Synthesis of ( Bi , Pb )-2223 superconductor powder by Pechini method

Guilherme Botega Torsoni gbtorsoni@yahoo.com.br orcid.org/0000-0001-7178-2191 Instituto Federal de Mato Grosso do Sul (IFMS), Naviraí, Mato Grosso do Sul, Brasil. Cicero Rafael Cena cicero.cena@ufms.br orcid.org/0000-0001-8766-6144 Universidade Federal do Mato Grosso do Sul (UFMS), Campo Grande, Mato Grosso do Sul, Brasil. Gustavo Quereza de Freitas gustavoquereza@yahoo.com.br orcid.org/0000-0001-5321-7009 Instituto Federal Goiano (IFGoiano), Rio Verde, Goias, Brasil. Claudio Luis Carvalho carvalho2dfq.feis.unesp.br orcid.org/0000-0003-0354-6765 Universidade Estadual Paulista (UNESP), Ilha Solteira, São Paulo, Brasil. In this paper, we present a detailed route of synthesis to produce ceramic superconductor Bi1,6Pb0,4Sr2Ca2Cu3Ox (Bi,Pb)-2223 powder by Pechini method. The obtained polymeric precursor solution was produced by using inexpensive chemical reagents, which showed a great stability for three weeks with high concentration of BPSCCO inorganic ions. The crystallization kinetic of BPSCCO powder was investigated by thermal analysis (DSC/TGA) and X-ray diffraction (XRD) techniques. The thermal treatment of the BPSCCO powder at different temperatures showed that complex phase equilibrium occurs to the system. The three superconductor phases seems to coexist in a large range of temperature, the Bi2201 phase was crystallized around 500 o C and then, after 840 o C the desirable (Bi,Pb)2223 phase appears with coexistence of the Bi-2212 phase at low quantity. Finally, the powder morphology was characterized by scanning electron microscopy (SEM), the results point to a typical plate like formation of the grains. PALAVRAS-CHAVE: Bi-2223 superconductor; Pechini Method; Low cost synthesis.


INTRODUCTION
Extensive studies have been made in new routes to synthesized oxide superconductors, mainly focused on improve the superconductor properties, such as higher critical temperature (T c ) and great electrical current density (J c ) (OHARA et al., 2015;SAFRAN et al., 2015;FREITAS et al., 2016-a).The BSCCO system -Bi 2 Sr 2 Ca n-1 Cu n O 4+2n+y (n = 1, 2, 3 or 4), is one of the main high temperature superconductors, and have been extensively investigated for technological applications and basic studies in materials science since its discovery by Maeda et al. (1988).The BSCCO system presents three different crystallographic phases with superconductor properties.In general, the Bi 2 Sr 2 Ca 1 Cu 2 O x (Bi-2212) is the most studied composition, because it´s present great superconductor properties and a relatively facile synthesis compared with other systems.The Bi-2212 have been produced by a large variety of techniques and different morphologies, such as films, bulk, wires, tapes and fibers (FREITAS et al., 2015;CENA et al., 2017FREITAS et al., 2016-b).Although, the Bi 2 Sr 2 Ca 2 Cu 3 O x (Bi-2223) superconductor phase is well known as a difficult stoichiometry to obtain, mainly due to an unstable phase equilibria, what makes it less explored for many research groups, who are dedicated to produce superconductor materials for basic studies and applications (PANDEY et al., 1990).However, the Bi-2223 is one of the most suitable for technological applications due to its higher critical current density (Jc) and critical temperature (Tc), when compared with the Bi-2212.
Regarding the superconductor phases of the BSCCO system, the Bi-2201 is the more common and stable phase, with Tc around -253 o C, presenting low crystallization temperature between 400 o C and 650 o C. The Bi-2212, considered a stable phase, exhibits Tc usually around -188 o C and its crystallization occur between 730 o C and 840 o C (WONG et al., 1992;MAJEWSKI, 1997).Finally, the Bi-2223 phase, with Tc around -165 o C, it is considered an unstable phase, because presents a short crystallization temperature range between 840 o C and 850 o C.Moreover, there is a complex multiphase equilibrium involved during the phase formation, that usually drives the system to secondary phases formation, such as (Ca,Sr) 2 CuO 3 , CuO and Ca 2 PbO 4 , and disturbs the aligning and coupling of superconductor plates (WANG et al., 1998;JIANG et al., 2001).Some studies, devoted to produce pure Bi-2223 phase have shown that the kinetic and stability of the phase formation can be improved by doping the system with lead ions (Pb+).The Pb ions replace Bi ions in the C-site, extending the crystallographic structure, and facilitating, the growth of new plane of Cu and Ca (TAKANO et al., 1988;TAKEDA et al., 1989).
Extensive studies have been made in new routes to synthesized oxide superconductors in the past few decades, mainly focused on improve the superconductor properties, such as higher critical temperature (Tc) and great electrical current density (Jc) (OHARA et al., 2015;SAFRAN et al., 2015;FREITAS et al., 2015).In this context, there is many advantages of produce (Bi,Pb)-2223 by Pechini method (LESSING, 1989), such as, (i) the method is able to produce thinner powder from a homogeneous reagent mixture, thereby increasing the reactivity of the powder; (ii) the low cost and simple laboratory facilities are required; (iii) great potential to produce many types of samples, such as thin films, nanoparticles, nanofibers, from a polymeric resin; and finally (iv) there is a facility to introduce others ions in the structure or even produce composite materials for basic studies and/or applications.
In this work, we present a detailed route for processing (Bi,Pb)-2223 powder by Pechini method.The dynamic viscosity studies of the resulting polymeric resin were performed to investigate the solution stability.The BPSCCO dried powder was studied by thermal analysis (TA), and after thermal treated at different temperatures, the phase evolution, microstructure and crystallographic phase formation of the samples were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively.

EXPERIMENTAL DETAILS
The BPSCCO powder with nominal composition Bi 1,6 Pb 0,4 Sr 2 Ca 2 Cu 3 O x was obtained by Pechini method (LIMA et al, 2014).As starting materials was employed carbonates (Bi 2 CO 5 , SrCO 3 , CuCO 3 , CaCO 3 , PbCO 3 ) all purchased from VETEC Chemical Company Ltd.A stable resin was obtained by dissolving, separately, the starting materials in deionized water and citric acid (C 8 H 8 O 7 ) (AC) at room temperature.The metallic ion/citric acid ratio employed was 1/3.The ion solutions mixtures followed the steps described in the Figure 1.Then, ethylene glycol (C 2 H 6 O 7 ) (EG) was added to the solution (citric acid/ethylene glycol = 60:40 mol%) and polymerized by heating up to 80 o C during 30 minutes.The final pH was ajusted, by adding few drops of ethylenediamine (C 2 H 8 N 2 ), between 7,0 and 8,0.The pH adjust was necessary to improve the resin stability, as result the resin coloration changes from light green to a dark blue.

RESULTS AND DISCUSSION
The dynamic viscosity studies performed as function of temperature, Figure 2, showed a typical exponential decay of Newtonian fluids.The same behavior was observed to the resin after 45 days, suggesting a great stability.A stable resin is very important to potential applications, it guarantees the reproducibility of the samples during long period of time.The viscosity of the resin at room temperature was around 200 cP, and density of 1.28 g/ml.(TORARDI et al., 1988).Although, some peaks associated with secondary phases such as SrCO 3 e CaCO 3 were also identified.These secondary phases correspond to an unreacted precursor material, that should react at higher temperatures with other elements to form another Bi-superconductor phase, the same behavior was also observed by Peng et al. (1998).By increasing the temperature of thermal treatment to 705 o C, Figure 4(d), the sample showed a remarkable presence of Ca 2 PbO 4 phase, this  SUNSHINE et al., 1988).Finally, the XRD results observed to the powder treated at 849 o C, Figure 4(f), showed the crystallization of the desirable phase (Bi,Pb) -2223, but with coexistence of Bi-2212(YAKHMI et al., 1993).: 1) CaBi 2 O 4 ; 2) Bi-2201; 3) SrCO 3 ; 4) CaCO 3 ; 5) Bi 2 CuO 4 ; 6) Ca 2 PbO 4 ; 7 Images obtained by scanning electron microscopy (SEM) of the treated powders are shown in Figure 5.The changes on powder morphology could be successfully evidenced.As evidenced by DRX analysis, the presence of the organic material at the sample treated at 220 o C were characterized by a smooth surface, Figure 5(a).By increasing the temperature of thermal treatment the organic compounds degraded, remaining just the inorganic content, which ones react to from the crystalline phases.Then, the initial smooth surface observed at 220 o C, evolves to small grain morphology typical of ceramic materials at intermediary temperature range.Finally, the morphology presents a plate-like grains structure above 743 o C, characteristic of Bi-superconductor phase, exhibiting a grain size distribution around 800 nm at 850 o C.
As discussed above at low temperature treatment (~220 o C) the presence of organic compounds could be confirmed by the large presence of nitrogen and carbon in the powder.By increasing the temperature of thermal treatment, the organic material has been removed, as observed on TGA analysis, remaining just a small amount of carbon atoms at 500 o C. Above 500 o C remains just the inorganic compounds that react to form the crystalline phases.The amounts of Bi and Sr in the stoichiometry did not changed substantially by increasing the temperature, however, the amounts of Ca and Cu present considerable variation, as expected because there is an evolution of the superconductor phase (from Bi-2201 to (Bi,Pb)-2223).The Pb ions could not be evidenced at EDS analysis probably by its low content in the material, and also experimental difficulties to measure samples with irregular surfaces with quantitative precision.

CONCLUSIONS
The Pechini method presented a great potential to produce thinner BPSCCO powders, with grain size around 800 nm, from polymeric solution.The crystallization kinetics study, based on TG/DSC and XRD results, are in accordance with the literature reports to the system produced by sol-gel method.The thermal treatment of the powder indicated formation of three superconducting phases Bi-2201, Bi-2212 and (Bi,Pb)-2223.The results showed that by add Pb ions in the BSCCO precursor can induce the (Bi,Pb)-222 phase formation, providing an easy and low cost route to produce superconductor materials for academic studies.

Figura 2 -
Figura 2-Rheological characterization of the BPSCCO polymeric resin produced by Pechini method.Dynamic viscosity behaviour as prepared and after 45 days

Figura 3 -
Figura 3-TGA/DSC results of BPSCCO powder, previously thermal treated at 500 o C/ 1 h.The derivative DSC curve inset as a secondary phase, but at 743 o C, Figure 4(e), it recombines with other phases to form the Bi-2212 phase (
Tabela 1-Atomic percentage of elements in the samples determined by EDS analysis Temperature 220 o C 500 o C 632 o C 705 o C 743 o C 849 o C