Segmented Thermoelectric Oxide-based Module

Since 1990s, oxide thermoelectrics have been considered as promising thermoelectric (TE) materials due to their non-toxicity, low-cost, and chemical stability at high temperatures. Studied results show great potential for applications in thermoelectric power generator (TEG) at high temperature and thus have drawn attentions over the years. However, oxides TEGs are still not used broadly due to their low performance. This thesis targets the research and development of exploring the use of these materials in high temperatures range using high conversion efficiency TEG based modules. This study demonstrates an effective way to improve the efficiency of oxide TEG by segmentation of oxide materials with other high-performance non-oxide materials, thereby, extending the temperature range.
This thesis was started by developing of n-type oxide material e.g. CaMnO₃ as possible alternative n-type candidate for a more stable high temperature material. In this study, thermoelectric properties from 300 to 1200 K of Ca_0.9Y_0.1Mn_1-xFe_xO₃ for 0 ≤ x ≤ 0.25 were systematically investigated in term of Y and Fe co-doping at the Ca- and Mn-sites, respectively. It was found that with increasing the content of Fe doping, the Seebeck coefficient of Ca_0.9Y_0.1Mn_1-xFe_xO₃ tended to increase, while the tendency towards the electrical conductivity was more complicated. Thermal conductivity of the Fe-doped samples showed a lower value than that of the non-doped sample. The maximum dimensionless figure-of-merit, zT was found to be improved about 20% for the sample with x = 0.05 as compared to that of the x = 0 sample at 1150 K.
High-performance segmented legs/unicouples based on oxide materials are first designed by numerical modelling. The criteria of material selection for segmentation are based on their “efficiency ratio” described the total conversion efficiency per the materials cost and their compatibility factors. The numerical modeling results (chapter 3) showed that the maximum theoretical conversion efficiency of segmented legs/unicouples could be over 10%, which is more than twice as compared with the one comprised from non-segmented oxide elements. The calculation also takes into account the interfacial contact resistances to evaluate the influence on the total conversion efficiency. The obtained modeling results provide useful tools for designing future low-cost, high-performance segmented TEGs. A high-performance segmented oxide-based module comprising of 4-unicouples using segmentation of the half-Heusler Ti_0.3Zr_0.35Hf_0.35CoSb_0.8Sn_0.2 and the misfit-layered cobaltite Ca₃Co₄O_9+δ as the p-leg and 2% Al-doped ZnO as the n-leg was successfully fabricated and characterized. The results (presented in Chapter 5) show that at a temperature difference of 700 K, the maximum output power density attains a value of ∼6.5 kW/m², which is three times higher than that of a non-segmented oxide module under the same condition. Initial long-term stability test of the module at hot and cold side temperature of 1073/444 K showed a promising result, although a slight degradation tendency could be observed after 48 hours of operating in air. Nevertheless, the total conversion efficiency of this segmented module is still low less than 2%, and needs to be further improved. A degradation mechanism was observed, which attributed to the increase in the interfacial contact resistance between the n-type material (doped ZnO) and the metal electrode. The next study (Chapter 6) focuses on enhancing the efficiency of a single oxide-based segmented leg by further reducing the contact resistance and employing materials with better TE properties, i.e. a p-type leg that consists of misfit-layered cobaltite Ca2.8Lu0.15Ag0.05Co4O9+δ nano-composite and the half-Heusler Ti_0.3Zr_0.35Hf_0.35CoSb_0.8Sn_0.2 alloy. For the first time, a maximum conversion efficiency as high as ∼5% at a ΔT ≈ 756 K was measured. This high efficiency segmented leg is also tested for over two weeks at the hot and cold side temperatures of 1153/397 K, showing good durability as a result of stable, low electrical resistance contacts.