Performance Study Of Medium Temperature Thermoelectric Materials On Ge_1-xCa_xTe_1-ySe_y And (Sn_0.7Ge_0.2Pb_0.1)_1-xMn_1.1xTe

Thermoelectric?TE?materials,as a kind of new energy materials,can achieve a direct conversion between thermal and electrical energy.It provides a safe,reliable and solid state device for thermoelectric power generation and refrigeration,which has widely potential application,especialy in this era of energy shortage.In this work,we report the investigation on the thermoelectric properties in Ge Te and Sn Te medium temperature thermoelectric materials by elements doping.Four series of alloys GeTe_1-xSe_x,Ge_1-x Cax Te,Ge_0.975-x Ca_xTe_0.9Se_0.1 and(Sn_0.7Ge_0.2Pb_0.1)_1-x Mn_1.1x Te were prepared by conventional melting,quenching and spark plasma sintering?SPS?techniques to study their phases,microstructures,solid solubilities,carrier concentrations and thermoelectric properties.?1?The investigation of GeTe_1-xSe_x?x = 0,0.1,0.2,0.3,0.4 and 0.5?alloys shows that all alloys formed rhombohedral Ge Te-based solid solution and minor cubic Ge phase without any phase arising from Se.All alloys keep p-type conduction of Ge Te.The substitution of Se for Te in the Ge Te has a little effect on the carrier concentration,but decreases the carrier mobility and thus increases its electrical resistivity and Seebeck coefficient.The enhancement of phonon scattering from the solute Se atoms and minor phase Ge was responsible for the reduction of the thermal conductivity significantly.As a result,the figure of merit ZT of the GeTe_1-xSe_x alloys can be enhanced with proper Se substitution.A maximum ZT of 1.17 at 773 K was eventually achieved in the sample with x = 0.3,which is 26% higher than that of pure Ge Te.?2?The investigation of Ge_1-xCa_x Te?x = 0,0.01,0.03,0.05,0.07 and 0.09?alloys shows that all alloys contain rhombohedral Ge Te-based solid solution and minor cubic Ge phase,while Ca Te minor second phase appears in the alloys with x ? 0.03?The solid solubility of Ca in Ge Te is only 1.5 mol %.All alloys keep p-type conduction of Ge Te.The substitution of Ca for Ge in the Ge Te reduces its carrier concentration,but increases the carrier mobility,and thus decreases its electrical resistivity and Seebeck coefficient in high temperature.Ca doping in Ge Te reduces the phase transformation temperature of Ge Te-based phase from rhombohedral to cubic structures.9 mol% Ca doping in Ge Te decreases the phase transition temperature from 673 K?400 ??to 573 K?300 ??.The enhancement of phonon scattering from the Ca solute atoms,grain boundary and Ca Te minor second phase was responsible for the reduction of the thermal conductivity.At room temperature,the thermal conductivity decaerse by 22%,from 8.54 W m^-1 K^-1 of Ca-free sample to 6.62 W m^-1 K^-1 in the alloy with x = 0.09.At 773 K,the thermal conductivity decaerse by 38%,from 3.23 W m^-1 K^-1 of Ca-free sample to 2.01 W m^-1 K^-1 of the alloy with x = 0.07.Low thermal conductivity compensates the loss of the power factor due to doping and thus optimizes the thermoelectric performance of alloys.As a result,A maximum ZT of 1.07 at 773 K was eventually achieved in the sample with x = 0.07,which is 22% higher than that of Ca-free sample.?3?The investigation of Ca and Se co-doped Ge_0.975-xCa_xTe_0.9Se_0.1?x = 0,0.01,0.03,0.05,0.07,0.09 and 0.11?alloys shows that the alloys with x = 0,0.01 formed rhombohedral Ge Te-based solid solution,while minor Ca Se second phase appears in the alloys with x > 0.03.Introduction of small amount of Se in Ge Te increases the solid solubility of Ca in Ge Te to 3 mol%.Although the power factor decreases at some degree,remains above 2400 ?W m^-1 K-2.A maximum ZT of 1.24 at 773 K was eventually achieved in the sample with x = 0.07,which is 32% higher than that of Ca-free sample and 16% higher than of the maximum ZT of Ca single doped Ge Te at same temperature.ZT values increase due to a combination of the following two aspects:?1?inreaseing the solid solubility of Ca in Ge Te due to the introduction of small amount of Se in Ge Te,which remains high power factor of 2760 ?W m^-1 K-2;and?2?decreasing the thermal conductivity due to enhancement of phonon scattering from Se solute atoms,grain boundary and minor Ca Se second phase.?4?The investigation of (Sn_0.7Ge_0.2Pb_0.1)_1-x Mn_1.1x Te?x = 0,0.1,0.15,0.2,0.25 and 0.3?alloys shows that those alloys form Na Cl-type structure?Sn,Ge,Pb,Mn?Te solid solution high entropy alloy main phase containing Ge,Pb,Sn,Mn and Te multiple principal elements with low thermal conductivity.A higher solubility limit of ²0 mol% of Mn in the Na Cl structure(Sn_0.7Ge_0.2Pb_0.1)Te was achieved as comparison with ?9 mol% for Mn in Sn Te,being better for valence band convergence and the enhancement of Seebeck coefficient.Mn doping in(Sn_0.7Ge_0.2Pb_0.1)Te increases its carrier concentration,reduces its mobility,enhances its resistivity and thus decreased carrier thermal conductivity.Meanwhile,the the lattice thermal conductivity can be reduced by the high entropy alloy main phase?Sn,Ge,Pb,Mn?Te and nano Mn Te second phase.At room temperature,the carrier thermal conductivity decaerse by 340%,from 1.67 W m^-1 K^-1 of the alloy with x = 0 to 0.38 W m^-1 K^-1 of the alloy with x = 0.3.At 873 K,the carrier thermal conductivity decaerse by 105%,from 1.11 W m^-1 K^-1 of the alloy with x = 0 to 0.54 W m^-1 K^-1 of the alloy with x = 0.3.The lattice thermal conductivity decaerse from 3.23 W m^-1 K^-1 to 1.22 W m^-1 K^-1 at 300 K,from 1.68 W m^-1 K^-1 to 0.37 W m^-1 K^-1 at 873 K,reducing by 160% and 350%,respectively as comparing the above alloys.A maximum ZT of ?1.37 at 873 K was achieved in the sample with x = 0.3 by above synergistic effects,which is 110% higher than 0.65 for Mn-free sample,1.22 for non-stoichiometric Sn0.9Mn0.11 Te alloy or that of other Mn single doped Sn Te at 873 K prepared by SPS.