Effect of substrate on the microstructure and thermoelectric performances of Sr-doped Ca3Co4O9 thick films

摘要

Ca3Co4O9 thermoelectric materials in form of thick films are very promising in practical applications due to their low costs and relatively high performance. In this work, two different suspensions have been used to produce different coatings on Al2O3 polycrystalline substrates with theoretical green thickness of 360 and 2000 mu m. Moreover, the effect of substrate has also been investigated using Al2O3 monocrystalline substrates and a 360 mu m green thickness. Sintering procedure at 900 degrees C for 24 h has drastically decreased coating thickness. XRD performed on the coatings surface has shown the formation of small amounts of Ca3Co2O6 secondary phase on the polycrystalline substrates, while it was more abundant, and accompanied by Ca2Co2O5 on the monocrystalline substrates. In spite of the higher secondary phases content, monocrystalline substrates produced a slight grain orientation which led to the highest thermoelectric properties between the samples (0.38 mW/K(2)m at 800 degrees C), and very close to the best reported values in the literature.