Fabrication of c-axis oriented higher manganese silicide by a high-magnetic-field and its thermoelectric properties

We report the preparation and thermoelectric properties of oriented higher manganese silicide (HMS) with a composition of MnSi_1.73 bulk. The grain alignment and densification were achieved by rotating high magnetic field and spark plasma sintering (SPS) techniques, respectively. The easy magnetization axis of MnSi_1.73 was found to be c-axis, and the applied magnetic field of 2 T was strong enough to rotate the powder with a mean grain size of 1 μm. The c-axis of grains was oriented when applying the magnetic field, and the degree of orientation was further increased after heat treatment. However, a secondary phase that was mono manganese silicide (MnSi) was observed as a result of oxidation on the surface of synthesized powder. The electrical conductivity of the c-axis oriented specimen along the ab-plane was about 40% larger than that for sample processed only by SPS, while the Seebeck coefficient of oriented and nonoriented specimens showed similar values regardless of existence of the second phase. Consequently, the power factor of the c-axis oriented specimen along the ab-plane was enhanced by about 35% compared to the nonoriented one. The proposed approach is found to be very effective not only in obtaining the oriented materials with nonductility but also in enhancing the thermoelectricity.