Poly(vinyl alcohol) Composite Sheets

Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 23714−23730

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Graphene-Wrapped MgO/Poly(vinyl 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 S. Srinath§ †

School of Engineering Sciences and Technology (SEST), University of Hyderabad, Gachibowli, Hyderabad 500046, India Advanced Centre of Research in High Energy Materials, School of Physics, University of Hyderabad, Gachibowli, Hyderabad 500046, India § School of Physics, University of Hyderabad, Gachibowli, Hyderabad 500046, India

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‡

ABSTRACT: Different amounts of graphene-wrapped magnesium oxide (G@MgO) powders are uniformly dispersed in poly(vinyl 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 t of pressure to finally obtain 1 mm thick freestanding G@MgO/PVA composite sheets in which the constituents, namely, graphene and MgO (in the form of G@ MgO), are the nanofillers in PVA matrix. During synthesis of G@MgO powder, MgO nanoparticles 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 Xray diffraction and Raman scattering data collected from different locations of the samples. Temperature (303−393 K) dependent complex permittivity of G@MgO/PVA composite sheets (including those prepared by casting) in low frequency (20 Hz to 2 MHz) and high frequency (i.e., X-band, 8.2−12.4 GHz) ranges are measured. In both frequency ranges, G@MgO/PVA composite sheets prepared by coagulation exhibited dielectric properties superior to those of PVA and G@ MgO/PVA composite sheets prepared by casting. A strong interfacial polarization is observed in coagulated and as-cast G@ MgO/PVA composite sheets. It is noticed from the calculated activation energies that conduction is the dominating mechanism for energy transfer in both 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 cast 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

1. INTRODUCTION In day-to-day life, the electromagnetic interference (EMI) shielding materials in 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. Therefore, metals and their alloys1−5 and ceramic materials such as 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 and graphene14−18 are found to be suitable EMI shielding materials due to their positive temperature coefficient of conductivity. Magnesia (MgO), an © 2019 American Chemical Society

ecofriendly 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 % GNSs showed enhanced electrical conductivity and superior dielectric and EM wave attenuation properties. The performance of such materials is improved by having interfaces at nanoscale in them. In the application point of view, such Received: March 22, 2019 Accepted: June 3, 2019 Published: June 3, 2019 23714

DOI: 10.1021/acsami.9b05137 ACS Appl. Mater. Interfaces 2019, 11, 23714−23730

Research Article

ACS Applied Materials & Interfaces

hot pressed at a temperature of 413 K and 3 t of pressure to form 1 in. diameter circular discs with 1 mm thickness as shown in Figure 1.

materials should be lightweight and possess corrosion resistance 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 2-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 t 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 from 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 in attenuating the incident EM wave through relaxation and conduction mechanisms, which are prominent at elevated temperatures. It may be noted that 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 to 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 stability 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.

Figure 1. Circular discs (diameter ∼ 1 in.) of PVA, as-cast, and coagulated (3.0G@MgO/PVA) samples (left to right). These samples are represented as 3.0G@MgO/PVA_Coag and 3.0G@MgO/PVA_ac for coagulated and as-cast composite samples with 3.0 wt % G@MgO filler material in PVA matrix, respectively. 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 a 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 Nd:YAG laser (wavelength ∼ 532 nm) in the backscattering geometry using a CRM spectrometer equipped with a confocal microscope (Alpha 300 of WiTec). The beam diameter of the laser was 680 nm (with 100× 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 (room temperature (RT, 303)−393 K at intervals of 10 K) in the frequency range of 20 Hz to 2 MHz with an LCR meter (model E4980A precession impedance analyzer, Agilent) in the parallel plate capacitor geometry. 2.3. Testing Electromagnetic Interference Shielding. EMI shielding ability of the synthesized G@MgO/PVA composite sheets was evaluated by Agilent 8722ES vector network analyzer (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, with respect to (wrt) 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 SEtotal = −10 log(Pt/Pi).29 When 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 below-given formulas:

2. 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 (MW, 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 a 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 333 K for 1 day. Similarly, other samples with 0.6, 0.8, 1.0, 3.0, 10.0, and 15.0 wt % G@MgO containing G@MgO/PVA composites were 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 cast in a borosilicate glass Petri dish. This cast solution was dried at 333 K overnight to obtain freestanding, and flexible G@MgO/PVA sheets. Later, the coagulated and as-cast composite samples are separately cut into small pieces and

R = (E R /E I)2 = |S11|2 = |S22|2 T = (E T /E I)2 = |S12|2 = |S21|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 electromagnetic radiation, expressed as SEtotal = SER + SEA + SEM. 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 23715


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Figure 2. (a) Secondary electron micrograph, (b) transmission electron micrograph (inset, SAED), (c) high-resolution transmission electron micrograph, and plots of (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. from S11 and S21 using the standard Nicolson, Ross, and Weir algorithms.30,31

the material, while the multiple reflections are the consequence of impedance mismatch at the two sample−air interfaces. When SEtotal ≥ 15 dB, it is usually assumed that SEM is negligible and thus, SEtotal ≈ SER + SEA. The effective absorbance (Aeff) can be therefore expressed as Aeff = (1 − R − T ) . The SEs due to reflection and absorption of the

3. RESULTS AND DISCUSSION 3.1. Microstructural and Phase Characteristics. Figure 2a displays secondary electron micrograph of G@MgO composite powder with ultrafine clustered particles, each of which is decorated with similar nanosized particles. The bright field transmission electron micrograph of one such ultrafine particle is shown in Figure 2b. 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. The high-resolution transmission electron micrograph shown in Figure 2c is evidence for 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 Figure 2c matches with the (200) plane of the hexagonal MgO nanoparticle. The selected area electron diffraction pattern shown in the inset of

(1 − R )

shielding material on the power of the effective incident electromagnetic wave inside the shielding material are shown as

SE R = − 10 log(1 − R ) SEA = − 10 log(T /(1 − R )) SEtotal = SE R + SEA = −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 a furnace consisting of a circular heating coil. Measurements at a particular temperature are taken after stabilizing the temperature for 30 min. The permittivity values are evaluated 23716


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

and graphene−MgO) in G@MgO are useful for EMI shielding application. Further, the XRD pattern (Figure 2d) is evidence for the presence of both graphene and MgO in the material. An average of 11 graphene layers is present in the G@MgO as calculated using the (002) diffraction peak. The calculated number of layers from XRD data is closely matching with the high-resolution TEM observations. Figure 2e shows the 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

Figure 2b also unveils the presence of both hexagonally crystalline graphene and MgO particles in the composite powder. In panels 1−4 (as insets) of Figure 2c inverse Fourier transform images of the high-resolution TEM image of G@ MgO are shown. Portions 1−3 show crossover, wrapped (wrapping MgO), and touching graphene layers, and portion 4 unveils defects (missed atoms) in the graphene layers. Figure 2c and portions 1−4 evidence 2−10 graphene layers in the G@MgO composite. As shown in Figure 2c, the presence of defects and multiple interfaces (between graphene−graphene, 23717


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

2D band are observed at ∼1336 and 2668 cm−1 respectively. The G and 2D bands indicate higher order graphene in G@ MgO. The intensity of the 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 a few layers of graphene. It is found from weight loss versus temperature and derivative weight loss versus temperature plots (Figure 2f) that G@MgO is constituted by 79 wt % graphene and 21 wt % 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. Secondary electron micrographs of hot pressed G@MgO/ PVA_Coag (panels a,b and e,f) and G@MgO/PVA_ac composites (panels c,d and g,h) are shown in Figure 3, respectively. As shown in Figure 3a,b, 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. A similar kind of morphology is observed in the case of 3.0G@MgO/PVA_Coag composite sample (Figure 3e,f). On the other hand, the dispersion of filler material in G@ MgO/PVA_ac composites appears 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-cast 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. XRD patterns of PVA and all the coagulated and as-cast (0.4 G@MgO/PVA_ac and 3.0G@MgO/PVA_ac) composites are shown in Figure 4a,b, respectively. In all of the 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. The (101̅) diffraction

peak is the major intense peak in monoclinic PVA.33 The variation of crystallite size of PVA (considering the (101̅) peak) with the addition of G@MgO is investigated by applying Scherer’s formula (Table 1). The crystallite size of the PVA is Table 1. 2θ, Crystallite Size, and ID/IG Ratio of Coagulated and As-Cast Samples X-ray diffraction material

2θ (deg)

crystallite size (Å)

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

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

∼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 Figure 4a) in the 0.6- and 1.0G@MgO/PVA_Coag composites the (101) diffraction peak is developed at the expense of the (101̅) 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 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 to G@MgO in all of 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 graphene’s (002) and MgO’s (200) 23718


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Figure 5. Dielectric constant and tangent loss versus frequency at different temperatures of (a) PVA and (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.

intensity. These observations are in line with XRD results. In both the G@MgO/PVA_Coag and _ac composites, the ID/IG value is continuously decreased, which is again an indication of good dispersion of the (formation of the 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 to 2 MHz and the X-band frequency (i.e., 8.2−12.4 GHz) ranges. 3.2. Dielectric Properties in Low-Frequency Range (20 Hz to 2 MHz). Equation 134 explains the variation of the dielectric constant of the material in an alternating electric field,

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. The Raman spectrum of PVA is discussed in detail in our previous report.29 The vibrational modes related to PVA, namely, the stretching mode of CH2, ν(CH2), the symmetric bending mode of the CH2 group, δ(CH2), and 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 (Figure 4c) a continuous decrease in the intensity of ν(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 as being that the filler material in the matrix tries to form a network type of structure and which influences 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-cast composite samples (Figure 4d) the intensity of the ν(CH2) peak increased as the weight fraction of G@MgO in PVA was increased. The reason for the increase in intensity was expected because loading of the G@MgO to PVA matrix causes CH2 to form some clustering which results in enhancement of

ε′ = ε∞ +

εs − ε∞ 1 + ω 2τ 2

(1)

where εs and ε∞ are dielectric constant values at static and infinite frequencies, respectively, ω is the angular frequency, and τ is the relaxation time. At very low frequencies (i.e., ω ≪ 1/τ), dipoles follow the field and ε′ = εs (dielectric constant at the quasi-static field). As the frequency increases (i.e., ω < 1/ τ), dipoles begin to lag behind the field and a slight decrease in 23719


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Figure 6. Dielectric constant and tangent loss versus frequency at different temperatures of (a) 0.4- and (b) 3.0G@MgO/PVA_ac composites, and a comparison of dielectric constant and tangent loss versus frequency at 120 °C of (c) 0.4- and (d) 3.0G@MgO/PVA_ac and _Coag composites, 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 wrt PVA Value) coagulated composite samples

as-cast composite samples

activation energy (eV) wt % G@MgO

ε′ at 120 °C, 1 kHz

0 0.4 0.6 0.8 1.0 3.0 10.0 15.0

25.5 28.8(1.13) 41.4(1.62) 71.0(2.78) 80.7(3.16) 159.9(6.27) 194.2(7.62) 1124.0 (44.78)

Ea, conduction 1.81 0.46 0.16 0.44 0.99 0.22 0.22 0.07

± ± ± ± ± ± ± ±

0.11 0.08 0.03 0.02 0.07 0.04 0.03 0.00

activation energy (eV)

Et, relaxation

ε′ at 120 °C, 1 kHz

Ea, conduction

Et, relaxation

± ± ± ± ± ± ± ±

25.5 39.7(1.55)

1.81 ± 0.11 0.47 ± 0.05

1.28 ± 0.04 1.07 ± 0.09

95.5(3.75)

0.87 ± 0.05

1.37 ± 0.11

1.28 1.59 1.37 1.04 1.35 0.58 0.37 0.12

0.04 0.03 0.05 0.07 0.08 0.04 0.05 0.01

ε′ value. When the frequency reaches the characteristic frequency (i.e., ω = 1/τ), the dielectric constant drops (this is called the relaxation process). At very high frequencies (i.e., ω ≫ 1/τ), dipoles can no longer 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 Figure 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 behavior of dipolar/polymer materials due to the presence of thermally active defects in the graphene and ionic groups in PVA.34 Similar behavior is observed in G@MgO/PVA_ac composite samples as shown in Figure 6a,b. Insets of Figure 5a−h and Figure 6a,b are the variations of tangent loss versus frequency at different temperatures. Two loss peaks (one in the lowfrequency range and other at the high-frequency edge) are observed in PVA and all of the composite materials (except in

10.0- and 15.0G@MgO/PVA_Coag), and as the temperature increases, they shifted toward the high-frequency 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 are 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 another 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 23720


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Figure 7. 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-, and (g) 15.0G@MgO/PVA_Coag composites, and (h) comparison of panels a−g at 120 °C, respectively.

accumulation of space charge, and consequently it results in decrease in the dielectric loss. 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. Panels c and d of Figure 6 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-cast 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-cast samples. When an 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 the frequency dependent imaginary part of the electric modulus at different temperatures is necessary. The electric modulus can be calculated from permittivity using eq 2 as follows, M * = M′ + iM″ =

ε′ ε″ 1 = 2 +i 2 ε* ε′ + ε″ 2 ε′ + ε″ 2

(2)

where M′ and M″ are real and imaginary parts of the electric modulus, respectively. 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-cast composites, the M″ versus frequency curves(M″ 23721


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Figure 8. Plots of M″ versus f and temperature of (a) PVA and (b) 0.4- and (c) 3.0G@MgO/PVA_ac composites, and (d) comparison of panels a−c at 120 °C, respectively.

Figure 9. Plots of (a, b) M″/M″max versus f/f m, (c, d) ln(τmax) versus 1000/T, and (e, f) ln(σdc) versus 1000/T of coagulated and as-cast G@MgO/ PVA composites, respectively.

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

versus f, at a different temperatures) of different samples are shown in Figure 7a−h and Figure 8a−d, respectively. As shown in Figure 7a−d and Figure 8a−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 be 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 Figure 8a), as the temperature increases, the segmental motion of polymer chains increases, the 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 toward a higher frequency side. On the other hand, 3.0−15.0G@MgO/ PVA_Coag composites (Figure 7e−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 of (or less dependent on) the temperature.36 Figure 7h and Figure 8d represent the frequency dependent imaginary part of the electric modulus of coagulated and PVA, and as-cast composite samples at 120 °C temperature, respectively. With increasing filler content in the matrix the loss peak shifts toward higher

frequencies due to an increase in new interfaces that reduces relaxation times. But in the present work, for first composition (i.e., at 0.4 wt % filler in both composite samples) the relaxation peak shifted to the lower frequency side, which is attributed to a reduction in crystallinity of the PVA matrix as shown in Table 1 and a lower amount of filler material. With further increase in filler content in the PVA the loss peak shifts toward higher frequencies. An appreciable amount of shift is observed in as-cast samples than in the coagulated samples. It is also observed that in the coagulated samples case, since the filler content is equal to or greater than 3.0 wt %, there is a huge shift in loss peak toward 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 Figure 9a,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″ versus f spectrum), and f is scaled by f max (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 of the composites are calculated from eqs 3 and 4, iE y τmax = τ0 expjjj t zzz k kT {

23723

(3) DOI: 10.1021/acsami.9b05137 ACS Appl. Mater. Interfaces 2019, 11, 23714−23730

Research Article


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

i −E y σdc = σ0 expjjj a zzz k kT {

σac = σdc + Aωs (4)

(5)

where ω, A, and s are angular frequency (i.e., 2πf) and parameters depending on temperature and filler concentration in the composite, respectively. The slope of ln(τmax) versus 1000/T and ln(σdc) versus 1000/T curves give activation energies due to relaxation and conduction mechanisms, respectively, and those values are presented in Table 2. As shown in Figure 9 a,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 composite samples, activation energy due to conduction is

where τmax is the relaxation time (i.e., τmax = 1/2πf max) for which the peak maximum in M″ versus log( f) appears, τ0 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. In eq 5, σdc is the dc conductivity and σ0 is the characteristic conductivity. σdc is calculated from the ac universality law (i.e., Jonscher Law).36 According to this law, at constant temperature ac conductivity can be expressed as 23724


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Figure 12. Plots of (a) EMI SE, (b) SEA, and (c) SER of G@MgO/PVA_ac composites, respectively.

lesser than the relaxation mechanism, and it is predominate in coagulated samples. In as-cast 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-cast samples. It is noticed that in both composite samples, the conduction mechanism is the dominating mechanism for energy transfer and it is more active in coagulated composite samples than in the as-cast composites. The excellent charge transport in coagulated samples than in as-cast samples is expected due to well dispersed G@MgO in a PVA matrix which associates with defects, and the conducting−conducting (graphene−graphene), dielectric−dielectric (MgO−PVA), and conducting− dielectric (graphene−MgO and graphene−PVA) interfaces. 3.3. EMI SE and Dielectric Properties in the X-Band Frequency Range (i.e., 8.2−12.4 GHz). EMI SE of PVA; 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 Figure 10a,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 Figure 10a, the EMI SE of coagulated composites is more than that of

PVA. Similarly, Figure 10b displays that EMI SE of 3.0G@ MgO/PVA_ac composite at all the measured temperatures is more than that of PVA. The decrease in EMI SE of the 0.4G@ MgO/PVA sample in both composites is expected due to the poorer crystallinity of PVA. At 120 °C and 8.2 GHz, EMI SEs of PVA and 0.4- and 3.0G@MgO/PVA_ac composites are ∼4.75, ∼3.0, and ∼5.91 dB, 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 3.0 wt % G@MgO is attributed to increased conductivity of the composite as observed in Figure 5−7. As shown in Figure 10c,d, a continuous increase in the EMI SE values of coagulated composite samples is observed as compared with as-cast composites, i.e., at 120 °C and 12.4 GHz increases of 1.35 and 3.74 times, compared to their ascast counterparts, are found in 0.4- and 3.0G@MgO/ PVA_Coag samples, respectively. This observation is in support of observed lesser activation energies (mentioned in Table 2) of coagulated samples than as-cast samples. The higher SEs of coagulated samples than those of as-cast samples at the same loadings of G@MgO in PVA are attributed to the formation of a 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 23725


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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.

observed that SE of each sample is almost constant in the entire “X-band” frequency range. Figure 11 and Figure 12 show EMI SE, SEA, and SER versus temperatures and frequencies 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 behavior 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. 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 (S 11 and S 21 ) using theoretical calculations given by Nicholson, Ross, and Weir algorithms.13

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. Panels a and b of Figure 13 show 3D plots of complex permittivity versus frequency and temperature of G@MgO/PVA_Coag composites, respectively. As shown in Figure 13c, it is observed that the dielectric constant of 0.4 wt % coagulated composite is slightly more than the PVA. An enormous increase in dielectric constant (real and imaginary parts) of the other composites (Figure 13d−f) is also observed; for example, complex permittivity (real and imaginary parts) of the 15.0G@MgO/ PVA_Coag composite sample is 5 times more than the value of PVA. Similarly, panels a and b of Figure 14 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 lower amount of G@MgO addition and the reduction of PVA crystallinity by adding 0.4G@MgO 23726


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

MgO and subsequent composite preparation can create defects such as 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 localized states near the Fermi level could be created by introducing lattice defects.20 Thus, the existence of defects in graphene favors absorption of electromagnetic energy by the transition from contiguous states to the Fermi level when the absorbing surface is irradiated. As observed from low-frequency dielectric data (Figure 5−Figure 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 (graphene−graphene, crossover layers), dielectric−dielectric (MgO−PVA), and conducting−dielectric (graphene−MgO and graphene− PVA) are the primary reasons for the polarization processes.

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 Figure 11a−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 parts) 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 of observed activation energies of these composite materials in the low-frequency range. The combustion synthesis of G@ 23727


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

First, the defects that are prominent in these systems get screened by free charges and can act as polarization centers, which would generate strong oscillating polarization under the alternating electromagnetic field and attenuate electromagnetic waves, 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 material;38 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 favorable 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 10-Figure 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 parts of dielectric constant are determined by relaxation loss and electrical conductive losses in the materials.25 And they are expressed as follows, ε′ = ε∞ +

ε″ =

SE R = 39.5 + 10 log[σ /2πfμ]

SEA = 8.7t πfμσ

(εs − ε∞)ωτ 1 + ω 2τ 2

+

4. 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 analyses of G@MgO powder are carried out via scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and Raman spectroscopy, confirming the presence of both graphene and MgO in the powder, and TGA-DTA also reveals the presence of 79% graphene and 21% MgO in the G@MgO. 1 mm thick amount of G@MgO/PVA_Coag composites are prepared via sonication and coagulation of different weight fractions of G@ MgO powder with poly(vinyl alcohol) solution and subsequent hot pressing at 140 °C temperature and 3 t 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 to 2 MHz), and high-frequency (i.e., X-band, 8.2−12.4 GHz) ranges. The dielectric properties of G@MgO/PVA_Coag composites are superior to PVA and G@MgO/PVA_ac composites in both frequency ranges. Electric modulus studies (i.e., M″ versus log( f) plots) reveal a high interfacial polarization exists in coagulated and as-cast G@MgO/PVA composites. It is noticed from calculated activation energies that conduction is the dominating mechanism for energy transfer in both composites than in 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 SEs of 10.0G@MgO/ PVA_Coag composite are ∼14.2 and ∼18.9 dB, and similarly, EMI SEs of ∼20.6 and ∼27.5 dB are found for 15.0G@MgO/ PVA_Coag composite, respectively. It is found the absorption is the dominating mechanism for EMI SE in all of 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 a PVA matrix.

(1)

σ (T ) ωε0

(6)

where ω is the angular frequency, τ is the temperature dependent relaxation time, εs 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 parts of complex permittivity are related to relaxation time (τ) and conductance (σ), which are relevant to the temperature as shown below10a, similar to eq 3:

τ=

1 U / kT e 2ν

(7)

where U is a potential barrier, v is the vibration frequency, k is the Boltzmann constant, and T is the temperature. The conductance of G@MgO is determined by25 similar to eq 4: σ(T ) = A e−E /2kT

(10)

where σ = ωεοε″, ω is the angular frequency of the electromagnetic wave, εo is the permittivity of free space = 8.854 × 10−12 F m−1, and ε″ 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 the 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 increase. At a constant thickness of the composite materials (here 1 mm), as the conductivity of the material increases with temperature, both SER and SEA increase and as a consequence of this the total shielding efficiency of the material increases.

εs − ε∞ 1 + ω 2τ 2

(9)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +91 40 2313 4453. E-mail: [email protected]. ORCID

Vadali V. S. S. Srikanth: 0000-0002-3021-0987 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.K.M. acknowledges Council of Scientific and Industrial Research for providing fellowship through the CSIR-DSRF scheme with Grant No. 09/414(1130)/2016 EMR-I.

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