Soft-chemistry synthesis of Ba2Ca2Cu3O x precursor and characterization of high-T c Hg0.8Pb0.2Ba2Ca2Cu3O8+δ superconductor

The paper describes the sol–gel process applied to synthesize a mercury-free Ba2Ca2Cu3O x precursor and the physicochemical properties of an Hg0.8Pb0.2Ba2Ca2Cu3O8+δ high-temperature superconductor (HTS) polycrystalline sample. The Ba2Ca2Cu3O x precursor with desired chemical and phase compositions was obtained using EDTA gel process followed by decomposition and calcination under optimized conditions. An Hg0.8Pb0.2Ba2Ca2Cu3O8+δ superconductor with a fine-grained microstructure, composed predominantly of the (Hg,Pb)-1223 phase in the form of plate-like crystallites or oval grains and with advantageous magnetic properties (T c(on) = 129.2 K, ΔT c = 6.5 K), was synthesized using the high-pressure crystallization method. At 20 K and 1 T, the critical current density of the studied sample was approximately 26 A/mm2, while at 4 K and 1 T it increased to 155 A/mm2. The high volume fraction of the (Hg,Pb)-1223 phase (89.1%) in the high-T c sample was associated with the low value of the average copper valence (2.11) in the calcinated Ba2Ca2Cu3O x precursor. Small amounts of non-superconducting secondary phases—BaCuO2, CaO, CaHgO2, CuO, Ca2CuO, BaPbO3—were also identified within the microstructure of Hg0.8Pb0.2Ba2Ca2Cu3O8+δ .