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This proposal aims to gain an understanding of surface and interfacial phenomena in thin films and superlattices of perovskite structured materials for improved performance of intermediate temperature solid oxide fuel cells (IT-SOFC's). Perovskite structured materials are chosen for the possibility of tuning their properties through chemical and structural parameters and the ability to meet stringent requirements associated with the operation of SOFC's. La_1-xSr_xMO₃ is the material of choice for high temperature cathode materials, however, the lack of oxygen vacancies leads to nearly exclusive electronic conduction and unacceptable performance at intermediate temperatures. In response, La_1-xSr_xMO₃ with transition metal cations (M=Fe, Co, and Ni) are investigated for mixed ionic and electronic conductivity, catalytic activity, and bulk diffusion.

The development of micro- and IT-SOFC's necessitates the use of thin film electrodes and electrolytes to minimize overall cell resistivity. These thin films experience strains which arise during processing and during operation due to factors such as thermal expansion coefficient differences. To understand real time performance of these SOFC's, one must understand the strain effect on parameters such as conductivity, mechanical properties, and materials compatibility. We utilize epitaxial thin films grown on various single crystal oxide substrates as a model system to systematically vary the amount and type of strain imposed on thin film cathode materials, La_0.67Sr_0.33MnO₃ (LSMO) and La_0.7Sr_0.3FeO₃ (LSFO). 

Table SEQ Table \* ROMAN I: Structural parameters of LSMO films

Table SEQ Table \* ROMAN II: Structural parameters of LSFO films.

In bulk, LSMO has a rhombohedral perovskite structure which can be approximated as pseudocubic (a=3.873Å), while LSFO is orthorhombic (a=5.501Å, b=5.544Å, and c=7.846Å). Thin films were grown by pulsed laser deposition on single crystal oxide substrates including (001)-oriented (LaAlO₃)_0.3(Sr₂AlTaO₆)_0.7 (LSAT, cubic, a=3.868Å), SrTiO₃ (STO, cubic, a=3.905Å), LaAlO₃ (LAO, pseudocubic, a=3.788Å), and (110)-oriented GdScO₃ (GSO, orthorhombic, a=5.488Å, b=5.746Å, c=7.934Å). The substrate temperature was 700ºC and chamber pressure was 500mTorr O₂. The samples were cooled in 300Torr O₂ to ensure the proper oxygen stoichiometry. The lattice mismatch between LSMO/LSFO and these substrates ranged from -2.16%/3.09% (LAO) to 2.51%/1.71%. For small lattice mismatch, fully strained films assume the in-plane lattice parameter, a, of the substrate, while the out-of-plane lattice parameter, c, freely contracts (tensile strain) or expands (compressive strain), depending on whether the substrate lattice parameter is larger or smaller than the film, respectively (Fig.1). In contrast, fully relaxed films retain their bulk lattice parameters as the film cannot support the strains associated with large lattice mismatch. Partially relaxed films lie somewhere in between, while a mixed state results when the film close to the substrate is fully strained and the remainder is partially or fully relaxed. The structural properties were determined by x-ray diffraction (XRD) and are summarized in Tables I and II. The amount of strain, characterized by the tetragonal distortion, c/a ratio, varies over a large range by the substrate choice.

Figure 2 plots T vs. 1000/T for LSMO and LSFO thin films grown on various substrates for temperatures from 20-500ºC in air. For small polaron hopping, the conductivity, , can be expressed as: where A is a pre-exponential factor and E_A is the activation energy. For LSMO films, T curves fall on a straight line for temperatures >150ºC suggesting that conduction occurs by small polaron hopping. At lower temperatures, the film transitions to a metallic ferromagnetic phase at the Curie temperature. These measurements are repeatable for multiple temperature cycles. With increasing strain, T decreases monotonically while E_A increases. These values differ from reported literature values for bulk samples such that on GSO, E_A is ~3X larger and sT is ~10X lower at 150ºC. This trend is attributed to the distortion of MnO₆ octahedra in the perovskite structure in terms of bond angles and bond distances.

For LSFO films, the same trend of decreasing T with increasing strain is observed, however, a single E_A cannot describe the entire temperature range studied as the slope decreases at high temperatures. In addition, we observe a sharp drop in T around 200ºC for many samples. This drop in >T coincides with a jump in film out-of-plane lattice parameter which is absent for the substrate, indicating a change in tetragonal distortion. Heating and cooling cycles display differing behavior: upon cooling the sharp drop in T disappears and the T value is higher than during heating cycle. The difference in T between the two cycles increases as the strain increases.  XRD scans taken after heating/cooling cycles indicate that the out-of-plane lattice parameter and surface roughness have increased. For LSFO on GSO, this increase in out-of-plane lattice parameter could correspond to strain relaxation, however, one would expect a decrease in out-of-plane lattice parameter upon relaxation for LSFO on LSAT, and no relaxation for relaxed and lattice matched films on LAO and STO, respectively. Instead, this change could result from an alteration of the concentration of oxygen vacancies. Further analysis in varying ambient (N₂, pure O₂) and differential scanning calorimetry and/or thermogravimetric analysis may clarify the origin of this effect.

These results highlight the importance of understanding the effect of lattice strain on the conductivity of thin film cathode materials. With increasing strain, T decreases and E_A increases. For LSMO, the measured T is reproducible upon repeated heating/cooling cycles, however, for LSFO films additional factors such as strain relaxation and oxygen stoichiometry lead to irreproducible results between cycles and highlight the need to fully characterize the films in order to understand their properties.