Lightweight graphene nanoplatelet/boron carbide composite with high EMI shielding effectiveness Yongqiang Tan, Heng Luo, Haibin Zhang, Xiaosong Zhou, and Shuming Peng Citation: AIP Advances 6, 035208 (2016); doi: 10.1063/1.4943977 View online: http://dx.doi.org/10.1063/1.4943977 View Table of Contents: http://aip.scitation.org/toc/adv/6/3 Published by the American Institute of Physics AIP ADVANCES 6, 035208 (2016) Lightweight graphene nanoplatelet/boron carbide composite with high EMI shielding effectiveness Yongqiang Tan,a Heng Luo,a Haibin Zhang,b Xiaosong Zhou, and Shuming Pengb Innovation Research Team for Advanced Ceramics, Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang, 621900, China (Received 14 November 2015; accepted March 2016; published online March 2016) Lightweight graphene nanoplatelet (GNP)/boron carbide (B4C) composites were prepared and the effect of GNPs loading on the electromagnetic interference (EMI) shielding effectiveness (SE) has been evaluated in the X-band frequency range Results have shown that the EMI SE of GNP/B4C composite increases with increasing the GNPs loading An EMI SE as high as 37 ∼ 39 dB has been achieved in composite with vol% GNPs The high EMI SE is mainly attributed to the high electrical conductivity, high dielectric loss as well as multiple reflections by aligned GNPs inside the composite The GNP/B4C composite is demonstrated to be promising candidate of high-temperature microwave EMI shielding material C 2016 Author(s) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/) [http://dx.doi.org/10.1063/1.4943977] With the ever faster development and miniaturization of wireless electronic devices and communication instruments, electromagnetic interference (EMI) which refers to undesired electromagnetic radiation, especially at high frequencies, has become a serious issue that could significantly influence the reliability of electronic devices Consequently, there is a growing interest in the exploration of high-performance EMI shielding materials with high shielding effectiveness (SE) in the gigahertz or terahertz frequency range.1–7 Metals have been traditionally utilized for EMI shielding applications due to their high electrical conductivity.8,9 However, the wide application of metals as EMI shielding materials is limited by their heavy weight, low mechanical strength and poor corrosion resistance.2 Electrically conducting polymer composites have attracted considerable attentions as promising EMI shielding materials in the past decade because they are lightweight, flexible, and resistant to corrosion.2–5,10–13 Generally, conducting polymer composites can be obtained by incorporating high aspect ratio conductive nanofillers, such as carbon nanotubes (CNTs), carbon nanofibers (CNFs), metal nanowires (MNWs), and graphene nanoplatelets (GNPs).2–5,10–15 The addition of dielectric or magnetic nanoparticles could further improve the EMI SE of polymer composites.7,16,17 Carbon materials, especially flexible graphene films with micro-sized thickness, have also been demonstrated to exhibit exceptionally high EMI SE as well as in-plane thermal conductivity.18–20 However, the major restrictions for the conducting polymer composites and flexible graphene are the inferior temperature stability and mechanical properties, respectively Recently, ceramic-based composites have gained growing popularity for EMI shielding applications because of their low density, high strength, excellent corrosion resistance and high temperature stability.21–26 Therefore, they are suitable in applications of electronic devices, aircrafts electronics and automobiles Generally, the main restriction for ceramics as efficient shielding materials is their low electrical conductivity Nevertheless, similar with those in polymer composites, the a Y Tan and H Luo contributed equally to this work b Authors to whom correspondence should be addressed Email: hbzhang@caep.cn, pengshuming@caep.cn 2158-3226/2016/6(3)/035208/6 6, 035208-1 © Author(s) 2016 035208-2 Tan et al AIP Advances 6, 035208 (2016) electrical conductivity and EMI SE of ceramics could also be significantly enhanced by incorporating highly conducting CNTs or carbon fibers.21–26 Xiang et al.21 prepared CNTs reinforced fused silica composites and a high SE of 68 dB at 36–37 GHz was obtained with 10 vol% CNTs Shi et al.22 investigated the effect of CNTs on the shielding properties of yttria-stabilized zirconiain in the Ku-band range (12.4–18 GHz) and an EMI SE value as high as 25–30 dB was achieved However, the application of CNTs in ceramic composites is hindered by the difficulty of homogenous dispersion due to their tendency to aggregate More recently, graphene nanoplatelet (GNP), a new two-dimensional (2-D) carbon nanostructure material with high electrical conductivity and aspect ratio was found to be more effective for the enhancement of electrical conductivity of ceramic composites.27,28 GNP shows a much better dispersability and lower cost than CNTs More importantly, the distinctive 2-D nature makes GNP to be a more fascinating candidate for EMI shielding filler compared with its counterpart one-dimensional (1-D) CNTs.4,6 Nevertheless, no studies on microwave shielding properties of GNP/ceramic composites have been reported yet In this letter, we propose to prepare GNP/ boron carbide (B4C) composites with the aim of obtaining highly conductive B4C composites and exploring the potential application of the composites as shielding materials at X-band frequencies (8.2-12.4 GHz) The choice of B4C ceramic as the ceramic matrix is mainly due to its unique combination of light weight, high elastic modulus, excellent corrosion resistance and extremely high melting point.29 GNPs with 6–8 nm in thickness and 1–5 µm in level dimensions were purchased from DeyangCarbonene Technology Co., Ltd The GNPs were first dispersed in ethanol and sonicated for h B4C powders (Grade HS, H C Starck GmbH, Germany) mixed with different amount of GNPs dispersion (0, 1, 2, and vol%) and 10 vol% Ti3AlC2 powders (purity>98%, Forsman, China) as sintering additives were homogenized by attrition milling in ethanol media for 10 h Afterwards, the slurry was dried and then sieved The mixed powders were placed in a 50 mm diameter graphite die and hot-pressing sintered in vacuum under 1850◦C for 30 A uniaxial pressure of 30 MPa was applied during sintering The electrical conductivity of the GNP/B4C composites was measured by four-point method The excitation wavelength used for the Raman spectra is 532 nm For the EMI SE characterization in the X-band frequency, 22.86 mm × 10.16 mm × 2.00 mm specimens were polished and the S-parameters (S11 and S21) of each samples was determined in X-band through wave-guide method with a vector network analyzer (Agilent N5230A) For accuracy of measurement, the device is carefully calibrated with Through-Reflect-Line (TRL) approach A MATLAB code based on S-parameters was developed to extract shielding by reflection, shielding by attenuation and the total SE Fig shows the Raman spectra of as-received GNPs and GNP/B4C composite with vol% GNPs measured on the polished surface perpendicular to the hot-pressing direction, and the inserted FIG Raman spectra of as-received GNPs and GNP/B4C composite with vol% GNPs 035208-3 Tan et al AIP Advances 6, 035208 (2016) FIG Typical fracture surfaces of GNP/B4C composite with (a) vol% and (b) vol% GNPs image displays the fracture surface of GNP/B4C composite with vol% GNPs The Raman peaks at 1340 cm−1 (D band) and 1589 cm−1 (G band) are the main features of sp2 crystalline graphitic structures.30 The peak at 2660 cm−1 (G’ band) reveals the few-layer nature of GNPs in as-received state as well as in GNP/B4C composite Compared with as-received GNPs, the intensity ratio of the D and G band (ID/IG) which is a characteristic of disorder became much higher in GNP/B4C composites.31 There begins to show a shoulder near G band, which is also an indicator of defects in GNPs The defective nature of GNPs in the composites may be ascribed to the contribution of the exposed platelets edges The relative lower G’ band in the composite suggest the possible overlapping of GNPs The blue shift of G bands in composites compared to GNPs may be attributed to the compressive strain induced by the B4C matrix Fig 2(a) and 2(b) exhibit the typical fracture surfaces of GNP/B4C composites with and vol% GNPs respectively The GNPs can be easily distinguished as the platelet protruding out of the surface, as indicated by the arrows in Fig Due to the high pressure applied on samples during sintering, the 2-D GNPs tend to align perpendicular to the pressing axis From Fig 2, it can be also observed that the GNPs were uniformly distributed in the B4C matrix indicating the homogenous dispersion of GNPs There is overlapping between adjacent GNPs, which is essential for the formation of GNPs conducting networks The DC electrical conductivity (σ) of the GNPs/B4C composites was determined using the standard four-point contact method to eliminate contact-resistance effects, and the σ of GNPs/B4C composites as a function of GNPs volume content is shown in Fig B4C composite without GNPs shows a low conductivity of 2.3 S/m which is lower than those of monolithic B4C ceramics reported in literature.32 This can be mainly ascribed to the insulating phases such as AlOC2 and Al8B4C7 produced by reactions between B4C, the surface oxidation layer of B4C and Ti3AlC2 respectively at FIG DC electrical conductivity of GNP/B4C composites as a function of GNPs loading 035208-4 Tan et al AIP Advances 6, 035208 (2016) high temperatures.33 When vol% GNPs is added, the conductivity of the composites experiences a dramatic enhancement to 1248.6 S/m which is three orders higher than that of composite without GNPs The great enhancement of electrical conductivity could be mainly ascribed to the formation of conduction network by the uniformly distributed GNPs.34 Reflection, absorption and multiple reflections are the three main mechanisms of EMI shielding.1,13 Reflection arises from the interaction between mobile charge carriers and the electromagnetic fields, and is thus closely related to the electrical conductivity of the shielding material; Absorption loss results from interactions between electric and/or magnetic dipoles and the electromagnetic fields; Multiple reflections mainly occur at various surfaces or interfaces in the shield Therefore, the total EMI SE (SET) of a shield can be expressed as1,13 SET = SE R + SE A + SE M (1) where SER, SEA and SEM denote the shielding effectiveness due to reflection, absorption loss, and multiple reflections, respectively Experimentally, SER and (SEA + SEM) can be calculated as follows,13 ( ) (2) SE R = −10 lg − |S11|2 , ( ) |S21| , (3) SE A + SE M = −10 lg − |S11|2 where S11 and S21 were normalized S-parameters which were obtained from the vector network analyser The frequency dependences of SER, SEA + SEM and the overall SE in X-band range for GNP/B4C composites with different GNPs loading are exhibited in Fig 4(a)-4(c) respectively B4C composite without GNPs exhibits an inferior SET of 18 ∼ 21 dB which is mainly due to the lower electrical conductivity With increasing the GNPs loading, SER and SEA + SEM both increase and experience a dramatic enhancement when the GNPs content is above vol%, which is similar with the electrical conductivity Correspondingly, the overall SR increases sharply with increasing GNPs loading and high EMI SE values of 34 dB and 38 dB were obtained in GNPs/B4C composites with and vol% GNPs, respectively The specific EMI shielding efficiency (i.e., attenuation per unit density) of GNPs/B4C composites with vol% GNPs was calculated to be around 15 dB cm3/g which is higher than that of typical metals (e.g., ∼10 dB cm3/g for solid copper).35 The increase of SER could be mainly attributed to the enhanced electrical conductivity due to the formation of three-dimensional conductive network of GNPs GNPs provide a high EMI SE enhancement efficiency and the 2-D nature of GNPs was thought to play a more important role in EMI shielding A closer observation reveals that the major contribution to the overall shielding is attenuation rather than that reflection, which is different from the composites containing CNTs or CNFs.36 This could be attributed to 2-D structure as well as the large surface area of GNPs The thickness of GNPs layers is much lower compared with their skin depth (δ) which is given by:1 δ= π f µσ , (4) where f , µ and σ are the frequency, magnetic permeability and electrical conductivity respectively The skin depth is approximately calculated to be around µm by assuming the f , µ and σ to FIG (a) Reflection, (b) absorption and multiple reflections, and (c) total microwave shielding effectiveness of GNP/B4C composites 035208-5 Tan et al AIP Advances 6, 035208 (2016) be 10 GHz, 4π × 10−7 N/A2 and 106 S/m respectively, and is thus much higher than the thickness of GNPs Therefore, EMI could penetrate the single GNP with only a small part being reflected However, due to the large surface area of GNPs and their tendency to align parallel to each other, multiple reflections are expected at interfaces inside the composites Further, the aligned GNPs act as numerous micro-capacitors inside the composites and therefore could increase the dielectric loss of EMI The multiple reflection and high dielectric loss leads to a higher attenuation of EMI and give rise to an absorption dominant shielding feature of the composites This absorptive attenuation of EMI makes GNP/B4C composites competitive in devices needing higher electromagnetic compatibility, and even could potentially extend the application of GNP/B4C composites to microwave absorption areas Although the DC conductivity is markedly improved with only wt% of GNPs, the EMI SE does not exhibit sudden jump at wt.% This discrepancy could be interpreted as follows: the improvement of electrical conductivity relies on the formation of conducting networks by GNPs, while the enhancement of SE relies on both the formation of conducting networks and the multiple reflections With wt% of GNPs the conducting networks can be formed, however, the contribution from multiple reflections was not significant In conclusion, GNP/B4C ceramic composites were prepared by hot-pressing and significantly enhanced EMI SE was achieved The EMI SE of the composite increases with increasing GNPs loading and a high value of 37 ∼ 39 dB has been obtained at X-band with a GNP loading as low as vol% The high EMI SE is mainly due to multiple reflections and dielectric loss by aligned GNPs as well as the reflection resulting from the high electrical conductivity of the GNP/B4C composites All these results indicate the 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