## 3 Results and Discussion
### Characterization of Microstructure
Similarly to a trend for the 3 YSZ ceramic, negligible fractions were determined for all the sintering temperatures for 1.5-1.8 \(\upmu\)m and >1.8 \(\upmu\)m pores. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{\ \begin{tabular}{c} **\% Y\({}_{2}\)O\({}_{3}\)** \\ **Sintering**, \({}^{\circ}\)C \\ \end{tabular}  } & \multirow{2}{*}{ \begin{tabular}{c} **Average Grain** \\ **Sinc**, \(\upmu\)m \\ **j**m \\ \end{tabular}  } & \multicolumn{2}{c}{**Power**} & \multicolumn{2}{c}{**Fantative Vials**} & \multicolumn{2}{c}{**Clustering Zacks**} \\  & & & & & & & & & \\ \hline \multirow{2}{*}{3} & 1550 & 10.8 \(\pm\) 0.5 & 0.9 \(\pm\) 0.6 & 0.028 & 2.9 & 0.7 \(\pm\) 0.4 & 0.009 & 8.0 \(\pm\) 4.9 & 0.00 & 34 \\  & 1500 & 0.7 \(\pm\) 0.3 & 0.7 \(\pm\) 0.4 & 0.0271 & 2.6 & 0.3 \(\pm\) 0.2 & 0.076 & 6.4 \(\pm\) 3.2 & 0.020 & 13 \\  & 1450 & 0.6 \(\pm\) 0.2 & 0.7 \(\pm\) 0.5 & 0.0724 & 4.0 & 0.4 \(\pm\) 0.2 & 0.142 &With a decrease in sintering temperature of the 3 YSZ ceramic from 1550 to 1450 \({}^{\circ}\)C, the area fractions of 0.3-0.6 \(\upmu\)m and 0.6-0.9 \(\upmu\)m grains increased from 4 to 21.5% and from 18 to 59%, respectively (Figures 3a-c and 4a). Conversely, for 0.9-1.2 \(\upmu\)m up to 1.8-2.1 \(\upmu\)m grains a decrease in the area fraction from 20.5 to 8% and from 2.5 to 1.5%, respectively, was observed. After sintering at 1550 \({}^{\circ}\)C, untypically high area fractions (up to 15%) were noticed for large 2.4-2.7 \(\upmu\)m and \(>\)2.7 \(\upmu\)m grains. Figure 4: Distributions of grain size for (**a**) 3 YSZ and (**b**) 6 YSZ ceramics sintered at three temperatures. Figure 3: SEM-SE microstructure of (**a–c**) 3 YSZ and (**d–f**) 6 YSZ ceramics sintered at (**a,d**) 1550, (**b,e**) 1500, and (**c,f**) 1450 \({}^{\circ}\)C.  
The microstructure of the 6 YSZ ceramic showed a quite different variation of the grain size distribution with the sintering temperature (Figures 3d-f and 4b). For this material, the area fraction of 0.3-0.6 \(\upmu\)m grains increased from 24 to 37% with the temperature decreasing. This means that the 6 YSZ ceramic generally exhibited finer microstructure at a low sintering temperature (1450 \({}^{\circ}\)C) compared to the 3 YSZ ceramic. Such a trend found for the 6 YSZ ceramic is not fully supported with the average grain size calculations, as no significant variation of this parameter was observed with a variation in sintering temperature (Table 1). Therefore, grain size distributions should always be considered for accurate characterization of microstructure evolution in YSZ ceramics, instead of the average grain size parameter. General trends in the microstructure variation with a decrease in sintering temperature from 1550 through 1500 to 1450 \({}^{\circ}\)C can be summarized as follows:  
* The average size of pores decreased by 30% in the 3 YSZ ceramic; however, it increased by 55% in 6 YSZ material (Table 1);
* The pore size distributions showed an increase in the fraction of finer pores (<0.6 \(\upmu\)m) and a decrease in the fraction of larger pores (>0.6 \(\upmu\)m) in the 3 YSZ ceramic (Figure 2a); however, this trend was opposite in the 6 YSZ ceramic (Figure 2b);
* The average grain size decreased in the 3 YSZ ceramic, but no significant variation was observed in the 6 YSZ one (Table 1);
* The 6 YSZ ceramic sintered at 1450 \({}^{\circ}\)C exhibited finer microstructure in terms of the area fractions for the grain size parameter, compared to the 3 YSZ ceramic;
* Grain size distributions should always be considered for accurate characterization of microstructure evolution in YSZ ceramics, instead of the average grain size parameter.