Continuous processing of Bi2Sr2CaCu2O8+δ precursor powders
Introduction
Within the Bi-Sr-Ca-Cu-O (BSCCO) family of superconductors [1], [2], Bi2Sr2CaCu2O8+δ (Bi-2212) and Bi2Sr2Ca2Cu3O10+δ (Bi-2223) with critical temperatures Tc ≈ 92 K and 110 K, respectively, have been the most widely studied phases in the past decades due to their possible technological interest [3], [4]. Despite its lower critical temperature, Bi-2212 phase presents certain advantages over Bi-2223 as it exhibits higher stability, allows simpler reaction conditions, and can be manufactured as round wires [5] instead of tapes, greatly preferred in magnet technology. However, its high superconducting anisotropy restricts applications to temperatures below ∼ 10 K in case of strong magnetic fields (> 10 T), or to low magnetic fields at higher temperatures. This superconductor has been used to manufacture current leads [6], and it is receiving nowadays increasing attention for further developments of existing applications requiring very high-field magnets beyond the limits of the current Nb3Sn technology, such as nuclear magnetic resonance (NMR) or particle accelerators [5].
In order to optimize the superconducting properties of the wires, tapes or bulk materials, their microstructure has to be controlled. In Bi-2212 materials, the required texture can be induced using melting techniques with a very controlled solidification process. In addition, this material exhibits incongruent melting. Different phases are thus present after solidification, essentially with stoichiometry close to Bi2Sr2CuO6+δ (Bi-2201) and some Bi-free oxides. In consequence, a final heat treatment is required to form the superconducting Bi-2212 phase while maintaining the grain texture achieved during solidification [7], [8]. Obviously, the characteristics of the precursors play an important role in the final properties of the wires and bulk textured materials. Several studies have explored the influence of the precursor composition on the properties of the final textured material [9], [10]. It was observed that changes in the precursor stoichiometry modify the proportions of the different phases that are present after the melting process, and affect the microstructure and superconducting properties of the final processed material.
As previously reviewed by Angurel et al. [11], [12], among the texturing methods reported in the literature to produce bulk textured materials, Laser Floating Zone (LFZ) and Laser Zone Melting (LZM) processing methods are appropriate to obtain highly textured BSCCO superconductors with a Bi-2212 stoichiometry.
On the other hand, precursor Bi-2212 powders, which are commercially available, can also be produced at the laboratory scale. Different synthesis routes have already been proposed [13], [14], [15], [16], and previous works have demonstrated a relatively low influence of the powder precursor synthesis method in subsequent LFZ and LZM processed materials [14], [15], [16]. Laser induced directional solidification processes produce phase segregation along the sample cross-section, with a highly textured granular microstructure in the regions close to its surface. The superconducting performance of these materials is mainly associated with the texture quality in this region. It has been observed that, when different powder synthesis methods are used, small changes in the microstructure of this external region barely affect the sample's superconducting properties.
Since the price of commercial Bi-2212 powders has been increasing within the last several years, reaching levels above 2 k€/kg, it seems appropriate to explore alternative Bi-2212 powder precursor synthesis routes, which may be industrially scalable and considerably cheaper than those presently available from commercial suppliers.
The most common methods to produce Bi-2212 powders follow conventional solid-state reaction processes, which, at the laboratory scale, are limited to small quantities of the order of 250–500 g. This is very low for an industrially scalable process because it requires processing the powder in several batches, compromising thus its stoichiometry homogeneity and, in consequence, its use for further processing.
The aim of this work is to overcome the above-mentioned problems by developing a novel continuous powder processing method based on the use of a continuous furnace. This methodology, which is used routinely by the ceramic industry, can be applied to prepare Bi-2212 precursor powders for melt texture processes in shorter times, without limitation regarding production volume and uniformity. This paper describes a continuous solid-state processing method, as well as the characteristics of the obtained powders at several stages of this process. A comparison between these precursor powders with those fabricated with the standard solid-state process [14], [15], as well as with commercially available powders, is also presented.
Section snippets
Powder preparation
Three different types of Bi-2212 powders have been compared in this study, including commercially available powders, as well as standard solid-state and continuous solid-state synthesized powders. Their synthesis is described as follows.
Evolution of the phase composition
The evolution of the phase composition during the different cycles of the continuous solid-state process has been monitored using XRD patterns. The obtained diffractograms are presented in Fig. 2. The pattern at the top of the figure corresponds to the initial raw mixture of oxides and carbonates (powder CSS0), which shows the expected phases: Bi2O3, SrCO3, CaCO3 and CuO (see Table 1). Below is the pattern of the precursor powders after the first thermal treatment in the continuous furnace
Conclusions
The results presented in this work show that it is possible to obtain Bi-2212 powders from oxide and carbonate raw precursors using a continuous solid-state fabrication process. The use of a 4-m long continuous furnace allows controlling the sample velocity and provides three zones where the temperature can be controlled independently, thus enabling prefixed temperature profiles and heat treatment durations. With this equipment, the Bi-2212 phase begins to form after three thermal cycles and
Acknowledgements
This work was supported by the Spanish Ministerio de Economía y Competitividad and the European FEDER Program (project ENE2014-52105-R), and by the Gobierno de Aragón (research groups T12, T87 and T54_17R). The authors acknowledge the use of Servicio General de Apoyo a la Investigación-SAI, University of Zaragoza.
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