Study On The Thermoelectric Properties Of Quais-two-dimensional GeSbTe Compounds

Thermoelectric technology can realize the direct conversion between heat and electrical power by Seebeck effect and Peltier effect.Due to its simple structure,free-pollution and high stability,thermoelectric technology has wide application prospects in recovery of waste heat and refrigeration fields.Exploring and investigating novel thermoelectric materials is an important aspect in thermoelectric application.Some materials serving in other fields with excellent electrical and thermal properties could show potentials in high performance thermoelectric materials.Recently,Ge-Sb-Te?GST?compounds are phase change materials?PCMs?,which have been gained wide attention.Ge₂Sb₂Te₅,GeSb₂Te₄ and GeSb₄Te₇ compounds are three typical GST materials.GST compounds are degenerate p-type semiconductors with high electrical conductivity and low thermal conductivity.GST compounds with trigonal phase exhibit Quasi-two-dimensional structure.In this dissertation,we mainly focus on three typical GST compounds?Ge₂Sb₂Te₅,GeSb₂Te₄ and GeSb₄Te₇?and systematically their thermoelectric properties.Then,we try to optimize their thermoelectric performance via element doping or alloying.In addition,combined with DFT calculations and experimental data,we explain underlying mechanism that affect crystal structure and thermoelectric properties.Finally,thin films of Ge₂Sb₂Te₅ are deposited by magnetron sputtering and the cubic phase of Ge₂Sb₂Te₅ is obtained by controlling annealing temperature.The main achievements of this dissertation are listed as follows:1.Three typical GST materials?GST225,GST124,GST147?were synthesized by the conventional melting-quenching-annealing method.XRD and SEM results reveal that GST materials are stable,hexagonal structure,which exhibits a typical lamellar morphology.GST compounds show good power factor with the maximum value of 13?W cm^-1 K^-2 at 750 K.Due to high electrical conductivity,the electronic part accounts for 80%of the total thermal conductivity.Bulk XRD and EBSD results indicate that sintered samples show weak textured,which is contributed to anisotropy in thermoelectric properties.Taking Ge₂Sb₂Te₅ as an example,we calculated band structure by DFT and it is concluded that appreciable anisotropy in Seebeck coefficient was observed and ascribed to the overall asymmetry of the band structure.Finally,thermoelectric performance in the parallel direction is higher than that in perpendicular direction.Maximal z T values of 0.46-0.60 were obtained at 750 K.2.In this work,focusing on the typical compound Ge₂Sb₂Te₅,we study the crystal structure and thermoelectric performance by doping indium at cation site.Firstly,doping In at Ge site leads to the lattice expansion along the a-axis and shrinkage along the c-axis.Considering the larger atomic radius of In than Ge,expansion along the a-axis is normal.Shrinkage along the c-axis comes from the reduced distance of specific layers along c-axis.The electrical conductivity as well as thermal conductivity monotonously decreases with increasing In content,while Seebeck coefficient obviously increases.The decreased thermal conductivity is ascribed to the reduced carrier thermal conductivity.By virtue of DFT calculations and modeling,the remarkable enhancement of Seebeck coefficient was attributed to the doping-induced local distortion in the electronic density of states.The z T and average z T values are significantly improved in In-doped samples.In addition,we attempted to dope In at Sb site to investigate their thermoelectric properties.It is concluded that In doping increases carrier concentration and decreases mobility.A remarkable increases of the apparent density-of-state effective mass is seen in In-doped samples.The peak z T value reaches 0.73 at 750 K,being about 40%higher than that of the pristine sample at the same temperature.3.We synthesize Ge₂Sb₂Te_5-xSe_x and Ge₂Sb₂Te_5-yS_y samples and all samples are trigonal phase without changing crystal structure.Because the ionic radius of Se/S is smaller than that of Te,the lattice constants gradually decrease with increasing Se content.With increasing Se/S content,electrical conductivity and thermal conductivity decrease and Seebeck coefficient increase.Compared with the pristine Ge₂Sb₂Te₅,Se-and S-alloyed samples exhibit lower carrier concentrations.This is due to the fact that Se and S atoms have larger electronegativities than Te,which means that the cation-Se/S bond is more ionic and“stronger”than the cation-Te bond.Therefore,the formation energy of cation vacancies should be higher in Se/S-alloyed materials,leading to a lower carrier concentration.According to SPB model,Se alloying leads to increase effective mass.By virtue of DFT calculations,Se alloying induces a small increase in electronic density of states near the Fermi level,which contributes to an unexpectedly large rise in effective mass.Maximum z T values of 0.71 and 0.74 at 800K have been achieved in Ge₂Sb₂Te_3.9Se_1.1 and Ge₂Sb₂Te_4.9S_0.1,respectively,which is30%higher than the pristine Ge2Sb2Te5.4.In this work,focusing on GeSb₄Te₇ compound,we successfully synthesized a series of GeSb_4-xBi_xTe₇ samples.GeSb₄Te₇ and Ge Bi₄Te₇ can form a complete solid solution.GeSb_4-xBi_xTe₇ samples are also trigonal phase.Owing to the larger ionic radius of Bi³⁺as compared with Sb³⁺,the lattice constant increases with increasing Bi content.The electrical conductivity as well as thermal conductivity monotonously decreases with increasing temperature,while Seebeck coefficient obviously increases.With increasing Bi content,electrical conductivity and thermal conductivity is decreased and Seebeck coefficient is increased.Hall measurement shows that the carrier concentration largely decreases and mobility gradually increases with increasing content of Bi.Power factor at room temperature is close to theoretical maximum value which is calculated by SPB model.Also,we use Callaway model to analyze lattice thermal conductivity.Finally,maximum z T value of 0.46 at 300 K is obtained in GeSb_0.5Bi_3.5Te₇ sample and maximum z T value of 0.69 at 550 K is obtained in GeSb_1.5Bi_2.5Te₇ sample.Maximum average z T value of 0.52 is achieved in GeSb_1.5Bi_2.5Te₇ sample.5.Thin films of Ge₂Sb₂Te₅ are deposited by magnetron sputtering method.The cubie phase of Ge₂Sb₂Te₅ is obtained by annealing.SEM results reveal the films after annealing are dense and homogeneous.EDS elemental mapping shows all three elements are homogeneously distributed and no obvious secondary phase or elemental segregation is observed.The cubie phase of Ge₂Sb₂Te₅ film shows high Seebeck coefficient as compared with trigonal phase.The electrical conductivity increases and Seebeck coefficient decreases at room temperature with increasing annealing temperature.