Thermal Conductivity of n-Aluminium Nitride-Added MgB2 Superconductor in Normal and Superconducting State

Abstract

Thermal conductivity (k) of 0, 5, 10, and 15 wt.% aluminium nitride (n-AlN)-added polycrystalline MgB 2 superconductors, synthesized by solid reaction is discussed both in the normal and superconducting states between 20 and 300 K. The prepared samples are characterized using X-ray diffraction (XRD) and field electron gun scanning electron microscope (FEG–SEM). Resistivity measurement confirms a decrease in superconducting transition temperature of MgB 2 (T c=38.5 K) with n-AlN addition and decreases to ∼35 K in case of 15 wt.% n-AlN-added MgB 2 sample. Thermal conductivity of both MgB 2 and n-AlN-added MgB 2 pellets does not show any hump around T c, and the absolute values of k decrease with increasing n-AlN in MgB 2. Temperature dependence of the thermal conductivity of MgB 2 and n-AlN-added MgB 2 has been analyzed, assuming the role of both electrons and phonons. The Wiedemann–Franz law does not work well for the present samples, which indicates inelastic scattering (L eff < L 0). Thermal conductivity of MgB 2 and n-AlN-added MgB 2 pellets is explained by assuming effective Lorentz number, L eff= 0.1 L 0. Electronic thermal conductivity in superconducting state ( $k_{\text {el}}^{\mathrm {s}} )$ follows “two-gap model” and has been used to estimate the values of band gaps, relative contribution of each band in thermal transport, and intraband scattering relaxation time. The estimated values are fairly consistent with the previously reported results for MgB 2. We further confirm that n-AlN addition in MgB 2 introduces disorders in π bands, which reduce the π band gapsand intraband relaxation time ( $\tau _{\pi }^{\text {im}})$ . The lattice contribution of thermal conductivity in both normal and superconducting states is analyzed in the terms of Callaway’s model, assuming various phonon scatterings. Our analysis indicates that the lattice thermal conductivity of MgB 2 is dominated by phonon-sheet-like fault scattering. Addition of n-AlN in MgB 2 enhances the phonon scattering from sheet-like faults, and dislocations induced strain field scattering by >7 times compared to that for pure MgB 2 pellets.