Zhang Qiang, Cheng Long, Liu Wei, Zheng Yun, Su Xianli, Chi Hang, Liu Huijun, Yan Yonggao, Tang Xinfeng, Uher Ctirad
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China.
Phys Chem Chem Phys. 2014 Nov 21;16(43):23576-83. doi: 10.1039/c4cp03468f. Epub 2014 Sep 2.
Mg2Si1-xSnx solid solutions are promising thermoelectric materials for power generation applications in the 500-800 K range. Outstanding n-type forms of these solid solutions have been developed in the past few years with the thermoelectric figure of merit ZT as high as 1.4. Unfortunately, no comparable performance has been achieved so far with p-type forms of the structure. In this work, we use Li doping on Mg sites in an attempt to enhance and control the concentration of hole carriers. We show that Li as well as Ga is a far more effective p-type dopant in comparison to Na or K. With the increasing content of Li, the electrical conductivity rises rapidly on account of a significantly enhanced density of holes. While the Seebeck coefficient decreases concomitantly, the power factor retains robust values supported by a rather high mobility of holes. Theoretical calculations indicate that Mg2Si0.3Sn0.7 intrinsically possesses the almost convergent double valence band structure (the light and heavy band), and Li doping retains a low density of states (DOS) on the top of the valence band, contrary to the Ga doping at the sites of Si/Sn. Low temperature specific heat capacity studies attest to a low DOS effective mass in Li-doped samples and consequently their larger hole mobility. The overall effect is a large power factor of Li-doped solid solutions. Although the thermal conductivity increases as more Li is incorporated in the structure, the enhanced carrier density effectively shifts the onset of intrinsic excitations (bipolar effect) to higher temperatures, and the beneficial role of phonon Umklapp processes as the primary limiting factor to the lattice thermal conductivity is thus extended. The final outcome is the figure of merit ZT ∼ 0.5 at 750 K for x = 0.07. This represents a 30% improvement in the figure of merit of p-type Mg2Si1-xSnx solid solutions over the literature values. Hence, designing low DOS near Fermi level EF for given carrier pockets can serve as an effective approach to optimize the PF and thus ZT value.
Mg2Si1-xSnx固溶体是有望用于500 - 800 K范围内发电应用的热电材料。在过去几年中已开发出这些固溶体出色的n型形式，其热电优值ZT高达1.4。不幸的是，到目前为止该结构的p型形式尚未实现可比的性能。在这项工作中，我们尝试通过在Mg位点上进行Li掺杂来提高和控制空穴载流子浓度。我们表明，与Na或K相比，Li以及Ga是更有效的p型掺杂剂。随着Li含量的增加，由于空穴密度显著提高，电导率迅速上升。虽然塞贝克系数随之降低，但功率因数因空穴具有相当高的迁移率而保持稳健值。理论计算表明，Mg2Si0.3Sn0.7本质上具有几乎收敛的双价带结构（轻带和重带），并且与在Si/Sn位点进行Ga掺杂相反，Li掺杂在价带顶部保持低态密度（DOS）。低温比热容研究证明Li掺杂样品中的DOS有效质量较低，因此其空穴迁移率更大。总体效果是Li掺杂固溶体具有较大的功率因数。尽管随着更多的Li掺入结构中热导率会增加，但增强的载流子密度有效地将本征激发（双极效应）的起始温度转移到更高温度，因此作为晶格热导率主要限制因素的声子Umklapp过程的有益作用得以扩展。最终结果是对于x = 0.07，在750 K时优值ZT约为0.5。这代表p型Mg2Si1-xSnx固溶体的优值比文献值提高了30%。因此，为给定的载流子口袋在费米能级EF附近设计低DOS可以作为优化功率因数从而优化ZT值的有效方法。
