Theoretical Investigations On Thermoelectric Transport In Several Systems And Their Applications

At present,many countries have proposed policies about peak carbon dioxide emissions and carbon neutrality to solve the problem which is the scarcity of fossil fuels and global warming.Many researchers are exploiting new energy to reduce carbon emissions.Thermoelectric（TE）devices can be used to achieve a direct energy conversion between thermal energy and electrical energy.The dimensionless parameter thermoelectric figure of merit（ZT）can be used to evaluate the performance of TE,hence this is critical to enhance the ZT of materials.This paper mainly explores the thermoelectric transport mechanism to improve the TE properties and predicts the TE properties of chalcogenide rare earths materials at high temperatures.Besides,the heat transport of organic/inorganic composites is also studied,and we design the TE generator made of organic/inorganic composites which have excellent physical properties.In Chapter 3,we investigate the effect of minority carrier blocking on the TE transport properties of nanocomposites（NCs）via the multi-band Boltzmann transport equation（BTE）under relaxation time approximation.Taking the p-type Bi_0.5Sb_1.5Te₃NCs as an example,we find that a heterojunction can effectively scatter the minority carriers,which suppress the bipolar effect.On this basis,we further considered the majority carriers low-energy filtering effect,both the power factor and ZT are enhanced in the temperature range of 300～500 K.The improvement of ZT value is mainly due to the majority carriers low-energy filtering effect at low temperature.The carriers participating in the thermoelectric transport will obtain a higher energy with increasing the temperature,and the majority carriers low-energy filtering effect will be weakened,but the concentration of minority carriers increases due to enhancement of the intrinsic excitation,the minority carriers blocking effect plays a significant role to enhance the ZT at high temperatures.Therefore,we summarized the main points to improve the performance of TE devices:（1）block the minority carriers as many as possible,and（2）filter majority carries with energy lower than 2～3 k _BT near the low-temperature region.In Chapter 4,we calculate the TE properties of La₃Te₄ and Ce₃Te₄ based on the first principles and the multi-band BTE.Some basic parameters of rare earth metal chalcogenides,such as the effective mass,band degeneracy,sound velocity,and optical phonon energy,are obtained by fitting the band structure and phonon spectrum.Then,the TE transport properties are calculated by BTE and we found that the TE performance of La₃Te₄ and Ce₃Te₄ are similar at low temperatures.This is mainly since the 4f electrons in Ce₃Te₄ are localized electrons nearly which have no contribution in the TE transport.This trend is consistent with the experiment.Besides,the linearly dependent optical phonon energy can explain the TE properties of La₃Te₄ at high temperatures.Based on this model,the TE performance of Ce₃Te₄ was predicted.We found the power factor of Ce₃Te₄ was slightly higher than that of La₃Te₄ at high temperatures.Simultaneously,the optimal carrier concentration of Ce₃Te₄ shifts upward with increasing temperature.When the temperature is 1200 K,the optimal carrier is 1.6×10²¹ cm^-3,and the corresponding power factor reaches 13.07μWcm^-1K^-2.This chapter summarizes the thermoelectric transport properties of Ce₃Te₄ at high temperature,which lays a theoretical research foundation for the application of rare earth chalcogenides.In Chapter 5,we proposed an analytical model to study the effect of space and temperature-dependent functional graded materials on the performance of TE refrigeration.We found that Joule heat does not flow uniformly to hot and cold ends when the electrical conductivity is not homogeneous.Assuming that the electrical conductivity at the cold end is larger than that at the hot end,the Joule heat flowing to the cold end will be reduced,which is beneficial to improve the TE cooling performance of the TE device.At the same time,the TE transport coefficient of unit volume can be determined based on the finite element method and BTE for a given carrier concentration,so that the TE cooling performance can be obtained.The results prove that selecting a suitable carrier concentration range can make the refrigeration performance of inhomogeneous TE materials be higher than that of homogenous materials.In Chapter 6,based on the single-leg thermoelectric device made of organic/inorganic composites,we consider waste heat and the huge temperature difference between a front and rear view in deep space,two different types TE generator are designed:（1）radiantional thermoelectric power generation device and（2）the power system is improved by using TE generator on the basis of the solar cell array power system,and we evaluate this system from the following aspect,such as rationality,reliability,economy,and advancement.Taking n-type PVDF/Ni nanowire and Cu electricity plate as an example,the output power and TE conversion efficiency are calculated.The results show that the contact effect is greatly affected when the leg length of the TE element is smaller.A higher output power density can be obtained by choosingan appropriate length of TE leg,we found a mass-specific power reached 7.21Wkg^-1 which is 2.4 times higher than that of Radioisotope Thermoelectric Generator（RTG）,besides the launch cost is only about 1/6 of that.This paper not only reveals microscopically the key to improving the thermoelectric performance of nanocomposites,but also improves the cooling performance of thermoelectric devices via constructing functionally graded materials on material structure design.At the same time,considering different heat sources and combining the characteristics of organic/inorganic composite materials,a corresponding thermoelectric generator is designed.this dissertation combines theory with application and aims to provide direction for practical application and save exploration time and cost.