Microstructure investigations and thermoelectrical properties of a P-type polycrystalline higher manganese silicide material sintered from a gas-phase atomized powder

Highlights: • Dense polycrystalline HMS samples are manufactured using a two steps process. • Such a process is compatible with mass production of sintered pellets. • ZT parameter is the best ever reported for this kind of material. - Abstract: A polycrystalline higher manganese silicide (HMS) material has been sintered from an aluminium-enriched gas-phase atomized powder using spark plasma sintering (SPS). After tailoring the SPS parameters, the polycrystal is almost fully dense, mainly constituted by the Mn₁₅Si₂₆ HMS phase and the average grain size is around 10 μm. Transmission electron microscopy investigations coupled to energy dispersive X-ray spectroscopy (EDS) measurements show that: (i) alumina is segregated at grain boundaries and multiple points; (ii) a small amount of residual silicon is homogeneously distributed in the sintered microstructure; (iii) intragranular nanometre-sized inclusions (averaged diameter around 20 nm, concentration of 8.9 × 10⁻⁴ inclusions/nm²) are observed in most of the individual grains constituting the polycrystal. Some are crystalline and made of metallic MnSi, some are residual holes/gas bubbles entrapped into the sintered microstructure during the manufacturing step; (iv) each individual grain contains around 1 at.% of aluminium that is dispersed in the Mn₁₅Si₂₆ elemental lattice and then acts possibly as a dopant. Thermoelectrical properties of the sintered material have been investigated in the 20–700 °C temperature range and compared to the literature. The material exhibits the desired P-type conduction, the Seebeck coefficient has a high value for all the temperature range and in the same time the thermal conductivity is especially low. It is postulated that aluminium doping and the presence of nanometre-sized inclusions in the sintered microstructure are responsible for the dimensionless figure of merit (ZT) around 0.7 measured at 500 °C. Such a value, obtained on a sample manufactured with a very simple process, is the best one ever reported for this kind of material