Physical property characterization of bulk MgB<Subscript>2</Subscript> superconductor

We report synthesis, structure/micro-structure, resistivity under magnetic field [ρ(T)H], Raman spectra, thermoelectric power S(T), thermal conductivity κ(T), and magnetization of ambient pressure argon annealed polycrystalline bulk samples of MgB<Subscript>2</Subscript>, processed under identical conditions. The compound crystallizes in hexagonal structure with space group P6/mmm. Transmission electron microscopy (TEM) reveals electron micrographs showing various types of defect features along with the presence of 3–4 nm thick amorphous layers forming the grain boundaries of otherwise crystalline MgB<Subscript>2</Subscript>. Raman spectra of the compound at room temperature exhibited characteristic phonon peak at 600 cm<Superscript>-1</Superscript>. Superconductivity is observed at 37.2 K by magnetic susceptibility χ(T), resistivity ρ(T), thermoelectric power S(T), and thermal conductivity κ(T) measurements. The power law fitting of ρ(T) give rise to Debye temperature (Θ<Subscript>D</Subscript>) at 1400 K which is found consistent with the theoretical fitting of S(T), exhibiting Θ <Subscript>D</Subscript> of 1410 K and carrier density of 3.81 × 10<Superscript>28</Superscript>/m<Superscript>3</Superscript>. Thermal conductivity κ(T) shows a jump at 38 K, i.e., at T<Subscript>c</Subscript>, which was missing in some earlier reports. Critical current density (J<Subscript>c</Subscript>) of up to 10<Superscript>5</Superscript> A/cm<Superscript>2</Superscript> in 1–2 T (Tesla) fields at temperatures (T) of up to 10 K is seen from magnetization measurements. The irreversibility field, defined as the field related to merging of M(H) loops is found to be 78, 68 and 42 kOe at 4, 10 and 20 K respectively. The superconducting performance parameters viz. irreversibility field (H<Subscript>irr</Subscript>) and critical current density J<Subscript>c</Subscript>(H) of the studied MgB<Subscript>2</Subscript> are improved profoundly with addition of nano-SiC and nano-diamond. The physical property parameters measured for polycrystalline MgB<Subscript>2</Subscript> are compared with earlier reports and a consolidated insight of various physical properties is presented. Copyright EDP Sciences/Società Italiana di Fisica/Springer-Verlag 2008