Influence of Oxygen Partial Pressure during Processing on the Thermoelectric Properties of Aerosol-Deposited CuFeO2

Abstract

In the field of thermoelectric energy conversion, oxide materials show promising potential due to their good stability in oxidizing environments. Hence, the influence of oxygen partial pressure during synthesis on the thermoelectric properties of Cu-Delafossites at high temperatures was investigated in this study. For these purposes, CuFeO2 powders were synthetized using a conventional mixed-oxide technique. X-ray diffraction (XRD) studies were conducted to determine the crystal structures of the delafossites associated with the oxygen content during the synthesis. Out of these powders, films with a thickness of about 25 µm were prepared by the relatively new aerosol-deposition (AD) coating technique. It is based on a room temperature impact consolidation process (RTIC) to deposit dense solid films of ceramic materials on various substrates without using a high-temperature step during the coating process. On these dense CuFeO2 films deposited on alumina substrates with electrode structures, the Seebeck coefficient and the electrical conductivity were measured as a function of temperature and oxygen partial pressure. We compared the thermoelectric properties of both standard processed and aerosol deposited CuFeO2 up to 900 °C and investigated the influence of oxygen partial pressure on the electrical conductivity, on the Seebeck coefficient and on the high temperature stability of CuFeO2. These studies may not only help to improve the thermoelectric material in the high-temperature case, but may also serve as an initial basis to establish a defect chemical model.

Effect of Oxygen Partial Pressure on the Phase Stability of Copper–Iron Delafossites at Elevated Temperatures

“…This result is in good agreement with previous investigations [64,65,66,67], whereas the calcination in pure N 2 leads to an elemental copper phase, due to the reduced oxygen partial pressure. Here, we focus on the influence of an elevated oxygen partial pressure on the phase stability of delafossites.…”

“…While the conduction mechanism of the delafossites is p -type, the arising mixed phase of CuO/CuFe 2 O 4 shows a mixed conduction, since copper (I) oxide is an n -type and the cuprospinel CuFe 2 O 4 is a p -type semiconductor, resulting in bipolar thermoelectric effects and therefore reducing the overall thermoelectric performance [68,69]. Previous defect-chemical studies with variations in the oxygen partial pressure on thin delafossite films prepared by aerosol deposition also showed an abrupt change in the conductivity at characteristic p O 2 levels at high temperatures, and this change results in bipolar thermoelectric effects [64]. These findings are supported by the results in the present work.…”

“…In order to obtain a high purity starting material, a synthesis route as reported in [64] was chosen, describing the formation of CuFeO 2 with no impurities or secondary phases. As starting materials, copper(I) oxide (99.9%, Alfa-Aesar, Karlsruhe, Germany) and iron(III) oxide (99%, Alfa-Aesar, Karlsruhe, Germany) were used and processed in a wet planetary ball mill (Fritsch, Idar-Oberstein, Germany) with cyclohexane as solvent.…”

“…In order to evaluate the phase composition of CuFeO 2 as a function of p O 2 at elevated temperatures, CuFeO 2 brick shaped pellets were cold-pressed uniaxially [64] and annealed at 900 °C under different oxygen concentrations (0, 1, 5, 10, 20, and 100%) mixed in nitrogen for 12 h. While the oxygen diffusivity of CuFeO 2 is low, previous investigations have shown that an annealing time of 12 h is sufficient for the samples to reach an equilibrium [64]. A scanning electron microscope (LEO 1450 VP, Zeiss, Oberkochen, Germany) was used for energy dispersive X-ray spectroscopy (EDX) and back-scattered electron (BSE) imaging of the samples.…”

“…Former studies have focused on the thermoelectric performance and the electrical conductivity of the delafossite-type oxide CuFeO 2 , its dependence on the oxygen partial pressure at high temperatures, and on the novel aerosol deposition coating technique [61,62,63,64]. In this work, we focus on the phase stability of delafossites at elevated temperatures and under varying oxygen concentrations.…”