Bi-2212 Grains Research Articles

Nucleation and texture formation of Bi 2Sr 2CaCu 2O x grains on (Sr,Ca) 14Cu 24O x plates in the Bi 2Sr 2Ca 1Cu 2.1O x/Ag system

During the partial melt-growth in the Bi 2Sr 2CaCu 2.1O x /Ag system, a Bi 2Sr 2CaCu 2O x (Bi-2212) phase formed via a reaction between a liquid and an (Sr,Ca) 14Cu 24O x (14:24) phase. The Bi-2212 grains nucleated and grew with a c-axis perpendicular to the interface of the 14:24 grains. A texture of Bi-2212 superconducting grains, having a c-axis perpendicular to silver sheaths, formed via two steps: the alignment of the textured Bi-free grains on the silver-oxide compact interface and the formation of the textured Bi-2212 grains on the 14:24 phase with the same crystallographic orientation.

Effect of MgO and Ag 2O on the microstructure and superconducting properties of the (Bi,Pb)-2223 phase in the partial-melting and sintering process

The effect of MgO and Ag 2O on the microstructure and superconducting properties of the (Bi,Pb)-2223 phase during sintering after partial melting has been investigated. In the Ag/oxide interface layer, a well-aligned Bi-2223 phase was preferentially formed. Both the degree of texturing and the volume fraction of the Bi-2223 phase decreased by increasing the distance from the Ag interface. Whereas, in the MgO/oxide interface layer, although well-aligned Bi-2223 grains existed at the MgO interface, a larger volume fraction of the Bi-2223 phase appeared in the center rather than at the MgO interface. The flux pinning was not clearly improved by the addition of MgO, Ag 2O or PtO 2 particles. However, MgO addition did not affect the formation rate of (Bi,Pb)-2223 phase, and could suppress the growth of Bi-free non-superconducting secondary phases. Ag 2O or PtO 2 addition resulted in coarse secondary phases, and slowed the formation rate of the Bi-2223 phase.

Optimization of calcination conditions on the Bi-2223 kineticformation and grain size

The effects of the calcination conditions (time, temperature,intermediate milling) on the kinetics of formation of Bi-2223 and on thegrain size were studied using a powder precursor synthesized by the polymermatrix method. The samples were characterized by XRD and SEM analysis. Thegrain size and the kinetics of formation of Bi-2223 strongly depend on thecalcination conditions and thus on the phase assemblages formed at the endof the calcination that determines the reactivity during sintering. Thehigher the calcination temperature is the larger the grain size. We haveobserved that 24 h of calcination at 820 °C without intermediatemilling allows one to obtain 79% of Bi-2223 phase in 60 h of sintering at835 °C.

The influence of precursor powders on the phase evolution, grain size and transport current properties of BSCCO 2223 Ag-sheathed tapes

The BSCCO 2223 Ag-sheathed tapes were prepared with spray-dried precursor powders, which are of different starting conditions. The J c in the short fully reacted tapes varied from 24 to 47 kA/cm 2 at 77 K and 0 T. The great improvement of J c in one tape, which is characterized by the large textured Bi-2223 grains, homogeneity of reactant and fewer tracks of second phases in the final superconducting matrices, is probably a result of optimizing the precursor powder.

Fabrication of laminated Bi-2212/Ag multifilamentary tape

The powder-in-tube (PIT) process has been successfully used to make long lengths of Ag-sheathed oxide superconductors. A modified PIT approach is proposed to fabricate conductors with laminated Bi-2212 configurations. Ag/Bi-2212 tapes consisting of seven laminae were produced with various thicknesses ranging from 0.38 to 0.25 mm. The use of a turkshead was found to be beneficial, yielding tapes with good dimensional integrity and consistency. Critical current density (Jc) values greater than 105 A cm-2 (0.01 T and Btape surface) have been attained for tapes of different thicknesses. Nonetheless, Jc was found to be related to the average thickness of individual Bi-2212 lamina. It is believed that excessive cold working accounts for the decrease in Jc with decreasing size below a threshold value. The enhancement of Jc is attributed to the high Bi-2212 grain alignment along the Ag-oxide interfaces and uniform dimensions within the laminate conductors.

Magnetic field and temperature dependence of critical current density in Hf-doped Bi-2223/Ag–Cu alloy sheathed tapes

Bi-2223/Ag–Cu alloy sheathed tapes have been fabricated by the powder-in-tube technique using Ag–10 at.% Cu alloy sheath doped with Ti or Hf of less than 0.1 at.%. Critical current density ( J c) was measured as a function of temperature between 30 and 90 K in the magnetic field up to 3 T. The flux-pinning force density ( F p= J c× B) was also calculated. Normalized pinning force ( f= F p/ F p max ) vs. normalized field ( b= B/ B s) curves for the Hf-doped samples were compared with those of non-doped Bi-2223/pure Ag and Bi-2223/Ag–Cu tapes. High J c values of 9.5×10 4 A/cm 2 at 30 K, 0 T and 4.5×10 4 A/cm 2 at 30 K, 3 T were obtained for the Hf-doped samples. The Hf-doping caused no substantial change in superconducting critical temperature ( T c). The Hf-doping seems to be effective to enhance Bi-2223 crystallization and to improve grain alignment as well as electrical connection between Bi-2223 grains. The n value of Hf-doped sample reached about 13 at 30 K, 1 T, which is lower than 23 of non-doped Bi-2223/pure Ag.

Processing of Ag-doped Bi2212 bulks in high magnetic fields: a strong correlation between degree of texture and field strength

A series of high magnetic fields up to 9 T have been applied to the preparation of Ag-doped Bi2212 bulks and a strong correlation is observed between the field strength and the degree of texture developed in the bulks. Two powders of Bi 2.2Sr 1.8CuO x and CaCuO 2 were synthesized separately and then 10 wt.% Ag was doped to their equimolar mixture, from which the Bi2212 bulks were formed by a partial melt–solidification process in the high magnetic fields. The degree of texture in the bulks was evaluated quantitatively by X-ray diffraction and magnetization measurement. It is found that the applied magnetic fields tend to orient Bi2212 grains with their c-axis parallel to them and obvious texture exists in all samples prepared in the magnetic fields; while in samples prepared without magnetic field Bi2212 grains orient randomly. In the field range of 0–9 T, the degree of texture increases notably with increasing the magnetic field. In 9 T Bi2212, bulks with magnetization anisotropy factor more than four and a density of 6.15 g/cm 3 are obtained and SEM observation also shows a highly oriented grain alignment.

Phase stability and grain growth in an Ag/Bi-2223 composite conductor prepared using fine-grained Bi-2223 as a precursor

We have investigated the stability and microstructural transformability of the Bi-2223 phase in a silver-sheathed monofilament composite tape fabricated using fine grained Bi/sub 1.7/Pb/sub 0.3/Sr/sub 1.9/Ca/sub 2.0/Cu/sub 3.0/O/sub y/ (Bi-2223) as the precursor powder. The fully formed Bi-2223 precursor was prepared using established procedures. The purpose of this study was to explore the prospects for growing textured, large-grain-size Bi-2223 from the fine-grained precursor by process parameter perturbations. These perturbations included thermal ramp up variations, programmed heat treatment temperature and oxygen pressure fluctuations, and parameter manipulations during cool-down. Our results show that the types of heat treatments used in conventional oxide-powder-in-tube (OPIT) processing do not facilitate Bi-2223 grain growth when the precursor powder is preconverted Bi-2223. We also observed that the Bi-2223 partially decomposed during conventional thermal ramp-up in 0.075 atm O/sub 2/, but that this decomposition can be inhibited by ramping up in a reduced oxygen pressure. A pathway was found for back-reacting the fine-grained Bi-2223 (to Bi-2212, Bi-2201 and nonsuperconducting secondary phases), then reforming large-grained Bi-2223 in a colony microstructure having some distinct differences from that produced during conventional OPIT processing.

Near-net-shape fabrication of continuous Ag-clad Bi-based superconductors

We have developed a near-net-shape process for Ag-clad Bi-2212 superconductors. This alternative to the powder-in-tube process offers the advantages of nearly continuous processing, minimization of processing steps, reasonable ability to control the Bi-2212/Ag ratio, and early development of favorable texture of the Bi-2212 grains. The powder is rolled into an Ag channel, a cap is applied, and the assembly is lightly rolled. To minimize distortion during heat treatment, it is especially important that the Bi-2212 powder be nearly free of volatile species such as carbon. A vacuum-calcination process was developed to control CO/sub 2/ evolution during heating. Superconducting properties are discussed.

Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub x//Ag multilayer tapes with J/sub c/ (4.2 K, 10 T) of 500,000 A/cm/sup 2/ by using PAIR process

We developed PAIR (pre-annealing and intermediate rolling) process, which is a fabrication procedure for Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub x/(Bi-2212)/Ag composite tapes, in order to improve their superconducting properties. PAIR process is the combination of pre-annealing (PA) and subsequent intermediate rolling (IR) processes and is performed prior to a melt-solidification process. By performing the PAIR process, Bi-2212 grain alignment and intergrain connectivity are much improved and a large J/sub c/ enhancement can be expected for Bi-2212/Ag conductors. We study the effects of pre-annealing and intermediate rolling processes on microstructure and superconducting properties of the Bi-2212/Ag multilayer tapes fabricated by the oxide-painting and lamination method. The results show that J/sub c/ has been increased strongly by performing pre-annealing at 1133 K in oxygen (1 atm) and intermediate rolling with 25% deformation. PAIR processed samples have transport J/sub c/-oxide (4.2 K, 10 T) exceeding 5.0/spl times/10/sup 5/ A/cm/sup 2/. Whereas J/sub c/-oxide for the samples melt-solidified without PAIR process remains 3.0/spl times/10/sup 5/ A/cm/sup 2/. The enhancement of J/sub c/ is assigned to the microstructure with high grain alignment and uniformity, as well as the improvement in grain connectivity achieved by the PAIR process.

Influence of Ag substrates on grain alignment and critical current density of Bi-2212 tape conductors

We investigated the influence of Ag substrate on the microstructure and J/sub c/ values of dip-coated Bi-2212/Ag tapes. The grain alignment of Bi-2212 in the oxide/Ag interface was much higher than that in the free surface of oxide. J/sub c/ increases with increasing oxide/Ag interface area and with decreasing oxide layer thickness, while J/sub c/ was almost independent of the free surface area. The smoothness of the Ag substrate is also an important factor. Micrometer-size roughness of the Ag substrate results in a decrease of Bi-2212 grain alignment and therefore J/sub c/. Addition of alloying elements to the Ag also degrades the grain alignment of Bi-2212 and decreased J/sub c/. Cold rolling before the heat treatment improved the grain alignment and enhanced J/sub c/.

Isotropic pinning in heavily Pb-doped Bi-2212/Ag tapes

Heavily Pb-doped Bi-2212/Ag tapes have been prepared using an isothermal partial melting (IPM) method. Under heavy Pb doping, irreversibility field ( H irr) for magnetic fields parallel to the c-axis ( H‖ c-axis) is elevated at temperatures higher than 20 K and is comparable to H irr for H‖ ab-plane. As a result, the original anisotropic pinning properties disappear in Pb-doped tape. At 30 K, the isotropic pinning force suppresses the anisotropy of transport critical current density ( J c) under magnetic field which is the most serious problem for power applications of Bi-2212 materials. Transmission electron microscopic (TEM) observation shows a disordered stripe structure and stacking faults along the c-axis in the bc-plane of Bi-2212 grains in Pb-doped tape. Such a stripe structure or stacking faults seem to be related to the production of strong pinning centers by heavy Pb doping.

Evidence for enhanced grain connectivity due to second phase reduction of Bi-2223/Ag tapes

Processing of Bi-2223/Ag tapes in a three-stage sintering process with reduced last processing temperature and intermediate cooling to room temperature leads to enhanced critical current densities. X-ray diffraction analysis shows that this is accompanied by the reduction of second phases like Bi-2201 and Bi-3221 in the superconducting core. Electron microscopy exhibits that these phases tend to precipitate between the Bi-2223 grains, where they can hinder the current path. The results are compared with those of a previously reported two stage sintering procedure. A complicated reaction between 3221, alkaline earth cuprates, Bi-2212 and Bi-2223 is proposed as a tentative explanation for the reduction of 3221.

Phase formation and microstructure development in silver-clad Bi-2223multifilament composite conductors

Phase evolution and microstructure development in silver-clad Bi-2223 composite conductors have been investigated as a function of filament count (mono, 19, 85 and 361), precursor powder type (metallic powder or oxide powder), oxygen partial pressure (<10-3 to 0.13 atm), temperature (790-855 °C) and time (0-6000 min). Favourable conditions for rapid Bi-2223 phase formation and stability were achieved with an oxygen partial pressure around 0.08 atm, regardless of precursor type or filament count. In general, the rate of the Bi-2223 formation reaction increased and the width of the temperature window for rapid kinetics became broader as the filament count increased. The most rapid kinetics and the broadest heat treatment window were observed for the 361-filament metallic powder composite. These trends in Bi-2223 formation rate and stability are believed to be associated with the increased proximity of powder to silver as the filament count increases and the filament transverse dimensions become smaller. For each conductor type (in terms of precursor form and filament type) there was an optimum processing temperature in ca 0.08 atm O2 that produced a well-developed Bi-2223 grain colony microstructure with the least amount of second-phase. This optimum temperature was near the peak of the reaction rate versus temperature profile.

Improvement of J c and Grain Alignment of Bi-2212/Ag Multilayer Tapes by Pre-Annealing and Intermediate Rolling Process

The high transport critical current density (Jc) > 500 kA/cm2at 4.2 K, 10 T is obtained in the Bi-2212/Ag multilayer tapes fabricated by using PAIR (Pre-Annealing and Intermediate Rolling) and melt-solidification process. This Jcvalue is twice higher than existing high-quality Bi-2212/Ag tapes (250 kA/cm2). By applying PAIR process to Bi-2212/Ag tapes, Bi-2212 grain alignment is much improved and a large Jcenhancement is achieved. Jchas been increased strongly by performing pre-annealing at 840°C in oxygen (1 atm) and intermediate rolling with 25% deformation.

Enhancement of critical current density for Bi-2212/Ag tape conductors through microstructure control

We investigated the microstructures of Bi 2Sr 2CaCu 2O y (Bi-2212) at the free-surface and at the interface with silver substrate. The alignment of Bi-2212 grains near the silver substrate was better than that near the free-surface. Based on this result, we fabricated tape conductors having various cross-sectional configurations. J c increased with increasing oxide/Ag interface area and with decreasing oxide layer thickness. However, J c was almost independent of the area of the free-surface. These results suggest that most of the superconducting current flows locally near the silver substrate. Furthermore, we found that the alignment of Bi-2212 grains, and hence, J c can be improved by decreasing cooling speed during the heat treatment. The application of cold-rolling to the tape before the heat treatment is also effective in improving the microstructure and J c of Bi-2212/Ag tape. About 2 times larger I c and J c were obtained by ∼50% reduction of the oxide layer. The highest I c and J c obtained in this experiment were I c=450 A and J c=3.5×10 5 A/cm 2 (in 10 T, at 4.2 K), respectively.

Phase development and kinetics of high temperature Bi-2223 phase

The two-dimensional nucleation (random)-growth mechanism were observed as a support for the related previous works, which is attributable to the growth of the Bi-2223 grain in the a– b plane direction of the Bi-2212 matrix is being much faster than in the c-direction, or that the early-formed plate-like 2212 phase confines the 2223 product. At the beginning of the reaction, the additional phases are partially melted. Because of the structure, composition and thermal fluctuation, the 2223 nucleates and grows in the phase boundary between the liquid phase and Bi-2212. It was shown here that the nucleation and the growth rate were relatively fast between 0 and 36 h. At the final stage, between 36 and 60 h, because of the impingement of the growth fronts of different nuclei, the high formation rate of 2223 is suppressed. The major reactant 2212 remains as a solid and its plate-like configuration determines the two dimensional nature of the reaction. The amount of liquid forms during reaction is small.

Recrystallization effects and grain size in Bi-2223 tapes

Bi-2223 precursor powders with different stoichiometries and synthesized using different heat treatments were compared with powders obtained from external sources in powder-in-tube tapes. In the final microstructure, it was found that the Bi-2223 grain size varied considerably between tapes made from different powders. The critical current density was found to increase with increasing grain size. Among the factors found to determine the Bi-2223 grain size were powder stoichiometry and Bi-2212 recrystallization rate.

Characterization of high critical temperature superconducting wires

Improvements in the current transport properties of high critical temperature superconductors would be facilitated by a quantitative description of the relationship between current capacity and superconductor microstructures. In this study, three types of silver–Bi2212 tapes were characterized in terms of critical current density and various microstructural features. Tapes given melt processing treatments were found to have a higher critical current density, larger apparent Bi2212 grain size, and stronger c-axis texture compared to the results of a lower temperature treatment. A comparison of (001) pole figure data and current transport measurements indicated that a 3-fold increase in the c-axis angular spread corresponded to an order of magnitude drop in critical current density.

Phase compatibility and preparation of composite

The chemical compatibility of the phase with was investigated, and a composite superconductor was prepared. Ceramic processing of the corresponding oxide and carbonate mixture at led to the two-phase species , while the solubility of In in Bi-2212 was less than 5 mol%. At indium appeared to be less soluble in the BSCCO melt, instead being concentrated in fine grains of phase suspended in the liquid. By slow crystallization of the melt the composite was obtained. The In-rich phase formed micron-sized rods which were both randomly embedded in Bi-2212 lamellas and agglomerated on the Bi-2212 grain boundaries. The presence of the indium-based surplus did not prevent Bi-2212 phase formation, had minor effect on and resulted in increase of flux pinning at K.