Preparation of Stacked Polypyrrole-Coated Non-Woven Fabrics for an Electromagnetic Wave-Absorbing Sheet

Yoshihiro Egami,Takashi Yamamoto, Kunio Suzuki, Tadashi Yasuhara and Hiroshi Inoue

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AZojomo (ISSN 1833-122X) Volume 6 November 2010

Topics Covered

Abstract

New electromagnetic wave-absorbing sheets was formed by piling up ten pieces of polypyrrole nanoparticles-coated non-woven fabrics with specific resistivity of 7.5 ¨C 700 ¦¸cm. The electromagnetic wave-absorbing sheets could absorb more than 95 % of electromagnetic waves in the 75 ¨C 110 GHz region although their thickness was less than 10 mm. The piled conductive non -woven fabrics are promising radar-absorbent materials which can absorb electromagnetic waves with extremely high frequencies of millimeter band.

Keywords

Polypyrrole Nanoparticle, Electromagnetic Wave-Absorbing Sheet, Conductive Non-Woven Fabrics, Broadband

Introduction

Information and communications technology is rapidly developing. Electromagnetic waves with ultra high frequencies (300 MHz¨C3 GHz) and super high frequencies (3¨C30 GHz) are used to transfer a great amount of information at high speeds. In practice, such electromagnetic waves have already been applied to cordless phones (0.8¨C2 GHz), wireless local area network (2.45¨C5.2 GHz), electronic toll collection (5.8 GHz) etc. In the near future, electromagnetic waves with frequencies over 30 GHz or extremely high frequencies will be required for higher speed communications and the spread of intelligent transportation system. However, the electromagnetic waves with extremely high frequencies cause some other serious problems such as the malfunction of electronic equipment, information leakage and the like. A solution to the problems is to develop radar-absorbent materials (RAMs) which can absorb broadband electromagnetic waves.

In general, RAMs are classified into three categories, dielectric, magnetic and conductive RAMs which have different mechanisms to absorb electromagnetic waves [1-6]. Most commercial dielectric and magnetic RAMs are heavy and just cover narrow ranges of frequencies. And the radar absorbency of the dielectric and magnetic RAMs depends on their nature, composition, loading amount and thickness. On the other hand, when conductive RAMs catch electromagnetic waves, current induced from the electromagnetic waves flows on the RAMs, leading to the decrease in the reflection of electromagnetic waves. Recently, we have developed succeeded in a new preparation method of polypyrrole (PPY) nanoparticles-coated non-woven fabrics with high conductivity [7]. In the present study, we investigate the radar absorbency of the conductive non-woven fabrics to clarify the usefulness as a new RAM material.

Experimental

Preparation of a Dopant Solution

BANS and BATS were used as a dopant in the present study because we have found experimentally that they were suitable for forming uniform PPY nanoparticles with high conductivity. 1-Butylamine (143 mL) was slowly added to a 1.57 M (= mol L^{- 1}) 2-naphthalenesulfonic acid aqueous solution while keeping the temperature below 20¡ãC to produce a 1.36 M BANS aqueous solution. To prepare 1.36 M BATS aqueous solution, the same procedure was performed, except that p-toluenesulfonic acid was used in place of 2-naphthalenesulfonic acid.

Preparation of Conductive Non-Woven Fabrics

As an oxidizing agent, ammonium persulfate (APS) was dissolved into distilled water, followed by adding the BANS aqueous solution, BATS aqueous solution, and ethanol for improving wettability of non-woven fabric and then stirring. The resultant solution is called ¡°solution A¡±. The concentration of ethanol was fixed to 5 M in the present study. The molar ratio of BANS to BATS, [BANS]/[BATS], and that of both dopants to APS, ([BANS]+[BATS])/[APS], were fixed to 1, respectively.

After immersing a non-woven fabric (WO-ME150, Mitsubishi Paper Mills Limited, 50¡Á50 cm) in the solution A for 10 min, the fabric was dehydrated at 0.3 MPa using the nip roll apparatus of ¦µ12 cm and then dried for 10 min at 60¡ãC. A laboratory dish containing 20 mL of pyrrole was placed at the bottom of a closed cubic acrylic box (100¡Á100¡Á100 cm). The pretreated fabric was laid in the box filled with pyrrole vapor. After 10 min, the fabric was removed from the box and dried for 10 min at 100¡ãC. The fabric was washed thoroughly using distilled water and then dried for 10 min at 100¡ãC again.

Characterization of Conductive Non-Woven Fabrics

Scanning electron microscopy (SEM; Model S-900, Hitachi Co.) was performed at an acceleration voltage of 6 kV to investigate the surface morphology of a conductive non-woven fabric. The surface of the fabric was thinly coated with platinum using an ion sputtering instrument for 3 min to form an electrical contact. Qualitative and quantitative analyses of sulfur were performed by X-ray fluorescence spectroscopy (XRF; Model Axios, PANalytical Spectris Co., 40 kV, 60 mA). The specific resistivity of the conductive non-woven fabric was measured with resistivity meters (Hiresta UP Model MCP-HT450 and Loresta GP Model MCP-T610, Mitsubishi Chemical Analytech Co.).

The radar absorbency was evaluated as reflection loss. The reflection loss was measured by a free space method using horn antennas [8]. As shown in Fig. 1, electromagnetic waves with frequencies of 75 ¨C 110 GHz were emitted from a horn antenna (sender) and detected by another antenna (receiver) after reflected at piled conductive non-woven fabrics on an Al sheet. The reflection loss is defined in the following equation.

Figure 1. Schematic illustration for the evaluation of reflection loss.

Reflection loss [dB] = 10 log (S_r/S_s)    (1)

where S_s and S_r represent the strength of the electromagnetic waves emitted by the sender and detected by the receiver, respectively.

Results and Discussion

Figure 2 shows SEM photographs of a fiber of the conductive non-woven fabric with specific resistivity of 70 ¦¸cm for example. As can be seen from these photographs, polypyrrole nanoparticles uniformly and compactly adhere on the fiber surface.

Figure 2. An SEM image (upper) and an enlarged SEM image of a rectangular area in the upper one (lower) for a fiber of the conductive non-woven fabric with a specific resistivity of 70 ¦¸cm.

Figure 3 shows specific resistivity (R) of conductive non-woven fabrics as a function of APS concentration. As the APS concentration increases from 0.01 M to 0.66 M, the R value significantly decreases from 8.5¡Á10⁷ ¦¸cm to 3.0 ¦¸cm. This suggests that the R value can be arbitrarily controlled by the APS concentration. The R value is usually lowered by increasing the content of BANS and BATS in polypyrrole. The R value can also be lowered by increasing the amount of deposited polypyrrole nanoparticles because contact resistance between them becomes low. If the content of the dopants is constant irrespective of the amount of deposited polypyrrole nanoparticles, the absolute quantity of dopants increases with an increase in the amount of deposited polypyrrole. Since both BANS and BATS include a sulfur element, the sulfur content in different conductive non-woven fabrics was evaluated by XRF and summarized in Fig. 4. The sulfur content was increased with an increase in the APS concentration or a decrease in the R value. This clearly indicates that the decrease in the R value is ascribed to the increase in the amount of deposited polypyrrole nanoparticles.

Figure 3. Specific resistivity as a function of APS concentration. [Ethanol] = 5 M.
[BANS]/[BATS] = 1. ([BANS] + [BATS])/[APS] = 1.

Figure 4. Content of sulfur element in the conductive non-woven fabric sheets as a function of APS concentration. [Ethanol] = 5 M.
[BANS]/[BATS] = 1. ([BANS] + [BATS])/[APS] = 1.

Figure 5 shows reflection loss as a function of frequency of electromagnetic waves when ten pieces of pristine non-woven fabrics and different pieces of conductive non-woven fabrics are piled up on an Al foil. The thickness of the piles of ten and twenty pieces of conductive non-woven fabrics was ca. 7mm and ca. 14 mm, respectively. If 90 % and 99 % of strength of electromagnetic waves are absorbed into a RAM, reflection losses are -10 dB and -20 dB, respectively. In general, reflection losses less than -10 dB are required for interrupting electromagnetic interference in the broadband.

Figure 5. Reflection loss as a function of frequency of electromagnetic waves for the stacks of (a) pristine non-woven fabric sheets with 10 pieces and conductive non-woven fabric sheets with (b) 1 piece, (c) 10 pieces and (d) 20 pieces. The specific resistivity of the conductive non-woven fabric sheet: 70 ¦¸cm.

The reflection loss of the pile of ten pieces of pristine non-woven fabric was -1 dB or more in the frequency range of 75 -110 GHz. This suggests that it scarcely absorbed electromagnetic waves and did not act as a RAM. In contrast, the reflection loss of the single conductive non-woven fabric was ca. -2 dB over the whole frequency range, suggesting that the conductive non-woven fabric partly absorbed electromagnetic waves. When ten pieces of conductive non-woven fabric were piled up, the reflection loss was ca. -20 dB, which showed that the pile could absorb ca. 99 % of electromagnetic waves in the frequency range of 75-110 GHz. Moreover, the pile of twenty pieces of conductive non-woven fabric absorbed ca. 99.99 % of the electromagnetic waves, in particular 99.999 % in the frequency range of 90-100 GHz. In this way, electromagnetic waves- absorbing effect is enhanced by piling the conductive non-woven fabric.

Figure 6 shows reflection loss as a function of frequency of electromagnetic waves for the piles of conductive non-woven fabrics with different specific resistivity and a rubber sheet of carbonyl iron/titanium slug (6:4) with the thickness of 0.7 mm for reference. Each pile was formed by ten pieces. The reference is a magnetic and dielectric RAM. It can effectively absorb electromagnetic waves in the relatively narrow frequency range of 90-100 GHz. On the other hand, the piles of conductive non-woven fabrics have the reflection loss less than -13 dB in the wide frequency range of 75-110 GHz irrespective of specific resistivity. This means that they can absorb more than 95 % of electromagnetic waves in the wide frequency range. In particular, the pile of ten pieces of conductive non-woven fabric with 700 ¦¸cm absorbed more than 99.9 % of electromagnetic waves at around 95 GHz. The electromagnetic wave-absorbing conductive non-woven fabrics developed in the present study have additional features such as light weight, flexibility, flat, relatively thin. Therefore, they must be a promising broadband electromagnetic waves-absorbing sheet for extremely high frequencies.

Figure 6. Reflection loss as a function of frequency of electromagnetic waves for the stacks of conductive non-woven fabric sheets with the specific resistivity of (a) 7.5 ¦¸cm, (b) 15 ¦¸cm, (c) 70 ¦¸cm and (d) 700 ¦¸cm and (e) a rubber sheet of carbonyl iron/titanium slug (6:4). The number of stacks is 10.

Conclusion

In this study, the finding obtained in this study will be summarized as follows.

1. We have prepared conductive non-woven fabrics by exposing non-woven fabrics immersed in mixed solutions of APS as an oxidizing agent and BANS and BATS as a dopant with pyrrole vapor. Their specific resistivity changed from 8.5¡Á10⁷ to 3.0 ¦¸cm with the concentrations of APS and dopants.
2. Piles of ten and twenty pieces of conductive non-woven fabric absorbed ca. 99 % and ca. 99.99 % of the electromagnetic waves in the frequency range of 75-110 GHz, respectively. In particular, the latter absorbed 99.999 % of them at frequencies of 90-100 GHz. These results clearly indicate that the present conductive non-woven fabrics are excellent broadband electromagnetic waves-absorbing materials.
3. Piles of ten pieces of conductive non-woven fabric with specific resistivity of 7.5-700 ¦¸cm had reflection losses less than -13 dB in the wide frequency range of 75-110 GHz, suggesting that they could absorb more than 95% of the broadband electromagnetic waves. In particular, the pile of ten pieces of conductive non-woven fabric with 700 ¦¸cm absorbed more than 99.9 % of electromagnetic waves at around 95 GHz.

References

Contact Details

R.Vijayalakshmi and K. V. Rajendran
Department of Physics, Presidency College
Chennai, TamilNadu, India

E-mail : [email protected]

Yoshihiro Egami
Osaka Research Laboratory, Tayca Corporation, 3-47, 1-Chome
Funamachi, Taisho-ku, Osaka 551-0022, Japan
Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University
1-1, Gakuen-cho, Sakai, Osaka 599-8531, Japan

E-mail : [email protected]

Takashi Yamamoto
Department of Communications Engineering, National Defense Academy
10-20, 1-Chome, Hashirimizu, Yokosuka, Kanagawa 239-8686, Japan

Kunio Suzuki
Osaka Research Laboratory, Tayca Corporation, 3-47, 1-Chome
Funamachi, Taisho-ku, Osaka 551-0022, Japan

Tadashi Yasuhara
Osaka Research Laboratory, Tayca Corporation, 3-47, 1-Chome
Funamachi, Taisho-ku, Osaka 551-0022, Japan

Hiroshi Inoueb
Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University
1-1, Gakuen-cho, Sakai, Osaka 599-8531, Japan

This paper was also published in print form in "Advances in Technology of Materials and Materials Processing", 12[1] (2010) 31-34.