Effect of Pore Morphology on the Dielectric Properties of Porous

Article pubs.acs.org/JPCC

Effect of Pore Morphology on the Dielectric Properties of Porous Carbons for Microwave Absorption Applications Yunxia Huang,* Yan Wang, Zhimin Li, Zi Yang, Chunhao Shen, and Chuangchuang He School of Advanced Materials and Nanotechnology, Xidian University, Xi’an 710071, China ABSTRACT: The porous carbons (PCs) with tunable morphologies and pore sizes were prepared by the sol−gel process via a freezedrying technique for microwave absorption applications. The results of Raman spectroscopy and nitrogen sorption analysis showed that the graphitization degree was barely influenced as the ratio of tertbutanol (T) to resorcinol (R) decreased, while the pore morphologies changed from the disordered slit-shaped pores to the uniform cage-like pores. Dielectric properties of the as-prepared carbon samples were determined by a vector network analyzer in the frequency range of 8.2−12.4 GHz. Results showed that the effect of pore morphology on the dielectric loss of PCs was dominant in the case of similar graphitization. When the T/R ratio was 7.5, the sample with cage-like pores revealed the maximum values in the real part ε′ and the imaginary part ε″ of complex permittivity, which were 13.2−6.5 and 15.6−10.1, respectively, suggesting a better capacity of dielectric loss in the 8.2−12.4 GHz range. The proposed mechanism for the effect of the pore morphologies on microwave absorption performance was discussed.

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Liu et al.13 prepared biomorphic porous graphitic carbon materials by using a kind of plant biomass for electromagnetic interference shielding application. The porous graphitic carbons exhibited a shielding effectiveness of 40 dB over the X-band frequency, and the shielding by absorption is as high as 91%. It is well-known that the carbonization temperature can greatly affect the degree of graphitization of carbon materials, which in turn influences the microwave absorption of carbon materials.13,16 In addition to graphitization degree, the pore structure is another influential role in microwave absorption performance.13 The studies discussed above only focus on the effect of the degree of graphitization on the microwave absorption performance of PCs. However, the change of porous structure will inevitably influence the microwave absorption performance of PC materials. In this research, the PCs with a tailored pore structure were prepared using the resorcinol−furfural polycondensation process through changing the solvent content. The dielectric properties of a series of PCs from different pore structures in Xband frequency were systematically investigated. The effects of the degree of graphitization and pore structures on absorption capacity are discussed in detail. Furthermore, the proposed mechanism for the effect of the pore structures on microwave absorption performance will be discussed herein as well.

INTRODUCTION With rapid developments of information technology and extensive applications of electrical and electronic devices, electromagnetic wave radiation, as a potential kind of environmental pollution, is drawing considerable attention. In order to reduce such pollution, more attention has been paid to the development of microwave absorbing materials, which can dissipate incidental electromagnetic waves by the mechanism of magnetic or dielectric loss.1−3 However, conventional absorbing materials, composed of magnetic metal powders or ferrite, are restricted in application because of high specific gravity and formulation difficulty.4−6 As one of the ways to overcome these problems, carbon-based materials, such as carbon nanotubes (CNTs), carbon nanofibers (CNFs), and porous carbons (PCs), play a leading role in the investigation and application of microwave absorbing materials due to their advantages of lightweight, large surface area, good conductivity, and thermal and chemical stability.7−13 Because of the three-dimensionally (3D) interconnected carbon framework, PCs with excellent microwave absorption capacity13−15 become increasingly interesting. Li et al.11 prepared porous carbon fibers (PCFs) with pore sizes of 0.1−3 μm in diameter and found that the composites containing 6 wt % PCFs exhibited the lowest reflection loss of −31 dB at 9.7 GHz. The bandwidth with reflection loss below −5 dB covered the whole X band. Du et al.12 investigated the electromagnetic properties and microwave absorption of a series of ordered mesoporous carbons (OMCs) from different carbonization temperatures. The results showed that the mesoporous carbon obtained at 650 °C displayed a very strong reflection loss of −27.1 dB at 16.2 GHz and a wide response bandwidth covered from 4.8 to18 GHz over −10 dB. © 2014 American Chemical Society

Received: July 14, 2014 Revised: October 10, 2014 Published: October 16, 2014 26027

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Figure 1. (a) XRD patterns and (b) Raman spectra of the TR12, TR7.5, and TR5 samples.

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Microwave Dielectric Properties. Because of no dielectric loss of paraffin,19 the samples for dielectric parameter measurements at room temperature were prepared by mixing the produced powders with paraffin in a mass ratio of 1:9. Then, the mixtures were molded into an aluminum flange to fabricate rectangular composite samples with the dimensions of 22.86 mm (width) × 10.16 mm (length) × 1.5 mm (thickness). The dielectric parameters were determined by the waveguide technique with mode TE10 in the frequency range of 8.2−12.4 GHz. The prepared samples were set in a brass holder, which is filled in the waveguide. After being calibrated with an intermediate of a short circuit and blank holder, reflection and transmission coefficients were obtained using a vector network analyzer (Agilent Technologies E8362B, Palo Alto, CA), and then the real and imaginary parts of the permittivity were determined.

EXPERIMENTAL SECTION Materials Synthesis. The synthetic procedure of resorcinol−furfural derived PC materials is similar to that reported elsewhere.17,18 The furfural (99%, Aladdin) to resorcinol (R, ≥ 99.5%, Sinopharm Chemistry Reagent Co., Ltd.) molar ratio was set to 2, and the resorcinol to hexamine (AR, Sinopharm Chemistry Reagent Co., Ltd.) molar ratio was 50. The resorcinol and furfural were mixed together first, followed by the addition of the solvent tert-butanol (T, CP, Sinopharm Chemistry Reagent Co., Ltd.). Finally, hexamine was added as the catalyst. The mixture was stirred after each step of addition. The samples were sealed and cured in an oven at 80 °C for 7 days to allow for gelation and aging to strengthen the newly formed 3D gel network. The gelled samples were frozen first in a refrigerator, followed by drying under vacuum at room temperature in a vacuum drying oven. The dried samples were then pyrolyzed at a temperature of 900 °C for 180 min, with a heating rate of 5 °C/min under a nitrogen atmosphere. The variation of pore structures was achieved by altering the ratio of tert-butanol to resorcinol. The ratios of tert-butanol to resorcinol (T/R) were set to 12, 7.5, and 5 (hereinafter referred to as TR12, TR7.5, and TR5), respectively. Characterizations. The PC products synthesized from the resorcinol−furfural polycondensation process were identified by an X-ray diffractometer (XRD, DX-1000, Dandong Fangyuan Instrument Co. Ltd., Liaoning, China) with nickelfiltered Cu-Kα (λ = 1.54 Å) radiation. A Raman spectrometer (InVia, Renishaw, Old town, Gloucestershire, U.K.) was utilized, using the 514.5 nm line of an Ar ion laser as the excitation source for spectroscopic measurements. The morphologies of the cross sections of the samples were observed by field emission scanning electron microscopy (FESEM, Hitachi-S4800, Japan), and the compositions were analyzed by energy-dispersive spectroscopy (EDS, NORAN System SIX Model 300, Thermo Electron Corporation, Waltham, MA). Nitrogen sorption isotherms were analyzed by means of nitrogen sorption at −196 °C using a Micromeritics ASAP 2020 HD88 (USA). Samples were degassed at 250 °C under vacuum for 4 h prior to measurement. The Brunauer− Emmett−Teller (BET) method was used to determine the total surface area. For the mesopore surface area, pore volume, and pore diameter, the Barrett−Joyner−Halenda (BJH) method was used. The micropore area and pore volume were determined using the t-plot method. The total pore volumes (Vt) were estimated from the adsorbed amount at a relative pressure P/P0 of 0.994.

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RESULTS AND DISCUSSION Structure and Morphology. XRD is used to determine the structure of the as-prepared resorcinol−furfural derived PCs. Figure 1a shows the XRD patterns of the samples TR12.5, TR7.5, and TR5, respectively. It can be seen that the diffraction of the as-prepared samples exhibits two hump peaks around 2θ = 22.3° and 43.4°, respectively. The former peak can be indexed to the (002) diffraction of graphite; the latter is the (100) peak of the graphitic structure.20,21 The results indicate that disordered graphitic carbons have been prepared. Raman spectroscopy is widely used to characterize the structural and disordered information in carbon materials.22 The Raman spectra of the samples TR12, TR7.5, and TR5 with tested wavelengths between 1000 and 1800 cm−1 are shown in Figure 1b. The two peaks are observed approximately at the 1344 and 1586 cm−1 region, which corresponds to the D- and G-band, respectively. The D (defect)-band can be attributed to the breathing mode of κ-point phonons of A1g symmetry, which is forbidden in perfect graphite and becomes active in the presence of disorder or finite-size crystals of graphite (nanographite crystals). The G (graphitic)-band is assigned to the E2g phonon of sp2 carbon atoms, which can be produced by all sp2 sites and not only by graphitic carbon.12,20,22−24 The integrated intensity ratio of D- and G-bands (ID/IG) is employed to quantitate the defect of graphitic materials. In general, a lower ID/IG ratio indicates a higher degree of the graphitization in carbon materials.25,26 The ID/IG ratios of the samples TR12, TR7.5, and TR5 are 0.85, 0.89, and 0.88, respectively. It should be noted that these samples exhibit approximate ID/IG values, implying a similar ordering of the graphitic structures. The result confirms that the graphitic 26028

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Figure 2. Typical FESEM images of TR12 (a), TR7.5 (b), and TR5 (c) samples.

Figure 3. Nitrogen adsorption/desorption isotherms of the TR12 (a), TR7.5 and TR5 (b) samples.

Table 1. Pore Structure Parameters of Samples TR12, TR7.5, and TR5

a

sample ID

S*BET (m2 g−1)

Smeso (m2 g−1)

V*total (cm3 g−1)

Vmeso (cm3 g−1)

D* (nm)

TR12 TR7.5 TR5

741 571 513

240 158 158

1.24 0.46 0.38

0.99 0.25 0.20

11.0−18.2 11.0

S*: surface area; V*: pore volume; D*: pore diameter.

Figure 4. Pore size distribution curves of the TR12 (a), TR7.5 and TR5 (b) samples obtained by using the BJH model.

at relative pressures close to the saturation vapor pressure, is associated with capillary condensation in mesoporous.27 The type-H3 hysteresis loop is indicative of the aggregates (loose assemblages) of plate-like particles giving rise to slit-shaped pores.29 The sorption isotherms of the TR7.5 and TR5 samples shown in Figure 3b have similar shapes and display a type-IV isotherm with a type-H2 hysteresis loop.30 These hysteresis loops with an obvious capillary condensation step at high relative pressures are associated with capillary condensation taking place in mesopores.28 A broad type-H2 hysteresis loop at a relative pressure at 0.5 with delayed capillary evaporation can be observed in isotherms of samples TR7.5 and TR5, indicating the existence of uniform cage-like pores in both samples.29,30 Table 1 shows the corresponding porosimetry data of the samples TR12, TR7.5, and TR5. It can be seen that the TR12

structures of the as-prepared PCs are practically unchanged with the varying T/R ratio. Figure 2 shows the FESEM images of the cross section of the TR12, TR7.5, and TR5 samples, respectively. The 3D interconnected carbon framework has been constructed in the three samples, and macro- and mesopores are visible in the images. According to the EDS analysis, these samples are only composed of carbon element. Pore Texture Analysis. The surface areas and pore structures of the samples with different the T/R ratios are further clarified by nitrogen sorption analysis. Figure 3 shows the nitrogen adsorption/desorption isotherms of the TR12, TR7.5, and TR5 samples. The sorption isotherm of the TR12 sample shown in Figure 3a exhibits a type-IV isotherm with a type-H3 hysteresis loop.27,28 This hysteresis hoop, out-of-level 26029

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Figure 5. Real parts (a) and imaginary parts (b) of complex permittivity in the frequency range of 8.2−12.4 GHz for the TR12/paraffin, TR7.5/ paraffin, and TR5/paraffin composites.

Figure 6. Schematic illustration of the microwave dissipated mechanism for the as-prepared porous carbon samples: (a) electromagnetic wave attenuated mechanism; (b) model of slit-shaped pores; (c) model of cage-like pores.

sample has a total surface area (741 m2 g−1) and pore volume (1.24 cm3 g−1). With decreasing the solvent level, the total surface area drops from 571 m2 g−1 (TR7.5) to 513 m2 g−1 (TR5). In addition, the pore volume also exhibits a decrease from 0.46 cm3 g−1 (TR7.5) to 0.38 cm3 g−1 (TR5). Meanwhile, the mesopore surface area decreases from 240 m2 g−1 for the TR12 sample, to 158 m2 g−1 for TR7.5 and TR5 samples. Figure 4 shows the pore size distribution curves of the TR12, TR7.5, and TR5 samples calculated by using the BJH model. Note, the pore size distribution of the TR12 sample is different from that of the TR7.5 and TR5 samples. As shown in Figure 4a, the pore size distribution of the TR12 sample is quite broad, showing that the mesoporous are not uniform and macropores are present in the sample. However, the pore size distributions of the TR7.5 and TR5 samples are both very narrow with a distribution peak, centered at 11.0−18.2 and 11.0 nm, respectively. In other words, the samples TR7.5 and TR5 possess a highly ordered mesostructure. These results show that the solvent content can effectively affect the pore morphology, surface area, and pore size distribution of the samples with the same pyrolysis temperature. Microwave Dielectric Properties. The interactions between absorbers and applied electromagnetic field in the microwave frequency band can be determined by the complex permittivity ε (ε = ε′ − jε″) and complex permeability μ (μ = μ′ − jμ″). The real parts ε′ and μ′ determine the storage capability of electric and magnetic energy. The imaginary parts ε″and μ″ represent the loss energy dissipative mechanisms in absorbers.31 It is beneficial for microwave absorption when the absorber has large imaginary parts of complex permittivity and permeability. Only the complex permittivities of the asprepared samples are investigated due to that PCs are nonmagnetic materials with nearly constant μ′ (∼1) and μ″ (∼0) values.12 Figure 5 shows the real parts ε′ and imaginary parts ε″ of the complex permittivity as a function of frequency

in the frequency range of 8.2−12.4 GHz for the TR12/paraffin, TR7.5/paraffin, and TR5/paraffin composite samples, respectively. With the reduced T/R ratio, the ε′ and ε″ first increase and then decrease. When the T/R ratio is 7.5, ε′ and ε″ values of the composite are 13.2−6.5 and 15.6−10.1, respectively, which are the maximum values in all samples. The higher ε″ suggests a better capacity of dielectric loss in the microwave range. The dielectric loss performance of the as-prepared samples is superior to that of the porous carbon fibers and similar to that of the mesoporous carbon in the frequency range of 8.2−12.4 GHz as compared to the results in refs 11and 12. In addition, as shown in Figure 5a, the ε′ values of the TR12/ paraffin and TR5/paraffin composites exhibit a fluctuation as the frequency increases. However, the ε′ value of the TR7.5/ paraffin composite shows a dramatic decrease with increasing frequency. It can also be noted that the ε″ values of these samples obviously reduce with increasing frequency. The characteristic behavior as a function of frequency, the significant decrease in the ε′ and ε″ with increasing applied frequency, is called frequency dispersion behavior, which is beneficial for the impedance matching of the incident electromagnetic wave.11,32 The mechanism for how the incident electromagnetic wave is dissipated in the PCs/paraffin composites is closely associated with the graphitic frameworks as well as the pore structure of the materials. A schematic diagram is proposed to describe microwave dissipated mechanisms of the as-prepared PC/ paraffin samples, as illustrated in Figure 6. An applied electromagnetic wave passing through the PC/ paraffin materials would be attenuated by reflection, absorption, and multiple reflections (shown in Figure 6a). First, the samples consist of two phases, PCs phase with higher conductivity and paraffin phase with extremely lower conductivity. This heterogeneous system will exhibit interfacial polarization also known as the Maxwell−Wagner−Sillars (MWS) effect or space charge polarization,33−36 because of 26030

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similar degree of graphitization. When the T/R ratio was 7.5, the composite sample had the maximum ε′ and ε″ values, which were 13.2−6.5 and 15.6−10.1, respectively. It suggested a better capacity of dielectric loss in the X-band frequency. The graphitization degree and pore size distribution were determined by Raman spectra analysis and nitrogen adsorption/desorption isotherms, respectively, from which the exact mechanism of microwave absorption was concluded. This research will provide a new insight for the design and preparation of lightweight and highly effective microwave absorbers in the future and offer some fundamental understanding of the mechanism that pore structures improve the microwave dielectric performance of PCs.

vastly different electrical conductivities of carbon and paraffin. The Maxwell−Wagner polarization can obviously increase the dielectric loss of the PCs/paraffin composite as compared to single paraffin.36 However, the influence of the interfacial polarization on the contribution of dielectric losses for the PCs/paraffin composites is almost alike due to their similar conductivity arising from graphitization degree of the PC samples according to Raman analysis. Next, the PC materials can be treated as an effective medium as a mixture of component and void space.12,13,37 The effective permittivity of a porous material can be determined by the Maxwell−Garnett theory.15,38 The presence of the pore structure facilitates the decrease of the effective permittivity and achieves impedance match, resulting in the improved microwave absorption.12,39 Furthermore, according to Raman spectra analysis, the ID/IG values of the samples display no significant change with decreased T/R ratio. The result shows that the solvent content barely has an impact on the graphitic structure of the PCs herein. In this case, the pore structures should play the key effect on microwave absorption performance through increasing the reflection and multireflections inside the PCs. From the nitrogen adsorption/desorption isotherms, the void space is composed of the inhomogeneous slit-shaped pores in the TR 12 sample and uniform cage-like pores in TR7.5 and TR5 samples, as shown in Figure 6b,c, respectively. The presence of uniform cage-like pores in the PC is equivalent to the occurrence of cavities for microwave, implying that an appropriate space allows for the reflection and multireflections of microwave in the cavities. The multiple reflections should cause more energy to be dissipated inside because they make a longer microwave travel in the PC.11,39 However, the slitshaped pores in the PC are similar to the tiny plane for microwave, which gives rise to the specular reflection of the microwave in the plane. The specular reflection leads to less energy loss due to a decrease in the propagation paths of the microwave in the PC. Therefore, the TR7.5 and TR5 samples have higher ε″ values than the TR12 sample, reflecting their good microwave absorption capacity. In addition, according to the pore size distribution curves, the pore sizes of the TR7.5 sample are larger than those of the TR5 sample, which is beneficial for multireflections of the microwave in the sample. As a result, the TR7.5 sample exhibits excellent absorption ability toward the incident electromagnetic wave at 8.2−12.4 GHz. To summarize, the pore size distributions and pore morphologies for the PCs would make an important contribution to microwave absorption by increasing the reflection and multireflections of the electromagnetic wave power transmitted into the samples with the similar graphitization nanostructures. However, more investigations are still required to clarify the effect of the pore structures on the microwave absorption.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86-29-81891324. Fax: +86-29-81891417. E-mail: h_ [email protected] (Y.H.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the Fundamental Research Funds for the Central University of China (JB141405). REFERENCES

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CONCLUSIONS Sol−gel processing via freeze-drying is a versatile chemical synthesis route for the fabrication of highly porous amorphous graphitic carbon for microwave absorption applications. The variation of the solvent content plays the important role in tuning the pore structure of the carbon materials. With the decreased T/R ratio, the degrees of graphitization were barely influenced while the pore structures changed from the disordered slit-shaped pores to the uniform cage-like pores. Furthermore, the pore structures were the primary factor to influence microwave dielectric properties of the PCs under the 26031

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