Carbon Aerogels toward High


Dec 13, 2017 - Center for Degradable and Flame-Retardant Polymeric Materials, College of Chemistry, National Engineering Laboratory of Eco-Friendly Po...
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Magnetic and conductive Ni/carbon aerogels toward high-performance microwave absorption Hai-Bo Zhao, Zhi-Bing Fu, Xue-Yi Liu, Xiao-Cao Zhou, HongBing Chen, Ming-Long Zhong, and Chao-Yang Wang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03612 • Publication Date (Web): 13 Dec 2017 Downloaded from http://pubs.acs.org on December 15, 2017

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Magnetic and conductive Ni/carbon aerogels toward high-performance microwave absorption

Hai-Bo Zhao1*, Zhi-Bing Fu2, Xue-Yi Liu3, Xiao-Cao Zhou2, Hong-Bing Chen4*, Ming-Long Zhong2, Chao-Yang Wang2* 1

Center for Degradable and Flame-Retardant Polymeric Materials, College of

Chemistry, National Engineering Laboratory of Eco-Friendly Polymeric Materials (Sichuan), Sichuan University, Chengdu, 610064, China 2

Research Center of Laser Fusion, China Academy of Engineering Physics, Mianyang

(Sichuan), 621000, China 3

Affiliated Middle School of Henan Normal University, Xinxiang (Henan), 453000,

China 4

Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics,

Mianyang (Sichuan), 621000, China

∗Corresponding authors: E-mail:

[email protected]

(Hai-Bo

Zhao),

[email protected]

(Hong-Bing Chen), [email protected] (Chao-Yang Wang)

Abstract Novel magnetic and conductive Ni/carbon aerogels have been successfully fabricated through an autocatalytic reduction process. The resultant Ni/carbon

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aerogels have nano-porous structure with high specific surface area, low densities, appropriate electrical conductivities and controllable magnetization. Due to these special attributes, the aerogel composites are demonstrated to be a great microwave absorbent possessing strong and controllable EM absorption with ultrathin thickness. Only with filler loading of 10 wt% in the wax, a minimum RL of -57 dB is found at 13.3 GHz with the thickness of 2mm. Even with the thickness of 1.5 mm, the Ni/carbon aerogel-3 composite can exhibit the low RLmin value of -32 dB and wide effective EM absorption bandwidth of 4.0 dB. The synergistic effect of the medium dielectric loss, weak magnetic loss and good impedance match, accounts for the high microwave absorption performance.

1. Introduction In recent years, electromagnetic (EM) absorption materials have attracted wide attention for their potential applications in the fields of EM interference shielding and radar cross section reduction.1-8 Ideal EM absorbents should be light in weight, as well as exhibiting a strong and wide absorption at low filler loading and thin coating thickness.9-11 Traditional EM absorbers with high magnetic permeability (Co, Ni, Fe3O4, or γ-Fe2O3, etc.) or dielectric constant (TiO2, SiO2, or BaTiO3, etc.) absorb EM microwave via magnetic or dielectric loss.12-14 These materials always show low permeability or permittivity, thus leading to weak EM absorption, while conductive composites with magnetic particles are considered as promising substitution and exhibit strong absorption strength in recent reports.15-17 However, current composite 2

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absorbents encounter severe problems such as large coating thickness (≥ 2 mm), high density and filler loading ratios (≥ 20 wt%), which limit their practical applications.7-19 Light weight, low filler ratio and thin thickness are the key factor for the next generation EM absorption materials. Among many kinds of composites used for microwave absorbents, carbon-based composites have attracted enormous attentions due to their relatively low density, great thermal stability, tunable properties and abundant resources.7-10 Graphene, carbon nanotube, carbon fiber and porous carbon, decorated with magnetic metals/oxides, have been developed as lightweight absorbents. Compared with other carbon materials, nanoporous carbon-based composites exhibit lower density and larger surface area. More recently, it is found that porous structure can improve microwave performances of absorbents interface polarization loss.7 Liu et al synthesized porous CNTs/Co composites, and the reflection loss of composites with filler content of 20 wt% can reach -60.4 dB with a thickness of 1.81 mm.7 Ji and Du et al. fabricated a series of magnetic metals/porous carbon composites via carbonization of metal-organic framework (MOF).8-10 The resultant composites possessed strong microwave absorption with low density and thin coating thickness. Despite great development, the reports on porous carbon-based composites are still rare for microwave absorption. Novel porous carbon-based composites could be great candidates for microwave absorbents with high microwave absorption performance. Carbon-based aerogels are highly porous solid nanomaterials with unique characteristics including chemical and thermal stability, lightweight, large pore 3

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volumes, high specific surface areas and tunable porosity.20-22 From this point, carbon-based aerogels pose as a good microwave absorber if its inherent electromagnetic property can be improved.23 Li et al. developed porous polypyrrole/graphene aerogel composites as microwave absorbents, and the minimum reflection loss can reach -45.12 dB.21 Xie et al. fabricated reduced graphene oxide modified sponge-like polypyrrole aerogel, and the the highest reflection coefficient of the composites reached -54 dB.11 Pure carbon aerogels always possess high conductivity and poor magnetism that will cause high reflection and low absorption.12 Based on the above considerations, the carbon-based aerogel with magnetic features may show great EM absorption (strong absorption strength, thin thickness, low density and filler loading). Unfortunately, there have been few reports focusing on the EM absorption, synthesis or properties of carbon-based aerogels with magnetism. In this study, we present an easy method to fabricate a Ni/carbon aerogel with suitable conductivity and controllable magnetism. The carbon aerogel (CRF) matrix was firstly synthesized from resorcinol/formaldehyde aerogels. Uniform Ni nanoparticles were electrolessly deposited on the surface of carbon aerogels via an autocatalytic reduction process. The resultant aerogels showed low density, high specific surface area, and suitable conductivity. Also, the magnetic properties of the aerogels can be controlled by the magnetic nanoparticles Ni contents, leading to different microwave absorption performances. The results from the microwave absorption behaviors of the aerogels revealed that the Ni/carbon aerogel had a great EM absorption behavior at the low filler loading ratio and thin coating thickness. 4

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Furthermore, the corresponding mechanisms of the aerogels were discussed in detail.

2. Experimental Chemicals: resorcinol (AR), formaldehyde (AR), ethanol (AR), sodium carbonate (AR), PdCl2 (AR), NiCl2*6H2O (AR), ammonia (AR), hydrazinium hydroxide (AR), and acetone (AR) were supplied by Chengdu Chemical Industries Co. (China). Ni nano-powder (AR) with the diameter of 20-100 nm was manufactured by Aladdin industrial corporation (Shanghai, China). Preparation of CRF aerogels: Carbon aerogels were synthesized according to our previous work (Figure 1).22 A mount of resorcinol, sodium carbonate, water and formaldehyde were added into vials and kept at 60 °С for 6 days. The obtained hydrogels were exchanged with water and acetone, and dried under supercritical CO2. After drying, the resorcinol-formaldehyde aerogels were further to pyrolysis under argon atmosphere at 1050 °С to obtain the CRF carbon aerogel.

Figure 1 Schematic illustration for the preparation of carbon aerogel.

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Preparation of Ni/Carbon aerogels: CRF aerogels were first immersed in the activating solution with HCl (1 mol/L, 5mL), ethanol (45 mL), and PdCl2 (0.05 g) for 24 hours. Consequently, the samples were soaked in the freshly Ni bath with NiCl2·6H2O (0.1/0.2/0.4 g), distilled water (20 mL), ammonia (2 mL), ethanol (5 mL), hydrazinium hydroxide (85%, 2 mL) until the solution became colourless. Then, the resultant materials were exchanged with water and acetone, and dried under supercritical CO2 to obtain Ni/Carbon aerogel. The Ni/Carbon prepared from different concentration of NiCl2·6H2O (3.3/6.6/13.3 mg/mL) are denoted as Ni/Carbon aerogel-1, Ni/Carbon aerogel-2 and Ni/Carbon aerogel-3, respectively.

Characterization: The X-ray diffraction (XRD) measurement was performed using a PANalytical X′Pert Pro X-ray diffractmometer. X-ray photoelectron spectroscopy (XPS) was carried out on XSAM 800 spectrometer (Kratos Co., UK). Thermalgravimetric analysis (TG) was analysized on the NETZSCH TGA (209 F1) at a heating rate of 10 °С min-1 under the air atmosphere. The morphology was characterized by transmission electron microscope (TEM, JEM-200CM, 200 kV) equipped with energy dispersive X-ray analysis (EDX) and field emission scanning electron microscope (FESEM, Nova 600i). High-angle annilar dark field (HAADF) STEM images of aerogels were obtained from transmission electron microscope in STEM mode. Surface area and pore-size distributions were measured by a Quantachrome Autosorb-1 Instructument. The magnetic properties were measured with a vibrating sample magnetometer (bkt-4500z). Electrical conductivity was tested using four-probe measurements (Guangzhou Four Probes Tech Co). The permeability 6

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and permittivity in the frequency range of 2-18 GHz were measured by a PNA-N5244A vector network analyzer using coaxial wire method for the calculation of reflection loss. The samples were firstly ground into powder (about 200 meshes). Then, the measured samples were prepared by uniformly mixing 10 wt% aerogels with wax by mechanical stirring at 85 °С. The mixture was cast into a ring mold with outer diameter of 7.0 mm, inner diameter of 3.0 mm, and thickness of 2.0 mm. The permeability and permittivity were calculated from the scattering parameters S11 (or S22) and S12 (or S21) according to the standard Nicolson-Ross-Weir (NRW) algorithm.

Figure 2. Illustration for the fabrication process of the Ni/carbon arogels.

3. Result and Discussion Figure 2 displays the synthesis route of the Ni/Carbon aerogels. Nanoporous CRF aerogels were first synthesized and used as carbon aerogel matrix (Figure 1). Then, a two-step chemistry process was developed to prepare the Ni/Carbon aerogels (Figure 7

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2). Ni nanoparticles were electrolessly deposited on the surface of carbon aerogels via an autocatalytic reduction process.24, 25 During the reduction process, the catalyst Pd was first deposited on the surface of the carbon aerogel as the catalytic center. After that, the reactants were introduced to trigger the formation reaction of Ni nanoparticles. And Ni ions were reduced into Ni nanoparticles by N2H4·H2O at the activated surfaces, as shown in Figure 2. This design make products are formed on the backbone of carbon aerogel. This process may create uniform Ni coatings over complex aerogels surfaces and insert Ni particles into aerogels nanochannels. It is noted that the Ni nanoparticles content can be designed and regulated in this process.

Figure 3. XRD spectrums for Ni/carbon aerogels.

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Table 1. Ni content and textural characteristics for the aerogels. Samples

Ni content

Ni content

BET

Total pore

Density

from TGA

from EDX

surface

volume

(g/cm3)

(wt%)

(wt%)

area (m2/g)

(cm3/g)

Pure aerogel

-

-

544

5.0

0.11

Ni/carbon

8.3

3.4

511

5.8

0.12

12.3

6.7

522

2.9

0.12

18.5

10.2

416

2.7

0.13

aerogel-1 Ni/carbon aerogel-2 Ni/carbon aerogel-3

The density of the Ni/Carbon aerogels are in the range of 0.1-0.13 g/cm3 range (Table 1), typical of carbon-based aerogels.22 Figure 3 depicts the XRD patterns of the Ni/Carbon aerogels. The broad peaks at 2θ = 23° and 2θ = 43° were identified for the typical amorphous carbon XRD patterns of the carbon aerogel matrix. The peaks at 44.4°, 51.7°, and 76.3° in the XRD patterns of the Ni/Carbon aerogels were well indexed to the (111), (200), and (220) planes of face-centered cubic Ni crystals (JCPDS 04-805). As for the diffraction peak at 40° and 32°, it may be attributed to the NiCx carbide that existed in the interfaces between Ni nanoparticles and the carbon matrix. Meanwhile, no NiOx oxide diffraction peaks appeared in the diffraction patterns. From the analysis, it is evident that the synthesized Ni/Carbon aerogels are 9

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mainly composed of crystalline Ni and amorphous carbon. The Ni contents of the aerogels were further determined by thermogravimetry in air (TGA, Figure S2) and energy dispersive X-ray (EDX, Figure S3), and the detailed data were listed in Table 1. From the results of TGA, the Ni contents in the Ni/Carbon aerogel-1, Ni/Carbon aerogel-2 and Ni/Carbon aerogel-3 were 8.3, 12.3, and 18.5 wt%, respectively. As can be observed in Table 1, the Ni contents determined by EDX were lower than that from TGA. This may be attributed to the fact that the incorporated Ni was mainly distributed inside the pore of the carbon matrix and the concentration on the surface was low. Also, it was noted that the Ni content results from TGA and EDX had the similar change trends.

Figure 4. SEM images for pure carbon aerogel (a, e), Ni/carbon aerogel-1 (b), Ni/carbon aerogel-2 (c), and Ni/carbon aerogel-3 (d, f).

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Figure 5. TEM images for pure carbon aerogel (a), Ni/carbon aerogel-1 (b), Ni/carbon aerogel-2 (c), and Ni/carbon aerogel-3 (d, e, f).

The microstructures of the Ni/Carbon aerogels were investigated by SEM and TEM. The SEM images (Figure 4 and Figure S4) showed that the Ni/Carbon aerogels were all highly porous with a similar network architectural structure. It was worthy that several Ni nanoparticles were evenly distributed in the matrix of the aerogels upon increasing the content of Ni. Especially in the Figure 4d, many Ni nanoparticles can be clearly found in the matrix of the Ni/carbon aerogel-3. To further observe these Ni nanoparticles, the TEM images of the Ni/carbon aerogels are shown in the Figure 5a-f, where the grey background is carbon aerogels and the dark dots stand for Ni nanoparticles. It is well illustrated that the dimension and distribution of the Ni nanoparticles in all aerogels are rather homogeneous. The diameter of these Ni nanoparticles is in size of 20-50 nm. Note that the high-solution TEM image 11

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(HR-TEM, Figure 5f) of the Ni nanoparticle edge in the Ni/Carbon aerogel-3 showed the unambiguous (111) lattice fringes, indicating the Ni nanoparticles were cyrstallized, consistent with XRD observations. The Selected Area Electron Diffraction pattern (SEAD, Figure S5) showed the crystal planes of Ni nanoparticles corresponded to (111) and (220) fringes. Figure S3 is the typical high-angle annilar dark field (HAADF) STEM image of the Ni/Carbon aerogel-3. The bright nanoparticles in the image are heavy Ni nanoparticles. Many Ni nanoparticles were observed to disperse in the aerogel without aggregation, which were were in agreement with SEM and TEM results. From the information above, the conclusion can be drawn that Ni nanoparticles in a form of nanocrystals are evenly introduced into the porous carbon aerogels. The characterization of Ni and carbon aerogel in the composite was further investigated by X-ray photoelectron spectroscopy (XPS). From the Figure 6, the Ni 2p3/2 (856 eV) and Ni 2p1/2 (873 eV) peaks were well indexed to the metallic Ni 2p peaks. Some satellite peaks at higher binding energy may belong to those of Ni2+, which could be ascribed to the formation of Ni-C or Ni-O-C at the Ni-Carbon interface.12

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Figure 6. XPS spectrum of the Ni 2p region for the Ni/carbon aerogel-3.

Figure 7. Nitrogen sorption isotherms (a) and pore size distribution (b) for aerogels.

The porous attributes of the Ni/carbon aerogels with different incorporated Ni nanoparticles contents were characterized by N2 adsorption-desorption isotherms. From Figure 7a, all aerogels exhibited similar curves of type IV isotherms with a H1 hysteresis loop, indicating a typical mesoporous structure. It was noted that these isotherms resembled each other but had different adsorption-desorption volumes. This was consistent with the SEM observations in that the essential microstructure of the 13

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carbon matrix was not changed by the dopping of Ni nanoparticles. From Figure 7b, it was observed that the peaks of the aerogels in the pore size distribution shifted towards left with the increase of the Ni content. This phenomenon was attributed to the partial mesoporous pore blocking from the Ni nanoparticals incorporated.22 Table 1 shows the parameters determined from N2 adsorption-desorption isotherms including the BET specific surface area and total pore volume of the aerogels. It can be found that all aerogels exhited high BET surface areas and total pore volumes. The Ni/carbon aerogel-3 displayed the lowest BET surface area (416 m2/g) and total pore volume (2.7 cm3/g). When the high content of Ni was incorporated into the aerogel, the pore of carbon aerogel may be blocked by particles. As a result, the BET surface area and total pore volume of the Ni/carbon aerogel-3 decreased. The highly porous attributes of the Ni/carbon aerogels may be a key to their enhanced microwave absorption performances.

Figure 8. Magnetization versus magnetic field for aerogels.

Figure 8 shows the magnetization of the aerogels at the room temperature. It can 14

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be clearly found that the pure carbon aerogel is the non-magnetic material. However, the Ni/carbon aerogels exhibited significant hysteresis with the nonsaturating reversible magnetization, typical for the ferromagnetic phases. The corresponding magnetic parameters of the aerogels, including the remanent magnetization (MR), saturation magnetizition (MS), and coercivity (HC), are listed in Table 2. As the expection, the introduction of Ni nanoparticles can effectively increase both the values of MR and MS. The values of MR, MS and HC for the Ni/carbon aerogel-3 are 1.1 emu/g, 4.7 emu/g and 190 Oe, respectively. The improvement of magnetization can improve the impedance matching and enhance the magnetic loss ability for the aerogels. In the present study, the magnetic properties of the aerogels were directly controlled by the magnetic nanoparticles Ni contents, leading to different microwave absorption performances. Meanwhile, the bulk electrical conductivity for the pure carbon aerogel, Ni/carbon aerogel-1, Ni/carbon aerogel-2, and Ni/carbon aerogel-3 are 1, 0.1, 0.1 and 0.1 S/m, respectively. The decoration of Ni nanoparticles slightly decreases the conductivity of the aerogels. Consequently, the Ni/carbon aerogels with low densities, high specific surface areas and nanoporous structure also have improved magnetism and appropriate electrical conductivities, which is considered as positive contribution to microwave absorption.

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Table 2. Magnetic parameters, electrical conductivities and densities for the aerogels. Samples

Remanent

Saturation

Coercivity

magnetization MR magnetization MS HC (Oe)

Pure

Electrical conductivity

(emu/g)

(emu/g)

(S/m)

-

-

-

1

0.03

0.4

112

0.1

0.17

1.2

64

0.1

1.1

4.7

192

0.1

aerogel Ni/carbon aerogel-1 Ni/carbon aerogel-2 Ni/carbon aerogel-3

Furthermore, the EM absorption properties of the Ni/Carbon aerogels are evaluated by the reflection loss (RL) which is calculated by the transmission line theory.26, 27 During the theory, the input impedance Zin is expressed as

Zin = µr ε r tanh  j ( 2π fd c ) µrε r 

(1)

where εr is the complex permittivity, µr is the complex permeability, f is the frequency of microwaves, c is the velocity of the electromagnetic waves, and d is the thickness of absorber. RL is further calculated according to the metal back-panel model.

RL=20 log

Zin - 1 Zin + 1

(2)

Lower RL values indicate higher microwave absorption. More than 90% of 16

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microwaves will be absorbed when the RL value is lower than -10 dB. This area is named as the effective EM absorption bandwidth.28-29

Figure 9. Reflection loss curves of carbon aerogels (a), Ni nanoparticles (b), Ni/carbon aerogel-1 (c), Ni/carbon aerogel-2 (d), and Ni/carbon aerogel-3 (e) samples in paraffin matrix with the filler content of 10 wt%, and their minimum reflection loss curves (f).

Figure 9 shows the microwaves absorption properties of the Ni/carbon aerogels with the thickness from 1.5 to 5.0 mm and the filler content of 10 wt% in the wax, and those of the pure carbon aerogel (CRF) and nano-Ni powder (diameter: 20-100 nm) are also listed as controls. The Nano-Ni powder almost exhibited no EM absorption behavior (Figure 9b), which may be caused by the fact that effective conductive or magnetic interconnections cannot be formed in the matrix at this low filler loading. Compared with the Ni nano-powders, CRF sample had the improved microwave 17

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absorption due to better dielectric loss. However, its EM absorption ability is still very weak, and the minimum RL (RLmini) value is only -14 dB in the thickness of 1.5 mm. The reason may be ascribed to its weak magnetic property and poor impedance matching, which lead to high reflection but low absorption. In contrast, with the introduction of Ni nanoparticles, the EM absorption abilities of the carbon aerogels significantly increased: the minimum RL value of the Ni/carbon aerogel-2 reached -36 dB with the thickness of 4mm (Figure 9d), nearly three times higher than that of the pure carbon aerogel. It is clear that Ni nanoparticles decorated aerogel can effectively improve the microwave absorption of the carbon aerogel. Improved magnetism and impedance matching may be the key to microwave attenuation. For this reason, the Ni/carbon aerogel-1 still showed very low EM absorption, and the low content of Ni nanoparticles cannot meet the desire of the requirement of impedance matching. As for the Ni/carbon aerogel-3 with the highest content of Ni nanoparticles, the best microwave absorption behavior can be observed as expected: a minimum RL of -57 dB was found at 13.3 GHz with the thickness of 2mm and filler content of 10 wt% in the wax (Figure 9e), and the effective microwave absorption bandwidth was about 3.6 dB. Noted that the RL values of the Ni/carbon aerogel-3 composites were all lower than -10 dB in the frequency range of 3.6-8.6 GHz and 10.1-18 GHz with the thickness of 1.5-5 mm. This wide bandwidth is very important, as the corresponding absorption efficiency is within the range of 90-100%. From the above results, the aerogels with different Ni contents exhibit totally different microwave absorption performances including RL values, microwave 18

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absorption frequency and effective bandwidth. With the increase of Ni contents in the aerogel, the RLmin values decreased, and the microwave absorption performances improved significantly for the aerogel composite. Table S1 lists the microwave absorption properties of some EM absorbents possessing strong EM absorption in the wax matrix in the recent years.10, 12,

30-31

It was noted that the Ni/carbon aerogel-3

sample showed the great RLmini (-56 dB), moderate effective EM bandwidth (3.6 dB), the lowest filler loading (10 wt%), and the thinnest thickness (2 mm). Even with the thickness of 1.5 mm, the Ni/carbon aerogel-3 composite exhibited low RLmin value of -32 dB and wide effective EM absorption bandwidth of 4.0 dB. The excellent EM absorption performance of Ni/carbon aerogel-3 indeed superior to most of other similar absorbents ever reported. More importantly, the microwave absorption performances of the aerogels in this work can be designed and regulated as we need via changing the magnetic nanoparticles contents incorporated. In addition, the frequency of RLmini moves towards the low frequency with the increase of the thickness of the composites, indicating that the quarter-wavelength attenuation requires the thickness to meet the demand of the phase match conditions.28 Consequently, the lightweight aerogel EM absorbent, possessing great and controllable microwave absorption with low filler loading and ultrathin thickness, has been demonstrated.

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Figure 10. Real part (ε′, a) and imaginary part (ε″, b) of permittivity, dielectric tangent loss values (ε″/ε′, c), real part (µ′, d) and imaginary (µ″, e) of permeability, and magnetic tangent loss values (µ″/µ′, f) for the carbon aerogel, Ni nanoparticles, Ni/carbon aerogel-1, Ni/carbon aerogel-2 and Ni/carbon aerogel-3 samples.

EM absorption performances of absorbents are highly associated with their complex permittivity (εr) and permeability (µr).32 The EM parameters including εr and µr of the pure carbon aerogel, nano-Ni powder, the Ni/carbon aerogel-1, the Ni/carbon aerogel-2 and the Ni/carbon aerogel-3 samples were investigated in detail (Figure 10). It is believed that εr and µr originate from electronic polarization, magnetic properties, and interface polarization for the carbon-based absorbents, on which their structures and morphologies also have an important influence.28 As observed from Figure 10a-b, Ni nanoparticle samples displayed the lowest ε′ (~ 2.5) and ε′′ (~ 0.1), which may be attributed that the Ni nanoparticles cannot form continuous electrical conductive network in the wax matrix.33 The low ε′ and ε′′ directly resulted in the weak EM 20

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absorption for Ni nanoparticles sample. On the contrary, the CRF carbon aerogel exhibited very high ε′ (11.4-21.7) and ε′′ (8.2-17.9), due to its low electrical conductive percolation threshold like most carbon materials.34 But too high permittivity always causes the bad impedance matching and brings out the weak absorption but strong reflection.28 The permittivity values of the Ni/carbon aerogels are lower than that of pure carbon aerogels, which was accordant with the result of the electrical conductivity test. The Ni/carbon aerogel-1, Ni/carbon aerogel-2 and Ni/carbon aerogel-3 samples had medium ε′ (8.2-10, 8.2-12.3, 10.1-15.5) and ε′′ (2.5-2.8, 3.0-6.1, 4.1-7.5), respectively. Also, it was found that the Ni/carbon aerogel-1, the Ni/carbon aerogel-2 and Ni/carbon aerogel-3 samples showed medium dielectric loss tangent values (0.25-0.32, 0.37-0.50 and 0.39-0.52) from Figure 10c. Interestingly, the dielectric loss tangent values increased with the increase of Ni content in the Ni/carbon aerogels. The Ni/carbon aerogel-3 composite showed the highest dielectric loss tangent value, indicating the strongest dielectric loss in all Ni/carbon aerogels samples. This phenomenon may be attributed to that Ni nanoparticles could bring out more interfacial polarization loss. It is known that the dielectric loss mainly derives from conductivity loss and interfacial polarization loss. Figure 11 shows the ε′ - ε″ plot of the testing sample. The semicircle in the ε′ - ε″ plot, regarded as Cole-Cole semicircle, represents the Debye relaxation process from Debye relaxation theory.35-37 It was observed that the Cole-Cole semicircle existed in the ε′ - ε″ plot of the Ni/carbon aerogel samples, which corresponded to the polarization process. In this work, the interface polarization loss, which occur in the 21

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interface between neighboring phase with different dielectric constant, could exist among Ni nanoparticles, porous carbon aerogel and wax matrix for Ni/carbon aerogels. The nanoporous structure of the Ni/carbon aerogel played an important role in the enhanced EM absorption. The nanoporous structure of the aerogels with high specific surface areas could cause the formation of dangling-bonded atoms and pore wall defects, further to bring out more polarization loss.12,31 It is illustrated that the Ni/carbon aerogels sample had medium dielectric loss (including conductivity loss and interfacial polarization loss) which can effectively dissipate electrical energy to attenuate incident microwave. 38

Figure 11. Plots of ε′ - ε″ for the pure carbon aerogel (a), Ni/carbon aerogel-1 (b), Ni/carbon aerogel-2 (c) and Ni/carbon aerogel-3 (d) samples with the thickness of 2 mm.

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As for the permeability, µ′ and µ′′ values are determined by the content of magnetic Ni incorporated for the testing sample in this paper. All samples displayed low µ′ values and µ″ values from Figure 10d-e, probably due to the low loading of Ni in the wax matrix. It was noteworthy that the Ni/carbon aerogel samples had enhanced µ′ and µ″ in comparison to those of the pure CRF carbon aerogel. Some negative µ″ values were observed in the testing samples, which were attributed that the magnetic energy radiated out from the samples.39,

40

Meanwhile, the magnetic loss tangent

values of the Ni/carbon aerogel-1, the Ni/carbon aerogel-2 and Ni/carbon aerogel-3 composites listed between those of Ni nanoparticles and CRF aerogel samples, proving the presence of magnetic loss (Figure 10f). Note that the maximum magnetic loss tangent values (0.02) of the Ni/carbon aerogel-3 composite were higher than that (0.01) of the Ni/carbon aerogel-1, indicating that the Ni/carbon aerogel-3 composite had stronger magnetic loss. A comparison of the dielectric loss tangent values and magnetic loss tangent values clearly demonstrated that the Ni/carbon aerogels composites exhibited medium dielectric loss and weak magnetic loss. More importantly, EM absorption performance is directly controlled by the impedance matching ratio.41 As mentioned above, the high ε′ and ε′′ values are not favorable for impedance matching and cause high reflection. So the pure CRF aerogel showed very weak EM absorption, despite the presence of strong dielectric loss. However, improved magnetization properties of the Ni/carbon aerogels can effectively improve the impedance matching with reduction of skin depth.42, 43 Thus, the appropriate complex permittivity and permeability values of the Ni/carbon aerogel 23

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composites can meet the requirement of impedance matching and bring out the strong microwave absorption instead of reflection action. The values of Z = |Zin/Z0| from equation (1) and (2) are used to evaluate impedance matching. 2, 44-45 When Zin = Z0 (Z=1), the material can meet an optimal impedance matching. Figure 12 displays the frequency dependence of impedance matching ratio (Z) for the aerogel samples with the thickness of 2 mm. It can be observed that pure carbon aerogel and Ni/carbon aerogel-1 samples both showed low Z value. Due to the significantly enhanced magnetism and medium ε′ and ε′′ values, Ni/carbon aerogel-2 and Ni/carbon aerogel-3 both had good impedance matching, leading to great microwave absorption in comparison to those of the pure CRF carbon aerogel. The Ni/carbon aerogel-3 had the best impedance matching (close to 1) and showed the best reflection loss (RL) at around 12 GHz in 2 mm thick sample. The good impedance matching ratio plays an important role in microwave absorption for the Ni/carbon aerogels composites.

Figure 12. The frequency dependence of impedance matching ratio (Z = |Zin/Z0|) for the aerogel composite samples.

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Figure 13. Illustration of microwave absorption actions for the Ni/carbon aerogel sample in the paraffin matrix.

Based on these discussions, the enhanced EM absorption mechanism for the Ni/carbon aerogels can be speculated as the following aspects. First, the appropriate conductivity endows the absorbers with medium conductivity loss at the low filler loading. Second, the nanoporous structure of the Ni/carbon aerogels with high specific surface areas can bring out more interface polarization loss. Third, magnetism of the aerogels contributes the weak magnetic loss, and what’s more, the magnetic property can effectively improve the impedance matching between the complex permeability and permittivity, further to contribute EM absorption instead of reflection for absorbers. For this reason, the microwave absorption performances of the aerogels can be controlled via changing magnetic nanoparticles contents. Overall, benefitting from lightweight, appropriate conductivity, magnetism, and nanoporosity with high specific surface areas for the Ni/carbon aerogels, the resultant medium dielectric loss (including conductivity loss and interfacial polarization loss), weak 25

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magnetic loss, as well as good impedance match together contributes the great enhanced microwave absorption (Figure 13).

4. Conclusion In summary, a novel and facile method was developed to fabricate microwave absorption aerogels with suitable conductivity and controllable magnetism. In the resultant aerogels, the crystallized Ni nanoparticles can be evenly coated on the pore surface of the CRF aerogel matrix without aggregation via an autocatalytic reduction process. The influences of different loadings of Ni nanoparticles on the microstructures, porous attributes, electrical conductivities, and magnetic properties were investigated in detail. The results showed that the Ni/carbon aerogels were highly porous with high specific surface area, appropriate electrical conductivities and controllable magnetism. Benefitting from these features, the Ni/carbon aerogels displayed greatly enhanced EM microwave absorption performances. Especially for Ni/carbon aerogel-3, a minimum RL of -57 dB was found at 13.3 GHz with the thickness of 2 mm and filler content of 10 wt% in the wax. Even with the thickness of 1.5 mm, the Ni/carbon aerogel-3 composite exhibited the low RLmin value of -32 dB and wide effective EM absorption bandwidth of 4.0 dB. The dielectric loss (including conductivity loss and interfacial polarization loss), weak magnetic loss, as well as good impedance match contributed greatly enhanced microwave absorption. This Ni/carbon aerogel can be considered as a great microwave absorption material possessing lightweight, low filler loading, strong EM absorption, wide EM absorption 26

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bandwidth and ultrathin thickness. The microwave absorption performances of the aerogels can be designed and regulated via changing magnetic nanoparticles types and contents, which provide a novel strategy for other microwave absorption materials.

Supporting Information Reaction of the generation of Ni on the surface of the carbon aerogel; TGA profile of the Ni/carbon aerogels up to 700 °С in air; STEM image, EDX mapping distribution images, and EDX pattern for the Ni/carbon aerogel-3; Digital photo, element mapping images and SEAD image for the Ni/carbon aerogel-3; Comparison of microwave absorption behaviors between other absorbents in recent reports and the Ni/carbon aerogels in the wax.

Acknowledgments This work was supported by the National Natural Science Foundation of China (Grants 51503191 and 51403192).

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