Thermal Transport Driven by Extraneous Nanoparticles and Phase Segregation in Nanostructured Mg₂(Si,Sn) and Estimation of Optimum Thermoelectric Performance

Abstract

Solid solutions of magnesium silicide and magnesium stannide were recently reported to have high thermoelectric figure-of-merits (ZT) due to remarkably low thermal conductivity, which was conjectured to come from phonon scattering by segregated Mg2Si and Mg2Sn phases without detailed study. However, it is essential to identify the main cause for further improving ZT as well as estimating its upper bound. Here we synthesized Mg2(Si,Sn) with nanoparticles and segregated phases, and theoretically analyzed and estimated the thermal conductivity upon segregated fraction and extraneous nanoparticle addition by fitting experimentally obtained thermal conductivity, electrical conductivity, and thermopower. In opposition to the previous speculation that segregated phases intensify phonon scattering, we found that lattice thermal conductivity was increased by the phase segregation, which is difficult to avoid due to the miscibility gap. We selected extraneous TiO2 nanoparticles dissimilar to the host materials as additives to reduce lattice thermal conductivity. Our experimental results showed the maximum ZT was improved from ∼0.9 without the nanoparticles to ∼1.1 with 2 and 5 vol % TiO2 nanoparticles at 550 °C. According to our theoretical analysis, this ZT increase by the nanoparticle addition mainly comes from suppressed lattice thermal conductivity in addition to lower bipolar thermal conductivity at high temperatures. The upper bound of ZT was predicted to be ∼1.8 for the ideal case of no phase segregation and addition of 5 vol % TiO2 nanoparticles. We believe this study offers a new direction toward improved thermoelectric performance of Mg2(Si,Sn).

Thermoelectric Transport in Nanocomposites

“…Simultaneously, the increased interface between the AZO and rGOs could remarkably reduce the lattice thermal conductivity due to the increasing phonon-boundary scattering [99]. Other materials such as manganese silicide based NCs [100] and magnesium silicide based NCs [101,102,103], polycrystalline SnSe embedded with PbTe nanoinclusions [104], Skutterudites CoSb 3 compounds [105] and Yb x Co 4 Sb 12 alloy [106] based NCs, n -type half-Heuslers alloy Hf 0.75 Zr 0.25 NiSn 0.99 Sb 0.01 based NCs [107], p -type half-Heuslers alloy Zr 0.5 Hf 0.5 CoSb 0.8 Sn 0.2 based NCs [108] and Zr 0.25 Hf 0.25 NiSn based NCs [109], BiCuSeO-based NCs [110] and InGaAs alloy embedded with ErAs nanoparticles [111,112] are also studied. Furthermore, some non-TE materials like carbon nanotube [113], C 60 [114,115], graphene [116,117], SiC [118] and platinum nanocrystals [119] are used as nanoconstituents to embed into Bi 2 Te 3 and Sb 2 Te 3 based materials to enhance ZT .…”

Uncertainty propagation in a multiscale CALPHAD-reinforced elastochemical phase-field model

“…Experimental determination of ZT even in a single alloy exhibits considerable variance, perhaps due to changes in the way these materials are synthesized and processed. Recently, the present authors and collaborators investigated the effect of non-equilibrium processing on the microstructure evolution (and transport properties) in the Mg 2 Si 0.7 Sn 0.3 system and found that instead of phase-separating, the system tended to form a solidsolution with superior TE performance, contrary to expectations and prior works [56,65]. This was ascribed to (elastic) coherency effects and was verified via quantitative multi-physics phase field simulations [39].…”

“…Here we return to the connection with performance in TE materials by noting that microstructures that correspond to non decomposed states could be associated with alloying/mass phonon scattering, while decomposed microstructures corresponds to interfacial phonon scattering. The length scales of different scattering mechanisms are different and are thus expected to change the phonon transport characteristics and the corresponding thermoelectric performance of the Mg 2 {Sn,Si} system [56]. Figure 10 illustrates the nature of such decision boundaries in the 2D input parameter regions which can be used to determine the desired regions (alloy/mass vs interface scattering) in the material parameter space.…”

Enhanced thermoelectric performance of Mg2Si1−xSnx codoped with Bi and Cr

“…TiO 2 nanoparticles are found to increase the thermoelectric efficiency when added in small amount in pure and Sn-doped Mg 2 Si. 18,[24][25][26] Even (110) surface thin films of Mg 2 Si are found to enhance the Seebeck coefficient over that of bulk. 25 In all, Mg 2 Si serve as very promising and economical TE material with suitable doping and nanostructuring.…”

“…ZT >1.45 at 750 K is reported for a combined optimal Sn, Sb and Ge doping for Si in Mg 2 Si. 16 Solid solutions of Mg 2 (Si,Sn) with nanoparticles and (unavoidable) segregated phases were formed, 18 with a maximum ZT of 0.9. At optimal Sn-doping in Mg 2 Si, 19 ZT as high as 1.1 is reported, comparable to the best PbTe and n-SiGe alloys.…”