Bi-2212 Grains Research Articles

Improvement of plastic deformation property and current carrying capacity of Bi2212 wires through low oxygen partial pressure post-annealing of precursor

The performance of superconducting magnets is greatly dependent on the size and uniformity of the superconducting filaments within the wire. However, in the fabrication of Bi2Sr2CaCu2Ox (Bi2212) wires using ceramic oxide as the superconducting filaments, we face multiple challenges including filament fragility, poor uniformity, and non-smooth Ag/superconductor interfaces. These challenges primarily stem from the significantly lower plastic deformation capability of ceramic oxide powder filaments compared to metal matrices. To address these challenges, this study focuses on the modification of ceramic oxide precursor powders. Through the utilization of low-temperature, low oxygen partial pressure (pO2) heat treatment, we have observed significant changes in the Cu chemical states and lattice parameters within the Bi2212 grains. These alterations have proven to be highly effective in reducing the energy required for grain sliding and, in turn, enhancing the plastic deformation properties of the Bi2212 oxide filament. Furthermore, post-annealing the oxide powder under low pO2 conditions narrows the gap in plastic deformability between the Ag sheath and the oxide filaments, resulting in improved filament uniformity and a smoother Ag/oxide interface in Bi2212 wires. Notably, the Bi2212 grains exhibit an orderly arrangement within the wires. As a result of these improvements, the critical current density (Jc) of the final wires increases by an impressive 70 %. The outcomes of this study are not only crucial for producing Bi2212 magnets with higher magnetic field strength, improved magnetic field uniformity, and enhanced operational stability, but also offer fresh insights and methodologies for advancing the field of superconducting materials.

The research on the influence mechanism of internal deformation for the performance of Bi-2212

Cable-in-conduit conductor (CICC), one of the most promising conductors for manufacturing magnets in magnetic confinement fusion devices, has attracted lots of attention. In the production of CICC, porosity control is necessary for its stability. The porosity control is usually realized by the diameter-reducing process, which would also lead to indentation damages to the elements of CICC-superconducting wires. In this article, systematic research of indentation damages was carried out on the next generation of high-temperature superconducting materials–Bi-2212 wires. The results indicate that the current carrying capacity of the indentation-damaged wire would first keep steady and then show exponential decline with the increase of indentation depth. The wires subjected to pre-overpressure (pre-OP) treatment exhibit slightly improved resistance against indentation damage at shallow indentation depths. However, their critical current (I c) deteriorates more rapidly at greater indentation depths when compared to wires that have not undergone the per-OP process. The following structural characterization analyzed the reasons for the property changes with the help of scanning electron microscopy, energy dispersive x-ray spectroscopy, computed tomography, and hardness tester. Misalignment between the Bi-2212 grains and shaped filaments was found in the indentation-damaged wires, with which the degradation in I c of the wires and the differences in properties of the two kinds of wires were further discussed.

Role of Y substitution for Ca-site on magneto-resistivity properties of Bi-2212 superconductor rods prepared by LFZ

This comprehensive study delves into the effect of partial Y3+ substitution for Ca2+ in Bi2Sr2CaCu2O8+y superconducting rods prepared by the laser floating zone method (LFZ). The critical transition temperatures (Tconset, Tc, Tcoffset), activation energy (U), irreversibility field (Hirr), upper critical magnetic field (Hc2), coherence length (ξ), and penetration depth (λ) are determined by means of magneto-resistivity measurements performed between 0 and 5 T. According to the resistivity-temperature graph for zero fields, Tcoffset values tend to decrease regularly from 90.8 K to 87.1 K, and the residual resistivity values increase steadily from 0.14 to 9.7 mΩ .cm with the Y-content. The change of these two parameters is a sign that Y-introduction into the Bi-2212 matrix undermines the inter-grain coupling and increases the level of permanent-defects in the crystalline structure. However, the relatively high Tcoffset values and extremely low residual resistivity, especially in the pure and low Y-substituted samples, reflect the success of the LFZ process in growing homogeneous and well-oriented Bi-2212 grains. The resistivity transition broadening is observed when the applied field increases and Tcoffset values decrease dramatically, i.e. from 90.8 K to 72.1 K for the pure sample and from 87.1 K to 48.2 K for the 0.20Y-substituted ones. The calculated U values using the TAFF model decrease with the increase of the Y-content and the magnetic field. In addition, the magnetic field and temperature dependence of U is expressed by using U(H,T) ∼ ΔT H−η equation. Deduced parameters of Hirr and Hc2 from magneto-resistivity curves and theoretical findings of ξ and λ also favor the degradation of U values with the augmentation of Y-content in the Bi-2212 rods.

Influences of Precursors Phase Composition on the Phase Evolution Dynamics of Bi-2223 Superconducting Tapes

Bi2Sr2Ca2Cu3O10+ (Bi-2223) precursors for the fabrication of high temperature superconducting tapes were prepared by spray pyrolysis technique followed by a calcination process. The influences of precursor powder phase composition on the phase evolution dynamics and final current capacity of Bi-2223 superconducting tapes were systematically studied. It was interesting to notice that slight increase of Cu content in chemical composition could cause obvious variations of both the phase composition and the morphology of CuO particles in precursors. Therefore, the Bi-2223 phase formation process during the sintering process of tapes could be affected, and the current capacity changed accordingly. On the other hand, during the calcination temperature optimization process, the Bi-2223 phase content in precursors changed accordingly, thus the Bi-2223 phase formation dynamics were also studied. More Bi-2223 nuclei were formed due to the pre-existed Bi-2223 grains, which worked as inhomogeneous nuclei centers. Thus, although the final Bi-2223 phase contents were similar in all the tapes, the particle size and texture structure of Bi-2223 varied greatly, which caused a dramatic decrease of critical current in Bi-2223 tapes with the formation of Bi-2223 in precursors. Therefore, it could be concluded that both the morphology of CuO particles and the Bi-2223 phase content in precursors were the factors controlling the phase evolution dynamics during Bi-2223 tape sintering process.

Microstructure and Fluctuation-Induced Conductivity Analysis of Bi2Sr2CaCu2O8+δ (Bi-2212) Nanowire Fabrics

Resistance measurements were performed on Bi2Sr2CaCu2O8+δ (Bi-2212) fabric-like nanowire networks or nanofiber mats in the temperature interval 3 K ≤T≤ 300 K. The nanowire fabrics were prepared by means of electrospinning, and consist of long (up to 100 μm) individual nanowires with a mean diameter of 250 nm. The microstructure of the nanowire network fiber mats and of the individual nanowires was thoroughly characterized by electron microscopy showing that the nanowires can be as thin as a single Bi-2212 grain. The polycrystalline nanowires are found to have a texture in the direction of the original polymer nanowire. The overall structure of the nanofiber mats is characterized by numerous interconnects among the nanowires, which enable current flow across the whole sample. The fluctuation-induced conductivity (excess conductivity) above the superconducting transition temperature, Tc, was analyzed using the Aslamzov-Larkin model. Four distinct fluctuation regimes (short-wave, two-dimensional, three-dimensional and critical fluctuation regimes) could be identified in the Bi-2212 nanowire fabric samples. These regimes in such nanowire network samples are discussed in detail for the first time. Based on this analysis, we determine several superconducting parameters from the resistance data.

Visualization of the grain structure in the filament cross sections of uniaxially textured high J c Bi-2223 tapes

In this study, we extensively used electron backscatter diffraction orientation imaging microscopy to visualize the grain structure in the flat-rolled (Bi,Pb)2Sr2Ca2Cu3O x (Bi-2223) tapes. The thermomechanical process made the grains’ c-axes oriented normal to the tape surface. The 24% difference in critical current density J c was caused by the ∼5° difference in the degree of out-of-plane texture. Although the in-plane orientations are not controlled, the Bi-2223 grains can form the domains, each of which consists of the grains with similar in-plane orientation. Controlling the domain formation could be the next protocol to raise the J c of Bi-2223 tapes.

A CLOSE CORRELATION BETWEEN DISORDERS-DEFECTS AND SUPERCONDUCTING TRANSITION TEMPERATURE OF Bi1.6Pb0.4Sr2Ca2-xNaxCu3O10+δ SUPERCONDUCTORS

The enhancements of zero superconducting transition temperature (Tc,0) in Bi1.6Pb0.4Sr2Ca2-xNaxCu3O10+δ(BPSCCO) are reported. The BPSCCO samples (with x ranging from 0.00 to 0.06) were prepared by using the conventional solid state reaction technique. The properties of the samples were examined by using the X-ray diffraction (XRD), scanning electron microscopy (SEM), and the resistance versus temperature (R-T) measurements. From the XRD resuts, all samples revealed the orthorhombic structure, and the volume fraction of the Bi-2223 phase was varied as increasing x, and reached the maximum value of 82.72 % for x = 0.05 sample. Improvements of connectivity between the Bi-2223 grains in the Na-substituted BPSCCO samples were obtained by using the surface SEM images and a quantitive analysis of a correlation between Tc,0 and residual resistance ratio (RRR). A similar variation of Tc,0 of the samples as increasing x was observed. The highest Tc,0 enhancement of 107.5 K was obtained for x = 0.05 sample. Variations of the hole carrier concentrations in the CuO2 layers were investigated, which also showed the highest value achieved for x = 0.05 sample.

Effect of high-temperature vacuum degassing on microstructure and current-carrying capacity of the Bi-2212 wire

The voids in Bi2Sr2CaCu2Ox (Bi-2212) long-length wire should be eliminated as far as possible in order to improve its current-carrying capacity. There are several reasons for the void formation in Bi-2212 wire, mainly including the gas adsorbed on powder and existed in the gaps among the powder particles. We have found that the high-temperature vacuum degassing, which can eliminate the gases in wire and suppress the void formation, is an effective method to manufacture the Bi-2212 long wire with high-performance. Moreover, the high-temperature vacuum degassing is beneficial for the arrangement of Bi-2212 grains and the improvement of the texture of filaments. The engineering critical current density (Je) of Bi-2212 wire made by this method reaches to 440 A/mm2 at 4.2 K and 5 T.

Density Enhancement for the Fabrication of Bi-2212 Superconducting Tapes

Abstract Bi2.1Sr1.96CaCu2.0O8+δ (Bi-2212) precursor powders were synthesized with co-precipitation process, and Bi-2212 single filament tapes were fabricated with powder-in-tube (PIT) technique. Based on SEM observation, the primary grain size of Bi-2212 precursor powders was 3∼5 μm. However, after sintering, Bi-2212 grains clustered together and the particle size distribution became quite wide, from 10μm to over 180 μm. By dividing the particles into four groups with different average particle sizes, the influences of average particle size on the filament density, microstructures and current capacity of final Bi-2212 tapes were investigated. Results show that with the decrease of particle size, the filament density increases over the entire cold-working process and heat treatment process. Therefore, the intergrain connection is enhanced, which leads to the obvious increase of current capacity.

A comparative study of enhancements of critical temperature in Bi-Pb-Sr-Ca-Cu-O superconductors

In this work, the effects of sintering temperature and Na subtitution on zero critical temperature (Tc,0) in Bi-Pb-Sr-Ca-Cu-O superconductors were investigated. The Bi1.6Pb0.4Sr2Ca2-xNaxCu3O10+d bulk samples with x = 0.00 and 0.05 (BPSCCO) were prepared by using the solid state reaction technique at elevated sintering temperatures of 840 and 850oC. To examine the improvements of structural and superconducting properties of the BPSCCO samples, the X-ray diffraction (XRD), scanning electronic microscope (SEM) and DC resistance versus temperature (R-T) measurements were carried out. The experimental findings exhibit a co-existence of the high Tc (Bi-2223) and low Tc (Bi-2212) phases in all samples. The estimated volume fraction of the Bi-2223 phase was increased from 71.4 to 75.7%, and the average size of the Bi-2223 grains was enlarged from 4.5 to 9.8 mm as changing the sample preparation conditions. The improved connectivities between the Bi-2223 grains were also obtained by quantitively analyzing the relation between Tc,0 and residual resistance ratio (RRR). These improvements were likely to be reasons for the enhancements of Tc,0 in the BPSCCO samples.

On the role of precursor powder composition in controlling microstructure, flux pinning, and the critical current density of Ag/Bi2Sr2CaCu2Ox conductors

Precursor powder composition is known to strongly affect the critical current density (Jc) of Ag/Bi2Sr2CaCu2Ox (Bi-2212) wires. However, reasons for such Jc dependence have not yet been fully understood, compromising our ability to achieve further optimization. We systematically examined superconducting properties, microstructural evolution and phase transformation, and grain boundaries of Bi-2212 conductors fabricated from precursor powders with a range of compositions using a combination of transport-current measurements, a quench technique to freeze microstructures at high temperatures during heat treatment, and scanning transmission electron microscopy (STEM). Samples include both dip-coated tapes and round wires, among which a commercial round wire carries a high Jc of 7600 A mm−2 at 4.2 K, self-field and 2600 A mm−2 at 4.2 K, 20 T, respectively. In the melt, this high-Jc conductor, made using a composition of Bi2.17Sr1.94Ca0.89Cu2Ox, contains a uniform dispersion of fine alkaline-earth cuprate (AEC) and copper-free solid phases, whereas several low-Jc conductors contain large AEC particles. Such significant differences in the phase morphologies in the melt are accompanied by a drastic difference in the formation kinetics of Bi-2212 during recrystallization cooling. STEM studies show that Bi-2212 grain colonies in the high-Jc conductors have a high density of Bi2Sr2CuOy (Bi-2201) intergrowths, whereas a low-Jc conductor, made using Bi2.14Sr1.66Ca1.24Cu1.96Ox, is nearly free of them. STEM investigation shows grain boundaries in low-Jc conductors are often insulated with a Bi-rich amorphous phase. High-Jc conductors also show higher flux-pinning strength, which we ascribe to their higher Bi-2201 intergrowth density.

Formation of Bi2Sr2CaCu2Ox/Ag multifilamentary metallic precursor powder-in-tube wires

Previously, a metallic precursor (MP) approach to synthesizing Bi2Sr2CaCu2Ox (Bi2212), with a homogeneous mixture of Bi, Sr, Ca, Cu and Ag was produced by mechanical alloying. Here, Bi2212/Ag round multifilamentary wire is manufactured using a metallic precursor powder-in-tube (MPIT) process. The MP powders were packed into a pure Ag tube in an Ar atmosphere and then sealed. After deformation, multifilamentary round wires and rolled tapes were heat treated in flowing oxygen through three stages: oxidation, conversion and partial-melt processing (PMP). Processing-microstructure-property relationships on 20–50 mm long multifilamentary round wires and rolled tapes were studied extensively. It is shown that conventional wire deformation processes, optimized for oxide-powder-in-tube wires, are not effective for deforming MPIT wires, and that as with prior studies of MPIT Bi2Sr2Ca2Cu3Oy conductors, hot extrusion is required for obtaining a multifilamentary structure with fine filaments. As a result, the Bi2212 MPIT wires reported here have low engineering critical current density. Nonetheless, by focusing on sections of wires that remain intact after deformation, it is also shown that the first heat treatment stage, the oxidation stage, plays a crucial role in chemical homogeneity, phase transformation, and microstructural evolution and three reaction pathways for MP oxidation are presented. Furthermore, it is found the Bi2212 grain alignment within an MPIT filament is significantly different from that found in OPIT filaments after PMP, indicating the formation of highly dense filaments containing Bi2212 fine grains and Ag particles before PMP aids the formation of large, c-axis textured Bi2212 filaments during PMP. These results show that, with improved wire deformation, high critical current density may be obtained via a MPIT process.

High critical current density Bi2Sr2CaCu2Ox/Ag wire containing oxide precursor synthesized from nano-oxides

Bi2Sr2CaCu2Ox (Bi2212)/Ag-alloy wires are manufactured via the oxide-powder-in-tube route by filling Ag/Ag-alloy tubes with Bi2212 oxide precursor, deforming into wire, restacking and heat treating using partial-melt processing (PMP). Recent studies propose several requirements on precursor properties, including stoichiometry, chemical homogeneity, carbon content and phase purity. Here, nanosize oxides produced by nGimat’s proprietary NanoSpray CombustionTM process are used as starting materials to synthesize Bi2212 oxide precursors via solid-state calcination. Oxide powders for wire fill (precursor powder) with precisely controlled stoichiometry and chemical homogeneity containing over 99 vol% of single Bi2212-phase are synthesized. Alkaline-earth cuprate are found to be the only impurity phase in the precursor powders. Phase transformation, carbon release and grain growth during calcination are studied through a series of quench studies. Effects of particle size, surface area, stoichiometry, chemical homogeneity and microstructures of the starting materials on Bi2212 formation and wire transport properties are discussed. Small particle size, high surface area and short diffusion length of the starting materials result in a rapid and homogeneous phase transformation to Bi2212, along with an early and rapid carbon release. The residual carbon in the precursor powder is between 50 and 90 ppm. The strong dependence of transport Jc on precursor stoichiometry indicates that compositional variations within precursor powders should be less than 1.5 mol%. Two Bi-rich and Ca-deficient stoichiometries give higher wire transport critical current density, with the highest being 2520 A mm−2 (4.2 K, 5 T) after 1 bar PMP and 4560 A mm−2 (4.2 K, 5 T) after 100 bar overpressure (OP) processing. The low residual carbon content results in smaller and fewer voids within an OP-processed wire filament. Bi-rich and Ca-deficient stoichiometries and small compositional variations within precursor powders may be a method for engineering uniformly-distributed and high-density Bi2201 intergrowths within Bi2212 grains after PMP.

Characterization by X-Ray Diffraction of Non- <inline-formula> <tex-math notation="LaTeX">$c$</tex-math> </inline-formula>-Axis Epitaxial <inline-formula> <tex-math notation="LaTeX">${Bi}_{2}{Sr}_{2}{CaCu}_{2}{O}_{8+\delta}$</tex-math> </inline-formula> Thin Films

Thin films of Bi 2 Sr 2 CaCu 2 O 8 (Bi-2212) with the non-c-axis (117) orientation were grown by MOCVD on (110) LaAlO 3 single crystal substrate. XRD θ - 2θ scans show that films contain also (119) and (011)Bi-2212 impurity grains. We propose and report characterization of the non-c-axis films by XRD φ - ψ scans. By this approach, the assumed theoretical film-substrate relationship of the (117) Bi-2212 grains is demonstrated experimentally. The result also confirms twinning in the span rooflike (117) Bi-2212 grains. The impurity (119) and (011) Bi-2212 grains are also rooflike and in-plane aligned according to film-substrate relation and AFM images so that the film can be considered in-plane epitaxial.

Enhancement of local microstructural distortions, boundary weak-interactions and crucial defeats performed via Bi+3/La+3 partial substitution in the Bi-2212 matrix

This exhaustive study examines not only the decrement in characteristic two-stage (pairing and coherence) transitions but also the characteristic variation of the mechanical performance with the partial substitution level of homovalent La nanoparticles on Bi sites in the Bi-2212 crystal structure with the aid of the detailed dc resistivity (ρ-T), bulk density, Vickers microhardness (Hv) experimental findings and evaluated theoretical calculations. The experimental results obtained show that both the electrical and superconducting properties being responsible for the potential technological and industrial applications degrade consistently with the increment of the La inclusions in the Bi-2212 superconducting matrix. In fact, the crucial characteristics disappear completely after the critical dopant value of x = 0.20 as a result of the full deformation between the Bi-2212 planes and grain boundaries. In other words, the excess La impurities lead to the disorder and hole localization (filling) problem in the system due to the normal-state electrical resistance transition from the truly-metallic state to the disordered metallic state, and the electrical and superconducting properties demolish totally due to the metal to insulator transition (MIT). In this paper, we focus only on the (Bi,La)-2212 materials exhibiting superconducting behavior to reveal granular superconducting nature of Bi1.6-xLaxPb0.5Sr2.0Ca1.1Cu2.1Oy. As for the theoretical values deduced from the resistivity curves, temperature derivatives of the sample resistivities (dρ/dT) confirm characteristic two-stage transitions in all the samples prepared and both the pairing (bulk genuine) and coherence transitions tend to suppress with the increase of the La inclusions inserted in the Bi-2212 superconducting core as a consequence of the presence of the increased structural inhomogeneities (distribution of oxygen) and grain boundaries. Moreover, Percolation model confirms that the decrease in the coherence transitions with the dopant level is explained with the enhancement of Josephson coupled energy and the degradation of coupling probability. The existence of the relationship between microstructure and strength of interaction between the grains is also confirmed by the bulk density and porosity surveys. As for the load dependent microhardness parameters deduced from Vickers measurements, the pure sample exhibits the Indentation Size Effect (ISE) behavior whereas the La nanoparticles inserted in the Bi-2212 superconducting system make the materials present Reverse Indentation Size Effect (RISE) feature. This is attributed to the fact that the local structural distortions and boundary weak-interactions between the superconducting grains enhance dramatically with the La doping. In other words, plastic (irreversible) deformation becomes a much more dominant character in the materials as a result of the loss of the elastic recovery. At the same time, we analyze the microhardness experimental findings with the aid of the available models such as Meyer's law, proportional sample resistance (PSR), modified PRS (MPSR), Elastic–Plastic Deformation (EPD), Hays–Kendall (HK) and Indentation-Induced Cracking (IIC) approach for the first time. The results determined show that HK approach is found to be the best descriptor to examine the mechanical performance of the pure sample, showing ISE nature while the IIC model is observed to be the most successful model in understanding the mechanical properties of the Bi-site La substituted Bi-2212 superconducting compounds, obeying RISE nature.

Diamagnetic, paramagnetic, and ferromagnetic properties of ball milled Bi $$_{1.65}$$ 1.65 Pb $$_{0.35}$$ 0.35 Sr $$_{2}$$ 2 Ca $$_{2}$$ 2 Cu $$_{3}$$ 3 O $$_{10+\delta }$$ 10 + δ powders

We have performed a systematic study of the general physical properties of (Bi,Pb) $$_2$$ Sr $$_2$$ Ca $$_2$$ Cu $$_{10+y}$$ (Bi-2223) powders that were ball milled for time intervals of up to $$t_{\mathrm{m}}\,=\,$$ 210 min. X-ray powder diffraction diagrams indicate that the phase composition of the samples seems to be preserved after the ball milling process, but the average grain size of the samples decreases appreciably with increasing milling time $$t_{\mathrm{m}}$$ . For $$t_{\mathrm{m}}\,=\,$$ 120 min, the transmission electron microscopy (TEM) images revealed the presence of Bi-2223 grains in the form of nanorods with a mean diameter of $$d_{\mathrm{g}} \sim$$ 20 nm. The temperature dependence of the magnetic susceptibility, $$\chi (T)$$ , showed the occurrence of superconductivity below $$T_{\mathrm{c}} \sim$$ 108 K in all samples studied and a progressive decrease of the diamagnetic contribution to $$\chi (T)$$ with increasing $$t_{\mathrm{m}}$$ . Such a decrease in the diamagnetic contribution to $$\chi (T)$$ is accompanied by an increase of a paramagnetic contribution that has its origin in a disordered shell of the milled grains. Increasing $$t_{\mathrm{m}}$$ results in a progressive increase of the width of disordered shell further increasing the paramagnetic contribution to $$\chi (T)$$ that shows a very small diamagnetic component in samples subjected to large milling times. We have also found that the occurrence of oxygen vacancies in the disordered shell is responsible for a ferromagnetic contribution to $$\chi (T)$$ at high temperatures. Such a shell–core morphology of the milled grains, comprising a superconducting and diamagnetic core and a disordered and paramagnetic shell, is sufficient for interpreting the magnetic properties of the samples.

In-plane transport anisotropy in BSCCO-Ag multi-filamentary tapes

Composite structures such as high-Tc multi-filamentary tapes display a complex anisotropy arising from the combination of the ‘intrinsic’ anisotropy of the Bi-2223 grains, and that associated to the superconducting phase distribution in the superconductor-metal composite, as well as cracks and other defects. In this paper we characterize the ‘in-plane’ anisotropy of tapes, i.e., the difference between the transport properties along the longitudinal axis and those along the transverse direction also lying on the wide face of the tape. In particular, we demonstrate that the dissipation associated to transport along the transverse direction approaches that of the longitudinal direction as the temperature or the current increases, which may be relevant to transport applications in situations where the superconducting properties have significantly degraded.

Bi-2223 grain growth in Ag/Ni composite-sheathed tapes sintered at different oxygen partial pressures

Bi-2223 ceramic grain growth in Ag/Ni composite-sheathed tapes sintered at different oxygen partial pressures was investigated systematically. The effect of oxygen partial pressure on the evolution of phases, texture, and microstructure development in tapes was characterized by X-ray diffraction and scanning electron microscope. Development curves of Bi-2223 phase at different oxygen partial pressures were different, indicating that they were in different formation stages of Bi-2223 during the sintering period. The appearance time of optimized texture in Bi-2223/Ag/Ni tapes was influenced greatly by oxygen partial pressure, which had an essential effect on Bi-2223 formation and growth. Grain size in nonfully reacted tapes sintered at higher oxygen partial pressure was smaller than that in tapes sintered at lower oxygen partial pressure, which was opposite to the result in fully reacted tapes. Depending on the above results, growth process of Bi-2223 platelets at different oxygen partial pressures was expounded in detail combined with kinetics.

Optimization of Partial Melting Process for Bi-2212 Multi-Filament Wires

Abstract Bi-2212 precursor powder was synthesized by a co-precipitation process, and 19×(36+1) multi-filament wires were prepared by traditional powder in tube (PIT) process. Both the maximum melting temperature, T max and cooling rate, R c1 have been optimized during partial melting process (PMP). The texture structures of Bi-2212 grains and secondary phase distributions were observed with the scanning electron microscopy. Differential scanning calorimetry (DSC) and thermogravimetric analyzer (TGA) were used to evaluate the thermodynamic properties of Bi-2212 wires. The phase transition process during cooling was discussed based on the analysis of both microstructure evolution and the thermodynamic investigation. It can be concluded that both T max and R c1 have great influences on the distribution of pores and secondary phase, such as alkali earth cuprate (AEC). The optimized critical current density of ∼4400 A·cm −2 at 77 K and self field can be obtained with the T max of 892 °C and a two-step cooling process at the R c1 of 40 °C/h.

Optimization of Current Carrying Capacity in Bi-2212 High Temperature Superconductors

Bubbles were considered to be a serious problem happened during the partial melting process of Bi-2212 superconducting wires and tapes, which can interrupt the textural alignment of Bi-2212 grains, block the current transport in single or several filaments, and even cause great damage of the entire wire. Therefore, it is necessary to suppress the formation of bubbles by optimization the traditional powder in tube process. In order to decrease the volume of absorbed H2O and CO2, an extra milling process was adopted on the precursor powders in oxygen atmosphere, as well as the filling process of precursor powders into Ag sheath. The influences of oxygen milling process on the lattice parameter, microstructure, phase evolution mechanism and transport properties were systematically analyzed. Due to the enhanced texture structures, optimized critical current was achieved in both single filament tapes and multi-filament wires. Further optimization of the modified powder in tube process is on the way in order to further improve the transport properties of Bi-2212 wires for practical applications.