Thermoelectric performance enhancement of lead-free germanium telluride-based materials

• Main Author: Tan, Xian Yi
• Other Authors: Alex Yan Qingyu
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2023
• Subjects: Engineering::Materials::Energy materials
• Online Access: https://hdl.handle.net/10356/165578
• Institution: Nanyang Technological University

The recovery of waste heat through the use of thermoelectric (TE) devices is an attractive solution to alleviate the growing global energy demands and fossil fuel shortage, considering that about two-thirds of the energy input to ubiquitous processes that involve heat engines are constantly being lost as waste heat. As the conversion efficiency of devices using traditional state-of-the-art materials remain too low to be widely commercially competitive, there is a desperate need for materials with high figure-of-merit (ZT) values. Such high ZT materials usually have combined traits of good heat insulation, metal-like electrical conductivities, and high voltage response to a temperature difference. The later two properties determine its electrical response. GeTe is an emerging semiconductor TE material with its peak performance in the medium-temperature range (350 – 550 °C), where most optimized state-of-the-art GeTe-based materials can achieve ZTs of >2 at ~450 °C, where those with the best efficiencies, wide-temperature range average ZTs and near-room-temperature (50 °C) ZTs all contain lead (Pb) atoms replacing some Ge atoms in the GeTe crystal (i.e. Pb doping), which are unfortunately highly toxic and will hinder the wide-spread usage of GeTe-based TE devices in the future. Therefore, the objectives of this thesis largely involve achieving comparable performances without Pb, by modifying Pb-free GeTe materials through: (i) alloying with a heat insulating phase, (ii) dilute doping for maximum electrical response properties at 50 °C, and (iii) stacking onto another TE material. The findings of this thesis will pave the way to less toxic GeTe-based devices with greater practical versatility. For the first objective, melting GeTe to blend with 8% AgInSe2 (i.e. AgInSe2 alloying) improves the heat insulation at 50 °C to a value comparable with that of a similar quantity of PbSe alloying, proving the capability of AgInSe2 alloying in replacing the role of Pb for heat insulation. In order to optimize the ZT further, 7% Bi doping was added. The resulting sample managed to achieve a heat insulation at 50 °C that is comparable to some of the best Pb-containing GeTe materials, due to the large irregularities in the GeTe crystal slowing down the heat transport. While its ZT of ~0.49 at 50 °C is one of the highest for Pb-free GeTe materials, however it is still lower than the best Pb-containing counterparts. For the second objective, GeTe was alloyed with 1.5% Cu2Te and doped with 1% In, achieving a record high electrical response among GeTe-based materials. Bi doping (4%) was added to improve the thermal insulation and achieve a ZT of ~0.43 at 50 °C and an average ZT of 1.35 at a temperature range of 50 – 500 °C, which are comparable to some of the best Pb-free GeTe materials, but are still lower than the Pb-containing counterparts. Future plans on improving the ZTs by introducing other microscopic structural features that slow down the heat transport were discussed. While the first two objectives were only partially fulfilled, the single bar device made with 75% bar length of Ge0.9Sb0.1Te joined with 25% bar length of Bi0.5Sb1.5Te3 was able to achieve the third objective. Its power conversion efficiency of 13.6%, generated from a temperature difference of 7 – 500 °C, is comparable to the best Pb-containing GeTe-based material. The strategy involves optimizing the length proportion of the two different materials and making sure that the electrical current to heat flow ratio is the same for both materials at the surface where they are joined. Therefore, even though sub-optimal common compositions of both materials were used, with a low estimated average ZT of 1.12 for the combined bar device sample, it was able to perform much closer to its theoretical maximum efficiency. Future plans to improve the efficiency by using more optimized compositions of GeTe and Bi2Te3-based materials were discussed.