Preparation and characterization of graphene paper for electromagnetic interference shielding

Abstract

Syntheses of multifunctional structures, both in two-dimensional and three-dimensional space, are essential for advanced graphene applications. A variety of graphene-based materials has been reported in recent years, but combining their excellent mechanical and electrical properties in a bulk form has not been entirely achieved. Here, we report the creation of novel graphene structures such as graphene pellet and graphene paper. Graphene pellet is synthesized by chemical vapor deposition (CVD), using inexpensive nickel powder as a catalyst. Graphene pellet can be further processed into a graphene paper by pressing. The latter possesses high electrical conductivity of up to 1136 ± 32 S cm⁻¹ and exhibits a breaking stress at 22 ± 1.4 MPa. Further, this paper-like material with thickness of 50 μm revealed 60 dB electromagnetic interference (EMI) shielding effectiveness.

Introduction

The proliferation of electronic devices in recent decades has greatly increased the potential for EMI. Electromagnetic radiation at high frequencies can easily interfere with electronic devices and is also harmful to human health. Consequently, there is a significant interest in the development of materials for EMI shielding. Generally, materials with good electrical conductivity show good shielding performance in reducing the energy of penetrating electromagnetic radiations. For example, metals with good electrical conductivity (e.g., copper, nickel, aluminum) show good performance for EMI shielding [1]. However, in many applications (such as aerospace electronics), the material for EMI shielding besides of being effective, needs to be lightweight and flexible, especially in applications of flexible electronics, aircrafts and automobiles. Thus the density and flexibility of shielding materials are usually evaluated in EMI shielding applications, making carbon materials [2], [3], [4], [5] competitive with metals.

For example, research suggests that graphene has the potential to be an excellent EMI shielding material with up to 500 dB cm³ g⁻¹ specific EMI shielding effectiveness when incorporated in a PMDS matrix [2]. Flexible graphite with an EMI shielding effectiveness of 130 dB was also reported [6]. However, the thickness is a major factor that needs to be considered when comparing the SE of different shielding materials. According to the plane-wave theory [7], larger thickness of shielding materials will yield higher shielding effectiveness in dB. This dependence is not linear, since both reflection and absorption are involved. Thus it is worth to mention that the thicknesses of the reported carbon shielding materials [2], [3], [4], [5], [6] are three orders higher compared to shield of copper film [1] which limit their applications as thin, protective layers for EMI shielding of sensitive instruments. The relative large thickness of these carbon based shielding materials is due to the poor mechanical properties that call for polymer coating, which enlarges the thickness and adds additional processing steps. This polymer coating reduces the carbon filler fraction in the carbon/polymer shield resulting in a low electrical conductivity of the composite material. This explains the absolute shielding effectiveness of these carbon/polymer composite materials [2], [3], [4], [5], [8], [9], [10], [11], [12], [13], [14], [15] is usually low compared to copper or nickel.

One approach to increase the electrical conductivity of carbon/polymer EMI shielding materials is to increase the carbon filler content, thus making paper-like pristine graphene materials [16], [17], [18], [19] promising for new shielding applications especially in the aerospace industry where the weight matters. This EMI shielding application of graphene paper is related to its low density, excellent flexibility and extraordinary electrical properties of graphene materials [20], [21], [22], [23]. Graphene paper [16] prepared by using graphene oxide (GO) as a template for synthesis and processing showed good mechanical properties with a breaking stress at 120 MPa. However, the poor conductivity of GO – resulting from the introduction of oxygen and surface defects during preparation – limits its applications [19]. Recently, highly electrical and thermal conductive GO paper prepared by direct evaporation demonstrated a EMI shielding effectiveness of 20 dB was reported [24], but GO paper prepared by similar method usually shows relatively high density [16], which limits its application as a light EMI shielding materials. Graphene made through CVD has an overall high quality. By using this method, graphene paper [18] with good electrical conductivity has been achieved by filtration of the CVD graphene foams [25], but the poor mechanical strength of this material requires supporting substrate and expensive catalyst template – nickel foam – which may be a barrier for industrial scale up.

Here, we report a novel, freestanding graphene paper prepared by CVD synthesis of 3D graphene pellets, extracting the Ni catalyst and pressing the remaining structure to form a paper-like material [26].