The Effects Of Phase Structures On The Thermoelectric Properties Of AgTaO₃:First-Principles Calculations

Humans are facing an increasingly energy shortage problem,and the total amount of solar thermal energy and industrial waste heat in daily life is huge.Recycling and utilizing these heat will alleviate the energy crisis.Thermoelectric materials can realize the conversion between thermal energy and electrical energy,so they have broad application prospects in waste heat recovery.As a common perovskite material,AgTaO₃ material can exhibit different phase structure changes during heating and cooling.Different phase structures are expected to exhibit different thermoelectric properties.Therefore,investigating the effects of phase structures on its thermoelectric properties is the key to understanding and enhancing its thermoelectric properties.However,the experimental research on the phase transition and doping optimization of materials usually demand high cost and long period.On theoretical side,there is a lack of systematic research on the effects of carrier concentration optimization and phase structures on the thermoelectric properties of AgTaO₃.Hence,in this thesis,for p-type and n-type AgTaO₃ materials,combined with the first-principles calculation method and the semi-classical Boltzmann theory,a systematical study of the effects of the carrier concentration,temperature-induced phase transition,and strain-driven phase structure change on its electronic structures and thermoelectric properties will be carried out.The optimal doping concentrations are estimated for AgTaO₃ materials,and phase structure change on its thermoelectric properties,and the optimal method of strain loading are obtained.The main work is as follows:（1）The effects of carrier concentration on the thermoelectric properties of AgTaO₃ materials are studied,and optimal doping concentration range are estimated of p-type and n-type AgTaO₃ thermoelectric materials.Based on the first principle,the crystal structure model,band structure and total density of states have been explored,and the electronic structure characteristics have been analyzed.Combined with Boltzmann theory,the variation of thermoelectric properties with respect to carrier concentration are explored,and the preferred carrier concentration range of AgTaO₃thermoelectric materials for different phase structures are determined,which provide the guidance for improving the thermoelectric properties of AgTaO₃ materials via doping in experiments.（2）The effects of temperature-induced phase structures change on the thermoelectric properties of AgTaO₃ material have been studied.The band structures,total density of states,partial density of states and charge density distributions of stable cubic,tetragonal and rhombohedral AgTaO₃ at different temperatures are calculated and analyzed by using the first-principles combined with the semi-classical Boltzmann transport theory.The variations of thermoelectric properties of p-type and n-type AgTaO₃ materials in different phase structures as function of temperature are analyzed and clarified.The thermoelectric anisotropy of AgTaO₃ in different phase structures has also been explored.It is found that the transformation from tetragonal to rhombohedral,and transformation from tetragonal to cubic can improve the thermoelectric properties of AgTaO₃.（3）The effects of the strain-driven phase structure changes on the electronic structures and thermoelectric properties of AgTaO₃ have been studied.For p-type and n-type AgTaO₃ materials,based on the first-principles and semi-classical Boltzmann theory,the variations of the electronic structures and thermoelectric properties of strain-driven tetragonal to orthogonal phase,and strain-driven cubic to tetragonal phase have been studied.It is found that the thermoelectric properties of AgTaO₃ can be improved by in-plane uniaxial strain induced the transformation of tetragonal to orthogonal phase.The power factor of p-type AgTaO₃ increases by 87%under in-plane uniaxial strain of 6%.Finally,the anisotropy of AgTaO₃ thermoelectric performance induced by strain in tetragonal-to-orthogonal phase change has also been explored.It is found that uniaxial strain in-plane induced tetragonal to orthogonal phase enhance the anisotropy of the thermoelectric properties of AgTaO₃.