## 3 Results  
### Phase Composition  
The phase composition of the initial powder mixtures was studied in the first step of the work. The formation of the cubic YSZ solid solution with the crystallinity values of \(\sim\)30% was confirmed for all studied compositions with the graphene contents of 0.25-2.5 wt.%. At the same time, any reflexes from rGO or carbon were absent in the XRD patterns of composite powders, indicating a high degree of homogeneity and the uniform carbon additive distribution in the ceramic matrix. Raman spectra analysis confirmed these assumptions and showed the formation of \"core-shell\" type composites. The optical photographs of the sintered Z0-Z2 samples are shown in Figure 2.  
The XRD patterns of sintered ceramic materials are presented in Figure 3. All peaks correspond to the zirconia-based cubic solid solution with 20 at 29.5, 34, 51, 59, 62.5 and 73.5\({}^{\circ}\), respectively. Peaks that can be related to GO at 20 \(\sim\)12\({}^{\circ}\) or rGO at 20 \(\sim\)25\({}^{\circ}\) are absent [28]. However, the main graphite peak at \(\sim\)26.5\({}^{\circ}\) is slightly seen in the X-ray patterns of all the samples with the intensity close to the signal of the baseline. Figure 3: XRD patterns of the obtained samples. The symbol “\(\star\)” refers to cubic YSZ phase. Figure 2: Optical photographs of the sintered Z0–Z2 samples. Crystallinity (K, %) and the crystallite size (d\({}_{\text{cryst}}\), nm) of the Z0-Z2 samples sintered from pure YSZ (Z0) and composites (Z0.25-Z2) are presented in Table 2. Z0 ceramics has the highest value of K \(\approx\) 98%, being close to the theoretical one. The data shown in Table 2 prove the completeness of the YSZ crystalline phase formation. With the addition of 0.25 wt.% of rGO, the crystallinity slightly decreases to ~94%, being in accordance with the previous studies [8; 9]. The (further) increase of the carbon content does not lead to the further decrease of crystallinity. Crystallite sizes for the Z0-Z2 samples are about 110-120 nm, which is higher than the value for YSZ ceramics sintered at 1150 \({}^{\circ}\)C for 3 h in air (35 nm) [29]. A low signal from the carbon phase in the ceramics may indicate either its homogeneous distribution (as it was observed for powder composites in [9; 30]) or rGO evaporation/burnout during the sintering process. For more accurate study of the carbon state in the samples, Raman spectroscopy was used. The spectrum of pure microwave-reduced rGO was obtained previously in [8]; it corresponds to some layered graphene having a decreased number of defects compared to the thermally reduced graphene oxide. Raman spectra of sintered Z0-Z2 ceramics are presented in Figure 4. A typical spectrum can be divided into two parts: an area from 0 to 1000 cm\({}^{-1}\), which is referred to as YSZ with characteristic bands at ~150, ~280 and 600 cm\({}^{-1}\)[31], and an area of rGO from 1000 cm\({}^{-1}\) to 3000 cm\({}^{-1}\) with 1350 (D), 1550 (G) and 2750 (2D) cm\({}^{-1}\) bands. However, a number of very intensive bands at the low shifts from 0 to ~1250 cm\({}^{-1}\) (observed at an excitation wavelength of both 532 nm and 785 nm) were registered for all samples studied (Figure 4a). In particular, they are: a band at 970 cm\({}^{-1}\) and a band at 480 cm\({}^{-1}\) with a shoulder at 320 cm\({}^{-1}\). All of them could be hardly attributed to the YSZ phase. The most intense broad band can be attributed to the transverse optic phonon of the amorphous Si and the second most intense peak to SiC [32]. The magnified areas of spectra at 1000-2000 cm\({}^{-1}\) corresponding to rGO main bands are presented in Figure 4b. Both \(D\) and \(G\) bands at ~1340 cm\({}^{-1}\) and ~1600 cm\({}^{-1}\) are distinguished for all samples. The intensities of the bands are comparable to the background level, which indicates that the carbon content in the sintered samples is low.