Calculation of the diffusion coefficient of Au in Bi-2223 superconductors

Abstract

Undoped Bi-2223 samples were prepared using a conventional solid-state reactionmethod. Doping of Au in Bi-2223 was carried out by means of diffusion from anevaporated Au film on pellets. We have investigated the effect of Au diffusion anddiffusion-annealing duration on the microstructure and superconducting properties ofAu-doped samples by performing x-ray diffraction (XRD), scanning electron microscopy(SEM), dc resistivity and critical current density measurements. Gold diffusion inBi1.8Pb0.35Sr1.9Ca2.1Cu3Oy has been studied over the temperature range500–800 °C using the technique of successive removal of thin layers and the measurementsof lattice parameters from XRD patterns at room temperature. The diffusiondoping of Bi-2223 by Au causes a significant increase of the lattice parameterc by about 0.19%. This observation is used for calculation of the Au diffusion coefficient inBi-2223. The Au diffusion coefficient decreases as the diffusion-annealing temperaturedecreases. The temperature dependence of the Au diffusion coefficient in the range500–800 °C is describedby the relation D = 4.4 × 10−4exp(−1.08 eV/kBT). Au doping of the sample increased the critical transition temperature and the critical current densityfrom 100 ± 0.2 to 104 ± 0.2 K andfrom 40 to 125 A cm−2, in comparison with those of undoped samples. The critical transition temperature and criticalcurrent density of Au-doped samples increased with increasing diffusion-annealing time from10 to 50 h. Au doping of the sample also improved the surface morphology and increased thehigh-Tc phase ratio. The possible reasons for the observed improvement in superconductingproperties of the samples due to Au diffusion are discussed.