Structural, thermal and superconducting properties of Ag2O-doped YBa2Cu3O7-x composite materials

A composite material with superconducting properties, consisting of YBa2Cu3O7-x doped with Ag2O, was obtained after 233 h of sintering at 960 °C and followed by a special cooling program in oxygen atmosphere. Preferential orientation of YBa2Cu3O7-x (YBCO-123) crystallites was evidenced through X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM). Small increase of c parameter for orthorhombic YBCO-123 crystalline unit cell together with a slight decrease of oxygen content (7-x) versus Ag content, were encountered. The dimensions of YBCO-123 crystallites are dependent on Ag content. Thermogravimetric analysis (TGA) during heating in flowing air atmosphere from room temperature (RT) to 1100 °C, showed that total mass loss is depending on Ag content. The cooling (from 1100 °C to RT, in air atmosphere) endows optimal oxygen accumulation only for 0 and 5 wt% Ag, while final masses for 10 and 20 wt% of Ag content, are more than 10% less. Differential scanning calorimetry (DSC) showed peritectic melting points for YBa2Cu3O7-x dependent on Ag percentage. DSC graphics also highlighted the endothermic effects: silver melting; melting and decomposition of Ag2O (in 225.81–345.12 °C temperature range) only for 10 and 20 wt%, but completely missing in the case of 5 wt% Ag content. We concluded that some of Cu sites from YBCO-123 superconductor were substituted by Ag ions. Quasi de Almeida-Thouless lines in the range of magnetic field amplitudes from 0.4 to 800 A/m, in the temperature range from 77 K to RT, were also investigated. Two cases are found concerning intergranular critical currents (Jci): depinning mechanism available for 5 wt% Ag content; the decoupling mechanism for 10, and 20 wt% Ag content.