Conducting Polypyrrole / Graphene Nanocomposites as Potential Electromagnetic Interference Shielding Materials in the Ku-band

Enormous progress in nanotechnology has made electronic systems smaller but has also created a new type of problem called electromagnetic interference (EMI). Carbon-based conducting polymer nanocomposites have potential applications as EMI shielding materials owing to their high conductivity and dielectric constant of the materials that contribute to the high EMI shielding efficiency (SE). In the present investigation, highly conducting polypyrrole (PPy)/graphene (GNS) nanocomposites were prepared by in-situ polymerisation with different concentration of functionalised GNS (1%, 3% and 5%). UV-VIS and FTIR show a systematic shifting of the characteristic bands of PPy, with the increase in the GNS phase suggesting significant interaction between the phases. The SEM images show thick and uniform coating of PPy over the surface of individual GNS. PPy/GNS nanocomposites showed a semiconducting behaviour similar to that of PPy as well as improved dielectric and EMI shielding properties. The EMI shielding effectiveness (SE) and dielectric constant of nanocomposites were found to increase with increasing GNS content and were found to be absorption-dominated, indicating that PPy/GNS nanocomposites are potential lightweight EMI shielding materials for the protection of electronic systems from electromagnetic radiation in the Ku-band.

aspect ratios such as carbon nanofibres (CNFs), carbon nanotubes (CNTs), and graphene for EMI shielding absorption.These carbon materials have attracted increasing interest because of their potential applications in ideal absorbers. 5,6mong these, graphene has emerged as a new member of carbon allotropes with exceptional carrier mobility and ballistic electron transport properties, making it the prime nanofiller employed in the preparation of nanocomposites for many applications. 7,8Graphene is a one-atom-thick planar sheet of sp 2 -bonded carbon atoms arranged in a hexagonal lattice.It is the thinnest and strongest material.It has remarkable physical, chemical, mechanical, electrical, thermal and microwave absorption properties.In view of the unique structural features of graphene such as its high surface area (theoretical specific surface area (SSA) of 2630 m 2 g -1 ), flexibility, high mechanical strength, chemical stability, and superior electric and thermal conductivities, graphene has been considered to be an ideal material for microwave absorption properties.Graphene nanosheets can be viewed as the building unit, and their reassembly provides opportunities to design and prepare specific structures and hybrids with improved properties for different applications.The most important property of graphene is its electron transport capacity.This means that an electron moves through graphene without much scattering or resistance.It has high electron mobility at room temperature.][11] Low-cost and solution-processable graphene can be produced from graphene oxide (GO), which in turn is produced from the aggressive oxidation of graphite.[14][15] The EMI shielding efficiency (SE) of a composite material depends on several important factors including the intrinsic conductivity and aspect ratio of the fillers.Based on these considerations, the composites of conducting polymers and CNTs have become promising materials for achieving high EMI SE.However, the expected EMI SE has not been achieved so far because the heterogeneous interface between the polymer and CNT components of the nanocomposite negatively affects the EMI SE. 2,16 To overcome this problem, composites based on graphene nanosheets (GNS) have been studied for EMI shielding. 17,18Liang et al. prepared graphene/epoxy composites and studied the composites that show a low percolation threshold of 0.52 volume%.The highest EMI SE of -21 dB was measured in the X-band. 19

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Complex Permittivity and Permeability
Permittivity and dielectric loss measurements were carried out using a vector network analyser in the microwave range of 12.4-18 GHz (K u band).To investigate the possible microwave absorption mechanism, we determined the real and imaginary parts of the complex permittivity (ε', ε") and permeability (μ', μ") from the scattering parameters using the Nicolson-Ross-Weir (NRW) method.The incident and transmitted travelling waves inside a vector network analyser can be represented by complex scattering parameters or the S parameters, i.e., S 11 or S 22 and S 12 or S 21 , which are related to the electromagnetic characteristics of permittivity and permeability.The NRW technique is formulated from the set of equations related to these S parameters and is useful for providing direct calculations for both permittivity and permeability.Our results show that permittivity values exhibit a trend of decreasing with the increase in frequency.This can be attributed to the decreasing ability of the dipoles present in the system to maintain the in-phase movement with the rapidly oscillating electric vector of the incident EM wave.At low frequencies, the electric dipoles have sufficient time for aligning with the field before the field changes its direction; consequently, the dielectric constant is high.However, at higher frequencies, the dipoles fail to follow the rapidly changing electric vector; consequently, the dielectric constant value decreases.The results also revealed that the dielectric constant (ε') as well as dielectric loss (ε") values of the composite exhibit a noticeable enhancement upon addition of GNS, as shown in Figures 5(a) and 5(b).
As the GNS concentration increases, the permittivity of the composites increases as well.This is due to the increase in the space charge build up caused by the interfacial polarisation.The presence of doping-induced localised charges (polarons or bipolarons) on the PPy backbone gives rise to strong polarization effects.Furthermore, in conducting polymers, the space charge formation due to the conductivity difference between the ordered or highly conducting (crystalline or metallic) islands and the electrically insulating amorphous matrix contributes toward the interfacial polarisation.The complete polarisation effects and associated loss mechanisms are responsible for the high dielectric constant values.Similarly, the associated relaxation effects lead to enhancement of the ε" values.The real and imaginary permittivity in PPy/GNS at the lower concentration is independent of frequency.For higher concentrations, the permittivity decreases with increasing frequency.The real permeability values of composites with higher GNS concentrations were found to be higher.This was due to the improvement of the magnetic properties along with the reduction of eddy current losses.Similarly, the composites with higher GNS concentration show higher magnetic losses.The introduced magnetic properties also lead to a better m and enh 6(b) sh approxim sulphate (μ") is compos any abs rather th plot sho

Electromagnetic Shielding Effectiveness
The electromagnetic interference shielding effectiveness is defined as the logarithmic ratio of the incoming (Pi) power input to the outgoing power (Po) of radiation.The efficiency of any shielding material is expressed in decibels (dB).
The higher the decibel level of electromagnetic interference shielding effectiveness, the lesser will be the energy transmitted through the shielding material. 20The shielding effectiveness (SE) of a shielding material is equal to the sum of the absorption factor, the reflection factor and the multiple reflections. 44e synthesised composite material consists of PPy/GNS, and the polymer matrix containing GNS enhances the interfacial polarisation and the effective anisotropy energy of the sheets; this contributes to more scattering.The materials also show a high shielding effectiveness compared to conventional materials.Addition of GNS as filler in PPy shows better microwave absorption properties that strongly depend on the volume fraction of the filler.Therefore, the high value of EMI SE is dominated by absorption rather than reflection.The EMI shielding effectiveness in the composites increases with increasing GNS content.With the increase in GNS from 1% to 5%, the volume resistivity of the composites decreases and the shielding and effectiveness increase.The number of percolating networks increases with the increase in the GNS amount.The conductive networks formed due to the dispersion of GNS behave as conductive meshes.With the increase in GNS loading, the size of the conductive mesh decreases, acting as a barrier to incident electromagnetic radiation and giving rise to a higher EMI SE.This is because the electrical conductivity of a composite tends to increase with increasing GNS content, and upon the action of electromagnetic radiation, an induction current generated on the interface or in the interior of the sample produces a reversal electromagnetic field, leading to the increase in surface reflection attenuation of electromagnetic waves and consequently increasing the EMI shielding effectiveness of the composite. 45e attenuation of the incident wave increases by increasing the absorption cross section and scattering cross section of the absorbent particle.The attenuation of the incident wave energy increases as well.A larger specific surface area of the GNS effectively increases the plane wave absorption cross section and scattering cross section of absorbing particles so that the electromagnetic wave loss is increased.It is well known that the total shielding effectiveness of PPy is dominated by absorption phenomena due to the presence of localised charges (polarons and bipolarons) leading to the strong divergence and relaxation effects.The PPy coating on the GNS can dominate the polarisation, and the functional groups of functionalised GNS give rise to the electromagnetic radiation absorption.The functional groups of GNS are also responsible for the absorption due to the increase in the content of GNS functional groups that are responsible for the absorption of electromagnetic radiation.The effect of conductivity on reflection and absorption loss (EMI SE) of a material depends on many factors such as conductivity, dielectric constant, aspect ratio, state of dispersion of conductive fillers, and thickness of shielding materials.Among all of these factors, conductivity is the primary factor for an EMI shielding material.In the case of a conductive material such as metal, EMI SE is mainly due to reflection of EM radiation, but for a material such as a conductive composite, EMI SE is mainly due to radiation absorption.The reflection shielding effectiveness (SE R ), absorption shielding effectiveness (SE A ) and total shielding effectiveness (SE T ) of PPy/GNS nanocomposites as a function of frequency are shown in Figures 7(a-c), for 1%, 3% and 5% GNS loading in PPy, respectively.The SE R is nearly linear for each composition in the entire frequency range of measurement and shows a negligible change even with the increase in GNS loading.The SE A is increased from 8 to 18 dB with the increase in GNS loading from 1 to 5 wt%.
The experimental results show that absorption is the primary shielding mechanism and that reflection is the secondary shielding mechanism.The SE T of the nanocomposites as a function of frequency shows that the nature of SE T for each composition is nearly linear with frequency, but the SE T of the composite is found to increase with increased GNS loading.The total shielding effectiveness values for PPy/GNS were 11, 24 and 26 for 1, 3, and 5 wt% GNS loading, respectively.For PPy/GNS composites, a high value of EMI SE is obtained that is higher than the value of the EMI shielding effectiveness (20 dB) required for commercial applications.Such a high value of EMI SE at a sufficiently low loading of GNS shows the efficiency of the compounding technique.The EMI SE results show that composites have an absorption-dominant mechanism and can be used as lightweight effective EMI shielding or Ku band microwave absorption materials for protection of electronic devices and components from electromagnetic radiation.Author (UGC), through

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Figure 7 Physical Scien