Perovskites and perovskite related materials are materials that are candidates for applications such as oxygen permeable membranes, cathodes for SOFC and high-temperature oxygen sensors. This arises from the potential high ionic conductivity and the chemical stability even at low partial pressures of oxygen. From an application point of view, it is important to have knowledge about the oxygen transport properties in these materials. Oxygen transport in mixed conducting oxides involves two inherently different processes; oxygen exchange between bulk gas and surface and solid state diffusion. The objective of this work has been to obtain fundamental understanding of these transport properties in mixed ionic and electronic conductors. For that purpose two materials systems with significant differences in electronic conductivty and oxygen vacancy concentration were chosen as objectives for the investigation, viz.: Sr-substituted LaCoO3 and Al-substituted SrTiO3.
All transport properties (diffusion and surface exchange) have been assessed by electrical conductivity relaxation, and the work also evaluate the pros and cons using this specific method to obtain transport data for the materials in question.
In the first two papers (Paper 1 and Paper 2) transport properties are derived for La1-xSrxCoO3-δ (x=0 (LC), 0.2 (LSC-02) and 0.5 (LSC-05)). In Paper 1 “chemical transport coefficients”, Dchem and kchem, are reported. More fundamental transport coefficients, such as oxygen component diffusion coefficient (DO) and vacancy diffusion coefficients (DV), are also deduced and discussed. Activation energies for DO and DV, were determined. The activation energies for DO varies from 279 kJ/mol for LC to 174-222 kJ/mol for LSC-02 and 90-105 kJ/mol for LSC-05, decreasing with increasing Srcontent. The activation energies for the vacancy diffusion coefficient, DV, are smaller than for the component diffusion coefficients and typical values are 77 kJ/mol for LC, 85 kJ/mol for LSC-02 and 75 kJ/mol for LSC-05, that is, almost independent of Sr-content. The enthalpies of vacancy formation decreases with increasing Sr content. The values are 206 kJ/mol for LC, 75 kJ/mol for LSC-02 and 15 kJ/mol for LSC-05, which agrees well with values reported in the literature. However, the vacancy diffusion coefficient showed an unexpected increase at high concentrations of oxygen vacancies, corresponding to δ=0.27-0.30. The phenomena with a PO2 dependent DV is discussed.
In Paper 2, the oxygen surface exchange coefficient, k0, is derived from “chemical values” reported in Paper 1, and used as a basis to deduce probable reaction mechanisms associated with surface exchange. The temperature dependency plots showed that for the composition with x = 0.5, the k0 made a shift in activation energy from ~120 kJ/mol to ~15 kJ/mol above 950 °C. It is suggested that this significant shift in activation energy might be due to an oxygen adsorption/desorption mechanism on the surface becoming rate controlling at high temperatures. The composition with x=0.2 did not show this shift in activation energy. Relations between possible rate controlling reactions and reaction rates (k0) were established, and formed the basis for discussions on probable rate controlling processes. There are reasons to assume that for oxidation prosesses a rate controlling reaction involving a direct “installation” of an oxygen molecule into two vacancies is dominating, while a dissociation of an oxygen molecule generally gives a better description for a reduction process.
In Paper 3 the oxygen transport properties in SrTi1-xAlxO3 (x=0 (ST), 0.02 (STA-02) and 0.05 (STA-05)) were determined in O2/N2 mixtures. In this contribution the electrical conductivity is also presented in a large PO2- interval (O2/N2- and CO/CO2-mixtures). Electrical conductivity for pure SrTiO3 (ST) in terms of PO2 applied well with defect chemistry reported in the literature. For the two Al-substituted compositions the electrical conductivity followed predicted behaviour at high and low PO2’s. However, in the medium PO2 range we were not able to describe the conductivity behaviour in terms of classical defect chemistry. Reasons for the discrepancy is discussed.
Dchem for ST and STA-02 are reported and are, along with their corresponding activation energies, 187 and 104-180 kJ/mol, respectively, in good accordance with values from literature. Furthermore, values for the component diffusion coefficient, DO, and the vacancy diffusion coefficient, DV, are reported for ST at 950 °C, the only composition where oxygen vacancy concentrations are available in the literature. Values for kchem in STA-02 and STA-05 are also reported, and show pronounced PO2 dependencies. For STA-05 the activation energy for kchem is found to vary between 90 and 105 kJ/mol. Due to a high uncertainty, activation energies are not reported for STA-02.
Reported Dchem and kchem values for related materials in literature indicate increasing numeric values with decreasing concentration of oxygen vacancies. It is reasoned that this is due to an ever increasing thermodynamic factor with decreasing population of vacancies. The implications for the component diffusion coefficient is discussed.
In Paper 4 the oxygen transport properties in SrTiO3 pure and with Al were investigated in mixtures of CO/CO2. Dchem are reported for ST and STA-02 while kchem are reported for ST, STA-02 and STA-05. The Dchem showed a PO2-dependency, which can be explained by the variation in the thermodynamic factor. The introduction of Al in the sample increases the value of Dchem, probably due to the introduction of more oxygen vacancies. STA-02 showed a discrete increase in Dchem in the CO-rich atmospheres, this may be due to phase transition or phase separation at PO2 ~10-17 atm. The kchem showed a maximum at PCO/PCO2 = 1 for STA-02 and STA-05. This behaviour corresponds well with a rate controlling reaction involving a charged and adsorbed CO2 molecule. The same maximum is also reported in the literature for BaTiO3 wihtout and with 1.8 % Al and for La0.9Sr0.1FeO3.
This work has examined chemical diffusion and surface exchange coefficients with electrical conductivity relaxation in two material systems with distinct differences in electrical conductivity and oxygen vacancy concentrations. The main focus has been to elucidate properties of the transport coefficients based on own measurements, but also include transport coefficients from other material systems from literature as references. The vacancy diffusion coefficients have been examined, showing that they increase with increasing concentration of oxygen vacancies in materials where the concentration of vacancies is high. No obvious reason for this behaviour has been found, however, it may be related to a change in activation energy. It is rather well established in the literature that for materials where the concentration of vacancies may be characterized as dilute, we should expect a DV independent of the population of vacancies. Finally, based on own results and data reported in the literature it appears that with respect to the oxygen surface flux the oxygen vacancy concentration seems to be the property of most importance. That is, for oxidation processes the oxygen exchange flux will increase with vacancy population.