Ag基快离子导体热电材料的研究进展

Recent Advances in Ag-Based Superionic Thermoelectric Materials

快离子导体材料中高的载流子迁移率和本征低的晶格热导率使其在室温区和中温区具有良好的热电性能。Cu基和Ag基快离子导体热电材料是其中研究最广泛的两个体系。本文详细介绍了Ag基快离子导体Ag2X(X=Te,Se,S)材料的结构、热电性能、结构相变及其对性能的影响。综述了Ag2X基材料的热电性能研究进展,主要包括制备方法的改进、载流子浓度的优化和能带结构的调节三个方面,同时对近来在该材料体系中报道的良好的塑性变形能力进行了阐述。此外,Ag2X基材料中自由迁移的Ag+虽然能够有效地降低晶格热导率,但也会给材料热电性能的稳定性和重复性带来不利影响,因此也总结了快离子导体热电材料提高稳定性的方法。最后,针对Ag2X基快离子导体材料中载流子浓度难以优化、离子迁移带来的成分均匀性和性能稳定性等问题展望了后续研究工作中的重点和难点。

Thermoelectric(TE)materials could realize direct conversion between thermal and electric energy by the transport of carriers and phonons,which had wide application in the fields of power generation and thermoelectric refrigeration.However,the relatively low conversion efficiency had limited the large-scale commercial usage of TE devices.Therefore,main issues of TE materials were still further improving the TE performance and developing new materials with intrinsically high TE performance.Currently,superionic conductors,possessing good electrical properties and intrinsically ultralow thermal conductivity were promising TE materials from room temperature to the middle temperature range.Cu-based superionic conductors were the most widely investigated system due to their excellent TE performance,low cost and low toxicity,and mainly exhibited p-type conduction due to the inevitable existence of Cu vacancies in the lattices.In contrast,most of the Ag-based superionic conductors showed n-type electrical transport behavior,which might be ascribed to the ionized interstitial Ag atoms inside the lattice.In this review,the structural characteristics of Ag2 Xbased(X=Te,Se,S)compounds were elaborated,which consisted of two independent sublattices,a rigid sublattice composed of chalcogenide atoms to form an anionic framework for electrons transport and a liquid-like sublattice composed of mobile Ag+to scatter phonons,decoupling the transport properties of carriers and phonons.Moreover,the carrier mobility of Ag2 X compounds was much higher than that of other typical semiconductor TE materials due to their small carrier effective mass.Subsequently,the impacts of structural phase transition on band structure,carrier concentration and carrier transport were summarized.For Ag2 Se and Ag2 S,the carrier concentration increased during the phase transition,which was mainly due to the decrease of the band gap during the phase transition.For Ag2 Te,the hole compensation by the emergency of Ag vacancies during the phase transition would result in a decrease in electron concentration.Moreover,the temperature dependence of carrier mobility(μ)showed that the scattering mechanism of carriers in Ag2 X compounds was dominated by acoustic phonon scattering.Furthermore,the recent research progress on Ag2 X superionic conductors and other ternary Ag-based chalcogenides was reviewed,including the improvement of preparation methods,the optimization of carrier concentration and the modification of electronic structure.Ultimately,the optimum TE performances were obtained in the room temperature phase of Ag2 Se compounds and superionic conductor phase of Ag2 Te respectively.For Ag2 Se,the introduction of facile room temperature synthesis method of manual mixing and cold press could maintain low carrier concentration and optimize the TE performance of room temperature phase.For Ag2 Te,alloying PbTe and Cu2 Te increased the band gap of superionic phase,and inhibited the occurrence of intrinsic excitation.Although the migration of Ag+in Ag2 X superionic conductors could effectively reduce the lattice thermal conductivity,it would also result in compositional inhomogeneity and electrical instability.On the one hand,Ag+underwent directional migration and precipitated at one side of the sample when electric current and/or temperature gradient were applied.On the other hand,the volatilization of chalcogen elements at high temperature led to compositional inhomogeneity.Both of them changed the carrier concentration of materials,resulting in poor reproducibility of TE performances.Hence,methods for enhancing the stability and reproducibility of TE performance in superionic conductors were outlined.For example,cation site doping to inhibit the free migration of ions,introduction of cation vacancy to adjust the critical voltage for metal ions precipitation and incorporation of secondary phase to hinder ions migration.Recently,Ag2 S-based compounds were reported to exhibit extraordinary metal-like flexibility.However,the plastic deformation mechanism of Ag2 S-based compounds had not been fully understood yet,and the possible related deformation mechanisms were elaborated.Meanwhile,the realization of both good flexibility and high TE performance for Ag2 X-based superionic conductors was briefly discussed.Finally,problems posed by accurate regulation of carrier concentration,migration of ions and composition control in Ag2 X-based superionic conductors were presented.With the summary of research progress and existing problems in Ag2 X-based superionic conductors,the outlook for future research direction was provided.Firstly,the TE performance for ntype conduction Ag2 X-based compounds could be further improved,especially for the superionic conductor phase.Then,more effective ways should be explored to improve the service stability of superionic conductors.Lastly,the plastic deformation ability and the TE performance for Ag2 X-based compounds should be balanced.