Ceramics-Silikáty 56 (2) 139-144 (2012)

THERMODYNAMIC BEHAVIOR OF Ca₃Co_3.93+xO_9+δ CERAMICS

O. Jankovský, D. Sedmidubsky, Z. Sofer, P. Simek, J. Hejtmanek

Based on the results of differential thermal analysis, differential scanning calorimetry, thermogavimetric analysis, low temperature calorimetry and X-Ray powder diffraction of equilibrated samples as well as on the assessed thermodynamic data phase a section of the Ca–Co–O diagram for pO₂ = 0.21 was calculated using the FactSage software. Among the involved phases the misfit cobaltite Ca₃Co_3.93+xO_9+δ is well known for its distinctive thermoelectric properties. As shown by our experiments and reproduced by the proposed thermodynamic model the Ca₃Co_3.93+xO_9+δ phase actually exhibits a narrow homogeneity range with respect to cation composition. The primary focus was put on the heat capacity of Ca₃Co_3.93O_9+δ collected in a broad temperature range and analyzed in terms of a combined Debye - Einstein model. This made it possible to separate two peaks detected at ~ 400 K and ~ 840 K and to evaluate the corresponding entropy and enthalpy changes. The second effect was interpreted as a spin transition from intermediate state to high state.

Keywords: Misfit cobaltites, Thermoelectric materials, Heat capacity, Thermodynamic properties, Debye-Einstein model, Phase equlibria