Synergetic thermal effects for oxygen sorption and order-disorder transition on perovskite-type oxides

Simultaneous DSC-TGA measurements were conducted on a perovskite-type oxide, La_0.1Sr_0.9Co_0.9Fe_0.1O _3-δ when it was subject to a sudden change of oxygen activity in the gas environment (from 1.3x10^-4 to 0.21 atm) at constant temperature. The DSC-TGA and XRD results show occurrence of synergetic thermal effects for exothermic oxygen sorption and endothermic oxygen vacancy order to disorder transition for the perovskite-type oxide in response to the sudden increase of oxygen activity in a large temperature range (350-800 °C). The perovskite-type ceramic transforms from the ordered (brownmillerite) to disordered (perovskite type) structure as temperature increases. The order to disorder phase transition temperature decreases with decreasing oxygen nonstoichiometry or increasing oxygen partial pressure, with the maximum transition temperature at 857.9 °C. The apparent heat measured for the perovskite-type ceramic sample during the oxygen sorption process changes from endothermic to exothermic one as temperature increases from 350 to 800 °C. The endothermic heat for phase transformation and exothermic heat for oxygen sorption for the perovskite-type oxide were estimated at 59.8 J/g and -180.5 kJ/mol, respectively, from the DSC-TGA measurements.