具有本征低晶格热导率的硫化银快离子导体的热电性能

硫化银(Ag_2S)是一种典型的快离子导体材料,前期关于Ag_2S的研究主要集中在光电和生物等领域.最近的研究表明, a-Ag_2S具有和金属一样的良好延展性和变形能力.但是, Ag_2S的热电性能尚无公开报道.本工作合成了单相Ag_2S化合物,系统研究了其在300—600 K范围的物相变化、离子迁移特性和电热输运性质.研究发现, Ag_2S在300—600 K温度区间表现出半导体的电输运性质.由于单斜-体心立方相晶体结构转变, Ag_2S的离子电导率、载流子浓度、迁移率、电导率、泽贝克系数等性质在455 K前后出现急剧变化.在550 K, Ag_2S的功率因子最高可达5μW·cm~(–1)·K~(–2). Ag_2S在300—600 K温度区间均表现出本征的低晶格热导率(低于0.6 W·m~(–1)·K~(–1)). S亚晶格中随机分布的类液态Ag离子是导致b-Ag_2S体心立方相具有低晶格热导率的主要原因.在573 K, Ag_2S的热电优值可达0.55,与Ag_2Se, Ag_2Te, CuAgSe等已报道的Ag基快离子导体热电材料的性能相当.

Thermoelectric properties of Ag2S superionic conductor with intrinsically low lattice thermal conductivity

Recently, Ag₂S superionic conductor has attracted great attention due to its metal-like ductility and deformability. In this work, the single phase Ag₂S compound is fabricated by the melting-annealing method. The crystal structure, ionic conduction, and electrical and thermal transports in a temperature range of 300－600 K are systematically investigated. The monoclinic-cubic crystal structure transition occurs around 455 K for Ag₂S. Significant reduction in the specific heat at constant volume below the Dulong-Petit limit is observed for Ag₂S after the monoclinic-cubic phase transition, which is attributed to the liquid-like Ag ions existing inside the sulfur framework. Ag₂S shows typical semiconducting-like electrical transport behavior in the whole measured temperature range. Around 455 K, the ionic conductivity, carrier concentration, carrier mobility, electrical conductivity, and Seebeck coefficient each show an abrupt change. The calculated ionic activation based on the ionic conductivity is 0.076 eV for the body centered cubic Ag₂S. The calculated band gap based on the electrical conductivity decreases from 1.1 eV for the monoclinic Ag₂S to 0.42 eV for the body centered cubic Ag₂S. The abrupt increase of electrical conductivity after the monoclinic-cubic phase transition leads to a maximum power factor around 5 μW·cm^-1·K^-2 at 550 K. In the whole measured temperature range, Ag₂S demonstrates an intrinsically low lattice thermal conductivity (below 0.6 W·m^-1·K^-1). The calculated phonon dispersion indicates that the weak chemical bonding between Ag and S is responsible for the low lattice thermal conductivity observed in the monoclinic Ag₂S. Likewise, the presence of liquid-like Ag ions with low ionic activation energy is responsible for the low lattice thermal conductivity for the cubic Ag₂S. Finally, the Ag₂S shows the maximum thermoelectric figure of merit of 0.55 at 580 K, which is comparable to the thermoelectric figure of merit reported before in most of Ag-based thermoelectric superionic conductors.