Variation of mechanical and electrical performances of Bi2Ca2Co1.7Ox ceramics above working conditions

This work explores the effect of exposing the Bi2Ca2Co1.7Ox-textured ceramics at temperatures above working conditions, on mechanical and electrical properties. Microstructural studies have shown a first improvement of microstructure with an important grain growth, followed by the formation of porosity and the appearance of cracks for larger times. These features were reflected on their mechanical and electrical properties. Three point bending tests have revealed an increase of bending strength with the thermal treatment, reaching the maximum at 24 h and decreasing for further treatment time, which is very slight for times larger than 196 h. On the other hand, electrical resistivity is drastically reduced with the thermal treatment when compared to the as-grown samples. Moreover, the samples behavior is modified from semiconducting-like for as-grown samples to metallic-like for the thermally treated ones. Accordingly, Seebeck coefficient is decreased with the thermal treatment, being very similar for all thermally treated samples. As a consequence of the drastic decrease of electrical resistivity, although with lower reduction on the Seebeck coefficient, all thermally treated samples display higher power factor values than the as-grown ones. The highest values at 650 °C (0.29 mW/K2 m) have been obtained in textured samples thermally treated for 48 h, which are comparable to the best values reported in the literature without the use of expensive materials.