67 research outputs found
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Kinetic model of silicate melts - Equilibrium case
Basing on the experimental evidence that in silicate melts silicon is always tetrahedrally coordinated by oxygen and that these structural units principally are able to form polyanionic structures by sharing oxygen atoms between two tetrahedra a kinetic model for the description of silicate melts is developed. As basic kinetic events in such systems the diffusive motion of individual poly anions and the interchange of structural units between different poly anions by condensation and splitting reactions are taken into account. The balance relations formulated for each polyanionic species separately lead to a system of differential equations which are coupled through the reaction terms. The calculation of polyanion distributions corresponding to a dynamical equilibrium is possible and allows principally the estimation of the degree of branching and intramolecular condensation in the melt
Oxygen exchange at gas/oxide interfaces: how the apparent activation energy of the surface exchange coefficient depends on the kinetic regime
In the dedicated literature the oxygen surface exchange coefficient KO and the equilibrium oxygen exchange rate [Fraktur R]0O are considered to be directly proportional to each other regardless of the experimental circumstances. Recent experimental observations, however, contradict the consequences of this assumption. Most surprising is the finding that the apparent activation energy of KO depends dramatically on the kinetic regime in which it has been determined, i.e. surface exchange controlled vs. mixed or diffusion controlled. This work demonstrates how the diffusion boundary condition at the gas/solid interface inevitably entails a correlation between the oxygen surface exchange coefficient KO and the oxygen self-diffusion coefficient DO in the bulk (“on top” of the correlation between KO and [Fraktur R]0O for the pure surface exchange regime). The model can thus quantitatively explain the range of apparent activation energies measured in the different regimes: in the surface exchange regime the apparent activation energy only contains the contribution of the equilibrium exchange rate, whereas in the mixed or in the diffusion controlled regime the contribution of the oxygen self-diffusivity has also to be taken into account, which may yield significantly higher apparent activation energies and simultaneously quantifies the correlation KO ∝ DO1/2 observed for a large number of oxides in the mixed or diffusion controlled regime, respectively
Nitrogen diffusion in nitrogen-doped yttria stabilised zirconia
Nitrogen self-diffusion was measured in single crystalline nitrogen-doped yttria-stabilised zirconia (YZrON) containing 10 mol% yttrium oxide. Samples containing two different nitrogen contents (5 and 6 mol% N on the anion sublattice) were investigated as a function of temperature (650–1000 K) using implanted 15N as a stable tracer. For a given temperature, the nitrogen diffusivity was nearly independent of the nitrogen content in the nitrogen-doped yttria-stabilised zirconia, which can be only partially understood using defect chemistry. The activation enthalpy of nitrogen diffusion was between 2 and 2.5 eV with a preexponential factor of the order of 100 cm2 s−1, which corresponds to a migration entropy of 5 kB. The surface exchange reaction between nitrogen and the oxonitride surface was investigated at 1073 K using 200 mbar gaseous 15N2 and was found to be slow but considerable. Decreasing the oxygen content in the gas phase can enhance the nitrogen incorporation into the oxonitrides.Instituto de Física La Plat
Lanthanide transport in stabilized zirconias : Interrelation between ionic radius and diffusion coefficient
The diffusion of all stable lanthanides in calcia stabilized zirconia (CSZ) and in yttria stabilized zirconia (YSZ) was measured. Lanthanide migration enthalpies were calculated using the Mott-littleton approach. The results suggest that the bulk diffusion coefficient depends on the ionic radius of the lanthanide tracer. It was also found that the experimental activation enthalpies are about 6eV for CSZ and between 4 and 5 eV for YSZ and almost constant for all lanthanides.Instituto de Física La Plat
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