Optimization Of Thermoelectric Properties Of Ge Se-based Compounds Using Crystal Structure Engineering And Ferroelectric Characteristic

In the context of the intensifying global energy crisis,the development of clean and efficient new energy conversion and utilization technologies is imminent.Thermoelectric technology can directly convert widely distributed thermal energy into electrical energy,which has important application prospects in alleviating the energy crisis.The core of this technology lies in the development of high-performance thermoelectric materials.GeSe is a typical two-dimensional layered material,its low-dimensional features endowing it with low lattice thermal conductivity,and it is a promising thermoelectric material.However,due to the excessively large intrinsic bandgap,low carrier concentration and doping efficiency,the electrical properties of GeSe are also poor.This work proposes to comprehensively optimize the thermoelectric transport properties of GeSe-based compounds by constructing a rhombohedral crystal network with ferroelectric characteristic by means of element solid solution combined with heat treatment process adjustment and crystal structure engineering.The main research contents and results of this paper are as follows:（1）A series of GeSe_1-xTe_x（x=0,0.05,0.15,0.25,0.35,0.45）polycrystalline samples were prepared by fusion quenching combined with spark plasma activation sintering process,and the phase structure and thermoelectric transport properties of GeSe compounds by GeTe solid solution were studied.The results show that with the increase of GeTe content,the crystal structure of GeSe gradually changes from orthorhombic phase to rhombohedral phase.This reduces the band gap of the material,and simultaneously increases the carrier concentration and mobility.Meanwhile,the energy band degeneracy of the compound increases significantly because of enhanced crystal symmetry in this process,thereby considerably improving the effective mass of carriers.Altogether,the power factor of the rhombohedral GeSe is increased by about 2 to 3 orders of magnitude compared with that of the orthorhombic phase GeSe.In addition,the rhombohedral phase GeSe has abundant cationic vacancy defects and softened phonons arising from its ferroelectric feature,leading the lattice thermal conductivity to be 60%lower than orthorhombic one.Crystal structure engineering could be considered as an effective way of improving the thermoelectric properties of GeSe compounds.（2）The above-mentioned rhombohedral phase GeSe_0.65Te_0.35 compound with ferroelectric characteristic was annealed at 615 K for 12 h to obtain a hexagonal phase sample with the same chemical composition but no ferroelectric characteristic.We systematically studied the phase transition behavior and process of rhombohedral and hexagonal GeSe_0.65Te_0.35,and revealed the effect of ferroelectric characteristic on the thermoelectric properties of GeSe_0.65Te_0.35.The results show that the phase transition from the rhombohedral phase to the hexagonal phase is a reconstructed phase transition,and the carrier concentration remains unchanged before and after the phase transition.The experimental and theoretical calculation results show that compared with the hexagonal phase,the rhombohedral phase has a larger band gap,higher carrier effective mass and Seebeck coefficient.In addition,due to the phonon softening effect caused by the ferroelectric characteristic,even though the rhombohedral phase is a three-dimensional lattice network,its lattice thermal conductivity is comparable to the hexagonal phase with a two-dimensional layered structure and can reach the amorphous limit.The sample of the rhombohedral phase achieves a peak ZT of 0.6 at 548 K,which is 6 times that of the hexagonal phase.The ferroelectric characteristic can significantly enhance the thermoelectric properties of GeSe0.65Te0.35.（3）On the basis of the above work,the effect of Sb as a doping element on the phase structure and thermoelectric transport properties of rhombohedral GeSe_0.65Te_0.35compounds was further studied.The results show that the introduction of Sb makes the crystal structure of GeSe_0.65Te_0.35 change from rhombohedral phase to cubic phase,and with the increase of Sb content,the phase transition temperature decreases continuously.During this process,the hole concentration of the samples decrease,simultaneously the Seebeck coefficient and power factor are greatly improved.In addition,due to the enhancement of point defect scattering and the decrease of electrical conductivity,the total thermal conductivity decreases significantly with the increase of Sb content.When the Sb content is 0.01,the maximum ZT of the sample is 1.06 at 623 K.Compared to the intrinsic sample,its performance is improved by 76%.