Performance Optimization Of GeTe-based Thermoelectric Materials And Devices Through Atomic Orbital Engineering

In the 21st century,energy crisis and environmental problems have become increasingly prominent.In order to alleviate the above dilemmas,thermoelectric technology,which can use waste heat to achieve direct conversion from the heat to the electricity,is gaining attention.Thermoelectric devices have many advantages such as miniaturization,light weight,long life,no mechanical movement,easy integration with other electronic devices,etc.,which is a promising technique in the temperature difference power generation and thermoelectric cooling fields.The temperature of most low-grade waste heat is 500-900 K.Therefore,thermoelectric materials in the mid-temperature region have become a research hotspot in recent years.Among the many mid-temperature thermoelectric materials,Pb Te based thermoelectric materials have been commercially available.However,due to the environmental hazard of Pb,researchers have focused their attention on its alternative materials,such as Sn Te and GeTe.Nevertheless,the thermoelectric performance of Sn Te-based materials is relatively low.Therefore,GeTe based thermoelectric materials have become a promising candidate.The intrinsic GeTe having much Gevacancies and large numbers of energy difference between the heavy and light bands at the top of the valence band,possesses a large carrier concentration(～10²¹cm^-1),which in turn makes the electrical and thermal conductivity high while keeps the Seebeck coefficient low.In addition,the volume change before and after the phase transformation affect its mechanical properties.The such mentioned shortcomings limit the application of GeTe-based thermoelectric materials.To solve the above problems,GeTe thermoelectric materials are prepared by high-temperature melting method combined with hot pressing process in this paper.The effect of spd orbitals on their crystal structure and energy band structure is analyzed by utilizing the orbital engineering system.The changes of phase components,phase transition temperature and microstructure in the electron and phonon scattering mechanisms are also further investigated.The thermoelectric and mechanical properties of the GeTe based thermoelectric materials were optimized synergistically.The main research works in this paper are shown as follows.（1）GeTe bulk materials were successfully synthesized by high-temperature melting method combined with hot pressing process.The Y-4d orbitals were introduced to increase the band gap and converge the energy band,reduce the Geenrichment,reduce the carrier concentration and enhance the carrier mobility.Meanwhile,point defects and grain boundaries introduced by Y doping,which contribute to phonon scattering.Thus resultings in a thermoelectric peak of～1.44 at 732 K and an average z T value（323 K-823 K）of 0.74for Ge_0.88Y_0.02Te.（2）On the basis of Y doping,we continue to dope Sb and introduce Sb-5p orbitals to increase the symmetry of the crystal structure of the sample,decrease the phase transition temperature,and reduce the band gap and light-heavy band energy level difference.Meanwhile,the carrier concentration is further reduced to improve electrical transmission characteristics.At the same time,the substitution of Sb for Gecan cause large fluctuation of mass and stress,consequently aggravating phonon scattering.Therefore,Ge_0.88Y_0.02Sb_0.10Te obtained the peak z T of～2.13 at 773 K,the average z T value（323 K-823 K）also reaches 1.31,and the Vickers hardness is 231 H_V.（3）The introduction of Ag continues based on Y and Sb doping,and the synergistic effect of Y-4d,Sb-5p and Ag-5s orbitals lowers the phase transition temperature.This further optimization of the energy band structure and the increase of carrier concentration and mobility significantly enhance the Seebek coefficient in the low temperature part.At the same time,the co-doping forms large mass and stress fluctuations,resulting in morphology of coherent twin crystal.Moreover,extremely low thermal conductivity was achieved by strongly scattering phonons,while a high power factor was kept.This led to a peak z T of～2.27 at 773 K,an average z T value（323 K-823 K）of 1.61 for Ge_0.870Y_0.02Sb_0.10Ag_0.010Te,and a Vickers hardness of 247 H_V,with all properties at the top of its class.（4）The p-type material MgAg_0.95Sb_0.99（cold end）was screened based on the compatibility factor theory to form a single-arm ladder device with the above Ge_0.870Y_0.02Sb_0.10Ag_0.010Te（hot end）.Through simulation with COMSOL Multiphysics software（Finite Element method,FEM）,the geometry of device are optimized.The conversion efficiency of the GeTe-Mg Ag Sb single-armder device can reach 19.3%when the optimized thermoelectric device parameters are adopted.The length of the high-temperature section（L₁）is 6.5 mm,the length of the low-temperature section（L₂）is2.7 mm,the croscross-sectionala is 4×4 mm,and the temperature difference is 500 K.This is a significant increase in the conversion efficiency of the GeTe-Mg Ag Sb single-arm ladder device.This lays a theoretical foundation for the preparation,application of GeTe-based thermoelectric devices.