Microwave Absorption and Infrared Stealth Characteristics of Bamboo Charcoal/Silver Composites Prepared by Chemical Reduction Method

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1 724 Journal of the Chinese Chemical Society, 2008, 55, Microwave Absorption and Infrared Stealth Characteristics of Bamboo Charcoal/Silver Composites Prepared by Chemical Reduction Method Tzu-Hao Ting a ( ), Kuo-Hui Wu a * ( ), Jen-Sung Hsu b ( ), Ming-Ho Chuang b ( ) and Cheng-Chien Yang b ( ) a Department of Applied Chemistry and Materials Science, Chung Cheng Institute of Technology, National Defense University, Tahsi, Taoyuan, Taiwan, R.O.C. b Chemical Systems Research Division, Chung Shan Institute of Science and Technology, Taoyuan, Taiwan, R.O.C. Bamboo charcoal coated with silver (BC/Ag) was prepared by activation and chemical reduction processes at different AgNO 3 contents (10-30 wt.%). The spectroscopic characterizations of the formation processes of BC/Ag composites were studied using X-ray diffraction, scanning electron microscopy and transmission electron microscopy. These composites were introduced in epoxy resin to be a microwave absorber and mixed polyethylene to be an infrared stealth plate. Microwave absorbing properties were investigated by measuring complex permittivity, complex permeability and reflection loss in the 2-18 and GHz microwave frequency range using the free space method. The thermal extinction measurements in the 3-5 and 8-12 m were done to evaluate the shielding affectivity of infrared. The results showed that a significant thermal extinction and a wider absorption frequency range could be obtained by adding silver to bamboo charcoal. Keywords: Bamboo charcoal; Silver; Microwave absorption; Thermal extinction. INTRODUCTION Microwave absorbing materials (MAM) have been widely used to prevent or minimize electromagnetic reflections from large structures such as aircraft, ships and tanks and to cover the walls of anechoic chambers. 1-5 But in general, the MAM layer is highly emissive and causes high temperatures. For an object with a high temperature, which is in the order of hundreds of degrees centigrade, such as the exterior surface of an aircraft, the peak radiation is distributed in the 3-5 and 8-12 m bands. 6 The high emittance of the exterior surface of the aircraft caused by the MAM greatly increases the probability of being detected by an infrared detector. And because military objects have higher temperatures than the background, they have higher brightness in the infrared image and the background has lower brightness and contrast In order to decrease the high temperature from the hot MAM layer, it is possible to add an additional material with low infrared emittance to the MAM layer. Two technical concepts can be employed in realizing the preceding purposes. 12 The first is to develop a structure which is effective in all the infrared and the microwave regions. This is a double-layer or multi-layer compatible absorbing plate. The second is to design the infrared stealth (low infrared emissivity) material and MAM separately and then to combine these materials into a singlelayer compatible absorber which is effective in the desired properties. Very little has been published on microwave absorbers with low infrared thermal exitance, such as bamboo charcoal coated with silver particles. Such materials have special interest because of new and often unusual microwave absorbance with infrared stealth properties. In this article, we present a novel approach to synthesizing bamboo charcoal/silver (BC/Ag) composites. The origin of their properties is also discussed on the basis of the structural characterization, including X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Microwave absorbing properties of the BC and BC/Ag reinforced epoxy resin composites were tested at 2-18 and GHz microwave frequencies using the free space method, which is known as an adequate method for measuring anisotropic and inhomogeneous materials like composites. 13 The characteristic of infrared stealth of the BC and BC/Ag polyethylene composite plate was studied with infrared thermal-imaging equipment in samples of 3-5 and 8-12 m.

2 Bamboo Charcoal Coated with Silver (BC/Ag) J. Chin. Chem. Soc., Vol. 55, No. 4, EXPERIMENTAL Preparation of BC/Ag and epoxy resin-bc/ag composites The BC powders (particle size < 10 m, Taiwan Paiho) were activated with surfactant sodium alginate under stirring for 1 h. The as-prepared BC powders (2 g) were immersed into 100 ml of biamminesilver nitrate ([Ag(NH 3 ) 2 ]NO 3 ) solutions, which were formed by adding aqueous ammonia into AgNO 3 solution at room temperature. After stirring for 1 h, dilute aqueous solutions of hydrazine monohydrate were separately prepared and were introduced to the BC-[Ag(NH 3 ) 2 ]NO 3 solutions in appropriate quantities (molar ratio 1:1 with respect to silver nitrate) by a syringe. The color of the solutions changed to either gray or gray black due to reduction of Ag + to Ag. Stirring was continued under inert atmosphere at room temperature for another 4 h. The BC/Ag particles were separated and washed with deionized water and ethanol, then dried in vacuum at 60 C overnight. The composite weight ratios of BC and Ag were 1:0.1 (BC/Ag-1), 1:0.2 (BC/Ag-2) and 1:0.3 (BC/Ag-3), respectively. The absorbing composite materials were prepared by molding and curing the mixture of BC and BC/Ag powders and a thermal-plastic epoxy resin. The mixing ratio of specimen powders to epoxy resin was 1:2 by weight. Molding was carried out in a hydraulic press at 5 Mpa pressure and 80 C for 1.5 h, obtaining specimens of 15 cm 15 cm with a thickness of 0.2 cm for reflectivity measurements. Each component of the infrared adsorption plates was mixed together by dry polyethylene to obtain a powder mixture. The composites of the polyethylene with BC and BC/Ag composite powders were controlled by weight. The powder mixtures having the above powders were carried out in a hydraulic press at 5 Mpa at 120 C for about 4 h to form a 15 cm 15 cm 0.1 cm for the infrared image test. Experimental techniques The phase identification of the samples was performed with X-ray diffraction with Cu K radiation. The particle morphology was observed with a JEOL JEM- 200CX scanning electron microscope as well as a Philips CM-200 transmission electron microscope. The permittivity (, ) and permeability (, ) of the composites were measured by using a HP8722ES network analyzer in the frequency range of 2-18 and GHz. The sample under test was positioned on an aluminum panel. For a microwave-absorbing layer terminated by a short circuit, the normalized input impedance related to the impedance in free space, Z, and reflection loss (R.L.) related to the normal incident plane wave are given by theory of the absorbing wall, 14 Z 2 t tanh j * * Z 1 RL..( db) 20log (2) Z 1 where is the wavelength and t is the thickness of specimen. * is the complex permittivity = j,where and are the real and the imaginary parts of *, respectively., associated with electric field loss, results mostly from electric-dipole polarization at microwave frequencies. * is the complex permeability = j,where and are the real and imaginary parts of *, respectively., associated with magnetic field loss, results from magnetic-dipole magnetization. Further, the loss tangent of the dielectric/ magnetic can be expressed as tan = / and tan = /, respectively. The infrared stealth effects were measured with FLIR MERLIN (3-5 m) and FLIR SC2000 (8-12 m) infrared thermal-imaging equipment. An infrared thermal image is an image captured by a thermal camera to display infrared radiation. Therefore an infrared thermal image is a suitable tool to measure the object surface temperature, and the reduced thermal exitance resulting from either low temperature or low emittance reduces the chance of thermal infrared detection. The test set-up is illustrated in Fig. 1. For this experiment the sample under test was a 15 cm 15 cm 0.1 cm low-density polyethylene (LDPE) plate. The amount of BC and BC/Ag hybrids in Fig. 1. Set-up of infrared thermal image testing. (1)

3 726 J. Chin. Chem. Soc., Vol. 55, No. 4, 2008 Ting et al. the LDPE was controlled to be ~0.002 g cm 3. RESULTS AND DISCUSSION Structure characterization Fig. 2 shows the X-ray diffraction patterns of the BC and BC/Ag composites, respectively. The XRD pattern measured from the BC is showed as an amorphous material. The broad peaks were from the (002) planes of the amorphous graphite phase in the bamboo charcoal. 15 Several small peaks besides the main broad peaks from (002) and (100) planes of amorphous graphite appeared in the X-ray diffraction patterns of the bamboo charcoal. In the spectrum of the BC, between 20 and 30 is seen as a broad plateau of mixed amorphous SiO 2 and C; also the graphite Fig. 2. XRD spectra of (a) BC, (b) BC/Ag-1, (c) BC/ Ag-2 and (d) BC/Ag-3 composites. Fig. 3. SEM photographs of (a) BC, (b) BC/Ag; TEM photographs of (c) BC, (d) BC/Ag-3 and (e) EDX spectrum of the BC/Ag-3.

4 Bamboo Charcoal Coated with Silver (BC/Ag) J. Chin. Chem. Soc., Vol. 55, No. 4, Fig. 4. Frequency dependences of the complex permittivity, complex permeability and loss tangent of dielectric/magnetic of epoxy-bc and epoxy-bc/ag composites in 2-18 GHz. and SiC phases are apparently formed in the bamboo charcoal. 16 Its intensity gradually decreases with increasing Ag content. The XRD of the BC/Ag composites are in good agreement with the literature values of silver nanoparticles. 17 All the prominent peaks at 2 values of about 38, 44, 65 and 77 represent the 111, 200, 220 and 311 Bragg s reflections of face-centered cubic crystalline silver. Furthermore, an increase of the amount of AgNO 3 in BC induced the obvious enhancement of characteristic peaks of silver, implying the development of larger and highly crystalline silver nanoparticles. Therefore, we can estimate the size of nano-ag grain (2 =38 ) using Scherrer s formula D = 0.9 / cos,whered is the crystallite size in nm, is the radiation wavelength ( nm for K 1), is the band-

5 728 J. Chin. Chem. Soc., Vol. 55, No. 4, 2008 Ting et al. width at half-height and is the diffraction peak angle. 18 The calculated crystallite sizes are 40, 55 and 75 nm for BC/Ag-1, -2 and -3, respectively. SEM and TEM photographs were used to evaluate the surface morphology and size distribution of silver deposited on the BC surface. As shown in Fig. 3, ultra-fine and aggregated silver particles were homogeneously distributed on the surface of the BC. The pure BC had a porous surface. The silver particles are granular in nature and seem to be nanosized, typically in the range of < 100 nm. This agreed with the XRD results. It shows that silver content increases with increasing the initial concentration of AgNO 3 solution. The surface composition of BC/Ag composites was distinctly determined with EDX (Fig. 3e). Therefore, it can be assumed that Ag nanoparticles are deposited on the surface of the BC particles. The microwave adsorbing properties of BC and BC/Ag composites Fig. 4 shows the relative permittivity *andtherelative permeability * of epoxy-bc and epoxy-bc/ag composites measured in the frequency range of 2-18 GHz. The real part and the imaginary part of permittivity (, ) for epoxy-bc and epoxy-bc/ag composites are larger than those composites of the real part and imaginary part of permeability (, ) in the frequency range of 2-18 GHz. The real and imaginary parts of permeability (, ) remained practically constant in the whole frequency range. The dissipation factors are represented by the dielectric loss (tan ) and magnetic loss (tan ). The values of tan for both epoxy-bc and epoxy-bc/ag composites are larger than those of tan in the whole frequency range, which suggests that microwave absorption enhancement of composites results mainly from dielectric loss rather than magnetic loss. These characteristic electromagnetic properties can be applied to the microwave absorbing material. Fig. 5 shows absorption characteristics of epoxy-bc composite and epoxy-bc/ag composites in the frequency range of 2-18 GHz. It can be seen that epoxy-bc/ag composites have a more obvious effect on microwave absorbing properties than epoxy-bc composite does. The specimen of the BC powder showed only one band at 6.5 GHz with -6.2 db in reflection loss. Among the epoxy-bc/ag composites, the powder prepared from the silver content of 20 wt.% (BC/Ag-2) exhibited two relatively pronounced absorption bands at 4.8 and 15 GHz, with the reflection losses of -13 db and -5 db, respectively. Clearly demonstrated is that the intensity and frequency of the reflection loss maximal for the composite also depends on the silver content. Fig. 6 shows frequency dependence of the complex permittivity, complex permeability and dielectric/magnetic loss tangent of epoxy-bc and epoxy-bc/ag composites in GHz. It can be seen that the real part and the imaginary part of permittivity (, ) for epoxy-bc and epoxy- BC/Ag composites are larger than those of the real part and the imaginary part of permeability (, ) in the frequency range of GHz. The values of tan for both epoxy- BC and epoxy-bc/ag composites are larger than those of tan in the whole frequency range, which suggests that microwave absorption enhancement of composite results mainly from dielectric loss rather than magnetic loss. Fig. 7 shows absorption characteristics of epoxy-bc composite and epoxy-bc/ag composites at the frequency range of GHz. Epoxy-BC composite achieves two absorbing values of -14 and -11 db at 18.2 and 31.5 GHz. For epoxy-bc/ag-1, -2 and -3 composites, their absorbing values are db at 25 GHz, -4.5 db at 25.5 GHz and -2.8 db at 25 GHz. The reflection losses of epoxy-bc/ag composites are found to depend on the sensitivity of the silver content. In comparison with the epoxy-bc sample, the epoxy-bc/ag composites showed that the attenuation peaks decreased from to -2.8 db and shifted to lower frequency from 31 to 25 GHz. This absorbing ability weakening and frequency shift decrease can be attributed to the fact that the addition of silver on the BC surface results in Fig. 5. Microwave absorbing properties of epoxy-bc and epoxy-bc/ag composites in 2-18 GHz.

6 Bamboo Charcoal Coated with Silver (BC/Ag) J. Chin. Chem. Soc., Vol. 55, No. 4, the surface reflectivity of composites as silver increases. 19,20 And thus the epoxy-bc composite wouldn t result in a remarkable improvement of the absorbing ability at GHz. Infrared thermal images Before the experiment, the plate sample was placed before the blackbody at 100 cm, in which the temperature was ~1000 C. Figs. 8(a) and (b) show the mid-wave infrared (3-5 m) and long-wave infrared (8-12 m) thermal images of the pure polyethylene, BC powders and BC/Ag-1, -2 and -3 composites mixed with polyethylene (PE), respectively. It can be seen from Fig. 8(a) that the surface maximum temperature (700 C) was not decreased and average temperature decreased significantly for the polyethylene-bc (PE-BC, 360 C) and PE-BC/Ag-1 (351 C), Fig. 6. Frequency dependences of the complex permittivity, complex permeability and dielectric/magnetic loss tangent of epoxy-bc and epoxy-bc/ag composites in GHz.

7 730 J. Chin. Chem. Soc., Vol. 55, No. 4, 2008 Ting et al. Fig. 7. Microwave absorbing properties of epoxy-bc and epoxy-bc/ag composites in GHz. Fig. 8. (a) The mid-wave infrared (3-5 m) and (b) long-wave infrared (8-12 m) thermal images of the PE, PE-BC and PE-BC/Ag composites. The value in parentheses is the maximum and average temperatures ( C). PE-BC/Ag-2 (340 C) and PE-BC/Ag-3 (330 C) composites in comparison with the value for the pure polyethylene (488 C). From Fig. 8(b), the surface temperature (maximum and average) decreased significantly following the increasing silver content of PE-BC/Ag-1, -2 and -3 composites ( C and C) in comparison with the value for the pure polyethylene (132 and 68 C). From the above results it could be concluded that adding the silver to the bamboo charcoal surface leads to higher infrared energy absorption and better shielding effect. We believe that, after some more improvement, the thermal energy absorbing and infrared shielding effects of the BC/Ag composites could be further increased. CONCLUSION In this work, bamboo charcoal/silver (BC/Ag) composites were successfully prepared by the chemical reduction method. XRD, SEM and TEM studies have established formation of BC/Ag composites. The complex relative permittivity, permeability, loss tangent value and their relationship with microwave absorbing properties were investigated. It is found that the complex permittivity and loss tangent value increase when the volume fraction of the silver particle increases, which results in microwave absorption properties in 2-18 and GHz. The microwave absorption in 2-18 and GHz of both epoxy-bc and epoxy-bc/ag composites results mainly from dielectric loss rather than magnetic loss. It is obvious that epoxy-bc/ag composites increase performance of microwave absorbing properties in 2-18 GHz. There is a little deterioration of the microwave performance in GHz, whereas the absorbing ability weakens in the low frequency before the epoxy-bc peak frequency. In infrared image test results, observation of temperature differences for infrared energy absorption is determined in the 3-5 and 8-12 m bands. Increasing silver on bamboo charcoal would effectively contribute to the increase of infrared energy absorption. Therefore, BC/Ag composite curtains mainly off infrared radiation in the 3-5 and 8-12 m bands. ACKNOWLEDGEMENT The authors thank the National Science Council of the Republic of China for supporting this work (Grant NSC M ). The authors wish to express their gratitude to Mr. K. C. Wang of the NSC Instrument Center for TEM analysis. Received February 19, REFERENCES 1. Pinho, M. S.; Gregori, M. L.; Reis Nunes, R. C.; Soares, B. G. Polym. Degrad. Stab. 2001, 1, Knott, E. F.; Shaeffer, J. F.; Tuley, M. T. Rader Cross Section; Artech House: New York, 1993; p Carvalho, A.; Gregori, M. L.; Chambers, B. Micro. Opt. Tech. Lett. 1997, 16, Smith,F.C.;Chambers,B.;Bennett,J.C. IEE Proc. Sci. Meas. Tech. 1994, 141, Petrov,V.M.;Gagulin,V.V. Inorg. Mater. 2001, 37, Liu, L. G.; Gong, R. G.; Cheng, Y. S.; Zhang, F. G.; He, H. H. Opti. Exp. 2005, 13,

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