Effect of (Bi, Pb)-2223 addition on thermal transport of superconducting MgB2 pellets

Polycrystalline MgB2 pellets containing 0, 3, 7, and 10 wt.% of (Bi, Pb)-2223 {Bi1.8Pb0.26Sr2Ca2Cu3O10+x} superconducting powder has been synthesized by solid state reaction. Micro-structural observations confirm a well developed MgB2 phase with randomly oriented hexagonal grains. XRD data shows absence of substitution effect and presence of unreacted 2223 as isolated grains or, clusters is indicated in FESEM. In addition, transition temperature (Tc) of MgB2 remains practically unaffected with 2223 addition in MgB2. Temperature dependence of thermal conductivity of the present set of samples does not show any anomalous features at/below Tc. However, the magnitude of thermal conductivity decreases substantially with addition of 2223. Ideal Wiedemann-Franz's law fails to provide acceptable estimation of the electronic part of the total thermal conductivity. Two band analysis of the electronic thermal conductivity in superconducting state reveals dominance of π band contribution. Analysis also confirms that addition of 2223 in MgB2 introduce extra defects/disorder that cause a substantial decrease in π band gap, as well as, intraband relaxation time of pure MgB2. On the other hand, σ band gap of MgB2 is not much affected due to addition of 2223. Lattice thermal conductivity analyzed using Callaway's expression indicates that it is mainly governed by phonon scattering from boundary, point defects and sheet-like faults. However, dominant role is played by phonon scattering from sheet like faults, which decides the lattice thermal conductivity of both pure and 2223 added MgB2.