Lightweight multi-walled carbon nanotube buckypaper/glass fiber–epoxy composites for strong electromagnetic interference shielding and efficient microwave absorption

Multi-walled carbon nanotube buckypaper (BP) reinforced glass fiber–epoxy (GF/EP) composites were selected to fabricate electromagnetic interference (EMI) shielding and microwave absorbing materials. Six different composite configurations with 3.0 mm thick have been conceived and tested over the X-band (8.2–12.4 GHz). Flexible and low-density (0.29 g/cm3) BP provided a high specific EMI SE of 76 dB with controlled electrical conductivity. GF/EP/BP111 and GF/EP/BP101 composites possess EMI SE as high as of 50–60 dB, which can be attributed to the number of BP inserted and variation in the wave-transmitting layer of the laminates. Furthermore, the shielding mechanism was discussed and suggested that the absorption was the dominant contribution to EMI SE. GF/EP/BP110 laminate demonstrated suitable EMI performance (~ 20 dB), whereas GF/EP/BP011 composite revealed excellent microwave performance, achieving an effective − 10 dB bandwidth of 3.04 GHz and minimum reflection loss (RL) value of − 21.16 dB at 10.37 GHz. On the basis of these results, GF/EP/BP composites prepared in this work have potential applications as both EMI shielding and microwave absorber materials given their facile preparation and lightweight use.


Introduction
The need for shields or absorber materials has been attracted more and more attention from both academic and industrial fields over the last years. The massive usage of electronic devices has generated several forms of electronic pollutions, such as electromagnetic radiation, electromagnetic interference (EMI), electronic noise, among others [1][2][3]. Electromagnetic radiation can also affect human health, with some works reporting severe diseases such as brain tumors and leukemia [4].
EMI shielding can be understood as the ability to reflect and/or absorb EM radiation by a barrier made of conductive materials [5,6]. Conventional metalbased EMI shielding materials (e.g., copper, nickel, aluminum, magnesium, and zinc) have been the target point of the researchers for years due to their notable EMI shielding effectiveness (SE) being attributed to high electrical conductivity [7]. However, some disadvantages are related to using metals. The high density of the material associated with its poor processability and corrosion issues can limit their EMI applications in real structures. Moreover, metallic materials most reflect or scatter the EM radiation into their surroundings, which may cause additional difficulty. Based on these features, novel EMI shielding materials are highly recommended, not only for better EMI shielding performance but also for developing microwave absorbing materials that efficiently convert the EM radiation into thermal energy.
Multiscale composites (MC) are a novel type of polymer composites that consist of a polymer matrix reinforced with microscale (fibers) and nanoscale materials [8][9][10]. Carbonaceous materials have been extensively studied as promising high-performance absorbing materials mainly due to their prominent electrical and thermal conductivities, low-density, chemical inertness, and suitable mechanical properties [11][12][13]. Carbon nanotubes (CNTs) pose a great potential as the ideal nanomaterials to be incorporated in the traditional fiber-reinforced polymer composites. Furthermore, CNT-polymer composites have been one of the major research topics over the last years due to the possibility of reaching high EMI SE properties and their significant advantages like lightweight and good processability over the metalbased shielding materials [14][15][16].
Uniform dispersion of the fillers is the major concern for CNT-polymer composites. However, the strong van der Waal's forces between the tubes make them easy to agglomerate when dispersed in polymer matrices, which may decrease the final shielding properties of the material. Huang et al. [17] prepared single-walled carbon nanotube/epoxy composites with high CNTs concentration (15%). The EMI results exhibited by the 2 mm thick composite were only 20-30 dB, which can be attributed to the poor dispersion level of the filler in the polymer matrix. As an alternative to dispersion issues, carbon nanotube films, also known as buckypapers (BP), have been employed in CNT-polymer composites revealing promising EMI applications. BP can be prepared by vacuum filtration of a suspended dispersion of randomly distributed carbon nanotube, showing a wide range of applications, including EMI shielding material [18]. Furthermore, as discussed by several authors [19][20][21], the high electrical conductivity of the BP (50-6000 S/m) can result in high SE properties. Jia et al. [22] prepared a carbon nanotube buckypaper by vacuum filtration with an average thickness of around 100 lm. The CNTs film reached high levels of electrical conductivity (* 3600 S/m) with suitable EMI SE performance of 35.6 dB in X-band . Qianshan Xia and collaborators [23] prepared buckypaper/polyacrylonitrile (BP/PAN) films based on electrospun and vacuum pressurized filtration methods. The BP/PAN films exhibited higher EMI SE values (65 dB) in Ku-band (12-18 GHz) than pristine BP (34.3-42.9 dB), with reflection being the dominant shielding mechanism. Zi Ping Wu et al. [24] prepared a free-standing CNTs mat using a floating catalyst chemical vapor deposition method, with a thickness of 1 lm and high electrical conductivity. The high EMI SE of 40.4-60.3 dB was achieved at the frequency range of 1-18 GHz, demonstrating the CNTs mat has potential applications as a shielding material.
Conventional fiber-reinforced polymer composites have been paid great relevance in the last decades once they provide high stiffness and strength associated with lightweight features [25,26]. This combination of properties makes these traditional composites particularly attractive for aerospace applications, where mechanical request and low density are a top priority. On the other hand, the potential applications of these structures can be underused in some cases. For instance, EMI demands once glass fiber polymer composites are non-conductive materials. As already discussed in this work, electromagnetic pollution is commonly associated with the malfunction of electronic devices (smartphones, laptops, radio, among others) and it should be treated as a severe concern because it harshly affects human health. In this perspective, microwave absorbing materials (MAMs) have received special attention from the scientific community over the last decade [27][28][29]. Also, they can be used not only for EM protection but also for stealth technology demands. Based on what was said, the incorporation of carbon materials, such as carbon nanotubes into glass fiber-reinforced polymer composites, could be an option to obtain electrically conductive materials for high EMI demands and also opening a new perspective for multifunctional structures.
Carbon nanotube buckypaper and their composites have recently received special attention from academic and industrial fields, appearing possibly as the next generation of EMI shielding materials due to their efficient microwave attenuation capacity and low density. A material that combines flexibility and tailored conductivity could be applied as shielding clothes keep saving the security of data information and people's health.
In this contribution, a flexible carbon nanotube buckypaper was prepared by vacuum filtration. The CNTs films were incorporated between the glass fiber/epoxy (GF/EP) layers, and the system cured in a hot press. Meanwhile, the EMI shielding properties of all samples were characterized over the X-band frequency range.

Materials
Multi-walled carbon nanotubes (MWCNTs), synthesized by chemical vapor deposition (CVD) method, were supplied by Bayer (Baytubes C150P). The average diameter of tubes is in the range of 10 nm and length 10-20 lm.
Glass fiber/epoxy layers were prepared by manual lamination using two main components: Epoxy resin (AralditeÒ GY 279 BR -Huntsman) attending the resin/harder ratio of 100:6 and glass fiber (Owens Corning-TLX0750), plain weave with areal fiber density of 600 g/m 2 .

Preparation of buckypapers
Vacuum filtration was used to prepare the buckypapers, and the procedure was described in detail in our previous work [30]. MWCNTs (50-200 mg) were ultrasonically dispersed in deionized water with 1 wt% of Triton X-100Ò surfactant for 40 min to obtain a uniform suspension. Before filtering, four layers of polyacrylonitrile nanofibers (PANF) were stacked over a nylonÒ microporous filter (0.45 lm), aiming for improved flexibility of the CNTs film as demonstrated in our previous work [31]. The CNTs/ water/surfactant suspension was filtered over the PANF/nylonÒ membrane under vacuum conditions. The obtained buckypaper was rinsed with acetone and isopropyl alcohol to remove the residual surfactant and then immersed into N,N-dimethylformamide (DMF) bath (50°C) several times to dissolve the PANF. The obtained buckypaper was dried at 80°C in a vacuum oven before any further test and use.

Preparation of glass fiber/epoxy/ buckypaper laminates
Glass fiber/epoxy layers were manually stacked over (70 9 70) mm 2 steel mold so that BPs were introduced between the prepregs layers forming a sandwich structure with a final thickness of 3 mm.
A KaptonÒ film was employed as a release agent, and the system was cured in a hot press at 150°C for two hours under 0.5 MPa. The position and the number of BP inserted in the laminate were deeply studied, generating several configurations described in Fig. 1.

Characterizations
The crystalline structure of the MWCNTs was evaluated at room temperature by wide-angle X-ray diffraction (XRD) (Rigaku X-ray diffractometer model Ultima IV) using Cu Ka radiation (k = 1.54178 Å ), at a scanning rate of 0.01°(fast detection mode) and a scan speed of 10°/min. Diffractograms were obtained in the angular region of 2h = 10-60°. X-ray photoelectron spectroscopy (XPS) analyses were performed with Axis Ultra DLD spectrometer using a monochromatized Al X-ray source (1486.6 eV) for both MWCNTs and BP. Casa XPS software was used to analyze the obtained data.
Morphological characteristics of BP were analyzed by scanning electron microscopy (SEM) (FEI Inspect S50) at an acceleration voltage of 20 kV. This equipment is assisted with the energy-dispersive X-ray spectroscopy (EDS). Before observation, the samples were sputtered with a thin golden layer under vacuum to avoid charging during electron irradiation, except those used for the EDS analyses. High-resolution transmission electron microscopy (HRTEM) (FEI Tecnai G2F20) was employed to evidence the morphology of the individual CNT at an acceleration voltage of 200 kV.
A two-point probe device (Agilent B2912A) was used to measure the DC conductivity of the samples at room temperature. The conductivity q v can be calculated based on Eq. 1: where A is the area of the laminate, t is thickness, and R v is the volume resistance of the specimen, respectively. The electromagnetic interference shielding performance of the composites was measured with a vector network analyzer from Agilent Technologies (PNA-L N5230C). The equipment has 4 signal capture ports, being used two, as demonstrated in Fig. 2a. The measurements were carried out under the frequency range of 8.2-12.4 GHz (X-band) with the assistance of a high precision rectangular waveguide from Agilent Technologies, model 00281-60016 (Fig. 2b). The magnitude of the scattering parameters associated with reflection (S 11 ) and transmission (S 22 ) was obtained placing the sample holder with the specimen attached between the waveguide and the port-1 coupler. The complex parameters (e and l) were calculated using the software 85071E (Agilent Technologies) based on the Nicolson Ross model.

Morphology of the buckypaper
The morphology of the prepared buckypaper in this work was investigated by SEM, as shown in Fig. 3a, b. The CNTs film has a thickness of about 50-100 lm, a bulk density of * 0.29 g/cm 3 , and its surface consists of randomly distributed MWCNTs revealing a porous network with no local aggregation of the tubes. Moreover, the van der Waals forces between the tubes make the MWCNTs bundles very close to each other, resulting in a denser film [32]. Figure 3c presents a different region of the buckypaper sample and Fig. 3d displays the cross-section view of the CNTs film showing tunnels/channels that were formed from the dissolution of the PANF by DMF solvent. As discussed in our previous work [31], the residual mass of PANF in the MWCNT network leads to improved flexibility to the BP, allowing it to be bent almost 180°C with no breaking. Also, this new feature made the lamination of the composites easier, thus resulting in a flexible macroporous film. Figure 3e, f shows the HRTEM image of the buckypaper. As can be observed, there are long entangled bundles with a diameter of 10-20 nm with no catalyst particles that are common from the synthesis process of the MWCNTs. Also, small fragments of the tubes can be visualized as well (Fig. 3e), indicating the CNTs nanofilaments were possibly damaged during the preparation of the buckypaper and/or during the functionalization process of the MWCNTs.  Fig. 4a. It can be observed that the MWCNTs sample has two broad diffractions peaks centered at about 25.9°and 43.5°, which can be ascribed to the characteristic planes (002) and (100) of graphitic carbons [33]. XPS analyses were used to investigate the incorporation of functional groups on the MWCNTs surface ( Fig. 4b) and in the prepared buckypaper (Fig. 4c). According to the full scan XPS spectra of the MWCNTs and the BP, both samples show the obvious commons peaks of C1s and O1s at 287 eV and 532 eV, respectively. C1s can be attributed to the C=O, C-O, and C-C [34], whereas O1s stand for -C=O, C-O, and -O-H bonds [35]. However, it is essential to point out the N1s spectra at 400 eV for the BP sample. The inset of Fig. 4c shows two predominant peaks around 400.3 eV and 401.5 eV, which can be assigned to N-H/N-O and C-N bonds, respectively. This result is directly associated with the residue of the PANF in the BP structure, as evidenced by SEM in Fig. 3c, d. As discussed in our previous work [31], after the dissolution of PANF by DMF solvent, the BP showed tunnels (channels) spread over the CNTs network, making easy the impregnation of the buckypaper by epoxy resin during the preparation of the laminates. Also, the EDS image of Fig. 4d shows traces of nitrogen in the BP sample (3.89 wt%), which confirms the residual amount of PANF in the CNTs film structure.

Electrical conductivity (DC) and EMI SE of the buckypaper
As widely discussed by several authors [36][37][38][39][40], the shielding performance is directly associated with the electrical properties of the material. The electrical conductivity of the BP measured in this work was 1982 S/m with a density of 0.29 g/cm 3 , which is slightly lower than other BP works available in the literature. For instance, Lu et al. [41] prepared vacuum filtered MWCNT BP with a density of 0.76 g/ cm 3 and reached an electrical conductivity of 3300 S/m, whereas Hu et al. [42] obtained MWCNT BP with density and conductivity of 0.45 g/cm 3 and 3800 S/m, respectively. The electrical conductivity of the BP is directly associated with the network formed by the physical entanglement of the tubes. The interconnected structure may offer charge carrier channels providing innumerous conductive pathways in the BP, which results in high electrical conductivity [43]. However, the use of PANF obstructs the dense network of the buckypaper (Fig. 3d) and consequently results in a lower density of the BP, thus reducing the formed conductive pathways and the electrical conductivity of the CNTs film. Moreover, it is important to mention that PANF residual mass present in the buckypaper network, especially between the tube's contacts, impacted the lower electrical properties obtained in this work. As detailed in our previous work [31], the residual mass of PANF after the removing process possibly isolated the contacts between the tubes, thus decreasing the electrical conductivity of the prepared buckypaper. The power balance of the BP can be studied by the scattering parameters, (S 11 and S 22 ) as introduced in Sect. 2.3. These S parameters can be associated with coefficients of shielding mechanisms as R ¼ jS 11 j 2 (reflection) and T ¼ jS 21 j 2 (transmission) [40,44]. The absorption coefficient (A) can be calculated from R and T for any incident EM wave, as follows:  Figure 5a shows the power balance (transmission, reflection, and absorption coefficients) as a function of frequency for the prepared buckypaper. As can be seen, all the calculated parameters are almost constant over the frequency range, so that 77% of the incident EM wave was reflected. The reflected power can be associated with the electrical conductivity of the shielding material that generates an impedance mismatch between air/BP. The small diameter and the high surface area of the tubes facilitate the reflection of the EM waves so that a more reflective surface occurs [45]. On the other hand, the polarization created among tube-tube contacts causes losses in the BP, revealing an absorption coefficient (A) of 0.19. Furthermore, the transmitted fraction of the EM wave power is close to 0.05, which can be attributed to the porosity of the CNTs film, as previously shown by SEM in Fig. 3d.
EMI shielding effectiveness can be clarified in which the propagating EM wave is blocked by employing conductive materials [46]. The total shielding effectiveness (SE T ) can be expressed in terms of the transmitted and reflected power of the EM wave, as demonstrated by Eq. 3: where P input and P output are the power of the incident and transmitted EM waves, respectively. When the EM wave interacts with a reflection shield, reflection, absorption, and multiple reflections can occur simultaneously [45]. Therefore, the total shielding effectiveness can be expressed by three main components: the primary reflection of the wave (SE R ), internal absorption (SE A ) and, secondary multiple reflections (SE M ) inside the shielding material [45,46]. The EMI SE can be expressed in terms of the scattering parameters (S 11 and S 22 ) as follows: where SE R = À 10log 1 À S 2 11 À Á ; SE A = À 10log S 2 21

1ÀS 2 11
; and SE M dB ð Þ ¼ À20 log 1 À 10 À SE A =10 ð Þ : Figure 5b shows the total shielding efficiency of the buckypaper. As can be seen, the SET value is * 22 dB, which means that the buckypaper blocks more than 99% of the incident EM wave. Also, the EMI result is in the range required for computers and electronic devices [47]. Compared to other works available in the literature, the EMI performance of the buckypaper prepared here is below average. Hu et al.  [42] achieved an EMI SE of 31.2 dB, whereas Wu et al. [48] presented a super high EMI SE of 61-67 dB for the CNTs film. The EMI performance obtained in this work can be attributed to the lower electrical conductivity and density of the BP, respectively. However, the buckypaper density demands special attention here. For instance, Zhang et al. [49] prepared both solid and foam PMMA/Fe 3 O 4 @MWCNT nanocomposites showing densities of 1.25 and 0.38 g/cm 3 , respectively. The solid and the foam composite presented an EMI SE of around 25 and 13 dB, respectively, which confirms the importance of the density in the EMI SE properties of the composites. It is worth mentioning that the buckypaper density is 0.29 g/cm 3 , a lower value than that obtained by Wu (0.45 g/cm 3 ). Therefore, the shielding values at such low-density materials could be more appropriately described in terms of specific EMI SE (EMI SE divided by density). The highly porous buckypaper prepared in this work displays a specific EMI SE value of 76 dB.cm 3 /g, which is similar to other buckypaper studies described above.
The reflection, absorption, and multiple reflection components can elucidate a complete understanding of the shielding mechanism of the BP. SE R and SE A components were calculated according to Eq. 4 and plotted in Fig. 5b. As can be seen, SE R values range around 8.4 dB, which is considered a relevant fraction of the total EMI SE (* 22 dB). The CNTs film has mobile charge carriers with high mobility that interact with the incident EM waves, providing several paths to transport the mass of electrons freely. Consequently, it favors a conductive interface between the tubes resulting in more interactions with the incident EM waves, thus improving the SE R and the total EMI SE values [50]. It is worth mentioning that the impedance of the conductive materials is small compared to the free space, resulting in a high impedance mismatch that conducts to reflective behavior of the incident waves [42]. The absorption component plays a crucial role in the shielding behavior of the BP once the SE A was superior to SE R , showing a value of around 13.6 dB. The mechanisms related to the absorption are associated with the MWCNTs entangled network of the buckypaper. The interaction of the high-frequency EM waves can stimulate delocalized p-electrons of MWCNTs for migrating, tunneling, and hopping mechanisms, which result in high ohmic loss and the conversion of the EM energy into heat [51]. Besides, when EM waves impinge on BP's surface, the polarization process in the MWCNTs network will lead to polarization losses [52]. The combination of these two factors was beneficial to enhance the absorption of the CNTs film. It is worth pointing out that the multiple reflections had a minor contribution to the EMI SE properties of the buckypaper. Its porous structure scattered and reflected the EM waves several times, dissipating them as heat, contributing to improving the shielding properties by absorption. Figure 5c presents the schematic diagram of the EM shielding mechanism of the buckypaper.
The skin depth (d) can be used to clarify the ability of BP to block electromagnetic waves. High-frequency EM waves only penetrate near the surface region of shielding material. This phenomenon is known as skin effect, and the field drops exponentially with depth [45]. The depth at which drops to 1/ e (e is the Neper number) of the incident value can be expressed as follows: where f, l, t, and r T are the wave frequency, permeability, thickness, and total electrical conductivity of the shielding material, respectively. As can be seen, d decreases with increasing frequency (Fig. 5d) and ranged from 0.11 to 0.09 mm. Interestingly, the d values are higher than the thickness of the BP. According to Eq. 5, the skin depth decreases with increasing conductivity; thus, it was expected that the use of carbon nanotube with high electrical conductivity would result in lower values of d and high EMI SE performance. As already discussed in this work, the measured conductivity of the buckypaper was below the average compared to other BP works available in the literature. This result can be attributed to the PANF residual mass in the BP structure. Hence, the use of PANF during the preparation of the buckypaper was twofold. Firstly, it provided great flexibility to the CNTs film, improving its handling and, consequently, the composites lamination. On the other hand, the residual mass of PANF in the CNTs network and the cavities/channels formed reduced the electrical conductivity of the BP, affecting the EMI SE properties detrimentally and, consequently, increasing the skin depth.

Power balance of GF/EP/BP composites
The GF/EP/BP composites with several configurations, as schematized in Fig. 1, were prepared by hot press from the stacking of GF/EP layers and BP films. All the prepared composites samples (GF/EP and GF/EP/BP) presented density values of around * 2.0 g/cm 3 . Figure 6a-f shows the reflection, absorption, and transmission coefficients for all BP composites prepared in the X-band frequency. As can be seen, for configuration 100 almost 90% of the EM wave power was reflected, 10% was absorbed, and 1-2% transmitted. It is worth noting that all three coefficients were nearly constant over the frequency range. The addition of one more BP film (configuration 110) led to similar results; however, small differences can be detected in Fig. 6b

EMI SE of GF/EP/BP composites
EMI SE results are closely associated with electrical conductivity, i.e., highly conductor materials will provide excellent EMI performance. Figure 7a-c shows the EMI results of the GF/EP/BP laminates in the X-band. The 100 laminate (Fig. 7a) (Fig. 7b) showed an ascending shielding Fig. 6 Power balance (reflection, transmission, and absorption components) of all composite samples over the X-band trend from 10 to 12.4 GHz, which contrasts with the previous architecture (100 and 110) that showed a tiny descending behavior. The most significant EMI results were presented by 101 and 111 laminates, as visualized in Fig. 7c. Both samples reached promising EMI performance of around 50-60 dB, with a short advantage for the GF/EP/BP 111 composite, which means at least 99.999% of the incident EM wave was blocked by both samples.
To fully understand the shielding mechanism in the GF/EP/BP laminates, it is necessary to evaluate the absorption (SE A ) and the reflection (SE R ) components to the total shielding effectiveness. It is worth noting that SEM is essential for porous materials, but it can be neglected for a thick absorbing shield due to the high value of the SEA. Therefore, when SE A-[ 10 dB, SE M can be safely ignored [37]. Figure 7d presents the SEA, SE R , and SE M for all samples prepared at 10 GHz. To further clarify the differences in the shielding properties, it is essential to study carefully how the buckypaper position in the composite can affect/change the shielding mechanism from reflection to absorption. In the first studied sample (GF/EP/BP 100 ), most of the EM wave is reflected, revealing a SE A /SE T ratio of 0.43. This behavior can be attributed to the impedance mismatch between the shield (BP) and the air, which means the SER is the dominant shielding mechanism. The addition of a second buckypaper layer in the mid-plane of the composite revealed an improvement towards absorption, showing an enhanced SE A /SE T of * 0.49. A short modification in the architecture of the composite (001 and 011) dramatically changed the shielding mechanism of the material. GF/EP/BP 001 composite demonstrated an absorption shielding mechanism showing an improved SE A /SE T ratio of 0.75. This behavior can be attributed to the interfacial polarization in which the accumulation of charges at the GF/EP/BP interfaces contributes to improving the conductivity so that the EMI shielding performance is boosted via absorption shielding mechanism [53]. The addition of a second buckypaper layer in the mid-plane of the composite (GF/EP/BP 011 ) significantly improved the absorption once SE A dominated at least 97% of the total shielding at 10 GHz. The EM waves can penetrate easily in the GF/EP/BP 011 composite since fewer EM waves are reflected on the surface. The incident waves were possibly trapped in the highly porous buckypaper, which enhanced the multiple reflections and prevented them from escaping the sample. As a result, the GF/EP/BP 011 composite exhibited the lower SE R values of all studied architectures. It is worth mentioning that the EMI SE results match with the power balance data presented previously in this work. The last two composites (101 and 111) showed the highest EMI SE values for all the configurations studied in this work, with a dominant absorption shielding mechanism. However, the absorption component of the sample GF/EP/BP 101 had a strong dependence on the insulating layer, showing higher EMI results than GF/EP/BP 111 at high frequencies. Figure 7c showed that SE R for both samples was practically the same over the X-band, whereas SE A101 was higher than SE A111 from 11 to 12 GHz. The introduction of two wave-transmitting layers of 1.5 mm (GF/EP/ BP 111 ) enhanced the SEA value to 37 dB over the studied frequency range. As already discussed in this work, the EM waves were possibly confined within the porous structure of the BP, which avoid them from escaping the sample, thus enhancing the absorption performance of the composite. On the other hand, a wave-transmitting layer of 3.0 mm (GF/EP/BP 101 ) slightly reduced the SE performance of the material but for frequencies from 11-12 GHz, an apparent peak was observed followed by ascending absorption performance. This phenomenon suggests that the increase of SE A by increasing the wave-transmitting layer can be associated with the constructive interference of the reflected electromagnetic waves in phase since they were absorbed by the BP structure and dissipated as heat. As discussed in the literature [42,47], the EM waves can be reflected several times between two BP layers, which leads to an enhancement of the shielding performance due to the constructive interference of the reflected EM waves in phase. As a result, the EMI SE values of GF/EP/BP 101 composite can reach 77 dB at 11.9 GHz, showing a robust shielding capacity. Table 1 presents the EMI SE results obtained in this work compared to other similar papers available in the literature.

Electrical conductivity, skin depth, and attenuation constant of GF/EP/BP composites
As widely discussed in this work, electrical conductivity has a significant contribution to the EMI SE results. Therefore, it should be carefully investigated for a solid comprehension of the shielding mechanisms. The r is composed of a frequency-dependent (AC) and independent (DC) components, as demonstrated by Eqs. 6 and 7: where f is the frequency (Hz), and e 0 is the permittivity of the free space (e 0 = 8.854 9 10 -12 F/m). Table 2 shows the DC and AC conductivities for all BP composites studied in this work. As can be seen, r AC presents an essential contribution to total conductivity, which explains the enhanced EMI SE results. However, it is crucial to add that r AC is directly affected by the permittivity of the samples. Permittivity is a compelling parameter to understand the mechanism of microwave absorption, and it can be described as follows: where e 0 and e 00 are the real and the imaginary components of permittivity. The real permittivity describes the electric energy storage, and the imaginary part represents the dielectric losses due to relaxation and polarization, resulting in the dissipation of microwaves as heat [58]. Thus, it is possible to assume the incorporated BP enhanced the dielectric losses of the composites, improving their electrical conductivity, and consequently, their shielding properties. Also, the mechanisms that explain the dielectric losses in BP composites should be investigated carefully. The interface between the buckypaper, epoxy, and glass fibers is responsible for interfacial polarization, contributing to dielectric losses. Since BP and EP/GF layers possess different dielectric properties, the accumulation of charges at the interface will likely occur, improving the conductivity. Such polarization contributes towards shielding performance via a secondary shielding mechanism (absorption) [59,60]. The skin depth (d) and the attenuation constant (a) for all studied composites are displayed in Table 2. Interestingly, d decreases with increasing r, which can be associated with the large concentration of charge carriers, facilitating the interaction with the incident EM at the composite interface. As already discussed in this work, an enhancement in the conductivity will result in strong EMI performance. The attenuation constant also provides valuable information about the capability of the composite to attenuate microwave radiation, as demonstrated by Eq. 9 [61]: l 00 e 00 À l 0 e 0 ð Þþ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi l 00 e 00 À l 0 e 0 ð Þ 2 þ l 0 e 00 À l 00 e 0 ð Þ 2 q r ð9Þ where f is the frequency, c is the velocity of the EM in the free space, l 0 and l 00 are the real and the imaginary components of permeability. As can be seen, 101 and 111 showed the highest attenuation constant, corresponding thus to the ideal attenuation effect for the incident EM wave. However, low absorption capacity can occur as a result of both high and low electrical conductivity. Also, high permittivity values are not ideal for the impedance matching once more reflections of the wave may occur, conducting to reduced absorption capacity. In this work, a highly porous carbon nanotube buckypaper was incorporated in glass fiber-reinforced epoxy composites (electrically isolating material). Both materials combined led to an optimized conductivity, improved impedance matching, and therefore high microwave capacity.

Reflection loss of GF/EP/BP composites
EMI shielding structures and microwave absorbing materials (MAMs) can effectively prevent the escape of undesired EM radiation. As previously discussed in this work, for shielding demands both absorption and reflection components contribute to the EMI SE of the BP composites. However, the reflected waves cannot be ignored and be still detrimental to electronic devices and human health. Therefore, the ideal material for effectively attenuating the EM waves should be absorption-dominant once the MAMs will attenuate the EM waves and transform them into heat [62,63]. The microwave properties of the GF/EP/BP laminates can be evaluated by the reflection loss (RL) based on the metal back-panel and the generalized transmission line theory. RL value can be calculated using complex electromagnetic parameters and the absorber thickness by the following equations [64][65][66]: where Z in is the input characteristic impedance, Z 0 is the characteristic impedance of the free space, l 0 is the permeability of the free space, e 0 is the dielectric constant free space, c is the velocity of light, e r ; and l r are the relative permittivity and permeability, respectively. The RL curves as a function of the frequency of GF/EP/BP composites over the X-band (8.2-12.4 GHz) are presented in Fig. 8a. As can be seen, the microwave absorption properties can be manipulated by changing the BP position in the laminate (in other words, varying the thickness of the wave-transmitting layer) and the number of the CNTs films inserted between the GF/EP prepreg. Only GF/EP/BP 011 sample displays strong attenuation performance, revealing a minimum RL value of -21.16 dB at 10.37 GHz, which means 99% of the incident EM wave was attenuated. Besides, the absorption bandwidth with reflection loss below -10 dB was 2.76 GHz (from 9.21 to 11.97 GHz). The improved attenuation of the GF/EP/BP 011 can be ascribed to the impedance match condition.
According to Eqs. 11 and 12, the ideal impedance matching condition is achieved when the Z in Z 0 gets closer to 1, which indicates all the incident EM waves can enter into the material and be absorbed. From the reflection loss curves, it is clear that GF/EP/BP 011 had better impedance matching than the other samples, suggesting the attenuation performance is strongly dependent on their electrical conductivity. As discussed in this work, high conductivity conducts more reflections of the EM wave resulting in poor attenuation behavior. As can be noted in Table 2, GF/EP/BP 011 sample presented one of the lowest total conductivity results, which was favorable to achieve the ideal impedance matching in the composite. Figure 8b, c shows a comparison between simulated and experimental results of GF/EP/BP 011 composite in the X-band. In addition to experimental work, a computational simulation study was carried out for a comparative purpose, aiming to design a robust absorbing material with a smaller thickness possible. The ideal thickness and the wave-transmitting layer of the composite were calculated through a commercial microwave software, FEKOÒ, based on the complex parameters (l r and e r ) of BP and GF/EP composite. The simulation results revealed a maximum RL value of -20.36 dB at the frequency of 9.74 GHz with a -10 dB absorption bandwidth of

Conclusions
Multi-walled carbon nanotube buckypaper was prepared by vacuum filtration showing great flexibility, low density (0.29 g/cm 3 ), and adequate conductivity. The correspondent EMI SE of the buckypaper was * 22 dB, whereas the specific EMI SE was as high as 76 dB cm 3 /g in the X-band. The CNTs film was incorporated between the GF/EP layers and the laminate cured in a hot press machine. Six different architectures were proposed in this work based on the number of the BP inserted and the length of the wave-transmitting layer. GF/EP/BP 111 and GF/EP/ BP 101 composites presented a remarkable EMI shielding performance of around 50-60 dB. Also, configuration 101 surpassed this capacity reaching 77 dB in the high frequencies (11)(12). GF/EP/ BP 011 laminate showed the most exciting results being applied both as shielding and absorbing material. The mentioned sample achieved an EMI SE of 16.07 dB, close to the shielding level (* 20 dB) for commercial applications. Also, it reached a minimum RL value of -21.36 dB, qualifying it to be used as a robust absorbing material. Based on the detailed study, it can be concluded that GF/EP/BP composites are suitable for both shielding and absorbing demands, providing some advantages such as lightweight, a simple manufacturing process, and multifunctional characteristics for applications in electronics and aerospace.