Synthesis of high phase pure cuprate superconductors via xerogel precursors
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Synthesis of high phase pure cuprate superconductors via xerogel precursors J. Macho, R. W. Schaeffer, G. H. Myer, and R. E. Salomon Center for Materials Research, Temple University, Philadelphia, Pennsylvania 19122
J. E. Crow MARTECH, Florida State University, Tallahassee, Florida (Received 31 May 1991; accepted 19 December 1991)
Phase pure YBa 2 Cu 3 O 7 high transition temperature superconductor powders were synthesized by a new precursor route. This new synthesis procedure is easy, leads to an atomic mixture of the relevant constituents, and is readily adaptable to many other systems or to the doping of existing systems. A gel of yttrium, barium, and copper salts was prepared using organic gelling agents. The gel was then dried and ground, leaving the xerogel precursor. These xerogels are perfectly stable even under high humidity atmospheric conditions and show an amorphous structure when characterized by x-ray diffraction, which is consistent with the existence of an atomic mixture of the three relevant metal salts. The organic gelling agent was selected after optimization studies focused on such issues as ash residue after calcination, solubility, and chemical interference with the metal salts. From a great variety of these gelling agents (agar, xanthan gum, gelatin, alginates, and others), gelatin was chosen as the ideal gelling agent for this specific application. The final product characterization shows very unexpected results. The crystalline phase purity of the YBa 2 Cu 3 0 7 superconducting phase was about 99%, with no traces of barium carbonate, as expected from pseudo-organic precursors (xerogels). The product shows superconducting properties even before oxygen anneal (Tc « 70 K), and after oxygen annealing of the powders the transition temperature was found to be 91 K, showing a linear behavior of the resistivity versus temperature before the drop at Tc, extrapolating to zero at 0 K. The particle size of these powders is smaller than 1 jim, as shown by SEM.
I. INTRODUCTION Different chemical approaches may be required to obtain superconducting powders with high phase purity, small particle size, and lower heat treatment temperatures with the approaches being tailored to the intended use of the powders. At the same time, segregation needs to be avoided at every step of the process. Many limitations of the ceramic method (grinding and sintering) in producing high phase purity ceramic oxide superconductors, particularly the well-known YBa 2 Cu 3 0 7 (YBCO), are discussed in almost every paper concerned with the synthesis of this high Tc superconductor. Some of the approaches explored by this group are (1) the use of a freeze drying process,1 (2) use of liquid ammonia as a solvent for precursor salts,2 and (3) use of fused eutectics of alkaline hydroxides to prepare YBCO precursors.3 Other researchers have also reported other synthetic techniques, such as spray drying, freeze drying, coprecipitation, and other solution techniques.4"8 Almost all of these methods have some disadvantages, such as product
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