Electromagnetic wave absorption and infrared camouflage of ordered mesoporous carbon–alumina nanocomposites

Microporous and Mesoporous Materials 134 (2010) 58–64

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Electromagnetic wave absorption and infrared camouflage of ordered mesoporous carbon–alumina nanocomposites Tao Wang, Jianping He *, Jianhua Zhou, Xiaochun Ding, Jianqing Zhao, Shichao Wu, Yunxia Guo College of Material Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR China

a r t i c l e

i n f o

Article history: Received 1 March 2010 Received in revised form 7 May 2010 Accepted 11 May 2010 Available online 31 May 2010 Keywords: Mesoporous C–Al2O3 nanocomposites Reflection loss Infrared emissivity Compatibility

a b s t r a c t Ordered mesoporous C–Al2O3 nanocomposites with different alumina contents were prepared via evaporation-induced triconstituent co-assembly method. As shown by X-ray diffraction (XRD), N2 adsorption– desorption and transmission electron microscopy (TEM), the C–Al2O3 nanocomposites exhibit ordered mesoporous structures, and the results indicate that the carbon as amorphous components in the mesostructured framework can stabilize the ordered mesostructures of the nanocomposites. Furthermore, the electromagnetic wave absorption properties and infrared emissivities of the ordered mesoporous C–Al2O3 nanocomposites were examined systematically. It was observed that the mesoporous nanocomposites have excellent electromagnetic wave absorption properties and low infrared emissivities. For example, the sample with 50 wt.% Al2O3 achieves a maximum reflection loss of 15.3 dB and the bandwidths being lower than 10 dB is 7.4 GHz with a thickness of 3 mm in 0.5–18 GHz, whereas its infrared emissivity can reach 0.46 in the wavelength range from 8 to 14 lm. Ó 2010 Elsevier Inc. All rights reserved.

1. Introduction Since the discovery of ordered mesoporous silica, [1,2] ordered mesoporous materials have attracted more and more attention owing to their potential applications in many fields, such as separation, [3] adsorption, [4] catalysis, [5,6] catalyst supports, [7] electrode materials, [8] energy storage, [9] drug delivery, [10] and sensors [11]. However, since ordered mesoporous material is a novel nanoscale material, it is necessary to fully explore the so far unknown potential applications. Recently, scientists find that the open frameworks, tunable porosities and various components endow ordered mesoporous materials with accessibility to electromagnetic wave absorption. Hence, the ordered mesoporous nanocomposites as electromagnetic wave absorption materials have a great potential application in military camouflage field. Liu et al. successfully fabricated mesoporous carbon (OMC)/fused silica composite, with 10 vol.% of OMC, its electromagnetic interference (EMI) shielding efficiency (SE) is as high as 40 dB in the X-band, which is higher than that of a carbon nanotube/fused silica composite with the same carbon content (30 dB) [12]. More recently, Zhao et al. have reported a novel and uniform hematite/ SiO2/mesoporous silica core–shell mesoporous nanocomposite via the combination of the sol–gel process and the surfactant selfassembly approach. Its EMI shielding effectiveness is enhanced by about 50% compared with that of the hematite materials at * Corresponding author. Tel./fax: +86 25 52112626. E-mail address: [email protected] (J. He). 1387-1811/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2010.05.007

the X-band region because of the unique multi-layered core–shell structure and the impedance attenuation of perpendicularly oriented mesoporous channels [13]. At the same time, with the appearance of the advanced detectors such as infrared detector, meter-band radar, centimeter-band radar and laser radar, the multi-band stealth compatibility, including meter wave, centimeter wave, infrared and laser, is the development trend of stealth technology. The radar and the infrared detector are often employed in modern wars, so the most important investigative objects of camouflage materials are aimed at stealthy technologies of infrared radicalization and radar wave. At present, a lot of research has been conducting on camouflage materials all over the world, and thereinto, the compatible stealth function in infrared and radar bands is an important direction of development and has great military value [14–16]. Radar adsorbing material requires a high absorptivity and low reflectivity, infrared stealthy material demands a lower emissivity. But it is very difficult to possess both a high absorptivity and a lower emissivity for a kind of material. There is a big difference between infrared wavelength and radar wavelength in the frequency, so it is possible to synthesize a sort of material with both an excellent absorption characteristic in the radar wave band and a lower emissivity in infrared wave band. However, to the best of our knowledge, there are no reported experimental results on the design of ordered mesoporous electromagnetic absorption materials with infrared camouflage. Hence, our manuscript presents a competitive work. In the present study, we designed an ordered mesoporous electromagnetic absorption material with infrared camouflage

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effect. Therefore, ordered mesoporous C–Al2O3 nanocomposites were synthesized by using resols as an organic precursor, less reactive metal alkoxides as an inorganic precursor, and amphiphilic triblock copolymer F127 as a template. Carbon was added into the mesostructured metal oxide framework, forming a glass-like phase, which was responsible for the stability of ordered mesostructures [17]. Such a performance can be easily accomplished via the templating effect of colloidal particles. The ordered mesoporous C–Al2O3 nanocomposite exhibits an effective electromagnetic adsorption property and a lower infrared emissivity in the wavelength range between 8 and 14 lm. 2. Experimental 2.1. Synthesis All the reagents were of analytical purity and used without further purification. Pluronic F127 (EO106PO70EO106, EO = ethylene oxide, PO = propylene oxide) was purchased from Sigma–Aldrich Corporation. Aluminum iso-propoxide, phenol, formaldehyde, hydrofluoric acid, hydrochloric acid and ethanol were purchased from Sinopharm Chemical Reagent Limited Corporation. The resol precursor (Mw < 500) was prepared according to the procedure reported previously [18]. In a typical preparation, 2 g of Pluronic F127 was dissolved in 12 ml of ethanol and stirred for 1 h at 40 °C as solution A. Meantime solution B was prepared from 1.5 ml of 37 wt.% hydrochloric acid mixed with 6 ml of ethanol. Aluminum iso-propoxide (2.04 g) was poured slowly to solution B under vigorous stirring for 1 h. Solution B and 2.5 g of 20 wt.% resols’ ethanolic solution were added into solution A. After being stirred for 2 h, the mixture was transferred into dishes. It took 24 h at room temperature to evaporate ethanol and 36 h at 70 °C in an oven to thermopolymerize. The as-made products, flaxen and transparent films, were scraped from the dishes. Calcination was carried out in a tubular furnace at 350 °C for 5 h and 500 and 700 °C for 2 h under N2 flow with a rate of 1 °C/min to remove the amphiphilic triblock copolymer templates and form mesoporous C–Al2O3 nanocomposite CA50-x, where x represents the heating temperature. The nanocomposites with different compositions in a wide range from 20 to 80 wt.% Al2O3 were prepared by varying the mass ratios of resol to aluminum iso-propoxide. The final products were labeled as CA-y-700, where y represents the weight percentage of alumina content in the nanocomposites. After mesoporous nanocomposite CA-50-700 being immersed in 10 wt.% HF solutions for 24 h, alumina was removed, with mesoporous carbon CA-50-C left. The calcination at 550 °C for 5 h in air was to burn off carbon and generate mesoporous alumina material CA-50-A. Moreover, ordered mesoporous carbon labeled as C-700 as a reference, was prepared according to the report [18].

stalled in FEI Tecnai G2 system was also used to analyze the microzone composition of the sample. Electromagnetic wave adsorption measurement: The as-prepared powders were uniformly dispersed into the commercial epoxy resin and then pressed into toroidal-shaped samples with the outer diameter of 7.0 mm, the inner diameter of 3.04 mm and the thickness of 3.0 mm. The scattering parameters S11, S21 of the samples with 40 wt.% as-prepared powders were measured using a vector network analyzer (Agilent E8363A) by using coaxial reflection/ transmission technique. The relative permittivity and permeability values were determined from the scattering parameters as measured in the frequency range of 0.5–18 GHz. To reduce the errors, all values were obtained by averaging over the data measured from three different toroids of each sample. Infrared emissivity measurement: Aluminum sheet, previously degreased in diluted NaOH at 50 °C and then chemically polished in diluted HNO3, was used as the substrate for the coatings, and EPDM dissolved in dimethylbenzene as the adhesive. The prepared powder was used as the filling and its weight ratio was set as 30% in the coating. The coating thickness was controlled at about 35 lm by the wire-wound rod coater (Tianjin Jingke Material Testing Machine Factory, China). Infrared emissivity value at the wavelength of 8–14 lm was measured by using IR-2 Infrared Emissometer (Shanghai Institute of Technological Physics of the Chinese Academy of Sciences). All values were obtained by averaging over the data measured from ten different regions of each coating.

3. Results and discussion 3.1. Structures of mesoporous C–Al2O3 nanocomposites The XRD patterns of these nanocomposites are shown in Fig. 1. A large and repeated lattice usually contributes to the presence of reflections at pseudo-small angles. The presence of these reflections suggests that this sample may consist of a highly regular or ordered structure. The sample CA-50-350 nanocomposite shows a very strong diffraction peak around 0.74°, which is still maintained when the calcination temperature reaches 700 °C. Moreover, the diffraction peak shifts rightward in Fig. 1, as can be ascribed to the shrinkage of pore channels (Table S1). The identification of this mesoporous structure will be discussed later with the assistance of TEM analysis. We have obtained a 2D hexagonal mesoporous C–Al2O3 framework after calcination in a tubular oven under nitrogen at 350 °C. A good mesoscopic order is reflected even at the calcination temperatures as high as 500 and 700 °C, respec-

The porous structures of the ordered mesoporous C–Al2O3 nanocomposites, mesoporous carbon, and mesoporous Al2O3 were measured by N2 adsorption–desorption isotherm using Micromeritics ASAP 2010 at 77 K. X-ray diffraction (XRD) patterns were recorded by a Bruker D8 ADVANCE diffractometer using Cu Ka radiation (k = 0.154056 nm). Transmission electron microscopy (TEM, FEI Tecnai G2) and Selected area electron diffraction (SAED) patternoperating at 200 kV was applied to characterize the morphology of the mesoporous materials. Samples for TEM measurements were prepared by ultrasonically suspending the powder in ethanol and placing a drop of the suspension on a carbon film supported by Cu grids. Energy dispersive X-ray spectrum (EDS) in-

Intensity (a.u.)

2.2. Characterization

CA-50-700 CA-50-500 CA-50-350 1

2

3 2theta (degree)

4

5

Fig. 1. XRD patterns of mesoporous C–Al2O3 nanocomposite CA-50 calcined at different temperatures.

T. Wang et al. / Microporous and Mesoporous Materials 134 (2010) 58–64

Volume adsorbed (cm3/g,stp)

400

(a)

CA-50-350 CA-50-500 CA-50-700

(b)

0.16

dV/dD (cm3g-1nm-1)

60

300

200

CA-50-350 CA-50-500 CA-50-700

0.12

0.08

0.04

100

0.00 0.0

0.2

0.4

0.6

0.8

Relative pressure (P/P0)

1.0

0

10 20 30 Pore diameter (nm)

Fig. 2. N2 adsorption–desorption isotherm (a) and pore size distribution (b) of mesoporous C–Al2O3 nanocomposite CA-50 calcined at 350, 500, and 700 °C in N2. The isotherm (a) for the CA-50-350 nanocomposite is offset vertically by 70 cm3/g.

Fig. 3. TEM images of mesoporous C–Al2O3 nanocomposite CA-50 calcined at (a) 350 °C (b) 500 °C and (c, d and e) 700 °C in N2, the insets are the corresponding FFT diffractograms.

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tively, indicating a high thermal stability of the mesoporous structure. Fig. 2 shows N2 sorption isotherms and pore size distributions of the C–Al2O3 nanocomposites calcined at different temperatures. The nanocomposites exhibit representative type IV isotherms with H1-type hysteresis loops [1]. The mesoporous nanocomposite CA50-350 has a surface area of 331 m2/g and a pore volume of 0.48 cm3/g. As the temperature increases, the specific surface area, pore size and pore volume decrease (Table S1). The decrease of BET (Brunauer-Emmett-Teller) surface area, pore size and pore volume may be attributed to the shrinkage of the mesopores in the frameworks, which mainly results from the glued carbon phase (Table S1). CA-50-700 was characterized by high angle XRD and the SAED pattern, the figures are provided in the revision (Fig. S1). We estimate that the walls of the mesostructure are amorphous at calcination temperature of 700 °C. A more detailed structural characterization is revealed by TEM images, as shown in Fig. 3. The nanocomposite calcined at 350 °C in N2 show large-area highly ordered stripe-like and hexagonally arranged images. The results indicate well-ordered hexagonal arrays of mesopores with 1D channels [1]. There are also some collapses around the left area, which may be resulted from the quick removal of the surfactant in the carbonization. After the nanocomposite is heated at 500 and 700 °C, the TEM images (Fig. 3b and c) also show well-defined hexagonal channels, suggesting that ordered mesostructure is retained, consistent with the result from the XRD patterns. We also attempt to prepare pure Al2O3 and carbon with ordered mesostructures by removing the carbon via the calcination in air at

550 °C and etching the Al2O3 with HF solution, respectively. Lesser changes are found between the mesoporous C–Al2O3 nanocomposite and the mesoporous alumina after the combustion of carbon, as can be proved by TEM (Fig. 4), XRD (Fig. S2), and nitrogen adsorption/desorption techniques (Fig. S3). The mesoporous alumina CA50-A derived from CA-50-700 exhibits a big pore size of 8.7 nm, a surface area of 324 m2/g and a pore volume of 0.71 cm3/g. It is worthy to note that mesoporous carbon CA-50-C gained from CA-50700 shows a small pore size of 3.7 nm calculated from N2 sorption isotherms (Fig. S3). TEM images provide further evidence for the small pores. The mesoporous carbon has a high BET surface area of 1393 m2/g and a large pore volume of 1.28 cm3/g. The ratios of C/Al2O3 in the carbon–alumina nanocomposites can be easily tuned by varying the mass ratios of resol to aluminum iso-propoxide in triconstituent co-assembling process. Energy dispersive X-ray spectrums (EDS) (Fig. S4) of CA-50-700 is offered in the ‘‘Appendix A. Supplementary data”, and we can obtain that the actual mass ratio is quite close to the theoretical mass ratio. N2 adsorption/desorption isotherms of the nanocomposites with the different C/Al2O3 ratios are shown in Fig. S5(a). CA-20-700 and CA-50-700 exhibit similar type IV curves with distinct capillary condensation steps, corresponding to the narrow mesopore size distributions at the mean values of about 2.9 and 5.6 nm calculated from the BJH model, respectively (Table S1). The isotherm of the CA-80-700 could also be classified as a type IV isotherm according to the International Union of Pure and Applied Chemistry (IUPAC) nomenclature, [19] suggesting that it is mesoporous, however, its hysteresis loop is not H1-type. Fig. S5(b) shows that

(a)

Complex permittivity (ε′)

20

C-700 CA-20-700 CA-50-700 CA-80-700

15

10

5

0

2

4

6

8 10 12 Frequency (GHz)

14

16

18

(b) Complex permittivity (ε″)

8

C-700 CA-20-700 CA-50-700 CA-80-700

6

4

2

0 0 Fig. 4. TEM images of the products from CA-50-700: mesoporous alumina CA-50-A (a), and mesoporous carbon CA-50-C (b). The TEM images were recorded along the [1 1 0] directions.

2

4

6 8 10 12 Frequency (GHz)

14

16

18

Fig. 5. Real (a) and imaginary (b) parts of the complex relative permittivity of the nanocomposites in the frequency range of 0.5–18 GHz.

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the CA-80-700 has a narrow BJH pore size distribution with a mean size of 7.5 nm. On the other hand, as the Al2O3 content decreases, the BET surface areas increase. It is mainly ascribed to the microporosity generated from the amorphous carbon phase in the nanocomposites [17]. TEM images (Fig. S6) further confirm that the nanocomposites with low Al2O3 content (20 wt.% or 50 wt.%) have well-ordered mesostructures. In contrast, the mesostructure of the CA-80 is unstable and partially destroyed as calcinated at 700 °C. It further provides evidence that the incorporation of carbon can effectively improve the thermal stability of mesoporous nanocomposites. 3.2. Electromagnetic wave absorption properties of mesoporous C–Al2O3 nanocomposites In order to investigate the intrinsic reasons for electromagnetic wave absorption of the ordered mesoporous C–Al2O3 nanocomposites, we measured the complex permittivities and permeabilities of the epoxy resin/CA-700 samples and the epoxy resin/C-700 via the coaxial line. Fig. 5a shows the real part (e0 ) and imaginary parts (e00 ) of complex permittivities of the epoxy resin/C-700 and resin/CA700 samples in the frequency range of 0.5–18 GHz. It can be observed that the values of e0 and e00 of the epoxy resin/C-700 sample are larger than those of the epoxy resin/CA-700 samples. The real permittivity of the epoxy resin/C-700 sample decreases from 19.4 to 10.9 and is constant after 14.3 GHz. The real permittivities of the epoxy resin/CA-700 samples all decline over the 0.5–18 GHz frequency range. Thereinto, the real permittivity of the epoxy re-

sin/CA-50-700 is larger than those of the epoxy resin/CA-20-700 and the epoxy resin/CA-80-700. The real part values of relative dielectric permittivity for the epoxy resin/CA-700 samples are comparatively smaller than those for the C-metal or C-oxide composites as reported in the literature [20–23]. The lower real part value of complex relative dielectric permittivity shows a great advantage in striking a balance between permeability and permittivity, thus decreasing the reflection coefficient of the absorber compared with other metal magnetic materials for electromagnetic wave absorption application. Usually, the porous structure contributes to the decrease of the permittivity, so porous structure definitely decreases effective permittivity and benefits impedance match [24,25]. The imaginary permittivity of the epoxy resin/CA80-700 sample is below 1, which indicates a very poor dielectric loss. The imaginary permittivities of the epoxy resin/CA-50-700 and the epoxy resin/CA-20-700 decline from 3.8 to 1.5 and from 2 to 0.8 over the 0.5–18 GHz frequency range, respectively. Therefore, this larger dielectric loss is attributed to the intrinsically dielectric loss of amorphous carbon in these nanocomposites. Fig. 6a and b depict the variations of the real part (l0 ) and imaginary part (l00 ) in the 0.5 ’ 18 GHz. The real permeability of the epoxy resin/C-700 is lower than that of the epoxy resin/CA-700 between 0.5–13 GHz, but higher than that of the epoxy resin/CA-700 at high frequency. The real permeabilities of all the epoxy resin/CA700, are around 1.1 and nearly independent of frequency. Interestingly, it is observed that the imaginary permeabilities of the epoxy

(a)

-5

Complex permeability (μ′ )

Reflection loss (dB)

1.2

1.1

1.0

0.9

2

4

-15 C-700 CA-20-700 CA-50-700 CA-80-700

-25 0

6

8 10 12 Frequency (GHz)

14

16

2

4

18

6 8 10 12 Frequency (GHz)

14

16

18

(b)

0

0.2

Reflection loss (dB)

(b)

0.1

Complex permeability (μ″ )

-10

-20

C-700 CA-20-700 CA-50-700 CA-80-700 0

(a)

0

0.0

-5

2 3 4 5 6 7 8 9

-10

-0.1 C-700 CA-20-700 CA-50-700 CA-80-700

-0.2

-15

0

-0.3 0

2

4

6 8 10 12 Frequency (GHz)

14

16

2

4

6 8 10 12 Frequency (GHz)

14

16

18

18

Fig. 6. Real (a) and imaginary (b) parts of the complex relative permeability of the nanocomposites in the frequency range of 0.5–18 GHz.

Fig. 7. (a) Reflection loss for the nanocomposites, with a sample thickness of 3 mm in the frequency range of 0.5–18 GHz. (b) Reflection loss for the CA-50 nanocomposites, with different thickness (2, 3, 4, 5, 6, 7, 8 and 9 mm) in the frequency range of 0.5–18 GHz. In measurement, 40 wt.% sample and 60 wt.% epoxy resin were used.

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resin/C-700 are negative between 0.5 and 18 GHz, which indicates that magnetic energies are radiated out from these samples. It is considered that the motion of charges is responsible for the magnetic energy. According to the Maxwell equations, in an electromagnetic field the motion of charges will produce an AC electric field, and as a result induce a magnetic field [22]. The reflection loss (RL) of electromagnetic waves was calculated from the relative permeability and permittivity at the given frequency and absorber thickness using the following equations [26,27]:

RL ¼ 20 log jðZ in  Z 0 Þ=ðZ in þ Z 0 Þj

ð1Þ

rffiffiffiffiffiffi Z0 ¼

l0 e0

ð2Þ

rffiffiffiffiffi



lr 2pfd pffiffiffiffiffiffiffiffiffi lr er tanh j c er

Z in ¼ Z 0

 ð3Þ

where l0 and e0 are the complex relative permeability and permittivity of free space, Z0 is the impedance of free space, Zin is the input impedance of absorber, lr and er are the complex relative magnetic permeability and dielectric permittivity of the composite medium, respectively, f is the frequency of the electromagnetic wave, d is the thickness of an absorber, and c is the velocity of electromagnetic waves. Fig. 7a shows the reflection loss of the epoxy resin/C-700 and epoxy resin/CA-700 samples with a thickness of 3.0 mm in 0.5– 18 GHz. The maximum RL value for the epoxy resin/C-700 is mea-

1.0 0.8

3.3. Infrared emissivities of mesoporous C–Al2O3 nanocomposites

(a)

C-700 CA-20-700 CA-50-700 CA-80-700

tan δε

0.6 0.4 0.2 0.0 0

2

4

6 8 10 12 Frequency (GHz)

14

16

sured to be 11.6 dB at 5.9 GHz, but the effective absorption bandwidth, lower than 10 dB, is only 1.1 GHz. For the epoxy resin/CA700 samples, they exhibit wide frequency absorption characteristics. For the 3 mm-thicked layer, the relative absorption peaks are about 18.3, 15.3 and 13.3 dB and the bandwidths lower than 10 dB are 5.5, 7.4 and 2.2 GHz for the samples with Al2O3 content increasing, respectively. The calculated results with different thickness of the absorbers are shown in Fig. 7b, which indicates that the maximum reflection loss reaches 15.91 dB at 18 GHz for a layer of 2 mm thickness, while the absorption range under 10 dB is from 2.7 to 18 GHz for an absorber thickness of 2– 9 mm. Because of the porous structures in the nanocomposites, the energy dissipation of incident electromagnetic waves could take place many times, in the form of reflection and scattering [12,13,22]. The dielectric dissipation factors (tande = e00 /e0 ) of the mesoporous C–Al2O3 nanocomposites are almost changeless, the tande of C-700 appears a large change especially at high frequency (Fig. 8a). The magnetic dissipation factors (tandl = l00 /l0 ) of the mesoporous C–Al2O3 nanocomposites are bigger than that of C700 (Fig. 8). The l00 and tandl of the mesoporous C–Al2O3 nanocomposites are almost zero, so the electromagnetic wave absorption of the mesoporous C–Al2O3 nanocomposites and C-700 result mainly from dielectric loss rather than magnetic loss. In general, the excellent electromagnetic wave absorptions are strongly dependent on the efficient complementarities between the relative permittivity and permeability. Only magnetic loss or dielectric loss leads to weak EM attenuation [28]. Hence, the electromagnetic wave absorptions of all the mesoporous C–Al2O3 nanocomposites are better than that of C-700.

18

Fig. 9 indicates the infrared emissivities curves of the nanocomposites with different alumina contents. The results show that the incorporation of the alumina made the infrared emissivity of the nanocomposite fall sharply, and the infrared emissivity reached the lowest value of 0.45 with the 80 wt.% content of aluminia. This experimental phenomenon can be interpreted as follows: as we all know, the sum of the emissivity and reflectivity is 1 for the opacity materials. The reflectivity of alumina is higher than that of carbon, which means alumina has a lower emissivity. Besides, surface effect and microdimension effect of the mesoporous material also leads to the decrease of infrared emissivity. So the alumina can adjust the infrared emissivity of nanocomposite. But too much aluminia may destroy the ordered mesostructure, and also reduce

0.2

(b)

0.65

0.1 Infrared emissivity

tan δµ

0.0 -0.1 -0.2

C-700 CA-20-700 CA-50-700 CA-80-700

-0.3 -0.4

0.60 0.55 0.50 0.45

-0.5 0

2

4

6

8 10 12 Frequency (GHz)

14

16

18

Fig. 8. (a) Frequency dependence of tande of the resin/CA-700 samples and the epoxy resin/C-700. (b) Frequency dependence of tandl of the resin/CA-700 samples and the epoxy resin/C-700.

0

20 40 60 The alumina content (wt%)

80

Fig. 9. The average infrared emissivity at 8–14 lm of the coatings (C-700, CA-20700, CA-50-700 and CA-20-700) with a thickness of 35 lm using EPDM as an adhesive.

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the electromagnetic absorption. Consequently, based on the above discussion, the optimal content of alumina should be kept at 50 wt.% for the nanocomposite to acquire excellent electromagnetic wave absorption property and low infrared emissivity.

Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.micromeso.2010.05.007.

4. Conclusions

References

In summary, the preparation of ordered mesoporous C–Al2O3 nanocomposites with different alumina contents using triconstituent co-assembly strategy was introduced. Amphiphilic triblock copolymer (Pluronic F127) as the structure directing agent was used to organize the carbon precursor and the metal precursor molecules into a well-ordered mesoporous structure during the solvent evaporation in ethanol with a controlled ratio of water and acid at elevated temperature. The nanocomposite possesses ordered 2D hexagonal mesostructure, high surface area and large uniform pore size. We obtain ordered mesoporous alumina with large meso-tunnels to remove carbon by burning CA-50-700 in air at 550 °C, and simultaneously, we can also obtain ordered mesoporous carbon with a pore size of 3.7 nm, a pore volume of 1.28 cm3/g, and surface area of 1393 m2/g by etching alumina in HF solution. Ordered mesoporous nanocomposite CA-50 showed good electromagnetic wave absorption effects. The alumina in the nanocomposite had a very definite effect on the electromagnetic wave absorption property of the nanocomposite. The relative absorption peaks are about 18.3, 15.3 and 13.3 dB and the bandwidths lower than 10 dB are 5.5, 7.4 and 2.2 GHz for the CA-20-700, CA-50-700 and CA-80-700, respectively, as the layer thickness is 3 mm. Electromagnetic absorption property can be adjusted simply by controlling the thickness of the sample in the required frequency bands. The infrared emissivity of the CA-50 can be reduced to 0.46. As a result, it is possible to make a new light and thin highperformance radar/infrared multifunctional camouflage material with the ordered mesoporous C–Al2O3 nanocomposite.

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Acknowledgements The authors appreciate the financial support of the National Natural Science Foundation of China (50871053) and the Aeronautical Science Foundation of China (2007ZF52061).

Appendix A. Supplementary data