The Thermoelectric Performance Of Low-symmetry Rhombohedral GeTe

Nowadays,the improvement of technology and science is severely limited by the lacking non-renewable resources,which also caused environmental pollution.Under this circumstance,it is extremely important to develop a new renewable and clean energy for reversing the above situation.Thermoelectric energy conversion technology is famous for enabling a clean conversion between heat energy and electric energy,used in semiconductor refrigeration,waste heat recovery power generation,aerospace power supply,and other fields.However,the low conversion efficiency is the main factor restricting the large-scale application of thermoelectric conversion technology,which depends on the dimensionless thermoelectric z T=S²σT/κ.This topic mainly focuses on Ge Te-based thermoelectric materials.Differ from previous researchers who mainly focus on the thermoelectric properties of high-temperature cubic Ge Te,this topic mainly focuses on the properties of rhombohedral Ge Te thermoelectric materials in the low-temperature region.The transformation of Ge Te from cubic phase to rhombohedral phase,leading to the cleavage of band structure as well as the energy reverses between the different valence bands,which enables a higher band degeneracy in rhombohedral phase Ge Te.In this topic,by establishing the relationship between the crystal structure of rhombohedral Ge Te and its energy band structure,the high band degeneracy is achieved at near room temperature and enable the high thermoelectric performance comparable to that of commercial Bi₂Te₃.Moreover,for the issue of the Ge Te intrinsic high carrier concentration,a high-efficiency dopant is developed to realize the rapid optimization of carrier concentration.Additionally,the thermoelectric performance of Ge Te is optimized from the material level to the device level utilizing defect control,interface design to reduce interface resistance,and module size optimization.This research will become the foundation of other researchers about low-temperature thermoelectric materials and thermoelectric devices.The main content of the study is clarified as follows:In this work,the single-phase solid solution Ge_1-xPb_xTe was obtained by annealing in the high temperature single-phase region of the Ge Te-Pb Te pseudo-binary phase diagram for a long time,realizing the continuous change of the angle between the axes in the rhombohedral phase.It is the first time to experiment declare the energy band structure is relative to the crystal structure.The high energy band degeneracy was achieved at near room temperature by adjusting the interaxial angle;After doping Pb Te,the cation to anion size ratio of solid solution is increased since the size of the Pb atom is larger than that of Ge,reducing the formation energy of Ge vacancy in the matrix,and optimizing the carrier concentration;On the other hands,decreasing the sound velocity and introducing a large number of Pb/Ge point defects to enhance phonon scattering,thus greatly reducing the lattice thermal conductivity.As a result,the high thermoelectric performance enabled in the rhombohedral Ge Te,which is comparable to the commercial p-type Bi₂Te₃.The hole carrier concentration of intrinsic Ge Te is up to～10²¹ cm^-3,which is higher than the optimized carrier concentration.To decrease carrier concentration of intrinsic Ge Te but induce less impurity element,the efficient dopant Cu₂Te has been developed.Just 1.5%Cu₂Te not only can largely reduce the carrier concentration to2×10²⁰ cm^-3,but also has a weak influence on the band structure and electronic structure,the carrier mobility is greatly improved.This work reveals the high electrical performance in intrinsic Ge Te.Moreover,the Bi Te and Pb Te are used to further optimize the carrier concentration,and a large number of point defects were introduced to reduce the lattice thermal conductivity.As a consequence,the optimal thermoelectric performance z T>2 is achieved in low temperature rhombohedral Ge Te,which is comparable to that of the cubic Ge Te in high temperature.In view of the characteristics of intrinsic high carrier concentration of Ge Te,this work has developed a new approach.Different from the traditional optimization of carrier concentration by heteropanent element doping,this work uses solid solution equivalent Cu₂Te and Pb Se to control the Ge vacancy concentration in intrinsic Ge Te to achieve a significant reduction of carrier concentration.The co-solution of Cu₂Te and Pb Se has less effect on the crystal structure of Ge Te,enabling the high carrier mobility in the Ge Te alloys In addition,there is a large amount of point defects form,reducing the lattice thermal conductivity.Finally achieved the peak figure of merit z T>2.5,the average z T>1.5.A design of Ag/Sn Te/Ge Te contact successfully enables both a prevention of chemical diffusion and an interfacial contact resistivity of only 8μΩ·cm² for the realization of the high output power density and high conversion efficiency have been achieved in an optimized Ge Te-based single-leg device.The performance of Ge Te-based thermoelectric materials has been improved much faster than the research on Ge Te-based thermoelectric modules,the research of thermoelectric modules is indispensable to the practical application of Ge Te based thermoelectric materials.Based on the excellent thermoelectric performance of p-type(Ge_0.98Cu_0.04Te)_0.88（Pb Se）_0.12 and n-type Mg_1.05Y_0.01Sb Bi alloys,the interface structure of Ag/Sn Te/Ge Te and Ni/Fe/Mg₃Sb Bi was designed to achieve low contact resistivity.This is very important to achieve the high output performance of the thermoelectric module.In addition,the size design of n-and p-type thermoelectric single-leg ensures the full display of the thermoelectric performance of the material.Finally,the thermoelectric conversion efficiency is>10%and the output power density is>15k W/m² when the temperature at the heat-source is<600 K.This work illustrates that non-Bi₂Te₃ thermoelectrics has the potential to realize even higher efficiency in recovering abundant low-grade waste heat.The large-scale application of p-Ge Te/n-Mg₃Sb Bi and Bi₂Te₃-based thermoelectric modules is difficult due to the limited abundances and prices of elements Te and Ge.Here we demonstrate cheap polycrystalline antimonides for even more efficient thermoelectric waste-heat recovery within 600 K than conventional tellurides.This is enabled by a design of Ni/Fe/Mg₃Sb Bi and Ni/Sb/Cd Sb contacts for both a prevention of chemical diffusion and a low interfacial resistivity,realizing a record and stable module efficiency at a temperature difference of 270 K.In addition,the output power to the raw-material cost ratio in this work is increased to 16 times of that of conventional Bi₂Te₃-modules.In this work,through an in-depth understanding of the relationship between interaxial angle and band structure of rhombohedral Ge Te,and taking the interaxial angle as an important parameter of band regulation of Rhombohedral Ge Te,the development of high thermoelectric performance at near room temperature and high efficiency dopant were realized.Furthermore,the high output performance of Ge Te-based thermoelectric devices was realized through interface structure design and leg-size design.A new type of cheap antimonide thermoelectric module with a high output power density and conversion efficiency was designed.This work provides a theoretical and experimental basis for the research of new thermoelectric materials and devices.