Formation Of Bi-2223 Research Articles

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.

Influences of Pb content on the critical current of Bi-2223 multi-filamentary tapes

Multifilament Bi-2223/Ag tapes with different Pb contents in precursor powders were fabricated by powder in tube (PIT) method. The influence of Pb contents on the phase formation, microstructure and critical current density of Bi-2223/Ag tapes were systematically investigated. X-ray photoelectron spectroscopy (XPS) demonstrated that the content and oxidation states change of Pb ions can influence the carrier concentration of superconducting phase. Scanning electron microscopy (SEM) revealed that the variation of Pb content can lead to the difference in microstructure due to the change of thermodynamic properties. The results showed that with the Pb content of x=0.35 in the nominal composition of Bi2−xPbxSr2Ca2Cu3Oy, proper content of liquid phase can be formed during the sintering process, thus maximum critical current (Ic) was obtained. The correlation between the transport property, carrier concentration and Pb content was discussed.

The Effect of Barium Doping on the Selective Structure of Bi-2223 Phase

The partial substitution of Sr by Ba in the two nominal compositions of Bi1.8Pb0.4Sr2−xBaxCa2.2Cu3Oy [x=0.0, 0.1, 0.2, and 0.3 (A group)] and Bi1.66Pb0.34Sr2−xBaxCa2Cu3Oy [x=0.0, 0.1, 0.2, 0.3, 0.4, and 0.5 (B group)] have been investigated by resistivity, ac susceptibility measurements and by XRD and SEM analysis. In general, the nature of the temperature dependence of resistivity and susceptibility measurements indicate the presence of a superconducting transition between grains coupled by weak links. However, the XRD and SEM analyses show that the relative composition of initial elements used in Bi-(2223) is essential to the site that is selected by the Ba ions. In the A group, Ba doping up to x=0.1 will improve the phase formation of Bi-2223, and improve the superconductivity properties of the samples. In the B group, although Ba doping up to x=0.1 will enhance the phase formation of Bi-2223, it will decrease the coupling between the grain and the superconductivity properties of these systems. The presence of lower Tc phases will begin to appear for x>0.1, in both of these systems. The superconductivity properties and the phase formation of Bi-(2223) will decrease as the Ba concentration increases.

Effect of filament twist and high resistive barrier on the properties of Ag-sheathed Bi2223 tape

Although the introduction of filament twisting with high resistive barrier between the filaments is effective to reduce the AC loss, the Jc value of the tape was easily decreased by the filament twisting and insertion of high resistive barrier. This decrease in Jc is a problem to achieve low AC loss and high Ic tape. In this study, the effects of filament twist and high resistive barrier on the microstructure and Jc property were investigated to overcome the Jc reduction. The Bi2223 tapes with filament twist or oxide barrier were fabricated by powder-in-tube method. The Jc value of twist tape gradually decreased as decreasing the twist pitch, and the degree of Jc reduction was small in slim tape compared to wide tape. This reduction would attribute to the small actual cross sectional area of the filament perpendicular to the current direction because the filament has a tilt angle from the tape axis. On the other hand, the conversion ratio of Bi2223 became small as increasing the thickness of SZO barrier layer. Since the SZO barrier has less reactivity with 2223 filament, this suppression of the Bi2223 formation would not be caused by the formation of impurity phase but would be caused by the suppression of gas release from the filaments by oxide barrier. The conversion ratio of Bi2223 increased in the short sample where the gas could be released from the both end of the tape. These results suggest that the gas diffusion from filaments would be important for the formation of Bi2223. Hence, the use of slim tape for decreasing the tilt angle of the filament and keeping the diffusion path of gas from the filaments may improve the Jc value of twist and barrier tapes.

Fabrication and properties of Bi2223 tapes with high resistive barrier formed by the in situ oxidation method

Ag sheathed Bi2223 tapes have the performances of I c values over 100 A and length of 1000 m which are enough for prototype applications. However, their AC loss is still too high for practical applications. To reduce the AC coupling loss, the introduction of high resistive barrier between filaments and the filament twisting are effective. In this study, the Bi2223 tapes with CuO and NiO barriers between filaments were fabricated through the in situ oxidation method, and the effect of 2223 addition in precursor on the formation of Bi2223 was also investigated. Mono-core wires covered with metal Cu or Ni were stacked and deformed to form 7-filamentary tapes. These tapes were sintered, and the metal Cu and Ni layer were oxidized to form CuO and NiO barrier layer, respectively. The continuity of CuO and NiO barriers were kept well in the final tapes, although the CuO grains in barrier layer easily agglomerated during sintering compared to NiO. The formation speed of Bi2223 phase was increased by the addition of Bi2223 in precursor. However, the formation speed of the tape with barrier was slower than that without barrier. This difference should be considered to control the processing for high J c value.

Effect of Bi2223 addition in precursor on the formation andJc property of Bi2223 tapes sheathed with the Ag–Cu alloy

The use of an alloy sheath is effective in improving the mechanical strength of Bi2223 tapes although it easilydecreases Jc properties. In the case of the tape sheathed with Ag–Cu alloy, theJc value was decreased by the formation of non-superconducting(Sr,Ca)14Cu24Ox (14:24) particles. In this study, the effect of Bi2223 addition in precursor on the formationand the properties of Bi2223 tapes was investigated. The addition of Bi2223 in precursorwas found to be effective in reducing the number and size of the 14:24 particles and inincreasing the formation speed of the Bi2223 phase. This rapid formation of Bi2223 isprobably due to the skipping of nucleation of Bi2223 since the Bi2223 crystals added inprecursor act as nuclei. In addition, the crystal size after the first sintering decreased onincreasing the content of 2223 powder. Due to the reduction of 14:24 particles,Jc valuesof about 2 × 104 A cm−2 were attained in the tapes with 2223 up to 5%, although theJc values were decreased by further addition of 2223 to more than 10%. This reduction ofJc would be caused by the poor alignment of Bi2223 crystals because of the small size in therolling step.

Influence of powder pre-annealing on the phase formation and critical current of Bi-2223/Ag tapes

The influence of precursor powders with different lead-rich phases, such as Ca 2PbO 4 and Pb 3(Sr,Bi) 3Ca 2CuO y (3 3 2 1), on the phase formation and critical current of Bi-2223/Ag tapes has been studied. Three precursors with different phase assemblages were prepared and used to make the tapes. The effects of different precursors were investigated by XRD, SEM/EDS and critical current measurements. It has been found that both the microstructure and phase formation depended strongly on the different lead-rich phases, which determined the reactivity of the precursor. Tapes fabricated using the precursor with Ca 2PbO 4 phase (tape T1) had lower transformation rate of 2223 phase than tapes fabricated using the precursor with 3321 phase (tape T2). SEM results show that a large fraction of secondary phases with big particle size was formed in the tape T1 during the subsequent sintering, which might degrade the critical current of the tapes. On the contrary, a small amount of secondary phases with small particle size was formed in the tape T2. Consequently, the critical current of tape T1 was lower than that of tape T2.

Fabrication of multifilament Bi-2223/Ag tapes by varying the lead content and calcination temperature of precursor powders

A set of multifilament Bi-2223/Ag tapes were fabricated by varying the lead content and calcination temperature of starting precursors. The influence of precursor conditions on the phase formation, microstructure and Jc property of Bi-2223/Ag tapes were explored with XRD, DTA, SEM-EDS and transport current measurements. The results showed that the Pb content of 0.20 in the nominal composition of Bi1.8PbxSr2.0Ca2.2Cu3.0Oz is too low to lead to a fully formation of Bi-2223 phase in tapes compared with Pb content of 0.25 and 0.30. In addition, despite the Pb variations, the powders calcined at 820 °C resulted in the highest Jc in final tapes against the powders calcined at 810 and 830 °C. The correlation between the transport property and the varied phase assemblages of precursors induced by lead and calcination temperature variations was discussed.

The affect of transients on secondary phase formation in multifilamentary (Bi, Pb)2223/Agcomposite tape

The processing of (Bi, Pb)2223 tapes has been investigated extensively since the firstOPIT tapes were made in 1989. The formation of Bi-2223 is via a Pb-inducedliquid phase, which is known to assist the Bi-2223 formation. However, there aremany unsolved microstructural inhomogeneities and defects in current OPIT tape;consequently research has begun to focus on the role of the liquid phase. In thisstudy differential thermal analysis results on green (Bi, Pb)2223 tape in 7.5% ppO2 give direct evidence for two phase transitions, with onset temperatures of 800 and820 °C. XRD phase analysis on tapes heat treated for prolonged periodsshow two distinct secondary phase formation regions: between 800 and820 °C; andabove 820 °C. A link is drawn between the phase transitions in the initial stages of Bi-2223 formationand the long-term secondary phase formation. The relevance of these results to transportproperties is discussed.

Phase formation and evolution of (Bi,Pb)2223 inside Ag tapes in air and reduced oxygen partial pressure using synchrotron X-rays

Abstract The phase formation of (Bi,Pb)2223 inside a silver sheath was investigated using high-energy X-ray photons (λ=0.5 A), in air at 840 °C and in reduced oxygen atmosphere, 7%O2 + 93%N2, at 832 °C. The data was analysed using the non-overlapping peaks method as well as the full pattern Rietveld refinement method. We identified seven crystalline phases, Bi2201, Bi2212, Bi2223, Bi3221, Ca2PbO4, (Sr,Ca)2CuO4 and CuO, plus silver. There is a significant difference in phase evolution under the two different heat treatment conditions. The heat treatment in reduced oxygen atmosphere at 832 °C favours the formation of Bi2223 with a reduced amount of Bi2201, Bi3221, Ca2PbO4 and (Sr,Ca)2CuO4 whilst a clearly noticeable amount of Bi2201 phase exists throughout the process for samples heat treated in air at 840 °C.

Kinetics of phase formation and transport properties of Bi-2223/Ag tapes fabricated by various calcined powders

The kinetics of phase formation and transport properties of Bi-2223/Ag tapes fabricated by precursor powders with different phase assemblages have been studied. Our results show that the formation process of 2223 phase is strongly related to the secondary phases in precursors. A variation of the reaction-order, n, deduced from the Avrami relation, was observed along with the whole sintering processing. Within a sintering period of 65 h, n keeps the value of 0.7 for the tape made from the powder calcined at 830/spl deg/C, while it changes from 0.3 or 0.4 to 1 for the tapes prepared with the powders calcined at 810/spl deg/C and 820/spl deg/C. Based on the analysis of phase formation and transport properties, the optimum calcination conditions for achieving high J/sub c/ tapes are discussed.

Effect of the precursor powders on the final properties of hot-forged Bi2223 textured discs

The effect of precursor powder assemblage on the phase formation, crystallite orientation and transport critical current density of Bi2223 textured discs has been studied. We observe that starting powder composed of Bi2212 and secondary phases results in a stronger grain alignment than if composed of nearly pure Bi2223. SEM observations and XRD analyses show that the liquid phase produced during the sinter-forging step is essential to achieve sharp orientation distributions and high critical current densities.

The effect of starting precursor conditions on the phase formation and transport properties of Bi-2223/Ag tapes

Precursor powders were prepared by the spray–drying method and varied in particle size distributions. The effect of starting precursor conditions on the phase formation and transport properties of Bi-2223/Ag tapes has been investigated. The results show that precursors with fine particle sizes (<2 μm in average) lead to a fast formation of 2223 phase at the early stage of sintering and a high conversion rate from precursors to Bi-2223 in fully processed tapes. The core density and grain texture are improved in the tapes made from fine powders, which correspond to the enhanced J c property. I c values, 60 and 62 A (77 K, in self-field), are achieved in short fully reacted tapes with decreasing the particle size of precursors. However, some large alkaline-earth-cuprate particles can also be found in the tape with the highest I c which indicates that the fine powder also triggers the formation of undesired secondary phase. It might be the reason for the rapid drop of J c at magnetic field higher than 0.05 T along H∥ c direction.

Optimization of precursor powders for manufacturing Bi-2223/Ag tapes

The phase assemblage and particle sizes of precursor powders have been optimized in a sequence for fabricating Ag/BSCCO-2223 composite tapes. Firstly, an optimal calcination temperature was determined based on the experimental results. Then, the precursors calcined at the optimal temperature were ball-milled for different dwell times to obtain varied particle sizes. The effects of both the phase assemblages and particle sizes of the precursor powder on the phase formation, microstructure and transport Jc of Bi-2223/Ag tapes have been investigated. The results show that the precursor phase assemblage has a large impact on the reaction routes, microstructure, and Jc property. Meanwhile, a fine powder is beneficial for the grain growth, alignment, and Jc enhancement in fully reacted tapes. The best Jc was achieved in the tape made from the powder after optimizing the phase assemblage and particle size.

Production of BiPbSrCaCuO Thin Films on MgO and Ag/MgO Substrates by Electron Beam Deposition Techniques

Superconducting BiPbSrCaCuO thin films were prepared on MgO(001) and Ag/MgO substrates using an electron beam (e-beam) evaporation technique. The effects of annealing temperature and Ag diffusion on the crystalline structure and some superconducting properties, respectively, were investigated by X-ray diffraction, atomic force microscopy, and by measurements of the critical temperature and the critical current density. It was shown that an annealing of both types of films at 845 or 860 °C resulted in the formation of mixed Bi-2223 and Bi-2212 phases with a high degree of preferential orientation with the c-axis perpendicular to the substrates. The slight increase of the critical temperature from 103 K to 105 K, the enhancement of the critical current density from 2 x 10 3 to 6 x 10 4 A/cm 2 , and the improved surface smoothness are due to a possible silver doping from the substrate.

Influence of low temperature–low oxygen pressure post-annealing on critical current density of Bi(Pb)2223/Ag superconductors

A Bi(Pb)2223/Ag multifilamentary tape was manufactured using the traditional PIT method. A series of the short samples cut from the tape were used to investigate the effect of low temperature–low oxygen pressure post-annealing on the critical current density of the samples. Two series of the air-sintered superconducting samples with the different intermediate pressing times and total sintering time were in situ post-annealed, respectively, under the reduced atmosphere taking the post-annealing temperature, time and oxygen partial pressure as variations. The results indicate that there are different effects of the post-annealing on the critical current density for the samples in the two series. Post-annealing at the temperatures from 760°C to 810°C for a short time can improve the J c values up to 1.5–2.5 times for the samples in series 1, and the maximum value can be achieved at 780°C, which makes the J c A/ J c B– T (A: after the post-annealing; B: before the post-annealing) curve present a single-peak form, whereas the J c values can be obviously improved up to 1.3–2.0 times at 820°C and slightly improved at 780°C for the samples in series 2, which makes the J c A/ J c B– T curve present a double-peak form. XRD, SEM and ac susceptibility measurements indicate that the two different effects of the post-annealing, which closely depends on the sintering time of the samples before post-annealing, should be attributed to the two different phase transformations relating to the formation of (Bi,Pb)2212 and Bi2223, respectively, during the post-annealing. Formation of (Bi,Pb)2212 from Pb-poor Bi2212 and (Pb,Bi) 4− α (Sr,Ca) 5− β CuO x (Pb451) at 780°C for the samples with a shorter sintering time has an effect of improving the transition temperature of 2212 from 68–74 to 76–80 K and, therefore, overcomes the obstruction of the phase on the critical current at liquid nitrogen temperature and obviously improves the critical current density of the samples. The improvement of the J c values for the samples with a longer sintering time after being treated at 820°C can be attributed to the formation of Bi2223 from Bi2212, Pb451 and (Sr,Ca) 14Cu 24O x . It seems that the effectiveness of post-annealing strongly depends on the sintering time of the samples before post-annealing: the formation of BiPb2212 at 780°C needs a shorter time (relatively larger amount of Bi2212) in advance, whereas the formation of Bi2223 at 820°C needs a longer time (decreased amount of Bi2212 and increased amount of (Sr,Ca) 14Cu 24O x ). Among the two phase transformations, the formation of Pb-enhanced Bi2212 at 780°C seems to be of utmost importance and has a universal meaning for the manufacture of the Bi2223 tapes, which solves the intrinsic problem of the partial decomposition of Bi2223 during the furnace cooling and may present a new idea of improving the J c values through adjusting the transition temperature of the superconducting secondary phases besides the exhausted effort of making the Bi2223 single phase. The J c values after post-annealed at 780°C for a short time for the samples with a shorter sintering time before the treatment can be comparable with that of the longer time sintered samples and the samples after post-annealed at 820°C, which also reflects the importance of the transformation of Bi2212 into BiPb2212 on the critical current density of the tapes and makes the mechanism deserve to be further investigated.

Influence of Ca2PbO4 on Phase Formation and Electrical Properties of (Bi,Pb)2Sr2Ca2Cu3O10/Ag Superconducting Composites

(Bi,Pb)2Sr2Ca2Cu3O10 (Bi2223) precursor powders with large and small amounts of Ca2PbO4 phase were prepared and used to make superconductor/silver composite tapes. The melting behavior of the powders and tapes was examined by differential thermal analysis (DTA). The influence of Ca2PbO4 on the formation and microstructure of Bi2223, and electrical properties of the tapes, was investigated by X‐ray diffraction (XRD), scanning electron microscopy (SEM), and critical current measurements. It was found that the melting onset temperature (Tm,onset) of precursor powders and composite tapes was strongly dependent on the amount of Ca2PbO4. Tapes with a small amount of Ca2PbO4 had a higher Tm,onset and a higher optimum sintering temperature compared with tapes with a large amount of Ca2PbO4. Also because of the higher sintering temperature, the total sintering time required for the former tapes was drastically shortened compared with the latter ones (250 vs 110 h). Furthermore, the microstructure and the current‐carrying capacity of the tapes were significantly improved by reducing the Ca2PbO4 content of the precursor powders. These results are of practical significance for the commercialization of Bi‐based high‐temperature superconductors.

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.

Filament decoupling and magnetization loss of multifilamentary Bi2223 superconducting tapes

Several types of multifilamentary Bi2223 superconducting tape were fabricated to study the filament decoupling and its effect on the magnetization loss reduction. First, twisted and untwisted multifilamentary Bi2223 tapes with pure silver matrix were prepared. The spacing between filaments was increased to ensure the suppression of the inter-filament bridging. Therefore, the engineering current density decreased, while the critical current density was kept at 150 A/mm/sup 2/. Measured magnetization loss of the tapes indicates that filaments are decoupled by twist in the parallel magnetic field, and the hysteresis loss is reduced. Second, the inter-filament resistive barriers were introduced. Several materials were tested as the barrier material to study their compatibility with the formation of Bi2223 phase during the fabrication process. The tapes with resistive barrier were fabricated successfully with a couple of meters of piece-length. DC magnetization of the fabricated tapes was measured by a SQUID magnetometer. The magnetization-temperature curves indicate that Bi2201, not superconducting at 77 K, and the Al/sub 2/O/sub 3/ do not affect the formation of Bi2223 superconducting phase. The transverse resistivity of a Bi2201-barrier tape, estimated from the measured coupling time constant, is 4.7/spl times/10/sup -8/ /spl Omega/m. Results of AC magnetization loss measurement indicate that the filaments can be decoupled by the Bi2201 barrier.

Fabrication and properties of some Ag-alloy sheathed Bi-2223 tapes

Mono and multifilamentary powder-in-tube tapes with sheaths of Ag, AgCu/sub 0.02/, Ag(AgCu/sub 0.02/), AgAl/sub 0.25/, Ag(AgAl/sub 0.25/), AgNi/sub 0.25/Mg/sub 0.25/, AgTi/sub 0.25/Mg/sub 0.25/, and AgTi/sub 0.25/ alloys have been fabricated and their physio-chemical properties and effect on the phase formation of Bi-2223 determined. Alloying was found to have a significant effect on phase formation of the Bi-2223 phase formation linked to the alloying elements reactive to oxygen, Cu<Ni<Al<Mg<Ti. Multifilamentary tapes, composed of an inner Ag sheath and an outer Ag alloy sheath showed no such effects. The resistivity and mechanical properties of the various Ag alloys used as sheathing materials are also reported on and their possible use in improving the performance of PIT tapes.