Preparation and characterization of calcium cobaltite for thermoelectric application

Starting powders of CaCO3 with different Ca:Co ratio and three different sources of cobalt (II,III) oxide were mixed and calcined at 700–1100 °C. DTA/TG, mass changes, density, grain size distribution, phase composition were examined. It has been found that phase composition of the resulted product was dependent on calcination temperature and properties of starting cobalt oxide. Three stages of cobaltite formation were established: simultaneous decomposition of CaCO3 with Ca3Co4O9 formation (800–900 °C), decomposition of Ca3Co4O9 and formation of Ca3Co2O6 (about 950 °C) and the last stage at 1050 °C was dependent on starting cation ratio and source of cobalt (II,III) oxide. It has been found that total decomposition of the previous phases was observed after calcinations at 1050 °C in the batches with Ca:Co = 1:1 ratio if very pure Co3O4 was used for preparation. Replacement of pure Co3O4 by cobalt oxide contaminated by cobalt hydroxide lead to re-formation of Ca3Co4O9 phase at 1050 °C in 1:1 batches. Residual Co3O4 and CaO were always present in calcined specimens. It seems that prolonged heat treatment did not lead to better reaction degree.