Phase analysis and determination of local charge carrier concentration in eutectic Mg2Si–Si alloys

Abstract

Multiphase materials attract attention due to possible combination of various properties attributed to each phase. The phase diagram of Mg–Si system shows that solidification of a melt containing about 45 and 55 at.% of Mg and Si should result in formation of Mg2Si and Si. Two alloys, Mg45Si55 and Mg46Si54 + 0.5 wt.% Cu have been synthesized and studied using XRD, SEM, and 29Si NMR at 300 K, and the Seebeck effect, electrical resistivity, and thermal conductivity in the temperature range of 300–750 K have been measured. 29Si NMR detects two distinct signals, at −177 and −80 ppm, in both materials, which are assigned to Mg2Si and Si phases, respectively. Both phases are slightly nonstoichiometric and doped with Mg. Two phases also are found by XRD and electron microscopy. 29Si NMR spin-lattice relaxation measurements in Mg2Si and Si phases show at least two components, short and long, which can be attributed to different local carrier concentrations, high and low, respectively, reflecting a local electronic inhomogeneity in each phase. The carrier concentrations range between 0.6 × 1019 and 9 × 1019 cm−3. The Seebeck coefficient in both alloys is mostly determined by the Si phase, while the thermal conductivity is limited by the Mg2Si phase with a lower value than that of the Si phase. By utilizing all characterization tools, we show how various experimental methods can be used as complementary methods to better understand the individual and combined properties of multiphase alloys.