Rational design of superior microwave shielding composites

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Article

Rational design of superior microwave shielding composites employing synergy of encapsulating character of alginate hydrogels and task-specific components (Ni NPs, FeO/CNTs) 3

4

Nan Zhou, Qingda An, ZuoYi Xiao, Shang-Ru Zhai, and Zhan Shi ACS Sustainable Chem. Eng., Just Accepted Manuscript • Publication Date (Web): 15 May 2017 Downloaded from http://pubs.acs.org on May 16, 2017

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ACS Sustainable Chemistry & Engineering

Rational design of superior microwave shielding composites employing synergy of encapsulating character of alginate hydrogels and task-specific components (Ni NPs, Fe3O4/CNTs) Nan Zhou a, Qingda An a,*, Zuoyi Xiao a, Shangru Zhai a,*, Zhan Shi b a

Faculty of Light Industry and Chemical Engineering, Dalian Polytechnic University, Dalian 116034, China

b

State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, China

*Corresponding authors. E-mail: [email protected] (Q.-D. An); [email protected] (S.-R. Zhai)

1

ACS Paragon Plus Environment


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Abstract Three-dimensional (3D) porous magnetic carbonaceous bead-like (MCB) composites (SA-Ni-(Fe3O4/CNTs)-X, SA stand for sodium alginate, CNTs means carbon nanotubes, and X means Fe3O4/CNTs percentage) have been successfully fabricated through a facile one-step encapsulation process, followed by carbonization at 600 oC in nitrogen atmosphere. These magnetic nickel nanocrystals and Fe3O4/CNTs were uniformly dispersed into the entire porous carbon matrix without aggregation; and various techniques like scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), X-ray photoelectron spectra (XPS) and vector network analyzer (VNA) were conducted to demonstrate the morphology, structure, chemical content, and electromagnetic parameters of the SA-Ni-(Fe3O4/CNTs)-X composites. The effect of the Fe3O4/CNTs molar percentage on the electromagnetic parameters and electromagnetic wave absorbing properties of the SA-Ni-(Fe3O4/CNTs)-X composites were investigated in the frequency range of 2-18GHz. It was proven that the composites would be superior lightweight microwave absorbers when the Fe3O4/CNTs molar percentages were relatively high. When an 25% Fe3O4/CNTs molar percentage was used, it could lead to a maximum reflection loss (RL) of -32dB at 10.8GHz even with a thickness of 2 mm; the effective microwave absorption bandwidth (RL < -10) reached 3.2 GHz (from 9.3 to 12.5GHz). The superior electromagnetic wave absorbing properties could be assigned to the high attenuation, Debye relaxation, electric polarization, interfacial polarization and high conductivity of the task-specific components. It is thus considered that the newly synthesized SA-Ni-(Fe3O4/CNTs)-X composite could be a promising candidate for novel types of light-weight and high-performance electromagnetic wave-absorbing materials with great potentiality in practice. 2


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Keywords: Alginate; Encapsulation; Metal NPs; CNTs; Electromagnetic-wave absorption Introduction Electromagnetic (EM) wave absorber, a kind of functional material, which could absorb electromagnetic waves validly and transform electromagnetic energy into thermal energy or dispel microwave by interference. During the past decade, for the purpose of the military and commercial application, an increasing number of materials have been investigated as microwave absorbers, leading to extensive research on the development of microwave absorbers with lightweight, thin coating thickness, broad eﬀective frequency and strong absorption properties and so on. 1,2 Comparatively, in past several years, 3D porous composites in the field of electromagnetic absorption have attracted particular attention, considering these matrices with a porous structure which would typically provide new interfacial effects, with complementary and synergetic behaviors between the host and guest species.3 Currently, the extensive studies on 3D porous composites in the field of electromagnetic waves absorption can be largely associated with its special heterogeneous interfaces. The presence of the heterogeneous interface can greatly improve the electromagnetic wave absorption performance. For instance, as a result of multi-interfacial polarizations, the dielectric loss performance could be enhanced by heterogeneous interfaces.4 In addition, the heterogeneous interface can also provide more active sites to promote the scattering and reflection of electromagnetic waves. The electromagnetic waves would be scattered and absorbed back and forth inside the composites to further achieve the purpose of weaken electromagnetic waves.5 Therefore, the 3D porous composites would be hopeful to have a major breakthrough in the field of electromagnetic wave absorption. As was demonstrated, Zhang and 3



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coworkers synthesized Fe-Co/nanoporous carbon (NPC) nano-composites and studied their electromagnetic wave absorption properties.6 The maximum reflection loss of the composites reached -21.7dB at 15.2GHz and the absorption bandwidths more than -10 dB were 5.8 GHz with a thickness of 1.2 mm. Besides, the authors have synthesized

three-dimensional

(3D)

hierarchical

cabbage-like

carbonaceous

Fe/CCMs-X (X means treating temperature) composites by a hydrothermal reaction and carbonization processes, and the composite materials exhibited a wider absorption band and a greater reflection loss.7 The maximum reflection loss (RL) of the composites reached -22.9dB and the absorption bandwidths were 4.32GHz with a thickness of 2.5 mm. Additionally, Zhu et al. prepared core-shell structured MnFe2O4@SiO2+polyvinylidene fluoride (PVDF) nano-composite and researched their electromagnetic wave absorption properties.8 It indicated that the absorption bandwidth with (RL) values less than -10 dB was up to 3.58GHz, and the maximum reflection loss (RL) of the composites gained -25.73dB. More recently, Wang et al. fabricated 3D net-like SnO2/Fe3O4/multi-walled carbon nano-tubes (MWCNTs) composites, and this kind of composite materials exhibited a wider absorption band and a greater reflection loss.9 As proposed, the designed porous structure can lead to some salutary physical effects to promote electromagnetic wave absorption, including the interaction of magnetic properties and dielectric properties, reflection and scattering within the material, and multi-polarization at the hetero-interface.10-12 However, although several kinds of 3D porous absorbers for electromagnetic wave absorption have been reported in previous works, it is still a great challenge to control high dispersion of magnetic species in related 3D materials in a facile, controllable manner. More unfavorably, the high-cost, high-density, low-yield and complicated preparation processes are extra limiting factors for their potential applications. 4


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In view of these issues, nowadays the researchers have turned to nature for new research ideas, among which is to make the biomass as a carbon-based material precursor. As a result, biomasses have been paid more and more attention in the preparation of useful carbonaceous functional materials. Amongst various agricultural/industrial by-products, the seaweed-derived alginate has attracted increasing interest to fabricate various functional materials, due to its easy availability and large annual production.13-16 More interestingly, alginate macromolecules can be easily cross-linked with metal ions, such as Fe3+, Co2+ and Ni2+, etc. Actually, in our previous study,17 for a binary phase hetero-structured electromagnetic wave absorber (SA-Fe-X), sodium alginate could be cross-linked with Fe3+ to form a gel, which was freeze-dried and carbonized at 600 oC, leading to a 3D porous composite material with favorable microwave absorption properties. The composite materials were found to exhibit a wider absorption band and a greater reflection loss. As compared to those related absorbers prepared by multi-steps, this kind of 3D composite materials not only have a facile preparation process, low-density, high-yield and developed nano-network, but also can effectively make the magnetic metal particles highly disperse into the carbonaceous matrix, hence improving the absorption of electromagnetic waves. However, in order to further enhance the absorption properties of this type of 3D composites, there is still significant improvement that can be achieved. To the best of our knowledge, MWCNTs are widely seen as the most competitive candidate because of its well conductivity, excellent mechanical strength, and unparalleled tubular structure, which make them a great potential for fabricating efficient electromagnetic wave absorbers at low filling levels.18,19 For instance, Liu et al. synthesized Fe3O4-doped MWCNTs composites and investigated their microwave absorption 5



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properties, finding that the composite exhibited great electromagnetic wave attenuation.20 Accordingly, based on the above proposal and our experience on fabricating 3D composites using alginate,21,

22

herein we present a simple fabrication method of

one-step encapsulation and controllable carbonization process to synthesize a novel porous graphitic carbon composites with excellent microwave absorbing performance. Firstly, the Fe3O4-doped MWCNTs composites was mixed with sodium alginate and then cross-linked by Ni2+ ions to form a gel via an easy-to-handle encapsulation. Secondly, a carbonization process was carried out at 600 °C to obtain 3D SA-Ni-(Fe3O4/CNTs)-X. The structure, morphology, magnetic properties and interfacial interactions SA-Ni-(Fe3O4/CNTs)-X were fully investigated. The effect of different percentage of filled-Fe3O4@MWCNTs on the electromagnetic wave absorption properties of synthesized composites was thoroughly studied.

Experimental section Materials Sodium alginate, ammonia solution, nitric acid (HNO3), FeCl3.6H2O, FeCl2.4H2O and Ni(CH3COO)2 ·4H2O were purchased from Tianjin Kermel Chemical Reagent Co. The MWCNTs (diameter, 20-40 nm; length, ~20 µm) were purchased from Shenzhen Nanoport Co. Ethanol was purchased from Tianjin Kermel Chemical Reagent Factory, China. All chemicals were of analytical grade and used without further purification. Preparation of oxidized MWCNTs In a typical synthesis, 1.0 g of the pristine MWCNTs was first dispersed in 100 mL 68% HNO3 by exerting ultrasonic dispersion and refluxed at 120 oC in an oil bath with constant stirring for 6 h. After the mixture was cooled down to room temperature, the treated MWCNTs was filtered and then washed with distilled water until the pH 6


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value reached neutral, the obtained MWCNTs was dried at 60 oC for further use. Preparation of MWCNTs/Fe3O4 composite Typically, 0.5 g of the above oxidized MWCNTs and 0.3 g of FeCl3.6H2O were dispersed in 100 mL distilled water under mechanical stirring at 70 oC. Afterwards, the reaction system was maintained for 15 min before N2 was purged into solution to remove oxygen. Then, 0.1 g of FeCl2·4H2O was added and keep stirring under N2 atmosphere for 30 min. Subsequently, 3 mL of ammonia solution was added into the above reaction system. The reaction was allowed to proceed for another 40 min under N2 atmosphere, and the Fe3O4/MWCNTs composites were prepared. Finally, the composites were separated using a magnet and washed with distilled water several times, Fe3O4/MWCNTs composites were obtained and dried at 60 oC for further use. Preparation of MCB SA-Ni-( Fe3O4/CNTs )-X Sodium alginate (SA) with determined amounts of MWCNTs/Fe3O4 (0%, 9%, 16.5%, 25%) was dispersed in 100 mL distilled water and magnetic stirring for 12 h. The mixture was then added dropwise to a 5% Ni(CH3COO)2 solution for cross-linked hydrogels beads left undisturbed in Ni(II) solution for 6 h for complete cross-linking to obtain nickel-alginate hydrogels beads. Lastly, the hydrogels beads were washed with distilled water followed by ethanol to remove extra CH3COO- and Ni2+ ions. Then, the as-prepared hydrogels beads were transferred to a freezer at -50 oC for 12 h to obtain 3D SA-Ni-(Fe3O4/CNTs)-X aerogels. Then, the obtained aerogels beads were placed in a tube furnace and heated at a rate of 5 oC min-1 to 600 oC and kept for 2h in N2 atmosphere. After being cooled in flowing N2, the obtained samples were recorded as SA-Ni-( Fe3O4/CNTs)-X (the number X refer to the mass percentage of Fe3O4/CNTs). Materials characterization 7



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The morphology and structure of the samples was investigated by high resolution transmission electron microcopy (HRTEM, Hitachi H9000NAR) and scanning electron microscope (SEM, JEM JEOL 2100) with an energy dispersive spectroscopy (EDS) images, respectively. The crystal structure was determined by X-ray diffraction, the XRD patterns were obtained with a Rigaku model D/max-2500 diffractometer using Cu Kα radiation (λ = 1.5406 Å) at 40 KV and 40 mA, scan rate (2θ) of 8o min-1 ranging from 10o to 70o. The chemical bonds were analysed by Fourier Transform Infrared spectroscopy was recorded on Perkin-Elmer FT-IR spectrophotometer 400-4000cm-1 resolution. The chemical composition was investigated by X-ray photoelectron spectroscopy (XPS) measurements were performed using the PHI 5000 Versa Probe systems. The magnetic properties of the composites were measured by vibration sample magnetometer (VSM) at room temperature. Electromagnetic measurements The electromagnetic wave absorption properties of SA-Ni-(Fe3O4/CNTs)-X composites were researched by using a vector network analyzer (Agilent N5222A) in the range of 2-18GHz. Coaxial specimens for electromagnetic parameters were fabricated by mixing paraffin with 15wt% and pressing them into a cylindrical-shaped (ψout of 7.0 mm, ψin of 3.04mm). The reflection loss (RL) was calculated according to the following equations:23 Zin = Z0 (µ r/ε r)1/2 tan h [ j(2πƒd)/c(µ r ε r)1/2] RL = 20 log | (Zin - Z0)/ (Zin + Z0) |

⑴ ⑵

where Zin stands for the input impedance of the absorber, Z0 is the impedance of free space, d is the thickness of absorber, ƒ is the frequency, c is the velocity of light, µr (µr = µ' - jµ'') and εr (εr = ε' - jε'') are the relative permeability and permittivity, respectively. 8


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Results and discussion Characterization of SA-Ni-( Fe3O4/CNTs)-X The manufacturing process of SA-Ni-(Fe3O4/CNTs)-X is illustrated in Scheme 1. Firstly, the Fe3O4/CNTs were synthesized by a co-precipitation method, followed by mixing with sodium alginate to form a sol. Then, by electronic attraction, the carboxyl group of SA can form a complex with Ni(II). By virtue of this one-step encapsulation process, plenty of nickel ions could be bound by ion-exchange between sodium and nickel ions. Secondly, the as-prepared hydrogels beads were transferred to a freezer at -50 oC for 12 h to obtain dried 3D SA-Ni-(Fe3O4/CNTs)-X aerogels. Then, the obtained aerogels spheres containing different percentages of Fe3O4/CNTs were placed in a tube furnace and carbonized at temperature 600 oC under an N2 atmosphere, and the obtained carbonaceous samples would be ready for electromagnetic wave absorption performance testing. Firstly, the morphologies and structures of the porous SA-Ni-(Fe3O4/CNTs)-X and SA-Ni hybrids were characterized by using (SEM) and (TEM). SEM and TEM images of SA-Ni-(Fe3O4/CNTs)-X and SA-Ni are shown in Fig. 1 and Fig. 2. The morphology of the SA-Ni composites could be clearly presented from SEM images Fig. 1(a1 and a2). Clearly, the surface of the carbon matrix is rough and the carbon skeleton is seriously coacervated, which is not beneficial to the formation of the electromagnetic wave track, further hampering the diffusion of electromagnetic waves inside the carbon skeleton. However, when Fe3O4/CNTs were introduced into SA-Ni by encapsulation and carbonization, it was found that Fe3O4/CNTs was thoroughly encapsulated by the carbon matrix, as shown in Fig. 1(b1 and b2). The structure was fluffy and the pore size increased, leading to the formation of a porous network. Nevertheless, with the increase of Fe3O4/CNTs content, the structure became more 9



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abundant, as shown in Fig. 1(c1 and c2). Hence, for SA-Ni-(Fe3O4/CNTs)-25%, the abundant channel structure might further promote the loss of electromagnetic waves.24 Moreover, elemental mapping by EDS (see inset of Fig. 1(d)) wildly used in analyzing the composition of the SA-Ni-(Fe3O4/CNTs)-25%, and furnished evidence that the distribution of C, Fe and Ni elements within the carbonaceous bead-like was homogenous. Meanwhile, available TEM was used to further investigate the microstructures of the SA-Ni-(Fe3O4/CNTs)-X and SA-Ni. The TEM image of the SA-Ni clearly shows that their shapes are spherical with diameters mainly from 17 to 27 nm and the particle distribution is uniform, without remarkable aggregation (see inset of Fig. 2(a)). Furthermore, the C layer covers Ni and NiO nanoparticles (see inset of Fig. 2(b and c)). Fig. 2(d) shows a lattices the image of Ni, it can be clearly seen that an ordered array of crystal lattices with an interplanar spacing of 0.25 nm, corresponding to standard (111) surface Ni lattices. Nothing that, from Fig. 2(d), one can see that the covered carbon layers has no stationary interplanar spacing, indicating that the microstructure of carbon is composed of relatively disordered arrays, which would be further supported by the XRD results given below. Besides, in contrast to pure SA-Ni, being incorporated magnetic CNTs, the internal structure surely became more complex. Fig. 2(e) displays the TEM image of as-prepared SA-Ni-(Fe3O4/CNTs)- 25% composites. It can be observed that the CNTs are not intertwined each other but relatively evenly distributed in the carbon layer, and the metal particles (Ni) uniformly distributed into the network. The result in Fig. 2(f) shows that Fe3O4 particles tightly adhere to the surface of the CNTs with a particle size distribution ranging from about 30-40 nm. Moreover, these Fe3O4 particles exposed on the surface of CNTs can easily attach to Ni particles, which are easy to form surface polarization and can promote the 10


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absorption of electromagnetic waves. Moreover, it is worth to noting that the C layers contains a number of structural defects, such as C-layer breakage and severe mixing, as shown in Fig. 2(h). These defects are prone to form the uneven and terraced fields.25 However, the presence of defective sites in C layers facilitates microwave absorption. The defect site generates an additional energy state near the Fermi level, which can reduce the electron transition energy.26 In addition, the outlines of Fe3O4/C, Fe3O4/CNTs and Ni/CNTs interfaces can be clearly observed, as shown in Fig. 2(g and h). These interfaces would induce plenty of beneficial substances that can exert effects on microwave absorption, such as interfacial polarization, microwave scattering and reflecting. With increasing Fe3O4/CNTs content, this heterogeneous interface also increased, the loss of electromagnetic waves would be constantly strengthened. The structures and phase composition of the synthesized SA-Ni-(Fe3O4/CNTs)-X and pure SA-Ni were further examined by XRD and the results are presented in Fig. 3. Clearly, as is shown by Fig. 3(a), the peaks at 2θ = 25.9o in pure SA-Ni show that the sample consists mainly of amorphous carbon, resulting from the carbonization of alginate organic polymers at high temperature treatment.27 Moreover, the characteristic diffraction peaks at around 2θ = 45o, 53o correspond to the (111), (200) planes to Ni (JCPDS card no. 04-0850). The other diffraction peaks at around 2θ = 42o, 47o can be corresponded to the NiO.28 When introducing the Fe3O4/CNTs into sodium alginate, besides the XRD diffraction peaks of Ni and NiO phases, there are also Fe3O4 peaks. Additionally, it was also observed that a broad diffraction peak at around 2θ = 26.1o was detected, which can be described to the (002) crystal plane of hexagonal graphite (JCPDS 65-6212). These results confirmed that MWCNTs species had been successfully encapsulated in the final samples,9 implying that 11



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SA-Ni-(Fe3O4/CNTs)-X composites were mainly composed of magnetic species of Ni crystals, Fe3O4 and carbonaceous substances of MWCNTs and alginate-derived carbonaceous network (Fig. 3(b)). More interestingly, with the increase of Fe3O4/CNTs content introduced in the samples, the intensity of diffraction peaks of the Ni phase, Fe3O4 phase and MWCNTs phase increased significantly (see inset of Fig. 3(c and d)). It suggested that the Ni, Fe3O4 and MWCNTs in SA-Ni-(Fe3O4/CNTs)-25% should be of higher degree crystallinity. It is well known that, for a typical magnetic loss material for EM wave absorption, the increment of Fe3O4 component would be beneficial to the EM wave absorption.29 Furthermore, according to the analysis of XRD results, it can also be deduced that the encapsulation of Fe3O4/CNTs species into SA-Ni had minor impact on the original crystal structure, which will be further supported by the following FT-IR results. Additionally, FT-IR results can provide information on the bonding of organic functional groups. The FT-IR spectra of the SA-Ni-(Fe3O4/CNTs)-X(X = 0%, 9%, 25%) were recorded within the range of 4000-400 cm-1 and were displayed in Fig. 4. It can be observed from the FT-IR spectrum of the SA-Ni that a band appeared at around ~3433 and ~1073 cm-1, which can be associated with O-H and C-O stretching vibrations, indicative of the presence of numerous hydroxyl groups in SA. The bands at ~1632 cm-1could be associated with (C=C) stretching vibrations, indicating that SA-Ni produced unsaturated groups during the pyrolysis process. Besides, the appearance of peaks at ~1384 cm-1 and ~2924 cm-1 can be corresponded to the C-H symmetric deformation vibration and -CH2 group. When Fe3O4/CNTs was introduced into sodium alginate, the SA-Ni-(Fe3O4/CNTs)-X (X = 9%, 25%) composites exhibited similar FT-IR results as SA-Ni. However, in contrast to the SA-Ni and 12


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SA-Ni-(Fe3O4/CNTs)-X FT-IR spectra of the characteristic peaks, small yet distinct differences can be observed in the 559 cm-1. This difference of characteristic peaks between SA-Ni and SA-Ni-(Fe3O4/CNTs)-X can be possibly due to the presence of Fe-O.

It

indicated

that

the

Fe3O4/CNTs combined

with

SA

forming

SA-Ni-(Fe3O4/CNTs)-X composites successfully. However, with the increase of Fe3O4/CNTs content, the Fe-O bond became stronger and stronger, which may be due to the high Fe3O4/CNTs content. XPS is a very useful technique to investigate the surface states of materials, which can deliver the information regard to the atomic compositions, also can identify the type of bonds between atoms. The XPS survey spectra of SA-Ni and SA-Ni-(Fe3O4/CNTs)-25% composites are shown in Fig. S1(a). Clearly, three strong peaks at 284.6ev, 529.8ev, and 860ev can be observed from the XPS survey spectra of SA-Ni composites, which can be ascribed to the C, O and Ni, respectively; and the atomic percentages of Ni, O and C in the surface were roughly estimated to be 1.91%, 8.6% and 89.49%, respectively. As a highly accurate result of XPS, the predominant presence of C in the surface (89.49%) indicates that Ni nanoparticles are well encapsulated by C layers, which can be deduced by the TEM results. In addition, the results of XPS analysis indicate that the atomic percentages of C, O, Ni and Fe in the surface of SA-Ni-(Fe3O4/CNTs)-25% composites were 91.87%, 6.78%, 1.1% and 0.25%, respectively. Compared with SA-Ni composites, the content of C increases obviously, which may be attributed to the introduction of MWCNTs. The O1s spectrum of SA-Ni-(Fe3O4/CNTs)-25% shows four peaks, e.g. 528.1ev, 529.5ev, 531.5ev and 530.6ev, which can be assigned to C-O, Fe-O, Ni-O and O-C=O,30 respectively (Fig. S1(b)). The formation of this carboxyl group (O-C=O) is possibly due to the addition of the oxidized MWCNTs. As shown in Fig. S1(c), the spectrum of 13



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C1s shows three peaks at 284.1, 284.6 and 285.3ev, corresponding to the C=C, C-H and C-O groups of SA-Ni-(Fe3O4/CNTs)-25%, respectively.31 The perfection degree of the carbon layers can be estimated by the intensity ratio of C=C. As can be seen from Fig. S1(c), the strong peak represents a relatively imperfect array of carbon layers. In the Ni 2p spectra (Fig. S1(d)), the fitting peak Ni 2p3/2 at 852ev corresponds to the Ni-Ni bond,28 whereas the Ni 2p1/2 peak at 866.7ev can be assigned to Ni2+ which stems from the NiO.28 In addition, considering the X-ray diffraction peaks of γ-Fe2O3 is very similar to XRD peaks of Fe3O4, an XPS measurement was conducted to ensure the valence states of the Fe element. As shown in Fig. S1(e), there are two peaks at binding energies of 709.2 eV and 723.5 eV, corresponding to the Fe 2p3/2 and Fe 2p1/2, surely indicating the appearance of Fe3O4 in the sample. Meanwhile, the XPS also showed C, O and Ni elements in the SA-Ni, which is consistent with the EDS results. Accordingly, on the basis of the above analysis results, it is confirmed that the preparation of SA-Ni-(Fe3O4/CNTs)-X composites with tunable composition, controllable dispersion has been achieved. . Magnetic properties The

above

characterization

results

indicated

that

the

porous

SA-Ni-(Fe3O4/CNTs)-X composites was successfully fabricated by an one-step encapsulation process combined with heating treatment. The characteristics of the composite

components

and

the

porous

structure

of

the

prepared

SA-Ni-(Fe3O4/CNTs)-25% composites are expected to possess good microwave absorption properties. To start, the magnetic properties of SA-Ni-(Fe3O4/CNTs)-X composites were researched at room temperature using a vector network analyzer (VSM). As shown in Fig. 5, two samples showed typical ferromagnetic hysteresis loops, mainly associating with the presence of metallic Fe3O4 and Ni nanoparticles. 14


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The saturation magnetization (MS), coercivity (Hc), and remnant magnetization (Mr) are 8.043 emu·g-1, 21.104 Oe, and 2.312 emu·g-1 for the SA-Ni-(Fe3O4/CNTs)-9%, and 22.829 emu·g-1, 25.654 Oe, and 0.9368 emu·g-1 for the SA-Ni-(Fe3O4/CNTs)-25%, respectively. The low MS of the as-prepared SA-Ni-(Fe3O4/CNT)-9% samples is mainly related to the lower Fe3O4/CNTs percentage, resulting in the relatively poor crystallinity. Moreover, the defects in the crystal structure surface may also lead to a significant decrease in the saturation magnetization of the porous carbon matrix.32 However, as compared to SA-Ni-(Fe3O4/CNTs)-9%, the SA-Ni-(Fe3O4/CNT)-25% sample has an enhanced coercivity. With the increase of Fe3O4/CNTs amount in the composites, the coercivity increased also. Therefore, it can be deduced that the increase of coercivity could be closely attributable to the increasing ratio of the heterogeneous component of MWCNTs in the composites. The larger Hc value might have led to a larger magneto-crystalline anisotropy energy, which would be helpful to enhance the microwave absorption properties.33 Electromagnetic wave absorption properties Generally, high-performance electromagnetic wave absorption is usually derived from the complementarities between the effective permeability and the permittivity of the materials. Therefore, our independent measurement of complex permittivity (εr=ε'-jε'') and permeability (µr=µ'-jµ'') of these samples to are used to betterly understand their microwave absorption properties. Fig. 6 shows the electromagnetic parameters (relative permittivity, εr=ε'-jε'' and relative permeability, µr=µ'-jµ'') of the composites containing 15wt% of SA-Ni-(Fe3O4/CNTs)-X (X = 0%, 9%, 16.5%, 25%), respectively, and the EM absorption properties were measured at room temperature in the frequency range of 2-18 GHz. The real parts (ε' and µ') represent the storage of electric and magnetic energy, respectively. The imaginary parts (ε'' and µ'') imply the 15



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loss of electric and magnetic energy. As illustrated in Fig. 6 (a), it can be seen that the real parts of complex

permittivity of the

SA-Ni-(Fe3O4/CNTs)-25% and

SA-Ni-(Fe3O4/CNTs)-16.5% samples tend to rapidly decrease with increasing frequency, whereas the ε' of the other two samples tend to slowly, steadily decrease with increasing frequency. According to those previous literature reports, this appearance can be put down to the fact that the dipoles exist in the material that more and more hard to keep the phase orientation with the electric vector of the incident radiation.34 Furthermore, the high conductivity of MWCNTs is denotative of a comparatively high dipole density,35 with the increase of Fe3O4/CNTs content, the density of the dipole also increased, which cannot reorient itself in the wake of the applied electric field, and may be responsible for observing quickly reduce in ε' of SA-Ni-(Fe3O4/CNTs)-25% and SA-Ni-(Fe3O4/CNTs)-16.5% samples. It is worth noting that the ε' values of the SA-Ni-(Fe3O4/CNTs)-25% sample is higher than that of the other three samples. This manifest better energy storage and polarization. As shown in Fig. 6(b), with increasing Fe3O4/CNTs amount, the ε'' values tend to decrease from

12.9,

8.2,

2.7 and 2.3 to 6.2,

4.9,

1.7 and

1.5 for

SA-Ni-(Fe3O4/CNTs)-X (X = 25%, 16.5%, 9%, 0%), respectively, in the frequency range of 2-18 GHz. It can be observed that the SA-Ni-(Fe3O4/CNTs)-25% sample was significantly larger than those of SA-Ni-(Fe3O4/CNTs)-X (X = 16.5%, 9%, 0%). As everyone knows, the ε'' values are in keeping with the dielectric loss ability. A higher ε'' values also imply that this structure could be beneficial for generating higher dielectric loss. When the electromagnetic wave approaches to the heterogeneous interfaces, the interfacial polarization would take place and a number of polarized charges would amass at the heterogeneous interfaces, strengthen dielectric loss. The real part (µ') and imaginary part (µ'') of the composites are shown in Fig. 6(c) 16


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and (d). It can be found that the µ' values exhibit an increase tendency in the 2-18 GHz frequency range with slight fluctuations, only the µ' values of the SA-Ni-(Fe3O4/CNTs)-25% composite increase significantly in the range of 15-17GHz. While the corresponding µ'' values exhibit a decrease tendency in the frequency region over 2-18 GHz for SA-Ni-(Fe3O4/CNTs)-X (X = 0%, 9%, 16.5%, 25%) respectively, among the four samples, the µ'' values are fluctuate around zero. The real part µ' SA-Ni-(Fe3O4/CNTs)-X composites kept almost constant in the low frequency range and then gently increased with the increase of frequency. The imaginary part µ'' of the SA-Ni-(Fe3O4/CNTs)-X composites behaves in contrast with respect to frequency as that of real part µ'. Compared with the SA-Ni-(Fe3O4/CNTs)-0%, the imaginary part µ'' of the SA-Ni-(Fe3O4/CNTs)-X composites would be lower because the amount of Fe3O4/CNTs increase; this then resulted in the decrease of the amount of eddy currents induced by the electro-magnetic waves in system.36 In general, magnetic loss is mainly due to the natural resonance, hysteresis, exchange resonance, domain-wall displacement and eddy current effect.37 However, the hysteresis loss in the weak field is negligible. The domain wall resonance is usually located at a much lower frequency range of the MHz frequency.38 Hence, eddy current effects and natural resonance can cause attenuation of EM waves in the 2-18 GHz frequency range. The eddy current loss can be expressed by the following equation:39 µ'' = 2πµo(µ')2σd2ƒ/3

(3)

where µo represent the permeability of vacuum, d represent the thickness, and σ represent the electrical conductivity of the composite. If magnetic loss only come from eddy current effects, the values of Co = µ''(µ')-2 ƒ-1 should be constant when the frequency changes. From Fig. 7, the values increased in the beginning, afterwards 17



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decreased in the frequency range of 2-3GHz and 3-4GHz singly, and four weak peaks consisted over the frequency range of 4-18 GHz. The values of Co = µ''(µ')-2ƒ-1 are nearly constant, which means that the eddy current loss mainly affected the dissipation of microwave energy, particularly in the range of 5-18 GHz. However, another mechanism for magnetic loss is natural resonance, which can be expressed by the following equation:40 Ha = 4|K|/3µ0MS

(4)

2πƒr =γ Ha

(5)

where γ mean the gyromagnetic ratio, Ha represent the anisotropy energy, and |K| represent the anisotropy coefficient. As we all know, the resonance frequency rest on the anisotropy field, which is related to the coercivity value of the materials.41 With the increase of Fe3O4/CNTs content, anisotropy increased also. The higher anisotropy energy would be greatly helpful to improve electromagnetic waves absorption properties. The two possible contributions to electromagnetic waves absorption are dielectric loss and magnetic loss.42 To further study which one is leading for the SA-Ni-(Fe3O4/CNTs)-X (X = 0%, 9%, 16.5%, 25%) composites, the magnetic loss tangent (tan δm =µ''/µ') and dielectric loss tangent (tan δe =ε''/ε') are calculated and shown in Fig. S2. Among the four samples, a decreasing tendency of dielectric and magnetic loss is generally observed as the frequency increases, respectively. Only the SA-Ni-(Fe3O4/CNTs)-16.5% tended to increase with frequency increasing from 1.9 to 0.78. In the high-frequency region, the appearance of the negative tan δm possibly resulted from the negative permeability. In the entire frequency range, the dielectric loss tangent is much larger than the magnetic loss tangent, implying that the dielectric loss played an important part in the EM absorption of these samples. As everyone 18


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knows, the dielectric loss of the EM energy principally roots in the interfacial polarization and the dipole polarization at microwave frequencies.43 In this process, the amount of the surface atoms with unsaturated bonds in Fe3O4/Ni particles will tremendously increase, finally resulting in the increase of the diploes. Therefore, a large amount of dipole polarization can be devoted to the dielectric loss. In addition, the fine dispersion of Fe3O4/CNTs in the porous carbon matrix can introduce more additional interfaces and consequently interfacial polarization, leading to the interfacial polarization and dielectric loss.44 Furthermore, the space-charge polarization, happening among adjacent metal nanoparticles, probably could lead to the increase of the dielectric loss to a certain degree.42 As is stated above, the composite is an electromagnetic wave absorber of dielectric loss type. To better understand the mechanisms of dielectric loss of the electromagnetic absorber, is could be explained by the Debye theory, and the relationship between ε' and ε'' can be expressed by the following equation:45 ε r =ε' + iε'' =ε∞ + (ε s - ε o)/(1+iωτo)

(6)

where ε∞ represent the relative dielectric permittivity at high-frequency limit, ε s mean the static permittivity, ω is frequency, τ is relaxation time, respectively. According to the above eqn 6, ε' and ε'' can be deduced as given below. ε' = ε∞ + (ε s - ε o)/[1+(ωτo)2]

(7)

ε'' = ωτo(ε s - ε o)/ [1+(ωτo)2]

(8)

According to eqn (7) and (8), the relationship between ε' and ε'' can be described as: [ε' - (ε s + ε∞) /2]2 + (ε'')2 = [(ε s - ε∞)/2]2

(9)

ε' = ε''/(2πƒτ) + ε∞

(10)

Based on eqn (10), the curves of (ε'-ε'') is a single semicircle, which is defined as a Cole-Cole semicircle, a semicircle corresponds to a Debye relaxation process. As 19



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shown in Fig. 8(a), no evident semicircle can be found in SA-Ni-(Fe3O4/CNTs)-X(X = 0.9%). For SA-Ni-(Fe3O4/CNTs)-16.5%, three small semicircles and a tiny semicircle is found in Fig. 8(b). Interestingly, a relatively large semicircle and five small semicircles are found for SA-Ni-(Fe3O4/CNTs)-25% sample (Fig. 8(c)), indicating

that

there

are

six

dielectric

relaxation

processes

in

the

SA-Ni-(Fe3O4/CNTs)-25% hybrid composite. It also demonstrates that the addition of Fe3O4/CNTs promotes the Debye dipolar relaxation process. According to those previous literatures, Ni nanoparticles possess high conductivity, when the electromagnetic wave is transmitted to the surface of the Ni particles, the high conductivity of metal Ni would make the significant skin effect happen.46 At the same time, it would result in its dielectric loss performance degradation. Nevertheless, the doped MWCNTs are a polar material with abundant electric dipole which shows a regular rotation in an alternating electronic field. This regular rotation would lead to a dielectric relaxation.47 Furthermore, the interfaces of the heterogeneous Fe3O4/CNTs nano-composites make interfacial polarization possible. On commutative electronic field, the contact of negative charges and positive charges at the interface of the heterostructure would lead to the variation in the charge density and generate an electric dipole moment, causing interfacial relaxation.48 Therefore, the dielectric loss of porous SA-Ni-(Fe3O4/CNTs)-X composites could be attributed to dipole polarization, interfacial polarization and space-charge polarization together. Additionally, in the interior of absorber, EM-wave attenuation plays an important part in the damping of the electromagnetic waves. The attenuation constant α means the attenuation effect of materials, as calculated according to the below formula:49 α = (√2πƒ/c) x √[(µ'' ε'' - µ' ε')+√[(µ'' ε'' - µ' ε')2 +(µ'' ε'' + µ' ε')2]]

(11)

where ƒ represent the frequency of the electromagnetic waves and c is the velocity of 20


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electromagnetic waves in free space. As shown in Fig. S3, the relying of attenuation constant α on frequency reveals that the SA-Ni-(Fe3O4/CNTs)-25% shows the largest α value among the four samples in the frequency range of 2-18GHz. Therefore, it can be inferred that the SA-Ni-(Fe3O4/CNTs)-25% sample might exhibit the excellent electromagnetic

wave

absorption

performance

than

those

of

the

SA-Ni-(Fe3O4/CNTs)-X (X = 0%, 9%, 16.5%,) samples. To further investigate the electromagnetic wave absorption performance of the as-synthesized SA-Ni-(Fe3O4/CNTs)-X composites, the reflection loss (RL) values are calculated from the relative complex permittivity (εr) and permeability (µr) at a given frequency (2-18GHz) and different absorber thickness. Fig. 9(a-e) depicts the calculated theoretical reflection loss (RL) curves of the composites-wax with different thicknesses in the frequency range of 2-18 GHz with a loading of 15 wt%. It can be observed that the microwave absorption performances of SA-Ni-(Fe3O4/CNTs)-X composites are better than those of pure SA-Ni hybrids. In particular, the SA-Ni-(Fe3O4/CNTs)-25% sample exhibits the best electromagnetic absorption properties, and the maximum RL is -32dB at 10.8GHz with only a thickness of 2 mm. The effective microwave absorption bandwidth (RL

Rational design of superior microwave shielding composites

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