Kinetics of the α→γ phase transformation in LiAlO2 under various atmospheres within the 1073-1173 K temperatures range

Stability of the LiAlO2 crystallographic forms within given temperature limits, especially the occurrence of alpha --> gamma-LiAlO2 phase transformation, is one of the crucial problems of application of this compound as solid phase matrix for electrolyte in molten carbonate fuel cells (MCFC). In this study, kinetics of this transformation has been investigated under atmospheres of CO2 and N-2, both dehydrated and water vapor containing, at constant temperatures 1073, 1123 and 1173 K, in the alpha-LiAlO2 powder prepared in a molten salts ambience. The increasing gamma-LiAlO2 phase concentration in the alpha-LiAlO2 sample was determined by X-ray diffractometry as dependent on time of heating the sample. Similar measurements have been performed with alpha-LiAlO2 samples after 5 h of heating at temperatures within 923 - 1223 K. The results of this investigation suggest the existence of some chemical steps of the overall phase transformation process, probably the decomposition of the double lithium-aluminum oxide at the alpha-LiAlO2 particle surfaces via a surface-nucleation mechanism. This step determines the overall transformation rate, strongly depending on gas atmosphere and on molten carbonate ambience (solution-deposition mechanism). This transformation does not take place below 973 K, which is the upper temperature limit of the MCFC operation. (C) 2004 Kluwer Academic Publishers.