The Study On Thermoelectric Properties And Ultrafast Spectroscopy Of Te-Based Compounds For Intermediate Temperature Range

In the past few decades,the increasing energy and environmental crises have made the development of the environment-friendly renewable thermoelectric materials that can accomplish efficient conversion between heat and electricity a research focus worldwide.Among these novel thermoelectric material systems,the ABTe₂（A=Cu,Ag;B=Ga,In）compounds with the diamond-type structure have drawn lots of research interests in the thermoelectric community for their outstanding thermoelectric performance.Some key issues for the ABTe₂-based compounds are as follows.Despite the same crystal structure and similar constituent elements,the four compounds（AgGaTe₂,AgInTe₂,CuGaTe₂and CuInTe₂）exhibit remarkably different electrical and thermal transport properties.The origin of such discrepancies is unknown and worth investigation.Moreover,the intrinsic lattice thermal conductivity of CuGaTe₂and CuInTe₂compounds is relatively high and needs to be optimized.In addition,owing to the p-type transport nature of most high-performance thermoelectric materials with the diamond-type structure,the n-type PbTe compound is chosen to match the ABTe₂（A=Cu,Ag;B=Ga,In）compounds for large-scale industrial applications.However,the n-type PbTe compound still possesses a relatively low ZT value,which requires a further optimization.Moreover,the thermoelectric transport properties are closely related to the ultrafast dynamic process of the particles including electrons and phonons in thermoelectric researches.Nevertheless,there is few research about the electron or phonon dynamics and their effects on the electrical or thermal transport properties in thermoelectric materials.Therefore,aiming at solving these issues focusing on the AgGaTe₂-based,PbTe-based and ultrafast spectroscopy,the main research results in this work are summarized as follow:（1）The mechanisms behind the distinct thermoelectric transport properties of ABTe₂（A=Cu,Ag;B=Ga,In）compounds.The deep-level in-gap states induced by the native A-site vacancies play a key role in the observed intrinsic semiconductor to degenerate semiconductor transitions exhibited in the temperature dependences of the electrical conductivity and are the origins of the distinct electrical conductivity among the four ABTe₂compounds.For example,a much larger electrical conductivity of 2.82×10³S m^-1at room temperature is obtained for the CuGaTe₂compounds compared with only 2.96×10^-2S m^-1for AgGaTe₂at the same temperature.Besides,the cryogenic heat capacity measurements and calculated phonon dispersion relations show that the acoustic and low-frequency optical modes of CuGaTe₂and CuInTe₂are governed by the vibrations of Te atoms while the counterparts of AgGaTe₂and AgInTe₂compounds are dominated by the vibrations of Ag-Te clusters,which increases the coupling between the acoustic and low-frequency optical modes.The stronger coupling will induce lower avoided-crossing frequencies,lower sound velocity together with stronger Umklapp process and thus,lower thermal conductivities of AgGaTe₂and AgInTe₂than CuGaTe₂and CuInTe₂.To be more specific,the lattice thermal conductivities of AgGaTe₂and AgInTe₂are 1.52 and 1.45W m^-1K^-1at 300 K,respectively,while much larger values of 7.38 and 6.01 W m^-1K^-1are obtained for CuGaTe₂and CuInTe₂,respectively,and CuGaTe₂sample possess a maximum ZT value of 1.00 at 873 K.（2）The influence of Ag alloying and Cu deficiency on the thermoelectric transport properties of Cu_1-xAg_xInTe₂and Cu_1-xAg_xGaTe₂quaternary compounds.Based on the experiment results,the force constant of the Ag-Te bond is reduced by the vibrations of Ag-Te clusters in samples with high Ag content,which plays a key role in the growing lattice structural deformation with rising temperature and negative coefficient of thermal expansivity（NTE）observed in the Cu_0.7Ag_0.3GaTe₂and Cu_0.9Ag_0.1GaTe₂samples.For the thermal transport properties,the reduced force constant of the Ag-Te bond in samples with high Ag content can lead to significant reduction in sound velocity,yielding the deviation in lattice thermal conductivity with Ag alloying from the calculation results based on the Callaway model.The lattice thermal conductivity is remarkably reduced through Ag alloying,which optimizes the thermoelectric properties.Considering both electrical and thermal properties,a maximum ZT value of 1.09 at 873 K is obtained in Cu_0.9Ag_0.1InTe₂,and the Cu_0.7Ag_0.3GaTe₂sample possess a maximum ZT value of1.32 at 883 K,which is 32%higher than CuGaTe₂.Besides,owing to the reduction in the defect formation energy of Cu vacancies,the electrical conductivity is promoted in the Cu_0.9-xAg_0.1GaTe₂and Cu_0.7-xAg_0.3GaTe₂via Cu deficiency.At room temperature,Cu_0.68Ag_0.3GaTe₂possess a larger electrical conductivity of 2.21×10⁴S m^-1,compared with 2.32×10³S m^-1for Cu_0.7Ag_0.3GaTe₂.（3）The actual form of existence and evolution of defects in Zn-doped PbTe-based compound and their influence on thermoelectric transport properties.Based on the experiment and calculation results,the tiny amount of dissolved Zn atoms will first fill these Pb vacancies in the Pb Zn_xTe samples.The reduction of Pb vacancies diminishes the carrier scattering,resulting in a significant increase in carrier mobility from 359.6 cm²V^-1s^-1for undoped PbTe to 1554.3 cm²V^-1s^-1for Pb Zn_0.02Te at room temperature.Whereas,with further increase of Zn content,the Zn _Pb+Zn_icomplex-defect possesses relatively low formation energy compared with the situation only considering Zn _Pbdefect or Zn_idefect.We attribute this phenomenon to the balance between the contraction stress induced by the smaller Zn atoms substituting on Pb sites and the swelling stress from Zn atoms occupying the interstitial sites in the lattice.Therefore,the solid solubility of Zn in PbTe can be enhanced by combining the substitutional Zn on Pb sites and the interstitial Zn.The interstitial Zn in PbTe was further explicitly corroborated by aberration-corrected scanning transmission electron microscopy（Cs-corrected STEM）.Benefited from the significantly enhanced electrical properties,the thermoelectric performance is remarkably promoted through Zn doping in PbTe.Accordingly,a maximum ZT value of 1.36 at 775 K is obtained for the Pb_0.995Zn_0.02Te sample,which is 84%higher than the ZT_maxof 0.74 for undoped PbTe.And a high average ZT value of 0.99 from 300 to825 K is achieved for Pb_0.995Zn_0.02Te.（4）The study of transient reflection spectrum and terahertz-time domain spectroscopy on the AgGaTe₂-based,PbTe-based,and Bi₂Te₃-based compounds.In the transient reflection spectrum results of AgInTe₂ and CuInTe₂,several peaks are found in the（35）OD and carrier lifetime within the probe wavelength range of 850 to 1100 nm,indicating several valleys with similar energy near the conduction band minimum or valence band maximum,resulting in stronger inter-valley scattering and lower carrier mobility.From the single exponential fitting results of the transient reflection spectrum in PbTe-based compounds,the Pb Zn_0.015Te and Pb_0.98Zn_0.035Te possess longer carrier lifetime than undoped PbTe sample,resulting from the reduction of the carrier scattering from Pb vacancies.A damped oscillation can be observed in the transient reflection spectrum results of Bi₂Te₃,attributed to coherent phonons.The terahertz-time domain spectroscopy was carried out on Cu_1-xAg_xInTe₂,Cu_1-xAg_xGaTe₂ and PbTe-based compounds.By fitting the results with Drude-Lorentz or Drude-Smith model,the plasma frequency,charge carrier scattering rate as well as the frequency and scattering rate of the phonon mode are attained,consistent with the thermoelectric transport properties.