Oxygen diffusion in the oxygen ionic conductor yttria-stabilized zirconia is investigated by means of the molecular dynamics simulation technique. Oxygen ions migrate by means of a discrete hopping process, mainly between neighbouring tetrahedral sites. Diffusion appears to occur in a short time and a long time regime. Only when the oxygen ions have moved over distances much larger than the characteristic distances of the underlying crystal structure, a linear relation is found between the mean square displacement and time. The oxygen tracer diffusion coefficient, obtained from this long time regime, is 1.86 × 10-6 and 3.23 × 10-6 cm2/s at 1759 and 2057 K, respectively. The ionic conductivity, calculated from the tracer diffusion coefficient, agrees well with experimental values.

Permporometry as well as nitrogen adsorption-desorption techniques have been applied to study the pore size distribution in y-alumina membranes with a pore radius ranging from about 2 nm to 10 nm. The permporometry technique measures the active pores only, while nitrogen adsorption-desorption measures the active as well as the passive pores. The pore size distribution in the porous ceramic membrane supported with an cy-alumina substrate (supported membrane) was determined by using permporometry, while the membrane without a supporting substrate (unsupported membrane) was characterized by nitrogen adsorption-desorption. The experimental results of permporometry and nitrogen adsorption-desorption indicate that the size distribution of the active pores in the supported membranes is similar to that of the active and passive pores in the unsupported membranes. The cu-alumina supporting substrate seems to have no detectable influence on the size distribution of the active pores of the ceramic membranes studied.

Chemical vapor deposition (CVD) and electrochemical vapor deposition (EVD) have been applied to deposit yttriastabilized-zirconia (YSZ) on porous ceramic media. The experimental results indicate that the location of YSZ deposition can be varied from the surface of the substrates to the inside of the substrates by changing the CVD/EVD experimental conditions, i.e., the concentration ratio of the reactant vapors. The deposition width is strongly dependent on the deposition temperature used. The deposition of YSZ inside the pores resulted in pore narrowing and eventually pore closure, which was measured by using permporometry. However, deposition of YSZ on top of porous ceramic substrates (outside the pores) did not result in a reduction of the average pore size. Ultrathin, dense YSZ layers on porous ceramic substrates can be obtained by suppressing the EVD layer growth process after pore closure.

No abstract

No abstract