Electromagnetic interference shielding in lightweight carbon xerogels

With the increasing use of high-frequency electronic and wireless devices, electromagnetic interference (EMI) has become a growing concern due to its potential impact on both electronic devices and human health. In this study, we demonstrated the performance of lightweight, electrically conducting 3D resorcinol-formaldehyde carbon xerogels, of 2.4 mm thickness, as an EMI shieldin the frequency range of 10–15 GHz (X-Ku band). The brittle carbon xerogels revealed complex porous structures with irregularly shaped pores that were randomly distributed. Electrochemical characterization revealed that the material behaved as an electrical double-layer capacitor. The carbon xerogels displayed reflection-dominated (∼ 84%) shielding behavior, with a total EMI shielding effectiveness (SE) value of ∼ 61 dB. The absorption process also contributed (∼ 16%) to the total SE. This behavior is attributed to the carbon xerogels' complex porous network, which effectively suppresses EM waves.


Introduction
Electromagnetic pollution resulting from the abundant use of electronic wireless equipment has become a serious concern in our society [1][2][3][4][5][6][7][8] and can often lead to the malfunctioning of sensitive electronic devices and the risk of severe human health issues [9,10]. Electromagnetic interference (EMI) shielding is essential to mitigate electromagnetic pollution.
EMI shielding results from the contribution of reflection, absorption, and multiple internal reflections. The total EMI effectiveness (SE) was related to the electrical conductivity and thickness of the shielding material [11] using the Simon formalism equation SE T (dB) = 50 + 10log(σ/f)+1.7t√(σf), where σ, t, and f represent the electrical conductivity (S/m), thickness (m), and frequency (Hz), respectively [12].
Three ranges of EMI shielding effectiveness can be identified: poor for SE 10 dB, acceptable, between 10 and 30 dB, and beyond acceptable, at SE 30 dB [13]. In aerospace applications, the requirements for EMI shielding generally vary between 40 dB and 120 dB at frequencies above 1 kHz [14]. Metals and metal-based composites are often used for electromagnetic shielding owing to their high electrical conductivity. However, they have some drawbacks, including high density, high-temperature processing, corrosion susceptibility, and relatively high cost [15]. A highly conducting (σ ∼ 8570 S cm −1 ) two-dimensional (2D) transition metal carbide, titanium carbide (Ti 3 C 2 T x ) MXene, with a thickness of ∼ 5 μm, showed an EMI shielding effectiveness (SE) of ∼ 55 dB with a significant contribution from reflection [16]. A highly reflection-dominated tunable EMI SE was found in a bilayer MXene/cellulose paper sheet between 35-60 dB [17]. Interestingly, MXene aerogels exhibited tunable reflection-to-absorption EMI shielding performance with a maximum EMI SE of ∼ 70 dB [18]. Recently, xerogel materials have attracted considerable attention for EMI shielding applications owing to their lightweight and efficient EM wave-absorbing properties. For example, 20 wt% graphitized reduced graphene oxide in a silica xerogel exhibited temperature-dependent EMI, attributable to hopping conductivity and dipole polarization. The graphene-silica xerogel demonstrated a thermally tuned absorption-dominated EMI shielding of up to ∼40 dB [19]. Our recent work reported a compressive strain-induced enhancement of EMI shielding properties (∼ 60%) in porous conducting polymer sponges, owing to the collapse of the porous structure [20]. Although there are several reports on new EMI shielding materials, only a few have explored electromagnetic multifunctional materials. Zhu et. reported a needle-like Co 3 O 4 /C array architecture with excellent EMI shielding, thermal insulation, and energy storage capabilities [21]. Zhang et al investigated the combination of highly effective EMI shielding with wearable electronics [22].
In this work, we demonstrated the EMI shielding properties in the frequency range of 10-15 GHz of lightweight, 3D electrically conducting carbon xerogels based on acid-catalyzed resorcinol-formaldehyde (RF). The carbon xerogels exhibited porous structures with randomly distributed pores and irregular shapes. The porous structures of the carbon xerogels lead to an exceptional total EMI shielding effectiveness (SE) of ∼ 61 dB, owing to the dominant reflection process. The porous carbon xerogel also acted as an electrical double-layer capacitor, with a specific capacitance of ∼ 3.5 F g −1 .

Preparation of carbon xerogels
Carbon xerogels were prepared from a resorcinol-formaldehyde (RF) mixture. First, a gel was prepared by mixing resorcinol, formaldehyde, and acetic acid at a molar ratio of 15:30:1. A solution of 28 g resorcinol in 34 g water was prepared under ultrasonication, followed by the addition of 41 g formaldehyde and 1 g acetic acid. The mixture was allowed to gel in a glass beaker for 24 h at room temperature before increasing the temperature stepwise up to 40°C and 60°C for 24 h, and then at 80°C for 72 h.
The gel was then allowed to cool to room temperature in an oven for another 72 h. The solidified cylindrical blocks were sliced into discs using a dicing saw (Advanced Dicing Technology, model ADT 7100 Provectus). To obtain the xerogel, sliced gelled discs were pyrolyzed in a tube furnace (Carbolite tubular STF) under argon at 900°C for 3 h at a rate of 2°C min −1 . The slow increase in temperature ensured that the pores in the xerogel were preserved. The xerogel disks were polished using a 6 μm grit diamond polishing pad to obtain smooth parallel-faced surfaces.

Morphology characterization
The surface and cross-sectional morphologies of the carbon xerogels were examined using a tabletop scanning electron microscope (SEM) (Hitachi TM3030) in mixed mode [secondary electron (SE) and backscattered secondary electron (BSE) mode].
The samples were also studied by 3D x-ray microscopy/micro-computed tomography (CT) using a ZEISS Xradia 520 Versa x-ray CT system.

Density and electrical conductivity measurements
The weight of the carbon xerogel having the dimension of 2 cm length, 1.8 cm breadth, and 0.24 cm thickness was measured using a sensitive analytical balance (model #: AL 104, Mettler Toledo, resolution: 10 -4 g).
The electrical conductivities of the carbon xerogels were measured using the two-probe method. The carbon xerogel samples were connected to crocodile clip electrodes wrapped with copper tape and connected to an electrical source measuring unit (Agilent B2902A). The electrical resistance (R) was evaluated from the slope of the linear current-voltage curve. The electrical conductivity (σ) was measured using the formula s r where ρ represents the resistivity, A is the cross-sectional area of the carbon xerogel, and l is the distance between the copper electrodes.

Mechanical properties
The mechanical properties (Young's modulus) of the carbon xerogels were investigated using a multiaxial mechanical tester (Mach-1 model v500csst, Biomomentum, Canada). First, a rectangular piece of carbon xerogel [dimensions: 2 cm (l) × 2 cm (b) × 0.244 cm (t)] was mounted on the sample stage of the multiaxial mechanical tester. A vertical force was then applied using a stainless-steel spherical indenter with a 6.35 mm diameter attached to a moving load cell (maximum force capacity of 70 N) on the sample at a velocity of 0.2 mm s −1 .
The multiaxial mechanical tester was calibrated in multiple steps to confirm that no force was experienced by the sample when the indenter touched its surface. The penetration depth of the carbon xerogel was measured as the load was applied. Young's modulus was calculated using the following equation derived from the Hayes model [23, 24]: where P is the recorded reaction force, v is the Poisson's ratio of the material, ω_0 is the indenter displacement, R is the radius of the indenter, and Χ and Κ are constants based on the ratio of the material thickness and indenter radius.

Electrochemical measurements
Cyclic voltammetry (CV) measurements were carried out with a potentiostat (Biologic SP 300) in a 0.1 M NaCl aqueous solution in a three-electrode cell. A Pt coil and Ag/AgCl were used as counter and reference electrodes, respectively. The carbon xerogel working electrode was connected to the electrochemical workstation via Cu wires attached to the sample edges with Ag paste (EG 8020, AI Technology Inc.) cured at 125°C for 3 h. CV curves were measured between −0.8 to 0.8 V with a scan rate of 50 mV s −1 . The cyclic stability of the CV measurements was evaluated for up to five cycles. The specific capacitance (C SC ) was calculated using the formula C SC = Q/(m × ΔV), where Q represents the charge, m is the mass of the carbon xerogel, and ΔV is the voltage window [25]. As Q can be written as the product of the current (I) and time (t), the integral of the I versus t curve represents the value of Q.

EMI shielding measurements
The EMI shielding effectiveness (SE) was measured using a vector network analyzer (Keysight PNA-X N5247B) in the range of 10-15 GHz (X-Ku band) using a WR-75 waveguide. Prior to the measurement, a two-port TRL (Thru-Reflect-Line) calibration was conducted between 10 and 15 GHz. This frequency range was selected to cover both the X-band (8-12 GHz), which is used for military and aerospace applications and the Ku-band (12-18 GHz), which is used for satellite communications. Next, carbon xerogel samples of ∼ 2.4 mm thickness completely covering the rectangular opening of the waveguide (0.750 × 0.375 in) were tightly clamped with screws. S-parameters were recorded and converted from decibels to linear values. The coefficients involved in EMI shielding are absorption (A), reflection (R), and transmission (T). They can be determined using linear S-parameters and the following equations [26]: The total EMI SE ( ) SE T in decibels (dB) of the carbon xerogels was expressed as the logarithmic ratio of the incident (P i ) and transmitted power (P t ).
The final equations connecting SE with reflection and absorption are [27], To account for the geometry of the sample, we evaluated the specific shielding effectiveness (SSE = SE/ρ, where ρ is the density of the sample, in g.cm −3 ) and the specific shielding effectiveness per unit thickness (SSE/t, where t is the sample thickness in cm) [28].
Finally, EMI efficiency was computed using the following equation [14]:
Error bars corresponding to the standard deviation were obtained from measurements on three different identical samples. The SEM images of the surface and cross-section of the carbon xerogel showed a complex 3D porous structure with anisotropic features (figures 1(a) and (b)). The pores had irregular geometrical shapes and were randomly distributed, making it difficult to estimate their exact size. We estimated the porosity of the carbon xerogel (i.e., both surface and cross-section) using ImageJ software (National Institute of Health). First, the color contrast between the porous and non-porous regions of the SEM images of the carbon xerogel was determined, and the area fraction corresponding to the porous region was calculated. The average surface and cross-sectional porosities were estimated to be (30 ± 0.8) % and (8 ± 0.3) %, respectively (see Supplementary  Information, figure S1). X-Ray Computed Tomography (CT) was performed to visualize the internal morphology of the material ( figure 1(c)). The computed 3-dimensional structure confirmed the existence of a porous structure inside the carbon xerogel. The pores were irregular in shape and size, as observed in the SEM images. Figure 2(a) shows a plot of the force as a function of the penetration depth of the carbon xerogel. The experiment suggests that there is a relatively sharp increase in the amplitude of the force as the indenter advances across the carbon xerogel thickness in the elastic region without significant plastic deformation. This behavior indicates the brittleness of the carbon xerogel, which can break suddenly under external loading without necking, unlike ductile materials. Experimentally, a maximum depth of ∼ 0.05 mm was realized, at which the spherical indenter applied a maximum external force of ∼ 20 N. The calculated average value of Young's modulus was found to be 384.14 ± 21.5 MPa, which is in sufficient agreement with the literature data, ranging between 80 MPa and 1 GPa [32,33]. Young's modulus value suggests that the carbon xerogel can withstand prolonged mechanical stress owing to its robustness while exhibiting repeatable and reproducible EMI shielding properties. Mechanically robust EMI shielding materials are particularly useful in portable and smart electronic devices [34].
Our carbon xerogels also possess charge storage properties. Although the capacitance is rather low with respect to the values reported in the literature for other xerogels, our results show that EMI shielding and energy storage can be combined in principle. The cyclic voltammetry curves revealed a slightly distorted rectangular shape which is an indication of an electrical double-layer capacitor with a small anodic peak observed at 0.2 V ( figure 2(b)). An electrical double layer was formed at the interface between the electrode and the electrolyte solution. The hysteresis phenomenon in the CV curves was likely attributed to the slow ion diffusion during the charging-discharging cycles [35]. The anodic peak in the CV curves can be tentatively attributed to the reversible oxidation of the electrode in aqueous solution, while the applied potential was scanned from lower to higher values. The specific capacitance was calculated to be (3.47 ± 0.30) F/g. It is mentioned in the literature that the electrochemical performances of the carbon xerogel depend on a few crucial factors including the pH value of initial resorcinol-formaldehyde-catalyst solution, resorcinol to formaldehyde molar ratio, gelation temperature, and pyrolysis temperature, etc Previously, a maximum capacitance of 37.6 F g −1 was obtained for a carbon xerogel catalyzed by NaOH using the selected optimum parameters reported in the literature [36]. This high value was attributed to the uniform structure of the carbon xerogel, which has a large specific surface area offered by nano-sized inner pores connected to the electrode surface through microchannels. This unique hierarchical morphology provided better conditions for enhanced specific capacitance via facile ion adsorption. Our material may show a lower capacitance owing to the absence of well-connected nano-sized inner pores.
Finally, we investigated the EMI shielding properties of a 3D carbon xerogel by averaging over four samples with a thickness of ∼ 2.4 mm in the frequency range of 10-15 GHz. Figure 3(a) shows the absorption (A) and reflection (R) coefficients calculated using S-parameters. The reflection was significantly higher than the absorption, with average values of0.84 and 0.16 respectively. Figure 3(b) shows a plot of the EMI SE as a function of frequency for the carbon xerogel samples. The average EMI Shielding between 10 and 15 GHz for the total shielding values of each sample were 60.20 dB, 60.03 dB, 59.62 dB, and 64.66 dB, for an average of 61.13 dB. One sample, which was not included in the average, exhibited EMI shielding above 80 dB.
Three shielding mechanisms come from Schelkunoff's theory [37]: reflection, absorption, and multiple reflection losses. When a vector network analyzer is used to measure EMI shielding, it is not possible to directly measure the absorption of the material. Instead, the reflected and received signals can be measured. By subtraction, we can obtain the absorption by the material. It has often been reported in the literature that Multiple Internal Reflections can be confused with absorption when using the S-parameter method.
The coefficients shown in figure 3(a) indicate that reflection is the main mechanism at play in the material. In porous structures, the holes at the surface reduce the impedance mismatch and cause electromagnetic waves to enter the material, thus reducing the interface reflection. The multiple interfaces created by the pores inside the material cause multiple reflection losses of electromagnetic waves [38]. Additionally, Hwang et al [39] explained that in the case of a high SE A and high reflection coefficient, the most probable cause is that multiple internal reflections are involved. Hence, we believe that for our samples, the main shielding mechanism comes from a combination of reflections and multiple internal reflections owing to its porous network.
The EMI shielding efficiency was estimated to be 99.999%, indicating that the carbon xerogel can be used in anti-EMI technologies. The availability of lightweight EMI materials opens the way for their use in applications where mass is critical, such as aerospace. Carbon xerogels can be used in a vacuum and provide EMI shielding or even some form of radiation protection for aircraft and spacecraft. Aircraft have stringent requirements in terms of EMI and electromagnetic compatibility (EMC), and mass is a key driver of their cost-effectiveness. In addition, with the electrification of transport, a low mass will likely become an even stronger driver in aircraft design. Several fibrous materials are effective radiation shields [40]. For example, high-density polyethylene (HDPE) fibers are used in the International Space Station (ISS) as a passive protection system. Carbon xerogel has a lower density (0.85 ± 0.01 g cm −3 versus > 0.941 g cm −3 for HDPE), does not suffer from the same outgassing issues, and provides higher EMI shielding effectiveness without the need for composite materials with complex manufacturing processes [41]. Our material performed at higher values than the general industrial and commercial standards of 30 dB and was shown to be efficient enough for aerospace applications, as it provided shielding above 60 dB, which is comparable to metal gaskets currently available in the aerospace industry [42]. The specifications for the EMC for different industrial environments differ substantially according to the application domain, and they are highly regulated by standards. For example, space applications from Europe rely on the ECSS-E-ST-20, whereas space systems from the US rely on MIL-STD-461, which can be adhered to in different ways (reduced emissions, directionality, shielding, etc). We do not claim that carbon xerogel is useful for a specific standard, but we note that it has substantial mass advantages: its density (0.85 ± 0.01 g cm −3 ) is lower compared with the current use of HDPE (> 0.941 g cm −3 ) and provides better shielding effectiveness (> 50 dB) compared with HDPE (< 20 dB). Given that mass is a critical factor in aviation and space, we believe that carbon xerogel could be an interesting shielding candidate in these areas.

Conclusion
In this work, we successfully synthesized lightweight and electrically conducting 3D carbon xerogels with a complex porous structure, and investigated their electrochemical and electromagnetic interference (EMI) shielding properties in the frequency range of 10-15 GHz. Our results demonstrate that the carbon xerogels exhibit a high total EMI shielding effectiveness (SE) value of approximately61 dB, mainly attributed to the dominant reflection process (~84%) and a significant absorption process (~16%) due to the presence of a complex porous network that suppresses the EM waves. These findings suggest that the carbon xerogels have potential applications in the aerospace and military industries, as well as in the development of low-cost supercapacitors.The present work sheds light onEMI shielding materials, that is, carbon xerogels, which can potentially be used for aerospace and military applications and low-cost supercapacitors.