Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Robust and Flexible Cellulose Nanofiber/Multiwalled Carbon Nanotube Film for High-Performance Electromagnetic Interference Shielding Haoruo Zhang, Xunwen Sun, Zhengguang Heng, Yang Chen,* Huawei Zou,* and Mei Liang
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The State Key Lab of Polymer Materials Engineering, Polymer Research Institute of Sichuan University, Chengdu 610065, China ABSTRACT: For the preparation of lightweight and high-performance electromagnetic interference shielding material, the poor dispersion of carbon nanotubes (CNTs) and weak interfacial strength degrade the mechanical properties of the polymer-based composite with extremely high filler contents. Herein cellulose nanofibers (CNFs) prepared by TEMPO-mediated oxidation exhibits a dispersive action for multiwalled carbon nanotube (MWCNTs) without chemical functionalization of the MWCNTs or the use of surfactant. Thus a robust and flexible CNF/MWCNT composite film can be fabricated by simple vacuum filtration and hot-pressing method. This composite film (thickness 0.15 mm) shows an electromagnetic interference shielding effectiveness (EMI SE) of 45.8 dB in the X-band. Thanks to the all-fiber structure and the association between CNFs and MWCNTs, it exhibits good flexibility and tensile strength up to 48 MPa, which is superior to other reported MWCNT-based films for electromagnetic shielding, giving it the potential to be used in flexible electronics and wearable devices.
1. INTRODUCTION With the widespread use of portable electronics and wearable devices, lightweight ultrathin electromagnetic interference (EMI) shielding materials are increasingly desirable. Considering the poor flexibility and high density of traditional metal electromagnetic shielding materials, conductive polymer composites with carbon-based conductive nanofillers are increasingly favored by researchers. Carbon nanotubes (CNTs) are one of the most promising carbon nanomaterials. CNTs have higher aspect ratios compared to carbon nanofibers and carbon black. Thus, the electrical percolation threshold concentration of CNT-based nanocomposites is lower than that of carbon nanofibers and carbon black based nanocomposites.1 The rapid development of production technology enables CNTs, especially multiwalled carbon nanotubes (MWCNTs), to be manufactured on a large scale, contributing to low cost,2 which gives MWCNTs a significant cost advantage over graphene and graphene oxide. In order to achieve relatively ideal electromagnetic shielding performance at low thickness, high conductive filler loading is usually required.3 The difficulty of dispersing CNTs homogeneously and the high viscosity of high filler loading limit the application of melt mixing processing. Some attempts have been made to prepare low-thickness films as high-performance electromagnetic shielding materials, in which composites with extremely high CNT content are typically produced by mixing CNTs with a polymer matrix in solution. Zeng et al. fabricated EMI shielding composites with extremely high CNT contents © XXXX American Chemical Society
by mixing MWCNTs with waterborne polyurethane (WPU) in water.4 However, its tensile strength is less than 3 MPa. Some strategies have been devised to improve the mechanical properties of the CNT-based film. Considering the intrinsic brittleness of CNT-based films, Jia et al.5 fabricated natural rubber/CNT film for flexible application. Zhang et al. reported a method to fabricate layer-structured films comprising poly(ethylene oxide) (PEO)/CNT layers and cellulose layers,6 which shows an advantage in thickness that obtained values of 30−40 dB shielding effectiveness (SE) in the X-band. In these studies, a tensile strength of up to 26.9 MPa was obtained. In general, surface covalent functionalization of the CNTs or addition of surfactants is required to ensure uniform dispersion of the CNTs, resulting in poor interfacial strength and undesirable mechanical properties. In addition, chemical functionalization substantially increases the CNT price and disrupts the electronic network of the nanotubes. The solvent volatilization process is inefficient and pollutes the environment. For the purpose of reaching the full potential of mechanical and electronic properties, improving the dispersion of CNT in composite matrix materials is important. Poor solubility in both aqueous and nonaqueous solutions imposes a considReceived: Revised: Accepted: Published: A
September 19, 2018 October 29, 2018 November 16, 2018 November 16, 2018 DOI: 10.1021/acs.iecr.8b04573 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research
Figure 1. Schematic of fabrication of CNF/MWCNT composite film.
free radical was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. NaClO and HCl were obtained from Sichuan Xilong Chemical Co., Ltd. (China). NaBr was purchased from Tianjin Kemiou Chemical Reagent Co., Ltd. NaOH was supplied by Tianjin Guangfu Fine Chemical Research Institute (Tianjin, China). All reagents in this work were used as received without further purification. 2.2. Preparation of CNF/MWCNT Composite Films. The fabrication of CNF/MWCNT composite film is shown in Figure 1. TEMPO-oxidized cellulose nanofibers (CNFs) were prepared as described in detail in previous reports.14−17 Specifically, 15 g of dry softwood pulp board was cut into small pieces and dispersed in distilled water with a mechanical stirrer for 24 h. Catalytic amounts of TEMPO (0.24 g, 1.5 mmol) and NaBr (1.5 g, 15 mmol) were subsequently dissolved into the slurry. The 8% NaClO solution (150 mmol) was added as a primary oxidant to initiate the TEMPO-mediated oxidation. During the reaction, the pH value of the system was maintained at 10 by the addition of 0.1 M HCl or 0.5 M NaOH. The pulp fibers were thoroughly washed with deionized water by filtration until pH values of the filtrate reached neutral (i.e., unchanged). Then, the TEMPO-oxidized cellulose fibers were suspended in water and sonicated for 120 min using a JY98-IIID ultrasonic processor (Ningbo Scientz Biotechnology Co., Ltd., China) at an output power of 600 W to give CNFs. For the preparation of composite films, MWCNT powder was added into the CNF dispersion with different CNF/MWCNT weight ratios (1/1, 1/2, and 1/3). Ultrasonication was also utilized to promote the dispersion process. A wet and stable hydrogel was formed by vacuum filtering the dispersion. The CNF/MWCNT composite films were finally fabricated by hot-pressing the hydrogels at a pressure of 10 MPa at 60 °C for 3 h. 2.3. Characterization. The CNF suspension and CNF/ MWCNT suspension (ca. 0.03 wt %) were deposited on carbon-coated grids. Excess liquid was blotted out with a filter paper and allowed to stand for drying by natural evaporation. The specimens were observed by a Tecnai G2 F20 transmission electron microscope (FEI, USA) at an acceleration voltage of 120 kV. The ζ-potentials of CNF suspension and CNF/MWCNT suspensions were measured using a Zetasizer Nano ZS (Malvern Instruments, Southborough, U.K.) at 25 °C. The thermal stability of the samples (about 4 mg) was investigated with a thermogravimetric analyzer (TGA; TA-Q600) under nitrogen from 30 to 800 °C at a
erable challenge for the separation and assembly of CNTs. As an alternative to synthetic polymers, cellulose nanofiber (CNF) is one of the most promising materials owing to its extraordinary mechanical properties combined with its sustainable nature.7 Compared to conventional cellulose fibers, CNFs have higher mechanical properties as well as high surface area, low density, and active interface.8,9 TEMPO-oxidized CNFs exhibit very high colloidal stability in water due to the negative charges on their surface.10−13 Therefore, they can assemble with other nanoparticles to create multifunctional composites. However, there is still no report of CNF/ MWCNT films for EMI shielding applications. In this work, we focus on the preparation of MWCNT-based films with excellent mechanical properties for high-performance electromagnetic interference shielding while controlling costs, which is beneficial for large-scale commercial applications. Water is used as a dispersion medium in this work and can be removed by vacuum filtration, which is environmentally friendly and efficient. At the same time, MWCNTs and CNFs can be dimensionally matched and assembled to each other after ultrasonication. Therefore, a strong and flexible CNF/ MWCNT composite film was fabricated without the need for chemical functionalization of the MWCNTs or the use of surfactant. At a thickness of 0.15 mm, an EMI SE of 45.8 dB, and a corresponding specific EMI SE as high as 3563.6 dB cm2/g were obtained in the X-band. Even for similar MWCNT content, the EMI shielding performance of CNF/MWCNT composite is better than that of most other reported MWCNT-based composites. Thanks to the all-fiber structure and the association between CNFs and MWCNTs, this composite film also has much better mechanical properties than previously reported, and shows a high tensile strength of 48 MPa. The combination of high EMI SE and mechanical properties offers great application potential to the CNF/ MWCNT composite films in the field of portable electronics and wearable devices, which require materials to be thin, lightweight, and flexible.
2. EXPERIMENTAL SECTION 2.1. Materials. Bleached softwood pulp was provided by the Institute of Paper Science and Technology at Georgia Tech (USA). MWCNTs (TNM3, purity >98 wt %, diameter 10−20 nm, and length 10−30 μm) were supplied by the Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences (CAS). 2,2,6,6-Tetramethylpiperidine-1-oxyl (TEMPO, AR) B
DOI: 10.1021/acs.iecr.8b04573 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research heating rate of 10 °C/min. The tensile strength was measured with a dynamic mechanical analysis machine (Q800) in tension film mode. The thermal diffusivity was measured by a Netzsch LFA 467 Nanoflash. The electrical conductivity of the films was measured by a KDB-1 four-point probe system (Guangzhou Kunde Technology Co., Ltd., China) with copper electrodes arranged in a straight line. Each of the electrical conductivity values reported was the average value obtained after at least five multiple measurements at different positions on the surface. The EMI shielding effectiveness of each sample was measured in the frequency range 8.2−12.4 GHz using an Agilent N5230 vector network analyzer. The Agilent N5230 vector network analyzer was calibrated using the standard APC-7 connector open, short, and 50 Ω loads. Samples with a diameter of 12 mm were placed in the sample holder and connected through the Agilent 85132F coaxial line to separate VNA ports.
indicating that the cellulose nanofibers have a positive effect on stabilizing the dispersion of carbon nanotubes in water. According to studies that have been reported, CNFs are assembled with CNTs in aqueous media due to fluctuations in counterions on their surface.23 In addition, the presence of surface charge of CNFs contributes to the electrostatic stabilization of the dispersion system. ζ-potential analysis was also performed on CNF/MWCNT dispersions of different weight ratios to further verify the stable function of CNFs. Figure 4 shows the ζ-potentials of various CNF/MWCNT dispersions presented in Figure 3. The ζpotentials of the CNF/MWCNT dispersions with weight ratios of 1/1 and 1/2 were consistent with the pure CNF dispersion, indicating that MWCNTs have been stably dispersed in water without altering the electrostatic repulsion between CNFs. Due to the approximation of the dispersion limit of the MWCNTs in this system, the ζ-potential value was significantly reduced when the weight ratio was 1/3. Transmission electron microscopy was used to further observe the dimension matching of CNFs and MWCNTs in water and confirm the interaction between them. It should be noted that ultrasonic treatment (600 W, 2 h) was also used to promote the dispersion process and plays a vital role. The results of TEM show that the original MWCNTs have a wide diameter distribution (10−20 nm) and different degrees of bending and torsion, ranging from 10 to 30 μm in length. It is after ultrasonication that MWCNTs and CNFs can be dimensionally matched and assembled to each other. Figure 3b,c shows that when the weight ratio of CNF/MWCNT was 1/1 or 1/2, all carbon nanotubes were well dispersed and aligned with CNFs. For the CNF/MWCNT dispersion with a weight ratio of 1/3, the number of CNFs in the field of view was significantly reduced. The MWCNTs that failed to assemble with CNFs showed aggregation and entanglement, indicating that the stability of the system decreased. The increasing unassembled MWCNTs in the dispersions imposes a rising ionic strength screening effect and squeezes the interfacial electrical double layer rendering a decline in the absolute value of the ζ-potential.24 As is well-known from colloidal science, potential values more negative than −30 mV are generally considered to represent sufficient mutual repulsion to ensure the stability of a dispersion.25 Therefore, the absolute value of the ζ-potential was significantly reduced when the weight ratio was 1/3, which is consistent with the results of ζ-potential analysis. 3.2. Characterization of CNF/MWCNT Composite Films. CNF/MWCNT composite films with a thickness of ∼0.15 mm were finally fabricated by vacuum filtering the CNF/MWCNT dispersions, followed by hot-pressing. Therefore, the interaction between CNFs and MWCNTs observed by TEM can be retained after preparation into a film. The density of the films reduced with the increase of MWCNT content, calculated as 0.89, 0.84, and 0.77 g/cm3. The results of TEM show that MWCNTs have a wide diameter distribution and different degrees of bending and torsion, thus showing a lower bulk density.26 In contrast, CNFs have a more uniform geometry. According to the literature, the density of films prepared from pure CNFs is about 1.5 g/ cm3.27 Therefore, the density of the film reduced with the increase of MWCNT content. The thickness of the composite film can be adjusted by controlling the concentration of the CNF/MWCNT dispersions, and the density also depends on the pressure and time of the hot-pressing.
3. RESULTS AND DISCUSSION 3.1. CNF Fabrication and CNF-Assisted Dispersion of MWCNT. Commercial bleached softwood pulp was used as the starting material to prepare an aqueous dispersion of CNFs by TEMPO-mediated oxidation and ultrasonication disintegration. Figure 2a shows a bottle of aqueous CNF dispersion that
Figure 2. (a) A bottle of CNF dispersion, which remains transparent and stable for 3 months. (b and c) TEM images of CNFs.
still exhibits high stability after being stored for 3 months. Transmission electron microscopy (TEM) was used to characterize the fiber morphology and nanometer size of CNFs. The TEM images (Figure 2b,c) show that the CNFs have lengths of 1−2 μm and widths of 5−8 nm. It is expected that the C6 primary hydroxyls of celluloses are selectively converted to C6 sodium carboxylate groups by TEMPOmediated oxidation, which is also derived from the oxidation of NaBrO and NaClO present in the system.10−13 CNFs can be quite stable in water due to the presence of negative carboxylate groups on the surface. The ζ-potential of CNF suspensions was measured at −36.1 mV, indicating the presence of negative charges on the surface of the nanofibers and a high dispersibility in water. For the preparation of CNF/MWCNT dispersions, MWCNT powder was added into the CNF dispersion with different CNF/MWCNT weight ratios, and ultrasonic treatment (600 W, 2 h) is also used to promote the dispersion process. Figure 3 shows that pure MWCNTs have poor dispersibility in water, and stable dispersion systems cannot be formed even at very low concentrations (0.072 wt %). The original MWCNTs tend to aggregate, due to substantial van der Waals attractions and specific hydrophobic interaction between tubes.18−22 However, the CNF/MWCNT dispersions in Figure 3 remained macroscopically stable and uniform, C
DOI: 10.1021/acs.iecr.8b04573 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research
Figure 3. (a) Dispersion of CNFs and MWCNTs in water with different weight ratios. TEM images of CNF/MWCNT dispersions in water with different weight ratios of (b, b′) 1/1, (c, c′) 1/2, and (d, d′) 1/3. (e, e′) TEM images of original MWCNTs.
at approximately 200 °C in a N2 atmosphere. The DTG curves consist of two peaks around 278 and 327 °C. The former DTG peak is probably due to the thermal degradation point of the sodium anhydroglucuronate units; the latter is that of cellulose chain, which is typical and agrees with previous reports.23,28,29 Note that, due to the formation of sodium carboxylate groups from the C6 primary hydroxyls by TEMPO-mediated oxidation, the thermal degradation point is reduced compared to that of untreated cellulose.28,29 The results of TGA show that the weight loss of the film mainly comes from the decomposition of CNFs during the heating process. The residual weights of the films at 700 °C were 57.8, 68.7, and 76.4%, respectively. Nevertheless, because the TGA results strongly rely on the residual carbon and the local sample structures, the MWCNT content of the films calculated from the TGA results is not completely consistent with the actual value.30 The typical stress−strain curves of CNF/MWCNT composite films with different CNF/MWCNT weight ratios are shown in Figure 6a. All composite films fractured in a brittle manner and with no distinct yielding or strain hardening due to the rigidity of the cellulose chains.6 The modulus reduced with the increase of MWCNT content, indicating that CNFs play a critical role in the mechanical strength of the film. The CNF/MWCNT film with a weight ratio of 1/2 has the largest tensile strength of 48 MPa. When the ratio further changed to 1/3, the tensile strength of the film dropped significantly. The obvious threshold ratio appears between 1/2 and 1/3, which is consistent with the TEM results. The extremely high content makes the aggregation of MWCNTs unavoidable, which breaks the continuity of the cellulose phase. The critical role of CNFs in dispersing MWCNTs was
Figure 4. ζ-potentials of CNF/MWCNT dispersions with different CNF/MWCNT weight ratios.
Figure 5 shows the TG and differential thermogravimetric (DTG) curves of CNFs, MWCNTs, and CNF/MWCNT composite films. The TG curve of MWCNTs is almost a straight line, and it has excellent thermal stability (weight loss < 2%) at 700 °C. As for CNFs and CNF/MWCNT composite films, the weight loss below 100 °C can be attributed to water evaporation. The thermal stability of the composite films gradually increases with the loading of MWCNTs, giving rise to the availability in real applications within a wide temperature range. Thermal degradation of the CNFs started D
DOI: 10.1021/acs.iecr.8b04573 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research
Figure 5. (a) TG and (b) DTG curves of CNF/MWCNT composite films with different CNF/MWCNT weight ratios.
Figure 6. (a) Stress−strain curve of CNF/MWCNT composite films with different weight ratios. (b) Digital photographs of CNF/CNT film with a weight ratio of 1/3, which show its flexibility.
further confirmed. Furthermore, the weight ratio of 1/3 means a low content of CNFs in the composite film. It should be noted that all of the above films have good flexibility due to the low thickness, the all-fiber structure, and the association between CNFs and MWCNTs. Figure 6b shows the flexibility of a CNF/MWCNT film with a weight ratio of 1/3, indicating potential applications in flexible electronics and wearable devices. Electrical conductivity is the inherent ability of EMI shielding materials to absorb electromagnetic energy, which is critical for electromagnetic shielding effectiveness (SE). Four-point probe method was used to measure the conductivity of the CNF/MWCNT films with different CNF/MWCNT weight ratios, shown in Figure 7. The electrical conductivity of the film with a weight ratio of 1/1 was 2356 S/m. Surface-charged CNFs improve the dispersibility and stability of MWCNTs in water, avoiding the agglomeration of MWCNTs in dispersion media at extremely high loadings. Thanks to the increase in dispersion limit, high electrical conductivity of the MWCNT nanocomposites was obtained,23 which is crucial for the application of EMI shielding. For composite films with MWCNT contents of 66.6 and 75 wt %, the conductivity is further increased to 3024 and 3187.3 S/m. Higher conductivity was obtained compared to values for most other MWCNT-based composites ever reported,3,4,6,31,32 which is attributed to the extremely high MWCNT contents and the increase in the dispersion limit of
Figure 7. Conductivity and thermal diffusivity of CNF/MWCNT composite films with different weight ratios.
MWCNTs in water without the need for chemical functionalization of the MWCNTs or the use of surfactant. For the application in electronic devices, thermal diffusivity is used to evaluate the response speed of CNF/MWCNT composite films to the environmental temperature change, which measures the heat transfer efficiency of the films.33 The thermal diffusivities of CNF/MWCNT films at 25 °C improve with increasing MWCNT loading, indicating enhanced heat E
DOI: 10.1021/acs.iecr.8b04573 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 8. (a) EMI SE in the X-band of CNF/MWCNT composite films with different weight ratios. (b) Comparison of SET, SEA, and SER at a frequency of 8.2 GHz for CNF/MWCNT composite films with different weight ratios. (c) EMI SE as a function of frequency for CNF/MWCNT1/2 composite films with various sample thicknesses. (d) Comparison of SET, SEA, and SER at a frequency of 8.2 GHz.
transfer efficiency.34 Generally, uniform dispersion of particles is an major factor in achieving high thermal diffusivity,35 further confirming the critical role of CNFs for dispersing CNTs. Since cellulose is a poor conductor of heat, a high CNT content results in a decrease in interfacial thermal resistance between MWCNTs. Furthermore, the hot-pressing process is essential for reducing the porosity of the film, reducing the thermal resistance caused by the air gap around the MWCNTs.36 In this work, the CNF/MWCNT composite film with a weight ratio of 1/3 has the largest thermal diffusivity of 11.4 mm2/s, which is even comparable to those of some metal-based materials. 3.3. EMI Shielding Properties of CNF/MWCNT Composite Films. The function of the electromagnetic shielding material is essentially to limit the transfer of electromagnetic energy from one side of the shielding material to the other. The attenuation of the propagating electromagnetic waves produced by the shielding material is evaluated by EMI shielding effectiveness (SE). The total EMI SE (SET) of the material is measured in decibels and can be expressed as SET (dB) = −10 log(Pt/P0), where Pt and P0 represent transmitted and incident electromagnetic power, respectively.37,38 The EMI SE values of the CNF/MWCNT composite films were measured in the microwave frequency range 8.2−12.4
GHz, which is widely used in communication applications, as shown in Figure 8a. The observations indicate that the EMI SE values of the 0.15 mm thick samples weakly depend on the frequency in the measured frequency region. It is worth noting that the EMI SE curves of all samples show a maximum at frequencies around 11.5 GHz. The EMI SE depends on the frequency of the electromagnetic wave and the electromagnetic properties of the shielding material.39 For a particular electromagnetic shielding material, its EMI SE will also fluctuate with the test frequency due to the test environment and its own structure, which will be more obvious at high conductive filler loadings.30,40 In this work, the EMI SE curves for all the samples show a maximum value at the frequency around 11.5 GHz. This can be explained by the fact that the samples are composed of the same CNFs and MWCNTs and have similar microstructures. Therefore, the trend of their EMI SE curves is basically the same, and this phenomenon is also widely found in other literature.41−43 The CNF/MWCNT composite films with MWCNT content of 50 wt % indicate EMI SE ∼ 40.9 dB in the whole X-band frequency range, much higher than ever reported for other typical films at similar thicknesses. Such a high SE value of our composite films may be mainly attributed to the high conductivity. Furthermore, the SE value increases with increasing MWCNT mass ratios and conductivities. As the F
DOI: 10.1021/acs.iecr.8b04573 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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in the microcapacitors, showing stronger absorption characteristics. As far as we know, there are few reports of electromagnetic shielding materials with such an all-fiberbased structure. It is expected that this result will provide more reference for the structural design of electromagnetic shielding materials. Considering that the film has a thickness of only 0.15 mm, higher electromagnetic shielding effectiveness can be obtained by stacking several films with a weight ratio of 1/2. As shown in Figure 8c,d, the EMI SE of samples substantially increases with the total thickness of the conductive layer, which is consistent with the theory of EMI shielding. The increase in SEA mainly contributed to the increased SET while SER is almost a constant. Specifically, the 0.15 mm × 5 multilayer sample exhibits an average EMI SE of 89.9 dB. The superior SE is also because the multilayer architecture caused multiple reflection loss. This result further suggests that the reflection might only happen at the incident surface of sample. Because the effect of coherent multiple reflections becomes evident as the number of the layers increases, the EMI SE amplitude of oscillation is enhanced, which has been reported for other layer-structured composites.6,46,47 Thanks to the auxiliary dispersion behavior of CNFs to MWCNTs, which solves the problem of MWCNT aggregation, the high utilization efficiency of MWCNTs is realized. For the applications that need lightweight shielding materials, the specific EMI SE (SSE) defined as the SE value divided by the surface density is often used to evaluate the absolute effectiveness of shielding materials.4,6,48 As shown in Table 1, high weight ratios of MWCNTs can be successfully dispersed by CNFs to prepare composite films that are much thinner than other MWCNT-based polymer composites to obtain an SE value of more than 30 dB in the X-band. Thus, a much higher SSE (4017.3 dB cm2/g) that corresponds to utilization of shielding materials was obtained. For example, an SE of 30 dB was achieved for a 50 wt % MWCNT/cellulose composite with a thickness of 0.64 mm,32 an SE of 35 dB was achieved for a 20 wt % MWCNT/PS composite with a thickness of 2.0 mm,49 and an SE of 40 dB was achieved for a 10 wt % MWCNT/ABS composite with a thickness of 1.1 mm.1 It is worth noting that the SSE of the CNF/MWCNT composite film is better than that of the layer-structured MWCNT− PEO/cellulose film (1372.4 dB cm2/g),6 even comparable with those of the ultralight porous structured MWCNT/WPU foam (2143.0 dB cm2/g)3 and MWCNT/CS foam (8556 dB cm2/ g).50 Even for similar MWCNT content, the EMI shielding performance of CNF/MWCNT is better than that of most other reported MWCNT-based materials. It should be noted that a more excellent electromagnetic interference shielding performance can be obtained by using a conductive polymer as a matrix or using expensive graphene or graphene oxide as a conductive filler,51 but that usually results in poor mechanical properties or high costs.
MWCNT loading rises to 66.6 and 75 wt %, the EMI SE increases to 45.8 and 46.4 dB, respectively. This result means that the further increase in the mass ratio of MWCNTs leads to a limited increase in SE value. The results of ζ-potential analysis and TEM show that MWCNTs aggregate when their loading rises to 75 wt %, which means that the utilization efficiency of CNTs is reduced. And the aggregation of MWCNTs brings about significant deterioration of mechanical properties. Based on the above factors, the CNF/MWCNT composite film with a weight ratio of 1/2 shows the best comprehensive performance. EMI shielding can be classified into three major mechanisms of reflection loss (SEA), absorption loss (SEA), and multiple reflection loss (SEM), which are mainly related to mobile charge carriers, electric (or magnetic) dipoles, and reflections at various surfaces or interfaces.1 SEM is often ignored because most of the multiply reflected power can be absorbed when the SET is higher than 15 dB. To clarify the EMI shielding mechanism, SET, SEA, and SER at the frequency of 8.2 GHz for CNF/MWCNT composite films with different weight ratios were calculated, as depicted in Figure 8b.44 For the composite film with three different weight ratios, the contribution of SEA is the main part of SET. When the weight ratio of CNF/ MWCNT was 1/1 or 1/2, micro-capacitors, derived from the coactions of MWCNT electrodes and dielectric cellulose, and structural defects (polarization centers) increased with increasing MWCNT contents. When the ratio was further changed to 1/3, the CNF/MWCNT film had a greatly reduced CNF content, which limits the number of micro-capacitors, resulting in a decrease in SEA value.45 As some literature reported, high SEA depends on the increase of complex permittivity (ε).1 Microcapacitors, derived from the coactions of MWCNT electrodes and dielectric cellulose, and structural defects (polarization centers) increase with increasing MWCNT contents,45 leading to high SEA values at high MWCNT loading. Attributed to a higher amount of mobile charge carriers, more charge carriers interact with the radiation and dissipate the EMI energy. To better understand the shielding mechanism, CNF/MWCNT film can be seen as composed of many microcapacitors, which derived from the coactions of MWCNT electrodes and dielectric cellulose nanofibers. As shown schematically in Figure 9, incident electromagnetic microwaves entering the film are attenuated by reflecting, scattering, and adsorption many times
4. CONCLUSIONS Robust and flexible CNF/MWCNT composite films were fabricated by a simple, efficient, and environmentally friendly strategy. TEMPO-oxidized CNFs improve the dispersibility and stability of MWCNTs in water, avoiding the agglomeration of MWCNTs in dispersion media at extremely high loadings. Thanks to the all-fiber structure and the association between CNFs and MWCNTs, this composite film shows a high tensile strength of 48 MPa. At a thickness of 0.15 mm, the
Figure 9. Schematic representation of microwave transfer across CNF/MWCNT composite films. G
DOI: 10.1021/acs.iecr.8b04573 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research
Table 1. EMI Shielding Effectiveness of Different MWCNT-Based Shielding Materials in the X-Band Frequency Range materials MWCNT/cellulose MWCNT−PEO/cellulose MWCNT/WPU MWCNT/WPU MWCNT/CS MWCNT/PS MWCNT/ABS MWCNT/PP MWCNT/epoxy MWCNT/PMMA MWCNT/PE MWCNT/CNF
filler content (wt %)
density (g/cm3)
thickness (mm)
electrical conductivity (S/m)
EMI SE (dB)
SSE (dB cm2/g)
ref
50 40 61.5 76.2 80 20 10 14 0.66 10 10 66.6 75
1.5 1.7 1.2 0.071 0.0176 1.84 0.9 0.97 0.98 1.28 1.0 0.84 0.77
0.64 0.15 0.32 2.3 2.5 2.0 1.1 1.0 2.0 2.1 3.0 0.15 0.15
133.3 20
