Flexible few-layered graphene/poly vinyl alcohol composite sheets: synthesis, characterization and EMI shielding in X-band through the absorption mechanism

Abstract

1 mm thick flexible few-layered graphene (FLG)/poly vinyl alcohol (PVA) composite sheets containing 0.1 and 0.5 vol.% of FLG are prepared using an easy solution mixing process followed by a simple casting process. As-synthesized FLG is used as the filler. The morphology, structure and phase characteristics clearly showed the formation of a composite. A maximum electromagnetic interference (EMI) shielding effectiveness (SE) of ∼19.5 dB (in the X-band, 8.2–12.4 GHz) was obtained in the case of the composite with 0.5 vol.% of FLG. Absorption is found to be the dominant mechanism for EMI shielding. The high EMI SE is attributed to the network-like features formed by FLG in the PVA matrix. EMI SE and absorption are understood by analyzing the dielectric behaviour, ac-conductivity and transparency to visible light of the composite sheets.

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1. Introduction

Electromagnetic interference (EMI) shielding is very important for the proper operation of electronic systems, especially for those operating at high frequencies such as X-band (8.2–12.4 GHz) frequencies. EMI shielding is understood in terms of shielding effectiveness (SE) which is the ratio of transmitted to incident electric (or magnetic) field powers and is typically measured in decibels (dB). Reflection, absorption and multiple internal reflections (with corresponding SE) contribute to the overall attenuation of the incident electromagnetic energy. Materials that exhibit EMI SE of ∼20 dB are suitable for commercial applications.

Of late, several polymer based composites with carbonaceous fillers have exhibited excellent EMI SE. These composites are light in weight, mechanically stable and flexible, and most importantly they are cost-effective when compared to other shielding materials.^1–5 However, in the case of some composites, time consuming and difficult multi-step procedures have to be followed to obtain the final product. In some cases synthesis is easy but scalability is a problem. In view of this, it has been suggested that the disadvantages (namely difficulty in synthesis, presence of impurities, intrinsic bundling, high price etc.) involved in the case of certain carbonaceous fillers could be mitigated by using graphene as a filler material.^5–7 Some of the important polymer based composites with carbonaceous fillers and their EMI SE are listed in Table 1.

Table 1 Different polymer composite materials with carbonaceous fillers for EMI shielding^a

Composite	Frequency range (GHz)	Maximum EMI SE (dB)
a MWCNT = Multi Wall Carbon Nanotubes, rGO = reduced graphene oxide.
MWCNT/poly (vinylidene fluoride)–poly (vinyl pyrrolidone)^8	0.0–1.5	∼20
MWCNT/poly (methylmethacrylate)^9	0.05–13.5	∼27
Polystyrene–MWCNT–graphite nanoplate microbeads^10	8.2–12.4	∼20.2
Carbon nanofibers filled PVA^11	0.85	1.6–4.8
Conductive carbon black and short carbon fiber filled natural rubber and ethylene vinyl acetate^12	8–12	≥ 20
PANI–graphite composites^13	0.01–1	∼27
Functionalized graphene/epoxy^14	8.2–12.4	∼21
Graphene/poly (ethylene oxide)^15	2–18	−38.8 reflection loss
Microcellular polyetherimide–graphene foams^16	8.2–12.4	∼44
Aligned graphene sheets in wax^17	8.2–12.4	∼12
rGO/SiO2 composite^18	8.2–12.4	∼37
rGO/wax and graphite nanosheet/wax^19	2–18	∼29.68 and ∼10

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In this work an easy and cost effective method to obtain flexible sheets of few-layered graphene (FLG)/poly vinyl alcohol (PVA) composite that is useful for EMI shielding has been demonstrated. PVA has been chosen as the matrix material owing to its water solubility, high transparency, very good flexibility and mechanical strength and wide commercial availability. In addition, PVA has excellent film forming, emulsifying and adhesion properties.²⁰ There are only two reports on graphene/PVA composites especially for EMI shielding in the X-band frequency range^21,22 while all other reports are on electrical conductivity and mechanical behavior. In these works, synthesis of graphene/PVA composites involves multiple steps such as unzipping MWCNT, in situ reduction of graphene oxide in polymer using hazardous reducing agents like hydrazine hydrate and so on. On the contrary, in this work, the synthesis procedure involves efficient use of ultrasonic waves, simple solution mixing of as-synthesized FLG with PVA to form FLG/PVA composite and simple casting process to form flexible FLG/PVA composite sheets which exhibit an excellent EMI shielding ability.

2. Experimental

2.1 Synthesis

Graphite oxide (GO) was prepared from graphite flakes (≤44 μm flake size, Alfa Aesar, 99.8% purity) using well-known Hummers method.²³ GO was then irradiated with microwaves (900 W and 2.45 GHz) to obtain graphene worms.²⁴ In a separate step, 2.5 g of PVA (M.W. 86000, Fisher Scientific) powder was mixed with 50 ml of de-ionized water and stirred at 333 K for 3 h to form a homogeneous solution. 0.2 wt% (∼0.1 vol.%) of the filler material (graphene worms²⁴) was dispersed in 50 ml of a solution (de-ionized water + small volume of ethanol AR) by using probe sonicator operated at 750 W for 1 h in continuous pulse mode to obtain independent FLG sheets dispersed in solution. This solution was added drop by drop to PVA solution and stirred for 2 h to form a homogeneous solution. Small bubbles present in this solution were removed using mild sonication for few seconds. The bubble free solution was then casted in a borosilicate glass Petri-dish and dried at 333 K to evaporate residual water and ethanol content. This resulted in obtaining free-standing, flexible and semi-transparent (to visible light) FLG/PVA sheets. Like-wise 0.5 vol.% of FLG containing FLG/PVA composite and FLG free PVA sheet were also synthesized. The thickness of all the sheets was maintained as 1 mm.

2.2 Characterization

Morphological studies were carried out by using field emission scanning electron microscope (FESEM, Ultra55 of Carl Zeiss) and transmission electron microscope (TEM, FEI Tecnai G² S-Twin) operated at accelerating voltages of 5 and 200 kV, respectively. X-ray diffraction (XRD) and Raman scattering studies were done to understand structural and phase characteristics of the samples, respectively. XRD experiments were carried out using Bruker AXS D8 Advanced system. XRD patterns were recorded from 10 to 100° using Cu Kα as the X-ray source (λ = 1.54 Å). Raman spectra were recorded in air using Nd–YAG laser (wavelength = 532 nm) in the back scattering geometry using CRM spectrometer equipped with a confocal microscope (Alpha 300 of WiTec). Beam diameter of the laser was 680 nm (with 100× objective) and the resolution of all the measurements was ∼3 cm⁻¹. Raman spectra were recorded in the spectral region 200–3500 cm⁻¹. AC conductivity of the samples was measured in the frequency range 20 Hz to 2 MHz with an LCR meter (model E4980A precession impedance analyzer, Agilent) in the parallel plate capacitor mode. Optical transmission spectra (in the wavelength range 190–2400 nm) from the samples were obtained using a dual beam UV-Vis-NIR spectrophotometer (Jasco V-570).

A special procedure was used to prepare TEM specimen from the samples. Initially borosilicate glass was cleaned in acetone and isopropanol alcohol using bath sonication and finally rinsed with deionized water. It was then dried in hot air oven at 100 °C for 1 h to remove residual alcoholic contents. Polystyrene was then spin coated on the glass at 1000 rpm. The polystyrene coated glass was then dried at 130 °C for 1 h. FLG/PVA composite solution was then spin coated at 8000 rpm on the surface of the polystyrene coated glass and subsequently dried at 130 °C. A part of this film (FLG/PVA composite on top of polystyrene film) was then peeled from the glass and placed on the TEM grid which was then dipped in toluene to dissolve polystyrene leaving a free standing FLG/PVA composite film on the TEM grid.

2.3 Testing EMI shielding effectiveness

EMI shielding ability of the synthesized FLG/PVA composite sheets was studied using Agilent 8722ES vector network analyzer (VNA). FLG/PVA composite sheets were placed in the X-band sample holder between the flanges of two standard X-band waveguides connected to coaxial waveguide adapters which are connected to the ports of the VNA. Full two-port Thru-Reflect-Line (TRL) calibrations were carried out on the adapter surfaces using the standard X-band waveguide calibration kit before placing the rectangular sample holder with samples for the measurements.

The measured scattering parameters S₁₁ and S₁₂ are related to the reflected and transmitted power, respectively w.r.t the power incident on the sample surface. The incident power (P_i) on a shielding material is divided into reflected power (P_r), absorbed power and transmitted power (P_t) at the output of the shielding. The EMI shielding effectiveness (EMI SE) of a material is defined as SE_total = −10 log(P_t/P_i).^7,10 When an electromagnetic radiation is incident on a shielding material the sum of absorption coefficient (A), reflection coefficient (R) and transmission coefficient (T) must be equal to 1. R, T and A can be calculated from S parameters using the formulae R = (E_R/E_I)² = |S₁₁|² = |S₂₂|², T = (E_T/E_I)² = |S₁₂|² = |S₂₁|² and A = 1 − R − T, respectively. Total EMI SE (SE_total) is the sum of reflection from the material surface (SE_R), absorption of electromagnetic energy inside the material (SE_A) and multiple internal reflections (SE_M) of electromagnetic radiation, expressed as SE_total = SE_R + SE_A + SE_M. The reflection is related to the impedance mismatch between air and absorber. Absorption is regarded as the energy dissipation of electromagnetic wave in the shielding material over multiple reflections at the interfaces and scattering from inhomogenieties inside the material while the multiple reflections are the consequence of impedance mismatch at the two sample–air interfaces. When SE_total ≥ 15 dB, it is usually assumed that SE_M is negligible and thus, SE_total ≈ SE_R + SE_A. The effective absorbance (A_eff) can be therefore expressed as . The shielding effectiveness due to reflection and absorption of the shielding material with respect to power of the effective incident electromagnetic wave inside the shielding material are expressed as SE_R = −10 log(1 − R) and SE_A = −10 log(T/(1 − R)), respectively whilst SE_total = SE_R + SE_A = −10 log(T).

3. Results and discussion

3.1 Morphological, structural and phase characteristics

Secondary electron micrographs of graphite flakes (starting material) and FLG are shown in Fig. 1. Graphite flakes are few μm thick as shown in Fig. 1(a). Fig. 1(b) shows randomly aggregated graphene sheets (few of them stacked together to form independent FLG particles) closely associated with each other. The folded regions of FLG have an average thickness of few nm. The absence of any surface charging during imaging indicates that the material is electrically conductive. It can also be observed from Fig. 1(b) that FLG sheets have lateral dimensions of at least 1 μm².

Fig. 1 Secondary electron micrographs of (a) graphite flakes and (b) FLG.

From Fig. 1 it is clear that the synthesis method used in this work converts large number of stacked graphene layers i.e., graphite flakes (Fig. 1(a)) into FLG (Fig. 1(b)). Cross-sectional micrographs of PVA and FLG/PVA composites are shown in Fig. 2. PVA (Fig. 2(a)) contains a mixture of flat and folded regions. A typical flat region at high magnification is shown in Fig. 2(b). 0.1 vol.% FLG/PVA composite contains (Fig. 2(c)) uniformly distributed FLG (Fig. 2(d)) particles which occupy very small regions in the PVA matrix. 0.5 vol.% FLG/PVA composite contains uniformly distributed network-like features (Fig. 2(e) formed by FLG (Fig. 2(f)) in PVA matrix.

Fig. 2 Cross-sectional secondary electron micrographs of (a), (b) PVA and FLG/PVA composites with different vol.%, (c), (d) 0.1, and (e), (f) 0.5 vol.% of FLG.

FLG sheets are transparent to the electron beam in TEM as shown in Fig. 3(a). This indicates the presence of few graphene layers in each FLG construction. Comparison of XRD data of FLG with graphite (Fig. 3(b)) shows typical (002) and (004) Bragg peaks with lower intensity indicating the formation of FLG. The X-ray diffractogram obtained from FLG also indicates that it is highly ordered. Selective area electron diffraction (SAED) pattern of FLG shows the typical six fold symmetry as expected for graphene. The regular outer and inner hexagon patterns with varied intensity of the diffraction spots as shown in the inset of Fig. 4(a) indicates multi-layer system that resembles A-B type of atomic stacking as in graphite.²⁵ PVA is a partially crystalline polymer that exhibits a strong (101) diffraction peak at 2θ = 19.5° and a weak (and broad) peak at 2θ = 40.4°. FLG has a characteristic (002) diffraction peak at 2θ = 26.5° as shown in Fig. 4.

Fig. 3 (a) Transmission electron micrograph of FLG (inset shows the corresponding diffraction pattern) and (b) comparison of XRD data of graphite and FLG.

Fig. 4 X-ray diffractograms of FLG, PVA and FLG/PVA composites.

In the case of composites, it is observed that the intensity of (101) peak of PVA is higher in the case of composites owing to the uniform distribution of FLG (in PVA matrix) which acts as nucleation sites for PVA chains to pack together resulting in large size crystallites in PVA sol–gel²⁶ and thus superior PVA crystallinity in composites. Increase in intensity of FLG diffraction peak in case of composites with increasing FLG loading is due to increase in the volume content of FLG.

XRD results are well complemented with transmission electron microscopy results. As shown in Fig. 5, the transmission electron micrographs show that all composites are transparent to electron beam. Fig. 5(a) and (b) show bright field micrographs of 0.1 vol.% FLG filled PVA composite and high resolution TEM micrograph, respectively. Selected area electron diffraction (SAED) pattern indicates one set of six-fold-symmetric spots which corresponds to six fold symmetry of graphene.²⁷ High resolution TEM micrograph clearly shows 2 to 4 graphene layers which confirms that the synthesized composite contains FLG. Fig. 5(c) and (d) show bright field TEM micrograph and high resolution TEM micrograph, respectively of 0.5 vol.% FLG filled PVA composite. SAED pattern contains family of spots, indicating that field of view contains several grains (individual FLG structures) in different orientations.²⁸ This is well supported by high resolution TEM micrograph (Fig. 5(d)) which shows the presence of several FLG structures in different orientations and the network-like formation by FLG in PVA.

Fig. 5 Transmission electron micrographs of FLG/PVA composites with (a & b) 0.1 vol.% FLG, (c & d) 0.5 vol.% FLG (insets: (a) and (c) are the corresponding SAED patterns).

PVA's Raman spectrum typically depicts 17 vibrational modes in (–CH₂–CHOH–) monomer. These modes are: 6 CH₂ (2 stretching, bending, wagging, twisting and rocking), 3 CH, 3 OH, 3 CO (each consists of a stretching mode plus 2 bending modes, perpendicular and parallel to the chain axis) and 2 CC (skeletal vibrations).²⁹ In each sample's case, an average of six Raman spectra is considered for good accuracy in analysis. A clear explanation for assignment of bands in PVA is available elsewhere.^30,31 The Raman spectra corresponding to different samples are shown in Fig. 6. The typical Raman bands at 2840, 2910, and 2942 cm⁻¹ correspond to the stretching modes of CH and CH₂ i.e., ν(CH) and ν_s(CH₂) (and ν_a(CH₂)), respectively.²⁹ In a separate work the same are tentatively assigned to undifferentiated C–H stretching vibrations.³⁰ The ambiguity for the assignment was removed in a separate work.³¹ Based on the previous work,³⁰ the bands at 2835, 2913 and 2935 cm⁻¹ in the present work are assigned to ν_a(CH₂), ν(CH), and ν_s(CH₂), respectively. As observed in Fig. 6(b) there is a small shift in the peak positions with the addition of FLG to the PVA matrix. The shift is attributed to stresses that are induced in the matrix owing the presence of FLG. The symmetric bending mode of the CH₂ group, δ(CH₂), is observed (Fig. 6(a)) in PVA at ∼1433 cm⁻¹ and in FLG/PVA composites in between ∼1435 and ∼1438 cm⁻¹. The wagging and rocking modes of the CH₂ group (γ_w(CH₂), γ_r(CH₂)) are observed (Fig. 6(a)) in PVA matrix at ∼1362 and ∼852 cm⁻¹ and in FLG/PVA composites at ∼1377 cm⁻¹ and in between 848 and 856 cm⁻¹, respectively. The twisting mode of CH₂, γ_t(CH₂) is too weak to be distinguished from noise of the spectrum. The stretching and wagging modes of the OH group, ν(OH) and γ_w(OH) are observed in PVA at 3361 and 639 cm⁻¹, respectively and the same are observed in FLG/PVA composites with slight peak shift. The peak at 1440 cm⁻¹ in PVA and at 1447 cm⁻¹ in FLG/PVA composites are attributed to a combined effect of bending mode vibration of CH and OH groups (δ(CH + OH)). It is not possible to assign the exact position of the peaks in the range 1000–1200 cm⁻¹ due to lack of crystallinity and limitation with instrument's resolution (∼3 cm⁻¹). The band at ∼1096 cm⁻¹ is probably due to ν(CO) in both PVA and FLG/PVA composites. The skeletal CC vibration is observed at 922 cm⁻¹ in PVA and at 914 cm⁻¹ in FLG/PVA composites. A broad band at ∼1723 cm⁻¹ is due to residual acetate groups in PVA.

Fig. 6 Comparison of Raman spectra of PVA, FLG and FLG/PVA composites in (a) 200–1500 cm⁻¹ and (b) 1500–3800 cm⁻¹ regions.

The Raman spectra of FLG and FLG/PVA composites (Fig. 6) showed distinctive D and G bands representative of graphitic material.³⁵ In FLG, D, G, M-K scattering, 2D, and 2D′ bands are observed at 1354, 1578, 2442, 2702 and 3234 cm⁻¹, respectively. Various Raman bands in FLG/PVA composites are shown in Table 2.

Table 2 Graphitic band positions in Raman spectra of FLG and FLG/PVA composites

Raman feature	FLG	0.1 vol.% FLG/PVA composite	0.5 vol.% FLG/PVA composite
D band	1354/cm	1356/cm	1356/cm
G band	1578/cm	1581/cm	1581/cm
M-K scattering band	2442/cm	2445/cm	2457/cm
2D band	2702/cm	2712/cm	2716/cm
2D′ band	3234/cm	3242/cm	3243/cm
ID/IG ratio	0.6	0.5	0.6
IG/I2D ratio	1.2	1.4	1.5

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Raman peak positions pertaining to FLG in the present study may correspond to Raman scattering in folded graphene, single, double, and FLG.^32–34 In the case of FLG, the intensity ratio I_D/I_G which is measure for the extent of disorder was ∼0.6 indicating high order graphitization while I_G/I_2D was ∼1.2 corresponding to less than 20 graphene layers.³⁵ It is well known that 2D band in the Raman spectrum of graphitic material is a fingerprint for number of graphene layers in the given graphitic material and one constraint for this technique is that, it is hard to distinguish more than 5 to 6 layers from graphite.³⁴ The maximum intensity count of 2D band in FLG was observed at 2702 cm⁻¹ with a good center of symmetry around 2700 cm⁻¹. This type of symmetry will be found for single or bilayer or highly ordered pyrolytic graphite (HOPG).³⁶ But in comparison to a work reported previously,³⁴ the symmetry of 2D band indicated that the sample in the present study (i.e., FLG) contains very few layers and it is definitely not HOPG. For an ideal and defect free graphene it is well known that G and 2D bands in Raman spectrum are observed at 1590 and 2685 cm⁻¹, respectively. As the number of layers increases the G band shifts towards lower wave number side, 2D band shifts towards higher wave number side and shape of the 2D band will change from sharp, symmetric peak to broad, asymmetric peak. For example, for a single layer graphene one sharp 2D band can be observed, for a bilayer graphene 2D mode can be decomposed in four components and for HOPG it is best fitted to two components with less intense 2D₁ component and high intense 2D₂ component. It was also well observed that as number of layers increase an increment of intensity of the higher frequency 2D₂ component compared to the 2D₁ component occurs. In case of FLG/PVA composites, the D and G band observed at 1356 and 1581 cm⁻¹ respectively and 2D band shifted to higher wave number side as loading level increased. As the loading level of FLG increased, number of graphene layers also increased (as observed in high resolution TEM micrographs) which is reflected as a change in 2D peak position and shape. G band peak position also shifted towards higher wave number side which is a contradictory result. Recently it has been reported^37–39 that such a blue shift in G peak position can be attributed to unintentional doping in graphene by the charge impurities present in the surrounding atmosphere (here due to the PVA). It was also reported⁴⁰ that defects like edges, dislocations, cracks or vacancies in the sample can cause unintentional doping. Due to this effect, even though the extent of disorder (i.e., I_D/I_G) increased as the loading of FLG increased, the intensity ratio of G to 2D (i.e., I_G/I_2D) decreased. It is anticipated that defects in FLG are created during ultrasonication process, which is supported by erosion of FLG edges as observed in Fig. 1(b).

3.2 AC-conductivity and optical transmittance

AC conductivity as a function of frequency pertaining to 0.1 vol.% FLG/PVA and 0.5 vol.% FLG/PVA composites is shown in Fig. 7(a). At low frequencies, conductivity of the 0.1 vol.% FLG/PVA composite is invariable with frequency. However at a certain frequency, conductivity started increasing with frequency. Conductivity of 0.5 vol.% FLG/PVA composite is invariant with frequency and is measured as 5 × 10⁻¹ S cm⁻¹. The high conductivity even for a very low volume fraction of FLG (in PVA matrix) is expected because of the presence of well dispersed and conducting network-like features of FLG inside the insulating PVA matrix as elucidated in the previous section. The (visible) light transmittance for PVA, 0.1 vol.% FLG/PVA composite and 0.5 vol.% FLG/PVA composite is shown in Fig. 7(b). As the FLG content in the PVA increased, optical transparency of the composite decreased. This is a clear indication of increase in conductivity. Optical transparencies of PVA, 0.1 vol.% FLG/PVA composite and 0.5 vol.% FLG/PVA composite are 61%, 42% and 0%, respectively. Digital photographs of PVA, 0.1 vol.% FLG/PVA composite and 0.5 vol.% FLG/PVA composite supported on a scale are shown in Fig. 7(c). From these photographs the transparencies of different samples can be qualitatively visualized.

Fig. 7 (a) ac conductivity vs. frequency, (b) transmittance spectra and (c) digital photographs of different samples.

3.3 EMI shielding characteristics and shielding mechanism

EMI SE of PVA and FLG/PVA composites is shown in Fig. 8(a). The average SE of PVA sheet is ∼0.2 dB. Similarly, average SE values of 0.1 and 0.5 vol.% FLG/PVA composites are 13.5 and 19.5 dB, respectively. High SE at low loadings of FLG in PVA is attributed to high aspect ratio of FLG and formation of network-like features by FLG in PVA, which leads to more surface sites available for the incoming wave to interact and communicate with the entire network. This inference is well-correlated with the observed morphology, structure and phase characteristics. It is also observed that SE of each sample is almost constant in the entire “X band” frequency range. Fig. 8(b) shows variation in average EMI SE due to absorption and reflection for different samples. From Fig. 8(b) it is clear that absorption is the dominant mechanism for EMI shielding. This behavior is again well-correlated with the observed morphology, structure and phase characteristics of the samples.

Fig. 8 (a) EMI SE of PVA and FLG/PVA composites, (b) average EMI SE vs. volume fraction of FLG in PVA matrix, (c) dielectric observables of PVA and FLG/PVA composites and (d) tangent loss of PVA and FLG/PVA composites (dotted (--------) line is guidance line).

Since ∼20 dB is the required EMI SE for a material to be useful in most of the commercial applications, 0.5 vol.% FLG/PVA composite fits in this league of materials. To further understand the reasons behind the observed increase in SE of FLG/PVA composites, complex permittivity ε* = ε′ − iε′′ is evaluated from the experimental scattering parameters (S₁₁ and S₂₁) using Nicholson and Ross, and Weir algorithms.^41,42 The dielectric constant (ε′) is concomitant with the degree of polarization occurring in the material and the imaginary part (ε′′) is a measure of dissipated energy. In the X-band frequency range, as shown in Fig. 8(c), ε′ and ε′′ values of PVA, 0.1 vol.% FLG/PVA composite and 0.5 vol.% FLG/PVA composite have slightly decreased (encircled regions in Fig. 8(c)) with increase in frequency and their average values are 2.8 and 0.2, 4.0 and 0.6 and 5.1 and 0.82, respectively.

It is well-known that the overall dielectric behavior of a material depends on the degree of ionic, electronic, orientational and space charge (or interfacial) polarization in the material. During the synthesis, combination of microwave irradiation of graphite oxide and subsequent exposure to ultrasonic waves induce residual groups and defects like missing carbon atoms and sheet corrugation (as shown in Fig. 1(b)) in the hexagonal carbon lattice of FLG. Energy transition of microwave band involves the electronic spin, which means greater spin states are required for microwave absorption. It has been reported that localized states near to the Fermi level could be created via introducing lattice defects.⁴³ The existence of defects in FLG (as also indicated by Raman analysis in this work) favors absorption of electromagnetic energy by the transition from contiguous states to Fermi level when the absorbing surface is irradiated with electromagnetic waves.

Cole–Cole plots give an idea about relaxation mechanisms occurring in the material under an external electromagnetic field. In Fig. 8(d) which is a Cole–Cole plot pertaining to 0.5 vol.% FLG/PVA composite, presence of three semi-circles representing three different relaxation processes (in the composite) could be identified. Presence of defects, other residual chemical groups in FLG and interfaces between FLG and FLG and FLG and PVA is the main reason for the relaxation processes.⁴⁴ As elucidated by Raman scattering and electron microscopy analyses, the defects that are prominent in FLG in the composites can act as polarization centers when they are screened by the free charges. These centers can then generate polarization relaxation under the alternating electromagnetic field and attenuate the field resulting in a profound energy loss. The existence of residual oxygen-containing chemical species such as C–O in the material (as indicated by Raman scattering analysis) can generate electric dipole polarization due to their ability to trap electrons between C and O atoms. Here, under an alternating electromagnetic field, electron motion hysteresis in these dipoles can induce additional polarization relaxation processes which are favorable in enhancing microwave absorption. Interfacial polarization (also known as Maxwell–Wagner polarization) can be another attribute to the absorption process. Interfacial polarization always occurs in a composite material constituted by materials with different dielectric constants (ε) and conductivities (σ). In the case of 0.5 vol.% FLG/PVA composite, the presence of well dispersed and network-like formation by FLG in the insulating PVA matrix (as shown in TEM micrographs) results in the formation conducting/dielectric (FLG/PVA) and conducting/conducting (FLG/FLG) interfaces to induce interfacial polarization. Due to different relaxation/spreading time constants (τ = ε/σ) of the free charge carriers of individual components in the composite material, the migration of these charge carriers through the composite material will be hindered (but differently) at various points (i.e., at interfaces) of the composite material. This hindrance leads to the accumulation of space-charge at the interfaces. Pertaining to the present work, when an external electric field is applied on FLG/PVA composite, redistribution of space-charges at the interfaces can distort the macroscopic field. This distortion appears as polarization of the charges (to an external observer) which can interact with the applied field (here, the microwave field).

4. Conclusions

Liu et al.⁴⁵ reported a maximum EMI SE of ∼16–17 dB for 20 wt% SWCNTs loading in the solution blended polyurethane (PU)/SWCNT composites. Gupta et al.⁴⁶ reported a maximum EMI SE of ∼20 dB for 7 wt% MWCNTs loaded in the solution casted PS/MWCNT composites. Kim et al.⁹ synthesized CNT–poly (methyl methacrylate) (PMMA) composites and reported highest EMI SE for raw CNT–PMMA composite as ∼27 dB. J. Liang et al.¹⁴ reported the EMI SE of ∼21 dB at 15 wt% loading of reduced graphene sheets for the epoxy/reduced graphene based composites, prepared by solution casting method followed by ultrasonication. Gupta et al.⁴⁷ obtained the EMI SE of ∼19 dB at 15 wt% loading of carbon nanofibers (CNF) in solution casted PS/CNF composites. The above mentioned carbonaceous/polymer composites synthesized through well-established chemical routes have used high wt% of carbon materials, their EMI SE are comparable to the EMI SE results pertaining to a very low vol.% (and therefore low wt%) of FLG filled PVA matrix composites presented in this work. Moreover the filler in this work is as-synthesized material (i.e., without any post-synthesis process such a surface functionalization). The synthesized FLG/PVA composite sheets are flexible. Cross sectional morphology, structure and phase studies of these sheets showed varied distribution of FLG in PVA matrix. The composite with 0.5 vol.% FLG showed optimum distribution of FLG in PVA matrix. In this composite which exhibited a high EMI SE of ∼19.5 dB, FLG formed network-like features in PVA matrix. Absorption was found to be the dominant mechanism for EMI shielding in the present study.

Acknowledgements

The authors thank Mr M. Durga Prasad, Center for Nanotechnology (University of Hyderabad) for helping us with imaging using transmission electron microscope.

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