2O8 and t is the sintering time. The size of the critical current of the tape is quantitatively related to the conversional fraction of (Bi,Pb)2Sr2CaCu2O8 to (Bi,Pb)2Sr2Ca2Cu3O10 phase. At low fraction regime of (Bi,Pb)2Sr2CaCu2O8, the critical current of a tape shows no clear dependence on the remaining Bi-2212 phase, other factors such as grain alignment, colony size, contact between colonies, and fine nonsuperconducting particles become important in controlling the Jc. Also, in this regime, the predominant weak links to be the colony boundaries rather than the Bi-2212 phase.

Anisotropy of the critical current in silver sheathed (Bi,Pb)2Sr2Ca2Cu3O10 tapes

The transport critical current of Bi:2223 tapes was measured in magnetic fields up to 15 T and at temperatures from 4.2 to 84 K. At high temperatures, the critical current density Jc is strongly anisotropic and the anisotropy increases rapidly with magnetic field, whereas at low temperatures the critical current is less anisotropic and the anisotropy is field independent above 1 T. In the former case we believe we are in a regime, where pinning limits Jc, at least within some parts of the tape, whereas in the latter case the limitation of Jc by Josephson weak links seems to be the dominant mechanism. In addition, a critical current hysteresis induced by flux trapping in a weak link network is observed, which is more pronounced at low temperatures. From transmission electron microscopy observations of the microstructure we find that the ‘‘brick’’ in the ‘‘brick wall’’ model turns out to be the colony instead of the grain inside the colony. Additionally it is found that colony boundaries parallel the ab pla...

Introduction of pinning centres into Bi - (Pb) - Sr - Ca - Cu - O superconductors

High critical current density is essential for most large-scale applications of high-temperature superconductors (HTSs). In addition to the weak link problem, weak flux pinning is a major cause of the rapid decline of with magnetic field at high temperatures. Through intensive research in the past 10 years the weak pinning problem has been partially overcome and has reached a level approaching the requirement for some commercial applications. In this article, a number of techniques by which effective pinning centres can be introduced to improve flux pinning in Bi - (Pb) - Sr - Ca - Cu - O HTSs are reviewed. These include surface pinning through a spiral growth, fine precipitates produced through phase transformation or phase formation - decomposition, inclusions through chemical doping or addition, cascades and columnar defects created by fast-neutron and heavy ion beam irradiation and various defects induced by mechanical deformation. In particular, the results of Bi-2223 formation - decomposition and cold deformation versus hot deformation have a significant implication for HTS wire fabrication. Comparative studies with comprehensive electrical, magnetic and microstructural characterizations have been carried out to assess the effectiveness of these techniques.

Angular dependence of critical currents and grain misalignment in Ag clad (Bi,Pb)2Sr2Ca2Cu3O10 tapes

Abstract Two-dimensional behaviour of the critical current in highly textured silver clad (Bi,Pb) 2 Sr 2 Ca 2 Cu 3 O 10 tapes was observed and analysed by introducing an effective grain misalignment angle, ϕ eff . This angle was found by SEM to be identical to the average crystallographic grain misalignment angle in the superconducting core. Furthermore, after fast neutron irradiation, which is isotropic, the J c of the tapes were modified by the introduction of artificiale defects, but the ϕ eff remained the same. This proves that the crystallographic misalignment of the grains determines the critical current anisotropy of the tape in a magnetic field, i.e. I c ( B tape plane) scales with I c ( B tape pane) with the factor sin ϕ eff . Therefore, ϕ eff is an intrinsic property of the tape. ϕ eff is found to be around 10° in different samples. We propose that this typical value is determined by intrinsic mechanical properties of the (Bi,Pb) 2 Sr 2 Ca 2 Cu 3 O 10 compound and is a result of the mechanical deformation of the tape during the tape fabrication. From a comparison of different samples, we propose that the pinning ability is the determining factor of J c when ϕ eff small enough.

Effect of hot pressing on the weak-link behaviour of Ag clad Bi based superconducting tapes

Abstract The effect of hot pressing on the critical current density of Ag clad Bi based superconducting multifilamentary tapes was investigated. The critical current density for the hot-pressed tapes was significantly improved in comparison with the cold-pressed tapes. The enhancement of J c is essentially attributable to the improvement of strong links between grains. The hot pressing has also resulted in a very smooth interface between Ag and oxide core, reducing the random growth of grains into silver sheath. The ratio of I c through strong links to the total I c increases from 21% for cold-pressed tapes to 33% for the hot-pressed ones. The SEM images show that the hot-pressed multifilamentary tape has higher density and better grain alignment than those without hot pressing.

Effect of mechanical deformation on the mass density of Ag-clad (Bi,Pb)2Sr2Ca2Cu3O10 wire and tape

Abstract The mass densities of Ag-clad (Bi,Pb) 2 Sr 2 Ca 2 Cu 3 O 10 wire and tape varies during the mechanical deformation process, which is one of the steps of the oxide-powder-in-tube technique used to fabricate the composite superconductor. Results show that the rolling has a more significant effect on densifying the tape core, whereas the drawing process can only densify the core to about 75% of the theoretical density. SEM observations of the rolled samples also reveal very dense morphology, consistent with the mass density calculations. SEM observation also shows that with increasing the deformation extent, the average grain size is reduced. It is proposed that although the rolling densifies the tape core significantly, it also destroys the crystallinity of the superconducting phases and results in the formation of an amorphous phase. Since the textured Bi-2223 phase forms by the epitaxial growth on the textured Bi-2212 seed crystals, the deformation induced texture is critical. Appropriate deformation extent is necessary, since too high an extent of deformation may change the well-aligned grains into amorphous phase. The formation of the amorphous phase is harmful to the texturing formation of the Bi-2223 phase, which finally leads to critical current degrading.

Effect of silver on the processing and properties of Bi-2223Ag tapes

The effect of Ag addition and Ag sheath on the processing and properties of Ag-clad Bi-2223 tapes has been studied. Ag addition in the Bi-2223 tapes lowers the annealing temperature and accelerates the formation of 2223 phase. Ag is inert to the oxide core and exists as an isolated phase within the tape. However, its undesirable shape and large size cause grain misorientation and hence leads to a decrease of Jc. For Ag-sheathed Bi-2223 tapes, the critical strains for bend tests show a dependence on the tape thickness, the number of filaments and Agoxide volume ratio, attributable to the improved strength and flexibility of the special interfacial layer between Ag and oxides. The results of magnetoresistance and the V-I curves of a well defined tape after correction for the parallel conduction through the Ag-sheathing and the core show a similar behaviour as single crystals and the epitaxial thin films. The effective activation energy for the flux creep is larger than those for the single crystals and thin films. Long length Ag-sheathed Bi-based superconducting multifilamentary tapes up to 45 m have been fabricated using powder-in-tube technique. A Ic up to 15 A and Jc of 8500 A/cm2 at 77 K over the entire length has been achieved. The fluctuation of Ic along the length is less than 10%.

Fabrication of Ag sheathed Bi2223 tape with supercold rolling process

Abstract A novel mechanical deformation process has been developed to fabricate high quality HTS tapes. In this process, the rolling step in the powder in tube technique is replaced by a supercold rolling conducted in liquid nitrogen. The critical current density of the Ag sheathed Bi2223 tapes is significantly enhanced by the supercold rolling. The microstructure observations of the tapes demonstrate that the supercold rolled tape has higher mass density and better grain alignment than the room temperature rolled tape, which are responsible for the high critical current density.

The effect of annealing and mechanical deformation on the grain alignment of (Bi,Pb)2 Sr2Ca2Cu3O8+x superconductors

Abstract Mechanical deformation and annealing treatments are widely used in the processing of (Br,P) 2 Sr 2 Ca 2 Cu 3 O 8+ x superconductor wires to produce grain alignment. Annealing texture has been suggested as one of the possible grain alignment mechanisms, but very experimental evidence and understanding have been reported. The present paper reports a detailed study of the relationships of mechanical deformation, annealing, grain alignment and grain size in Ag-sheathed Bi2223 tapes. Results indicated that (1) the (00 l ) grain alignment on the specimen surface after annealing is considerably higher than before annealing, (2) the surface grain alignment decreases with the increasing mechanical deformation, and (3) the grain size after crystallisation decreases with the increasing mechanical deformation. Based on the experimental results, the mechanisms of annealing/recrystallisation induced grain alignment were discussed. This understanding can be used to control the microstructure of high temperature superconductors in order to improve their transport properties.

Cryogenic deformation process of high temperature superconductors

A cryogenic deformation process has been developed and investigated for use in the processing of Ag/Bi-Pb-Sr-Ca-Cu-O superconducting wires. A typical cryogenic deformation process used comprised rolling or pressing the wires into tapes at 77 K and then heat treating the tapes at 838- for a period of 40-60 h. It was found that the cryogenic deformation improved the density, grain alignment, Ag/oxide core interface and critical current density. Critical current densities for cryogenically rolled and pressed Ag-clad mono- and multifilamentary tapes showed a 10 to 20% increase over those observed in normal processed tapes. In comparison with hot deformation and processed tapes, the cryogenic processed tapes showed improved flux pinning capability whereas the hot deformation was found to only improve weak links. During cryogenic deformation, the hardness of the silver sheath was significantly increased, which allowed a greater pressure to be applied during the deformation process. This large pressure was found to be the principal cause of the enhanced densification, texturing and increased dislocation densities.