## 3 Results and Discussion
Additionally, aging the nanopowder at 3 \({}^{\circ}\)C for 24 h resulted in a 6.6% increase in volume fraction and a 10% increase in permeability (Figure 7). \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Ratio** & **Pressure (ton/cm\({}^{2}\))** & **Permeability (\(\mu\))** & **Volume Fraction (\%)** \\ \hline \multirow{8}{*}{Fe-6.5 Si} & \multirow{8}{*}{8:2} & 1.8 & 29.35 & 72.11 \\ \cline{3-5}  & & 2.5 & 32.1 & 73.45 \\ \cline{2-5}  & & 3.75 & 36.73 & 75.73 \\ \cline{2-5}  & & 6.25 & 45.08 & 79.29 \\ \cline{2-5}  & & 1.8 & 30.82 & 79.77 \\ \cline{2-5}  & & 2.5 & 32.42 & 80.89 \\ \cline{2-5}  & & 3.75 & 33.67 & 81.39 \\ \cline{2-5}  & & 6.25 & 35.17 & 82.37 \\ \cline{2-5}  & & 1.8 & 35.03 & 80.2 \\ \cline{2-5}  & & 2.5 & 35.66 & 80.75 \\ \cline{2-5}  & & 3.75 & 38.14 & 82.21 \\ \cline{2-5}  & & 6.25 & 40.33 & 83.27 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Results of evaluation of magnetic properties and volume fractions by molding pressure and mixing ratio of magnetic bulk composites. Figure 4: SEM images of various bulk composites fabricated by different molding pressure. The mixing ratios of the Fe-6.5 wt.% Si powder and fine iron powder were (a,b) 10:0, 1.8 ton/cm\({}^{2}\), (c,d) 10:0, 6.25 ton/cm\({}^{2}\), (e,f) 8:2, 1.8 ton/cm\({}^{2}\), (g,h) 8:2, 6.25 ton/cm\({}^{2}\), (i,j) 7:3, 1.8 ton/cm\({}^{2}\), (k,l) 7:3, 6.25 ton/cm\({}^{2}\). Figure 5: Volume fraction and permeability measurements, using [PERSON]’s theory for each magnetic bulk composite. In this experiment, silane coupling agents were prepared in a ratio of 4:1 with distilled water, for surface treatment of nanopowders. However, rapid surface reactions can form non-uniform coatings. To induce the formation of a uniform coating layer, a slow and uniform surface-treatment method was applied by aging for 24 h at a temperature of 3 \({}^{\circ}\)C. As a result, a uniform coating layer with a thickness of 5 nm was successfully formed on the surface of the nanopowder. Such a surface treatment works to fill the void space between the coarse powder and the fine powder when the magnetic slurry is made. As a result, the volume fraction is improved, and the permeability is increased (Figure 8). Figure 6: Roadmap of technology changes of power inductors required by the size change in various IT devices. Figure 7: Permeability and volume fraction evaluation results of magnetic bulk composites by surface-treatment conditions of nanopowders. In the previous experiment using the bulk magnetic composite, the best mixing rate was found at the magnetic powder mixing ratio of 7:2.5:0.5. Iron nanopowder was given the same aging treatment, 3 \({}^{\circ}\)C for 24 h. For the magnetic sheet using only Fe-6.5 wt.% Si powder with an average particle size of 40 \(\upmu\)m, it can be seen that the inside of the sheet is not uniformly filled (Figure 9a). For the magnetic sheet produced with the coarse and the fine powder at 7:3, the empty spaces between the coarse powders were confirmed to be uniformly filled, as shown in Figure 9b. To improve the volume fraction, the magnetic sheet was manufactured by using nanopowder, without surface treatment. However, all the magnetic sheets cracked during sheet casting; this is because, due to its large specific surface area, the epoxy binder was not uniformly adsorbed on the surface of the nanopowder. Hence, the surface treatment process using a 1% silane solution is the key technology for casting flexible magnetic sheets. As a result of applying the process developed through this study, we have succeeded in developing highly densified magnetic sheets by using nanopowder (Figure 9c,d). The characteristics of the magnetic sheet produced under each condition were analyzed. Analyzing the surface properties of the magnetic sheet shows that Ra = 2.01 \(\upmu\)m and Rz = 12.7 \(\upmu\)m for the mixing ratio 7:3.