Chemical Expansion of Mixed Ionic and Electronic Conducting Perovskites

Mixed ionic and electronic conducting oxides, employed in, e.g., catalysts and solid oxide fuel cells (SOFCs), often store and release large quantities of oxygen during operation. Upon oxygen release (i.e., during exposure to lower oxygen pressure or higher temperature), multivalent cations reduce their oxidation state, resulting in an expansion that typically outweighs the small lattice contraction around the oxygen vacancies that form [1]. This chemo-mechanical coupling between lattice expansion and change in chemical composition can be described quantitatively by the chemical coefficient of expansion (CCE). When materials with high CCEs are incorporated into operating devices, such as SOFCs, the large stresses accompanying stoichiometry changes can lead to mechanical failure and shortened device lifetimes. Therefore, CCEs of new materials need to be characterized, and factors controlling the chemical expansion need to be investigated in order to engineer materials with low CCEs.

     In this work the CCEs of two candidate SOFC cathodes, La_0.9Sr_0.1Ga_1-xNi_xO_3-δ (LSGN; 0 ≤ x ≤ 0.5) and SrTi_0.65Fe_0.35O_3-α (STF), were determined as part of an ongoing effort to understand factors governing chemical expansion in perovskites. Prior computational simulations from our group identified that the degree of charge localization on the multivalent cations can play a significant role in determining the CCE, with delocalized charges (i.e., metallic behavior) resulting in decreased changes in cation radii upon reduction [2]. In this presentation, experimental evidence for the effect of charge localization on chemical expansion in LSGN, recently measured for the first time, will be discussed. Bulk samples prepared by the Pechini method were measured from 600 – 900 °C in N₂/O₂ mixtures by thermogravimetric analysis (TGA), to determine oxygen non-stoichiometry and defect equilibria, and by dilatometry and in situ X-ray diffraction, to determine the corresponding lattice expansion.

     For LSGN, increasing charge delocalization was achieved by increasing the Ni content, as demonstrated through electrical conductivity measurements [3] and subsequent analysis of electronic mobilities using carrier concentrations derived from TGA data with defect modeling. Correspondingly, the measured CCE decreased by 13% at 800 °C upon increasing the Ni concentration from 0.1 to 0.5 [4], in agreement with the theoretical prediction of decreasing CCE with increasing charge delocalization. Additionally, recent work on measuring the CCE for STF will be presented. STF displays similar chemical expansion behavior to LSGN, with the CCE at 800 °C being comparable to the highest value measured for LSGN.

[1] D. Marrocchelli, S. R. Bishop, H. L. Tuller, and B. Yildiz, Advanced Functional Materials, 22 (9) 1958-1965 (2012).

[2] D. Marrocchelli, S. R. Bishop, H. L. Tuller, G. W. Watson, and B. Yildiz, Physical Chemistry Chemical Physics, 14, 12070-12074 (2012).

[3] N. J. Long, F.’ Lecarpentier, and H. L. Tuller, Journal of Electroceramics, 3 (4), 399-407 (1999).

[4] N. H. Perry, J. E. Thomas, D. Marrocchelli, S. R. Bishop, and H. L. Tuller, ECS Transactions, 57 (1) 1879-1884 (2013).