Porous Thick Film Low Temperature Lanthanum Strontium Ferrite Oxygen Surface Exchange Coefficient Measurements By Curvature Relaxation

Introduction

Mixed ionic electronic conducting (MIEC) materials, such as Lanthanum Strontium Ferrite (LSF), are widely used in solid oxide fuel cell cathodes, catalytic converters, and other applications. Changes in temperature or oxygen partial pressure (pO₂) give rise to simultaneous changes in oxygen nonstoichiometry, cation oxidation state, and lattice strain in these materials.(1) This interplay between the mechanical and chemical behaviors of these materials allows the oxygen chemical surface exchange coefficient (k) of a mechano-chemically active film bonded to an inert substrate to be measured via the curvature relaxation (κR) technique.(2) Upon instantaneous small step changes in pO₂, the k of a porous film can be determined using Equation [1], where κ, κ₀, and κ_∞ are the instantaneous, initial, and new-pO₂-equilibrated curvature, respectively, k_i is the surface exchange coefficient for a portion of the film with faction of surface area, A_i, t is time, V_V is the volume fraction porosity, and S_V is the volume specific surface area.(3-6) This κR technique provides an in-situ, electrode-free method to measure porous film k’s, and also allows the film stress to be evaluated from the bilayer curvature via Stoney’s Equation.

Experimental Methods

5-µm-thick porous La_0.6Sr_0.4FeO_3-δ (LSF64) films were spray deposited onto single crystal (Y₂O₃)_0.13(ZrO₂)_0.87 (YSZ) substrates. The curvature relaxation of the bilayers was measured at different pO₂’s and temperatures using an in-situ Multi-beam Optical Stress Sensor (MOSS) apparatus, as described in Yang et al.(2)

 

Results and Discussion

Figure 1 shows the κR of LSF64|YSZ bilayers subjected to multiple pO₂ cycles at two different temperatures. Reproducible relaxation kinetics was observed, and equilibrium curvature levels were consistent with the thermal expansion mismatch at these temperatures.

As shown in Figure 2, a normalized curvature relaxation can be fitted to a single surface exchange coefficient. As shown in Figure 3, the k’s of LSF64 were measured down to 250^oC for the first time in this study and agreed well with the low temperature extrapolation of the bulk sample k’s measured by ten Elshof et al.(7) A similar activation energy (E_A=1.3 eV) is also observed. The stress in the film is smaller than that of previously measured sputtered thin films. The k and E_A’s are different than those measured for pulsed laser deposited thin films, possibly caused by difference stress levels in the film, the presence/absence of electrodes, or other effects. The errors of both kand stress measurements are smaller than the symbols.

 

Conclusions

Surface exchange coefficients and stresses of porous thick film LSF64 were measured for the first time using a curvature relaxation technique. The film had relatively low stress and similar k and E_A’s compared to low temperature extrapolations of bulk LSF64 measurements in the literature.

Acknowledgements

This work was funded by a Michigan State University faculty startup grant.

References

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