Mixed conductivity, stability and thermomechanical properties of Ni-doped La(Ga,Mg)O3-d, A.A. Yaremchenko, V.V. Kharton, E.N. Naumovich, D.I. Shestakov, V.F. Chukharev, A.V. Kovalevsky, A.L. Shaula, M.V. Patrakeev, J.R. Frade, F.M.B. Marques, Solid State Ionics 177 (2006) 549-558
LaGaO3-based solid electrolytes and mixed ionic–electronic conductors attract significant attention during the last decade due to their potential application as materials of intermediate-temperature solid oxide fuel cells (IT-SOFCS), electrochemical oxygen sensors, and membrane reactors for conversion of natural gas to synthesis gas. In particular, a very high level of ionic transport is observed for La1-xSrxGa1-yMgyO3-d (LSGM) solid solutions with x=0.10-0.20 and y=0.15-0.20. The aim of this paper is to estimate oxygen stoichiometry from the weight changes by thermogravimetric analysis.
Estimation of oxygen stoichiometry of LaGa0.65Mg0.15Ni0.20O3-d ceramic was carried out in a Setsys TGA.
The following program is used:
- Heating at 3K.min-1 in flowing air with equilibration steps at 1073, 1123 and 1173K for 2h at each temperature;
- Flushing of the apparatus with argon for 1h;
Reduction at 1173K in flowing 10%H2-90%N2 mixture (cf. figure);
The values of oxygen nonstoichiometric in air at 1073-1173K, calculated from the TG data, are listed in the table below:
The average oxidation state of nickel cations is +2.95 at 1173 K, and increases up to +3.13 on cooling down to 1073K. This indicates co-existence of Ni2+, Ni3+ and Ni4+ states in the lattice of LaGa0.65Mg0.15Ni0.20O3-d under oxidizing conditions. The formation of tetravalent nickel should still be understood as a hypothesis and requires additional experimental confirmation, particularly to verify exact location of the electron holes formed due to oxygen incorporation.