Research On The Microstructure And Thermoelectric Performance Of New Zintl Phase

Thermoelectric material,which can realize the reversible conversion between waste heat and electricity through carrier transport,is one of the functional materials.Zintl phase compounds are new thermoelectric materials with promising performances due to their complex crystal structures.In recent years,the development of new Zintl phase thermoelectric materials with high thermoelectric performances in both n-type and p-type transports have been the hot research topics.For example,Mg₃Sb₂-based materials have realized the n-type and p-type transport properties and obtained highly optimized thermoelectric performances.Besides,the maximum figure-of-merit,ZT of the n-type Mg₃Sb₂-based thermoelectric material has exceeded 1.5.Therefore,the optimization of thermoelectric performances of p-type materials has been the key to realize the application of power generation.In this dissertation,the structure and thermoelectric performances of YbMg₂Pn₂（Pn=Sb and Bi）Zintl phase compounds were studied,and the thermoelectric performances were improved through optimizing the carrier concentration and intensifying the phonon engineering.（1）By introducing Na into the Yb site of YbMg₂Sb₂,the carrier concentration was optimized,the microstructure and thermoelectric performances of the p-type Yb_1-xNa_xMg₂Sb₂ samples were systematically studied.It demonstrated that the voids along the grain boundaries were derived from the loss of unknown metallic oxide phases,which might be observed in the hot-pressed bulk species with oxygen-affine elements,such as Yb,Na,and Mg in our work.The electrical resistivity and Seebeck coefficient of the Na-doped YbMg₂Sb₂-based samples decreased obviously with increasing Na concentration.Therefore,the power factor significantly improved.Besides,the thermal conductivity was slightly lower in the Na-doped samples.Finally,a higher ZT of 0.6 was obtained in the Yb_0.98Na_0.02Mg₂Sb₂ sample at 773 K.By using the first-principle calculations,the band structure of YbMg₂Sb₂ showed that the band gap was 1.23 eV,and it contained multiple bands near the valence band maximum,indicating that high thermoelectric performance might be achieved in the p-type YbMg₂Sb₂-based compounds.（2）The thermoelectric performances of the p-type YbMg₂Sb₂-based materials were enhanced by doping with Ag and alloying with YbMg₂Bi₂.With increasing Ag-doping concentration,the electrical resistivity,Seebeck coefficient and thermal conductivity of Yb_1-xAg_x Mg₂Sb₂ samples decreased significantly.When the Ag-doping concentration increased to x=0.01,the Yb_0.99Ag_0.01Mg₂Sb₂ sample had higher power factor and lower thermal conductivity,and the maximum ZT reached to 0.5 at 773 K.Based on the lower thermal conductivity of the Yb_0.99Ag_0.01Mg₂Sb₂ sample,the thermoelectric performances were further enhanced by preparing YbMg₂Sb₂-YbMg₂Bi₂ solid solutions.With increasing Bi concentration,the electrical resistivity and Seebeck coefficient further decreased,higher power factor of 11μW cm^-1 K^-2 was achieved.The thermal conductivity also further decreased significantly in the Yb_0.99Ag_0.01Mg₂SbBi sample due to the significantly enhanced alloying scattering.Therefore,a higher ZT reached up to 1at 773 K was obtained,indicating that YbMg₂Sb₂ was a promising p-type thermoelectric material.（3）n-type YbMg₂Sb₂-based materials were synthesized by doping at the Yb site and increasing Mg concentration at the Mg site.With increasing Y-doping and Mg concentration,the electrical resistivity,Seebeck coefficient and thermal conductivity of Yb_1-xY_x Mg_2.2+δSb₂ samples decreased significantly,resulting in improved power factor and ZT.When the Y-doping and Mg concentration increased to x=0.02 andδ=0.1,respectively,the Yb_0.98Y_0.02Mg_2.3Sb₂ sample had higher ZT of 0.2 at 773 K.Based on the first-principles calculation,both conduction band and phonon spectrum mainly from Yb ion in the structure were expected to contribute significantly to the electronic and thermal transport properties,respectively.Therefore,the thermoelectric performances of the n-type materials were further enhanced by preparing YbMg₂Sb₂-Mg₃Sb₂ solid solutions,i.e.Yb_0.98-yY_0.02Mg_y Mg_2.3Sb₂.As expected,both electrical resistivity and Seebeck coefficient were further optimized,leading to higher power factor of 6μW cm^-1 K^-2.Besides,the intensification of alloy scattering led to a significant decrease in thermal conductivity.When increasing the Mg concentration to y=0.5,lower thermal conductivity was achieved in the Yb_0.48Y_0.02Mg_0.5Mg_2.3Sb₂ sample.Therefore,a higher ZT up to 0.75 at 773 K was obtained in this sample,indicating that YbMg₂Sb₂ was also a promising n-type thermoelectric material.（4）The lattice thermal conductivity and average ZT of the YbMg₂Bi₂-based samples were optimized via phonon engineering.Through alloying YbMg₂Bi₂ with Mg₃Bi₂,significant phonon scattering was realized due to the substantial mass difference between the host atom Yb and the alloying atom Mg,which significantly reduced the lattice thermal conductivity,especially near room temperature.As a result,the average ZT increased from 0.46 for YbMg₂Bi_1.96 to 0.61 for Yb_0.8Mg_0.2Mg₂Bi_1.96,a reduction of 33%.And the conversion efficiency increased from 7.5%for YbMg₂Bi_1.96to 10%for Yb_0.8Mg_0.2Mg₂Bi_1.96,a reduction of 33%.Furthermore,based on the optimal thermoelectric performance of the Yb_0.8Mg_0.2Mg₂Bi_1.96 sample,the bipolar thermal conductivity was inhibited by increasing the band gap and carrier concentration,leading to optimized maximum ZT while maintaining higher average ZT,indicating that increasing the band gap and carrier concentration were two effective ways to enhance the thermoelectric performance.