Relevance of Solidification Kinetics for Enhanced Thermoelectric Performance in Al-Doped Higher Manganese Silicides

Abstract

The solidification kinetics of an alloy from its liquid state forms an underlying basis for microstructural engineering, wherein the state of thermodynamic equilibrium associated with the melt-grown crystal and the quenched amorphous solid denotes the two limits for crystallinity in the alloy synthesis. In this study, we report the implication of the crystalline state on the thermal and electrical transport properties of partially substituted Mn(Si 1−x Al x ) γ by comparing the single crystals melt-grown by the Bridgman method, and polycrystals synthesized from melt spinning (MS) and subsequent spark plasma sintering (SPS). The rapidly solidified alloys exhibited nanocrystalline microstructures in MS ribbons, while melt-grown single crystals displayed characteristics evolution of MnSi striations with limited solubility of Al. It was observed that Al as a p-type dopant enhances the carrier concentration and electrical conductivity, while nanocrystallinity in MS + SPS polycrystals and secondary phases in monocrystals were effective in enhancing the phonon scattering. Maximum zT values of ∼0.54 (±0.05) at 823 K and 0.75 (±0.05) at 873 K were attained for the single crystal (directed perpendicular to the c-axis) and melt-spun polycrystals (along the in-plane direction), respectively. These results present the efficacy of aliovalent Al substitution and demonstrate the critical role of the solidification kinetics in optimizing the carrier concentration and enhancing the phonon scattering in higher manganese silicide crystals for thermoelectric applications.

Lattice anharmonicity in charge compensated higher manganese silicide single crystals

“…For comparison with the previously reported single crystals of MnSi γ , the attained maximum zT is displayed in Fig. 11, wherein the synthesized partially co-substituted (Mn 1−2 x V x Fe x )Si 1.74 crystals, outperformed previously reported HMS single crystals synthesized from chemical vapor deposition (CVD), 44 or partially substituted V, 22 Al, 45 and Ge 35 melt grown by Bridgman methods, which emphasizes a remarkable prospect for compensated co-doping in HMS crystals to attain higher zT , comparable to its polycrystal counterpart.…”

Discordant Doping and Labile Silicon Atoms in Higher Manganese Silicide

“…(a) Temperature-dependent thermoelectric figure of merit ( zT ) and (b) comparison of maximum zT attained for synthesis with previously reported B-, Ge-, , and Al-doped higher manganese silicide.…”

“…For discordant dopants, different phases may emerge upon solidification, introducing defects, inhomogeneity, and altered crystal structure. To enhance substitutability, rapid consolidation methods, particularly melt spinning, have proven effective in achieving a supersaturated solid solution of HMS, as demonstrated in the case of Al- and Re , -substituted HMS. Remarkably, the zT was significantly enhanced to 0.75 at 873 K in Al-doped HMS polycrystals prepared by melt-spinning .…”

“…To enhance substitutability, rapid consolidation methods, particularly melt spinning, have proven effective in achieving a supersaturated solid solution of HMS, as demonstrated in the case of Al- and Re , -substituted HMS. Remarkably, the zT was significantly enhanced to 0.75 at 873 K in Al-doped HMS polycrystals prepared by melt-spinning . Within the realm of doping at the silicon (Si) site in HMS, the inclusion of germanium (Ge) and boron (B) offers a novel prospect among the available alternatives.…”

Charge-Compensated (V, Ru) Co-Substitution in Higher Manganese Silicide Single Crystals for Enhanced Thermoelectric and Mechanical Performance

“…Moreover, limited control of various synthesis methodologies and the ineffectiveness of nanoengineering approaches in deterring the inevitable segregation of MnSi striation, further skewed efforts aiming toward zT optimization. − Thus, a marginal improvement in power factors and complex microstructural control had long limited the peak zT < 0.6 despite approaching the minimum κ L ∼ 1.6 W m –1 K –1 at 723 K in re-substituted HMS . Although maximum zT has been enhanced by nonequilibrium methods such as melt spinning, − the prospects of a single crystal have remained practically ignored owing to its lower zT ∼ 0.2 at 800 K and the anisotropic segregation of metallic MnSi as a secondary phase during crystal growth. − Single-crystal HMS specimen aligned perpendicular to the c -axis (i.e., along the cleavage ab -plane) exhibited high electrical and thermal conductivity, while maximum S was measured along the c -axis direction . The anisotropic transport properties thus require careful consideration, particularly in single crystal which can be exploited to attain higher zT in HMS.…”