Variation of citrate to metal cation ratio for dense and phase pure BaZrO 3 via auto‐combustion synthesis

Abstract In this work, we demonstrate how the metal–citrate complex structure formed in the gel reflects the true stoichiometry of the redox mixtures to yield a highly dense BaZrO 3 ceramic obtained as a combustion product in the sol‐gel autocombustion technique. By systematically varying the citric acid to metal cation ratio (C/M) between 0.56, 1.0, 1.5, and 2, we show that the optimal synthesis parameters correspond to C/M ratio of 1.5, at which ratio complete chelation of citrates happen with nitrates. This was confirmed using TGA, which showed a single‐step weight loss peak corresponding to C/M =1.5 but multiple peaks in C/M ratios of 0.56 and 1. PXRD and FTIR measurements on all the burnt powders showed multiphase peaks for the C/M ratios 0.56 and 1 but only peaks corresponding to BaZrO 3 for C/M = 1.5, apart from a very tiny BaCO peak. Combustion behavior of each sol‐gel corresponding to C/M ratios 0.56, 1.0, 1.5, and 2.0 is described in detail, wherein 0.56 and 1.0 sol‐gels were found to belong to the smouldering/less intense flame‐type combustion, while 1.5 and 2 belonged to the intense flame combustion, contradicting oxidant to reductant ratio (O/R) calculations that require a stoichiometric combustion for C/M ratio of 0.56 to result in an intense flame combustion. This discrepancy was explained by the optimal usage of NO ions in the gel structure corresponding to C/M ratio of 1.5 since a complete chelation of citrates to nitrates ions happen for C/M = 1.5. The observed combustion behavior, phase purity, and other characteristics of stoichiometric/ nonstoichiometric combustion products are illustrated by means of the extent of the 3D metal–citrate complex network formed as a function of C/M. The density of the sintered pellets that were sintered at a temperature of 1650C was found to be the best at 99.6 for the C/M ratio 1.5. This is the highest ever reported density for BaZrO 3 ceramic demonstrating that C/M ratio of 1.5 represents that metal to cation ratio for which best chelation of citrates happen with nitrates resulting in the formation of dense BaZrO 3 ceramics. A comparison of BaZrO pellet made by solid‐state synthesis technique yielded a low value of density of 97.5. Impedance spectroscopy measurements done on all BaZrO pellets yielded the lowest grain‐boundary resistance for the pellet made by the sol‐gel technique with C/M ratio of 1.5.