## 4 Final Remarks  
Based on the simulations presented above, at first glance, it appears that the composites with metallic ingredients with a lower dimension are more adequate for usage. At higher frequencies, the power loss density is larger for both metals, so the usage of higher frequencies is recommended for technological applications. The particles with a larger dimension (800 nm) absorb a much lower energy compared to the ones with a small dimension (50 nm), at the same mass ratio, because the equivalent external surface exposed to microwave radiation is lower. On the other hand, due to a higher conductivity, Al particles within composite materials can absorb a larger amount of energy compared to Fe particles, at both insertion dimensions. However, the impact of the metallic type might lose significance when considering the energy consumption that is just 30% more for Fe, and/or the technological productivity, alongside the expense of powders, as Al powders are considerably more expensive than Fe powders. Figures 19 and 20 compare the S11 reflection coefficient parameters, and Table 3 presents some data considering the most relevant frequencies for microwave technology. The reflection coefficient is influenced by the dimension of the insertion, decreasing for higher-dimensional cases. It also relies on frequency, decreasing as the frequency increases. Concerning the effect of the metal type, the Al-containing composite shows a marginally lower reflection at both sizes or analyzed frequencies (a less than 5% difference). Overall, based on the data, the reflection phenomena are minimal; however, it is important to also examine these phenomena when taking into account the types of insertion and frequency dependence, as it confirms that the key factor for bonding technology is the absorption of radiation energy. Analyzing the \(S_{21}\) transmission parameters, the findings are shown in Figures 21 and 22, which illustrate that incorporating nanometric inserts will reduce transmission through the material. It is clear that the transmission significantly declines as the frequency rises in the GHz range, yet a saturation effect was observed after 2-2.5 GHz. The significant reduction in transmission, linked to minimal reflection, verifies that, for composites with added metallic powder, microwave energy is extensively absorbed by the materials, particularly at frequencies above 1.5 GHz, confirming their potential functionality as hot melts. Figure 19: \(S_{11}\) reflection coefficient comparison for composites with 50 nm inserts. Figure 21: S\({}_{21}\) transmission coefficient comparison for composites with 50 nm inserts. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Frequency** & **Al—50 nm** & **Al—800 nm** & **Fe—50 nm** & **Fe—800 nm** \\ \hline
1 GHz & 83.5 & 60 & 86 & 62 \\ \hline
3 GHz & 73 & 49 & 76 & 51.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: S\({}_{11}\) magnitude [dB] vs. insertion type. Figure 20: S\({}_{11}\) reflection coefficient comparison for composites with 800 nm inserts. The reduction in transmission shown in Figures 21 and 22 may not appear significant, but it is important to note that the simulated layers are here merely 2 \(\upmu\)m thick for 50 nm insertions, and 33 \(\upmu\)m thick for 800 nm insertions, respectively. However, when transforming the absorption into dB/unit length, notably high values for composite layers, such as those 1 or 2 mm in thickness, are achieved, which is the typical dimension in bonding applications utilizing hot melt technology. The figures in Table 4 are pertinent in this manner. Overall, a value exceeding 80 dB is regarded as adequate for a hot melt layer of 1 mm thickness; however, it can be noted that the peak values are attained for Al insertions and, respectively, for both metallic particles at a size of 50 nm, reaffirming the results obtained from the power loss simulations.