SOL-GEL SYNTHESIS AND SUPERCONDUCTING PROPERTIES OF HGBA2CACU2O6+DELTA

Composition and superconducting properties of powder samples of Hg-1212, HgBa2CaCu2O6+delta were investigated. The specimens were prepared under various synthetic conditions by reacting the precursor Ba2CaCu2O5+/-delta, synthesized by the sol-gel technique, and HgO in closed ampoules at two different temperatures. The initial ratio (HgO:precursor) was in most cases larger than 1.0, which gave a Hg-1212 phase of better purity. An excess of 40% HgO gave the purest product. The thermal decomposition of the precursor gel and the subsequent formation of the superconducting phases are described. The synthesized materials were characterized by X-ray powder diffraction, IR spectroscopy, and scanning and transmission electron microscopy. Common impurities were Hg-1201, CaHgO2 and unreacted precursor. After annealing in flowing oxygen at 300 degrees C for 6 h the samples became superconducting at a T-c (onset) of 124-127.5 K, determined by AC susceptibility measurements. The purest superconducting powder contained 95% of Hg-1212, judged from XRD, and had a superconducting volume chi' approximate to -2 X 10(-4) m(3)/kg (-20 X 10(-3) emu/g) corresponding to approximately 100 vol.% of the sample.