Defect and Dopant Perspectives of Thermoelectric Materials

熱電材料的缺陷和摻雜劑研究

Student thesis: Doctoral Thesis

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Supervisors/Advisors
Award date14 Apr 2020

Abstract

Thermoelectric materials are evolving with continuous improvement in their performance through nano-structuring, introducing resonant states, doping process and multi-phase engineering. However, their intrinsic defects such as vacancies, interstitial, stack fault and grain boundary defect play an inevitable role in influencing the thermoelectric performance of the material. Thus native and extrinsic defect engineering is the future of thermoelectric power conversion process. Presence of microstructural defects in thermoelectric materials plays significant role in modifying their properties by influencing the behavior of electrons and phonons. Dopants with unique properties can directly cause distortions in electronic density of states (eDOS) and phonon transport mechanism by intentionally inducing defects in their lattice.

In this thesis, we present the theoretical and experimental outcomes of engineered vacancy defects, by intentional doping of f-block rare earth elements in β-Zn4Sb3. Thermoelectric behavior breaks down the inverse relation and exhibits a parallel increase in both Seebeck coefficient and electrical conductivity for rare earth doped β-(Zn0.997Ce0.003)4Sb3 and β-(Zn0.997Er0.003)4Sb3. This synergistic response triples the power factor of thermoelectric β-Zn4Sb3 system realized by the impurity induced resonant distortion in eDOS. From first principle GGA + U calculations, the above-mentioned unconventional properties are attributed to the effect of doping induced vacancy formation and the formation of resonant impurity levels. Further, defect engineering in thermoelectric materials leads to the formation of exotic thermal transport properties. Specifically, reduction in lattice thermal conductivity (кL) can be realized through scattering of low and high-frequency phonons by interfacial and point defects respectively. Herein we explore such phenomena by inducing dense dislocations through doping of rare earth (RE) impurities in β-(Zn1-xREx)4Sb3 [x=0.3-0.5 at.%] as phonon scattering source of all frequencies. Lattice anharmonicity created results in an ultra-low кL of ~0.15 W/mK for β-(Zn0.997Yb0.003)4Sb3. Vibrational properties and phonon scattering altered by the lattice anharmonicity is studied in detail through terahertz and infrared spectroscopies.

We also present another experimental works on 2D layered Tin Selenide (SnSe), where we demonstrate a record of high zT of 2.4 at 800 K through controlled optimization of intrinsic defects in polycrystalline SnSe. Theoretically, through detailed calculation defect formation energies and lattice dynamic phonon dispersion studies, we demonstrate that the presence of intrinsic charged Sn vacancies can enhance the hole concentration via Fermi level shifting and distort the lattice thermal conductivity through phonon-defect scattering. Supporting our theoretical calculations, the experimental enhancement in the electrical conductivity lead to a massive power factor of 8.9 μW/cm/K2 and an ultralow lattice thermal conductivity of 0.22 W/m/K was achieved by vacancy-phonon scattering effect in polycrystalline SnSe.

Furthermore, we have demonstrated a thermoelectric generator (TEG) with shape conformable geometry for sustaining low-thermal impedance and large temperature gradient (ΔT) is fundamental for wearable and multi-scale energy harvesting applications. This approach not only decreases the thermal impedance but also multiplies the temperature gradient, thereby increasing the power conversion efficiency (PCE) as comparable to bulk TEG. Intact thin films of Tin telluride (p-type) and Lead Telluride (n-type) are deposited on flexible substrate through physical vapor deposition and a thermoelectric module possessing a maximum output power density of 8.4mW/cm2 is fabricated. We have established the performance of p-SnTe/n-PbTe based TEG as a flexible wearable power source for electronic gadgets, as a thermal touch sensor for real-time switching and temperature monitoring for exoskeleton applications.