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

Abstract

Ca 3 Co 4 O 9 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 Al 2 O 3 polycrystalline substrates with theoretical green thickness of 360 and 2000 µm. Moreover, the effect of substrate has also been investigated using Al 2 O 3 monocrystalline substrates and a 360 µm green thickness. Sintering procedure at 900 ºC for 24 h has drastically decreased coating thickness. XRD performed on the coatings surface has shown the formation of small amounts of Ca 3 Co 2 O 6 secondary phase on the polycrystalline substrates, while it was more abundant, and accompanied by Ca 2 Co 2 O 5 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 ºC), and very close to the best reported values in the literature.

Enhancement of electrical conductivity of Ca2.93Sr0.07Co4O9 thick films via hot uniaxial pressing

“…S is positive in the whole measured temperature range, pointing out to holes as major charge carriers. Moreover, it is not observed any effect of the processing route on the S values, which increase with temperature, in agreement with previous works . The highest S value has been reached at 800°C in the sintered coatings (204 μV/K), slightly higher than the measured in textured ones (198 μV/K).…”

“…Ca 2.93 Sr 0.07 Co 4 O 9 nominal composition has been used due to previous results, indicating the enhancement of the Seebeck coefficient without drastic modifications of electrical resistivity . The initial suspension, with 29 vol.% of solid, was prepared from CaCO 3 (≥99%, Aldrich), SrCO 3 (≥98%, Aldrich), and CoO (99.99%, Aldrich) commercial powders following previous procedure . Polycrystalline Al 2 O 3 substrates were used for deposition, as they provide the highest amount of TE phase after sintering.…”

“…These substrates were then coated with the suspension by the dip‐coating technique as previously reported, and totally dried under air at 50°C for 24 hours. All samples were then thermally treated under air at 450°C for 2 hours to eliminate organics, and sintered at 900°C for 24 hours, followed by slow cooling to room temperature (sintered films).…”

The efficiency of short sintering time on thermoelectric properties of delafossite CuCr0:85Mg0:15O2 ceramics

“…However, it is noted that Na x CoO 2 will be decomposed into insulating Co(OH) 2 as being exposed in a high humidity environment 2 . In the case of the Ca 3 Co 4 O 9 compound, the anisotropic electric properties are caused by its crystal structure and the less densification because of the large difference of temperature between the eutectic point and the stable range of Ca 3 Co 4 O 9 phase are the two main disadvantages of this material 5 . Delafossite, known as an inherited p-type material, has the layered-type structure belonged to cobaltite oxide family like Ca 3 Co 4 O 9 and Na x CoO 2 .…”