## 3 Results and Discussion
### Microwave Absorption Performance of Composites
Figure 9: 3D plots of RL for CB/PLA composites with different CB contents: (**a**) 0% CB; (**b**) 5% CB; (**c**) 10% CB; (**d**) 15% CB; (**e**) 20% CB; and (**f**) 25% CB. The 25% CB/PLA exhibited excellent absorbing properties. Figure 11a shows the 25% CB/PLA RL curves under different thicknesses in the frequency range of 2-18 GHz. The peak value of RL gradually moved from high frequency to low frequency as the thickness increased, and the peak value first increased and then decreased. It can be observed from the projection diagram of Figure 9f that the whole displacement of the minimum RL was an approximate fan shape, which can be explained by the quarter-wavelength cancelation theory [47; 48]. Figure 11b shows the EAB corresponding to a 1.5-4.2 mm thickness. When the thickness was 2.1 mm, the EAB reached a maximum of 5.36 GHz, and the RL value reached \(-\)45.47 dB. Generally, the RL value is lower than \(-\)20 dB, indicating that > 99% of electromagnetic waves can be absorbed [49]. In addition, microwave absorption performance can be adjusted by changing the thickness. As shown in Table 3, compared with other composites using CB absorbers, 25% CB/PLA is advantageous with strong absorption performance, broad bandwidth, small thickness, low cost, and low density, meeting the development requirements of \"thin, light, wide and strong\" MAMs. Meanwhile, the microwave absorption performance of single-component single-layer composites is similar to that of some multicomponent multilayer materials, avoiding complex processing problems, low efficiency, and high cost. Based on the above investigation, the microwave absorption mechanism of CB/PLA composites is speculated and illustrated in Figure 12. With increased CB content, the conductive network is gradually completed and continues increasing, so the conduction loss is significantly enhanced. When CB particles are uniformly distributed in PLA, many interfaces, such as the CB/PLA interface, are formed. The number of interfaces increases correspondingly, and the interface loss increases significantly, as the CB content increases. Dipole polarization is promoted due to polar functional groups and surface defects in CB. Figure 11: The 25% CB/PLA (**a**) RL curves with different thicknesses; (**b**) EAB with a thickness of 1.5–4.2 mm. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Absorber and Content** & **EAB/GHz** & **Minimum RL/dB** & **Thickness at Minimum RL/mm** & **Reference** \\ \hline (6\%) CB/CF/EP & 2.4 & \(-\)31.8 & 2 & [50] \\ (7\%) CB/T-ZnO/EP & 4.25 & \(-\)17.36 & 3 & [51] \\ (30\%) CB/nmCIP/PU & 3.6 & \(-\)25.8 & 2 & [52] \\ (5.5\%) CB/SiO2/PI & 3.95 & \(-\)46.18 & 1.6 & [53] \\ (8\%) CB/PAN/EP & 6.7 & \(-\)17.0 & 6.3 & [54] \\ (30\%) CB/CNTs & 4.1 & \(-\)53.6 & 2.7 & [55] \\ (17\%) CB/EP & 0.6 & \(-\)32.1 & 3 & [56] \\ (25\%) CB/PLA & 5.36 & \(-\)45.47 & 2.1 & This work \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Performance comparison with other CB-based composites. Additionally, CB and PLA form protrusions and wrinkles at the contact surface, which enhances the reflection and scattering of electromagnetic waves inside the material [25, 28]. The uniform distribution of CB particles also induces multiple reflection behaviors, which promote the attenuation of electromagnetic waves; thus, the CB/PLA composite exhibits superior microwave absorption effects.