Synthesis of high phase pure cuprate superconductors via xerogel precursors

Abstract

Phase pure YBa₂Cu₃O₇ 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 YBa₂Cu₃O₇ 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 (T_c ≍ 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 T_c, extrapolating to zero at 0 K. The particle size of these powders is smaller than 1 μm, as shown by SEM.