Thermoelectric Properties of Bi‐Doped Mg₂Si_0.6Sn_0.4 Solid Solutions Synthesized by Two‐Step Low Temperature Reaction Combined with Hot Pressing

Abstract

The goal of this study is to investigate the influence of the amount of Bi doping on the microstructure and thermoelectric properties of the Mg 2 Si 0.60 Sn 0.4 material studied by X-ray diffraction, scanning electron microscopy, and thermoelectric measurements. The results of the present study provide strong evidence for the presence of a number of secondary phases with different stoichiometries, including Si-rich and Sn-rich phases, besides the main matrix and the MgO. Phase separation could be attributed to preferential occupancy of the dopant (bismuth in the present case) on tin sites in the lattice, leading to segregated darker regions that are observed in the SEM micrographs, which arose as a result of nondiffusion of Si in the matrix. The electron concentration increases with increasing Bi concentration, similar with Sb. The optimum amount of Bi seems to correspond to Mg 2 Si 0.57 Sn 0.4 Bi 0.03 with maximum dimensionless figure of merit of %1.2 at 850 K, arising from a high power factor 36 mW cm À1 K À2 and a low lattice thermal conductivity of %1.5 W m À1 K À1 . These materials exhibit a high thermoelectric figure of merit, which is one of the best among all reported Mg 2 Si 1Àx Sn x compounds on the Si-rich side.

Thermoelectric Properties of Bi-Doped Magnesium Silicide Stannides

“…24 The former sample exhibits higher σ than most known high-performance materials in this family, including Mg 2 Si 0.57 Sn 0.4 Bi 0.03 with 1780 Ω −1 cm −1 at 320 K, which was synthesized by a two-step solid-state reaction combined with hot-pressing, and exhibited a zT max (850 K) = 1.2. 34 As the lattice parameter continued to increase from 3 to 4.5% Bi, at least part of the additional Bi atoms is present in the crystals of the 4.5% Bi sample and should provide additional electrons. Thus, we postulate that the experimentally observed lower electrical conductivity of the 4.5% Bi sample results either from the ionized impurity scattering between carriers as found in other n-type Mg 2 Si-doped materials 35−37 and/or from the presence of small additional side products as Mg 3 Bi 2 , as found recently.…”

“…325 Sn 0.645 Bi 0.03 has the highest electrical conductivity (2490 Ω –1 cm –1 ) compared to 2140 Ω –1 cm –1 for Mg 2 Si 0.30 Sn 0.67 Bi 0.03 at 300 K. The results exhibit comparable σ to Mg 2 Si 0.3 Sn 0.665 Bi 0.035 with σ = 2400 Ω –1 cm –1 at 320 K, which was prepared via two-stage ball milling, followed by annealing using tantalum crucibles sealed by arc melting method . The former sample exhibits higher σ than most known high-performance materials in this family, including Mg 2 Si 0.57 Sn 0.4 Bi 0.03 with 1780 Ω –1 cm –1 at 320 K, which was synthesized by a two-step solid-state reaction combined with hot-pressing, and exhibited a zT max (850 K) = 1.2 …”

“…At higher dopant concentrations, the additional electrons from the Bi atoms populate the parabolic conduction bands, and therefore, the Seebeck coefficient is reduced . The Seebeck coefficient values range from −145 μV K –1 (1.5% Bi) to −119 μV K –1 (3% Bi), and −113 μV K –1 (4.5% Bi) at 300 K. The compounds with 0.03 Bi and different Sn contents exhibit no significant difference in their S values, all increasing from about −115 μV K –1 at 300 K to −215 μV K –1 at 773 K in agreement with the reported values for Mg 2 Si y Sn 0.97– y Bi 0.03 . ,, …”

“…38 The Seebeck coefficient values range from −145 μV K −1 (1.5% Bi) to −119 μV K −1 (3% Bi), and −113 μV K −1 (4.5% Bi) at 300 K. The compounds with 0.03 Bi and different Sn contents exhibit no significant difference in their S values, all increasing from about −115 μV K −1 at 300 K to −215 μV K −1 at 773 K in agreement with the reported values for Mg 2 Si y Sn 0.97−y Bi 0.03 . 24,31,34 The total thermal conductivity is composed of an electrical (k e ) and a lattice contribution (k L ). k e can be calculated via the Wiedemann−Franz relation, k e = LσT.…”

Key properties of inorganic thermoelectric materials—tables (version 1)

“…Figure 13. The maximum thermoelectric figure of merit, zT, of selected n-type magnesium silicides: Mg2Si0.97Bi0.03 [806], Mg2Si0.5875Sn0.4Sb0.0125 [807], Mg1.995La0.005Si0.58Sn0.42 [808], Mg2.10Si0.38Sn0.6Sb0.02 [809], Mg2Si0.487Sn0.5Sb0.013 [810], Mg2(Si0.3Sn0.7)0.975Sb0.025 [811], Mg2Si0.6Ge0.4B i0.02 [812], Mg2Si0.385Sn0.6Sb0.015 [813], Mg2Si0.3925Sn0.6Sb0.0075 [814], Mg2(Si0.4Sn0.6)0.97Bi0.03 [815], Mg2Si0.57Sn0.4Bi0.03 [816], Mg2.2Si0.49Sn0.5Sb0.01 [817], (Mg2.06Si0.3Sn0.68Bi0.02 [818], Mg2Si0.35Sn0.62Bi0.03 [819], Mg2(Si0.4Sn0.6)0.82Sb0.18 [820], Mg2Si0.53Sn0.4Ge0.05Bi0.02 [821], Mg2.08Si0.364Sn0.6Sb0.036 [789], Mg2.08Si0.37Sn0.6Bi0.03 [822], Mg1.98Cr0.02(Si0.3Sn0.7)0.98Bi0.02 [788]. Material Material Material Material Material 3.12.…”