Abstract
Electrical conductivities and Seebeck coefficients of Fe-doped Bi-2212 type superconductors have been reported in the temperature range 300--10 K. These superconductors are obtained by annealing the Fe-doped amorphous ${\mathrm{Bi}}_{3}{\mathrm{PbSr}}_{3}{\mathrm{Ca}}_{3}{\mathrm{Cu}}_{4\ensuremath{-}m}{\mathrm{Fe}}_{m}{\mathrm{O}}_{x}$ $(m=0.0,$ 0.02, 0.04, 0.05, and 0.06) system around 840 \ifmmode^\circ\else\textdegree\fi{}C for 40 h in air. The superconducting transition temperature ${T}_{c}$ decreases continuously with a gradual increase of Fe content and finally diminishes for $m>~0.06$ supporting the Abrikosov-Gor'kov-type pair-breaking mechanism. Unlike undoped glass ceramics, semiconducting behavior exhibited by all the Fe-doped glass ceramics in their normal states can be explained by Mott's variable range hopping model. The Seebeck coefficients (S) of the glass-ceramic superconductors showed interesting nonlinear variation with temperature. Such nonlinear variation of thermopower is considered to be associated with phonon-drag effect which is, however, not the case in undoped (Fe free) glass-ceramic superconductors. Formation of impurity bonds such as Cu-O-Fe, Fe-O-Fe, etc., is considered to be more important than the magnetic effect for the depression of ${T}_{c}$ and nonlinear thermal variation of Seebeck coefficient (S) in these superconductors.
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