Department of Mechanical Engineering, Texas A&M University , College Station, Texas 77843, United States.
Korea Electrotechnology Research Institute , Changwon, Korea.
ACS Appl Mater Interfaces. 2016 Mar 23;8(11):7003-12. doi: 10.1021/acsami.5b12060. Epub 2016 Mar 8.
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).
最近有报道称，镁硅化物和镁锡化物的固溶体由于热导率显著降低而具有较高的热电优值（ZT），这被推测是由于分离的 Mg2Si 和 Mg2Sn 相的声子散射所致，但没有进行详细研究。然而，确定进一步提高 ZT 的主要原因以及估计其上限是至关重要的。在这里，我们合成了具有纳米颗粒和分离相的 Mg2（Si，Sn），并通过拟合实验得到的热导率、电导率和塞贝克系数，从理论上分析和估计了分离相分数和外加纳米颗粒添加对热导率的影响。与之前认为分离相加剧声子散射的推测相反，我们发现晶格热导率由于混溶性间隙而增加，这是难以避免的。我们选择与宿主材料不同的外来 TiO2 纳米颗粒作为添加剂来降低晶格热导率。我们的实验结果表明，在 550°C 时，与没有纳米颗粒的样品相比，添加 2vol%和 5vol% TiO2 纳米颗粒后，最大 ZT 从约 0.9 提高到约 1.1。根据我们的理论分析，纳米颗粒添加引起的 ZT 增加主要来自于晶格热导率的降低，以及高温下双极热导率的降低。在没有相分离和添加 5vol%TiO2 纳米颗粒的理想情况下，预测 ZT 的上限约为 1.8。我们相信这项研究为提高 Mg2（Si，Sn）的热电性能提供了一个新的方向。
