Effect of fluorine doping on phase formation and properties of Bi(Pb)-2223 ceramics

Superconducting ceramics of Bi1.6Pb0.4Sr2Ca2Cu3OyFx (x = 0–0.6) are prepared in air by conventional solid state reaction and characterized. The study shows that the melting point of the samples decreases as fluorine content increases. As a consequence, the grain size increases with the doping level and for x = 0.6, the sample is completely deformed and presents a concave shape making impossible the measurements on it. The Vickers microhardness reaches its maximum for x = 0.2. The analysis of the X-ray diffraction results reveals that all the samples are composed of only Bi(Pb)-2212 and Bi(Pb)-2223 phases. The highest proportion of the high Tc phase (Bi(Pb)-2223) is also observed for x = 0.2 and is about 67.32%. The refinement of cell parameters is done by considering the structural modulation. The results show that the doping leads to a reduction of cell volume as well as the a axis component of modulation. From resistivity versus temperature measurements, it is shown that the doped phases exhibit higher onset critical transition temperatures than the undoped one. The residual resistivity increases with fluorine content suggesting that the doping introduces structural defects and disorder into the samples. The obtained critical current density at 77 K under zero magnetic field also increases with fluorine doping.

► The doping with fluorine of Bi(Pb)-2223 phase reduces its melting temperature.
► Increasing fluorine content will increase the grain size of the samples.
► The doping reduces the structural modulation and cell volume.
► The volume fraction of the phase and the superconducting properties are improved.