The oxygen exchange process between the gas phase and a mixed conducting solid oxide was simulated by application of finite element modeling (FEM). The solid oxide was covered by inert surface particles of different shape and size. The effect of spherical, ellipsoidal, rectangular, and dendritic surface particles on the oxygen exchange kinetics was elaborated in detail. Relaxation curves for the total amount of exchanged oxygen upon an instantaneous change of the oxygen activity (partial pressure) of the surrounding atmosphere were calculated. One-dimensional analytical solutions of the diffusion equations were fitted to the numerically calculated relaxation curves, yielding apparent chemical diffusivities and surface exchange coefficients. The FEM simulations of the oxygen exchange process were carried out as a function of surface coverage for the various particle types. The apparent surface exchange coefficient decreased with increasing surface coverage. In case of small spherical and ellipsoidal particles with a fairly small extension of the shorter axis (large aspect ratio) the surface exchange coefficient decreased almost linearly with increasing surface coverage caused by the reduction of the active (uncovered) surface area. An additional decrease of the surface exchange coefficient (non-linear variation with surface coverage) was observed with respect to dendritic particles. This peculiar behavior could be interpreted in terms of flux constriction effects, i.e. lateral diffusion processes underneath the inert surface particles. The surface area blocked for the exchange reaction seems to be significantly larger than the cross-section of the dendritic surface particles.

中文翻译：

---

枝晶表面颗粒覆盖的混合导电氧化物陶瓷的氧交换动力学

通过应用有限元建模（FEM）模拟气相和混合导电固体氧化物之间的氧气交换过程。固体氧化物被不同形状和尺寸的惰性表面颗粒覆盖。详细阐述了球形、椭圆形、矩形和树枝状表面颗粒对氧交换动力学的影响。计算周围大气氧活度（分压）瞬时变化时交换氧总量的弛豫曲线。将扩散方程的一维解析解拟合到数值计算的松弛曲线，产生表观化学扩散率和表面交换系数。氧交换过程的有限元模拟是根据各种颗粒类型的表面覆盖度进行的。表观表面交换系数随着表面覆盖率的增加而降低。对于具有相当小的短轴延伸（大纵横比）的小球形和椭圆形颗粒，表面交换系数随着活性（未覆盖）表面积的减少而导致的表面覆盖率的增加几乎呈线性下降。对于树枝状颗粒，观察到表面交换系数进一步降低（随表面覆盖度的非线性变化）。这种奇特的行为可以用通量收缩效应来解释，即惰性表面颗粒下方的横向扩散过程。用于交换反应的表面积似乎明显大于树枝状表面颗粒的横截面。