## 3 Results and Discussion  
### Characterization of Microstructure  
Optical microscopy of the microstructures showed uniformly distributed pores in all the studied materials (Figure 1). For the 3 YSZ ceramic, a decrease in the average size of pores can be recognized with a decrease in sintering temperature from 1550 through 1500 to 1450 \({}^{\circ}\)C (Figure 1a-c). However, the opposite tendency was found for the 6 YSZ ceramic (Figure 1d-f). Thus, the average size of pores decreased in the 3 YSZ ceramic from 0.91 to 0.69 \(\upmu\)m whereas this in the 6 YSZ ceramic increased from 0.49 to 0.76 \(\upmu\)m (Table 1). The most distinct differences were found for the pore number density and area fraction of the 3 YSZ ceramic with a decrease in sintering temperature from 1500 to 1450 \({}^{\circ}\)C and for those of the 6 YSZ ceramic with a decrease in sintering temperature from 1550 to 1500 \({}^{\circ}\)C. These results indicate the most essential microstructural changes in the mentioned temperature ranges; namely, the highest temperature in the range promotes a more advanced densification of corresponding ceramics. Figure 1: Optical 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 evolution of porosity with a decrease in the sintering temperature was characterized by the pore size distributions analysis (Figure 2). With a decrease in sintering temperature of the 3 YSZ ceramic from 1550 to 1450 \({}^{\circ}\)C (Figure 2a), the fraction of 0.3-0.6 \(\upmu\)m pores increased from 26 to 54%. At the same time, the fraction of 0.6-0.9 \(\upmu\)m pores decreased from 25 to 15%, this of 0.9-1.2 \(\upmu\)m pores decreased from 22 to 14.5%, and this of 1.2-1.5 \(\upmu\)m pores decreased from 13 to 12% (Figure 2a). For 1.5-1.8 \(\upmu\)m and >1.8 \(\upmu\)m pores, negligible fractions were estimated for the sintering temperatures of 1500 \({}^{\circ}\)C and 1450 \({}^{\circ}\)C. The exception was found for 1550 \({}^{\circ}\)C, where the fractions reached 7.5%. An increase in the sintering temperature to 1550 \({}^{\circ}\)C is suggested to provoke a rapid recrystallization of the initial powder in contrast to a lower sintering temperature. This is followed by substantial shrinkage in local areas where homogeneous distribution of the initial powder was not reached. As a result, a comparatively high number of large pores are formed under such conditions. It should be noted that the average pore size is maximal (i.e., 0.9 \(\upmu\)m, see Table 1) and the area fraction of pores is 2.9% (see Table 1) for these conditions. According to the work [46], for the sample series prepared by high-speed sintering (1500 \({}^{\circ}\)C, 5 min) the porosity was in the range 0.7-1.2% and for those conventionally sintered (1500 \({}^{\circ}\)C, 2 h) it was about 0.3% that exhibited larger deviation of the measured values from the average for the samples sintered rapidly. It was suggested that both high-speed sintering and non-uniform distribution of yttria in 5 YSZ zirconia caused an increase in size of residual pores. Similarly, the authors of the work [47] have studied evolution of microstructural components during the sintering of yttria-zirconia compacts at 1400 \({}^{\circ}\)C for 2 h. They found that along with domination of small pores, some quantity of much larger pores occurred in the ceramics due to formation of the isolated clusters which consisted of sub-micrometer particles of pure zirconia. A quite different pattern of the porosity evolution was revealed for the material with a higher content of yttria (Figure 2b). With a decrease in the sintering temperature of the 6 YSZ ceramic from 1550 to 1450 \({}^{\circ}\)C (Figure 2b), the fraction of 0.3-0.6 \(\upmu\)m pores decreased from 76 to 40%. At the same time, the fraction of 0.6-0.9 \(\upmu\)m pores increased from 9 to 15%, this of 0.9-1.2 \(\upmu\)m pores increased from 7 to 22%, and this of 1.2-1.5 \(\upmu\)m pores increased from 5 to 10% (Figure 2b).