Abstract

Oxide-based materials are promising candidates for use in high temperature thermoelectric generators. While their thermoelectric performance is inferior to commonly used thermoelectrics, oxides are environmentally friendly and cost-effective. In this study, Cu-based delafossites (CuFeO2), a material class with promising thermoelectric properties at high temperatures, were investigated. This work focuses on the phase stability of CuFeO2 with respect to the temperature and the oxygen partial pressure. For this reason, classical material characterization methods, such as scanning electron microscopy, energy dispersive X-ray spectroscopy, and X-ray diffraction, were combined in order to elucidate the phase composition of delafossites at 900 °C at various oxygen partial pressures. The experimentally obtained results are supported by the theoretical calculation of the Ellingham diagram of the copper–oxygen system. In addition, hot-stage X-ray diffraction and long-term annealing tests of CuFeO2 were performed in order to obtain a holistic review of the phase stability of delafossites at high temperatures and varying oxygen partial pressure. The results support the thermoelectric measurements in previous publications and provide a process window for the use of CuFeO2 in thermoelectric generators.

Highlights

  • In past years, huge efforts have been undertaken to cope with global warming and climate change, mainly originating of ever-growing economics and societies in industrial countries

  • Whereas Stöcker et al showed an in-situ phase transition by measuring the thermopower of CuFeO2 with increasing oxygen partial pressure, this study aims on a holistic material characterization of CuFeO2 and its stability for their application in thermoelectric generators at high temperatures

  • Previous investigations indicated a change in the conduction mechanism of CuFeO2 at high oxygen concentrations, assuming a phase change of the material and reducing the thermoelectric performance

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Summary

Introduction

Huge efforts have been undertaken to cope with global warming and climate change, mainly originating of ever-growing economics and societies in industrial countries. Whereas commonly used material classes such as chalcogenides [19,20,21,22,23,24,25,26,27,28,29], skudderudites [30,31,32], and polymers [33,34,35,36,37] exhibit good thermoelectric performance at low- and mid-temperature ranges, oxides show their advantages at elevated temperatures above 700 ◦ C. Oxide materials follow the prevailing trend to substitute costly and less abundant thermoelectrics in favor of inexpensive materials. Whereas their thermoelectric performance might be inferior to prevalently used materials, oxides exhibit a remarkable relationship between thermoelectric performance and Materials 2018, 11, 1888; doi:10.3390/ma11101888 www.mdpi.com/journal/materials

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