Investigating The Role Of Grain Boundaries And Nanoscale Interfaces In Zinc-oxides Based Thermoelectrics

In this thesis,intrinsic grain boundary and nanometer interface in oxide were discussed.Combining with experimental results and physical models,and considering the interfacial thermal resistance₆)and the effective electronpotential barrier heigh?-?as starting point,we studied the electrical and thermal transport properties of grain boundary and interface from qualitative and quantitative aspects.The research contributed to further understand the electron and phone transport characteristic in grain boundary and interface,which proved a new idea and theoretical basis to optimize thermoelectric performance by interface engineering.First part,grain boundary could play an important role in energy carrier transport.By choosing ZnO as experimental object,we synthetized ZnO polycrystals with a wide range of grain boundary spacing from 80 nm to 4.6?m.Synthetic routes involved the solvothermal,SPS sintering and annealing,etc.By measuring the thermal and electrical transport properties of ZnO polycrystals with different grain sizes,we estimated the interfacial resistance of ZnO grain boundary₆)=4.0±0.7×10^-9 m² K W^-1,which was independent of temperature and grain size.The effective electron potential barier height and the depletion width of grain boundary increased monotonically with spacing,but they collapsed blow¹00 nm and became almost invariant above¹?m.While the ZnO grain boundary was locally modified by ZnS thin film of several nanometer in thickness,the interfacial resistance increased by more than three times,up to 12.9×10^-9 m² K W^-1,and the depletion region expanded more than twice.The charge carrier concentration was influenced by the effective potential barrier height due to grain boundary energy filtering effect,whereasthe electron mobility was related to the depletion width.Our investigations demonstrated the significance of grain boundary characteristics for interfacial and effective bulk transport properties.The findings and the approach are broadly important for polycrystalline materials of which the functional performance can be adjusted via grain boundary engineering.Second part,the electrical and thermalt transport properties of nanometer superlattice interface were studied in depth.The typical structure of In₂O₃?ZnO?_k superlattice was the stacking of the single atomic InO₂ layer along c-axic of the ZnO wurtzite structure seperating the blocks of the?k+1?ZnO atomic layers.In particular,the ZnO block existed the randomly distributed indium ions with an average occupancy probability of 20 mol.%on each zinc site.The low thermal conductivity and high seebeck coefficience were mainly attributed to special InO₂ superlattice interface,thus it promised to be potential high temperature thermoelectric materials.We investigated the high temperature thermoelectric properties of the In₂O₃?ZnO?₄ superlattice bulk polycrystals that were singly and dually doped with Al and Ce.Transport property measurements revealed that Al and Ce did not only enter the ZnO blocks but also modified the InO₂ single atomic layers.The effective electron potential barrier height of the superlattice interfaces can be adjusted by doping,and the optimal value that maximizes the power factor is of the order of6)above the Fermi level.The interfacial thermal resistance of the InO₂atomic sheets dramatically increased with doping,primarily accounting for the bulk thermal conductivity reduction.At the optimal crossing of the interfacial thermal resistance and the effective potential barrier height,a maximum(5of?0.22 was achieved at 800°C in the 1.6 mol.%Al doped superlattice,which was an enhancement of?200%over the pristine In₂O₃?ZnO?₄.This work provided a new perspective on enhancing the high temperature thermoelectric performance of nanostructured oxides by synergistically optimizing the interfacial phonon and electron transport properties.