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A4: Theory - bulk materials
In the field of thermoelectric energy conversion, oxide materials show a promising potential due to their good stability in oxidizing environments. Delafossite materials exhibit a good thermoelectric performance as p-type as well as n-type oxide semiconductors at elevated temperatures.
In this study, the influence of the oxygen partial pressure during the synthesis and on the thermoelectric properties of Cu-Delafossites at high temperatures were investigated. For these purposes, CuFeO2 was synthetized by a conventional mixed-oxide technique. X-ray diffraction studies were conducted to determine the crystal structures of the Delafossites associated to the oxygen content during the synthesis. By using a modulation heater to impress an oscillating temperature gradient over the samples, the Seebeck coefficient and electrical conductivity were measured between 973 K and 1173 K under defined oxygen partial pressures.
Measurements on pressed CuFeO2 pellets have shown that Cu-Delafossites exhibit a very low oxygen diffusion coefficient. Thus, the new Aerosol-Deposition (AD) coating technique was employed. This technology bases upon 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. By employing this AD method, it was possible to measure the oxygen dependency of the Seebeck coefficient, the electrical conductivity and to calculate the diffusion coefficient of oxygen vacancies in CuFeO2.
Our results indicate that CuFeO2 runs through a phase-transition at low oxygen partial pressures, resulting in a change of the conductivity mechanism and therefore the appearance of bipolar effects. In addition to that, we elucidated an increase in the electrical conductivity when calcinating the samples under a defined oxygen partial pressure. This study helps to gain knowledge of the kinetics of oxygen diffusion and defect mechanism of CuFeO2. Both are essential for the understanding of Cu-Delafossites as material for high temperature applications.
