| Energy conversion technology based on Seebeck effect,which directly converts thermal energy into electrical energy,is a green technology to solve the increasingly issues of environmental pollution and energy shortage.Since the electric energy is generated from the carrier in solid materials,it has the advantages of no transmission parts and zero emission,and has become an alternative stratege to achieve sustainable use of energy and low carbon emissions.However,the energy conversion efficiency of thermoelectric materials is lower than that of traditional energy sources,so its large-scale commercial application still faces challenges.PbTe-base alloys are traditional thermoelectric materials with high thermoelectric properties.There high performances are mainly due to the high-degeneracy valence band structures.When the 4 L pockets and 12 ? pockets contribute to electrical transports together,the high electrical conductivity and power factor could be attained.However,the conduction band structure of PbTe is really poor,and it cannot achieve band convergence through doping or solid solution.Therefore,the ZT value of p-type PbTe can reach 1.8@850 K,while that of n-type PbTe is only 0.8 under the same carrier concentration.As we know,thermoelectric devices need n-type and p-type modules with matching thermoelectric performance,so it is of great significance to improve the thermoelectric performance of n-type PbTe.The thermoelectric properties of n-type PbTe can be enhanced by reducing the lattice thermal conductivity via introducing nanostructures.However the nanostructures would sacrifice the carrier mobility and power factor in some extent.Focus on high average ZT is more significant for thermoelectric applications.Construct local nanostructures can significantly reduce lattice thermal conductivity via enhanced low-middle frequence phonon scattering;meanwhile the high carrier mobility can be guaranteed by microsized-PbTe frameworks.Combined with dynamic doping effect,the temperature-dependent carrier concentration can be optimized,so optimization of thermoelectric performance can be achieved.The main research contens are listed as follows:(1)PbTe0.5Se0.5 nanowires with diameter of 19 nm were synthesized by simple liquid-phase method at a lower temperature.By changing the reaction conditions,the controllable method of the phase and morphology of the reaction products was attained,and the formation mechanism of nanowires was investigated.It was found that KOH concentration played a special role in the balance of chemical reactions,which solved the problem of phase separation in the synthesis of one-dimensional chalcogenide at large doping density.On this basis,the bulk thermoelectric materials based on PbTe0.5Se0.5 nanowires were prepared by spark plasma sintering process,and the thermoelectric properties were studied.The results show that the microstructure of nanowires transformed into nanoparticles after sintering,dense nanograin boundary and other microstructure defects were formed in the bulks.These microstructure defects enhanced the electron and phonon scattering,led to reduced electrical conductivity,and reduced the lattice thermal conductivity less than 0.7 W m-1K-1 between 300 K and 723 K.This strategy provides a new idea for obtaining low lattice thermal conductivity materials.(2)Due to the destroy of nanostructures during heating history,the nanocrystals treated by carbon coating acted as stabilization method are adopted here,so that the morphology and size of nanograins after annealing and sintering will be maintained.Based on the difference of the mean free path of electron and phonon in n-type PbTe,the local nanostructure was designed,the micrometer sized regions which were grown from PbTe nanocubes during the spark plasma sintering process acted as the conductive frame,while nanometer sized region originated from PbTe@C nanoparticles acted as the low-middle-frequency phonon scattering centers,so as to maintain the considerable carrier mobility while scattering the phonons substantially.Through transmission electron microscope,Raman spectrum and other characterization methods,it was confirmed that the carbon shell as a diffusion barrier layer was the key to stabilize the growth of nanocrystalline in sintering process.In addition,the Ag nanocrystalline decorated around grain boundary compensates interfacial resistance at low temperature,and diffused into the lattice at high temperature,forming dynamic doping effect which can continuously provide carriers to the matrix,inhibiting bipolar effect,and improving electrical conductivity and Seebeck coefficient.At the same time,the interstitial Ag atoms that increase with increasing temperature would scatter high-frequency phonons greatly,which effectively reduces the high-temperature thermal conductivity.Finally,the power factor exceeded 20 ?W cm-1 K-1 at 723 K,which was one of the highest reported values.The lattice thermal conductivity was 0.39 W m-1 K-1 approached the theoretical minimum,and the ZTpeak reached 1.65,which was one of the highest values reported in n-type PbTe.After repeated preparation and cycle tests,the system has high repeatability and good thermal stability.This novel nanostructure design provides a new strategy for the construction of high-performance thermoelectric materials.(3)On the basis of local nanostructures,the PbTe matrix was doped with Cu to optimize its low-middle temperature power factor,so as to optimize its average ZT value.The results showed that Cu doping can't change the band structure of PbTe.However,the carrier mobility and carrier concentration in the whole temperature range were optimized,and the power factors exceeded 20 ?W cm-1 K-2 in the temperature range of 400-823 K.For thermal properties,the local nanostructures and atomic point defects greatly enhanced phonon scattering,leading to a low lattice thermal conductivity,and the ZT reaches 1.68 at 823 K.What is particularly important is that the ZT value of room temperature was optimized to 0.4,and the ZTavg value in temperature range from 300 K to 823 K reached 1.14,which is one of the highest values reported so far.Good electrical performance and high ZTavg value are very important in aspect of practical value.(4)Compared with other methods that rely on the change of solubility,introduce magnetic nanoparticles and change the effective mass to optimize the temperatue-dependent carrier concentration,this work verify In doping is more universal.The ZTavg value of In-doped PbTe was improved by melting and combining with local nanostructures.Doping did not change the carrier effective mass of PbTe,and the impact on the carrier mobility is also smaller,but will introduce deep level defects in PbTe matrixs,the defect level would trap electrons in low temperature and reduce the doping efficiency.As temperature elevated to high temperature,the electrons on the deep level motivated and carrier concentration increased,the bipolar motivation effect can be inhibited,thus improved the Seebeck coefficient and reduced the lattice thermal conductivity.The point defects formed by In-doping can effectively scatter phonons.Combined with the scattering of phonons by various defects introduced by the local nanostructures,the lattice thermal conductivity drops significantly.Finally,a higher power factor than Cu doping was obtained from 300 K to 823 K,and the ZT value was 1.49 at 823 K,and the average ZT value was 0.84. |
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