| Energy consumption and environmental pollution have become increasingly prominent problems nowadays.The development of thermoelectric materials and the application of thermoelectric devices have become one of the research hotspots all over the world.The study topic of this thesis is about GeTe-based thermoelectric material.The thermoelectric performance is optimized by means of phase structure regulation,introduction of various scattering mechanisms to reduce lattice thermal conductivity,optimization of carrier concentration,and inhibition of bipolar transport in high temperature regions.In this paper,three series of(Ge0.87Pb0.13Te)1-x?Bi2Te3?x,Ge0.8Pb0.1Bi0.1Te1+x and(Ge1-xPbxTe)0.93?Bi2Te3?0.07were prepared by high-temperature melting,spark plasma sintering processes.The doping effects of Bi2Te3,Pb,Te and heat treatment process on the phase,microstructures and thermoelectric properties were studied.The main conclusions are shown below.?1?The study of the system(Ge0.87Pb0.13Te)1-x?Bi2Te3?x show that Bi2Te3 doping can effectively increase the solubility of Pb in GeTe and gradually transform the GeTe from rhombohedral structure into cubic structure,thus realizing the control of phase structure.Moreover,Bi2Te3 doping can also reduce the concentration of carriers,which will also affect the effective mass and carrier mobility of the samples.Besides other traditional phonon scattering mechanisms in this system,stacking fault phonon scattering mechanism targeting intermediate frequency phonons can be introduced by Bi2Te3 alloying.Combined all kinds of phonon scattering mechanisms to scatter phonons in different frequencies,the lattice thermal conductivity can be greatly reduced.In this system,the highest ZT value1.67 is achieved in the samples with x=0.05 at 573 K.The samples has also been treated with liquid nitrogen rapid cooling after thermal annealing,and the corresponding thermoelectric test showed that the electrical conductivity of the samples with x=0.05 and 0.07 can be significantly improved in the low temperature region?<373 K?.This enhanced electrical properties and ZT values in the low temperature can be ascribed to the formation of high-speed carrier channel by the heat treatment.?2?The study of the system(Ge1-xPbxTe)0.93?Bi2Te3?0.07 show that the lattice constant and the interaxial angles of GeTe increases with the Pb content and the carrier concentration can also be reduced with the increase of Pb content.Moreover,point defect scattering mechanism can be enhanced by Pb doping,which effectively reduce the lattice thermal conductivity.However,due to the poor electrical properties and bipolar effect,the maximum ZT value of this system can only reach 1.47.The diffraction peak of(Ge1-xPbxTe)0.93?Bi2Te3?0.07.07 samples can be broadened after annealing treatment,and PbTe phase precipitation can also show up in some samples.Meanwhile,the carrier concentration and the lattice thermal conductivity can be increased by thermal annealing,which can be ascribed to the extrusion of the Pb atom from the matrix of GeTe.Annealing treatment can also deteriorate the properties of samples with high Pb content,and the thermal stability is poor.Annealing treatment can worsen the thermoelectric performance and thermal stability of the samples with high Pb content.?3?The study of the system Ge0.8Pb0.1Bi0.1Te1+x show that the increase of Te content can effectively eliminates the Ge phase,increase the carrier concentration and enhance the electrical properties of the samples.Moreover,the bipolar effect can be effectively inhibited by increasing Te content,which can gradually decrease the bipolar thermal conductivity in the high temperature region.Thus,the increase of Te content promotes the electrical performance and the inhibition of bipolar thermal conductivity,which improves the ZT value and the average ZT value of the system.The highest ZT value of1.92 at 637 K and the average ZT value of1.33 are obtained in the samples with x=0.06. |
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