| Lead telluride(PbTe)has long been become a good thermoelectric material in the middle temperature region due to its properties such as narrow band gap(0.32 eV),intrinsic low thermal conductivity and high melting point.The thermoelectric figure of merit of the p-type PbTe material has been improved to more than 2.0,but the n-type PbTe has a relatively poor performance.This thesis aims to optimize the electrical and thermal transport performance through the double doping in anion and cation sites of PbTe,thereby improving the thermoelectric figure of merit of n-type PbTe and realizing n-p matching in thermoelectric devices.Furthermore,in order to broaden the application temperature range of thermoelectric devices and obtain high-temperature stable thermoelectric materials,we will also apply the optimization strategy to adjust the carrier concentration and construct core-shell structure for the preparation of high-temperature thermoelectric materials of strontium titanate(SrTiO3).(1)We have successfully prepared the n-type Pb1-xBixTe1-xIx(x=0,0.05,0.1,0.2,0.3,0.4,0.5%)thermoelectric materials doped with bismuth(Bi)and iodine(I)dual elements via the solid phase method,and the carrier concentration of doped samples reached the order of 1019 cm-3,which was in the optimal carrier concentration range.Since Bi3+and I-occupy the cation and anion sites of PbTe as donor impurities,respectively,as the increase of the doping amount,the resistivity of the samples decrease and the absolute Seebeck coefficient increase significantly,and the power factor of the x=0.5%samples reaches 25 ?WK-2cm-1.Dual doping leads to a large amount of point defects and nano-precipitations inside the sample,which make the phonons to be scattered forcefully and then the thermal conductivity of lattice reduced.The total thermal conductivity of dual element doping concentration for x=0.05%drops to 1.21 Wm-1K-1(T=732 K),which is 14%lower than the pure PbTe.In addition,the lowest lattice conductivity in all the samples is 26%lower than that of the PbTe sample.After the collaborative optimization of electrical and thermal transport performance,the sample with x=0.20%achieved the highest zT?0.9 at 725 K,and the sample of x=0.05%achieved an average thermoelectric figure of merit?0.7 in the range of 323-723 K,which is more than six times the performance of PbTe.On the basis of the above Bi and I dual doping,we further studied the thermoelectric properties of trace antimony(Sb)and iodine(I)co-doping PbTe.Sb can be used as an amphoteric dopant in PbTe,that is,as a donor impurity when it is at a cation site,and as an acceptor impurity when it occupies an anion site.The doping of Sb element directly causes the energy band inversion,and the main carriers are transformed from holes to electrons,which realizes the transformation from the intrinsic p-type to the n-type doped with donor impurities.The power factor of the sample with the doping amount x=0.3%obtains an extreme value of?22.5 ?WK-2cm-1 at about 300 K.(2)In order to achieve a temperature-dependent change in carrier concentration,we introduce monovalent silver(Ag)or copper(Cu)cations at the cation site.The Ag atoms in the PbTe sample doped with Ag and I will preferentially fill the Pb vacancies(VPb2-),and Ag as an acceptor impurity indirectly generates holes,which leads to an increase in the absolute Seebeck coefficient and the resistivity.When the doping concentration continues to increase to x=2.0%,4.0%,the interstitial Ag atoms gather around the Te element to form Ag2Te,which acts as a donor in the matrix to release electrons and increase the electron carrier concentration.In terms of thermal transport property,due to the dynamic doping effect of the Ag2Te nano-scaled second-phase in the Pb0.98Ag0.02Te0.98I0.02 sample,nano-sized precipitation enter the main phase to accelerate the process of phonon scattering,which manage to diminish the thermal conductivity of lattice.As the temperature increases,the lattice thermal conductivity continues to drop from 1.35 Wm-1K-1 to the minimum value of 0.67 Wm-1K-1.Due to the improvement of electrical transport performance and the decrease of lattice thermal conductivity,the Pb0.98Ag0.02Te0.98I0.02 sample achieved the maximum zT value of?1.0 at 773 K.Similar to Ag doping,Cu atoms occupy cation sites as acceptor impurities when the doping amount is small,resulting in an abnormal increase in the conductivity of the sample and a decrease in the absolute Seebeck coefficient.As the impurity content increases,more copper atoms in the sample forms interstitial impurities to release electrons,so the resistivity of the sample with high doping concentration decreases significantly.The power factor of the Pb1-xCuxTe1-xIx(x=1.0-4.0%)sample at low temperature and high temperature is about 10 ?WK-2cm-1.In addition,the heat-carrying phonons of different wavelengths can be fully scattered by introducing zero-dimensional point defects,two-dimensional grain boundaries,and nanometer-sized precipitation into the sample,and the lattice thermal conductivity of the sample is greatly reduced.The lattice thermal conductivity of the sample doped with 3%CuI was as low as 0.4 Wm-1K-1 at 623 K,and the zT value of the Pb0.97Cu0.03Te0.97I0.03 sample reached 1.0 at 777 K.(3)In addition to the introduction of impurity elements to adjust the carrier concentration,the regulation of the microstructure is also an important means to improve the thermoelectric properties of materials.We synthesized PbTe-PbS coated structures with different ratios,the size of which was about 10 ?m.The sintering method is replaced from traditional hot-press sintering to spark plasma sintering(SPS)with a shorter reaction time.The smaller PbS grains gather around the PbTe grains,and the grain size of the sintered sample is significantly reduced.We also optimized the microstructure of the high temperature thermoelectric materials of SrTiO3(STO),and successfully prepared(La0.10STO)0.90(Nb0.05STO)0.10 nano-core-shell structure with an average grain size of about 60 nm and a uniform size distribution through the hydrothermal synthesis method.Finally,after optimizing the molding pressure and sintering conditions,the sintered samples are hard and retain the nano-structure to the greatest extent. |
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