Three-dimensional and highly ordered porous carbon–MnO₂ composite foam for excellent electromagnetic interference shielding efficiency

Abstract

In this study, high-performance carbon–MnO₂ composite foam was developed by using a simple sacrificial template technique from a mixture of MnO₂ nanoparticles and phenol formaldehyde resin as a carbon source. It is observed that composite foam possesses density between 0.25–0.35 g cm⁻³. The addition of MnO₂ endows the dielectric and electromagnetic interference (EMI) shielding efficiency with absorption as the dominating constituent in EMI shielding mechanism in X-band frequency region (8.2–12.4 GHz). The obtained carbon–MnO₂ composite foam with MnO₂ loading of 4 wt% demonstrates excellent specific EMI shielding effectiveness (SE) of −150 dB cm³ g⁻¹. This enhancement is due to the dielectric constant and electromagnetic wave attenuation resulting from the introduction of MnO₂. However, the compressive strength of carbon foam improved by 77.3% (7.8 MPa) by the incorporation of 8 wt% MnO₂ particles in a carbon matrix. This technique is very fast, highly reproducible, and scalable, which may facilitate the commercialization of such composite foam and it can be used as EMI shielding materials in the fields of aerospace applications.

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1. Introduction

Nowadays the significant use of electronic and communication devices is generating high electromagnetic interference or signal interference, in the form of radiation, which can disturb electrical circuits, cell phones, televisions, satellite communications etc. This has become a big issue for the environment such as air pollution, water contamination, noise pollution as well as human life.¹ To solve this problem, there are two practical ways for shielding against electromagnetic interference (EMI): the first is to protect certain components from the radiation by reflecting or absorbing the waves by using traditional conducting materials, another is to reduce the reflection and increase the absorption by incorporating dielectric or magnetic particles into newer materials.

Consequently, the search for materials that can act as microwave absorbers or shield against electromagnetic waves become essential for proper functioning of next generation electronics devices. The currently available broad range of microwave absorbing materials generally show its best possible absorbing power at particular frequency range and over certain control parameters. Carbon materials such as MCMB,² carbon nanotube,^3,4 carbon black,^5,6 carbon fibers,⁷ graphite,⁸ graphene,⁹ reduced graphene oxide¹⁰ etc. are acquired high efficiency EM wave attenuation because of their high thermal/electrical conductivity, low density, good corrosion resistance and thermally stability. But light weight and porous carbon materials are generally favorable for the practical EMI shielding application in the areas of aircraft and spacecraft because it would save materials and energy.

During recent decades, porous carbon materials have undergone rapid development and have become one of the key material in modern research. They are being extensively used as radar absorbing materials,¹¹ EMI shielding materials,¹² electrode materials for batteries,¹³ fuel cells, super capacitors,¹⁴ and as supports for many important catalytic processes.^15,16 Their use in such diverse applications is directly related to low density, electrical and specific thermal conductivity, chemical stability and also wide availability. Porous carbon, especially carbon foam has recently gained revolutionary aspiration because of its remarkable properties such as low density, high surface area, high specific electrical and thermal conductivity, high EMI shielding.^9,17,18

There are few studies reported in literature on carbon foam specifically as EMI shielding materials.^19–23 It is demonstrated that some modification of carbon foam during their processing, incorporation of carbon nanomaterials and metal nanoparticles in it, have given some consoling results in fabricating EMI shielding materials.^24–28 Furthermore, in our previous study, we have reported EMI shielding of coal tar pitch based carbon foam with ferrocene and MWCNTs and they exhibited high shielding of −81 dB and −85 dB, respectively which was basically contributed by the reflection losses by −60 dB and −65 dB and absorption losses −21 dB and −20 dB, respectively.^29,30 In both the cases shielding was dominated by reflection due to high conductivity of the pitch carbon foam. Therefore, in the present study, efforts are made to the development of absorption dominated microwave shielding carbon foam. The complex dielectric permittivity is an important parameter for describing the properties of absorption dominated shielding or radar absorbing materials (RAM). Excellent dielectric properties are needed for making good RAM. Previously, metal based microwave shielding materials are widely explored and utilized but the high specific gravity, corrosion proneness and cumbersome processing methods made these materials unsuitable for both the researchers and users. Furthermore, metals mainly reflect the radiation and cannot be used for applications where absorption is mandatory. However, among the different materials, manganese oxide (MnO₂) offers a new path for the development of advance microwave shielding materials with a combination of carbon materials because of its high dielectric constant, low cost and environment friendly nature.^31,32 There are only a few reports are available on MnO₂ as a microwave shielding material in literature.^33,34 Yuping et al.,⁶ studied the microwave absorbing properties of MnO₂–carbon black composites and showed that these composites can absorb microwaves in a wide band range of microwave frequency with lower reflection loss of −10 dB (90% absorption) due to dielectric loss. Microwave absorption properties of MnO₂ composites were also studied by Guan et al.³⁵ In their study, MnO₂–SiO₂–PVA composites exhibited the strongest absorbing peak at 8.5 GHz with −25 dB with absorption loss 30 vol% MnO₂. Gupta et al.,³⁶ demonstrated that chemically synthesized MnO₂ decorated graphene nanoribbons (GNRs) have high shielding effectiveness −57 dB and blocked >99.9999% radiation in the frequency range 8.2–12.4 GHz.

In this study, light weight carbon–MnO₂ composite foams are developed by sacrificial template technique from a mixture of phenolic resin and MnO₂ nanoparticles. The resultant carbon–MnO₂ composite foams are characterized by various spectroscopic techniques. The composite foam EMI-SE in the frequency range of 8.2–12.4 GHz is measured at room temperature by waveguide using vector network analyzer. It is expected that the carbon–MnO₂ composite foam would exhibit superior EMI SE dominated by absorption phenomena and improved mechanical properties.

2. Experimental

2.1. Materials and chemicals

Phenol formaldehyde resin was supplied by Noida Polychem Pvt. Ltd., Noida, Uttar Pradesh. Acetone (99.0%) and MnO₂ particles were purchased from Qualigenes Fine Chemicals, Navi Mumbai, India. Polyurethane foam (PU) was procured from S. G. & Company, New Delhi.

2.2. Preparation of MnO₂ particles and carbon–MnO₂ composite foams

The carbon foam was prepared by sacrificial template technique in which the polyurethane (PU) foam (density 0.030 g cm⁻³ and average pore size 0.45 mm) was used as template.³⁷ The PU foam was impregnated by solution of phenolic resin mixed with different weight proportions (viz. 0, 2, 4 and 8 wt%) of MnO₂ nanoparticles. For the development of carbon foam, the first MnO₂ powder was grounded with NaCl through ball milling for 48 hours to get the fine powder and then washed with distilled water 3–4 times. After that, MnO₂ nanoparticles was sonicated in acetone for 1 hour and then phenolic resin was mixed in this solution followed by stirring for 30 minutes to make a homogenous slurry of phenolic resin and MnO₂ nanoparticles. The PU foam slabs (size 50 × 40 × 10 mm) were then dipped in the homogenous solution in order to obtain phenolic resin and MnO₂ impregnated PU foam. Impregnated foams were dried at 70 °C for 12 h. The dried impregnated foams were cured at 300 °C in presence of air atmosphere for 12 h to increasing the cross-linking between the polymeric chains. These cured foam slabs were carbonized at 1000 °C in an inert atmosphere to get MnO₂ incorporated highly ordered porous carbon foam. The carbon foam was named as CFMn0 for as such carbon foam whereas CFMn2, CFMn4 and CFMn8 named for 2, 4, and 8 wt% of MnO₂ loading in carbon foam, respectively. The schematic representation for fabrication of carbon–MnO₂ composite foam is given in Fig. 1.

Fig. 1 Schematic representation of fabrication process of carbon–MnO₂ composite foam.

2.3. Characterization

Surface morphology of carbon–MnO₂ composite foam was studied by scanning electron microscope (SEM, VP-EVO, MA-10, Carl-Zeiss, UK) operating at 10 kV. X-ray diffraction (XRD) studies of the samples were performed in the scattering range of 10–80° at scanning rate of 5° min⁻¹ on D-8 Advanced Bruker diffractometer using CuKα radiation (λ = 1.5418 Å). Raman spectra of the carbon foam samples were recorded using Renishaw Raman spectrometer, UK, with laser as an excitation source at 514 nm. Transmission electron microscope (TEM) (FEI, Tecnai T30) was used to investigate the crystallite size of MnO₂ nanoparticles. Thermal stability of the carbon foam samples carried out on thermo gravimetric analyzer (on Mettler Toledo TGA/SDTA 851^E thermal analysis system in air) at a heating rate of 10 °C min⁻¹. The compressive strength of carbon foam samples was measured using Instron Universal Testing Machine (model 1122) at the rate of 0.5 mm per minute. Classical four probe contact method with Keithley 224 programmable current source and Keithley 197A auto ranging microvolt DMM was used for calculating electrical conductivity. EMI-shielding effectiveness in the frequency range of 8.2–12.4 GHz (X band) was measured at room temperature by waveguide using vector network analyzer (VNA E8263B Agilent Technologies). For EMI SE carbon foam samples were cut in a rectangular shape of dimension dimensions 26.8 × 13.5 × 2.5 mm to fit within the cavity of the sample holder. A full two-port calibration was performed using a quarter-wavelength offset and termination and keeping the input power level at −5.0 dBm.

3. Result and discussion

3.1. Structural and morphological analysis of carbon–MnO₂ composite foams

The MnO₂ nanoparticles morphology observed by TEM is depicted in Fig. 2(a), from Fig. 2(a) it is clearly evident that the sizes of MnO₂ nanoparticles are irregular in shape and most of MnO₂ nanoparticles are approximately rectangular shape with mean size ∼40 nm. Fig. 2(b) shows the X-ray diffraction pattern of MnO₂ nanoparticles. A sharp diffraction peak arises at 2θ = 28.86° belong to (110) plane of β-MnO₂ and interlayer spacing is 0.309 nm, which is calculated by Bragg's equation. The mean crystallite size of MnO₂ (37.4 nm) was calculated from the XRD curve according to the line width of the (110) peak using Scherrer equation. These results are in agreement with the results obtained from the TEM analysis. However, the particle size from TEM measurements is slightly larger than that of crystal sizes measured by XRD, this is due to the presence of noncrystalline surface layers. The other peaks of MnO₂ nanoparticles in XRD pattern (Fig. 2(b)) at 2θ = 28.7, 37.5, 40.9, 42.8, 56.8, 59.3, 64.9, 67.3 and 72.4° corresponding to the diffraction plane (101), (200), (111), (210), (211), (220), (002), (310) and (112), which is corresponds to the tetragonal phase of β-MnO₂ (JCPDS card no. 24-0735). Additionally, the XRD patterns for CFMn0 and CFmn4 and CFMn8 are shown in Fig. 2(c). According to Fig. 2(c), the XRD pattern of CFMn0 shows a peak at 2θ angle of 24.2 and 44.4 and 54.2, corresponding to the amorphous carbon of (002), (100) and (004) lattice planes. Meanwhile, CFMn4 and CFMn8 have several additional phase which corresponding to the diffraction plane (101), (200), (111), (210), (211), (220), (002), (310) and (112) indicate the presence of tetragonal phase of β-MnO₂. The smoothening diffraction peaks of the CFMn8 (Fig. 2(c)) compared with CFMn4 and CFMn2 indicate that the presence of higher content of MnO₂ related phases in the composite foam.

Fig. 2 (a) TEM image of β-MnO₂, (b) XRD pattern of β-MnO₂, (c) XRD and (d) Raman spectra of carbon foam with different wt% of MnO₂.

The Raman spectroscopy is used to further investigate the composition and microstructure of the carbon–MnO₂ composite foams. Raman spectroscopy is an effective technique to identify carbon materials. As shown in Fig. 2(d), the Raman spectrum of CFMn0 and CFMn4 exhibits two peaks at ∼1600 and ∼1355 cm⁻¹ represent G and D bands, respectively. The G band is the radial C–C stretching mode of sp² bonded carbon, while the D band is a first-order zone boundary phonon mode associated with the defects in the carbon or graphene layer.³⁸

The D band indicates defects and disorder of the carbon structure. The intensity ratio of the D and G bands (I_D/I_G) further quantifies the relative levels of disordered and graphitic carbons in the carbon composite foams. It is observed that I_D/I_G ratio for CFMn0 is 0.834 while 0.917 in CFMn4. A higher intensity ratio of I_D/I_G is in case CFMn4 composite foam than that for CFMn0, which exhibits the presence of a large amount of defects generated due to the addition of MnO₂. The defects in graphitic materials are formation by C–MnO₂ complex which is important for improving the shielding properties of carbon based materials. These defects are generally in the form of actives sites and these are in the between the graphene layers. The defects presence on the CFMn4 basal plane caused by the adverse excessive restacking of carbon layers could act as polarized centers for the dipole/electrons polarization. Therefore, when the EM field is in the high frequency, the dipole and electrons polarizations could not match up with the changes of EM field, which will lead to the Debye relaxation contributing to enhancing the dielectric loss and electromagnetic energy dissipation.

Carbon foam is a solid structure based on cell connected through open faces depicted in Fig. 3(a)–(f). It reveals that pores are open, highly ordered, and uniform in size although some pore walls in the micrographs are broken during sample preparation for SEM analysis. The pore walls are called ligaments, which are interconnected to each other and responsible for overall properties of carbon foam. It can be seen that CFMn0 (Fig. 3(a)) appears less bright compared to CFMn2, CFMn4 derived from the MnO₂ addition (Fig. 3(b) and (c)). The analysis of cell and pores of carbon foam reveals that the average pore diameter is in the range of 350–400 μm. From Fig. 3(b) and (c), it is clearly seen that MnO₂ nanoparticles are uniformly decorated over the ligaments, surface as well as inside the pore. This resulted in a decrease in open porosity of carbon foam. Fig. 3(d) shows the EDX spectrum of CFMn4 which confirms the presence of the elemental composition of C (87.75%), O (9.70%) and Mn (2.55%) in the carbon foam matrix. However, increased amount (8 wt%) of MnO₂ particles may cause agglomeration on the surface of carbon foam as appeared in Fig. 3(e) and high magnification SEM image of CFMn8 (Fig. 3(f)), this is also responsible for decrease porosity and surface area of carbon foam.

Fig. 3 SEM micrographs of (a) CFMn0, (b) CFMn2, (c) CFMn4, (d) EDX analysis of CFMn4 showing the presence of MnO₂, (e) CFMn8 and (f) showing agglomeration of MnO₂ inside the pore of CFMn8.

To confirm further, actual uptake of MnO₂ wt% in carbon foam, thermal gravimetric analysis (TGA) of CFMn0, CFMn2 and CFMn8 composite foam was performed. TGA is a most important tool to study the weight loss of materials, presence of impurities and to find out the percentage of materials. Fig. 4 represents the TGA curves of carbon foam and carbon–MnO₂ composite foams containing 2 and 8 wt% MnO₂ nanoparticles (CFMn2 and CFMn8) which have been studied in the oxidative environment at the rate of 10 °C min⁻¹ upto 1000 °C. It is observed that carbon foam experiences weight loss in two systematic steps. At first it shows very minute wt. loss which started at temperature below 300 °C. However in later step major weight loss takes place between temperatures 300 °C to 650 °C. The final major wt. loss step corresponds to the complete pyrolysis of carbon fragments into smaller fraction and gaseous byproducts.

Fig. 4 TGA curves carbon–MnO₂ composite foams.

While in the case of carbon–MnO₂ composite foams (CFMn2 and CFMn8) first weight loss step started at temperature below 350 °C and major weight loss takes place between temperatures 350 °C to 650 °C. This evidently shows MnO₂ nanoparticles have stabilizing effect on thermal pyrolysis of carbon matrix and increases initial thermal decomposition temperature to 350 °C. These nanoparticles are thermally more stable compared to carbon foam and enhances the weight loss process in the oxidative environment. Additionally, as evident from the TG curve, with the increase of MnO₂ nanoparticles content in carbon foam, the corresponding ash residue also increases from 1.8% (CFMn2) to 4.0% (CFMn2) and 9.4% (CFMn8) indicating the presence of MnO₂ nanoparticles in the carbon foam.

3.2. Physical and mechanical properties of the carbon–MnO₂ composite foams

Fig. 5(a) shows the variation in bulk density and compressive strength of carbon foams with increasing MnO₂ contents. The density of carbon–MnO₂ composite foams increase continuously and it is 0.25, 0.28, 0.30 and 0.35 g cm⁻³ for CFMn0, CFMn2, CFMn4 and CFMn8, respectively. Improvement in the bulk density of carbon–MnO₂ composite foams has a positive effect on the mechanical properties. The compressive strength of CFMn0 is 4.4 MPa while the in case of CFMn8 it increases from 4.4 to 7.8 MPa. There is 77.3% increment in the compressive strength of carbon composite foam with 8 wt% MnO₂ loading.

Fig. 5 (a) Variation in bulk density and compressive strength and (b) open porosity and BET surface area of carbon foam with different wt% MnO₂.

The compressive strength of carbon composite foam depends mainly on three factors namely microstructure, bulk density and porosity. The microstructure mainly includes width of the ligaments and quantity of micro cracks. In case of CFMn0, load is transferred through ligaments; therefore, cracks in the ligaments can be responsible for low load bearing capacity of carbon foam. While in case MnO₂ loaded carbon foam, MnO₂ nanoparticles are infiltrated in the cracks, deposited on the surface of ligaments, inside and on the surface of pores. This resulted in an increase in bulk density and reduced the cracks density, which can help to deflect the cracks and improve load bearing capacity of carbon composite foam and hence improvement in the compressive strength. Furthermore, the addition of MnO₂ particles reduced the open porosity. The open porosity decreases with increasing the MnO₂ loading. The linear decrease in open porosity is the result of complex changes in carbon foam. The CFMn8 with 67.3% open porosity is much stronger than the pure foam (CFMn0) with an open porosity of 80.4%. This can be attributed to the fact that closed-cell structure can disperse more stress than the open-cell structure. Additionally, the variation in open porosity and BET surface area of carbon–MnO₂ composite foams are shown in Fig. 5(b). It is observed that the porosity of carbon foam slightly decreases with increase in the MnO₂ content. The porosity of carbon composite foams; CFMn0, CFMn2, CFMn4 and CFMn8 is found to be 80.4, 76.5, 71.5 and 67.3%, respectively. The decrease in porosity is due to the incorporation of MnO₂ particles inside the pore of carbon foam. However, the surface area of carbon foam increases from 6.5 to 13.7 m² g⁻¹ with 4 wt% MnO₂ loading in a carbon matrix. Whereas in the case of carbon foam with 8 wt% MnO₂ loading (CFMn8), the surface area is decreased to 8.4 m² g⁻¹, which is due to the agglomeration of MnO₂ particles over the surface of carbon foam as shown in SEM image Fig. 3(f).

3.3. Electrical conductivity

Fig. 6(a) shows the electrical conductivity (EC) of the carbon foam having different MnO₂ loading. Initially, the electrical conductivity CFMn0 is 24.5 S cm⁻¹. However, the electrical conductivity of carbon composite foam decreases with the increase of MnO₂ additive. When the MnO₂ amount is 8 wt%, the electrical conductivity of CFMn8 decreases to 6.0 S cm⁻¹. The decrease in electrical conductivity can be ascribed to the intrinsic nature of MnO₂. The dielectric and poor conductive nature of MnO₂ is able to block the conduction path of electrons in the carbon networks. Therefore, a significant reduction in the electrical conductivity of carbon composite foam is observed with the increasing MnO₂ nanoparticles content. Furthermore, it is accomplished that conductivity values are very close to the endorsed range for microwave shielding and suppose to provide more promising EMI shielding response. It is worth mentioning here that the electrical conductivity of materials plays an important role in influencing the shielding property. However, a material with low electrical conductivity does not always show poor EMI shielding.³⁹

Fig. 6 (a) Electrical conductivity, EMI-SE of carbon composites foams: SE_T (b), SE_A (c) and SE_R (d) in frequency range of 8.2 to 12.4 GHz (X-band) with MnO₂ nanoparticles loading.

3.4. Electromagnetic interference (EMI) shielding

The EMI shielding effectiveness (SE) of a material is the ability to attenuate electromagnetic (EM) radiation that can be expressed in terms of the ratio of incoming (incident) and outgoing (transmitted) power. The EMI attenuation offered by a shield may depend on the three mechanisms: reflection of the wave from the front face of shield, absorption of the wave as it passes through the shield's thickness and multiple reflections of the waves at various interface.⁴⁰ Therefore, SE_T of EMI shielding materials is determined by three losses i.e. reflection loss (SE_R), absorption loss (SE_A) and multiple reflection losses (SE_M), which can be expressed is as i.e.,^24,41

SE_T (dB) = SE_R + SE_A + SE_M = 10 log(P_t/P_i) (i)

where, P_i and P_t are the incident and outgoing power, respectively. If the distance between the reflecting surface or interface is comparatively larger than the skin depth, one can ignore the shielding due to multiple reflections. This suggests that a shielding material must shield against the EM waves either by reflection or absorption phenomenon and hence the total shielding effectiveness (SE_T) can be written as^42,43

SE_T (dB) = SE_R + SE_A (ii)

For reflection, the shield must have mobile charge carriers and tends to be electrically conducting. Although, not required very high conductivity. On the other hand absorption of the radiation by shield depends on electric and magnetic dipoles. Materials with high values of dielectric constant provide the electric dipoles while the materials having high values of magnetic permeability provides the magnetic dipoles.

In this study, MnO₂ nanoparticles (dielectric material) are incorporated in carbon foams and their EMI shielding measurements are carried out in X-band frequency range (8.2–12.4 GHz). Fig. 6(b)–(d) shows the results obtained for absorption, reflection and total EMI SE with frequency as a function of MnO₂ nanoparticles loading in carbon composites foams. When the electromagnetic wave falls on carbon foams; the dominant loss of the incident electromagnetic wave is through absorption. The energy is consumed mainly due to dielectric loss because of the presence of dielectric material, β-MnO₂. A small part of the incident electromagnetic wave is reflected from the front and back interfaces of the material. It is observed from Fig. 6(b) that SE_T value of CFMn0 is −27.5 dB at a thickness of 2.5 mm and SE_T is equally shared by SE due to the absorption (−13.9 dB) and reflection (−13.6 dB) at a frequency of 10.7 GHz. From the experimental measurement, it is well evident that on the incorporation of 2 wt% MnO₂ in carbon foam SE_T increases initially to −33.6 dB and the SE is due to absorption and reflection phenomena, is found to be −23.6 dB and −10 dB, respectively at a frequency of 10.7 GHz. Whereas on increasing the MnO₂ nanoparticles loading 4 wt% (CFMn4) the SE_T reaches to −45 dB, SE is dominated by absorption losses (SE_A −40.8 dB) as compared to reflection losses (SE_R −4.2 dB) at a frequency of 10.7 GHz.

The extreme change in absorption and reflection component (SE_A and SE_R) in CFmn4 can be expressed mathematically as:⁴⁴

σ is electrical conductivity of shield material, hence reflection loss is directly proportional to conductivity.

SE_A (dB) = −8.68{t/δ} = −8.68αt (iv)

where ‘α’ is the attenuation coefficient which describes the extent to which the intensity of an electromagnetic wave is reduced when it passes through a specific material.where n is the refractive index of the material and λ₀ is the wavelength in vacuum. Refractive index (n) is defined by the following relation:

n = (ε_rμ_r)^1/2 (vi)

where, ε_r and μ_r are the relative permittivity and permeability respectively.

For non-magnetic materials μ_r is very close to 1.³⁶ Therefore, n = (ε_r)^1/2 and we know that for a medium, ε_r is equal to ε′. Therefore,

n = (ε′)^1/2 (vii)

where, ε′ is the real part of permittivity. Wavelength in vacuum (λ₀) is equal to λ₀ = 2πc/ω because ω = 2πn = 2πc/λ₀, where ‘c’ is the velocity of light and is described as:

c = (ε_rμ_r)^−1/2 (viii)

Therefore,

For nonmagnetic materials, μ_r = 1. Thus, high permittivity is very important for the enhancement of SE_A and pacifies of SE_R, respectively. Thus, incorporation of very high permittivity material, MnO₂ nanoparticles in carbon foam enhanced the overall shielding effectiveness. Furthermore, according to the electromagnetic theory, dielectric losses in the material occur because of complex phenomena such as dipolar relaxation, natural resonances, electronic polarization and its relaxation and unique structure (ordered porous structure) of the material. In MnO₂ loaded carbon foams, MnO₂ acts as a polarized center in the presence of microwave which gives rise to better microwave shielding due to absorption. However in the case of CFMn8, total shielding SE_T slightly decrease to −39.0 dB with absorption losses (SE_A) −32.6 dB and reflection losses (SE_R) of −6.4 dB at the frequency of 10.7 GHz. Thus, decreasing shielding effectiveness in CFMn8 may be due the agglomeration of MnO₂ nanoparticles and decreasing porosity. By decreasing the porosity in carbon foam not only increases the density but it would also decreases the EMI shielding but this trend is not always since the EMI shielding also depends on introducing materials, dielectric constant and magnetic dipoles. Although it requires more study to demonstrate the contribution of the porosity to increasing the shielding performance. The interaction mechanism of electromagnetic waves with carbon foam and MnO₂ is shown in Fig. 7 through a schematic diagram.

Fig. 7 Schematic representation of EMI shielding mechanism.

The EM radiation at high frequencies penetrates only near surface region of an electrically conducting material. This phenomenon is known as skin effect. The depth, at which field drops to 1/e of the incident value is called as skin depth (δ) and expressed as:¹

where f is frequency, σ is electrical conductivity and μ is permeability and t is the thickness of carbon composite foam. Fig. 8(a) shows the variation in skin depth of carbon composite foams with different loading MnO₂ (wt%) at frequency range 8.2–12.4 GHz. From eqn (v) and it is clearly observed that skin depth (δ) is inversely proportional to SE_A. Therefore, CFMn0 having the maximum δ of 1.6 mm exhibits minimum absorption loss of −13.9 dB at a frequency of 10.7 GHz. Alternatively, CFMn4 exhibiting maximum absorption loss of −40.8 dB with a minimum skin depth of 0.53 mm.

Fig. 8 Frequency dependence of (a) skin depth (δ), (b) real part of permittivity (ε′), (c) imaginary part of permittivity (ε′′) and (d) dielectric tangent loss (tan δ_E) with MnO₂ nanoparticles loading.

The increase in EMI shielding effectiveness due to absorption has further correlated with relative complex permittivity (ε* = ε′ − iε′′) as a function of frequency. Fig. 8(b) and (c) show the real (ε′) and imaginary (ε′′) part of permittivity of the relative complex permittivity of MnO₂ incorporated carbon foams in the frequency range of 8.2–12.4 GHz. These parameters are calculated by scattering parameters (S₁₁ and S₂₁) and standard Nicholson–Ross and Weir theoretical calculations is used to study the variation in these parameters with frequency. The real part of permittivity (ε′) is mainly associated with the amount of polarization occurring in the material and the imaginary part (ε′′) is a measure of dissipated energy (loss energy).

The dielectric performance of the material depends on electronic, ionic, orientation and space charge polarization. The contribution to the space charge polarization appears due to the heterogeneity of the material. The presence of dielectric materials (MnO₂) in conducting foam results in the formation of more interfaces and a heterogeneous system due to some space charge accumulating at the interface that contributes toward the higher microwave absorption in the composite foam. In Fig. 8(b), it can be observed that the real part of permittivity (ε′) of carbon–MnO₂ composite foams increases with increasing concentrations of MnO₂. The real part of permittivity (ε′) at fixed frequency of 8.2 GHz is 70.4, 87.8, 102.6 and 123.90 for CFMn0, CFMn2, CFMn4 and CFMn8 respectively. In MnO₂ loaded carbon composite foam, ε′ is much higher than that of pure carbon foam. Meanwhile, the values of an imaginary part of permittivity (ε′′) at fixed frequency of 8.2 GHz are 53.5, 65.9, 76.1 and 90.5 for CFMn0, CFMn2, CFMn4 and CFMn8 respectively. Similarly, in MnO₂ loaded carbon composite foam, ε′′ is also much higher as than that of pure carbon foam. It is proposed that the addition of MnO₂ increases the electric polarization since ε* is an expression of the polarizability of a material, which consists of dipolar polarization and electric polarization at microwave frequency.^45,46 Furthermore, all ε′ and ε′′ shows decrease tendency with the increasing frequency, which can be attributed to the phenomenon of dispersion of materials and can be further confirmed by the Debye equations⁴⁷ i.e.,

where the decrease in ε′ and ε′′ is attributed to the increase in angular frequency (ω), where ε_s and ε_∞ are the static permittivity and relative dielectric permittivity at the high-frequency limit respectively and τ is the polarization relaxation time. The accumulation of MnO₂ particles in carbon matrix significantly enhanced the properties of dielectric loss and microwave absorption. In light of EM theory, there are several factors that lead to this phenomenon. The first reason is the domain factor that caused by improving the dielectric loss capability of the carbon–MnO₂ composite foams. As confirmed by the Raman spectrum analysis, the defects on the carbon composite foam basal plane caused by the adverse restacking of carbon the layer and the interface between the MnO₂ nanoparticles and the carbon layers could act as polarized centers for the dipole/electrons polarization. Consequently, when the EM field is in the high frequency, the dipole and electrons polarizations could not match up with the changes of EM field, which will lead to the Debye relaxation contributing to enhancing the dielectric loss and electromagnetic energy dissipation.⁴⁸ Secondly, the existence of complex phases in carbon–MnO₂ composites foam and the unique micro structures of β-MnO₂ facilitate the transportation of electron, which are beneficial to enhance dipole polarization and contribute to the dielectric loss and microwave absorbing properties. Furthermore, according to the free electron theory i.e.,where ε₀ is the permittivity in vacuum, σ is the material conductivity and f is the radiation frequency, the high conductivity of pure carbon foam enables a higher ε′′ than carbon–MnO₂ composite foam, finally leading to an increase in the dielectric loss. Above all, the synergetic effect of carbon and MnO₂ contributes to the improved microwave attenuation properties of carbon–MnO₂ composite foams. The dielectric tangent loss (tan δ_E = ε′′/ε′) based on the permittivity of the sample has also been evaluated and the values are in the range of 0.76 to 0.73 and are shown in Fig. 8(d). As illustrated in the earlier section, the total SE of carbon foam is mainly dominated by absorption hence, β-MnO₂ nanoparticles are responsible for the SE enhancement of the carbon composite foams.

4. Conclusion

In this study demonstrated the development of low cost, highly reproducible approach to preparing high performance carbon composite foams with excellent electromagnetic radiation absorption properties. The composite foam is developed by a sacrificial template method by incorporating different content of MnO₂ into the carbon matrix. The as-prepared foams possessed microcellular cell structure and their density is in the range of 0.25 to 0.35 g cm⁻³. The EMI SE of these composite foams significantly enhanced with the increase of MnO₂ content in composite foam. At 4 wt% MnO₂ loading, it is demonstrated maximum improvement in EMI SE (−45.0 dB) over a frequency range of 8.2–12.4 GHz. The EMI shielding efficiency is mainly attributed to the absorption rather than the reflection in the investigated frequency range. This is due to the improvement in dielectric constant and electromagnetic wave attenuation resulting from the introduction of MnO₂, as well as the existence of multiple reflections, most of the electromagnetic wave are adsorbed rather than reflected back from the composite foams. Also, the compressive strength of composite foam is increase by 77.3% indicating composite foam can be used as an excellent shielding material for structural applications.

Acknowledgements

Authors are highly grateful to the Director, CSIR-NPL to publish the results. The authors are also thankful to Dr S. K. Dhawan and Dr Vidhyanand for EMI shielding measurements and TEM characterization, respectively. Authors also wish to thank Mr Naval Kishor and Mr Jai Tawale for doing XRD measurements and SEM characterization, respectively. The authors (Pinki Rani Agrawal) would like to thanks CSIR for JRF fellowship and the author Rajeev Kumar thankful to DST, Govt. of India for Inspire Faculty Programme.

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