基于纳米晶与层状异质界面协同效应的Mg2(Sn,Si)基热电复合薄膜制备与性质研究

Layer-structured SnSe or MoS2 materials have higher thermoelectric properties, especially their layered characteristic possess lower heat conductivity. Mg2(Si,Sn)-based materials have higher Seebeck coefficient, moderate electrical conductivity, but higher heat conductivity. In the study, the nanocomposite structure composed of the Mg2(Si,Sn) and SnSe or MoS2 with Bi doping are designed on the basis of their advantage to further improve the thermoelectric properties. The influence of the nano-grain boundary and layered hetero-interface on the electron, phonon transport process and interface electron state is studied by controlling the thickness of SnSe or MoS2 single layer and the grain size between layers and by measuring the parameters such as electron concentration, mobility, Seebeck coefficient, electrical conductivity and heat conductivity. It is expected that the two dimensional layered structure of the SnSe or MoS2 in composite films accelerate the electron motion along layer direction and the mobility increase. The heat conductivity reduces because the longitudinal and transversal phonon transport process is suppressed by the nano-grain boundary and layered hetero-interface, respectively. Meanwhile, the Seebeck coefficient increases due to the increasing electron states density by the limit effect of the nanocrystalline between layered hetero-interfaces. The thermoelectric figure of merit is greatly improved by above collaboration effect.

层状结构的SnSe或MoS2热电性能较高，特别是其层状特征具有较小热导率， Mg2(Si,Sn)基材料具有较高的Seebeck系数和中等电导率，但热导率较大。本项目基于这两类材料的优势，设计Bi掺杂的Mg2(Si,Sn)与SnSe或MoS2的纳米复合结构，从而提高热电性能。将控制复合薄膜中SnSe或MoS2的单层厚度，以及夹层纳米晶粒的尺寸，并通过测量复合薄膜的载流子浓度、迁移率、Seebeck系数、电导率和热导率等参数，研究纳米晶面、层状异质界面及它们的复合作用对电子、声子的传输过程和界面电子态的影响。预期复合结构中SnSe或MoS2的二维层状结构将加速电子沿层界面方向的迁移，增加电子的迁移率，纳米晶界与层界面还分别抑制声子的纵向和横向的传输，降低热导率；同时，利用层状异质界面间纳米晶的限制效应，增加费米能附近电子态密度，增加Seebeck系数。综合协同上述效应，达到大幅度提高热电优值。

国家面上自然科学基金“基于纳米晶与层状异质界面协同效应的Mg2(Sn,Si)基热电复合薄膜制备与性质研究(No. 51772193)”是利用纳米晶和层界协同效应来提高复合薄膜的Seebeck系数进而提高功率因子和优值。研究了Mg和Sn含量对Mg2(Sn,Si)基薄膜热电性能的影响；研究了Cu含量对沉积Cu-Se薄膜的相组成和热电性能的影响；研究了Ag, Al, Ni, Mn掺杂对β-Cu2Se相薄膜的热电性能的影响；研究了Mg2Sn/Mg，Cu2Se/a-C和Cu2Se/SiC纳米多层膜的热电性能。结果表明，在Mg2(Si,Sn)薄膜中存在少量的金属Mg或Sn相，可提高Seebeck系数，进而提高功率因子。沉积薄膜中Cu/Se比率由0.49增加到5.74，其相组成由p-CuSe2 到 p-CuSe2+γ-CuSe,到γ-CuSe,到γ-CuSe+β-Cu2-xSe,到β-Cu2-xSe,到α-Cu2Se+未知相,到 α-Cu2Se+未知相+Cu。室温载流子浓度以CuSe2, CuSe 和 β-Cu2-xSe相顺序减小而迁移率呈现相反的趋势；Seebeck系数和功率因子都以β-Cu2-xSe, CuSe2 和 CuSe 相顺序减小。具有单一β-Cu2Se相薄膜具有较高的Seebeck系数和功率因子，是一种理想而实用的热电材料。掺杂Ag的β-Cu2Se薄膜中形成纳米尺寸的CuAgSe相，其它掺杂元素形成替位式固溶体，适当含量的元素掺杂都可获得最佳功率因子。Mg2Sn/Mg、β-Cu2Se/a-C和β-Cu2Se/SiC纳米多层膜随着调制周期的减小，载流子浓度和迁移率都减小，而Seebeck系数和电阻率都增加。导电性能良好的金属Mg和非晶碳（a-C）作为隔层构成多层膜不仅显著提高Seebeck系数还使其电阻率增加很小，这对提高复合膜层的热电性能是非常有益的。