  The recent emergence of multi-species multi-phase materials provides
intriguing opportunities to maximize electrochemical performance in various
electrochemical devices. This work summarizes the current understanding of the
coupled transport reactions in multi-phase multi-species ionic conductors. We
also provide experimental results of the fabrication of multi-phases
Na-beta"-alumina+20mol% Scandia Doped Ceria(20SDC) as simultaneous sodium and
oxygen ion conductor by a cost-effective vapor phase process demonstrating
higher conductivity achieved in a much shorter time than other published
results. In this study, two-phase contiguous composites of Al2O3+20SDC are
fabricated by conventional ceramic processing and sintering in the air at 1400C
1500C, and 1600C, for 3 hours. The samples are heat-treated while exposed to a
sodium oxide vapor source at different time lengths. The conversion mechanism
involves coupled transport of sodium ions through newly formed Na-beta"-alumina
and oxygen ions through 20SDC. The experimental data are analyzed using
diffraction and spectroscopy methods. The samples with finer grains show faster
kinetics compared to coarse microstructures due to the presence of more
extended triple-phase boundaries (TPB). As a result, the total conductivity of
the multi-phase sample compared to that of pure 20SDC is improved by three
times, while fabrication time is decreased by 60% compared to
Na-beta"-alumina+YSZ.
