Lightweight, flexible and ultra-thin sandwich architectures for screening electromagnetic radiation

Abstract

A lightweight and flexible multilayer structure consisting of poly(vinylidene fluoride) and iron particles deposited (electroless) on to a carbon nanofiber (CF) mat was successfully fabricated for electromagnetic interference shielding (EMI) application. The CF was pre-activated before depositing the iron particles by an electroless deposition technique. To enhance the shielding, a poly(vinylidene fluoride) PVDF composite containing multiwalled carbon nanotubes (MWNT) was sandwiched as an inner layer between two outer layers of iron particles deposited onto the CF mat (Fe@CF). The electroless deposition of iron particles onto the CF mat is reflected in a dramatically enhanced SE of ca. −54 dB (at 18 GHz) for an ultra-thin sheet of 0.6 mm as compared to the controlled sandwich structure (consisting of PVDF/CF) due to the presence of a conducting MWNT/PVDF film and magnetically active Fe@CF layer. This particular structure exhibits >95% absorption of the incoming EM radiation. Such a lightweight and flexible sandwich structure promises protection against EM microwave radiation.

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Introduction

At the present time, developing novel architectures with characteristics like lightweight, flexibility, easy to design structures has become the prime focus of research. The materials with a perfect combination of properties and performance are highly desirable in various devices and electronic circuitry.¹ A new problem, electromagnetic interference (EMI), has popped up with heavy usage of electrical devices and is extensively harmful for the device itself and nearby circuitry. So, to make an effective material which can absorb the electromagnetic noise and hence shield the device is the need of the hour. There are two important ways by which EM radiations could be arrested, either by reflection or absorption or sometimes by a combination of both.² It is established that conducting materials (like metals) attenuates EM radiations by reflection whereas magnetic materials (like ferrites) by absorption.^1c,3 Thus, combining the advantages of these materials, maximum attenuation could be attained. Recently, layer by layer sandwich structures are explored due to the ease with which the individual property of material could be tailored in order to achieve the right combination of properties.⁴ Hence, one such attempt has been made to demonstrate the feasibility of such approach which could lead to the development of unique architectures, exhibiting the synergistic effect of both conducting as well magnetic property. Since the multi-layer structures incorporated with fibrous material or decorated nanoparticles play a vital role in absorbing EM radiations,^1c the aim of this study is to develop a sandwich structure which can attenuate the radiations mostly by absorption. Poly(vinylidene fluoride) (PVDF) is an insulating polymer and has attracted a good deal of attention due to its high permittivity, high dielectric constant and easy processability and moldability. Hence, to design lightweight and flexible architectures for applications like EM interference shielding materials, it could be a good choice.⁵ Since PVDF is transparent to EM radiation, the effects from the fillers can be quite well alienated. The conventional way to develop EM shielding materials is to incorporate metallic particles (Fe, Ni, Co etc.) or more recently carbon derivatives such as carbon nanotubes, graphene sheets, carbon black, carbon fibers etc. in polymer matrix.^1c,6 The latter particles have showed some promises to be a perfect candidate for EMI shielding due to their excellent electrical conductivity, high strength, high aspect ratio and low density.^1c However, these particles alone can shield only in a specific frequency window and often require high concentration to show shielding that is required for commercial applications. However, high loading often deteriorates the flexibility and few other physical properties of the host polymer thereby restricts their use in structural applications.⁷ On the other hand sandwich structure offers design flexibility, ease of processing, high strength and also provides the scope of intelligently integrating different particles by selection of desired parameters such as core and skin thickness, functionality of core etc.⁸ Thus, it is expected that carbon material decorated with metallic particle will possess the advantages of electrical conductivity as well as magnetic properties.⁹ Amongst the different metallic particles, iron, cobalt and nickel are extensively used for EMI shielding application due to their good magnetic properties.¹⁰ The purpose of choosing iron particles is to take advantage of ferromagnetic property of iron which could be useful in absorbing the EM noise. Although there are various methodologies available to alter the surface properties of carbon material in order to achieve the proper adhesion of metallic particle, electroless deposition is envisaged to be more cost effective and most suitable method due to its ability to uniform coat the particles on the substrate. This technique also doesn't require any external power (electricity) during deposition unlike other ways like electro-deposition, thus making the process much cheaper.¹¹ For instance, Dai and coworkers fabricated nickel coated carbon fiber in paraffin wax and showed shielding effectiveness of −41 dB.^8a Cao et al.¹² showed that carbon nanofiber–graphene nano sheets–carbon nanofiber heterojunction results in significant enhancement in EM attenuation (25–28 dB). Zou et al.¹¹ successfully deposited nickel particles on to carbon fiber by creating sufficient activation sites and reported their use in EMI applications. Zhong and coworkers^5a designed multi-layer sandwiched structure with carbon fiber (CF) and PVDF to enhance the dielectric constant of the designed structure. Table 1 lists the available literature on carbon fiber and decorated carbon fibers used for EMI shielding application.

Table 1 Comparative study of shielding effectiveness of different composite containing CF and electrodeposited CF

Material	SE total (dB)	Frequency	Reference
Ni@CF/ABS	44	30–1000 MHz	13
Cu@CF/ABS	15	30–1000 MHz	14
Ni@CF/ABS	35	30–1000 MHz	14
Ni@CF/PC/ABS	47	1000 MHz	15
CF/LCP	41	15 MHz	16
Ni@CF/ABS	50	1000 MHz	17
CF/PP foamed	25	8–12 GHz	18
CNF/ABS	35	8–2 GHz	19
CF/PP solid	15–20	8–2 GHz	18
CF/PBT	10	100–1400 MHz	20
PFe@CF (sandwich structure with 0.6 mm thickness)	56	12–18 GHz	This work

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The present paper reports the fabrication of multilayer sandwich structure with outer layers consisting of PVDF/CF laminates (hereafter denoted as PCFP and will be used as control sample) and the inner layers consists of PVDF/MWNTs composites. In addition, to maximize the absorption of the incoming EM radiation, a unique strategy was adopted here. The outer layers were replaced with CF mat containing iron particles that were deposited by electroless deposition technique and was embedded within insulating PVDF films by compression molding (hereafter denoted as PFe@CFP). The inner layer consists of a thin film of conducting multiwalled carbon nanotubes filled PVDF composites (PVDF/MWNT). The rationale behind this architecture (PFe@CFP–PVDF/MWNT–PFe@CFP) is to attenuate the incoming EM radiation to a maximum extent. The outer layers can shield both by reflection (from CF) and absorption (iron particles) and the EM waves that penetrate the outer layers will be reflected back by the conductive inner layer and will be scavenged by the outer layers. This assembly can be fabricated by simple hot pressing the different layers.

Experimental

Materials

Analytical grade reagents such as SnCl₂ and PdCl₂ were supplied by Sigma Aldrich and used as received. FeSO₄·7H₂O, KNaC₄H₄O₆·4H₂O, NaH₂PO₂·H₂O, C₁₂H₂₂O₁₁, H₃BO₃ were obtained from SDFCL. The pristine multiwalled carbon nanotube (MWNTs, NC7000, average diameter and length of 9.5 mm and 1.5 μm respectively with 90% purity) was obtained from Nanocyl, Belgium. Commercially available PVDF (Kynar 761 with molecular weight 440 000 g mol⁻¹) was procured from Arkema. Bidirectional carbon fiber, grade 200GSM (10 μm diameter, 0.25 mm thickness) was supplied by Hindustan Technical Fabrics Ltd.

Electroless deposition via single-step activation of carbon fibers

Pre-activation of carbon fibers was accomplished by single step activation method.²¹ Here, the CF sheet was sonicated in activating solution for 2 min. After the activation process, CF was washed in 1 M HCl and deionised water. Iron decorated CF was obtained by immersing the CF in 50 ml plating bath for 3 h. The plating bath was prepared by dissolving the reagents into de-ionised water. The composition of the bath is as follow: FeSO₄·7H₂O 25 g L⁻¹, KNaC₄H₄O₆·4H₂O 110 g L⁻¹, NaH₂PO₂·H₂O 15 g L⁻¹, C₁₂H₂₂O₁₁ 10 g L⁻¹, H₃BO₃ 10 g L⁻¹. The pH was maintained at 12 and the deposition was carried out at room temperature. Fig. 1 shows the schematic of electroless deposition of iron particle onto carbon fiber mat.

Fig. 1 Schematic showing electroless deposition of iron particles onto carbon fiber mat.

Design and preparation of sandwich structure

The PCFP (PVDF–CF–PVDF) sandwich structure was prepared by making two thin films of PVDF embedded with iron particles deposited CF mat. Additionally, a thin film of PVDF–MWNT composite was also prepared as inner layers. For this a composite containing 3 wt% of MWNTs in PVDF was melt mixed at 220 °C, 60 rpm and 20 min. Finally, the as prepared PVDF–MWNT composite film was sandwiched between two PFe@CFP layers to design the multilayered sandwich structure using a hot press as shown in Fig. 2. The thickness of the sandwich structure (PFe@CFP–PVDF/MWNT–PFe@CFP) is only 0.6 mm and is quite flexible as shown in Fig. 2.

Fig. 2 Schematic showing the fabrication of PCFP/PVDF–MWNT/PCFP sandwich structure.

Characterization

The morphology of electroless deposited carbon fiber was studied using field emission scanning electron microscopy (SEM) (ULTRA 55, FESEM, Carl Zeiss). Energy dispersive X-ray technique attached with an ESEM (Quanta 200, FEI) was used to collect the spectra. Raman analysis was done using a Horiba LabRAM HR with a 532 nm monochromatic laser. EMI shielding of thin sample (ca. 0.6 mm) was measured using a Anritsu MS4642A two port Vector Network Analyser coupled to a Keycom waveguide to measure the scattering parameters in the frequency range of 12–18 GHz. Thin samples for EMI measurement were prepared by hot press.

Results and discussion

The electroless deposition of iron particles on CF mat was confirmed by scanning electron microscopy. Fig. 3a shows the morphology of bare carbon fibers. The electroless deposited iron particle onto CF mat is shown in Fig. 3b. The presence of iron particles was confirmed using EDS technique as shown in Fig. 3c.

Fig. 3 (a) SEM morphology of bare CF mat; (b) iron particles decorated CF mat. (c) EDAX spectra of electroless deposited CF mat.

Raman analysis can reveal the structural integrity of CF. Fig. 4 shows the Raman spectra of CF and Fe@CF. Two signature peaks at 1355 and 1582 cm⁻¹ corresponding to the D and G band respectively are well evident.²² D band corresponds to the disordered structure and G band corresponds to the ordered graphitic structure. Thus the ratio of I_D/I_G manifests the crystalline nature of C materials.²² The I_D/I_G ratio of the different CF is estimated to be ca. 1.07 for both the types of CF. It is interesting to note that the ratio is unaltered manifesting in the fact that electroless deposition is rather less harsh technique which helps in preserving the integrity of the CF unlike other techniques like electro-deposition, chemical routes etc.

Fig. 4 Raman spectra of CF and Fe@CF.

The EMI shielding effectiveness (SE) of a material is defined as the ratio of incoming power (P_in) to the transmitted power (P_out). The EMI SE of a composite material depends on many factors such as electrical conductivity, magnetic permeability, size, shape and aspect ratio of filler.²³ The higher value of SE means the total incoming radiations have been attenuated by the material. The SE was obtained from the scattering parameters measured using a VNA coupled to a waveguide set-up (see Fig. 5a). Fig. 5b illustrates the shielding effectiveness of various sandwich structures, where a single layer of control PCFP shows almost similar SE as compared to two layers of PCFP. Since the bare carbon fibers are electrically conducting and embedded within two insulating PVDF films which are transparent to EM radiations, the observed attenuation could be achieved partially by reflection mechanism and the impedance mismatch between two different medium. Surprisingly, the PVDF–MWNT composite film sandwiched between two PCFP layers also showed almost unaltered SE as compared to control PCFP because of extensive surface reflection governed by the outer layers. The total SE is controlled only by the outer layers and the effect of inner layer is almost masked. Similarly, single layer of PFe@CFP showed unaltered SE at lower frequencies however at higher frequencies a slight increment was observed. On the other hand, two (outer) layers of iron nanoparticles deposited CF (PFe@CFP) sandwiched with one (inner) layer of PVDF–MWNTs composite film showed significantly higher SE of −54 dB at 18 GHz. The dramatic increase in SE for the designed architecture (PFe@CFP–PVDF/MWNT–PFe@CFP) is attributed to the presence of magnetically active iron particle on to CF mat. We have demonstrated that PVDF/MWNTs showed better shielding effectiveness due to the presence of electrically conducting MWNTs.^3b It is envisaged that the incident radiations could be utilized in aligning the magnetic domains and hence leading to magnetic loss. The radiation that penetrates these outer layers will be reflected back from the inner conductive layers (due to the presence of nomadic charge carriers in PVDF/MWNT composite films) and eventually will be scavenged by the outer layers. Thus, a synergistic effect can be expected from these architectures due to associated electric and magnetic dipole in the sandwich architecture that interacts with the EM waves.

Fig. 5 (a) Set-up of Vector Network Analyzer coupled to a waveguide (b) SE of different sandwich structure as a function of frequency (PCFP 1 layer indicate that a CF mat is sandwiched between two PVDF films; PCFP 2 layers indicate that a PVDF/MWNT inner layer is sandwiched between two PCFP outer layers; PFe@CFP 1 layer indicate that Fe@CF mat is sandwiched between two PVDF films; PFe@CFP 2 layers indicate that a PVDF/MWNT composite film is sandwiched between two PFe@CFP outer layers).

It is important to tailor the properties of material in order to make it useful for a particular application. In this regard, it is possible to modify the surface of CF to make them applicable for EMI shielding application. The decoration of CF with iron particle has contributed significantly in attenuating the incoming EM radiation mostly by absorption. It is believed that the additional contribution in shielding mechanism could be due to the interfacial polarization created between CF and the attached magnetic nanoparticles. The inter linkages of CF resulting in conducting pathways when stacked in layer helps in absorbing the radiations.²⁴ Additionally, the presence of magnetic particles leads to enhancement in absorption mechanism as compared to neat CF.²⁴ As shown in Fig. 6a and b the reflection and absorption components of different composites were evaluated from the scattering parameters. It is interesting to see that our sandwich structures attenuate the incoming EM radiation mostly by absorption and the maximum absorption (>95%) is observed in PFe@CFP–PVDF/MWNT–PFe@CFP structure. This begins to suggest that the outer layers are less reflecting and more absorbing the incoming EM radiation due to the presence of magnetic particles on the CF mat. In addition, the interface between the layers also helps in multiple scattering due to heterogeneous dielectrics. When interacting with the incoming EM radiation the CF mat behave like a waveguide when they are cross linked and their absorption characteristics strongly depend on the geometric parameters (see Fig. 7). The waves that transmit the outer layers then interact with the inner layers consisting of PVDF/MWNT composite films. In Fig. 7, an equivalent circuit of MWNTs embedded in an insulating polymer is shown where it explains the interaction of these nano entities with EM signals by interconnection between capacitance and resistance. The inter-cluster polarization creates a virtual connection between the MWNTs leads to charge transport which is further explained by enhancement in increased conductivity of the composite.²⁵ Our sandwich structure clearly demonstrates promising microwave absorbing materials.

Fig. 6 (a) Reflection and (b) absorption components of total shielding effectiveness of different composites.

Fig. 7 Schematic explaining the shielding mechanism.

Conclusions

Our study demonstrates the potential of multilayered architecture, consisting of PVDF and iron particles deposited on to CF mat by electroless deposition technique, in blocking EM radiation. The iron particles were deposited onto CF mat via single step activation process. The rational construction of two (outer) layers of PFe@CFP sandwiched between one (inner) layer of PVDF/MWNT composite showed a significant improvement is SE (ca. −54 dB) at 0.6 mm stack thickness as compared to control sandwich structures. The designed multilayered architecture is lightweight, flexible and easy to assemble; making them potential candidates as EM shielding materials. Such architectures can further be explored for various applications that require blocking of EM microwave radiation.

Acknowledgements

The authors would like to thank DST and INSA for the financial support and also Dr Jafar Hasan for his assistance in SEM imaging.

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