manganese zinc ferrite hybrid fillers

manganese zinc ferrite hybrid fillers

Journal of Magnetism and Magnetic Materials 401 (2016) 472–478 Contents lists available at ScienceDirect Journal of Magnetism and Magnetic Materials...

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Journal of Magnetism and Magnetic Materials 401 (2016) 472–478

Contents lists available at ScienceDirect

Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm

Electromagnetic interference shielding performance of epoxy composites filled with multiwalled carbon nanotubes/manganese zinc ferrite hybrid fillers C.H. Phan a, M. Mariatti a,n, Y.H Koh b a b

School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia Motorola Solutions Malaysia Sdn. Bhd., Plot 2, Bayan Lepas Technoplex Industrial Park, Mukim 12, S.W.D., 11900 Penang, Malaysia

art ic l e i nf o

a b s t r a c t

Article history: Received 4 May 2015 Received in revised form 2 October 2015 Accepted 17 October 2015 Available online 19 October 2015

An effective electromagnetic-interference (EMI) shielding epoxy composite has been fabricated with a combination of multiwalled carbon nanotubes (MWCNTs) and manganese zinc ferrite (MnZn ferrite) fillers. MWCNTs were functionalized to improve dispersibility while manganese zinc ferrite nanoparticles were synthesized via the citrate gel method. The EMI-shielding performance of the fabricated composites was examined. It was found that the composite with a filler ratio of MWNCTs to MnZn ferrite ¼3:1 obtained the highest EMI shielding effectiveness (SE), with the shielding mechanism dominated by absorption. In addition, the EMI shielding performance of composites was improved by increases in the filler loading and thickness of composites. Composites with a filler loading of 4.0 vol% and thickness of 2.0 mm achieved an SE of 44 dB at 10 GHz with the assistance of conductive silver backing. This EMI SE is better than that of composites filled with single conductive filler and comparable with that of commercial EMI absorber. & 2015 Published by Elsevier B.V.

Keywords: Carbon nanotubes Manganese zinc ferrite Epoxy composites Electromagnetic interference Shielding mechanism

1. Introduction The electromagnetic interference (EMI) problem is getting serious due to increase in sensitivity of electronic devices, since currently most electronic gadgets are wireless and higher chip speeds [1,2]. Thus, an effective method of providing shielding from unwanted electromagnetic waves is in strong demand. The primary mechanism of conductive EMI shielding is usually reflection [3]. However, the reflected wave will bring “secondary electromagnetic pollution” to the surroundings. In order to obtain low reflection of the shield, impedance matching between the shield and free space is crucial [4]. Recently, a new strategy for providing EMI shielding was implemented by introducing both dielectric and magnetic lossy nanofillers into composites to give a very effective EMI absorption in a broad frequency range with controllable permittivity and permeability of composites [5,6]. Carbon nanotubes are widely used as an EMI shielding material due to their excellent dielectric properties, and they have demonstrated enhanced EMI attenuation when a low filler loading is embedded into a polymer matrix [7,8]. On the other hand, manganese zinc ferrite (Mn1  xZnxFe2O4) n

Corresponding author. Fax: þ60 4 5941011. E-mail address: [email protected] (M. Mariatti).

http://dx.doi.org/10.1016/j.jmmm.2015.10.067 0304-8853/& 2015 Published by Elsevier B.V.

is gaining popularity due to its fascinating magnetic and electromagnetic properties. It has good chemical stability, high permeability, high resistivity, and a large saturation magnetic flux density, which make it a good electromagnetic-wave absorber [9,10]. Therefore it can be foreseen that the combination of carbon nanotubes and manganese zinc ferrite filler will be able to improve the effectiveness of EMI shielding. In the present work, EMI-shielding composites were obtained by mixing epoxy resin with hybrid fillers consisting of functionalized MWCNTs (F-MWCNTs) and manganese zinc ferrite nanoparticles. MWCNTs were functionalized with nitric acid for controlled formation of the functional group with better dispersibility, while manganese zinc ferrite nanoparticles were synthesized by the citrate gel method. The effect of the hybrid filler ratio, filler loading, and thickness of composites on the shielding effectiveness (SE) was investigated, and the EMI-shielding mechanism of these EMI-shielding composites is discussed.

2. Materials and methods 2.1. Materials MWCNTs were purchased from USAINS Holding, Universiti Sains, Malaysia. These MWCNTs were produced via a catalytic

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chemical vapor deposition process, with carbon purity of more than 80% [11]. The average diameter and average length of MWCNTs were 10 71 nm and 1–5 μm, respectively. Residuals of the MWCNTs may include molybdenum and cobalt. The precursors used for MnZn ferrite synthesis were iron (III) nitrate nonahydrate (Fe(N03)3  9H2O, analytical grade), zinc nitrate hexahydrate (Zn(N03)2  6H2O, 98%), and manganese (II) nitrate hexahydrate (Mn(N03)2  6H2O,4 98%). All three precursors were purchased from Acros Organics BVBA. The chelating agent citric acid monohydrate (C6H8O7  H2O, analytical grade) was purchased from Fisher Scientific. Epoxy resin used in this research was D.E.R. 332 Epoxy Resin, a bisphenol A diglycidylether (DGEBA), with an epoxide equivalent weight of 171–175 g/eq. Epoxy resin was supplied by Penchem Technologies Sdn. Bhd. Polyetheramine D230 M was used as a hardener in this study. Here, the mixing ratio was set to 100 epoxy resin to 32 hardener. Also, 65% nitric acid for analysis (max.: 0.005 ppm Hg) EMSURE ISO was purchased from Merck Millipore, 25% ammonia solution for analysis EMSURE was purchased from Merck Millipore, and CMOS grade acetone was purchased from JT Baker. 2.2. Functionalization of MWCNTs The functionalization process used in this study was based on previous works [12–14]. However, modifications of process parameters and a pre-sonication step before reflux were considered in the present study. Here, 0.3 g of pristine MWCNTs was added to a 250 ml Pyrex round-bottomed flask containing nitric acid aqueous solution (65%). The mixture of MWCNTs and nitric acid was sonicated in a conventional ultrasonic bath (Sono Swiss SW12H) for an hour to promote disentanglement of MWCNTs within the acid solution. Next, the mixture was heated by a heating mantle under reflux at a temperature of 120 °C. After 2 h of reflux, the mixture was filtered using vacuum filtration with a 0.45 μm polytetrafluoroethylene (PTFE) membrane (Merck Millipore), and the filtration residue was rinsed with deionized water until all the nitric acid was neutralized. The functionalized MWNCTs (F-MWCNTs) were dried in a vacuum oven for 24 h at 80 °C. 2.3. Synthesis of manganese zinc ferrite The experimental procedure for synthesis of manganese zinc ferrite (MnZn ferrite) is based on previous work [15]. However slight modification was done, where calcination was carried out in an inert environment without a further annealing step. In this method, first the precursors Fe(NO3)3  9H2O, Zn(NO3)2  6H2O, and Mn(NO3)2  6H2O were dissolved in deionized water with respective stoichiometry under stirring. After all the metal nitrate precursors had dissolved, 10 ml of citric acid was added. The mole ratio of citric acid to total metal ions was controlled at 1:1. Ammonium hydroxide solution (NH4OH) with a concentration of 0.10 mol/L was slowly added into the mixture in order to reach pH 5–7. Then the mixture was stirred at 30 °C for 24 h to complete the reaction. After 24 h of stirring, the solution was heated to 80 °C until gel formed. The produced gel was dried in an oven at 100 °C for 12 h. The final product was obtained after 2 h of calcination at 1000 °C in an argon atmosphere. X-ray diffraction analysis (shown in Fig. 1) was performed on the final product to confirm the presence of manganese zinc ferrite phase. 2.4. Sample preparation

Fig. 1. The presence of manganese zinc ferrite phase found on synthesized ferrite powders by using X-ray diffraction analysis (XRD). Table 1 Identification of fabricated EMI-shielding composites. Sample Name

Fillers

C 100 C75F25 C50F50 C25F75 F100

100 vol% MWNCTs 75 vol% MWNCTsþ 25 vol% MnZn ferrite 50 vol% MWNCTsþ 50 vol% MnZn ferrite 25 vol% MWNCTsþ 75 vol% MnZn ferrite 100 vol% MnZn ferrite

sonicator (Hielscher – Ultrasound Technology, UP200S) at room temperature, for 10 min, with 50% amplitude and a 0.5 sonication cycle. Once the sonication process was completed, the curing agent (polyetheramine, D230 M) was added into the mixture at a ratio of 100:32 by weight (epoxy to curing agent). The mixture was immediately sonicated for another 10 min, in the presence of an ice-water bath, to slow down the curing process. After sonication, the mixture was placed in a vacuum oven (National Appliance Model 5831) for 45 min for the degassing process in order to remove the air entrapped during sonication. In the meantime, soapy water was applied to the glass mold as a releasing agent. Once the degassing process was finished, the mixture was poured into a glass mold and dried at room temperature for 12 h. Lastly, the composite sheet was cured at 80 °C for 2 h. 2.5. Characterization The EMI-shielding effectiveness (SE) measurement method used in this study is a waveguide system with a measured frequency range of 7 to 12 GHz. This set-up was done according to previous work [16]. A vector network analyzer was connected to two coaxial-waveguide adapters. The composite was located between the adapters during the measurement. The incident and transmitted waves in a two-port vector network analyzer are mathematically represented by complex scattering parameters (or S-parameters). The S-parameters, S11 (or S22) and S12 (or S21) were correlated with reflectance (R) and transmittance (T), respectively, based on Eq. (1). 2

The EMI-shielding composites were fabricated by the conventional casting method and the sample identification is described in Table 1. F-MWCNTs and synthesized MnZn ferrite were added to epoxy resin (D.E.R. 332). The mixture was sonicated with a

473

T=

2

ET E = S12 2 ; R= R = S11 2 EI EI

(1)

The absorbance, A, was represented as A ¼(1  R  T). Once the transmittance and absorbance parameters had been obtained, the

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reflection loss, SER, and absorption loss, SEA, were calculated based on Eq. (2),

SER (dB) = 10 log10R; SEA (dB) = 10 log10A

(2)

Hence, the values of the EMI-shielding effectiveness, SET, of the composite and commercial EMI absorber were determined by summing up the absorption loss and the reflection loss as shown in Eq. (3).

SET (dB) = SER + SEA

Table 2 Maximum reflection loss and absorption loss of F-MWCNTs/manganese zinc ferrite epoxy composite with different filler ratios. Samples

C100

C75F25

C50F50

C25F75

F100

Reflection loss (dB) Frequency Absorption loss (dB) Frequency

4.36 7 2.0 10

3.0 7 4.1 7

2.8 7 2.1 9.6

2.3 7 3.5 11.5

2.1 7 3.6 12

(3)

The vector network analyzer was calibrated with electronic calibration (ECal) modules before the measurement. After calibration, the waveguide adapters were connected to the vector network analyzer via a coaxial cable. The composite was placed between the adapters and the S parameters were measured from 7 to 12 GHz. The dispersion of both fillers in the epoxy matrix was observed using field emission scanning electrons microscopy (FESEM, Zeiss SupraTM 35VP), on the fracture surface of the composite, with 10.0 kV under 10 K magnification.

3. Results and discussion 3.1. Effect of filler ratio on EMI-shielding performance of the composites The effect of the filler ratio on EMI SE was studied by incorporation of F-MWCNTs and MnZn ferrite into epoxy composites and the measurement of EMI SE was carried out in a frequency range of 7–12 GHz. Based on Fig. 2, the EMI SE of C75F25 epoxy composites (with F-MWCNTs to MnZn ferrite filler ratio¼3:1) is higher than that of other composites and achieved the highest EMI SE value of 7 dB at the frequency of 7 GHz. The highest value of EMI SE for C75F25 composites is believed that contributed by the acceptable reflection loss and absorption loss. Fig. 2 provides further understanding of the effect of the filler ratio on the reflection loss and absorption loss of composites. Generally, the trend of the reflection loss value decreased with increases in frequency for all the composites while the value of the absorption loss increased with frequency for all the composites. As reported by Chung [17], the primary shielding mechanism is usually reflection, followed by absorption as the secondary shielding mechanism. The reflection mechanism takes place when there is a difference between the characteristic impedances of the wave and the shielding materials [18]. Hence, shielding materials

Fig. 2. Effect of filler ratio on EMI-shielding effectiveness of F-MWCNTs/MnZn ferrite epoxy composites.

which are higher in electrical conductivity and lower in magnetic permeability exhibit a larger reflection loss due to the impedance mismatch [18]. This condition is represented by C100 composite, which achieved the highest reflection loss with a value of 4.36 dB at a frequency of 7 GHz and the lowest overall absorption loss, as shown in Table 2. Apparently, C75F25 composite has an acceptable reflection loss and the highest overall absorption loss in this frequency range, which is consistent with the result of EMI SE shown in Fig. 2. According to Gama et.al. [19], an effective EMI absorber needs to satisfy the impedance matching condition, where the magnetic permeability value must be close to the electric permittivity value of the shielding material. By introducing a suitable filler ratio of F-MWCNTs and MnZn ferrites into the composite, the effect of impedance mismatch was minimized and the absorption of EMI was maximized. Hence, the overall absorption loss for C75F25 composite is the highest among the composites evaluated, with a maximum of 4.1 dB at a frequency of 7 GHz, achieved with the help of the magnetic loss contributed by MnZn ferrite and the dielectric loss contributed by F-MWCNTs. This finding is consistent with those of Zhou et. al. [20], where the proper amount of iron (Fe) was incorporated with mesoporous carbon  silica nanocomposites to form an effective electromagnetic-wave absorber. The FESEM images of MWCNTs/manganese zinc ferrite filled epoxy composite (C75F25) are shown in Fig. 3. As seen in Fig. 3, both MWCNTs and manganese zinc ferrite are dispersed homogeneously in an epoxy matrix without obvious agglomeration since the surface charges on functionalized MWCNTs help in facilitated the dispersion of fillers. Fig. 4 shows that the reflection loss decreased with increasing frequency while the absorption loss increased with frequency. The decrease of reflection loss with increasing frequency occurs

Fig. 3. FESEM micrographs of the fracture surface of MWCNTs/manganese zinc ferrite filled epoxy composite (C75F25). (MWCNTs – Brighter spot, MnZn Ferrite – dull spot).

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Fig. 4. Effect of filler loading on reflection loss and absorption loss of F-MWCNTs/ MnZn ferrite epoxy composites.

Fig. 6. Effect of filler loading (0.1, 0.5, 1.0, 2.0, and 4.0 vol%) on reflection loss and absorption loss of C75F25 composites.

because the shield impedance increases with frequency, while the absorption loss increases with frequency because the skin depth decreases [21]. In addition, based on the reflection loss (SER) and absorption loss (SEA) equations in Eq. (2), the SER and SEA for a plane wave can be represented using Eq. (4) [22],

Table 3 Maximum reflection loss and absorption loss of F-MWCNTs/manganese zinc ferrite epoxy composite with different filler loadings.

SER = 39.5 + 10 log

σ d , SEA = 8.7 = 8.7d πfμσ 2π fμ δ

Filler loading (vol%)

0.1

0.5

1.0

2.0

4.0

Reflection loss (dB) Absorption loss (dB)

2.6 2.3

1.7 3.5

2.9 4.0

3.8 4.3

5.7 5.8

(4)

where s is electrical conductivity, μ is permeability, and δ is skin depth. It is clear that this equation shows the relationship between reflection loss and absorption loss with frequency, where reflection loss decreases with frequency and absorption loss increases with frequency. As a result, the composites with higher amounts of MnZn ferrite such as C25F75 and F100 tend to give increased EMI SE values in higher frequency ranges since absorption loss is dominating the shielding mechanism of these two composites, as shown in Fig. 4. 3.2. Effect of filler loading on EMI-shielding performance of composites The effect of filler loading from 0.1 to 4.0 vol% on the EMI SE of epoxy composite in the frequency range of 7–12 GHz is shown in Fig. 5. Composites with an F-MWCNTs to MnZn ferrite filler ratio of 3:1 or C75F25 composites were used in the study. It can be seen

Fig. 5. Effect of filler loading (0.1, 0.5, 1.0, 2.0, and 4.0 vol%) on EMI-shielding effectiveness of C75F25 composites.

that the overall EMI SE of composites increased with increases in the filler loading. The composite with a filler loading of 4.0 vol% have the highest overall EMI SE among the other composites evaluated, with a maximum value of 11.3 dB at a frequency of 7 GHz. The increase in the EMI SE of composites with the filler loading is mainly due to the increase in both reflection loss and absorption loss. Besides, the overall trend of EMI SE for all the composites decreased gradually with increasing frequency. This phenomenon was observed mainly because the reduction in the magnitude of reflection loss was greater than the increase in the magnitude of absorption loss in the higher frequency range. Fig. 6 shows that the overall reflection loss and absorption loss curves for all the composites increased with increases in the amount of added fillers, and this result is consistent with the result of EMI SE in Fig. 5. The reflection loss is governed by the conductivity of the composites, where reflection loss increases with electrical conductivity [23]. The greater the amount of F-MWCNTs introduced into the composites, the more densely conductive the network that is formed, and thus the distance between MWCNTs decreases, which helps to facilitate the flow of electrons through the network formed. Therefore the electrical conductivity as well as the reflection loss increased with filler loading (Table 3). The increase of absorption loss with filler loading is mainly contributed by dielectric loss and magnetic loss. Based on electromagnetic theory, dielectric loss is attributed to natural resonance, dominant electronic and dipolar polarization, and interfacial polarization [24]. The interfacial polarization was enhanced when number of interface between both fillers increased. In addition, the magnetic loss is due to the resonance phenomenon, which is governed by magnetic domain wall resonance and ferromagnetic resonance [25]. The addition of a greater amount of magnetic filler, MnZn ferrite, to the composite might result in an increase in the magnetic domain within the composite, and a larger amount of energy will be dissipated as a result of the resonance phenomenon.

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Table 4 Maximum reflection loss and absorption loss of F-MWCNTs/manganese zinc ferrite epoxy composite with different composite thicknesses. Thickness (mm)

2.0

1.0

0.6

0.2

Reflection loss (dB) Frequency Absorption loss (dB) Frequency

3.9 7 6.7 7

2.9 7 4.0 7

1.4 7 2.8 7

0.6 9.6 1.4 9.6

by current induction because of an alternating magnetic field. The eddy current loss can be expressed by Eq. (5) [28], where e is the eddy current loss coefficient, d is the thickness, and s is the electrical conductivity. Fig. 7. Effect of composite thickness (0.2, 0.6, 1.0, and 2.0 mm) on EMI-shielding effectiveness of C75F25 composites.

3.3. Effect of composite thickness on EMI-shielding performance of composites The effect of the thickness of C75F25 composites on EMI SE at a frequency of 7 to 12 GHz is shown in Fig. 7. Generally, the overall EMI SE are increased with the thickness of the composites. The maximum SE value increased from 1.0 to 10.7 dB at the frequency of 7 GHz when the thickness of the composites increased from 0.2 to 2.0 mm. The enhancement in the EMI SE of composites with the increase in the thickness is caused by increases in reflection and absorption loss. This result is consistent with the previous works [24,26]. Besides, the maximum EMI SE was shifted to a higher frequency when the composite thickness decreased. The frequency shift for maximum SE is due to the resonant absorption, and this absorption is attributed to quarter-wavelength attenuation when the reflected EM waves and incident EM waves meet in composites [27]. Seemingly, the reflection loss and absorption loss for all the composites are increased with increasing composite thickness as shown in Fig. 8. Based on Table 4, the composite with a thickness of 2.0 mm shows the highest reflection and absorption, with maxima of 3.9 and 6.7 dB respectively at the frequency of 7 GHz. Besides, the increase in absorption loss was far greater than the increase in reflection loss. The increase in reflection loss was suspected to be due to eddy current loss, where energy dissipates

e=

4π 2μ 0 d2σ 3

(5)

Based on this equation, materials with large thickness and high in electrical conductivity will possess higher eddy current loss. On the other hand, according to the equation SEA ¼8.7d/δ, the absorption loss is proportional to the thickness of the composite [29]. Increasing the composite thickness would increase the amount of filler interacting with the incident EM wave and would enhance the dielectric and magnetic loss of the composite, as shown in Fig. 9. 3.4. Comparison of EMI-shielding performances A comparison of EMI-shielding performances obtained in the present work and previous works is shown in Table 5. Seemingly, in the present work, the composite without conductive silver backing layer exhibited a lower EMI SE when compared with composite with applied conductive silver backing. This conductive backing layer served as a secondary shield by promoting multiple reflection within the shield and minimized the transmission of the EM wave through the shield. Thus, the absorption of the EM wave within the shield was improved significantly. By applying the conductive silver layer behind the fabricated composite, the SE of the composite was clearly enhanced. With the filler loading of 4.0 vol % and thickness of 2.0 mm, EMI shielding of the fabricated composite of 44 dB was observed at 10 GHz, which is much higher than cured epoxy resin, with merely 0.75 dB in EMI shielding performance. In addition, the EMI-shielding performances of hybrid F-MWCNTs/MnZn ferrite filled epoxy composites were obviously better than those of composites with single conductive fillers (MWCNT or SWCNT) [23,30–37], as shown in Table 5. Seemingly, the used of hybrid fillers enhances the overall SE of composites with the combination of dielectric loss and magnetic loss of these two fillers. Furthermore, the SE performances of these hybrid-filler-filled composites are comparable with that of commercial EMI absorber, as shown in Table 5.

4. Conclusions

Fig. 8. Effect of filler loading (0.2, 0.6, 1.0, and 2.0 mm) on reflection loss and absorption loss of C75F25 composites.

This work provides promising EMI-shielding polymer composites by incorporating both MWCNTs and MnZn ferrites into epoxy matrix. Composite with the filler ratio of MWNCTs to MnZn ferrite¼ 3:1 obtained the highest EMI SE, compared with the composites with other ratios. It was found that the shielding mechanism of the composite was dominated by absorption. Besides, the results showed that the EMI-shielding performance of the composites was enhanced by increases in the filler loading and composite thickness. With the assistance of conductive silver

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477

Fig. 9. Illustration of the interaction between hybrid fillers and EM wave in the composites with different thicknesses.

Table 5 Comparison of EMI SE in present and previous works. Matrix þ fillers

Fillers content

Thickness (mm)

MWCNTsþ MnZn ferritesþ Epoxy MWCNTsþ MnZn ferritesþEpoxy (silver backed) Commercial EMI absorber Cured Epoxy MWCNT þPolypropylene SWCNT (long) þEpoxy MWNCTþ Polyacrylate MWNCTþ Polyacrylate MWCNT þPolyurethane SWCNTþ PANIa MWNCTþ Polyurethane MWNCTþ Polyurethane MWNCTþ Epoxy SWCNTþ ABSb

4.0 vol%

2.0

17

Present work

4.0 vol%

2.0

44

Present work

– 0 vol% 7.5 vol% 15 wt% 2 wt% 10 wt% 5 wt% 20 wt% 10 wt% 10 wt% 20.4 wt% 5 wt%

0.55 2.0 1.0 2 1.5 1.5 0.1 2.4 2.5 1.5 0.35 2.0

40 0.75  34 25  4  25  5  19  41.6  29  19  9

Present work Present work [23] [30] [31] [32] [32] [33] [34] [35] [36] [37]

a b

EMI SE (dB) at 10 GHz

References

Polyaniline. Acrylonitrile butadiene styrene.

backing, the EMI-shielding performance of fabricated hybrid-fillers-filled epoxy composite with a filler loading of 4.0 vol% and thickness of 2.0 mm achieved an SE of 44 dB at 10 GHz, which is better than that of composites filled with single conductive filler and comparable with that of commercial EMI absorber.

Acknowledgments The authors gratefully acknowledge the support provided by Motorola Solutions (M) Sdn Bhd.

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