Lattice Dynamics And Phonon Transport Mechanisms In Filled Skutterudites

Thermoelectric materials have a great promising prospect in the filed of the energy material for its advantages on transforming heat to electricity and vice versa.The crucial factor in the design of thermoelectric materials is optimizing the transport properties of electric carriers and phonons,i.e.enhancing the power factor and suppressing the thermal conductivity.The strategy of“phonon-glass”makes a great success in the design of thermoelectrics by inducing dirorder impurities and nanostructures in the lattice impeding the phonon transport.The filled skutterudites are the prototype of this concept since that its lattice framework made by covalent bonds constitutes the electron conductive network while the nanocages contain filled atoms greatly scattering the phonons.The concept of“chemical bond hierarchy”generalized from the structure feature of filled skutterudites also boost lots of non-caged themoelectic compounds with ultra-low thermal conductivites,including Cu-based materials known for their“Part Crystalline-Part Liquid”and“Phonon Liquid”features.However,the role of filled atoms in skutterudites played on the thermal transport is still in debate.The argument issue is whether the paradigm of the perturbative theory is appropriate or not for the scattering machenism induced by the filler vibrations,which breakdown the hypothesis of the perturbative approximation for the large-amplitude vibrations of fillers.If not,what is the physical nature of the scattering mechanism of fillers beyond the traditional three-phonon paradigm?The work presented in this thesis is devoted to investigate the relationships between the guest-host interactions,vibration characteristics and phonon transport mechanisms in filled skutterudites,and to reveal the physical process of fillers impeding the phonon pragation.In the thesis,we first consider the dynamic effects of the filler motions in the crystal cages through ab initio molecular dynamics?AIMD?and the tempeprature dependent effective potential?TDEP?approaches.Regular finite displacement method?FDM?is hard to capture the dynamic disorder of filled atoms in skutterudites via the zero-temperature first principles calculations.AIMD enables us to simulate the dynamic process of filled skutterudites considering all the interactions at the finite temperatures.TDEP method is also applied to extract the harmonic and anharmonic interatomic force constants from the AIMD trajectories and evaluate the lattice thermal conductivity.The calculation shows that the large-amplitude quasi-localized vibrations of fillers greatly enhance the anharmonic scattering strength through the coupling between fillers and the framework.It causes much lower lifetimes for the filler-dominant modes than the?^-2 relationship for the Umklapp process.Comparing with the results of FDM,the theoretical lattice thermal conducitivity is closer to the experimental values due to the more accurate frequencies of the fillers and therefore phonon scattering channels.Nonetheless,the overestimations of theoretical lattice thermal conductivities got by this methodology are both existed in many filled skutterudites.It demenstrates that the influence of fillers on thermal transport in skutterudites is beyond the three phonon paradigm.Moreover,introducing resonant scattering for the filler-dominant modes additional to the finite-temperature three phonon process could diminish these deviations between the calculated and experimental lattice thermal conductivities of these filled skutterudites.To investigate the physical origin of the resonant scattering mechanism,the time-evolutional power spectra and correlation properties of the filler vibration are studied through the wavelet analyses on the AIMD trajectories.The time-varing localized filler vibrations fluctuate between the ground and excited states through coupling and decoupling periodically with neighboring Sb atoms.Lattice phonons would be absorbed and emitted out during these dynamical process owing to the fluctuated guest-host interactions.This phenomenon strongly eliminates the contributions of the low-frequency phonons to the heat currents.Thus,we demonstrate the physical picture of resonant scattering in the filled skutterudites.Recent experimental studies report that some elements with high electronegativities,i.e.Ga,In and S,also occupay the cage void sites through the covalent guest-host interactions.Different from those of ionic fillers,the covalent interactions in these systems results in many abnormal properties of lattice dynamics.Our calculations of the Ga-filled skutterduites shows that the covalent bonds and the long-range polarization effect between filled-site Ga and the framework cause the low frequency vibrations modes comparable with those of heavy REs fillers.These modes improve the anharmonicities and the scattering channels leading to prominently strong three-phonon scattering.In addition,the large-amplitude vibrations of the filled-site Ga also scatter the low-frequency phonons by resonant scattering and greatly reduce the lattice thermal conductivity.