## 4 Discussion  
As previously stated, microwave absorbing materials (i.e., materials that limit the reflection and transmission) require high dielectric losses (imaginary part of the complex permittivity) in order to convert the energy of the impinging radiation into heat and dissipate it. Needless to say, nanostructured materials such as CNTs have the capability to maximize both terms of the dielectric loss: relaxation and conductivity. A few deductions can be inferred from Figure 7 and 8. First, the high aspect ratio of the tubes (\(\approx 1400\)) in conjunction with their high specific surface area (\(\approx 300\) m\({}^{2}\)/g) brings about both a large interfacial polarization relaxing in the microwaves region and a good connectivity of the conductive structure across the composite [25]. Second, the presence of defects, as emerged from the Raman analysis, may lead to a further increase of the interfacial polarization. Hence, the relaxation of the interfacial polarization can account for the minimum in the reflection coefficient of the composite. Furthermore, a high aspect ratio and a high specific surface area can also cause multiple reflections (scattering), actually entrapping the radiation inside the material. Finally, both the high MWCNT aspect ratio and the relatively high (8 wt.%) content of metallic impurities enhance the composite conductivity and therefore the material losses. In order to verify such hypotheses, a comparison of the results with a higher load (7 wt%) of structurally different MWCNTs was carried out. Figure 9 displays the real part of the complex permittivity and the conductivity for an Epilox resin loaded with 7 wt% of MWCNTs possessing the following characteristics: diameter (D) 22-45 nm, length (L) \(>\)10 \(\upmu\)m, aspect ratio (AR = L/D) \(\approx 300\), catalyst residue 1.5 wt%, specific surface area (SSA) 200-250 m\({}^{2}\)/g. For the sake of simplicity, this sample will be referred to as AR300 while the one employing 4 wt% of high aspect ratio (AR \(\approx 1400\)) MWCNTs will be indicated as AR1400. Although the concentration of MWCNTs in the AR300 composite was almost doubled (i.e., compared to the AR1400), the both the dielectric constant and the conductivity dropped (for instance \(\varepsilon^{\prime}\)r (AR1400) = 12.1 and \(\sigma\)(AR1400) = 0.075 S/m; \(\varepsilon^{\prime}\)r (AR300) = 7.4 and \(\sigma\) (AR300) = 0.025 S/m at 1 GHz). Figure 8: Microwaves reflection coefficient measurements: bare Epilox (dotted line) and 4 wt% MWCNT-loaded resin (dashed line). A comparison between the simulated (solid line) and the measured (black) reflection coefficient for the sample containing the carbon nanotubes is shown. Thus, the morphological properties of the carbon nanotubes proved to be crucial in determining the dielectric characteristic of the composite. In fact, the lower SSA and aspect ratio of the AR\({}_{300}\) sample inevitably causes a reduction of the interfacial polarization. Consequently, the real component of the complex permittivity decreases. Apparently, also the conductivity seems to be affected by the smaller aspect ratio of the 7 wt% composite. However, the tube length is similar for both kinds of MWCNTs, and other parameters also need to be taken into account. For instance, the two samples differ in the wt% of the metal catalyst residue. Hence, the higher amount of metal particles in the AR1400 composite may also partially account for its higher conductivity. Such morphological (i.e., aspect ratio and specific surface area) and compositional (catalysts residue) features also reflect into the microwave absorption properties of the two materials. Figure 10 illustrates the reflection coefficient spectrum (for the AR300 composite. Despite the high concentration of carbon nanotubes compared to the AR1400 sample, the lower conductivity and dielectric constant resulted in similar amplitudes of the reflection coefficient.