Variation of citrate to metal cation ratio for dense and phase pure BaZrO3 via autocombustion synthesis

Abstract

AbstractIn 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 BaZrO3 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 BaZrO3 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 BaZrO3 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 BaZrO3 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.