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In this chapter, the production of carbon nanotube (CNT)-cellulose composite sheets and their electrical conductivity have been studied. Good electroconductive CNT-cellulose composite sheets and good electromagnetic interference (EMI) shields have been obtained. Single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) were used to produce the CNT-cellulose composite sheets. In the ecofriendly process, CNTs, cellulose, and gelatin were mixed well to prepare the sheets. The performance of the SWCNTs-cellulose and MWCNTs-cellulose composite sheets was studied for 8.9, 13, and 17 wt% of CNT contents. Their electrical properties were measured by the DC current method, and their optical properties (reflection and absorption) were measured using a microwave transmitter of 10.5 GHz. Herein, significant differences in the electric properties and the EMI shielding effects between the MWCNTs-cellulose and the SWCNTs-cellulose composite sheets were found. The SWCNTs-cellulose composite sheets showed good characteristics for electromagnetic wave (EMW) shielding. In the microwave region, the SWCNTs-cellulose composite sheets showed lower transmission and higher reflection properties compared to the MWCNTs-cellulose composite sheets. The shields do not generate dust or particles and possess flexibility and toughness. With these sheets, 3D flexible circuits, flexible electric heaters, EMW shields, EMW polarizers, and cone-shaped EMW absorbers could be prepared.
Graduate School of Science and Technology, Shizuoka University, Shizuoka, Japan
Weni Yulistia
Faculty of Science, Department of Physics, Shizuoka University, Shizuoka, Japan
*Address all correspondence to: mieno.tetsu@shizuoka.ac.jp
1. Introduction
The progress of telecommunication systems will raise the integration of harmony between cyberspace (virtual space) and physical space (real space), which has been studied as part of the “Society 5.0” project [1]. In these scenes, the utilization of electromagnetic waves (EMWs) in the communications using electronic devices will be very important. In these cases, electromagnetic wave interference (EMI) causes practical problems. And strong EMW fields may cause negative effects on human health [2]. Therefore, good electromagnetic interference shielding (EMS) is required to suppress or attenuate electromagnetic leakage from EMW emitters and protect devices from EMI [3, 4, 5]. Here, new functional materials have been studied to develop flexible devices and EMI-shielding materials. It can be considered that new polymer composite materials have advantages for EMI shielding. Composites using carbon nanomaterials (such as carbon nanotubes) and polymers are gaining attention for their many advantages [6, 7, 8]. In this study, nanocomposites using carbon nanotubes and cellulose fibers were used.
Carbon nanotubes (CNTs) were discovered and have been studied since 1991 as nanoscale carbon materials [9, 10, 11, 12]. CNTs are classified into single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs). CNTs have an excellent electric conductivity of 108 S/m and thermal conductivity of ca. 3200 W/mK. Their Young’s modulus reaches ca. 1 TPa, and they have anticorrosive properties [13]. These outstanding properties inspire the use of CNTs as effective electrical and electronic materials [14, 15, 16, 17, 18, 19]. CNTs can be dispersed into some matrices, and the preparation of nanotubes containing composite materials is possible [13, 20, 21, 22, 23]. Recently, the eco-friendly production of CNTs-cellulose composite sheets has been developed [24, 25, 26, 27, 28]. These sheets have good flexibility, good toughness, and no dust formation [24, 25]. Here, the electric properties of the composite sheets have been investigated and highly electroconductive sheets have been developed. Then, their electrical and optical properties in the microwave region [29] have been studied to develop EMS and microwave parts.
CNTs tend to form bundle states because of their van der Waals force, and pristine CNTs do not disperse well in water [30]. Under some conditions, CNTs are separated into individual tubes or bundles of only a few tubes [25]. CNT coagulates can be decomposed by ultrasonication in water. However, the CNTs form coagulates again when the ultrasonication is stopped. Therefore, some dispersing methods are required to stably disperse CNTs in water to prevent their self-agglomeration. In this study, biocompatible, non-immunogenic gelatin was used to wrap the surfaces of CNTs [24, 25]. Gelatin is a safe environment-friendly material. Therefore, gelatin was used as the dispersant in the preparation of composite materials. It was also confirmed that gelatin-wrapped CNTs effectively mix well with cellulose fibers in water at room temperature.
The mechanical properties of polymer composites were enhanced by adopting a suitable combination of high-modulus carbon and polymer fibers. Cellulose fibers are one of the most popular polymers in the world. They are biodegradable, have high tensile and compressive strength, and are stable under thermal condition [31]. Therefore, in this study, cellulose fibers were used as polymer fibers, and CNTs were coated on the fibers to produce electrically conductive composite sheets, as shown in Figure 1.
Figure 1.
Schematic of cellulose fibers coated with CNTs.
The new-generation CNT-cellulose composite sheets would show high electric conductivity, good flexibility, low costs and light weight [25]. Because of their EMW-shielding properties, they have potential to be used as EMW shields in future communication technology.
The SWCNTs provided by Zeon Nano Technology Co., Tokyo, Japan, ZEONANO SG101 (diameter of 3∼5 nm, length of 100∼600 μm, and purity >99%) and the MWCNTs provided by C-Nano, Inc., CA, USA, FT-9000 (outer diameter of 10∼20 nm, inner diameter of 3∼10 nm, length of 10∼30 μm, and purity >95%) were used. Gelatin (Wako Chemical, Inc., 1st grade, Osaka, Japan) was used to make water-dispersible CNTs. Cellulose fibers were made from filter papers (Toyo Roshi Co., No. 1, Tokyo, Japan). The papers were blended using a blender (Fukai Co., FJM-601, Osaka, Japan). To optimize the quality of the produced composite sheets, composites with different mixing rates of CNTs, cellulose, and gelatin were studied.
As shown in Figure 2, to prepare CNTs-cellulose composite sheets, the cellulose suspension is made by soaking the filter paper in distilled water (Kyoei Techno Co., Chiba, Japan) and blending for 20 min [24, 25]. The initial mass of the paper is 8.8 g. Then, to prepare gelatin/CNT dispersion in water, different concentrations of MWCNTs or SWCNTs (8.9, 13, and 17 w% of CNTs) are mixed in 90 ml of distilled water and sonicated in an ultrasonic mixer (Sonics & Material, Inc., CT, USA, VC-130, f = 20 kHz, and 6 mm probe) at an input power of 20 W for 60 min. Next, the cellulose suspension is added to the dispersion with 90 ml of MWCNTs or SWCNTs suspension and mixed well in a blender (Braun Co., Hessen, Germany, MQ 5200) for 10 min. The final solution is poured onto a screen plate and dried after sandwiching it between two flat metal plates to obtain a flat composite sheet. As shown in Figure 3, the produced composite sheet size is 24 × 17.5 cm with a thickness of ca. 0.2 mm.
Figure 2.
Production process of the CNT-cellulose composite sheet.
5. Scanning electron microscope (SEM) and transmission electron microscope (TEM) observations of the CNT-cellulose composite sheets
The morphology of the CNT-cellulose composite sheet was observed using an SEM (JEOL Co., Tokyo, Japan, JSM 6510LV) at an operating voltage of 20 kV. Figure 4 shows that MWCNTs uniformly cover the cellulose fibers. It was observed that the SWCNTs are bundle-shaped and are densely coated on the cellulose fibers [32]. On the other hand, the MWCNTs on the cellulose fibers are agglomerates, resulting in rougher surfaces than with the SWCNTs-cellulose composite sheets.
Figure 4.
SEM image of the MWCNT-cellulose composite sheet.
A TEM device (JEOL Co., Tokyo, Japan, JEM1400) was used to observe each nanotube at an acceleration voltage of 100 kV. It was confirmed that the SWCNTs and MWCNTs are mainly bundled.
6. Physical properties of the CNT-cellulose composite sheets
6.1 Electrical properties of the CNT-cellulose composite sheets
To measure the electric conductivity of the SWCNTs and the MWCNTs composite sheets, the DC current method (the two-point probe method) was used. The composite sheet was set as shown in Figure 5, and the electric conductivity, σ, as a function of nanotube content was measured. The result is shown in Figure 6. The conductivity increases with increasing the CNTs content. In addition, the composite sheet with 8.9 wt% SWCNTs already has a much higher conductivity (4.4 × 102 S/m) than that with 8.9 wt% MWCNTs (10 S/m).
Figure 5.
Illustration of measurement of electric conductivity of the composite sheet.
Figure 6.
Electric conductivity σ versus CNT content.
6.2 Investigation by using an infrared camera
By applying a DC current, the CNT-cellulose composite sheet can generate heat. While the composite sheet was electrified, its temperature was measured with a thermometer gun (J&C Co., Toyohashi, Japan, type IT-122). An infrared camera (FLIR LLC, OR, USA, i3, emissivity 0.85) was used to obtain thermal images of the composite sheet. Figure 7 shows that the SWCNT-cellulose composite sheet conducts current much better than the MWCNT-cellulose composite sheet. Electrified with merely 30 V, the 13 wt% of SWCNT-cellulose composite sheet already conducts current (1.32 A) with a resistance of ca. 30 Ω and the center temperature reaches ca. 97°C. On the other hand, in the 17 wt% of MWCNT-cellulose composite sheet, the center temperature of ca. 93°C could be obtained by applying 70 V.
Figure 7.
FLIR images of the CNT-cellulose composite sheet. (a) 13 wt% SWCNTs electrified at 30 V. (b) 17 wt% MWCNTs electrified at 70 V.
6.3 Flexibility properties and tensile strength test
To measure the flexibility of the composite sheets, bending and tensile tests were carried out by using a force meter, model-1308 (Aikoh Engineering Co., Nagakude, Japan), as shown in Figure 8. In the bending test, the Young’s modulus of elasticity E (GPa) is determined as
Figure 8.
Illustration of measurement of Young’s modulus.
E=FL34BH3d,E1
where F is the applied force, L is the distance between the supports, B is the width of the specimen, H is the height of the specimen, and d is the deflection corresponding to load F [33].
By the measurement, as shown in Figure 9, it was obtained that the composite sheet becomes harder by increasing the CNT content. The MWCNT-cellulose composite sheet is harder than the SWCNT-cellulose composite sheet. The tensile test revealed that the MWCNT-cellulose composite sheet has higher toughness than the SWCNT-cellulose composite sheet. It is conjectured that the SWCNTs are more flexible and softer than the MWCNTs.
Figure 9.
(a) Young’s modulus and (b) maximum tensile strength of the composite sheets.
6.4 Optical properties of the CNTs-cellulose composite sheets
To clarify the microwave transmission and reflection properties of these composite sheets, several experiments have been carried out by using a microwave transmitter with a constant frequency of 10.5 GHz (input power P = 15 mW and wavelength λ = 2.85 cm), as shown in Figure 10 [25]. The transmission and reflection properties of the SWCNT-cellulose and the MWCNT-cellulose composite sheets as a function of CNT content are shown in Figure 11, where the intensity shows the detected microwave power. Their thickness is about 0.2 mm. The SWCNT-cellulose composite sheet has higher transmission and lower reflection. There is significant difference in EMI shielding effectiveness (SE) between the SWCNT-cellulose and the MWCNT-cellulose composite sheets. Here, the definition of EMI SE in dB is
Figure 10.
Schematic representation of (a) the microwave transmission measurement, and (b) the reflection measurement of the sheets.
Figure 11.
Microwave absorption and reflection properties of the SWCNT and the MWCNT composite sheets.
where Porg and Ptrans are original and transmitted microwave powers, respectively. Iorg and Itans are original and transmitted signal intensities, respectively. The results are shows in Table 1. The produced SWCNT and MWCNT cellulose composite sheets show good EMI-shielding properties in the microwave range.
Species
CNT content (wt%)
EMI SE (Itrans/Iorg)
EMI SE (dB)
MWCNTs
8.9
0.26
− 5.8
MWCNTs
17
7.2 × 10−3
− 21
SWCNTs
8.9
1.1 × 10−3
− 30
SWCNTs
17
4.0 × 10−4
− 34
Table 1.
The EMI shielding effectiveness (SE) of the sheets.
The estimated absorption power of the EMWs in the sheet, Pabs, can be obtained as
Pabs=Porg−Pref−Ptrans,E3
where Pref is the reflected power of the EMWs. For a copper plate, it is assumed that the reflection is 100% and there is no transmission.
The estimated absorption of the EMWs of the composite sheet is shown in Figure 12. For the SWCNTs-cellulose composite sheet, by increasing the amount of SWCNTs, the reflection of the EMWs increases and the absorption decreases. On the other hand, for the MWCNTs-cellulose composite sheet, by increasing the amount of MWCNTs, the absorption of the EMWs increases and then decreases with 17 wt% MWCNTs. Therefore, the 13 wt% MWCNT-cellulose composite sheet has good EMW absorption. And the 8.9 wt% SWCNT-cellulose composite sheet has good EMW absorption. The 17 wt% MWCNT-cellulose and the 17 wt% of SWCNT-cellulose composite sheets also reflect the EMWs effectively. In other words, these composite sheets can be used as EMW absorbers or reflectors with these properties.
Figure 12.
Estimated absorption of EMWs for (a) the SWCNT composite sheet and (b) the MWCNT composite sheet.
7. Mechanism of electric conduction in the CNTs-cellulose composite sheet
The combination of cellulose fibers with CNTs leads to a significant change in electric conductivity. Such a significant change has a great potential for use in various interesting applications [34, 35, 36, 37]. To elucidate the mechanism of electric conduction of the produced CNT-cellulose composite sheets, resistive load cells are made using the MWCNT-cellulose composite sheets. As the cellulose fibers are electrically insulating, the MWCNTs coated on the fibers are responsible for the electric conduction [35]. The transition from an insulator to a conductor upon the addition of CNTs follows the percolation theory based on the large disparity in electrical conductivity between the cellulose fiber matrices and the CNTs.
Figure 13(a)–(c) show the change of the sheet structure when a vertical force F is applied. When F is increased, the fiber density and the CNT density in the sheet increase. Figure 13(d) shows that all the composite fibers are in contact with each other and form a conductive network. By applying an external force, the change in conductance can be measured. When the applied force increases, the fibers are pressed together and deformed, causing the contacting areas among the fibers to increase and the contact resistance to decrease. The resistivity of the composite material, ρ, is obtained as
Figure 13.
Compression model of the load cell with external forces of (a) F = 0, (b) F = F1 and (c) F = F2. (d) Model figure of the conduction paths without and with the force.
ρ=RAt,E4
where R is the resistance, A is the area, and t is the thickness of the composite sheet. The electric conductivity, σ, is defined as σ = 1/ρ. A schematic of the measurement method is shown in Figure 14. The sensor sample is pressed by a handpress. The resistance R was measured by a circuit as shown in Figure 14.
Figure 14.
Illustration of the resistance versus pressure measurement of the composite sheet.
Figure 15 shows the experimental results. The resistance versus the applied force was measured for the three samples (one sheet, two stacked sheets, and three stacked sheets). The resistance monotonically decreases with increasing the force. In particular, the one-sheet sample shows the largest change in resistance.
Figure 15.
Sheet resistance, R, versus applied force, F, for the 1-, 2-, and 3-layered MWCNTs-cellulose composite sheets.
A microwave polarizer was made from the MWCNT-cellulose composite sheet, as shown in Figure 16(a). Using the experimental setup as shown in Figure 10(a), the transmission of the microwave was measured. The original EMW was vertically polarized, and the polarizer is rotated with an angle φ. The transmitted wave intensity versus the angle φ is shown in Figure 16(b). It shows good polarization properties at f = 10.5 GHz, which is almost the same as the case of a metal polarizer.
Figure 16.
(a) Photograph of the polarizer. (b) Transmission intensity versus rotation angle.
If nonpolarized EMWs arrive, only the component perpendicular to the slit direction can propagate through the slit.
8.2 A cone-array-type microwave absorption plate
When many conical structures made from the composite sheets are lined up on a flat plate, a multi-cone-array plate can be made as shown in Figure 17(a). Using the experimental setup as shown in Figure 10(b), the reflection intensity versus reflection angle θ was measured. It shows good microwave absorption property. When a flat copper plate was set instead of the cone-array, the reflected intensity at θ = 0 was 670 mV. If the larger plates are made, they would be a good EMW absorption wall.
Figure 17.
(a) Photograph of the cone-array-type microwave absorption plate. (b) Reflected intensity versus reflection angle θ by the measurement using the method as shown in Figure 10(b).
The CNT-cellulose composite sheets have good electric conductivity. By adjusting the content of CNTs, the conductivity of the sheet can be controlled. The SWCNT-cellulose composite sheet has a higher electric conductivity (σ=6×102S/m) than the MWCNT-cellulose composite sheet (σ=3.2×102S/m). By the mechanical test, it was confirmed that the MWCNTs-cellulose composite sheet is harder than the SWCNT-cellulose composite sheet. The SWCNT-cellulose composite sheet is more flexible. For the flat SWCNT-cellulose composite sheet (24 × 17.5 × 0.2 mm), a DC power of 60 W was applied and the surface temperature became ca. 97°C at the center.
The composite sheets can be applied as 3D conductors in lanterns, flat heater panels, electromagnetic shields, polarizers, dot-like structures, and cone-array-type microwave absorption walls. The reflection increases by increasing the CNT content in the composite sheets. The composite sheets have good EMI shielding effectiveness (SE). The EMI SE values were 4.0 × 10−4 (ca. −34 dB) for the 17 wt% SWCNTs and 7.2 × 10−3 (ca. −21 dB) for the 17 wt% MWCNTs. As a polarizer, the CNT-cellulose composite sheet shows good polarization property. Many electroconductive structures can be designed using these composite sheets.
This study was supported by the Graduate School of Integrated Science and Technology, Shizuoka University, and Asia Bridge Program of Shizuoka University.
Conflict of interest
The authors declare no conflict of interest.
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Written By
Tetsu Mieno and Weni Yulistia
Submitted: 29 December 2023Reviewed: 01 February 2024Published: 22 February 2024
Open access peer-reviewed chapter - ONLINE FIRST
