Band Structure Optimization And Phonon Engineering Synergistic Tuning Of Thermoelectric Properties For P-type CuGaTe₂-based Materials

The rapid development of society and accelerated industrial modernization drive the overconsumption of traditional energy sources,leading to the search for new renewable energy sources and high-efficiency energy conversion technologies.Thermoelectric materials,capable of converting thermal energy into electrical energy,hold great promise in various fields,including energy harvesting and environmental conservation.CuGaTe₂-based thermoelectric materials exhibit characteristics such as stability,a high Seebeck coefficient,abundant elemental reserves,and environmental friendliness,making them promising candidates in the field of thermoelectricity.However,the thermoelectric performance of this material is limited by low carrier concentration and high thermal conductivity,which hinder its competitiveness in the mid-temperature range among thermoelectric materials.This dissertation focuses on addressing these issues by conducting optimization research on its thermoelectric properties through both theoretical calculations and experimental studies,summarized as follows:Theoretical calculations reveal that substituting Zn atoms for Ga atoms in CuGaTe₂increases the bandgap and band degeneracy.Experimental results further demonstrate that Zn forms acceptor defects in the system,increasing hole carrier concentration and optimizing the power factor.Additionally,element doping introduces point defects as phonon scattering centers,further reducing thermal conductivity.As a result,the（CuGaTe₂）_0.9975（2Zn Te）_0.0025 sample achieves a ZT value of 0.74 at 810 K.Theoretical calculations evaluated the electronic band structure of CuGaTe₂ with Ni,Mn,Fe,and Ti elements doping at the Ga site.Results indicate that Mn and Ni doping increases the band degeneracy at the valence band top and Mn doping enhances the electron state density near the Fermi level,thereby improving charge carrier concentration and raising the average power factor of CuGaTe₂ by 59%,achieving effective p-type doping.Experimental results show that Cu Ga_0.99Ni_0.01Te₂ exhibits excellent thermoelectric performance with a maximum ZT value of 0.76 at 823 K,while Mn-doped samples exhibited excellent electrical properties,high electronic thermal conductivity impacted the thermoelectric performance,resulting in a maximum ZT value of only 0.64.Fe has low solubility in CuGaTe₂,and during melting,it generates a second phase,Fe Te₂.regulating the doping ratio of Fe controls the size of the second phase,creating nano-sized phonon scattering centers and lowering lattice thermal conductivity.Ti doping significantly improves the conductivity of CuGaTe₂,thus obtaining a higher power factor.Increasing intrinsic defects is an effective approach to enhancing CuGaTe₂thermoelectric performance.Electronic band structure calculations show that Cu defects cause the valence band to approach the Fermi level,leading to a significant improvement in electronic performance.Phonon structure calculations reveal a"phonon-avoided crossing"phenomenon between the low-frequency optical and acoustic branches in Cu defect samples,reducing sound velocity and lattice thermal conductivity,which synergistically enhances thermoelectric performance.Experimental results show that Cu defect samples achieve a significantly higher carrier concentration of 10²⁰ cm^-3,greatly optimizing electronic performance,along with the formation of a high-density dislocation defect,further enhancing thermoelectric performance.Ga defect samples exhibit numerous second-phase impurities and reduced densification,resulting in relatively average electronic performance.Ultimately,the ZT value of Cu_0.96Ga Te₂ is significantly improved,reaching 1.23 at 823 K,an increase of 114%compared to pristine CuGaTe₂,with an average ZT value improvement of 171.4%.The high thermal conductivity of CuGaTe₂ remains a major challenge in improving its thermoelectric performance.Through AgCl composites,Ag/Cu substitutions can generate mass strain disturbances,and Cl in AgCl escapes to form nanoscale pores during SPS sintering process,inducing multiscale phonon scattering centers and significantly reducing lattice thermal conductivity.Consequently,the（CuGaTe₂）_0.96（AgCl）_0.04 sample achieves a remarkable ZT value of 1.4 at 823 K.