## 5 Discussion  
### Measurement of 28 GHz Band  
Table 1 summarizes the amount of radio wave absorption in the 28 GHz band measured for each coating thickness. Figure 18: Radio wave absorption of graphite paint: (**a**) Coating thickness 15 \(\upmu\)m upper part (0.77 dB); (**b**) Coating thickness 36 \(\upmu\)m lower part (0.93 dB); (**c**) Coating thickness 74 \(\upmu\)m upper part (0.71 dB). The maximum value of the radio wave absorption in the 28 GHz band was confirmed when 35 \(\upmu\)m of CNT spray paint was applied. The carbon black paint obtained a radio wave absorption amount of about 60% of that of the CNT spray paint. It was confirmed that the graphite paint did not have much absorption effect even when the coating thickness was changed. The amount of radio wave absorption of the CNT spray paint with a coating thickness of 16 \(\upmu\)m is smaller than that of other paints, which is considered to be due to the fact that the coating thickness in the vicinity of the irradiation of radio waves is thin. In order to verify this, a part of the coating film of the test material used for the measurement was peeled off, and the coating thickness at the center of the test material was measured. The coating thickness was measured using a KEYENCE double-scan high-precision laser measuring instrument (LT-9500) and a sensor head with a camera function (LT-9010M). Figures 19 and 20 show screen copies during measurement. The black part on the right is the coating film, and the gray part on the left is the part where the coating film is peeled off. The numerical value at the top is the coating thickness at the center of the screen. The coating thickness was about 6-9 \(\upmu\)m, and the thinnest part was 5.8 \(\upmu\)m. (See Figure 19). In order to confirm the variation in coating thickness, the coating thickness at the top and bottom of the sample was also measured. The coating thickness was about 13-19 \(\upmu\)m, and although there was a large variation, it could be seen that the coating was thicker than the central part (see Figure 20). \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & & \multicolumn{3}{c}{**Coating Thickness (\(\upmu\)m)**} \\ \cline{3-5}  & & **16** & **35** & **76** \\ \hline \multirow{2}{*}{Radio wave absorption (dB)} & CNT spray paint & 0.52 & 5.91 & 3.29 \\  & Carbon black paint & 3.29 & 2.32 & 1.62 \\  & Graphite paint & 1.27 & 1.44 & 0.57 \\ \hline \end{tabular} Note: Graphite coating thickness is 15 \(\upmu\)m, 36 \(\upmu\)m, 74 \(\upmu\)m.  
\end{table} 
Table 1: 28 GHz band radio wave absorption. Figure 19: Screen copy when measuring the thinnest part. Figure 20: Coating thickness at the top and bottom of the sample: (**a**) top 13.2 \(\upmu\)m; (**b**) bottom 18.6 \(\upmu\)m.  
From this, it was confirmed that the radio wave absorption amount of the CNT spray paint with a coating thickness of 16 \(\upmu\)m was lower than that of other carbon-based paints due to the influence of the variation in the coating thickness. The difference in the amount of radio wave absorption depending on the coating thickness was observed in all the test materials, but it could be seen that the appropriate coating thickness differed depending on the material. In addition, when the coating thickness was 76 \(\upmu\)m, the amount of radio wave absorption decreased in all the test materials. It is considered that this is because the amount of radio wave absorption decreased due to the increase in the amount of reflection. The shape of CNTs tends to be elongated and difficult to spray compared to carbon grains and graphite grains. The coating sample used this time was prepared by Fukoku Co., Ltd., but it seems that there is a problem with the shape of their spray head, and it is presumed that this led to the above results being obtained. Next time, it will be necessary to reconfirm with a spray [24; 25; 26; 27] that applies a nozzle developed by [PERSON] and [PERSON]. This spray system has been developed to make it easier to create electrodes for dielectric elastomers [32; 33].