dielectric and electromagnetic interference shielding properties at ele

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Surfaces, Interfaces, and Applications

Graphene-wrapped MgO/polyvinyl alcohol composite sheets: dielectric and electromagnetic interference shielding properties at elevated temperatures Sandeep Kumar Marka, Vadali V. S. S. Srikanth, Bashaiah Sindam, Binoy Krishna Hazra, K.C. James Raju, and Prof. S. Srinath ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b05137 • Publication Date (Web): 03 Jun 2019 Downloaded from http://pubs.acs.org on June 3, 2019

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ACS Applied Materials & Interfaces

Graphene-wrapped MgO/polyvinyl alcohol composite sheets: dielectric and electromagnetic interference shielding properties at elevated temperatures Sandeep Kumar Marka1, Vadali V S S Srikanth1,*, Bashaiah Sindam2,3, Binoy Krishna Hazra3, K. C. James Raju3, and S. Srinath3 1School

of Engineering Sciences and Technology (SEST), University of Hyderabad, Gachibowli, Hyderabad 500046, India

2Advanced

Centre of Research in High Energy Materials, School of Physics, University of Hyderabad, Gachibowli, Hyderabad 500046, India

3School

of Physics, University of Hyderabad, Gachibowli, Hyderabad 500046, India ABSTRACT

Different amounts of graphene-wrapped magnesium oxide (G@MgO) powders are uniformly dispersed in polyvinyl alcohol (PVA) solution in different experiments to obtain solutions which are coagulated to obtain solid materials, which are then hot pressed at 413 K and 3 tonnes of pressure to finally obtain 1 mm thick free-standing G@MgO/PVA composite sheets in which the constituents namely graphene and MgO (in the form of G@MgO) are the nano-fillers in PVA matrix. During synthesis of G@MgO powder, MgO nano-particles are in-situ wrapped by the graphene nanosheets as revealed by electron microscopy. Uniformity of G@MgO dispersion in PVA was confirmed by secondary electron micrographs and the consistency in x-ray diffraction and Raman scattering data collected from different locations of the samples. Temperature (303393 K) dependent complex permittivity of G@MgO/PVA composite sheets (including those prepared by casting) in low frequency (20 Hz - 2 MHz), and high frequency (i.e., X-band, 8.2-12.4 *

Corresponding author. Tel: +91 40 2313 4453; E-mail address: [email protected] (V. V.

S. S. Srikanth) 1 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

GHz) ranges are measured. In both the frequency ranges, G@MgO/PVA composite sheets prepared by coagulation exhibited superior dielectric properties than PVA, and G@MgO/PVA composite sheets prepared by casting. A strong interfacial polarisation is observed in coagulated, and as-casted G@MgO/PVA composite sheets. It is noticed from the calculated activation energies that conduction is the dominating mechanism for energy transfer in both the composite sheets' cases while it is predominating in coagulated composite sheets due to the better network formation of the fillers in the coagulated samples than in the casted composite samples. The electromagnetic interference shielding effectiveness (EMI SE) values in the X-band frequency range (i.e., 8.2-12.4 GHz) of the G@MgO/PVA composite sheets prepared by coagulation are more than those prepared by casting for a particular weight fraction of G@MgO. At 393 K, for a particular G@MgO/PVA composite sheet prepared by coagulation, an excellent EMI SE of ~27.5 dB is measured. It is also experimentally elucidated that the absorption is the dominating mechanism for EMI SE in the prepared composite sheets. KEYWORDS: EMI shielding, Graphene, Dielectric, PVA, Composite, Elevated Temperature. INTRODUCTION In day-to-day life, the electromagnetic interference (EMI) shielding materials in the electronic devices used in civil, military, and aerospace applications work at elevated temperatures. For example, battery management systems, which are often operated at temperatures above 50 °C require good EMI shielding materials that perform at elevated temperatures are required. Therefore, metals and their alloys,1-5 ceramic materials like AlN,6 Al2O3,7 and SiBCN8 are considered as efficient EMI shielding materials. But the electrical conductivity of metals and their alloys decrease at elevated temperatures while EMI shielding capacity of ceramics is limited by their poor electrical conductivity. In this context, materials such as SiC,9,10 carbon nanotubes,11-13

2 ACS Paragon Plus Environment

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and graphene14-18 are found to be suitable EMI shielding materials due to their positive temperature coefficient of conductivity. Magnesia (MgO), an eco-friendly ceramic material with high melting point, high thermal conductivity at elevated temperatures, good chemical stability, high thermal shock resistance, and excellent dielectric behavior can be an excellent EMI shielding material at both room- and elevated- temperatures, if compounded with a conducting material. Recently, graphene nanosheets (GNSs) filled magnesia (GNSs/MgO) composites were synthesized by sintering at 1700 °C.19 The prepared sample with 3 vol% of GNSs showed enhanced electrical conductivity and superior dielectric and EM wave attenuation properties. The performance of such materials are improved by having interfaces at nanoscale in them. In the application point of view, such materials should possess light weight and be corrosion resistant besides exhibiting high EMI shielding efficiency. In this work, nanoscale interfaces between few-layered graphene20 and MgO are made by synthesizing graphene wrapped magnesium oxide (G@MgO) composite.21 As compared to several excellent studies on graphene/polymer composites for EMI shielding,22,23 the novelty of the present work is of two-fold: one, graphene wrapped magnesium oxide (G@MgO) is synthesized using simple combustion of Mg in the presence of solid carbon dioxide and subsequent purification with acid treatment and second, the obtained G@MgO is compounded with PVA using simple solution mixing and subsequent coagulation in ethanol and casting to obtain G@MgO/PVA composites. These G@MgO/PVA composites were hot pressed at 140 °C temperature and 3 tonnes of pressure to get 1 mm thick G@MgO/PVA composite circular sheets, which exhibited excellent EMI shielding ability. Here it should be noted that graphene/polymer composites are already proven to be better candidates in the point of view of applications.24 In general, the carbonaceous materials (here, G@MgO) interact with the polymer matrix (here, PVA) through hydrogen bonding. In the composite, the presence of conducting filler, dielectric matrix



and interaction (i.e., chemical bonding) between them play a key role to attenuate the incident EM wave through relaxation and conduction mechanisms, which are prominent at elevated temperatures. It may be noted that the literature on the temperature dependent EMI shielding materials is very limited.25-27 In this work, dielectric and EMI shielding properties of G@MgO/PVA composite sheets are measured at elevated temperatures (i.e., 303-393 K) in the low-frequency (i.e., 20 Hz-20 MHz) and high-frequency (X-band, i.e., 8.2-12.4 GHz) regions and some of these sheets are found to be commercially viable owing to the commercially viable materials’ preparation processes used and the measurement of EMI shielding effectiveness (EMI SE) values around or/and above 20 dB. The reasons for choosing the temperature range 303-393 K (i.e., 30-120 °C) are: i) the intended applications (for example, the electronic circuits) require to be stable at temperatures in the range of 50-150 °C during operation and ii) the melting point of PVA is around 200 °C, which is well above the maximum temperature in the considered temperature range. EXPERIMENTAL PROCEDURE 2.1 Synthesis of G@MgO/PVA Composite. G@MgO powder was first synthesized using a combustion process as mentioned in our earlier reports.28 Then, 1.992 g of PVA (M.W. 86000, Fisher Scientific) powder was mixed with 20 ml of deionized water and stirred at 333 K for 3 h to form a homogeneous solution. Then 0.4 wt. % of the filler material (G@MgO) was uniformly dispersed in 50 ml of ethanol (AR) in a bath sonicator operated at 100 W for 2 h. This solution was added drop by drop to the PVA solution and stirred for 2 h to form a homogeneous and viscous G@MgO/PVA composite solution. Small bubbles formed during the mixing were removed using mild sonication for few seconds. The bubble free solution was coagulated using ethanol, and then to remove excess ethanol, the obtained material was washed with water for 3-4 times and dried at


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333 K for one day. Similarly, other samples with 0.6, 0.8, 1.0, 3.0, 10.0, and 15.0 wt.% of G@MgO containing G@MgO/PVA composites are prepared. In another batch, 0.4 and 3.0 wt.% (i.e., at and above the percolation threshold) G@MgO/PVA composite solution was prepared, and then this solution was casted in a borosilicate glass petri-dish. This cast solution was dried at 333 K overnight to obtain free-standing, and flexible G@MgO/PVA sheets. Later, the coagulated and ascasted composite samples are separately cut into circular pieces and hot pressed at a temperature of 413 K and 3 tonnes of pressure to form one-inch diameter circular discs with 1 mm thickness as shown in Fig. 1. These samples are represented as 3.0G@MgO/PVA_Coag and 3.0G@MgO/PVA_ac for coagulated and as-casted composite samples with 3.0wt.% of G@MgO filler material in PVA matrix, respectively. Here it may be noted that

Figure 1. Circular discs (diameter ~1 inch) of PVA, as-casted and coagulated (3.0G@MgO/PVA) samples. 2.2 Characterization of Materials. Morphological features of the synthesized materials were recorded using field emission scanning electron microscope (FEI, NovaNanoSEM 450) and transmission electron microscope (TEM, model FEI Technai G2 S-Twin) operated at 5 and 200 kV, respectively. The crystallinity of the samples was studied using the X-ray diffraction (XRD) technique with Bruker AXS model D8 Advanced system. XRD patterns were recorded from 5 to 90° using Cu Kα as the X-ray source (λ = 1.54 Å). Raman spectra were recorded in air using a NdYAG laser (wavelength ~532 nm) in the backscattering geometry using CRM spectrometer equipped with a confocal microscope (Alpha 300 of WiTec). The beam diameter of the laser was 5 ACS Paragon Plus Environment


680 nm ( with 100x objective lens), and the resolution of all the measurements was 3 cm-1. Raman spectra were recorded in the spectral region 0-3500 cm-1. Complex permittivity and loss tangent of the samples were measured at elevated temperatures (RT (303)-393 K at the interval of 10 K) in the frequency range of 20 Hz to 20 MHz with an LCR meter (model E4980A precession impedance analyser, Agilent) in the parallel plate capacitor geometry. 2.3 Testing Electromagnetic Interference (EMI) Shielding. EMI shielding ability of the synthesized G@MgO/PVA composite sheets was evaluated by Agilent 8722ES vector network analyser (VNA). G@MgO/PVA composite sheets were placed in an 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 TRL calibrations were carried out on the adapter surfaces using the standard X-band waveguide calibration kit before placing the sample holder with samples for the measurements. The measured scattering parameters S11 and S21 are related to the reflected and transmitted power, respectively w.r.t the power incident on the sample surface. The incident power (Pi) on a shielding material is divided into reflected power (Pr), absorbed power and transmitted power (Pt) at the output of the shielding. The EMI SE of material is defined as

29 P SE total  10 log  t  .  Pi 

When an electromagnetic radiation 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 below-given formulae: R  E R /E I   S11  S22 2

2

T  E T /E I   S12  S21 2

2

2 2

A  1 R  T

Total EMI SE (SEtotal) is the sum of reflection from the material surface (SER), absorption of electromagnetic energy inside the material (SEA) and multiple internal reflections (SEM) of 6 ACS Paragon Plus Environment

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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 inhomogeneity 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 SEM is negligible and thus,

The effective absorbance ( A

eff

) can be therefore expressed as

A eff =

SE total  SE R  SE A

.

1-R-T  . The SE due to 1-R 

reflection and absorption of the shielding material on power of the effective incident electromagnetic wave inside the shielding material are shown as SE R  10 log(1  R) SE A  10 log(T/(1  R)) SE total  SE R  SE A  10 log(T)

The temperature dependent S parameters were measured by placing the sample in an X-band sample holder between the flanges of two standard X-band waveguides connected to coaxial to waveguide adapters in furnace consisting a circular heating coil. Measurements at a particular temperature are taken after stabilising the temperature for 30 min. The permittivity values are evaluated from S11 and S21 using the standard Nicolson, Ross and Weir algorithms.30,31 RESULTS AND DISCUSSION Microstructural and Phase Characteristics. Figure 2(a) displays secondary electron micrograph of G@MgO composite powder with ultrafine clustered particles each of which is decoraed with similar nanosized particles. Bright field transmission electron micrograph of one such ultrafine particle is shown in Fig. 2(b). This semi- transparent ultrafine particle is decorated with nanosized MgO particles, and the edges of these particles are darker in contrast implying that they are graphene layers. High-resolution transmission electron micrograph shown in Fig. 2(c) 7 ACS Paragon Plus Environment


evidences the presence of graphene wrapped MgO nanoparticles, and some of the graphene layers acquired the cubic shape of MgO particles. The measured d-spacing in Fig. 2 (c) matches with the (200) plane of hexagonal MgO nanoparticle. The selected area electron diffraction pattern shown in the inset of Fig. 2 (b) also unveils presence of both hexagonally crystalline graphene and MgO particles in the composite powder. In panels 1-4 (as insets) of Fig. 2(c) inverse Fourier transform images of high resolution TEM image of G@MgO are shown. Panels 1-3 show cross-over, wrapped (wrapping MgO), touching graphene layers, and panel 4 unveils defects (missed atoms) in the graphene layers. Fig. 2(c) and panels 1-4 evidence 2-10 graphene layers in the G@MgO composite. As shown in Fig. 2 (c), the presence of defects and multiple interfaces (between graphene-graphene, and graphene-MgO) in the G@MgO are useful for EMI shielding application. Further, the XRD pattern (Fig. 2 (d)) evidences the presence of both graphene and MgO in the material. An average of eleven graphene layers is present in the G@MgO as calculated using (002) diffraction peak. The calculated number of layers from XRD data is closely matching with the high resolution TEM observations. Figure 2(e) shows Raman spectrum of G@MgO. The Raman active E2g mode at ~1569 cm-1 (G band), confims the sp2-hybridized carbon-carbon bonds in the G@MgO,32 while the defective D band and the 2D band are observed at ~1336, and 2668 cm-1 respectively. The G and 2D band indicate higher order graphene in the G@MgO. The intensity of D band is greater than that of the G band (i.e., ID/IG=1.18) indicating that the defects are present in the G@MgO while the sharp and intense 2D band indicates that G@MgO contains few layers of graphene. It is found from weight loss versus temperature, and derivative weight loss versus temperature plots (Fig. 2(f)) that G@MgO is constituted by 79 wt.% of graphene and 21 wt.% of MgO. The presence of conducting graphene with defects and dielectric MgO in G@MgO is thus a potential candidate for EMI shielding application when combined with PVA.


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Secondary electron micrographs of hot-pressed G@MgO/PVA_Coag and G@MgO/PVA_ac composites shown in Figs. 3 (a,b&c,d) and (e,f&g,h), respectively. As shown in Fig. 3 (a-d), the ultrafine G@MgO filler material in the 0.4G@MgO/PVA_Coag composite is well dispersed in PVA, and thick PVA layer coats the G@MgO particle. Similar kind of morphology is observed in the case of 3.0G@MgO/PVA_Coag composite sample (Fig. 3 (e-h)). On the other hand, the dispersion of filler material in G@MgO/PVA_ac composites appear to be more agglomerated than their coagulated counterparts. From the morphological observations, it is understood that the distribution of G@MgO particles in coagulated composite samples is better than that in as-casted composites. It is also observed that agglomeration of the filler material in the matrix increases as the weight fraction of filler in the PVA matrix increases.



Figure 2. (a) secondary electron micrograph, (b) transmission electron micrograph (inset, SAED), (c) high-resolution transmission electron micrograph and (d) intensity versus 2θ (x-ray diffractogram), (e) intensity versus wavenumber (Raman spectrum), and (f) weight versus temperature (TGA) and derivative weight loss versus temperature (DTA) of G@MgO powder, respectively. XRD patterns of PVA, all the coagulated and as-casted (0.4 G@MgO/PVA_ac and 3.0G@MgO/PVA_ac) composites are shown in Figs. 4 (a) and (b), respectively. In all the 10 ACS Paragon Plus Environment

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composites, PVA and G@MgO crystallized in monoclinic and hexagonal crystal systems, respectively. The process of hot pressing has enhanced the crystallization of PVA as evidenced from the appearance of new diffraction peaks (unlike in our previous report29) in all the composite samples. (10-1) diffraction peak is the major intense peak in monoclinic PVA.33 The variation of crystallite size of PVA (considering (10-1) peak) with the addition of G@MgO is investigated by applying Scherer’s formula (Table 1). The crystallite size of the PVA is ~65.5 Å while it decreased in coagulated samples with increasing filler weight fraction, except for 0.6 and 1.0G@MgO/PVA_Coag composites. It is observed that (as shown in Fig. 4 (a)) in the 0.6 and 1.0G@MgO/PVA_Coag composites (101) diffraction peak is developed at the expense of (10-1) diffraction peak intensity. It is also observed that the addition of G@MgO creates compressive stresses in the PVA matrix and these stresses help PVA crystals to grow.29



Figure 3. Secondary electron micrographs of (a,b&c,d) 0.4, and (e,f&g,h) 3.0G@MgO/PVA_Coag, and _ac composites at low and high magnifications, respectively. 12 ACS Paragon Plus Environment

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Similarly, the crystallite size in the case of 3.0G@MgO/PVA_ac is greater than that in the case of 0.4G@MgO/PVA_ac. The absence of peaks related G@MgO in all the composites (except 10.0 and 15.0G@MgO/PVA_Coag) is an indication of excellent dispersion of the filler in the PVA matrix. The appearance of graphenes (002) and MgO’s (200) diffraction peaks in the 10.0 and 15.0G@MgO/PVA_Coag composite samples are expected due to the high amount of G@MgO powder in the PVA matrix.

(b)

1.0_Coag. 0.8_Coag. 0.6_Coag. 0.4_Coag. PVA

10

20

30

40

50

60

0.4_ac *(101)

*( 20 0)

10.0_Coag. 3.0_Coag.

G@MgO

3.0_ac

Intensity (arb. units)

Intensity (arb. units)

*(100) *(001) *(101) *(*(10-1) 20 0)

(#(100), @(200)) *PVA, #Graphene @MgO15.0_Coag. @(220) #(002)

*(100) *(001)

(a)

G@MgO

70

10

20

2 (Degree)

PVA

#(100), @(200) #(002) @(220) @(111)

30

40

50

60

70

2 (Degree)

(d) Intensity (arb. units)

1346 1583 15.0_Coag.

2683

(c) Intensity (arb. units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2917

10.0_Coag. 3.0_Coag. 1.0_Coag. 0.4_Coag.

G@MgO 1148 859 916

500

1445

2910

PVA

1000 1500 2000 2500 3000 3500

3.0_Coag.

0.4_Coag.

G@MgO 1148 1366 916 859 1074

500

-1

1445

2910

PVA

1000 1500 2000 2500 3000 3500 -1

Wavenumber (cm )

Wavenumber (cm )

Figure 4. X-ray diffractograms and Raman spectra of (a, and c) coagulated, and (b, and d) ascasted G@MgO/PVA composites, respectively.



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Table 1. 2Theta, crystallite size, and ID/IG ratio of coagulated and as-casted samples. Material PVA G@MgO 0.4G@MgO/PVA_Coag 0.6G@MgO/PVA_Coag 0.8G@MgO/PVA_Coag 1.0G@MgO/PVA_Coag 3.0G@MgO/PVA_Coag 10.0G@MgO/PVA_Coag 15.0G@MgO/PVA_Coag 0.4G@MgO/PVA_ac 3.0G@MgO/PVA_ac

X-ray diffraction 2Theta Crystallite Size (Degrees) (Å) 19.97 65.50 19.99 20.17 19.98 20.28 20.01 19.97 19.98 19.92 20.01

60.90 114.03 57.69 113.09 64.04 56.16 53.85 56.30 77.52

Raman ID/IG 1.18 1.12 0.97 1.08 0.98 0.95 1.11 0.84

Raman spectrum of PVA is discussed in detail in our previous report.29 The vibrational modes related to PVA namely, stretching mode of CH2, (CH2), symmetric bending mode of the CH2 group, δ(CH2), wagging, and rocking modes of the CH2 group (γw(CH2), and γr(CH2)) appeared at 2910, 1445, 1366, and 859 cm-1, respectively. In the coagulated composite samples (Fig. 4(c)) a continuous decrease in the intensity of (CH2) peak and a slight shift in this peak position are observed. This change is attributed to stresses in the matrix and the decrease in intensity can be understood that the filler material in the matrix try to form a network type of structure and which influence the crystalline phase of PVA (it is clearly observed as increased intensity of D, G, and 2D bands in the G@MgO/PVA_Coag composites). On the other hand, in the as-casted composite samples (Fig. 4(d)) the intensity of the  (CH2) peak increased as the weight fraction of G@MgO in PVA is increased. The reason for the increase in intensity was expected because loading of G@MgO to PVA matrix causes the CH2 to form some clustering which results in enhancement of intensity. These observations are in line with XRD results. In both the G@MgO/PVA_Coag and _ac composites, ID/IG value is continuously decreased, which is again an indication of good 14 ACS Paragon Plus Environment

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dispersion of the (formation of network type) filler in the PVA. It is predictable that combination of conducting graphene, dielectric MgO, and PVA in the synthesized composite materials can produce novel dielectric properties. The dielectric properties of these composite materials are presented in the 20 Hz-2 MHz and the X-band frequency (i.e., 8.2-12.4 GHz) range. Dielectric Properties in low-frequency range (20 Hz-2 MHz). Eq.134 explains the variation of the dielectric constant of material in an alternating electric field,

εs  ε ..........(1) 1  ω2 τ 2 where ε s and ε  are dielectric constant values at static and infinite frequencies respectively,  is ε'  ε 

the angular frequency, and τ is th e relaxation time. At very low frequencies (i.e., ω  1

τ

),

dipoles follow the field and ε '  ε s (dielectric constant at the quasi-static field). As the frequency ' increases (i.e., ω  1 ), dipoles begin to lag behind the field and a slight decrease in ε value. τ

When the frequency reaches characteristic frequency (i.e., ω  1 ), the dielectric constant drops τ (this is called relaxation process). At very high frequencies (i.e., ω  1 ), dipoles can no longer τ follow the field and ε '  ε  (the value of ε ' at high frequencies). The dielectric constant versus frequency of the PVA and G@MgO/PVA_Coag composites are shown in Fig. 5 (a-h). A monotonic decrease in the dielectric constant is observed at low temperatures and it is increased with frequency and achieves a constant value at high frequencies. It is also noted that dielectric constant increases with temperature and it is a typical behaviour of dipolar/polymer materials due to the presence of thermally active defects in the graphene and ionic groups in PVA.34 Similar behaviour is observed in G@MgO/PVA_ac composite samples as shown in Figs. 6 (a) and (b). Insets of Figs. 5 (a-h) and Figs. 6 (a and b) are the variation of tangent loss versus frequency at



different temperatures. Two loss peaks (one in low-frequency range and other at the highfrequency edge) observed in PVA and all the composite materials (except in 10.0 and 15.0G@MgO/PVA_Coag) and as the temperature increases, they shifted towards the highfrequency side. The loss peaks and their change with temperature suggest thermally activated relaxation mechanisms. In the G@MgO/PVA_ac composites, and 0.4-1.0G@MgO/PVA_Coag composites a slight variation in loss tangent with an increase in temperature and on the other hand a massive shift of loss tangent in 3.0-15.0G@MgO/PVA_Coag composites with temperatures is observed. This is expected due to increased conductivity of the 3.0-15.0G@MgO/PVA_Coag composites. In the amorphous phase, dipolar molecules can orient from one equilibrium position to other equilibrium position relatively quickly and contribute to the absorption over a broad frequency range or temperature range. The high dielectric loss observed at low frequencies is due to the accumulation of space charge at the boundary between filler and matrix and at high frequencies periodic reversal of the electric field is so high such that less charge can diffuse in the electric field direction which reduces accumulation of space charge, and consequently it results in decrease in the dielectric loss.


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40 C 60 C 80 C 100 C 120 C

60

1

10

(a)

0

10

1

10

(b)

0

10

tan 

30 C, 50 C, 70 C, 90 C, 110 C,

tan 

70 60 50 40 30 20 10

40

-1

-1

10

10

-2

10

-2

10

3

2

3

10

5

10

10

Frequency (Hz)

4

10

4

10

5

10

6

10

2

2

10

10

Frequency (Hz) 80

(c)

-1

120

3

10

20 2

10

3

4

10

4

5

10

10

Frequency (Hz)

5

10

6

10

6

10

10

Frequency (Hz) 250

1

10

(e)

200 0

tan 

150

10

-1

10

100 50

2

3

10

2

10

3

4

10

6

10

10

Frequency (Hz)

4

10

5

10

5

10

Dielectric Constant (')

10

2

5

6

10

6

10

10

6

10

(d)

10

10

0

10

-1

10

-2

10

80

-3

10

2

3

10

40 2

10

3

10

4

10

4

5

10

10

Frequency (Hz)

5

10

6

10

6

10

10

Frequency (Hz) 600 500 400 300 200 100

1

10

(f)

0

10

-1

10

2

10

Frequency (Hz)

3

2

3

10

10

4

10

4

5

10

10

Frequency (Hz)

5

10

6

10

6

10

10

Frequency (Hz) 8000

(g)

1

10

tan 

750 600 450 300 150 0 2 10

4

1

0

10

10

200 160

40

5

10

Frequency (Hz)

Frequency (Hz)

10

tan 

60

4

10

10

tan 

1

10

3

10

tan 

100

3

10

20

6

10

3

10

2

3

10

10

4

5

10

10

Frequency (Hz)

4

10

5

10

6

10

6

10

0 2 10

1

10

0

10

2000

-1

10

10

4000

0

10

(h)

2

6000 tan 

2

10


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2

3

10

3

10

10

4

5

10

10

Frequency (Hz)

4

10

5

10

6

10

6

10

Frequency (Hz)

Frequency (Hz)

Figure 5. Dielectric constant and tangent loss versus frequency at different temperatures of (a) PVA, (b) 0.4, (c) 0.6, (d) 0.8, (e) 1.0, (f) 3.0, (g) 10.0, and (h) 15.0G@MgO/PVA_Coag composites, respectively.



0

10

-1

10

40 2

10

30

3

10

4

5

10

10

Frequency (Hz)

6

10

20 10 2 10

3

10

4

5

10


0

tan  -1

10

2

10

3

10

4

5

10

10

Frequency (Hz)

6

10

20 2

10

3

10

4

10

5

10

3

4

10

5

10

10

Frequency (Hz)

6

10

0 2

10

40

2

10

3

10

0.4G@MgO/PVA_ac

60

-2

10

(d)

0.4G@MgO/PVA_Coag. 101

-1

10

100

10

Frequency (Hz)

(c) 80

0

10

200

6

10

1

10

tan 

50

(b)300

1

10

10

4

5

10

6

10

10

Frequency (Hz) 1

3.0G@MgO/PVA_Coag. 10 3.0G@MgO/PVA_ac

400 tan 

60

40 C 60 C 80 C 100 C 120 C


30 C, 50 C, 70 C, 90 C, 110 C,

tan 


(a)70


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 42

0

10

-1

10

200

6

10

0 2 10

Frequency (Hz)

2

10

3

10

4

5

10

10

6

10

Frequency (Hz)

3

10

4

10

5

10

6

10

Frequency (Hz)

Figure 6. Dielectric constant and tangent loss versus frequency at different temperatures of (a) 0.4, (b) 3.0G@MgO/PVA_ac composites, and a comparison of dielectric constant and tangent loss versus frequency @120 °C of (c) 0.4 and (d) 3.0G@MgO/PVA_ac, and _Coag composites, respectively. As shown in Table 2, at 120 °C and 1 kHz, the dielectric constant of G@MgO/PVA_Coag and G@MgO/PVA_ac composites increase in the ranges 1.02-44.78, and 1.55-3.75 times, respectively, than the PVA. Figs. 6 (c) and (d) display, the comparison of dielectric constant and loss tangent (at 120 °C) of 0.4 and 3.0G@MgO/PVA_Coag and _ac composite samples. It is noticed that at a higher weight fraction of the filler in PVA, dielectric constant and loss tangent of coagulated samples is more than the as-casted composite samples. This is expected in coagulated samples due to good dispersion of filler material in the matrix. This type of distribution creates more filler-matrix interfaces in the coagulated samples than the as-casted samples. When an


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electric field is applied on a material, due to different dielectric properties and conductivities of the filler and matrix, charge accumulation takes place at the interface, and it causes interfacial or Maxwell-Wagner-Sillars (MWS) polarization.35 To understand the interfacial polarization in the prepared composites, the study of frequency dependent imaginary part of electric modulus at different temperatures is necessary. The electric modulus can be calculated from permittivity using Eq. 2 as follows,

1 ε' ε" M  M  iM  *  '2 "2  i '2 "2 ..........(2) ε ε ε ε ε *

'

"

where M’ and M” are real and imaginary parts of the electric modulus, respectively. Table 2. Real part of dielectric constant, and activation energies due to conduction and relaxation of G@MgO/PVA_Coag, and _ac composites (values in bracket represents enhancement w.r.t. PVA value) Coagulated composite samples wt.% G@MgO

of

as-casted composite samples

Activation Energy (eV)

ε ' @120

1kHz

Ea-conduction

Et-relaxation

& 1kHz

Ea-conduction

Et-relaxation

0

25.5

1.81±0.11

1.28±0.04

25.5

1.81±0.11

1.28±0.04

0.4

28.8(1.13)

0.46±0.08

1.59±0.03

39.7(1.55)

0.47±0.05

1.07±0.09

0.6

41.4(1.62)

0.16±0.03

1.37±0.05

-

-

-

0.8

71.0(2.78)

0.44±0.02

1.04±0.07

-

-

-

1.0

80.7(3.16)

0.99±0.07

1.35±0.08

-

-

-

3.0

159.9(6.27)

0.22±0.04

0.58±0.04

95.5(3.75)

0.87±0.05

1.37±0.11

10.0

194.2(7.62)

0.22±0.03

0.37±0.05

-

-

-

15.0

1124.0 (44.78)

0.07±0.00

0.12±0.01

-

-

-

ε ' @120

°C &


°C

Activation Energy (eV)


To investigate the influence of interfacial polarization or MWS polarization on the substantial increase in dielectric constant and loss tangent of the G@MgO/PVA coagulated and as-casted composites, the M″ versus frequency (M″ vs f, at a different temperature) curves of different samples are shown in Figs. 7 (a-h), and Figs. 8 (a-d), respectively. As shown in Figs. 7 (a-d), and Figs. 8 (a-c) at a certain temperature (here 70°C or above) in the composite materials, the intrinsic immobilized free charge can move freely under the influence of applied field and blocked at the interface between insulating matrix and conducting filler due to different dielectric response to the applied electric field. Similarly, in the PVA (as shown in Fig. 8 (a)), as the temperature increases the segmental motion of polymer chains increases and response of free charges to the applied electric field increases and also charge carrier mobility increases. This causes a decrease in relaxation time of the charges, and it appears as a shift in loss peak towards higher frequency side. On the other hand, 3.0-15.0G@MgO/PVA_Coag composites (Figs. 7 (e-g)) show a huge shift in loss peak compared with PVA and other composites and they mildly respond to the temperature. This is expected due to increased conductivity of these composites. In the conducting materials, charge transport (with temperature) takes place via hopping mechanism or tunneling mechanism 36.

It is observed in literature that, if the composite contains conductive fillers dispersed and

separated by insulating matrix hopping mechanism dominates or if the composite contains conductive fillers dispersed by forming a network of fillers throughout the matrix and are separated by thin insulating matrix then tunneling mechanism dominates.36 The salient feature of these mechanisms is that hopping mechanism depends on temperature whereas tunneling mechanism is independent (or less dependent) of the temperature.36 Figure 7 (h) and Fig. 8 (d) represent frequency dependent imaginary part of the electric modulus of coagulated and PVA, as-casted composite samples at 120 °C temperature, respectively. With increasing filler content in the matrix


Page 20 of 42

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the loss peak shifts towards higher frequencies due to increase in new interfaces that reduces relaxation times. But in the present work, for first composition (i.e., at 0.4 wt.% of filler in both the composite samples) relaxation peak shifted to lower frequency side which is attributed to a reduction in crystallinity of PVA matrix as shown in Table 1 and less amount of filler material. With further increase in filler content in the PVA the loss peak shifts towards higher frequencies. An appreciable amount of shift is observed in as-casted samples than in the coagulated samples. It is also observed that in the coagulated samples case, as the filler content is equal to or greater than 3.0 wt.% there is huge shift in loss peak towards higher frequencies while the loss peak is not observed at the percolation threshold. A new curve is observed. From the above discussion, it is understood that for a critical amount of filler addition (percolation threshold) to PVA different loss mechanism (may be conduction loss) is dominating the interfacial or MWS polarization. To further confirm the above mechanisms the spectra of M″ versus f are scaled as shown in Figs. 9 (a) and (b) for both the composites.37 In the scaling process, M″ is scaled by M″max (where M″max is the maximum value of M″ in M″ vs. f spectrum) and f is scaled by fmax (i.e., the frequency of which maximum of the loss peak appears). It is noted that spectra of all the composites do not merge onto a single master curve, which concludes that relaxation in the material depends on the amount of filler material in the composite. To understand composition dependent transportation, the activation energies30 of all the composites are calculated from the Eqs. 3 and 4, E τ max  τ 0 exp ( t )..........(3) kT

-E

and σ dc  σ 0 exp ( a )..........( 4) kT where  max is the relaxation time (i.e., τ max  1 2f ) for which peak maximum in M” vs. log max (f) appears,  0 is the characteristic relaxation time, k is the Boltzmann constant, Et is the activation



energy for the migration of free charges, Ea is the activation energy for the conduction of charge carriers, and T is the temperature.

20

(b)

40 C, 50 C 70 C, 80 C 90 C, 100 C 110 C 120 C

10

0 2 10

(c)

30 C, 60 C,

3

10

4

10

5

10

Frequency (Hz)

(d)

20

5

3

10

4

10

5

10

5

3

10

4

10

5

10

5

10

5

10

10

Frequency (Hz)

6

20 15

Frequency (Hz)

(f)

20

-3

10 5

3

10

4

10

5

10

Frequency (Hz)

(h) -3

M" (x 10 )

-3

5

0 2 10

3

10

4

10

5

10

Frequency (Hz)

3

10

6

10

4

10

10

Frequency (Hz)

6

10

5

0 2 10

6

10

10

5

15

M"(x10 )

15

0 2 10

10

0 2 10

6

10

-3

M"(x10 )

10

-3

M"(x10 )

-3

M"(x10 )

10

0 2 10

(g)

15

0 2 10

6

10

15

(e)

20

-3

30

M"(x10 )

-3

M"(x10 )

(a)

M"(x10 )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 42

3

10

4

10

10

Frequency (Hz)

6

30

20

PVA, 0.6_COag. 1.0_Coag. 10.0_COag.

0.4_Coag. 0.8_COag. 3.0_Coag. 15.0_COag.

10

0 2 10

3

10

4

10

5

10

Frequency (Hz)

6

10

Figure 7. The plots of M″ versus f and temperature of (a) 0.4, (b) 0.6, (c) 0.8, (d) 1.0, (e) 3.0, (f) 10.0, (g) 15.0G@MgO/PVA_Coag composites, and (h) comparison of (a), (b), (c), (d), (e), (f), and (g) at 120 °C, respectively. 22 ACS Paragon Plus Environment

30 C, 50 C, 70 C, 90 C, 110 C,

-3

M" (x 10 )

25 20

(b) 15

40 C 60 C 80 C 100 C 120 C

-3

(a) 30

15 10

10

5

5

(c) 15

4

10

5

10

0 2 10

6

10

Frequency (Hz)

(d) 30

10

-3

-3

3

10

M" (x 10 )

0 2 10

M" (x 10 )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


M" (x 10 )

Page 23 of 42

5

0 2 10

3

10

4

10

5

10

4

10

5

6

10

10

Frequency (Hz) PVA 0.4_ac 3.0_ac

20

10

0 2 10

6

10

3

10

Frequency (Hz)

3

10

4

10

5

10

6

10

Frequency (Hz)

Figure 8. Plots of M″ versus f and temperature of (a) PVA, (b) 0.4, (c) 3.0G@MgO/PVA_ac composites, and (d) comparison of (a), (b), and (c) at 120 °C, respectively. In Eq. 5, σdc is dc conductivity; σ0 is the characteristic conductivity. σdc is calculated from the acuniversality law (i.e., Jonscher Law)36. According to this law, at constant temperature ac conductivity, can be expressed as, σ ac  σ dc  A s ..........(5)

where  , A, and s are angular frequency (i.e., 2πf), parameters depending on temperature and filler concentration in the composite, respectively. The slope of ln( τ max ) vs. 1000/T and ln( σ dc ) vs. 1000/T curves give activation energies due to relaxation and conduction mechanisms, respectively and those values are presented in Table 2. As



shown in Figs. 9 (a) and (b), the activation energy due to relaxation mechanism in PVA is less than the conduction mechanism. Which means polarization is the dominant mechanism for energy transfer in PVA. In both the composite samples, activation energy due to conduction is lesser than the relaxation mechanism, and it is predominate in coagulated samples. In as-casted specimens, activation energy due to conduction mechanism is less than that in PVA due to higher relaxation in comparison to that in PVA. Similarly, in coagulated samples activation energy due to conduction mechanism is less than that in PVA. When compared, the activation energies due to conduction in coagulated samples are less than those in the as-casted samples. It is noticed that in both the composite samples conduction mechanism is the dominating mechanism for energy transfer and it is more active in coagulated composite samples than in the as-casted composites. The excellent charge transport in coagulated samples than as-casted samples is expected due to well dispersed G@MgO in PVA matrix which associates with defects, and the conductingconducting (graphene-graphene), dielectric-dielectric (MgO-PVA), and conducting-dielectric (graphene-MgO and graphene-PVA) interfaces. EMI SE and Dielectric Properties in the X-band Frequency Range (i.e., 8.2-12.4 GHz). EMI SE of PVA, and 0.4, 3.0, 10.0, and 15.0G@MgO/PVA_Coag, and 0.4 and 3.0G@MgO/PVA_ac composites at 303, 333, 363, and 393 K are shown in Figs. 10 (a) and (b). A slight increase in the EMI SE value of PVA than both 0.4G@MgO/PVA composites at all the measured temperatures is observed. As shown in Fig. 10 (a), the EMI SE of coagulated composites is more than the PVA. Similarly, Fig. 10 (b) displays, EMI SE of 3.0G@MgO/PVA_ac composite at all the measured temperatures is more than PVA. The decrease in EMI SE of 0.4G@MgO/PVA sample in both the composites is expected due to poorer crystallinity of PVA. At 120 °C and 8.2 GHz, the EMI SE of the PVA, 0.4 and 3.0G@MgO/PVA_ac composites is ~4.75, ~3.0, and ~5.91 dB, respectively.


Page 24 of 42

(a)

1.2

PVA, 0.4_Coag., 0.8_Coag. 1.0_Coag. 3.0_Coag. 10.0_Coag. 15.0_Coag.

M"/M"max

1.0 0.8 0.6 0.4

-3

(c)

-2

10

-1

0

10

10

f/fmax

1

10

2

10

0.5

1

10

2

10

3

10

-5 -6 -7 -8 -9 -10 -11 -12 -13 2.5

-8

(d)

ln (max)

ln (max)

0

10

f/fmax

-6

-10 -12 -14

(e)

0.0 -3 -2 -1 10 10 10

3

10

-4

2.5

PVA 0.4_ac 3.0_ac

1.0

0.0 10

(b)

1.5

0.6_Coag.

0.2

2.6

2.7

2.8 -1 1000/T (K )

2.9

2.6

2.7

2.8 -1 1000/T (K )

2.9

(f)

-10 -6

-15

ln (dc)

-9 ln (dc)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


M"/M"max

Page 25 of 42

-20

-12

-25

-15 -18

-30

-21 2.6

2.8

3.0 3.2 -1 1000/T (K )

3.4

-35

2.6

2.8

3.0 3.2 -1 1000/T (K )

3.4

Figure 9. Plots of (a, b) M″/M″ max versus f/fm, (c, d) ln( τ max ) versus 1000/T and (e, f) ln( σ dc ) versus 1000/T of coagulated and as-casted G@MgO/PVA composites, respectively. On the other hand, EMI SE values of 0.4, 3.0, 10.0, and 15.0G@MgO/PVA_Coag composites are ~4.05, ~16.1, ~19.3, and ~28 dB, respectively. A huge increase in EMI SE values from 3.0wt.% G@MgO is attributed to increased conductivity of the composite as observed in Figs. (5-7). As shown in Figs. 10(c) and (d), a continuous increase in the EMI SE values of coagulated composite 25 ACS Paragon Plus Environment


samples are observed as compared with as-casted composites, i.e., at 120 °C and 12.4 GHz an increase of 1.35 and 3.74 times is found in 0.4 and 3.0G@MgO/PVA_Coag samples, respectively than in their as-casted counterparts. This observation is in support to observed lesser activation energies (mentioned in Table 2) of coagulated samples than as-casted samples. The high SE of coagulated samples than of as-casted samples at the same loadings of G@MgO in PVA is attributed to the formation of network type structure (inside PVA), which leads to more surface sites available for the incoming wave to interact and communicate throughout the network. It is also observed that SE of each sample is almost constant in the entire “X band” frequency range. Figure 11 and Fig. 12 show EMI SE, SEA and SER versus temperatures and frequency of G@MgO/PVA_Coag and G@MgO/PVA_ac composites, respectively. From these figures it is clear that absorption is the dominant mechanism for EMI shielding which is quite important for most of the shielding applications especially for reducing electromagnetic interference (EMI). This behaviour is well correlated with observed morphology, structure and phase of the G@MgO/PVA composites. The EMI SE required for most of the commercial applications is ~20 dB. In the present work, 10.0 and 15.0G@MgO/PVA_Coag composites can be used as EMI SE materials for industrial applications.


Page 26 of 42

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Figure 10. 3D plots of EMI SE versus frequency and temperature of (a) coagulated, (b) as-casted composite samples, and a comparison between (c) 0.4 wt.%, and (d) 3.0 wt.% of G@MgO/PVA composite samples, respectively.



Figure 11. Plots of (a) EMI SE, (b) SEA, and (c) SER, of G@MgO/PVA_Coag composites, respectively.


Page 28 of 42

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Figure 12. Plots of (a) EMI SE, (b) SEA, and (c) SER of G@MgO/PVA_ac composites, respectively. To further understand the reasons behind the observed increase in EMI SE of G@MgO/PVA composites, complex permittivity, (ε∗ =ε’-iε”) is evaluated from experimental scattering parameters (S11 and S21) using theoretical calculations given by Nicholson, Ross and Weir algorithms13. The dielectric constant (ε’) is mainly associated with the amount of polarization occurring in the material, and the imaginary part (ε”) is a measure of conduction loss. Figs. 13 (a) and (b) show 3D plots of complex permittivity versus frequency and temperature of G@MgO/PVA_Coag composites, respectively. As shown in Fig. 13 (c), it is observed that the dielectric constant of 0.4 wt.% coagulated composite is slightly more than the PVA. An enormous 29 ACS Paragon Plus Environment


Page 30 of 42

increase in dielectric constant (real and imaginary parts) of the other composites (Figs. 13 (d-f)) is also

observed;

for

example,

complex

permittivity

(real

and

imaginary

parts)

of

15.0G@MgO/PVA_Coag composite sample is 5 times more than the value of PVA. Similarly, Figs. 14 (a) and (b) represent 3D plots of real and imaginary parts of the dielectric constant of G@MgO/PVA_ac composites, respectively. The complex permittivity of 0.4 G@MgO/PVA_ac composite is less than PVA, and it is attributed to the less amount of G@MgO addition, and the reduction of PVA crystallinity by adding 0.4G@MgO in the PVA matrix (as shown in Table 1). The complex permittivity of 3.0G@MgO/PVA_ac composite is increased when compared with PVA. As shown in Figs. 11 (a-c) (i.e., permittivity versus temperature at 8.2, 10.2, and 12.2 GHz frequency) in all the composites, it is noticed that dielectric constant (real and imaginary part) is increased with temperature and slightly decreased with the frequency at a constant temperature. In all the composites, the dielectric constant (real and imaginary parts) increased with temperature and decreased slightly with increasing frequency. The excellent dielectric performance of the coagulated samples is attributed to the ionic, electronic, and orientational and space charge (or interfacial) polarization, and especially due to improved conduction in the materials with a developed network of interconnected conducting fillers in the PVA. In PVA, the observation is attributed to segmental motion of polymer chains with temperature rise.34 This observation is in support to observed activation energies of these composite materials in the low-frequency range. The combustion synthesis of G@MgO and subsequent composite preparation can create defects like missing carbon atoms and sheet corrugation in the hexagonal carbon lattice of graphene. Energy transition of microwave band involves the electronic spin, which means greater spin states are required for microwave absorption. It has been reported that localised states near to the Fermi level could be created by introducing lattice defects.20 Thus the existence of defects in graphene


Page 31 of 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


favours absorption of electromagnetic energy by the transition from contiguous states to Fermi level when the absorbing surface is irradiated. As observed from low-frequency dielectric data (Fig. 5-Fig. 9), relaxation and conduction (domination) are the main reasons for microwave attenuation in the composite materials. The presence of defects in the graphene, dielectric MgO, the segmental motion of the PVA chains, and interfaces such as conducting-conducting (graphenegraphene, cross over layers), dielectric-dielectric (MgO-PVA), and conducting-dielectric (graphene-MgO and graphene-PVA) are primary reasons for the polarization processes. First, the defects that are prominent in these systems get screened by free charges and can act as polarization centres, which would generate strong oscillating polarization under the alternating electromagnetic field and attenuate electromagnetic wave, resulting in a profound effect on the loss of microwave energy (There is no relaxation observed in the microwave range. Dielectric constant has come down drastically because of relaxation at lower frequencies). Second, MgO is a good dielectric material38 on the application of electric field a charge separation takes place, which generates electric dipole polarization. Therefore, under the alternating electromagnetic field, electron motion hysteresis in these dipoles can induce additional polarization processes which are favourable in enhancing microwave absorbing ability. Third, interfacial polarization (or space charge polarization) appears due to the local heterogeneity in the material. The presence of well dispersed and network type (as demonstrated in morphological, structural and phase studies) G@MgO in the insulating matrix results in the formation of more interfaces. Due to the difference in the dielectric constant and conductivity of G@MgO and PVA, some charge carriers present in G@MgO are trapped, and, as a result, some space charge polarization is developed on the interface between the PVA molecules and the G@MgO. This generates space charges at the various interfaces leading to the distortion of field when an electromagnetic wave passes through the material. Fourth, when



electromagnetic energy incident on the composite material part of the energy is absorbed and dissipated as heat due to formed network type filler in the composite material.

Figure 13. 3D plots of complex permittivity versus frequency and temperature of (a, b) coagulated, and complex permittivity versus temperature at a constant frequency for (c) 0.4, (d) 3.0, (e) 10.0, and (f) 15.0G@MgO/PVA_Coag composites respectively. 32 ACS Paragon Plus Environment

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Figure 14. 3D plots of complex permittivity versus frequency and temperature of (a, b) ascasted, and complex permittivity versus temperature at a constant frequency for (c) PVA, (d) 0.4, and (e) 3.0G@MgO/PVA_ac composite samples respectively.



Fig. 10-Fig. 14 show the EMI SE, SEA, SER, and complex permittivity of the G@MgO/PVA_Coag and _ac composites which increase with temperature rise. It is required to understand temperature dependent dielectric and shielding properties of the composite materials. According to Debye theory of dielectrics, the real and imaginary part of dielectric constant is determined by relaxation loss and electrical conductive losses in the materials25. And they are expressed as follows, ε'  ε  ε'' 

εs  ε ..........(1) 1  ω2 τ 2

(ε s  ε )ω σ(T)  ..........(6) 1  ω2 τ 2 ωε0

where ω is the angular frequency, τ is temperature-dependent relaxation time, ε s is the static permittivity (i.e., stationary dielectric constant), and ε  is the relative permittivity (i.e., optical dielectric constant). The drastic changes in real and imaginary part of complex permittivity are related to relaxation time (), and conductance (σ), which are relevant to the temperature as shown below10a, U

1 kT τ e ..........(7) (Similar to Eq. (3)) 2ν

where U is a potential barrier, ν is vibration frequency, k is the Boltzmann constant, and T is the temperature. The conductance of G@MgO is determined by25,

σ(T)  Ae

E 2kT

..........(8) (Similar to Eq. (4))

where A is a constant, and E is the band gap energy of G@MgO. Shielding effectiveness39 in terms of dielectric constant (), conductivity (σ), and permeability () is,


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SER  39.5  10  log10 [ σ

2fμ

]..........(9)

SE A  8.7t f ..........(10)

where =",  is the angular frequency of the electromagnetic wave, o is the permittivity of free space = 8.854 x 10-12 F m-1, " is the imaginary part of complex permittivity. From Eqs. 7 and 8, and Eqs. 9 and 10, it is noticed that as the temperature of dielectric material increases, its relaxation time decreases and the conductance of the material increases. As a result, both real and imaginary parts of the complex permittivity increases. At a constant thickness of the composite materials (here 1mm), as the conductivity of the material increases with temperature both SER and SEA increases and as a consequence of this the total shielding efficiency of the material increases. CONCLUSIONS Graphene-wrapped magnesium oxide (G@MgO) powder is synthesized by ignition and combustion of magnesium metal turnings in the presence of waste solid carbon dioxide and subsequent acid treatment. Morphological and structural and phase characteristics of G@MgO powder is carried out via scanning electron microscopy, transmission electron microscopy, x-ray diffraction, and Raman spectroscopy confirmed the presence of both graphene and MgO in the powder, and TGA-DTA also reveals that presence of 79% graphene and 21% MgO in the G@MgO. 1 mm thick G@MgO/PVA_Coag composites are prepared via sonication and coagulation of different weight fractions of G@MgO powder with polyvinyl alcohol (PVA) solution, and subsequent hot pressing at 140 °C temperature and 3 tonnes of pressure. Similarly, for comparison, G@MgO/PVA_ac composites are also prepared. Characterization of these materials with a multitude of techniques confirmed the formation of composite materials. Temperature (303-393 K) dependent complex permittivity measurements of these composites are carried out in low frequency (20 Hz - 2 MHz), and high frequency (i.e., X-band, 8.2-12.4 GHz) 35 ACS Paragon Plus Environment


ranges. The dielectric properties of G@MgO/PVA_Coag composites are superior to PVA and G@MgO/PVA_ac composites in both the frequency ranges. Electric modulus studies (i.e., M” vs. log (f) plots) reveals a high interfacial polarization exists in coagulated and as-casted G@MgO/PVA composites. It is noticed from calculated activation energies that conduction is the dominating mechanism for energy transfer in both the composites than the PVA and it is predominating in G@MgO/PVA_Coag composites than the G@MgO/PVA_ac composites. EMI SE of G@MgO/PVA_Coag composites is more than G@MgO/PVA_ac composites of the same amount of G@MgO in PVA. At 303 and 393 K, the average EMI SE of 10.0 G@MgO/PVA_Coag composite is ~14.2 and ~18.9 dB, and similarly, an EMI SE of ~20.6 and ~27.5 dB is found for 15.0G@MgO/PVA_Coag composite, respectively. It is found the absorption is the dominating mechanism for EMI SE in all the composites. It is concluded that observed superior EMI SE of G@MgO/PVA_Coag is attributed to the excellent dispersion of G@MgO in PVA matrix. ACKNOWLEDGEMENTS Sandeep Kumar Marka acknowledge Council of Scientific and Industrial Research for providing fellowship through CSIR-DSRF scheme with grant no. 09/414(1130)/2016 EMR-I. REFERENCES 1. Lee, T.-W.; Lee, S.-E.; Jeong, Y. G. Highly Effective Electromagnetic Interference Shielding Materials Based on Silver Nanowire/Cellulose Papers. ACS Applied Materials & Interfaces 2016, 8 (20), 13123-13132. 2. Hu, M.; Gao, J.; Dong, Y.; Li, K.; Shan, G.; Yang, S.; Li, R. K.-Y. Flexible Transparent PES/Silver Nanowires/PET Sandwich-Structured Film for High-Efficiency Electromagnetic Interference Shielding. Langmuir 2012, 28 (18), 7101-7106.


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10. Fan, X.; Yin, X.; Chen, L.; Zhang, L.; Cheng, L. Mechanical Behavior and Electromagnetic Interference Shielding Properties of C/SiC–Ti3Si(Al)C2. Journal of the American Ceramic Society 2016, 99 (5), 1717-1724. 11. Chen, Y.; Zhang, H.-B.; Yang, Y.; Wang, M.; Cao, A.; Yu, Z.-Z. High-Performance Epoxy Nanocomposites

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Two

types

of

graphene

(G)

wrapped

MgO/polyvinyl

alcohol

(PVA)

namely

G@MgO/PVA_Coaglated and G@MgO/PVA_as-casted composites are prepared and compared for their EMI shielding effectiveness (SE). At room temperature, G@MgO/PVA_Coaglated sample (with the same filler content) as that of G@MgO/PVA_as-casted sample showed 75% improvement in EMI SE. Coaglated composite showed EMI SE as high as 27.5 dB even at an elevated temperature.

16 14

EMI SE (dB)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 42 of 42

12 10 8

75%

3GMgO/PVA_ac 3GMgO/PVA_Coag

6 4 2 0

9

8.0x10

9

9.0x10

10

1.0x10

10

1.1x10

10

1.2x10

Frequency (Hz)


10

1.3x10

dielectric and electromagnetic interference shielding properties at ele

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