Elsevier

Thin Solid Films

Volume 520, Issue 21, 31 August 2012, Pages 6547-6550
Thin Solid Films

Electromagnetic interference shielding effectiveness of nanoreinforced polymer composites deposited with conductive metallic thin films

https://doi.org/10.1016/j.tsf.2012.07.018Get rights and content

Abstract

The effect of using conductive metallic thin films deposited on high density polyethylene (HDPE) and styrene butadiene copolymer (SBC) in conjunction with carbon nanofiber (CNF) reinforcement of HDPE and SBC was investigated in order to improve the electromagnetic interference shielding effectiveness (EMI SE) of the structures. Thin films of copper, silver and aluminum were deposited by thermal evaporation onto the polymeric matrices and its composites (0–20 wt.% of CNFs). Results show a synergistic effect of the two approaches (metallic coating and CNF reinforcement) toward improving the EMI SE. The chemical composition, surface morphology, carbon nanofiber distribution, thickness and microstructure of metallic coated polymers are examined using X-Ray Diffraction and Scanning Electron Microscopy.

Highlights

► Metallic thin films were evaporated on carbon nanofiber reinforced polymers. ► The electromagnetic shielding effectiveness of the structures was evaluated. ► Thin films and carbon nanofibers synergistically improved the shielding effectiveness.

Introduction

There is an increased interest in developing materials that could protect the signal from EMI produced by an increased number of wireless electronic devices. EMI affects communications and proper functioning of electronic devices and concerns had developed about their impact on human health [1], [2], [3], [4]. SE represents the ability of a material to reduce the propagation of the electromagnetic fields by reflection, absorption, and/or multiple reflection mechanisms [4]. The 0.1 to 1300 MHz range is commonly targeted since electromagnetic emissions from computers and other wireless electronic equipment are included in this range [5], [6]. A level of EMI SE of 30 decibel (dB) is considered acceptable for some applications and attenuates 99.9% of electromagnetic radiation [7], [8]. For more sensitive applications (e.g. automotive and aerospace industry) the requirements for EMI SE have higher demands, sometimes as high as 100 dB [9]. In order to protect from the undesirable effects of EMI a conductive or magnetic material can be used to shield. Metals are the first option for providing the required EMI shielding due to high surface reflection capability ensured by free electrons and low skin depth. However, metals are heavy, prone to corrosion, expensive to manufacture and rigid [4]. Polymers as an attractive alternative to metals are light, noncorrosive, easy to manufacture, inexpensive, flexible, but generally transparent to EMI. Their shielding capability is usually enhanced by either process of coating with a conductive material [4], [9], [10], [11], [12], [13], [14], or by reinforcing the polymeric matrices with conductive fillers [4], [15], [16], [17], [18], [19], [20], [21], [22]. In the case of coatings, the process is limited since low deposition temperatures are needed, and adhesion related problems at the metal–polymer interface are commonly encountered. The microstructure, adhesion and electromagnetic properties of metal–polymer structures are related to the interface formation, possibility of metal diffusion into the polymer and intermixing [13], [14]. In the case of composite materials consisting of an insulating polymer matrix and homogeneously dispersed conductive fillers, the SE is governed by a percolation process where a minimum amount and an efficient arrangement of the fillers within the polymer matrix are required to form an electrical conductive network. A published article has previously reported a study on the SE improvement within certain frequency range by using the ceramic titanium carbide coating and CNF reinforcement of a liquid crystalline polymer matrix [23]. The present research is reporting on EMI SE improvement resulted by using conductive metallic thin films and CNF reinforcement of SBC and HDPE matrices and related adhesion analysis.

Section snippets

Material and methods

HDPE (Chevron Phillips Marlex® HXM 50100 Polyethylene) and SBC (K-Resin®) were supplied by Chevron Phillips Chemical Company and fabricated by compression molding in a Carver heated press to a final substrate size of 40 × 40 × 1 [mm] initially with no CNF reinforcement. The vapor grown CNF Pyrograf III™ (PR-24-AG) was provided by Applied Sciences Inc. (50 to 200 nm range in diameter) and was purified using procedures described in detail elsewhere [24]. Following the purification procedure increased

Results and discussion

The EMI SE represents the protection from the propagation of the electromagnetic field and is evaluated by measuring the attenuation or the insertion loss (IL) of the electromagnetic field passing through the test sample. The IL is the ratio in dB of the received powers Pref and Pload measured using the reference and the load specimen [29].IL=10logPref/PloaddB

The contribution to SE improvement of the composite materials obtained by using different ratios of CNF reinforcement for SBC and HDPE is

Conclusions

The SE of two different polymeric materials (SBC and HDPE) was tested after CNF reinforcement of the matrix and metallic thin film depositions and results show a higher SE when the two methods are used together. The SE was also tested for 100 nm and 300 nm thicknesses of metallic coatings and no significant improvement was observed for the case of higher thickness. Reflection as the prime mechanism in metallic thin films SE proves to be independent of film thickness but dependent on the material

Acknowledgments

Authors gratefully acknowledge the National Science Foundation, Partnership for Research and Education in Materials (PREM) for support under grant no. 0934157 and US Army Research Laboratory Instrumentation grant W911NF-08-1-0353. We would also like to thank student Alma Perez.

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