SHORT-TERM AND LONG-TERM CHEMICAL EXPANSION EFFECTS IN THE MIXED-CONDUCTOR La_(1-x)Sr_xCoO_(3-δ)

摘要

The thermal and chemical expansion properties of polycrystalline La_(1-x)Sr_xCoO_(3-δ)(LSC) (x = 0.2, 0.4, and 0.7) at 600°C < T < 900°C and 10~(-4) atm < P_(O2) < 1 atm were measured by using controlled-atmosphere dilatometry. The thermal expansivity, β_T, was found to be nearly constant from 25 to 900°C, indicating that increases observed in the coefficient of thermal expansion (CTE) at high temperature and constant P_(O2) is caused primarily by changes in cobalt oxidation state and/or structure. This "chemical" expansion was found to involve two components: a larger, rapidly-equilibrating component associated with short-term changes in oxygen vacancy concentration, x_v, and a smaller, slowly-equilibrating component, possibly associated with cation rearrangement following a change in average cobalt oxidation state. The oxygen vacancy chemical expansivity, β_C, based on the faster component was found to increase with x_v, in a slightly nonlinear behavior. The possible reasons for the observed nonlinearity and the probable explanation for the slower expansion component are also discussed.