AN546 – Oxygen stoichiometry of LaGa0.65Mg0.15Ni0.20O(3-d)

REFERENCE

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

INTRODUCTION

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.

AN-546

Example of reduction kinetics of LaGa0.65Mg0.15Ni0.20O3-d in flowing 10%H2–90%N2 mixture at 1173 K. The sample was kept for 2 h in air, 1 h in argon, and then 144 h in the H2-containing mixture. Dashed lines correspond to the theoretical weight changes upon reduction into metallic nickel and binary oxides, calculated assuming that there is no gallium oxide volatilization and that all nickel cations in air are in 2+, 3+ or 4+ oxidation states.

EXPERIMENTAL

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);

RESULTS

The values of oxygen nonstoichiometric in air at 1073-1173K, calculated from the TG data, are listed in the table below:

AN-546a

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.