and D 0 !- GHE < D 0 !- HIJ , therefore D 9;; 2E@0/GHE < D 9;; F ,, /HIJ , also Na " O flux (j 2, ! 0 ) is a function of diffusivity of mobile species, then j 2, ! 0 2E@0/GHE < j 2, ! 0 F ,, /HIJ resulted in K 9;; 2E@0/GHE < K 9;; F ,, /HIJ and lower slop of the fit. In other words, Na -β′′ -alumina is a two-dimensional sodium-ion conductor compared to NZGO which is a one-dimensional Na ! conductor, therefore, there will be less maneuverability for Na ! ions within the structure. Besides, 20SDC has higher oxygen ionic conductivity compared to that of YSZ, which results in a lower conversion kinetic rate for NZGO + YSZ system. The conversion mechanism of Ga " O ' + YSZ to NZGO + YSZ can be explained in three steps as illustrated in figure (a)-3(c). In step 1, a thin layer of NZGO formed on the pellet's surface (x ( > 0 at t = 0) (which initiated below reaching the conversion temperature 1250 °C) where