- Email: [email protected]
The electromagnetic properties and microwave absorption of mesoporous carbon
Materials Chemistry and Physics 135 (2012) 884e891
Contents lists available at SciVerse ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
The electromagnetic properties and microwave absorption of mesoporous carbon Yunchen Du a, *, Tao Liu a, Bin Yu a, Haibin Gao a, Ping Xu a, Jingyu Wang a, Xiaohong Wang b, Xijiang Han a, * Chemistry Laboratory Center, Department of Chemistry, Harbin Institute of Technology, Harbin 150001, China Beijing Institute of Aeronautical Materials, Beijing 100095, China
h i g h l i g h t s < Electromagnetic property and microwave absorption of mesoporous carbon are studied. < Effects of graphitization degree and pore structure are systematically investigated. < The presence of pore system can improve microwave absorption of carbon materials. < Optimum mesoporous carbon can be used as excellent microwave absorbers.
g r a p h i c a l a b s t r a c t 5
Disordered Meosporous Carbon Ordered Mesoporous Carbon Nonporous Carbon
0 -5
Incident Microwave
b
Reflection loss (dB)
a
-10 -15 -20 -25 2
4
6
8
10
12
14
16
18
Frequency (GHz)
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 December 2011 Received in revised form 30 March 2012 Accepted 25 May 2012
The electromagnetic properties and microwave absorption of a series of ordered mesoporous carbon from different carbonization temperatures are investigated. The degree of graphitization of mesoporous carbon is gradually increased with increasing carbonization temperature, leading to a sudden increase in complex permittivity and negative imaginary part of complex permeability. The microwave absorption behaviors calculated from measured complex permittivity and complex permeability suggest that the optimum sample, MC-650 (mesoporous carbon obtained at 650 C), exhibits very strong reflection loss (27.1 dB at 16.2 GHz) and wide response bandwidth (4.8e18 GHz over 10 dB) due to its well matched characteristic impedance and dielectric loss. By comparing the electromagnetic properties and microwave absorption of disordered mesoporous carbon and nonporous carbon materials, it can be concluded that the degree of graphitization is the primary factor affecting the microwave absorbing properties of carbon materials. In the case of a relatively high degree of graphitization, the carbon materials with different porous structure show very similar reflection losses. Only for the samples with the appropriate degree of graphitization, pore structure and degree of order will also play important roles, which can not only improve characteristic impedance, but also increase the reflection loss. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Amorphous materials Microstructure Raman spectroscopy Dielectric properties
1. Introduction With the expanded applications of electromagnetic technology in civil and military fields, there is an explosive increase in GHz range electromagnetic waves in recent years. It becomes urgent to design * Corresponding authors. Tel.: þ86 451 86413702; fax: þ86 451 86418750. E-mail addresses: [email protected], [email protected] (Y. Du), [email protected] (X. Han). 0254-0584/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2012.05.074
and fabricate microwave absorbers to eliminate or decrease the ensuing problems in electromagnetic interference, human health, environmental pollution, etc. As a kind of typical candidates, carbon materials receive intensive attention due to their tunable properties, relative low density, abundant resource, easy preparation, and low cost. Up to now, a series of carbon materials with different forms, such as carbon nanotubes (CNT), carbon nanofibers (CNF), carbon nanocoils, carbon foams, graphene, etc., have been utilized as main components of novel microwave absorbers [1e10]. Although most of
Y. Du et al. / Materials Chemistry and Physics 135 (2012) 884e891
them can attenuate electromagnetic waves effectively by dielectric and magnetic losses, it remains a challenge to find new carbon materials with excellent properties. Recently, highly ordered mesoporous carbon materials with large surface area, high porosity, uniform pore size, appear as a class of multi-functional nanomaterials with promising applications in the fields of catalysis, adsorption and separation, storage of hydrogen and methane, supercapacitor, etc [11e15]. It is generally accepted that a porous material is composed of two phases: solid and void. If the size of voids is much smaller than an incident wavelength, scattering will not occur and the material will behave as an “effective medium” [16], whose effective permittivity can be determined by MaxwelleGarnet theory [17], MG
3 eff
¼
ð3 2 þ 23 1 Þ þ 2fr ð3 2 3 1 Þ 3 ð3 2 þ 23 1 Þ fr ð3 2 3 1 Þ 1
(1)
where 3 1 and 3 2 are the permittivity of the solid and void, respectively, and fr corresponds to the volume fraction of the voids in the effective medium. Based on this theory, it can be concluded that the presence of pore structure can decrease the effective permittivity and optimize characteristic impedance, resulting in the improved microwave absorption [8,9,18]. By considering the uniform pore structure in mesoporous carbon, it should be very significant to design and prepare novel microwave absorbing materials on the basis of mesoporous carbon. However, there are only a few literature concerning the microwave absorption of mesoporous carbon composites. For example, Wang et al. reported an as-sintered dense ordered mesoporous carbon (CMK-3) and fused silica composite, which exhibited excellent electromagnetic interference shielding efficiency in X band, and the contribution of microwave absorption was found to be much higher than that of the reflection [19]; He et al. also designed ordered mesoporous carbon composites CeSiO2eFe [20] and CeAl2O3 [21] as novel microwave absorbers that showed considerable reflection loss in the frequency range of 2e18 GHz. Imperfectly, all of these studies only focus on the microwave absorptions of composites without paying attention to the change of mesoporous carbons during the preparation process. It is well known that the carbonization temperature and the amount of transition metal can greatly affect the degree of graphitization of carbon materials, which closely relates to the microwave absorption of carbon materials [22]. Different preparative conditions and chemical compositions of composites will inevitably lead to different contribution of mesoporous carbon to microwave absorption, thus it is unfavorable to formulate and normalize mesoporous carbon-based microwave absorbers. Only by accurately understanding the electromagnetic behaviors of pristine mesoporous carbon materials, it will be conducive to discuss the underpinning mechanism of microwave absorption and design excellent microwave absorbers. To make clear the afore-mentioned problems, the electromagnetic properties and microwave absorption of a series of pristine ordered mesoporous carbon from different carbonization temperatures are systematically investigated in this article. Moreover, the effects of the degree of graphitization and pore structure are also discussed in detail by comparing with the electromagnetic properties and microwave absorption of disordered mesoporous carbon and nonporous carbon.
885
1.1 g of F127 were dissolved in a mixture solution containing 4.5 mL of EtOH and 4.5 mL of HCl aqueous solution (3.0 M). To this solution, 1.3 g of formaldehyde (37%) was then added. After stirring for 40 min at room temperature, the phenolic resins gel was obtained by centrifugation at 9500 rpm for 4 min, which was then loaded on a large Petri dish, dried at room temperature overnight, and cured at 80 C and 120 C for 24 h, respectively. Subsequently, the resultant polymers were carbonized in a horizontally tubular furnace under N2 atmosphere for 3 h with a heating rate of 1 C min1, where the carbonization temperatures are 600 C, 650 C, 700 C and 750 C, respectively. The final black solids are denoted as MC-x, where x refers to the carbonization temperature. For comparison, another two kinds of carbon materials, disordered mesoporous carbon (DC-x) and nonporous carbon (NC-x), were also prepared. DC-x was typically synthesized as following: (1) 1.1 g of resorcinol was dissolved in a mixture solution containing 4.5 mL of EtOH and 4.5 mL of HCl aqueous solution (3.0 M). Then 1.3 g of formaldehyde (37%) was added; (2) About 1 h later, a solid gel was obtained and cut into small pieces; (3) These small pieces of gel were heat-treated and carbonized under the same condition as that of MC-x. NC-x was prepared from resol precursors, a lowmolecular-weight and soluble phenolic resin [24], which was poured into a Petri dish to evaporate ethanol at room temperature for 5e8 h, followed by heating in an oven at 100 C for 24 h. Carbonization was carried out in a horizontally tubular furnace under the same condition as MC-x. 2.2. Characteriztion Nitrogen adsorption isotherms were obtained at 196 C on a Micromeritics ASAP 2020. Samples were normally prepared for measurement by degassing at 120 C until a final pressure of 1103 Torr was reached. The specific surface area was calculated using the BrunauereEmmeteTeller (BET) method from the nitrogen adsorption data in the relative range (P/P0) of 0.05e0.20. Pore size distributions were derived from the adsorption branch of the isotherm using the BarretteJoynereHalenda (BJH) method, and collected from pores between 1 and 1000 Å diameter. The total pore volume was determined from the amount of N2 uptake at P/ P0 ¼ 0.99. Powder X-ray diffraction (XRD) data were recorded on a Rigaku D/Max-2550 (50 kV, 200 mA) using nickel-filtered Cu Ka radiation with wavelength of k ¼ 1.5406 Å. Transmission electron micrograph (TEM) image was obtained on Tecnai G2 F30 operating at an accelerating voltage of 300 kV. Raman spectra were performed on a Jobin Yvon HR 800 micro-Raman spectrometer at 457.9 nm. A HP-5783E vector network analyzer was applied to determine the complex permeability and permittivity in the frequency range of 2e18 GHz for the calculation of reflection loss. A sample containing 60 wt % carbon powders was pressed into a ring with an outer diameter of 7 mm, an inner diameter of 3 mm, and a thickness of 2 mm for microwave measurement in which paraffin wax was used as the binder. The reflection loss, R(dB), of an absorber can be deduced from the following equation,
Z 1 RðdBÞ ¼ 20log in Z þ 1
(2)
in
Zin refers to the normalized input impedance of a metal-backed microwave absorbing layer and is given by [25,26]
2. Experimental section
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi mr pffiffiffiffiffiffiffiffiffi 2p fd mr 3 r tanh j 3r c
2.1. Sample preparation
Zin ¼
Highly ordered mesoporous carbon was prepared according to previous literature [23]. In a typical synthesis, 1.1 g of resorcinol and
where 3 r (3 r ¼ 3 0 j3 00 ) and mr (mr ¼ m0 jm00 ) are the complex permittivity and permeability respectively, of the composite
(3)
886
Y. Du et al. / Materials Chemistry and Physics 135 (2012) 884e891
medium, c is the velocity of electromagnetic waves in free space, f is the frequency of microwave, and d is the thickness of absorbers.
Table 1 Porous parameters and ID/IG values of various samples. Sample
BET surface (m2 g1)
Pore volume (cm3 g1)
Pore size (nm)
ID/IG
MC-600 MC-650 MC-700 MC-750 NC-650 NC-700 DC-650 DC-700
633 640 630 631 57 72 544 557
0.68 0.68 0.65 0.64 0.023 0.029 0.57 0.55
6.5 6.6 6.5 6.5 e e e e
0.688 0.735 0.790 0.837 0.738 0.792 0.733 0.794
3. Results and discussion Fig. 1 shows the N2 adsorptionedesorption isotherms and pore size distributions of various carbon materials (MC-x, DC-x and NCx), and their textural parameters are summarized in Table 1. As observed, mesoporous carbons, MC-x, give standard IV-type isotherms with a sharp capillary condensation step at relative pressure (P/P0) from 0.4 to 0.8 and narrow pore size distributions centered at 6.5e6.6 nm (Fig. 1a), which are quite similar to those of mesoporous carbon prepared by the same method [23], indicating their highly ordered mesostructures. Additionally, small-angle XRD patterns and TEM images of MC-650 and MC-750 are shown in Fig. 2. It is clear that they display three well-resolved peaks that can be indexed into (100), (110), and (200) diffractions of 2-D hexagonal symmetry (P6mm), as well as uniformly hexagonal pore system, further confirming their highly ordered mesostructures. It is very interesting to note that the texture parameters, such as surface area and pore size, of MC-x carbonized at different temperature are very close, implying that mesoporous carbons prepared in this method
a
4.5
700 3.5
500
(D)
400
(C)
300
(B)
200
(A)
dV/dlog(D) (cm3/g · nm)
3
Adsorbed Volume (cm /g, STP)
4.0
600
3.0
(D)
2.5 2.0
(C)
1.5 1.0
(B)
0.5
100
(A)
0.0
0 0.0
0.2
0.4
0.6
0.8
1.0
0
700
0.8
600
0.7
dV/dlog(D) (cm3/g · nm)
500
3
Adsorbed Volume (cm /g, STP)
b
400
(H) 300 200
(G)
100
(F)
10
20
30
Pore size (nm)
Relative pressure (P/P0)
(H)
0.6
(G) 0.5 0.4 0.3 0.2
(F) 0.1
(E) 0 0.0
(E) 0.2
0.4
0.6
0.8
Relative pressure (P/P0)
1.0
0.0 0
20
40
60
80 100
Pore size (nm)
Fig. 1. N2 adsorption/desorption isotherms and pore size distribution of MC-x (a) and NC-x and DC-x (b). (A) MC-600, (B) MC-650, (C) MC-700, (D) MC-750, (E) NC-650, (F) NC-700, (G) DC-650, and (H) DC-700. Isotherms B, C, D, F, and H have been offset by 100, 200, 300, 50, 150 cm3 g1 along the vertical axis for clarity, respectively.
possess good thermal stability, which is consistent with previous report [23]. Pore volume presents a slight decrease with increasing carbonization temperature (Table 1), this can be attributed to the slight shrinkage of mesoporous framework during the carbonization process [24]. Also, a small abnormity can be detected in MC600 at relative pressure 0.4e0.5 (marked with a circle in Fig. 1a), which may arise from the surfactants that are not completely removed at current temperature and the trace residues that block part of mesopore system [27]. For comparisons, NC-x and DC-x are both prepared in the absence of surfactants, however, they exhibit completely different N2 isotherms and structural parameters (Fig. 1b and Table 1). NC-650 and NC-700 give extremely poor N2 adsorption, very low BET surface area and pore volume, and ambiguous pore size distribution, suggesting that they are almost free of porosity. DC-650 and DC-700 show isotherms between IItype and IV-type, characteristic of mesoporous materials according to the IUPAC classification [28]. However, their pore size distributions (Fig. 1b) are quite broad and nitrogen amounts absorbed rise very steeply at high relative pressure (P/P0 > 0.7), indicating that these mesopores are not uniform and some macropores are also present [28,29]. The existence of porosity in DC650 and DC-700 stimulates the increase in BET surface area and pore volume, but still smaller than those of MC-x (Table 1). As shown in Fig. 3, all mesoporous carbons, MC-x, display two distinguishable peaks in the range of 800e2000 cm1. One is a broad peak centered at about 1350 cm1 (D band), the other is a relatively sharp peak at about 1590 cm1 (G band). The value of the intensity ratios of these two bands, ID/IG, monotonously increases from 0.688 to 0.837 with the increase of carbonization temperature (Table 1). According to previous literature [30e32], D band is a breathing mode of A1g symmetry involving phonons near the K zone boundary, which is forbidden in perfect graphite and becomes active in the presence of disorder or finite-size crystals of graphite (nanographite crystals); G band corresponds to the E2g mode due to stretching vibrations of sp2 bond, which can be produced by all sp2 sites and not only by graphitic carbon. It is reasonable to conclude that D band and G band herein originate from finite-size crystals of graphite and amorphous C sp2 sites, respectively, and the increasing ID/IG value stands for the gradually increased degrees of graphitization. Otherwise, there should be less amount of D band and more amount of G band, as well as gradually decreased value of ID/IG with increasing carbonization temperature. According to the phenomenological three-stage model proposed by Ferrari and Robertson [32], the gradually increased ID/IG values (0.675e0.818) of MC-x (x ¼ 600e750) are exactly in the transitional stage from amorphous carbon to nanocrystalline graphite, further confirming the formation of tiny crystalline domains at the surface of the materials. However, these tiny crystals are very difficult to detect, even undetectable by high-resolution transmission electron microscopy (HR-TEM) [33]. The increased ID/IG values can be also observed in other Raman spectra studies about amorphous carbonbased materials [34e36]. Very similar to the Raman spectra of MC-
Y. Du et al. / Materials Chemistry and Physics 135 (2012) 884e891
887
Intensity (a.u.)
Fig. 2. Small-angle XRD patterns (a) of MC-650 (A) and MC-750 (B). TEM images of MC-650 (b) and MC-750 (c) in the direction of (110) (Bar: 50 nm), and TEM image of MC-650 (d) in the direction of (100) (Bar: 20 nm).
(D)
(C)
(B) (A) 800
1000
1200
1400
1600
1800
2000
-1
Raman shift (cm ) Fig. 3. Raman spectra of (A) MC-600, (B) MC-650, (C) MC-700, and (D) MC-750.
x, NC-x and DC-x also exhibit two characteristic peaks in the range of 800e2000 cm1 and increasing ID/IG values with the increase of carbonization temperature (Fig. 4 and Table 1). Of particular note is that the samples carbonized at the same temperature, such as NC650, DC-650 and MC-650, give almost identical ID/IG values, indicating that these samples have comparable degrees of graphitization, although they have quite different textural parameters. Fig. 5 shows the real and imaginary parts of complex permittivity and complex permeability of MC-x (x ¼ 600e750) in the frequency range of 2e18 GHz. The real (3 0 ) and imaginary part (3 00 ) of complex permittivity of MC-600 are almost constant with nearly no variation throughout the whole frequency range (3 0 z 2.5, 00 3 z 0), implying its very poor dielectric loss. Compared with those of MC-600, both 3 0 and 3 00 of MC-x (x ¼ 650e750) are simultaneously improved, especially for MC-750, whose 3 0 and 3 00 are larger than 30 between 2 and 18 GHz and even exceed 100 in low frequency range, which should be attributed to the formation of more nanocrystalline graphite domains at high carbonization temperature and the consequent increase of electrical conductivities. In addition, it is discovered that the real (m0 ) and imaginary parts (m00 ) of complex permeability of these samples are also quite different. The m0 and m00 of MC-600 and MC-650 are approximately
888
Y. Du et al. / Materials Chemistry and Physics 135 (2012) 884e891
Intensity (a.u.)
(D)
(C)
(B) (A) 800
1000
1200
1400
1600
1800
2000
-1
Raman shift (cm ) Fig. 4. Raman spectra of (A) NC-650, (B) NC-700, (C) DC-650, and (D) DC-700.
constant and close to one and zero, respectively, indicating the negligible magnetic loss for incident electromagnetic wave [26]. In contrast, the m0 of MC-700 is significantly dependent on frequency, and it is lower than those of MC-600 and MC-650 in 6.4e12.8 GHz
b
140
Imaginary parts of permittivity (ε'')
Real parts of permittivity (ε')
a
frequency range, but higher than those in 12.8e18 GHz frequency range. This trend of m0 is further expanded in MC-750, while the intersecting point is at 11.6 GHz. Very interestingly, the m00 of MC700 and MC-750 are gradually decreased and negative in the whole frequency range, and MC-750 always retains smaller than MC-700. According to Maxwell equations, these phenomena, caused by the motion of charges in an electromagnetic field, suggest that the interaction between MC-x (x ¼ 700e750) and incident electromagnetic wave can induce variation of magnetic field and so that some magnetic energies can be radiated out from these samples [18,21]. Using equations (2) and (3), the reflection loss curves of MC-x are deduced with a thickness of 2.5 mm (Fig. 6). MC-600 exhibits a reflection loss of 0 dB in the whole frequency range due to its very poor dielectric loss and magnetic loss (Fig. 5). MC-650 shows an intensively increased reflection loss with a maximum of 21.5 dB at 14.2 GHz, and the reflection loss less than 10 dB (90% absorption) is in the range of 11.0e18 GHz. The improved microwave absorbing ability of MC-650 is superior or comparable to some of ever reported carbon-based composites, although there are no any profitable magnetic particles. With higher carbonization temperature, MC-700 and MC-750 show increased reflection losses at lower frequency accompanied by intense attenuation at higher frequency bands. The maximum reflection losses of MC-700 and MC-750 are only 5.1 dB at 4.8 GHz and 2.8 dB at 2.6 GHz, respectively. It is generally accepted that the real parts of complex permittivity (3 0 ) and permeability (m0 ) represent the storage capability of electric
MC-600 MC-650 MC-700 MC-750
120 100 80 60 40 20
140 MC-600 MC-650 MC-700 MC-750
120 100 80 60 40 20 0
0 2
4
6
8
10
12
14
16
2
18
4
6
Frequency (GHz)
d
1.4
Imaginary parts of permeability (μ '')
Real parts of permeability (μ ')
c
1.2
1.0
0.8 MC-600 MC-650 MC-700 MC-750
0.6
8
10
12
14
16
18
14
16
18
Frequency (GHz)
0.4
0.2 0.0 -0.2 -0.4 MC-600 MC-650 MC-700 MC-750
-0.6 -0.8 -1.0
2
4
6
8
10
12
Frequency (GHz)
14
16
18
2
4
6
8
10
12
Frequency (GHz)
Fig. 5. The real parts (a) and imaginary parts (b) of complex permittivity, and the real parts (c) and imaginary parts (d) of complex permeability of MC-x in the frequency range of 2e18 GHz.
Y. Du et al. / Materials Chemistry and Physics 135 (2012) 884e891
Reflection loss (dB)
0
-5
-10 MC600 MC650 MC700 MC750
-15
-20
-25 2
4
6
8
10
12
14
16
18
Frequency (GHz) Fig. 6. The reflection loss of MC-x with an absorber thickness of 2.5 mm in the frequency range of 2e18 GHz.
and magnetic energy, and the imaginary parts (3 00 and m00 ) represent the loss of electric and magnetic energy [37]. Hence it is beneficial to microwave absorption if the absorbers have large imaginary parts of complex permittivity and permeability. Additionally, some published papers also propose that negative imaginary parts of complex permeability can produce geometrical effects to improve microwave absorption [38e40]. Herein, MC-700 and MC-750 show considerable imaginary parts of complex permittivity and negative imaginary parts of complex permeability, but they still fail to exhibit good reflection losses. It results from another important parameter, the concept of matched characteristic impedance, where the characteristic impedance of the absorbing materials should be equal/close to that of the free space (377 U sq1) to achieve zero-reflection at the front surface of the materials [41]. Overlarge difference between the complex permittivity and complex permeability cannot bring considerable microwave absorption since most of microwave will be reflected off at the surface of absorbers. In contrast, MC-650 exhibits better reflection
0
Reflection loss (dB)
-5 -10 -15
2.0 mm 2.2 mm 2.5 mm 3.0 mm 3.5 mm 4.0 mm 5.0 mm
-20 -25 -30 2
4
6
8
10
12
14
16
18
Frequency (GHz) Fig. 7. The reflection loss dependent on the thickness of MC-650 in the frequency range of 2e18 GHz.
889
loss than MC-700 and MC-750 owing to its suitable characteristic impedance, although it has lower 3 0 and 3 00 , as well as negligible magnetic loss. According to equation (3), the thickness (d) of absorbers can also affect the reflection loss; thus, the relationship between the thickness and the reflection loss of MC-650 is investigated in Fig. 7. The microwave frequency corresponding to the maximum reflection loss shifts negatively with the increase of thickness, and the value of reflection loss exceeding 10 dB can be obtained in the range of 4.8e18 GHz with a variation in thickness from 2.0 to 5.0 mm. A maximum of 27.1 dB at 16.2 GHz can be achieved when the thickness is 2.2 mm. These results further suggest that MC-650 will be a promising microwave absorber, whose absorption band can be simply modulated by manipulating the thickness to satisfy the applications in different frequency bands. As mentioned above, He et al. prepared ordered mesoporous carbon-based absorbers CeSiO2eFe and CeAl2O3 at 700 C, and they had to incorporate a large amount of SiO2eFe and Al2O3 particles (even up to 80 wt%) to optimize characteristic impedance and improve reflection loss, although it was at the expense of some structural properties, such as BET surface, pore volume, density, and so forth [20,21]. In fact, direct incorporating different amount of metal or metal oxides particles, particularly Fe species, into mesoporous carbon matrix will result in different degrees of graphitization of carbon materials during carbonization process and disordered complex permittivity of mesoporous carbon composites [20,22]. In these cases, it will be difficult to evaluate the contribution of mesoporous carbon on microwave absorption and design mesoporous carbon composites microwave absorbers. In this work, we optimize the characteristic impedance by adjusting the degrees of graphitization of mesoporous carbons and make pristine mesoporous carbon (MC-650) possess good microwave absorption comparable with those of CeSiO2eFe and CeAl2O3, which is more favorable for the design and preparation of novel and light-weight microwave absorbers. To study the effect of pore structure, the real and imaginary parts of complex permittivity and complex permeability of NC-x and DC-x are also investigated, as shown in Fig. 8. Very similar to those of MC-x, improving the degree of graphitization can increase complex permittivity and induce the variation of complex permeability. However, even with the similar degree of graphitization, the real and imaginary parts of complex permittivity of DC-x are obviously smaller than those of NC-x, and a little larger than those of MC-x. For example, the 3 0 values of DC-650, NC-650 and MC-650 decrease from 13.1, 22.9, and 11.4 to 5.3, 7.4, and 4.5 in the 2e18 GHz range, respectively; while the 3 00 values decrease from 10.5, 12.1, and 9.7 to 5.1, 7.6, and 3.2, respectively. It is reasonable to attribute these differences to their distinguishable pore structure, and conclude that rich pore structure can decrease the real and imaginary parts of complex permittivity, which is quite consistent with previous reports and MaxwelleGarnet theory [8,9,17,18]. Moreover, it can be found that the presence of rich pore structure can also decrease the variation of complex permeability. For instance, DC-700 and MC-700 exhibit relatively small m0 and negative m00 as compared with those of NC-700. Fig. 9 shows the calculated reflection loss curves of NC-x and DCx with a thickness of 2.5 mm in the frequency range of 2e18 GHz, which suggests that the reflection loss characteristics are sensitive to the degree of graphitization and pore structure. NC-700 and DC700 exhibit weak reflection loss properties in the whole frequency range, and their maximums are 5.0 dB at 5.1 GHz and 5.3 dB at 4.6 GHz, respectively, which are very close to that of MC-700 (Fig. 6), indicating that the difference of pore structure cannot bring improved reflection losses in current state. That is, the degree of graphitization is the primary factor for the microwave absorbing properties of carbon materials. However, pore structure will play an
890
Y. Du et al. / Materials Chemistry and Physics 135 (2012) 884e891
b
60 50
Imaginary parts of permittivity (ε'')
Rear parts of permittivity (ε')
a
NC650 NC700 DC650 DC700
40 30 20 10
60 NC650 NC700 DC650 DC700
50 40 30 20 10 0
0 2
4
6
8
10
12
14
16
2
18
4
6
d
1.4
Imaginary parts of permeability (μ'')
Real parts of permeability (μ ')
c
1.2
1.0
0.8 NC-650 NC-700 DC-650 DC-700
0.6
8
10
12
14
16
18
16
18
Frequency (GHz)
Frequency (GHz)
0.2 0.0 -0.2 -0.4 NC-650 NC-700 DC-650 DC-700
-0.6 -0.8 -1.0
0.4 2
4
6
8
10
12
14
16
18
2
4
6
8
10
12
14
Frequency (GHz)
Frequency (GHz)
Fig. 8. The real parts (a) and imaginary parts (b) of complex permittivity, and the real parts (c) and imaginary parts (d) of complex permeability of NC-x and DC-x in the frequency range of 2e18 GHz.
important role when these carbon materials have appropriate degrees of graphitization. For example, DC-650 with rich porosity exhibits larger reflection loss and wider response frequency range than NC-650 with poor porosity. Additionally, we summarize the reflection loss curves of NC-650, DC-650, and MC-650 in Graphical
0
Reflection loss (dB)
-5
Abstract. It is very interesting that the reflection loss of DC-650 is obviously less than that of MC-650, although both of them are rich porous materials with similar degree of graphitization. The difference in reflection loss may be explained from two aspects: (1) MC650 has larger BET surface and pore volume than DC-650, which are beneficial to lower 3 0 and 3 00 (Figs. 5 and 8) and thus improve matched characteristic impedance; (2) Compared with disordered and random pore system in DC-650, the pore system in MC-650 is highly homogenous and uniform, which is favorable for the multiple reflections and repeated consumption of incident electromagnetic wave inside the pores [20]. These two aspects are believed to lead to the significant improvement in microwave absorption of MC-650.
-10 4. Conclusions
-15
NC-650 NC-700 DC-650 DC-700
-20
-25 2
4
6
8
10
12
14
16
18
Frequency (GHz) Fig. 9. The reflection loss of NC-x and DC-x with an absorber thickness of 2.5 mm in the frequency range of 2e18 GHz.
By systematically investigating the degrees of graphitization, electromagnetic properties and microwave absorption of ordered mesoporous carbon, disordered mesoporous carbon and nonporous carbon materials, it can be concluded that the degree of graphitization is the primary factor affecting the microwave absorbing properties of carbon materials. When the degree of graphitization is relatively high, pore structure can decrease the real and imaginary parts of complex permittivity to some extent, but fails in producing improved reflection loss due to poor characteristic impedance. However, for the samples with the appropriate degree of graphitization, pore structure and degree of order
Y. Du et al. / Materials Chemistry and Physics 135 (2012) 884e891
will play quite important roles not only in improving characteristic impedance, but also in increasing reflection loss. For example, the optimum one, MC-650 with suitable degree of graphitization and uniform pore structure, exhibits very strong reflection loss and wide response bandwidth, even in the absence of any profitable magnetic particles. We believe this work will be helpful for the design and preparation of light-weight and highly effective microwave absorbers in the future. Acknowledgments This work is supported by National Natural Science Foundation of China (21003029, 21071037 and 21101041), Special Fund of Harbin Technological Innovation (2010RFXXG012) and Postdoctoral Science-research Development Foundation of Heilongjiang Province (LBH-Q11098). References [1] R.C. Che, L.M. Peng, X.F. Duan, Q. Chen, X.L. Liang, Adv. Mater. 16 (2004) 401. [2] X.C. Gui, W. Ye, J.Q. Wei, K.L. Wang, R.T. Lv, H.W. Zhu, F.Y. Kang, J.L. Gu, D.H. Wu, J. Phys. D: Appl. Phys. 42 (2009) 075002. [3] H. Zhu, H.Y. Lin, H.F. Guo, L.F. Yu, Mater. Sci. Eng. B 138 (2007) 101. [4] X.G. Liu, B. Li, D.Y. Geng, W.B. Cui, F. Yang, Z.G. Xie, D.J. Kang, Z.D. Zhang, Carbon 47 (2009) 470. [5] Y. Fan, H. Yang, X. Liu, H. Zhu, G.T. Zou, J. Alloy Compd. 461 (2008) 490. [6] N.J. Tang, Y. Yang, K. Lin, W. Zhong, C. Au, Y.W. Du, J. Phys. Chem. C 112 (2008) 10061. [7] M. Itoh, J.R. Liu, T. Horikawa, K. Machida, J. Alloy. Compd. 408e412 (2006) 1400. [8] Z.G. Fang, C.S. Li, J.Y. Sun, H.T. Zhang, J.S. Zhang, Carbon 45 (2007) 2873. [9] J. Yang, Z.M. Shen, Z.B. Hao, Carbon 42 (2004) 1882. [10] C. Wang, X.J. Han, P. Xu, X.L. Zhang, Y.C. Du, S.R. Hu, J.Y. Wang, X.H. Wang, Appl. Phys. Lett. 98 (2011) 072906. [11] R. Ryoo, S.H. Joo, S. Jun, J. Phys. Chem. B 103 (1999) 7743. [12] D.S. Su, J.J. Delgado, X. Liu, D. Wang, R. Schlogl, L.F. Wang, Z. Zhang, Z.C. Shan, F.S. Xiao, Chem. Asian J. 4 (2009) 1108. [13] Z.X. Wu, D.Y. Zhao, Chem. Commun. 47 (2011) 3332.
891
[14] K.S. Xia, Q.M. Gao, C.D. Wu, S.Q. Song, M.L. Ruan, Carbon 45 (2007) 1989. [15] A.B. Fuertes, G. Lota, T.A. Centeno, E. Frackowiak, Electrochim. Acta 50 (2005) 2799. [16] N.Z. Wing, B. Wang, J.W. Halloran, J. Am. Ceram. Soc. 89 (2006) 3696. [17] O. Levy, D. Stroud, Phys. Rev. B 56 (1997) 8035. [18] Q.L. Liu, D. Zhang, T.X. Fan, Appl. Phys. Lett. 93 (2008) 013110. [19] J.C. Wang, C.S. Xiang, Q. Liu, Y.B. Pan, J.K. Guo, Adv. Funct. Mater. 18 (2008) 2995. [20] J.H. Zhou, J.P. He, G.X. Li, T. Wang, D. Sun, X.C. Ding, J.Q. Zhao, S.C. Wu, J. Phys. Chem. C 114 (2010) 7611. [21] T. Wang, J.P. He, J.H. Zhou, X.C. Ding, J.Q. Zhao, S.C. Wu, Y.X. Guo, Micropor. Mesopor. Mater. 134 (2010) 58. [22] Y.C. Du, J.Y. Wang, C.K. Cui, X.R. Liu, X.H. Wang, X.J. Han, Synth. Met. 160 (2010) 2191. [23] X.Q. Wang, C.D. Liang, S. Dai, Langmuir 24 (2008) 7500. [24] Y. Meng, D. Gu, F.Q. Zhang, Y.F. Shi, L. Cheng, D. Feng, Z.X. Wu, Z.X. Chen, Y. Wan, A. Stein, D.Y. Zhao, Chem. Mater. 18 (2006) 4447. [25] S.S. Kim, S.B. Jo, K.I. Gueon, K.K. Choi, K.S. Churn, IEEE Trans. Magn. 27 (1991) 5462. [26] L. Du, Y.C. Du, Y. Li, J.Y. Wang, C. Wang, X.H. Wang, P. Xu, X.J. Han, J. Phys. Chem. C 114 (2010) 19600. [27] F. Schuth, A. Wingen, V. Sauer, Micropor. Mesopor. Mater. 44e45 (2001) 465. [28] M. Kruk, M. Jaroniec, Chem. Mater. 13 (2001) 3169. [29] T.Y. Ma, X.J. Zhang, Z.Y. Yuan, J. Phys. Chem. C 113 (2009) 12854. [30] R.J. Nemanich, S.A. Solin, Phys. Rev. B 20 (1979) 392. [31] R.O. Dillon, J.A. Woollam, V. Katkanant, Phys. Rev. B 29 (1984) 3482. [32] A.C. Ferrari, J. Robertson, Phys. Rev. B 61 (2000) 14095. [33] K.X. Wang, Y.G. Wang, Y.R. Wang, E. Hosono, H.S. Zhou, J. Phys. Chem. C 113 (2009) 1093. [34] R. Sergiienko, E. Shibata, S. Kim, T. Kinota, T. Nakamura, Carbon 47 (2009) 1056. [35] P.K. Chu, L.H. Li, Mater. Chem. Phys. 96 (2006) 253. [36] F. Li, J.S. Lannin, Appl. Phys. Lett. 61 (1992) 2116. [37] N. Chen, G.H. Mu, K.K. Gan, M.Y. Gu, Mater. Sci. Eng. B 139 (2007) 256. [38] A.N. Yusoff, M.H. Abdullah, S.H. Ahmad, S.F. Jusoh, A.A. Mansor, S.A.A. Hamid, J. Appl. Phys. 92 (2002) 876. [39] L.J. Deng, M.G. Han, Appl. Phys. Lett. 91 (2007) 023119. [40] C. Wang, X.J. Han, P. Xu, J.Y. Wang, Y.C. Du, X.H. Wang, W. Qin, T. Zhang, J. Phys. Chem. C 114 (2010) 3196. [41] P. Xu, X.J. Han, X.R. Liu, B. Zhang, C. Wang, X.H. Wang, Mater. Chem. Phys. 114 (2009) 556.
Contents lists available at SciVerse ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
The electromagnetic properties and microwave absorption of mesoporous carbon Yunchen Du a, *, Tao Liu a, Bin Yu a, Haibin Gao a, Ping Xu a, Jingyu Wang a, Xiaohong Wang b, Xijiang Han a, * Chemistry Laboratory Center, Department of Chemistry, Harbin Institute of Technology, Harbin 150001, China Beijing Institute of Aeronautical Materials, Beijing 100095, China
h i g h l i g h t s < Electromagnetic property and microwave absorption of mesoporous carbon are studied. < Effects of graphitization degree and pore structure are systematically investigated. < The presence of pore system can improve microwave absorption of carbon materials. < Optimum mesoporous carbon can be used as excellent microwave absorbers.
g r a p h i c a l a b s t r a c t 5
Disordered Meosporous Carbon Ordered Mesoporous Carbon Nonporous Carbon
0 -5
Incident Microwave
b
Reflection loss (dB)
a
-10 -15 -20 -25 2
4
6
8
10
12
14
16
18
Frequency (GHz)
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 December 2011 Received in revised form 30 March 2012 Accepted 25 May 2012
The electromagnetic properties and microwave absorption of a series of ordered mesoporous carbon from different carbonization temperatures are investigated. The degree of graphitization of mesoporous carbon is gradually increased with increasing carbonization temperature, leading to a sudden increase in complex permittivity and negative imaginary part of complex permeability. The microwave absorption behaviors calculated from measured complex permittivity and complex permeability suggest that the optimum sample, MC-650 (mesoporous carbon obtained at 650 C), exhibits very strong reflection loss (27.1 dB at 16.2 GHz) and wide response bandwidth (4.8e18 GHz over 10 dB) due to its well matched characteristic impedance and dielectric loss. By comparing the electromagnetic properties and microwave absorption of disordered mesoporous carbon and nonporous carbon materials, it can be concluded that the degree of graphitization is the primary factor affecting the microwave absorbing properties of carbon materials. In the case of a relatively high degree of graphitization, the carbon materials with different porous structure show very similar reflection losses. Only for the samples with the appropriate degree of graphitization, pore structure and degree of order will also play important roles, which can not only improve characteristic impedance, but also increase the reflection loss. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Amorphous materials Microstructure Raman spectroscopy Dielectric properties
1. Introduction With the expanded applications of electromagnetic technology in civil and military fields, there is an explosive increase in GHz range electromagnetic waves in recent years. It becomes urgent to design * Corresponding authors. Tel.: þ86 451 86413702; fax: þ86 451 86418750. E-mail addresses: [email protected], [email protected] (Y. Du), [email protected] (X. Han). 0254-0584/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2012.05.074
and fabricate microwave absorbers to eliminate or decrease the ensuing problems in electromagnetic interference, human health, environmental pollution, etc. As a kind of typical candidates, carbon materials receive intensive attention due to their tunable properties, relative low density, abundant resource, easy preparation, and low cost. Up to now, a series of carbon materials with different forms, such as carbon nanotubes (CNT), carbon nanofibers (CNF), carbon nanocoils, carbon foams, graphene, etc., have been utilized as main components of novel microwave absorbers [1e10]. Although most of
Y. Du et al. / Materials Chemistry and Physics 135 (2012) 884e891
them can attenuate electromagnetic waves effectively by dielectric and magnetic losses, it remains a challenge to find new carbon materials with excellent properties. Recently, highly ordered mesoporous carbon materials with large surface area, high porosity, uniform pore size, appear as a class of multi-functional nanomaterials with promising applications in the fields of catalysis, adsorption and separation, storage of hydrogen and methane, supercapacitor, etc [11e15]. It is generally accepted that a porous material is composed of two phases: solid and void. If the size of voids is much smaller than an incident wavelength, scattering will not occur and the material will behave as an “effective medium” [16], whose effective permittivity can be determined by MaxwelleGarnet theory [17], MG
3 eff
¼
ð3 2 þ 23 1 Þ þ 2fr ð3 2 3 1 Þ 3 ð3 2 þ 23 1 Þ fr ð3 2 3 1 Þ 1
(1)
where 3 1 and 3 2 are the permittivity of the solid and void, respectively, and fr corresponds to the volume fraction of the voids in the effective medium. Based on this theory, it can be concluded that the presence of pore structure can decrease the effective permittivity and optimize characteristic impedance, resulting in the improved microwave absorption [8,9,18]. By considering the uniform pore structure in mesoporous carbon, it should be very significant to design and prepare novel microwave absorbing materials on the basis of mesoporous carbon. However, there are only a few literature concerning the microwave absorption of mesoporous carbon composites. For example, Wang et al. reported an as-sintered dense ordered mesoporous carbon (CMK-3) and fused silica composite, which exhibited excellent electromagnetic interference shielding efficiency in X band, and the contribution of microwave absorption was found to be much higher than that of the reflection [19]; He et al. also designed ordered mesoporous carbon composites CeSiO2eFe [20] and CeAl2O3 [21] as novel microwave absorbers that showed considerable reflection loss in the frequency range of 2e18 GHz. Imperfectly, all of these studies only focus on the microwave absorptions of composites without paying attention to the change of mesoporous carbons during the preparation process. It is well known that the carbonization temperature and the amount of transition metal can greatly affect the degree of graphitization of carbon materials, which closely relates to the microwave absorption of carbon materials [22]. Different preparative conditions and chemical compositions of composites will inevitably lead to different contribution of mesoporous carbon to microwave absorption, thus it is unfavorable to formulate and normalize mesoporous carbon-based microwave absorbers. Only by accurately understanding the electromagnetic behaviors of pristine mesoporous carbon materials, it will be conducive to discuss the underpinning mechanism of microwave absorption and design excellent microwave absorbers. To make clear the afore-mentioned problems, the electromagnetic properties and microwave absorption of a series of pristine ordered mesoporous carbon from different carbonization temperatures are systematically investigated in this article. Moreover, the effects of the degree of graphitization and pore structure are also discussed in detail by comparing with the electromagnetic properties and microwave absorption of disordered mesoporous carbon and nonporous carbon.
885
1.1 g of F127 were dissolved in a mixture solution containing 4.5 mL of EtOH and 4.5 mL of HCl aqueous solution (3.0 M). To this solution, 1.3 g of formaldehyde (37%) was then added. After stirring for 40 min at room temperature, the phenolic resins gel was obtained by centrifugation at 9500 rpm for 4 min, which was then loaded on a large Petri dish, dried at room temperature overnight, and cured at 80 C and 120 C for 24 h, respectively. Subsequently, the resultant polymers were carbonized in a horizontally tubular furnace under N2 atmosphere for 3 h with a heating rate of 1 C min1, where the carbonization temperatures are 600 C, 650 C, 700 C and 750 C, respectively. The final black solids are denoted as MC-x, where x refers to the carbonization temperature. For comparison, another two kinds of carbon materials, disordered mesoporous carbon (DC-x) and nonporous carbon (NC-x), were also prepared. DC-x was typically synthesized as following: (1) 1.1 g of resorcinol was dissolved in a mixture solution containing 4.5 mL of EtOH and 4.5 mL of HCl aqueous solution (3.0 M). Then 1.3 g of formaldehyde (37%) was added; (2) About 1 h later, a solid gel was obtained and cut into small pieces; (3) These small pieces of gel were heat-treated and carbonized under the same condition as that of MC-x. NC-x was prepared from resol precursors, a lowmolecular-weight and soluble phenolic resin [24], which was poured into a Petri dish to evaporate ethanol at room temperature for 5e8 h, followed by heating in an oven at 100 C for 24 h. Carbonization was carried out in a horizontally tubular furnace under the same condition as MC-x. 2.2. Characteriztion Nitrogen adsorption isotherms were obtained at 196 C on a Micromeritics ASAP 2020. Samples were normally prepared for measurement by degassing at 120 C until a final pressure of 1103 Torr was reached. The specific surface area was calculated using the BrunauereEmmeteTeller (BET) method from the nitrogen adsorption data in the relative range (P/P0) of 0.05e0.20. Pore size distributions were derived from the adsorption branch of the isotherm using the BarretteJoynereHalenda (BJH) method, and collected from pores between 1 and 1000 Å diameter. The total pore volume was determined from the amount of N2 uptake at P/ P0 ¼ 0.99. Powder X-ray diffraction (XRD) data were recorded on a Rigaku D/Max-2550 (50 kV, 200 mA) using nickel-filtered Cu Ka radiation with wavelength of k ¼ 1.5406 Å. Transmission electron micrograph (TEM) image was obtained on Tecnai G2 F30 operating at an accelerating voltage of 300 kV. Raman spectra were performed on a Jobin Yvon HR 800 micro-Raman spectrometer at 457.9 nm. A HP-5783E vector network analyzer was applied to determine the complex permeability and permittivity in the frequency range of 2e18 GHz for the calculation of reflection loss. A sample containing 60 wt % carbon powders was pressed into a ring with an outer diameter of 7 mm, an inner diameter of 3 mm, and a thickness of 2 mm for microwave measurement in which paraffin wax was used as the binder. The reflection loss, R(dB), of an absorber can be deduced from the following equation,
Z 1 RðdBÞ ¼ 20log in Z þ 1
(2)
in
Zin refers to the normalized input impedance of a metal-backed microwave absorbing layer and is given by [25,26]
2. Experimental section
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi mr pffiffiffiffiffiffiffiffiffi 2p fd mr 3 r tanh j 3r c
2.1. Sample preparation
Zin ¼
Highly ordered mesoporous carbon was prepared according to previous literature [23]. In a typical synthesis, 1.1 g of resorcinol and
where 3 r (3 r ¼ 3 0 j3 00 ) and mr (mr ¼ m0 jm00 ) are the complex permittivity and permeability respectively, of the composite
(3)
886
Y. Du et al. / Materials Chemistry and Physics 135 (2012) 884e891
medium, c is the velocity of electromagnetic waves in free space, f is the frequency of microwave, and d is the thickness of absorbers.
Table 1 Porous parameters and ID/IG values of various samples. Sample
BET surface (m2 g1)
Pore volume (cm3 g1)
Pore size (nm)
ID/IG
MC-600 MC-650 MC-700 MC-750 NC-650 NC-700 DC-650 DC-700
633 640 630 631 57 72 544 557
0.68 0.68 0.65 0.64 0.023 0.029 0.57 0.55
6.5 6.6 6.5 6.5 e e e e
0.688 0.735 0.790 0.837 0.738 0.792 0.733 0.794
3. Results and discussion Fig. 1 shows the N2 adsorptionedesorption isotherms and pore size distributions of various carbon materials (MC-x, DC-x and NCx), and their textural parameters are summarized in Table 1. As observed, mesoporous carbons, MC-x, give standard IV-type isotherms with a sharp capillary condensation step at relative pressure (P/P0) from 0.4 to 0.8 and narrow pore size distributions centered at 6.5e6.6 nm (Fig. 1a), which are quite similar to those of mesoporous carbon prepared by the same method [23], indicating their highly ordered mesostructures. Additionally, small-angle XRD patterns and TEM images of MC-650 and MC-750 are shown in Fig. 2. It is clear that they display three well-resolved peaks that can be indexed into (100), (110), and (200) diffractions of 2-D hexagonal symmetry (P6mm), as well as uniformly hexagonal pore system, further confirming their highly ordered mesostructures. It is very interesting to note that the texture parameters, such as surface area and pore size, of MC-x carbonized at different temperature are very close, implying that mesoporous carbons prepared in this method
a
4.5
700 3.5
500
(D)
400
(C)
300
(B)
200
(A)
dV/dlog(D) (cm3/g · nm)
3
Adsorbed Volume (cm /g, STP)
4.0
600
3.0
(D)
2.5 2.0
(C)
1.5 1.0
(B)
0.5
100
(A)
0.0
0 0.0
0.2
0.4
0.6
0.8
1.0
0
700
0.8
600
0.7
dV/dlog(D) (cm3/g · nm)
500
3
Adsorbed Volume (cm /g, STP)
b
400
(H) 300 200
(G)
100
(F)
10
20
30
Pore size (nm)
Relative pressure (P/P0)
(H)
0.6
(G) 0.5 0.4 0.3 0.2
(F) 0.1
(E) 0 0.0
(E) 0.2
0.4
0.6
0.8
Relative pressure (P/P0)
1.0
0.0 0
20
40
60
80 100
Pore size (nm)
Fig. 1. N2 adsorption/desorption isotherms and pore size distribution of MC-x (a) and NC-x and DC-x (b). (A) MC-600, (B) MC-650, (C) MC-700, (D) MC-750, (E) NC-650, (F) NC-700, (G) DC-650, and (H) DC-700. Isotherms B, C, D, F, and H have been offset by 100, 200, 300, 50, 150 cm3 g1 along the vertical axis for clarity, respectively.
possess good thermal stability, which is consistent with previous report [23]. Pore volume presents a slight decrease with increasing carbonization temperature (Table 1), this can be attributed to the slight shrinkage of mesoporous framework during the carbonization process [24]. Also, a small abnormity can be detected in MC600 at relative pressure 0.4e0.5 (marked with a circle in Fig. 1a), which may arise from the surfactants that are not completely removed at current temperature and the trace residues that block part of mesopore system [27]. For comparisons, NC-x and DC-x are both prepared in the absence of surfactants, however, they exhibit completely different N2 isotherms and structural parameters (Fig. 1b and Table 1). NC-650 and NC-700 give extremely poor N2 adsorption, very low BET surface area and pore volume, and ambiguous pore size distribution, suggesting that they are almost free of porosity. DC-650 and DC-700 show isotherms between IItype and IV-type, characteristic of mesoporous materials according to the IUPAC classification [28]. However, their pore size distributions (Fig. 1b) are quite broad and nitrogen amounts absorbed rise very steeply at high relative pressure (P/P0 > 0.7), indicating that these mesopores are not uniform and some macropores are also present [28,29]. The existence of porosity in DC650 and DC-700 stimulates the increase in BET surface area and pore volume, but still smaller than those of MC-x (Table 1). As shown in Fig. 3, all mesoporous carbons, MC-x, display two distinguishable peaks in the range of 800e2000 cm1. One is a broad peak centered at about 1350 cm1 (D band), the other is a relatively sharp peak at about 1590 cm1 (G band). The value of the intensity ratios of these two bands, ID/IG, monotonously increases from 0.688 to 0.837 with the increase of carbonization temperature (Table 1). According to previous literature [30e32], D band is a breathing mode of A1g symmetry involving phonons near the K zone boundary, which is forbidden in perfect graphite and becomes active in the presence of disorder or finite-size crystals of graphite (nanographite crystals); G band corresponds to the E2g mode due to stretching vibrations of sp2 bond, which can be produced by all sp2 sites and not only by graphitic carbon. It is reasonable to conclude that D band and G band herein originate from finite-size crystals of graphite and amorphous C sp2 sites, respectively, and the increasing ID/IG value stands for the gradually increased degrees of graphitization. Otherwise, there should be less amount of D band and more amount of G band, as well as gradually decreased value of ID/IG with increasing carbonization temperature. According to the phenomenological three-stage model proposed by Ferrari and Robertson [32], the gradually increased ID/IG values (0.675e0.818) of MC-x (x ¼ 600e750) are exactly in the transitional stage from amorphous carbon to nanocrystalline graphite, further confirming the formation of tiny crystalline domains at the surface of the materials. However, these tiny crystals are very difficult to detect, even undetectable by high-resolution transmission electron microscopy (HR-TEM) [33]. The increased ID/IG values can be also observed in other Raman spectra studies about amorphous carbonbased materials [34e36]. Very similar to the Raman spectra of MC-
Y. Du et al. / Materials Chemistry and Physics 135 (2012) 884e891
887
Intensity (a.u.)
Fig. 2. Small-angle XRD patterns (a) of MC-650 (A) and MC-750 (B). TEM images of MC-650 (b) and MC-750 (c) in the direction of (110) (Bar: 50 nm), and TEM image of MC-650 (d) in the direction of (100) (Bar: 20 nm).
(D)
(C)
(B) (A) 800
1000
1200
1400
1600
1800
2000
-1
Raman shift (cm ) Fig. 3. Raman spectra of (A) MC-600, (B) MC-650, (C) MC-700, and (D) MC-750.
x, NC-x and DC-x also exhibit two characteristic peaks in the range of 800e2000 cm1 and increasing ID/IG values with the increase of carbonization temperature (Fig. 4 and Table 1). Of particular note is that the samples carbonized at the same temperature, such as NC650, DC-650 and MC-650, give almost identical ID/IG values, indicating that these samples have comparable degrees of graphitization, although they have quite different textural parameters. Fig. 5 shows the real and imaginary parts of complex permittivity and complex permeability of MC-x (x ¼ 600e750) in the frequency range of 2e18 GHz. The real (3 0 ) and imaginary part (3 00 ) of complex permittivity of MC-600 are almost constant with nearly no variation throughout the whole frequency range (3 0 z 2.5, 00 3 z 0), implying its very poor dielectric loss. Compared with those of MC-600, both 3 0 and 3 00 of MC-x (x ¼ 650e750) are simultaneously improved, especially for MC-750, whose 3 0 and 3 00 are larger than 30 between 2 and 18 GHz and even exceed 100 in low frequency range, which should be attributed to the formation of more nanocrystalline graphite domains at high carbonization temperature and the consequent increase of electrical conductivities. In addition, it is discovered that the real (m0 ) and imaginary parts (m00 ) of complex permeability of these samples are also quite different. The m0 and m00 of MC-600 and MC-650 are approximately
888
Y. Du et al. / Materials Chemistry and Physics 135 (2012) 884e891
Intensity (a.u.)
(D)
(C)
(B) (A) 800
1000
1200
1400
1600
1800
2000
-1
Raman shift (cm ) Fig. 4. Raman spectra of (A) NC-650, (B) NC-700, (C) DC-650, and (D) DC-700.
constant and close to one and zero, respectively, indicating the negligible magnetic loss for incident electromagnetic wave [26]. In contrast, the m0 of MC-700 is significantly dependent on frequency, and it is lower than those of MC-600 and MC-650 in 6.4e12.8 GHz
b
140
Imaginary parts of permittivity (ε'')
Real parts of permittivity (ε')
a
frequency range, but higher than those in 12.8e18 GHz frequency range. This trend of m0 is further expanded in MC-750, while the intersecting point is at 11.6 GHz. Very interestingly, the m00 of MC700 and MC-750 are gradually decreased and negative in the whole frequency range, and MC-750 always retains smaller than MC-700. According to Maxwell equations, these phenomena, caused by the motion of charges in an electromagnetic field, suggest that the interaction between MC-x (x ¼ 700e750) and incident electromagnetic wave can induce variation of magnetic field and so that some magnetic energies can be radiated out from these samples [18,21]. Using equations (2) and (3), the reflection loss curves of MC-x are deduced with a thickness of 2.5 mm (Fig. 6). MC-600 exhibits a reflection loss of 0 dB in the whole frequency range due to its very poor dielectric loss and magnetic loss (Fig. 5). MC-650 shows an intensively increased reflection loss with a maximum of 21.5 dB at 14.2 GHz, and the reflection loss less than 10 dB (90% absorption) is in the range of 11.0e18 GHz. The improved microwave absorbing ability of MC-650 is superior or comparable to some of ever reported carbon-based composites, although there are no any profitable magnetic particles. With higher carbonization temperature, MC-700 and MC-750 show increased reflection losses at lower frequency accompanied by intense attenuation at higher frequency bands. The maximum reflection losses of MC-700 and MC-750 are only 5.1 dB at 4.8 GHz and 2.8 dB at 2.6 GHz, respectively. It is generally accepted that the real parts of complex permittivity (3 0 ) and permeability (m0 ) represent the storage capability of electric
MC-600 MC-650 MC-700 MC-750
120 100 80 60 40 20
140 MC-600 MC-650 MC-700 MC-750
120 100 80 60 40 20 0
0 2
4
6
8
10
12
14
16
2
18
4
6
Frequency (GHz)
d
1.4
Imaginary parts of permeability (μ '')
Real parts of permeability (μ ')
c
1.2
1.0
0.8 MC-600 MC-650 MC-700 MC-750
0.6
8
10
12
14
16
18
14
16
18
Frequency (GHz)
0.4
0.2 0.0 -0.2 -0.4 MC-600 MC-650 MC-700 MC-750
-0.6 -0.8 -1.0
2
4
6
8
10
12
Frequency (GHz)
14
16
18
2
4
6
8
10
12
Frequency (GHz)
Fig. 5. The real parts (a) and imaginary parts (b) of complex permittivity, and the real parts (c) and imaginary parts (d) of complex permeability of MC-x in the frequency range of 2e18 GHz.
Y. Du et al. / Materials Chemistry and Physics 135 (2012) 884e891
Reflection loss (dB)
0
-5
-10 MC600 MC650 MC700 MC750
-15
-20
-25 2
4
6
8
10
12
14
16
18
Frequency (GHz) Fig. 6. The reflection loss of MC-x with an absorber thickness of 2.5 mm in the frequency range of 2e18 GHz.
and magnetic energy, and the imaginary parts (3 00 and m00 ) represent the loss of electric and magnetic energy [37]. Hence it is beneficial to microwave absorption if the absorbers have large imaginary parts of complex permittivity and permeability. Additionally, some published papers also propose that negative imaginary parts of complex permeability can produce geometrical effects to improve microwave absorption [38e40]. Herein, MC-700 and MC-750 show considerable imaginary parts of complex permittivity and negative imaginary parts of complex permeability, but they still fail to exhibit good reflection losses. It results from another important parameter, the concept of matched characteristic impedance, where the characteristic impedance of the absorbing materials should be equal/close to that of the free space (377 U sq1) to achieve zero-reflection at the front surface of the materials [41]. Overlarge difference between the complex permittivity and complex permeability cannot bring considerable microwave absorption since most of microwave will be reflected off at the surface of absorbers. In contrast, MC-650 exhibits better reflection
0
Reflection loss (dB)
-5 -10 -15
2.0 mm 2.2 mm 2.5 mm 3.0 mm 3.5 mm 4.0 mm 5.0 mm
-20 -25 -30 2
4
6
8
10
12
14
16
18
Frequency (GHz) Fig. 7. The reflection loss dependent on the thickness of MC-650 in the frequency range of 2e18 GHz.
889
loss than MC-700 and MC-750 owing to its suitable characteristic impedance, although it has lower 3 0 and 3 00 , as well as negligible magnetic loss. According to equation (3), the thickness (d) of absorbers can also affect the reflection loss; thus, the relationship between the thickness and the reflection loss of MC-650 is investigated in Fig. 7. The microwave frequency corresponding to the maximum reflection loss shifts negatively with the increase of thickness, and the value of reflection loss exceeding 10 dB can be obtained in the range of 4.8e18 GHz with a variation in thickness from 2.0 to 5.0 mm. A maximum of 27.1 dB at 16.2 GHz can be achieved when the thickness is 2.2 mm. These results further suggest that MC-650 will be a promising microwave absorber, whose absorption band can be simply modulated by manipulating the thickness to satisfy the applications in different frequency bands. As mentioned above, He et al. prepared ordered mesoporous carbon-based absorbers CeSiO2eFe and CeAl2O3 at 700 C, and they had to incorporate a large amount of SiO2eFe and Al2O3 particles (even up to 80 wt%) to optimize characteristic impedance and improve reflection loss, although it was at the expense of some structural properties, such as BET surface, pore volume, density, and so forth [20,21]. In fact, direct incorporating different amount of metal or metal oxides particles, particularly Fe species, into mesoporous carbon matrix will result in different degrees of graphitization of carbon materials during carbonization process and disordered complex permittivity of mesoporous carbon composites [20,22]. In these cases, it will be difficult to evaluate the contribution of mesoporous carbon on microwave absorption and design mesoporous carbon composites microwave absorbers. In this work, we optimize the characteristic impedance by adjusting the degrees of graphitization of mesoporous carbons and make pristine mesoporous carbon (MC-650) possess good microwave absorption comparable with those of CeSiO2eFe and CeAl2O3, which is more favorable for the design and preparation of novel and light-weight microwave absorbers. To study the effect of pore structure, the real and imaginary parts of complex permittivity and complex permeability of NC-x and DC-x are also investigated, as shown in Fig. 8. Very similar to those of MC-x, improving the degree of graphitization can increase complex permittivity and induce the variation of complex permeability. However, even with the similar degree of graphitization, the real and imaginary parts of complex permittivity of DC-x are obviously smaller than those of NC-x, and a little larger than those of MC-x. For example, the 3 0 values of DC-650, NC-650 and MC-650 decrease from 13.1, 22.9, and 11.4 to 5.3, 7.4, and 4.5 in the 2e18 GHz range, respectively; while the 3 00 values decrease from 10.5, 12.1, and 9.7 to 5.1, 7.6, and 3.2, respectively. It is reasonable to attribute these differences to their distinguishable pore structure, and conclude that rich pore structure can decrease the real and imaginary parts of complex permittivity, which is quite consistent with previous reports and MaxwelleGarnet theory [8,9,17,18]. Moreover, it can be found that the presence of rich pore structure can also decrease the variation of complex permeability. For instance, DC-700 and MC-700 exhibit relatively small m0 and negative m00 as compared with those of NC-700. Fig. 9 shows the calculated reflection loss curves of NC-x and DCx with a thickness of 2.5 mm in the frequency range of 2e18 GHz, which suggests that the reflection loss characteristics are sensitive to the degree of graphitization and pore structure. NC-700 and DC700 exhibit weak reflection loss properties in the whole frequency range, and their maximums are 5.0 dB at 5.1 GHz and 5.3 dB at 4.6 GHz, respectively, which are very close to that of MC-700 (Fig. 6), indicating that the difference of pore structure cannot bring improved reflection losses in current state. That is, the degree of graphitization is the primary factor for the microwave absorbing properties of carbon materials. However, pore structure will play an
890
Y. Du et al. / Materials Chemistry and Physics 135 (2012) 884e891
b
60 50
Imaginary parts of permittivity (ε'')
Rear parts of permittivity (ε')
a
NC650 NC700 DC650 DC700
40 30 20 10
60 NC650 NC700 DC650 DC700
50 40 30 20 10 0
0 2
4
6
8
10
12
14
16
2
18
4
6
d
1.4
Imaginary parts of permeability (μ'')
Real parts of permeability (μ ')
c
1.2
1.0
0.8 NC-650 NC-700 DC-650 DC-700
0.6
8
10
12
14
16
18
16
18
Frequency (GHz)
Frequency (GHz)
0.2 0.0 -0.2 -0.4 NC-650 NC-700 DC-650 DC-700
-0.6 -0.8 -1.0
0.4 2
4
6
8
10
12
14
16
18
2
4
6
8
10
12
14
Frequency (GHz)
Frequency (GHz)
Fig. 8. The real parts (a) and imaginary parts (b) of complex permittivity, and the real parts (c) and imaginary parts (d) of complex permeability of NC-x and DC-x in the frequency range of 2e18 GHz.
important role when these carbon materials have appropriate degrees of graphitization. For example, DC-650 with rich porosity exhibits larger reflection loss and wider response frequency range than NC-650 with poor porosity. Additionally, we summarize the reflection loss curves of NC-650, DC-650, and MC-650 in Graphical
0
Reflection loss (dB)
-5
Abstract. It is very interesting that the reflection loss of DC-650 is obviously less than that of MC-650, although both of them are rich porous materials with similar degree of graphitization. The difference in reflection loss may be explained from two aspects: (1) MC650 has larger BET surface and pore volume than DC-650, which are beneficial to lower 3 0 and 3 00 (Figs. 5 and 8) and thus improve matched characteristic impedance; (2) Compared with disordered and random pore system in DC-650, the pore system in MC-650 is highly homogenous and uniform, which is favorable for the multiple reflections and repeated consumption of incident electromagnetic wave inside the pores [20]. These two aspects are believed to lead to the significant improvement in microwave absorption of MC-650.
-10 4. Conclusions
-15
NC-650 NC-700 DC-650 DC-700
-20
-25 2
4
6
8
10
12
14
16
18
Frequency (GHz) Fig. 9. The reflection loss of NC-x and DC-x with an absorber thickness of 2.5 mm in the frequency range of 2e18 GHz.
By systematically investigating the degrees of graphitization, electromagnetic properties and microwave absorption of ordered mesoporous carbon, disordered mesoporous carbon and nonporous carbon materials, it can be concluded that the degree of graphitization is the primary factor affecting the microwave absorbing properties of carbon materials. When the degree of graphitization is relatively high, pore structure can decrease the real and imaginary parts of complex permittivity to some extent, but fails in producing improved reflection loss due to poor characteristic impedance. However, for the samples with the appropriate degree of graphitization, pore structure and degree of order
Y. Du et al. / Materials Chemistry and Physics 135 (2012) 884e891
will play quite important roles not only in improving characteristic impedance, but also in increasing reflection loss. For example, the optimum one, MC-650 with suitable degree of graphitization and uniform pore structure, exhibits very strong reflection loss and wide response bandwidth, even in the absence of any profitable magnetic particles. We believe this work will be helpful for the design and preparation of light-weight and highly effective microwave absorbers in the future. Acknowledgments This work is supported by National Natural Science Foundation of China (21003029, 21071037 and 21101041), Special Fund of Harbin Technological Innovation (2010RFXXG012) and Postdoctoral Science-research Development Foundation of Heilongjiang Province (LBH-Q11098). References [1] R.C. Che, L.M. Peng, X.F. Duan, Q. Chen, X.L. Liang, Adv. Mater. 16 (2004) 401. [2] X.C. Gui, W. Ye, J.Q. Wei, K.L. Wang, R.T. Lv, H.W. Zhu, F.Y. Kang, J.L. Gu, D.H. Wu, J. Phys. D: Appl. Phys. 42 (2009) 075002. [3] H. Zhu, H.Y. Lin, H.F. Guo, L.F. Yu, Mater. Sci. Eng. B 138 (2007) 101. [4] X.G. Liu, B. Li, D.Y. Geng, W.B. Cui, F. Yang, Z.G. Xie, D.J. Kang, Z.D. Zhang, Carbon 47 (2009) 470. [5] Y. Fan, H. Yang, X. Liu, H. Zhu, G.T. Zou, J. Alloy Compd. 461 (2008) 490. [6] N.J. Tang, Y. Yang, K. Lin, W. Zhong, C. Au, Y.W. Du, J. Phys. Chem. C 112 (2008) 10061. [7] M. Itoh, J.R. Liu, T. Horikawa, K. Machida, J. Alloy. Compd. 408e412 (2006) 1400. [8] Z.G. Fang, C.S. Li, J.Y. Sun, H.T. Zhang, J.S. Zhang, Carbon 45 (2007) 2873. [9] J. Yang, Z.M. Shen, Z.B. Hao, Carbon 42 (2004) 1882. [10] C. Wang, X.J. Han, P. Xu, X.L. Zhang, Y.C. Du, S.R. Hu, J.Y. Wang, X.H. Wang, Appl. Phys. Lett. 98 (2011) 072906. [11] R. Ryoo, S.H. Joo, S. Jun, J. Phys. Chem. B 103 (1999) 7743. [12] D.S. Su, J.J. Delgado, X. Liu, D. Wang, R. Schlogl, L.F. Wang, Z. Zhang, Z.C. Shan, F.S. Xiao, Chem. Asian J. 4 (2009) 1108. [13] Z.X. Wu, D.Y. Zhao, Chem. Commun. 47 (2011) 3332.
891
[14] K.S. Xia, Q.M. Gao, C.D. Wu, S.Q. Song, M.L. Ruan, Carbon 45 (2007) 1989. [15] A.B. Fuertes, G. Lota, T.A. Centeno, E. Frackowiak, Electrochim. Acta 50 (2005) 2799. [16] N.Z. Wing, B. Wang, J.W. Halloran, J. Am. Ceram. Soc. 89 (2006) 3696. [17] O. Levy, D. Stroud, Phys. Rev. B 56 (1997) 8035. [18] Q.L. Liu, D. Zhang, T.X. Fan, Appl. Phys. Lett. 93 (2008) 013110. [19] J.C. Wang, C.S. Xiang, Q. Liu, Y.B. Pan, J.K. Guo, Adv. Funct. Mater. 18 (2008) 2995. [20] J.H. Zhou, J.P. He, G.X. Li, T. Wang, D. Sun, X.C. Ding, J.Q. Zhao, S.C. Wu, J. Phys. Chem. C 114 (2010) 7611. [21] T. Wang, J.P. He, J.H. Zhou, X.C. Ding, J.Q. Zhao, S.C. Wu, Y.X. Guo, Micropor. Mesopor. Mater. 134 (2010) 58. [22] Y.C. Du, J.Y. Wang, C.K. Cui, X.R. Liu, X.H. Wang, X.J. Han, Synth. Met. 160 (2010) 2191. [23] X.Q. Wang, C.D. Liang, S. Dai, Langmuir 24 (2008) 7500. [24] Y. Meng, D. Gu, F.Q. Zhang, Y.F. Shi, L. Cheng, D. Feng, Z.X. Wu, Z.X. Chen, Y. Wan, A. Stein, D.Y. Zhao, Chem. Mater. 18 (2006) 4447. [25] S.S. Kim, S.B. Jo, K.I. Gueon, K.K. Choi, K.S. Churn, IEEE Trans. Magn. 27 (1991) 5462. [26] L. Du, Y.C. Du, Y. Li, J.Y. Wang, C. Wang, X.H. Wang, P. Xu, X.J. Han, J. Phys. Chem. C 114 (2010) 19600. [27] F. Schuth, A. Wingen, V. Sauer, Micropor. Mesopor. Mater. 44e45 (2001) 465. [28] M. Kruk, M. Jaroniec, Chem. Mater. 13 (2001) 3169. [29] T.Y. Ma, X.J. Zhang, Z.Y. Yuan, J. Phys. Chem. C 113 (2009) 12854. [30] R.J. Nemanich, S.A. Solin, Phys. Rev. B 20 (1979) 392. [31] R.O. Dillon, J.A. Woollam, V. Katkanant, Phys. Rev. B 29 (1984) 3482. [32] A.C. Ferrari, J. Robertson, Phys. Rev. B 61 (2000) 14095. [33] K.X. Wang, Y.G. Wang, Y.R. Wang, E. Hosono, H.S. Zhou, J. Phys. Chem. C 113 (2009) 1093. [34] R. Sergiienko, E. Shibata, S. Kim, T. Kinota, T. Nakamura, Carbon 47 (2009) 1056. [35] P.K. Chu, L.H. Li, Mater. Chem. Phys. 96 (2006) 253. [36] F. Li, J.S. Lannin, Appl. Phys. Lett. 61 (1992) 2116. [37] N. Chen, G.H. Mu, K.K. Gan, M.Y. Gu, Mater. Sci. Eng. B 139 (2007) 256. [38] A.N. Yusoff, M.H. Abdullah, S.H. Ahmad, S.F. Jusoh, A.A. Mansor, S.A.A. Hamid, J. Appl. Phys. 92 (2002) 876. [39] L.J. Deng, M.G. Han, Appl. Phys. Lett. 91 (2007) 023119. [40] C. Wang, X.J. Han, P. Xu, J.Y. Wang, Y.C. Du, X.H. Wang, W. Qin, T. Zhang, J. Phys. Chem. C 114 (2010) 3196. [41] P. Xu, X.J. Han, X.R. Liu, B. Zhang, C. Wang, X.H. Wang, Mater. Chem. Phys. 114 (2009) 556.