Thermal Conductivity of Bulk MgB2 Produced by Infiltration of Different Boron Powders

The thermal conductivity (TC) of superconducting bulk materials is a key property in determining its stability at the cryogenic conditions and the superconducting behavior in applications as the current leads and the resistive current limiters. The thermal conductivity of bulk MgB₂ materials obtained by the reactive liquid Mg infiltration technique has been measured, in the temperature range from 4.2 K to 300 K, on samples having different grain size and derived from boron powders of different crystallinity and different granulometry. A wide spread of the TC values has been detected for the different samples and a typical maximum of TC at temperatures around T_c (critical temperature) has been detected only in the materials derived by very pure and macro-crystalline boron. On the contrary, the MgB₂ materials derived from less pure commercial so called "amorphous" boron, in which finer crystalline grains are detectable, show very low values of the thermal conductivity without any maxima in the lower temperatures range. The different behavior can be phenomenologically explained in terms of dependence of the electronic thermal conductivity on the lattice defects and impurities in the samples, with a larger amount of these defects in the microcrystalline samples. An interesting contribution is suggested to be due to the lately described Mg₂B₂₅ structure.