Molten salts synthesis and electrical properties of Sr- and/or Mg-doped perovskite-type LaAlO3 powders

Abstract

Lanthanum-based LaBO3 oxides adopting the very stable perovskite structure are currently considered attractive materials for a growing number of applications in the field of solid-state ionics. In particular, LaAlO3-based perovskites are promising electrolyte materials for solid oxide fuel cells because they show almost pure oxygen ion conductivity at low oxygen partial pressures and high tem- peratures as well as excellent thermal and chemical stability under the standard operating conditions. This article describes a low-temperature synthesis of pure and acceptor- doped perovskite-type LaAlO3 nanopowders via a facile and environmental-friendly molten salts method. Using hydrated metal nitrates and sodium hydroxide as raw materials, the proposed methodology consists of two steps: a mechanically induced metathesis reaction and short firing above NaNO3's melting point. The purpose of the first is twofold: i.e., to generate in situ the NaNO3 flux and to obtain a suitable precursor for the synthesis of the target materials in molten nitrates. Accordingly, pure and Mg- and/or Sr-doped LaAlO3 powders were obtained directly without using any purifica- tion step at temperatures B500 C. When preparing the Mg-containing samples, NaNO2 was also added to the reaction mixture to increase melt reactivity. The formation of the target series in the molten salt is thought to proceed through a ''dissolution-precipitation'' mechanism with LaAlO3 particles precipitating during cooling from a solution oversaturated with reactants. Electrical properties of the as-prepared materials were measured as a function of temperature and frequency by means of impedance spec- troscopy and found comparable to those shown by similar materials prepared using more complicated routes.