Enhanced thermoelectric figure of merit in nano-structured Si dispersed higher manganese silicide.

Higher manganese silicide (HMS) is considered as a promising thermoelectric material at intermediate temperatures. Samples of Si-rich HMS were prepared by arc melting followed by ball milling combined with densification by an induction hot uni-axial pressing (HP) and spark plasma sintering (SPS), respectively. Powder X-ray diffraction, SEM and EPMA studies confirmed the presence of Si in HMS matrix. TEM micrographs on milled powders further confirmed the presence of Si particles with an average size of ~5–10 nm in HMS matrix. Microstructure investigations on densified samples revealed that SPS process seems to be a beneficial tool for embedding the nanostructures of Si (~20–50 nm) particles in HMS matrix. SPS also controls the grain growth of HMS during densification, which in turn reduces the total thermal conductivity from ~4.4 W/m.K to 2.10 W/m.K. On the other hand, samples processed by HP showed the value of ~2.44 W/m.K with similar sintering parameters as in SPS used for densification. Huge reduction in lattice thermal conductivity of about ~55%, and a considerable increase in Seebeck value was observed in Si-rich BMed HMS. However, reduction in electrical conductivity associated with insulating Si particles in HMS matrix limited the z T to ~0.26 at 725 K. Scattering of mid-to-long wavelength phonons by increased grain boundaries of HMS and embedded Si nano-particles and their consequence in lattice thermal conductivity of Si-rich HMS as a function of temperature. Image 1 • Higher manganese silicide (HMS) with embedded Si nano-particles was prepared by arc melting and ball milling. • Pertitectic formation of long-striped MnSi in the matrix of HMS was completely suppresses via addition of excess Si. • Thermal conductivity of MnSi 2 was drastically reduced due to increased scattering of mid-to-long wavelength phonons. • Si-rich HMS sample thus densified by SPS for 2 min shows the highest z T value of ~0.26 at 725 K. [ABSTRACT FROM AUTHOR]