Papers by Author: Manfred Martin

Abstract: In oxides which are exposed to thermodynamic potential gradients, transport processes of the mobile components occur. These transport processes and the coupling between different processes are not only of fundamental interest, but are also the origin of degradation processes, such as kinetic demixing, kinetic decomposition, and changes in the morphology of the material. The kinetics of high temperature oxidation processes of metals can be studied in situ by X-ray absorption spectroscopy (XAS), optical microscopy and X-ray diffraction (XRD) at elevated temperatures and defined oxygen partial pressures. As an example, the in situ XAS investigation of the oxidation of cobalt, forming layers of CoO and Co₃O₄, will be discussed.

Abstract: Through the use of a high-pressure modification of the spark plasma sintering method, it was possible to consolidate functional oxides (yttria- stabilized zirconia and doped ceria) to high densities and retain a grain size of < 20 nm. The role of the pressure on densification and on the grain size of the sintered samples was demonstrated. The pressure had a marked effect on density at relatively low temperature but an insignificant effect at relatively high temperature. It was found that when prepared with such small grain sizes, these oxides conduct protonically even at temperatures as low as room temperature. The dependence of the protonic conductivity is stronger dependence on grain size than what can be anticipated from a geometric consideration based on an increase in grain boundary area. This observation strongly suggests that factors other than an increase in grain boundary area play a role, a consideration that is being further investigated.

Abstract: In this paper, we discuss different possible cation diffusion mechanisms in YSZ and LSGM. Monte Carlo simulations are also reported of tracer diffusivities in LSGM for a postulated cluster mechanism. These simulations extend earlier simulations on un-doped material. The limits of the ratio of the diffusivities are consistent with experimental tracer diffusion findings over a wide range of cation-vacancy exchange frequencies. We also develop relationships between the phenomenological coefficients and use these relationships to predict possible demixing and interdiffusion experimental outcomes.

Abstract: It was recently shown by Schulz and co-workers that in doped lanthanum gallate: La(1-x) SrxGa(1-y)MgyO(3-(x+y)/2) (LSGM) the three cations: La, Sr and Mg have tracer diffusivities that are nearly identical. To explain these findings, a bound defect cluster mechanism containing a cation vacancy from both the A- and the B- sublattices and an anion vacancy was proposed as the principal vehicle for cation diffusion in LSGM. In this paper, implications of this mechanism are considered for the first time. Sum-rule expressions for the collective correlation factors are derived and found to be in excellent agreement with Monte Carlo calculations. Expressions are also developed for the tracer correlation factors of lanthanum and gallium for diffusion via the cluster mechanism and tested by Monte Carlo computer simulation. Good agreement was found. The ratio of the tracer diffusivities of lanthanum and gallium are shown to be consistent with the cluster mechanism.

Abstract: In this paper, the phenomenon of steady-state cation demixing in yttria-stabilized zirconia in an electric field is examined for the case of an open system wherein the defects are everywhere close to internal surfaces which allow the Schottky-defect reaction to occur (very fine grain material). The effect of the constant electric field assumption as a substitute for the constant current condition is analyzed for diffusion by vacancy-pairs and by vacancy-triplets over a wide range of applied electric fields. It is shown that the assumption of a constant electric field provides only a moderate correction to the shape of the demixing concentration profile.

Abstract: In this paper, the nature of steady state demixing of yttria-stabilized zirconia in an electric field is examined for the case of open system conditions (very fine grain material) wherein the defects are everywhere close to internal surfaces which allow the Schottky defect reaction to occur. It is shown that under such conditions the applied force needs to be some 20 -25 times larger than for the case of closed system conditions (Schottky defect reaction occurs only at the external surfaces) in order to achieve the same degree of demixing of the cation components. The effect of the constant electric field assumption as a substitute for the constant current condition is also analyzed for a wide range of applied electric fields. It is shown that in most cases, the assumption of a constant electric field provides only a moderate correction to the shape of the concentration profile.