Measurement of surface exchange coefficients in thin films using isotope exchange depth profiling
Date
2012
Authors
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Publisher
University of Delaware
Abstract
Optimizing the performance of mixed ionic electronic conductors (MIECs) will greatly increase the performance of solid oxide fuel cells, allowing them to produce more power and operate at lower temperatures. The performance of MIECs is governed largely by two material properties: the diffusion coefficient and the surface exchange coefficient. Doping most commonly optimizes these coefficients, however the optimal doping combination is often quite time consuming to determine. Traditional measurement methods are very capable of measuring the bulk diffusion coefficient but lack precision when measuring the surface exchange coefficient. A new technique is required to enable precision measurement oft he surface exchange coefficient over a wide range of values. In this work, the technique of isotope exchange depth profiling (IEDP) of thin films is developed.IEDP of thin films is shown to be a powerful tool for the optimization of MIEC performance. IEDP is a technique where an isotope enriched atmosphere is allowed to diffuse into a material for a given exchange time. Secondary ion mass spectrometry is used to measure the concentration of isotope as a function of depth giving a diffusion profile in the material. This diffusion profile is then fitted to the diffusion model to extract the surface exchange coefficient. Using this technique on thin films requires films to be produced that are less than 10 μm thick. The diffusion profiles are then fit to the solution to diffusion in a plane wall instead of the traditional semi-infinite solution. This accounts for the finite diffusion distance that is not seen in IEDP of bulk materials. First, numerical modeling of the technique is used to predict the range of experimental surface exchange coefficient values that can be measured using practical experimentation. The model uses the solution of the diffusion equation in a plane wall using the surface evaporation boundary condition. The modeling was done in Mathematica version 8. This model assumes 1-dimensional diffusion with a zero flux boundary condition at the substrate. It is shown that IEDP of thin films is able to accurately measure surface exchange coefficients from 5 10-16 cm/sec to 5.5 10-5 cm/sec. Next, two model MIEC materials, LaCoO3 and SrCoO3, were measured using this technique at 400 ̊C and 500 ̊C. Sputtering targets were made of these materials using traditional ceramic processing techniques. These films were deposited using magnetron sputtering to a thickness near 100 nm. These films were exchanged in a highly enriched atmosphere for times between one minute and one hour. Secondary ion mass spectrometry provided a diffusion profile in the material and this profile was fitted to the plane diffusion model. Surface exchange coefficients of LaCoO3 were measured to be 2.274 10-8 cm/sec and 1.061 10-7 cm/sec at 400 ̊C and 500 ̊C respectively. Surface exchange coefficients of SrCoO3were measured to be 9.891 10-10 cm/sec and 2.117 10-9 cm/sec at 400 ̊C and 500 ̊C respectively.