SURFACE EXCHANGE COEFFICIENT MEASUREMENTS ON POROUS La_(0.8)Sr_(0.2)FeO_(3-δ) BY CONDUCTIVITY RELAXATION

摘要

The conductivity relaxation technique was used on porous samples to determine the oxygen surface exchange coefficient, k_(chem), of mixed conducting La_(0.8)Sr_(0.2)FeO_(3-δ)(LSF20). Conductivity relaxation experiments were carried out on samples of varying porosity and specific surface area from 600 to 800°C for small p_(O2) step changes, both decreasing and increasing, from 0.02 to 0.21 atm. The k_(chem) obtained for porous bodies of LSF20 at 800°C was found to increase from 6 × 10~(-6) to 8 × 10~(-5) cm/s over the p_(O2) range from 0.02 to 0.21 atm, with some leveling off at the higher p_(O2)'s close to 0.21 atm. Over the temperature range investigated, k_(chem) was found to increase with decreasing temperature for all samples. The increasing k_(chem) with decreasing temperature in porous samples is believed to be due to chemisorption of oxygen, which is expected to lead to increased surface coverage over some temperature range.