Highly anisotropic Cu oblate ellipsoids incorporated polymer composites with excellent performance for broadband electromagnetic interference shielding

Highly anisotropic Cu oblate ellipsoids incorporated polymer composites with excellent performance for broadband electromagnetic interference shielding

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Composites Science and Technology 144 (2017) 57e62

Contents lists available at ScienceDirect

Composites Science and Technology journal homepage: http://www.elsevier.com/locate/compscitech

Highly anisotropic Cu oblate ellipsoids incorporated polymer composites with excellent performance for broadband electromagnetic interference shielding Seung Hwan Lee a, b, 1, Seunggun Yu a, c, 1, Faisal Shahzad a, d, 1, Jun Pyo Hong a, Woo Nyon Kim b, **, Cheolmin Park c, Soon Man Hong a, d, Chong Min Koo a, d, e, * a

Materials Architecturing Research Center, Korea Institute of Science and Technology, 5, Hwarang-ro 14-gil, Seongbuk-gu, Seoul, 02792, Republic of Korea Department of Chemical and Biological Engineering, Korea University, Seoul, 136-713, Republic of Korea Department of Materials Science and Engineering, Yonsei University, Seoul, 120-749, Republic of Korea d Nanomaterials Science and Engineering, University of Science and Technology, 217, Gajung-ro, Yuseong-gu, Daejeon, 34113, Republic of Korea e KU-KIST Graduate School of Science and Technology, Korea UniversityAnam-ro 145, Seongbuk-gu, Seoul, 02841, Republic of Korea b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 18 August 2016 Received in revised form 14 December 2016 Accepted 14 March 2017 Available online 15 March 2017

In this study, highly anisotropic Cu oblate ellipsoids incorporated polymer composites were prepared that revealed excellent broadband electromagnetic shielding effectiveness of 80.0 to 62.1 dB at a frequency region between 300.0 KHz and 12.0 GHz, at low Cu contents. Cu-coated hollow polymer beads were fabricated through electroless plating of Cu on the polymer hollow beads. The hollow polymer beads were prepared through thermal expansion of acrylonitrile-based polymer beads containing a blowing agent. These beads were capable of incorporating highly anisotropic 2-dimensional (2D) Cu oblate ellipsoids into polymer composite through simple compression molding process. The resulting broadband electromagnetic shielding performance attributes to a low percolation behavior of the composites due to high anisotropy of the conductive Cu filler and their multilayered structure in the composites. © 2017 Elsevier Ltd. All rights reserved.

Keywords: EMI shielding Percolation threshold Cu ellipsoid High anisotropy Nanocomposite

1. Introduction Recent advances in telecommunication and electronic technologies have brought innovative prosperity in the ubiquitous life of human beings. Despite these advances, however, the unwanted electromagnetic (EM) signal interference produced through the excessive use of electronic devices has led to deterioration in performance of neighboring electronic gadgets, also causing invisible hazards to the health of humans [1e4]. Metals have excellent electrical conductivity among the materials; therefore, they are the most promising materials for electromagnetic interference (EMI) shielding because this characteristic is proportional to their excellent electrical conductivity [5,6].

* Corresponding author. Materials Architecturing Research Center, Korea Institute of Science and Technology, 5, Hwarang-ro 14-gil, Seongbuk-gu, Seoul, 02792, Republic of Korea. ** Corresponding author. E-mail addresses: [email protected] (W.N. Kim), [email protected] (C.M. Koo). 1 These authors contributed equally. http://dx.doi.org/10.1016/j.compscitech.2017.03.016 0266-3538/© 2017 Elsevier Ltd. All rights reserved.

However, metals have several disadvantages, including corrosion, large density, and relatively high cost. To mitigate these disadvantages, many carbon-filled polymer composite systems were explored for EMI shielding applications. Carbon filler-coated polymer composites were researched to get 20.2 dB at extremely low loading of MWCNT and graphite nanoplatelets [7] and 45.1 dB at 3.47 vol% of reduced graphene oxides [3]. However, the EMI shielding efficiency was required to be further improved. Recently, metal-filled polymer composites also received a great attention. But, to obtain a sufficient EMI shielding performance, large amount of particulate metallic fillers were incorporated in the polymer matrix to exceed the percolation limit [8e13]. The high percolation not only increases the density and cost but also decreases the processability and mechanical reliability of the product [12e15]. Recently, the incorporation of anisotropic metallic fillers, such as 1-dimensional (1D) metal nanowire [16,17] and 2-dimensional (2D) metal flakes [18] has gained tremendous interest to reduce the percolation threshold concentration. Arjmand et al. [16] and AlSaleh et al. [17] reported that 1D Ag nanowire/PS (Ag nanowire 2.5 vol%) and 1D Cu nanowire/PS composites (Cu nanowire 2.1 vol

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%), due to the huge shape anisotropy of 1D nanowire fillers, showed low percolation and resultant high EMI shielding effectiveness (SE) values of 31.8 dB and 35.0 dB in X-band, respectively. However, the synthesis cost of 1D fillers, such as Cu and Ag nanowires is much higher than the common particular fillers and therefore, the use of 1D nanowire fillers are not economically viable to use in practical applications where high EMI shielding >50 dB is required. Li et al. [18] fabricated electrically conductive polyethersulphone (PES) composites containing 2D Al flakes (30 vol%) and an EMI SE value of above 50 dB was obtained at 1 GHz. EMI shielding performance of 2D flakes is better than isotropic particular filler systems [14,15], but it need to be improved compared with the 1D fillers. Meanwhile, EMI shielding materials that cover a broadband frequency region are inevitably required to be developed for many arising future applications, such as satellite communications for GPS and navigation applications (3.0e30.0 MHz), aerospace communications (1.0e1.2 GHz), cellular devices (mobile phones and global positioning systems) (0.3e6.0 GHz), wireless LAN (2.4e5.0 GHz), and radar systems for self-driving car that can work in further expanded range of frequency (3.0e30.0 GHz). However, so far, most of the researches on EMI shielding materials have been focused on a limited frequency range, notably the X-band. This study demonstrates that highly anisotropic Cu incorporated polymer composites, fabricated via a simple electroless plating of Cu onto the expanded polymer hollow beads followed by compression molding, provided excellent EMI shielding ability at a very broad frequency range. 2. Experimental section 2.1. Materials Expandable polymer beads (Expancel 461 DU 40, Akzo Nobel, Sweden) were made of an acrylonitrile-based copolymer as shell and isobutane as blowing agent in core with diameter of 1e15 mm. Polystyrene (PS) and dimethylformamide (DMF) were purchased from Daejung Chemicals & Metals, Republic of Korea. All electroless plating reagents, including conditioner, sensitizer, activator, accelerator and electroless Cu plating solution were purchased from PI Tech, Republic of Korea. Cu solid beads were purchased from Sigma Aldrich. 2.2. Preparation of Cu coated expanded polymer beads (CuEBs) Expanded polymer beads (EBs) were prepared through the thermal treatment of the expandable beads in oven at 130  C for 5 min. The prepared EBs had the diameter of 20e40 mm. For the Cu coating on the surface of EBs, they were initially rinsed in 1 M NaOH solution for 5 min followed by neutralizing with the HCl solution. Then, the EBs were treated with tin and palladium catalyst and were stirred in the plating solution containing CuSO4 for 30 min at 60  C to perform the electroless Cu plating. The resulting Cu-coated EBs (CuEBs) were rinsed with distilled water and dried in oven at 70  C for 18 h. 2.3. Preparation of CuEB/PS composites PS was dissolved in DMF (10.0 mg mL1) under sonication for 30 min. The CuEBs were mixed with the PS solution in DMF while stirring for 30 min. CuEB/PS composites were precipitated in excessive methanol and the precipitate was dried in an oven overnight at 70  C. The products were put into a stainless steel mold with a toroidal shape (fout ¼ 7.0 mm, fin ¼ 3.0 mm) and pressed under 5.0 MPa at 95  C using a compression molding machine (Auto series, Carver Inc., U.S.A), (Fig. S1). Cu solid beads

incorporated Cu/PS composites were compared as a control system. Isotropic Cu solid beads had the diameter of 40 mm. 2.4. Characterizations The morphologies of CuEBs and CuEB/PS composites were examined using optical microscopy (DM2500P, Leica, Germany) and field-emission scanning electron microscopy (FE-SEM, Inspect F50, FEI Company, USA) techniques. Electrical conductivity of composites was examined using a four-pin probe (MCP-TP06P PSP) method with a low resistivity Loresta GP meter (MCP-T610, Mitsubishi Chemical, Japan). EMI SE of the samples was examined using a vector network analyzer (ENA5071, Keysight Technologies, USA) with a coaxial air line sample holder. Toroidal-shaped composite specimen (fout ¼ 7.0 mm, fin ¼ 3.0 mm) were prepared for EMI measurements [19,20]. 3. Results and discussion The fabrication process of CuEB/PS composites with highly anisotropic Cu oblate ellipsoids is exhibited in Fig. 1a. Optical microscopic images of polymer beads before (Fig. 1b) and after (Fig. 1c) expansion, and Cu-coated expanded polymer beads (Fig. 1d) are also shown. The expandable polymer beads were consisted of acrylonitrile-based copolymer shell and volatile isobutane blowing agent core. The expandable polymer beads with diameter of 1e15 mm were gradually foamed into expanded beads (EBs) with diameter of 20e40 mm through evaporation and escaping of blowing agent out of the polymer beads at 130  C. The resulting EBs had completely spherical shape and an extremely low density of approximately 0.02 g cm3. CuEBs were prepared through uniform Cu coating on the surface of EBs through electroless plating method [21,22]. The EBs with inner empty space play as a template for the fabricating of Cu hollow beads. Average thickness of Cu shell was about 1 mm, which was confirmed by SEM analysis (inset of Fig. 1e). Cu coating layers formed very smooth surface on the EBs and exhibited high crystallinity without the formation of oxidation layer, as shown in Fig. S2. The CuEB/PS composites with highly anisotropic Cu oblate ellipsoids were successfully fabricated through compression molding of CuEB and PS blend under high pressure. Spherical CuEB beads were simultaneously deformed into highly anisotropic oblate ellipsoids, as shown in Fig. 1e and f. The Cu oblate ellipsoid morphology was examined through the observation of disc-shape morphology in a top view and squeezed ring shape morphology in a cross-sectional view. The Cu oblate ellipsoids were well distributed in the CuEB/PS composite and there were not found any voids between ellipsoids and matrix, indicating that there is no any interfacial debonding between Cu fillers and matrix polymer after compression molding process. Due to the highly anisotropy of Cu ellipsoid, CuEB/PS composites formed a percolative structure even at 7.0 vol% CuEBs. Additionally, it is worth noting that CuEB/PS composite revealed much larger compressive modulus and compressive strength than neat PS, as shown in Fig. S3. These are attributed not only to good dispersion quality of Cu ellipsoid dispersions in a matrix polymer, but also to larger modulus of Cu filler than PS matrix. PS is an insulating polymer with an electrical conductivity of less than 1016 S cm1. The electrical conductivities of CuEB/PS composites increased with the increase in CuEB content and exhibited the values of 1.1  107, 7.8  102 and 4.2  103 S cm1 at 2.7, 7.0 and 15.0 vol% of CuEBs, respectively (Fig. 2). Isotropic Cu solid beads with a diameter of 40 mm were used as a control system. The Cu/PS composite with 15.0 vol% isotropic Cu solid bead revealed very low electrical conductivity of 5  105 S cm1, which is much smaller than that of CuEB/PS composite at the same Cu

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Fig. 1. (a) Schematic illustration of fabrication process of polymer composites with highly anisotropic Cu oblate ellipsoids. Optical microscope images of: (b) expandable polymer beads, (c) expanded polymer beads, (d) Cu-coated EBs. Insets show highlighted SEM micrographs. SEM images of the cross-section view (e) and top view (f) of CuEB7/PS composite. Inset shows magnified cross-section image of highly deformed Cu oblate ellipsoid in the composite.

content. In general, when an EM wave strikes onto a surface, the waves are either absorbed, reflected, or transmitted through the target material. Shielding effectiveness (SE) can be defined as the logarithmic ratio of incident power to the transmitted power as;

  P Shielding effectiveness SEðdBÞ ¼ 10 log I PT

(1)

Where PI is the incident power and PT is the transmitted power of EM waves. Total EMI SE (SET) in the composites can be divided into effects caused by absorption (SEA) and reflection (SER). Theoretically, the shielding due to absorption (SEA) and reflection (SER) can be expressed as [23];

  s SER ¼ 39:5 þ 10 log 2pf m

(2)

pffiffiffiffiffiffiffiffiffiffiffi SEA ¼ 8:7d pf ms

(3)

Where s is the electrical conductivity, f is the frequency, d is the thickness of shield and m is the magnetic permeability. Overall, EMI SE strongly depends on electrical conductivity and thickness of the shield. Therefore, a material with high electrical conductivity and thickness is expected to provide large EMI SE performance. Experimentally, SEA and SER can be calculated by using the reflection (R) and transmission (T) coefficients through the scattering parameters that are directly obtained from network analyzer. When an EM wave hits a surface, the reflection (R), absorption (A), and transmission (T) must add up to 1 as [23,24];

RþAþT ¼1

Fig. 2. Electrical conductivity values of neat PS, Cu15/PS, CuEB2.7/PS, CuEB7/PS, and CuEB15/PS composites.

 Aeff ¼

1RT 1R

(7)

SER and SEA can be expressed in terms of scattering parameters as;

 SER ¼ 10 log

(4)

 2  2 R ¼ S11  ¼ S22 

(5)

SEA ¼ 10 log

 2  2 T ¼ S12  ¼ S21 

(6)

¼ 10 log

The effective absorbance (Aeff), a measure of the absorbed EM waves in a material can be described as;



 1 ¼ 10 log 1R

1  2  1  S11 

!

!   1 1R ¼ 10 log 1  Aeff T  2 ! 1  S11   2 S21 

SET can be deduced from equations (8) and (9) as;

(8)

(9)

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Fig. 3. Variations of EMI shielding effectiveness for neat PS and Cu15/PS, CuEB2.7/PS, CuEB7/PS, and CuEB15/PS composites in: (a) total, (b) absorption and (c) reflection mode at various frequencies.

SET ¼ 20 logðS21 Þ

(10)

In the examinations of SET for PS, CuEB/PS and Cu/PS composites, PS showed 0 dB in the whole frequency range due to its insulating nature (Fig. 3a). The EMI SE values for CuEB/PS composites rapidly increased with CuEB content, providing an

extremely large value of 80.0 dB at 300 KHz for CuEB/PS composite containing 15.0 vol% CuEB. The composite materials maintained an excellent EMI SE value above 60.0 dB with bandwidth of over 12.0 GHz. Meanwhile, Cu/PS composite containing 15.0 vol% Cu solid beads showed extremely small EMI SE values of 5.3  103 dB at 300 KHz and 1.2 dB at 12.0 GHz, which are much

Fig. 4. SEM micrographs of: (a) CuEB/PS and (b) Cu/PS composites with deformed Cu sheet and isotropic solid Cu bead of 15.0 vol%. Schematic illustration of PS composites with: (c) highly deformed Cu multilayer, (d) conventional solid Cu bead.

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Fig. 5. EMI SE vs filler content (vol%) comparison of CuEB/PS with other polymer composite systems containing different metallic fillers.

smaller than the EMI SE value for CuEB/PS composite. Both SEA and SER values increased with CuEB content, which is similar with the SET trend, as shown in Fig. 3b and c. Meanwhile, shielding due to absorption was dominant mechanism for all CuEB/PS composites in the whole frequency range. For CuEB/PS composite with 15.0 vol% CuEB, absorption loss reached 57.2 dB out of the SET of 80.0 dB at 300 KHz. The significant difference between CuEB/PS and Cu/PS composites in the electrical conductivity and EMI SE can be explained by the morphological differences of Cu in both the composites, as shown in Fig. 4a and b. Unlike the use of solid Cu beads, the expanded beads with cellular structure were observed beneficial to incorporate the highly anisotropic sheet-like Cu oblate ellipsoids into the polymer composites. During compressing molding of CuEB/PS composites, the CuEB hollow beads were flattened with the widened surface. Due to the high anisotropy of Cu oblate ellipsoids, CuEB/PS composite formed a percolation network of Cu even at small CuEB content of 7.0 vol%. Furthermore, the inner hollow framework formed an insulating polymer gap with the thickness of approximately 100 nm between Cu layers, leading to formation of a multilayered sandwich structure. Such high anisotropic Cu ellipsoids and the resulting network structure of Cu fillers in the CuEB/PS composites are attributed to the significant improvement in the electrical conductivity and EMI SE. That is, the percolative Cu structure significantly increases the electrical conductivity as well as EMI SE of the CuEB/PS composites. Additionally, one can expect the multilayered structure as advantageous to effectively absorb the EM wave [19,25e31]. The incident EM waves penetrating into the first Cu layer, is reflected back by the second layer. The reflected EM is re-reflected again by the first layer towards the second layer. During the multi-reflection process, EM wave energy is efficiently absorbed in the composite, leading to a decrease intensity of EM waves. Such a high absorption loss contribution in CuEB/PS composite is caused not only by the excellent electrical conductivity but also by unique multilayer architecture of highly deformed Cu ellipsoids. Meanwhile, Cu/PS composite with Cu solid beads revealed the isolated Cu bead morphology. The low volume fraction (15.0 vol%) of Cu solid beads failed to reach the critical percolation threshold and conducting pathways, providing insulating feature and an extremely small EMI SE value. The structural effect on EMI shielding was well understood by the schematic illustration in Fig. 4c and d. Thus, the highly

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anisotropic Cu multilayer microstructure was effective to inhibit incident EM waves because the highly deformed Cu ellipsoids provide widely spread multilayered structure with respect to perpendicularly irradiated EM waves by enlarging surface area with a radius of approximately 100 mm. This feature presents an efficient shielding effect compared with conventional PS composite containing isotropic Cu solid beads. Fig. 5 shows the EMI SE comparison of CuEB/PS with those of other known polymer composites containing different types of metallic fillers as a function of filler content and geometry. It is obvious that increasing the filler content in all polymer composites will increase the EMI shielding effectiveness. Additionally, EMI SE strongly depends on the shape of filler. Isotropic particular fillers that are the most common and cheap among the metal filled polymer composite suffers the disadvantage of having large percolation threshold [12,14,15]. That means, EMI SE increase very slowly and the large amount of fillers are needed to achieve a high EMI shielding value. Whereas, 1D fillers with a huge aspect ratio like Ag and Cu nanowires exhibit very fast increase in EMI SE with the filler content, indicating low percolation threshold [16,17]. However, due to the high cost issue, 1D nanowire fillers are not economically viable to use in practical EMI shielding applications. In this work, the EMI SE of 2D CuEB/PS composites is not only much better than conventional isotropic particles and 2D flakes systems, but also it is comparable to the 1D fillers. It arises from the unique architecture of the composite that provides conducting pathways as well as a multilayered structure beneficial to shield EM waves. Considering the cost disadvantage of 1D nanowire fillers, CuEBs are much easier and cheap to synthesize on bulk scale and offers better opportunity for synthesizing polymer composites. This feature is particularly important in applications requiring higher EMI Shielding values > 80 dB where large amount of filler are needed to synthesize polymer composites to shield big structures and in a broad frequency range. 4. Conclusions We demonstrated a facile route to manufacture highly anisotropic Cu oblate ellipsoids incorporated polymer composite for lightweight and high performance EMI shielding materials. The acrylonitrile-based polymer beads containing a volatile blowing agent were transformed into hollow polymer beads with extremely low density through rapid evaporation of the blowing agent at elevated temperature. Cu-coated hollow polymer beads, fabricated through electroless plating of Cu on the hollow beads, were capable of incorporating highly anisotropic 2D Cu oblate ellipsoids into polymer composite using compression molding process. Due to the low percolation thresholds and multilayered morphology of the anisotropic Cu fillers, the resulting composite exhibited excellent electrical conductivity of 4.2  103 S cm1 and EMI SE in the range of 80.0 to 62.1 dB at a wide broadband frequency region between 300.0 KHz and 12.0 GHz. As a result, the excellent broadband EMI shielding ability makes it a strong choice for many arising future applications, such as next generation of 5G mobile phones and radar system for self-driving cars. Notes The authors declare no competing financial interest. Acknowledgment This work was supported by Fundamental R&D Program for Core Technology of Materials and the Industrial Strategic Technology Development Program funded by the Ministry of Trade,

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