## 3 Results
### Y\({}_{2}\)O\({}_{3}\):Eu@MF Prepared by the Variation in pH and Solvents
After the coating reaction completed in the initial solution at pH 8, the final pH of the resultant solution was slightly higher than pH 8 at 85 \({}^{\circ}\)C and 95 \({}^{\circ}\)C, but was slightly lower than pH 8 at 75 \({}^{\circ}\)C. As can be seen from the zeta potential curve in Figure 3, that the potential difference is larger and the coating property is better at pH 8. As the temperature increases, the ions in the solution move actively; this affects its pH. It is considered that the attractive force of the Y\({}_{2}\)O\({}_{3}\):Eu\({}^{3+}\) nanoparticles is strengthened as they get coated on the surface of MF. Especially, the results with a stirring temperature of 95 \({}^{\circ}\)C are not significantly different from those of 85 \({}^{\circ}\)C; the stirring temperature at 85 \({}^{\circ}\)C was considered to be the most suitable. Figure 9 shows the FE-SEM images of the obtained Y\({}_{2}\)O\({}_{3}\):Eu@MF for different calcination temperatures. Figure 9a clearly shows that the Y\({}_{2}\)O\({}_{3}\):Eu@MF calcined at 650 \({}^{\circ}\)C is stably attached on the mica surface. Compared to the samples calcined at 750 \({}^{\circ}\)C, Figure 9b, and 850 \({}^{\circ}\)C, Figure 9c, it was found that the Y\({}_{2}\)O\({}_{3}\):Eu\({}^{3+}\) nanoparticles were detached from the mica surface. This phenomenon occurs due to the different thermal expansions of the Y\({}_{2}\)O\({}_{3}\):Eu\({}^{3+}\) nanoparticles and mica. Figure 10 depicts the PL emission spectra fixed at 254 nm excitation of the Y\({}_{2}\)O\({}_{3}\):Eu@MF powders as red phosphoror shown in Figure 9. The emission intensity of the sample calcined at 650 \({}^{\circ}\)C was higher than that of the two samples calcined at 750 \({}^{\circ}\)C or 850 \({}^{\circ}\)C. The brightness of these red-emitted phosphors in the images inserted in Figure 9 is decreased accordingly with the increase in the calcination temperature. This result was considered to be proportional to the coating coverage or the number of Y\({}_{2}\)O\({}_{3}\):Eu\({}^{3+}\) nanoparticles coated on mica, and to be detached due to the difference in the coefficient of the thermal expansion between the Y\({}_{2}\)O\({}_{3}\):Eu\({}^{3+}\) and mica particles according to the increase in the temperature, as shown in Figure 9. Also, it was confirmed that the luminescent property of the nanoparticles with low crystallinity, caused by milling, increased with the increasing crystallinity of the coated nanoparticles after the calcination process of Y\({}_{2}\)O\({}_{3}\):Eu@MF. [PERSON] group had reported that the luminescent property of Y\({}_{2}\)O\({}_{3}\):Eu\({}^{3+}\) nanoparticles was recovered after the calcination process in spite of the decreased crystal structure of the nanoparticles due to the bead-mill process [21]. Figure 11a shows the cross-sectional FE-SEM image of the Y\({}_{2}\)O\({}_{3}\):Eu@MF (10 wt%) particle, fixed in epoxy resin, captured to observe the coating thickness. The powder was mixed with epoxy resin and then dried and prepared to measure its coating thickness. Based on the FE-SEM image, it has been concluded that the MF coated with the Y\({}_{2}\)O\({}_{3}\):Eu\({}^{3+}\) nanoparticle was surrounded by epoxy resin. Herein, the bright plate of the top and bottom layer indicates epoxy resin and the dark middle layer consists of MF. The mica flake of  
the middle layer was coated by Y\({}_{2}\)O\({}_{3}\):Eu\({}^{3+}\) nanoparticles of the bright part. However, the coated nanoparticles on MF were observed only on the bottom side due to the covered epoxy plate on the top side. The thickness range of the mica and Y\({}_{2}\)O\({}_{3}\):Eu\({}^{3+}\) nanoparticles was about 284 nm, and 80-105 nm, respectively. Multi-elemental EDS mapping images of Y, Si, and Ti are shown in Figure 11a. Apparently, the corresponding Y and Si maps evidenced bright spots corresponding to the Y\({}_{2}\)O\({}_{3}\):Eu red phosphor and mica area, and illustrated the coating layers of these elements in the field of view of the cross-section.