Electrochemical Impedance Spectroscopy (EIS) (Solartron-Impedance/Gain-Phaser Analyzer SI 1260 & Electrochemical interface SI 1287) was performed on samples within a frequency range of 1 MHz to 1 Hz, with 10 mV amplitude from ~200 °C to ~800 °C. Energy Dispersive Spectroscopy (EDS) was utilized to analyze possible elemental changes adjacent to the electrodes after the test. Porous platinum electrodes (Heraeus) were screen printed symmetrically on both sides of the fully converted pellets (t~10 µm) and fired at 950 °C for 45 minutes. The EIS analysis was carried out using the circuit model shown in figure . The inductive resistance and contribution of lead wires and testing setup were obtained by shorting the leads in the empty cell setup (without the sample) and subtracting this measurement from EIS data points. The obtained data were fitted to semi-circles. The intercept of the extrapolation of the x-axis at the high-frequency regime was considered as the total sample resistance. The microstructure of the sintered samples showed an increase in grain size as expected. However, newly formed fine grains were observed in the sample sintered at 1600 °C (dashed circles in figure ). These fine grains are formed due to the high sintering temperature with respect to the formation of the liquid phase in Ga " O ' -Y " O ' and Ga " O ' -ZrO " systems. Therefore, an uneven distribution of grains was observed, which will be discussed in detail in figure . to Na -β′′ -alumina using 20SDC or 10Sc1CeZr as oxygen-ion conductors which occurred to be sharper and highly porous. however, the relative density of the sample sintered at 1600 °C unexpectedly decreased to 92% of relative density. Most sintered ceramics show a sigmoidal densification behavior where a plateau occurs after achieving high densities . The expected sigmoidal fit to obtain density data (1200 °C to 1400 °C) without considering unexpected behaviors is shown in figure , in case the typical sintering behavior would be observed. However, some oxide systems deviate from this common trend, known as inadequate densification due to full or partial surface melting in the high energy regions such as grain boundaries at temperatures close to the melting point of one of the components or possible phase transformation reactions such as eutectic reactions. In the binary systems of Ga " O ' -Y " O ' , and Ga " O ' -ZrO " , a heterogeneous microstructure consisting of both large and fine grains and also a reduction in density was unexpectedly observed for samples sintered at T > 1500 °C. The β -Ga " O ' has a much lower melting point than the other oxides in this study (T ? @, ! 0 ) = 1725 °C) caused an undesired liquid phase during the solid-state sintering along with the presence of a eutectic reaction. The same densification behavior has also been studied in other systems such as Ga " O ' -TiO " , Ga " O ' -La " O ' , Ga " O ' -Sm " O ' , and Ga " O ' -Gd " O ' . Figure schematically illustrates the inadequate densification in Ga " O ' -Y " O ' -ZrO " system. The dissolution of air in newly melted regions at T > 1500 °C, additionally to the subgrain formation while there was insufficient time for pores to pass the grain boundaries, caused inadequate densification. In such a system, the average grain size decreases close to the eutectic point due to an increase in the number of newly formed fine-grains, resulting in an increase in the total number of grains (considering grains in both phases) per unit area . A solution to inadequate densification can be addressed by using pressure-assisted sintering or increasing the time and temperature of the process to thoroughly perform the liquid-phase sintering process, which was not the case for this study. Figure shows the average converted thickness of as-sintered Ga " O ' + YSZ to NZGO + YSZ via vapor phase process after 5 to 30 hours of exposure to Na " O vapor source. The samples sintered at 1400 °C which exhibited the highest achieved density in this study, showed a higher average converted thickness within the same exposure time. In figure , the obtained data of the current work were compared to the similar vapor phase conversion of a different system of alumina + 20SDC to Na -β $$ -alumina + 20SDC as mixed sodium and oxygen ion conductors where the exact conversion mechanism happened using Na -β $$ -alumina as Na " O vapor source. The conversion process consists of coupled diffusion of sodium and oxygen ions through sodium and oxygen ion conductors. The comparison of Al " O ' and Ga " O ' conversion process derived from the extensive structural similarities between gallia and alumina systems where most phases are analogous . The samples with finer grains and higher densities show a faster kinetic rate and higher converted thickness than coarse-grain microstructures in both systems. This is consistent with previous theoretical and experimental works on vapor phase conversion of Na -β $$ -alumin + YSZ , and Na -β $$ -alumin + SDC. Hence, fine-grained structures and a higher density of sintered samples facilitated the conversion of vapor phase. Parthasarathy and Virkar suggested a general equation for such a conversion to predict the kinetics of the vapor phase conversion for two-phase materials