Journal of Alloys and Compounds 769 (2018) 961e968 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom Enhanced electromagnetic wave response of nickel nanoparticles encapsulated in nanoporous carbon Bin Quan, Guoyue Xu, Heng Yi, Zhihong Yang**, Junxian Xiang, Yutian Chen, Guangbin Ji* College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing, 211100, PR China a r t i c l e i n f o a b s t r a c t Article history: Received 3 July 2018 Received in revised form 3 August 2018 Accepted 7 August 2018 Available online 11 August 2018 Nickel nanoparticles embedded nanoporous carbon (NPC/Ni) was fabricated via thermal treatment of metal organic framework precursors. Nickel particles were uniformly dispersed in the nanoporous carbon network under certain sintering temperature. Meanwhile, the obtained magnetic metal Ni could improve the electrical conductivity of carbon due to its graphitization catalysis on amorphous carbon. The obtained multiple interfaces, porous structures as well as tunable electrical conduction are all beneﬁcial to the enhanced microwave absorption. The sample prepared at 700 C exhibits nice microwave absorption capacity with the maximum reﬂection loss (RL) value of 39.4 dB and effective bandwidth of 4.2 GHz. In addition, the prepared sample exhibit huge potential as light absorber. When sample loading content decreases from 30 wt% to 25 wt%, its maximum RL value reaches 39.4 dB with an effective bandwidth of 4.2 GHz. This work not only demonstrates that the NPC/Ni composites are excellent absorbers with wide bandwidth, strong absorption, thin thickness and light weight, but also initiates a new pathway for artiﬁcially designed magnetic metal/dielectrics composites nanostructures with objective functionalities. © 2018 Elsevier B.V. All rights reserved. Keywords: Ni nanoparticles Nanoporous carbon CPO-27-Ni Microwave absorption 1. Introduction With the growing electromagnetic interference in daily life, high-efﬁciency microwave absorbing materials with low thickness, lightweight, wide absorption frequency and strong absorption capacity are eagerly required [1e3]. Among them, the magnetic/ dielectric composites exhibit huge potential compared to pure dielectric or magnetic absorbers [4,5]. As we all know, electromagnetic wave absorption is related to complex permittivity and pﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ permeability Zin ¼ Z0 mr =εr , hence, ideal microwave absorbers should possess large saturation magnetization/high Curie temperature and moderate permittivity values [6e8]. When the intrinsic impedance matching is close to the free space impedance, more microwaves can be introduced into the interior of absorbers [9,10]. Thus the construction of magnetic metal/carbon matrix composites is to be a reliable option. Of all the magnetic metals, nickel has attracted broad attention due to its relative high magnetization as well as tunable size/shape anisotropy [11,12]. However, * Corresponding author. ** Corresponding author. E-mail addresses: email@example.com, (Z. Yang), firstname.lastname@example.org (G. Ji). https://doi.org/10.1016/j.jallcom.2018.08.069 0925-8388/© 2018 Elsevier B.V. All rights reserved. email@example.com conventional fabrication methods are heavily restricted by the extra factors. The solvent reaction is easily affected by solution environment and addition agent , and the magnetic metal particles at the nanoscale tend to reunite into bulk . In consideration of architectural design, nanoporous structure is favor of the microwave attenuation, however, the traditional template method requires complicated operation procedures [15e17]. In view of the above-mentioned situation, Ni-based CPO-27-M metal organic framework  is used as precursor to construct the nickel particles embedded nanoporous carbon (NPC/Ni). During high sintering temperature, Ni nanoparticles are well dispersed into the nanoporous carbon matrix, and magnetic metal nickel could also improve the conductivity of carbon by the graphitization catalysis on amorphous carbon. The sample prepared at 700 C exhibits excellent performance, whose maximum RL value is 39.4 dB with an effective bandwidth of 4.2 GHz. When the sample loading content decreases from 30 wt% to 25 wt%, the maximum RL value reaches 39.4 dB with an effective bandwidth of 4.2 GHz, which demonstrates that the prepared sample has huge potential as light absorbers. This work demonstrates that the NPC/ Ni composites are nice absorbers with wide bandwidth, strong absorption, thin thickness and light weight. Moreover, it also opens up a new pathway for artiﬁcially designed magnetic metal/ 962 B. Quan et al. / Journal of Alloys and Compounds 769 (2018) 961e968 dielectrics composites functionalities. nanostructures with objective 2. Experiment 2.1. Materials preparation Synthesis of CPO-27-Ni: 2.9 g 2, 5-dihydroxyterephthalic acid was dissolved in 50 mL of THF to obtain solution A. Then 7.4 g Ni(CH3COOH)2$4H2O was added in 50 mL distilled water to form solution B. The obtained mixed solution (solution A þ solution B) was heated at 100 C for 72 h. Yellow powder can be obtained after ﬁltration with water as well as subsequent drying at vacuum oven. Synthesis of NPC/Ni composites: The as-prepared CPO-27-Ni composites were calcinated at 700, 800, 900 C for at N2 atmosphere with the heating rate of 3 C min 1, and the obtained nickel particles embedded nanoporous carbon (NPC/Ni) was denoted as S700, S800 and S900, respectively. 2.2. Measurement The as-obtained samples were characterized by X-ray diffractometer (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM), Raman spectroscopy, BET and so on. Electromagnetic parameters were detected using Agilent PNA N5244A vector network analyze. The mixture was previously synthesized by homogeneously mixing wax and samples, then compacted them into toroidal-shaped samples (Fin: 3.04 mm, Fout: 7). 3. Results and discussion Sample S700, S800 and S900 were obtained by calcining the same precursor CPO-27-Ni at various temperatures (700, 800, 900 C). As can be seen from Fig. 1, three samples basically maintain the identical diffraction peak of Ni , which demonstrates that the crystal structures of as-obtained NPC/Ni composites largely remain the same when prepared at different temperatures. In addition, carbon peak could not been found in the XRD patterns, which is derived from its amorphous feature. In addition, the stability of sample was also tested. As seen in Fig. S2, the XRD pattern of S700 exposed in heating oven at 50 C for one week (S700-One week) exhibits similar crystal structure as that of S700, indicating the relative stability of NPC/Ni composites. The existence of carbon matrix and the effect of preparation condition on carbon graphitization degree were further Fig. 1. XRD patterns of sample S700, S800 and S900. Fig. 2. Raman spectra of sample S700, S800 and S900. characterized by Raman spectra (Fig. 2). The samples treated at various temperatures display different graphitization characteristics, as revealed by the typical carbon features of D band (1356 cm1) and G band (1554 cm1). G band, ﬁrst-order scattering of E2g phonons by sp2 carbon atoms, is on behalf of graphite; and D band, breathing mode of k-point photons of A1g symmetry, signiﬁes the amorphous forms . As can be seen from the calculation results, the intensity ratio of D band to G band is 1.33, 1.26 and 1.18 for S700, S800 and S900, respectively, which is cause by the increasing defect as rising temperature. SEM images of the precursor CPO-27-Ni and S700, S800 as well as S900 were provided in Fig. 3. Laminated NPC/Ni composites (Fig. 3d, e, g, h, j, k) were obtained after the heat treatment of bulk CPO-27-Ni composites (Fig. 3aec) at different calcinations temperature. Obvious nanoparticles embedded in the carbon matrix can be observed in Fig. 3f, i, l, which may be derived from the nickel nanoparticles as demonstrated in the XRD patterns. More clear porous structures of Ni particles dispersed carbon texture can be seen in Fig. 4aed. Clear interfaces between carbon matrix and Ni particles can be seen from Fig. S3, which is beneﬁcial to the generation of interfacial polarization and enhanced microwave absorption. In addition, the EDS elemental mapping of C, N, Ni for the selected image (Fig. 4e) are exhibited in Fig. 4feh to demonstrate the distribution of each element in prepared sample. Obviously, Nickel particles are homogeneously distributed in the carbon sheets with N element doping, revealed by the uniform distribution of C, N, Ni. Nitrogen adsorption-desorption measurements were conducted to evaluate the variation of speciﬁc surface as well as porosity features of three NPC/Ni composites. Typical IUPAC type IV pattern with sharp upturn in high-relative-pressure region  can be found in all the samples, as shown in Fig. 5aec. The BET surface areas of NPC/Ni composites are 112.9311 m2/g, 149.0714 m2/g, 136.3568 m2/g, respectively (Fig. 5def). Moreover, the pore size distribution based on BJH method exhibits the average pore size is in mesoporous range (23.07314 nm, 19.39422 nm, 20.26114 nm). All the above-mentioned information indicates the nanoporous characteristics of NPC/Ni composites, which makes for the dissipation of incident microwave . Electromagnetic wave absorbing capacity is closely related to the electromagnetic parameters [23,24]. Fig. 6a and b exhibit the histograms of ε0 values and ε00 values for three samples at S, C, X, B. Quan et al. / Journal of Alloys and Compounds 769 (2018) 961e968 963 Fig. 3. SEM images of precursors (aec), S700 (def), S800 (gei) and S900 (jel) at different magniﬁcation. Fig. 4. TEM images of S700 at the magniﬁcation of 1.0 mm (a), 0.5 mm (b), 0.2 mm (c) and 100 nm (d). Corresponding elemental mapping of elements C (f), N (g), Mg (h) for the selected image (e). and Ku bands. Gradually increasing complex permittivity values at all measured bands can be observed from S700, S800 to S900, which demonstrates the improving storage capacity of electrical energy and dielectric loss ability. Accordingly, electrical conductivity of three samples are also obtained based on the conductivity equation: s ¼ εo ε00 w ¼ 2εo ε00 pf , which exhibits the same trend as that of complex permittivity, as can be seen from Fig. S1. Complex permeability values are also presented in Fig. 6c and d. The roughly same m0 values m00 values demonstrate that nanoscale nickel particles embedded in carbon matrix do not possess big differences in magnetic energy storage as well as magnetic loss ability. The microwave reﬂection maps of NPC/Ni composites, as shown in Fig. 7aec, exhibit that the samples have huge dependence on matching thickness and frequency. As the matching thickness and 964 B. Quan et al. / Journal of Alloys and Compounds 769 (2018) 961e968 Fig. 5. N2 adsorption-desorption isotherms and corresponding BJH pore-size distribution of S700 (a, d); S800 (b, e) and S900 (c, f). frequency changing, the related microwave absorption capacities vary a lot. Moreover, the microwave absorbing activity decreases from S700 to S900, and sample S700 shows the optimal performance with maximum absorption value of 20.65 dB at 1.4 mm and effective absorption bandwidth of 3.28 GHz. Magnetic dissipation mechanisms were detected by the magnetization measurement at the applied ﬁeld of 10 kOe < H < 10 kOe for the three samples. As shown in Fig. 8, the saturation magnetization (Ms) values of S700, S800 and S900 (36.8, 35.1, and 32.3, respectively) are lower than some reported saturation magnetization values of Ni nanoparticles due to the smaller size of nickel particles and massive carbon matrix in the composites. Furthermore, the coercivity (Hc) values are 224.1, 93.1, and 136.3 Oe, respectively, which is due to the crystalline anisotropy and shape anisotropy. Due to the relatively weak loss ability compared to that of permittivity loss, the magnetic parameters could not exhibit big inﬂuence on magnetic dissipation activity. In order to explore the intrinsic attenuation mechanisms, the frequency dependence of dielectric/magnetic tangent (tandε, a/ tandm, b), attenuation constant (c) and impedance matching ratio (d) were gained by the following equations [9,25,26]: 00 tan dε ¼ ε tan dm ¼ m . ε0 00 . m0 (1) (2) rﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ pﬃﬃﬃ qﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ 2pf 00 00 00 00 00 00 ðm ε m0 ε0 Þ þ ðm ε m0 ε0 Þ2 þ ðm0 ε þ m ε0 Þ2 a¼ c (3) Z ¼ Z0 pﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ mr =εr (4) B. Quan et al. / Journal of Alloys and Compounds 769 (2018) 961e968 965 Fig. 6. Histograms of real permittivity (a) and imaginary permittivity (b) values at various microwave bands for three NPC/Ni composites. Real (c) and imaginary (d) permeability curves for NPC/Ni composites at 2e18 GHz. Fig. 7. Microwave reﬂection maps of S700 (a), S800 (b) and S900 (c). As can be seen from Fig. 9a and b, the dielectric dissipation capacity varies a lot and can be ranked as S900 > S800 > S700, while the magnetic loss ability has little difference . Likewise, the comprehensive dissipation capacity, expressed by attenuation constant, is also ranked as that of dielectric loss ability (Fig. 9c). Impedance matching is also explored as shown in Fig. 9d. It can be found that the three samples exhibit opposite trend compared to dissipation ability, and can be ranked as S700 > S800 > S900. Therefore, it can be concluded that one-sided pursuit of strong loss ability is not rational behaviors. An excellent absorber needs both impedance matching and dissipation capacity at the same time. Relatively low ε0 value can result in nice impedance matching; however, it could not provide enough loss ability to dissipate the incident microwave, ﬁnally making little contribution to effective microwave absorption . In order to further explore the potential application in light absorbing materials, the electromagnetic behaviors for sample S700 with 30 wt%/20 wt% of parafﬁn loading were compared. As shown in Fig. 10a and b, both of the complex permittivity and Fig. 8. MH loops measure of three samples measured at room temperature. 966 B. Quan et al. / Journal of Alloys and Compounds 769 (2018) 961e968 Fig. 9. Dielectric (a) and magnetic (b) loss factor, attenuation constant (c) and impedance matching ratio (d) of NPC/Ni composites. Fig. 10. Electromagnetic parameters (ε0 , ε0 0 , m0 , m00 ) of S700/70 wt% (a) and S700/75 wt% parafﬁn composites; attenuation constant (c) and dielectric loss factor (d) of S700 with different parafﬁn loading. permeability decrease when the parafﬁn loading content increases from 70 wt% to 75 wt%, which is derived from the microwave penetrating effect of wax . In view of that dielectric constant act as a dominant role in microwave dissipation, the integral loss ability of S700/70 wt% composites should be higher than that of S700/75 wt% composites, which can be conﬁrmed by the B. Quan et al. / Journal of Alloys and Compounds 769 (2018) 961e968 967 Fig. 11. 3D reﬂection loss curves of S700/70 wt% (a) and S700/75 wt% parafﬁn composites. attenuation constant, and dielectric loss factor as shown in Fig. 9c and d. 3D reﬂection loss curves of S700/70 wt% and S700/75 wt% parafﬁn composites are plotted. As can be seen from Fig. 11, the sample S700/75 wt% parafﬁn composites exhibit much enhanced microwave absorption performance compared to that of S700/ 70 wt% parafﬁn composites. Its maximum RL value reaches 39.4 dB with an effective bandwidth of 4.2 GHz, which demonstrate that the prepared sample has huge potential as light absorbers. 4. Conclusion Thermal treatment of metal organic framework was performed to prepare Ni nanoparticles embedded nanoporous carbon (NPC/ Ni). Uniformly dispersed nickel particles in nanoporous carbon can be obtained during certain pyrolysis temperature. The introduction of metal nickel can effectively regulate the electrical conductivity. With multiple interfaces, tunable conduction and porous structures, excellent microwave absorption performance can be gained. The maximum RL value of 39.4 dB and effective bandwidth of 4.2 GHz could be obtained when calcining at 700 C. When parafﬁn loading content increases from 70 wt% to 75 wt%, the maximum RL value reaches 39.4 dB with an effective bandwidth of 4.2 GHz, which demonstrates that the prepared sample has huge potential as light absorbers. This work demonstrates that the NPC/Ni composites are nice absorbers with wide bandwidth, strong absorption, thin thickness and light weight. Moreover, it also opens up a new pathway for artiﬁcially designed magnetic metal/dielectrics composites nanostructures with objective functionalities. Acknowledgments Financial supports from the National Nature Science Foundation of China (No.:11575085, 51602154), the Aeronautics Science Foundation of China (2017ZF52066), the Qing Lan Project, Six Talent Peaks Project in Jiangsu Province (No.: XCL-035), Funding for Outstanding Doctoral Dissertation in NUAA (BCXJ 18-07), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) are gratefully acknowledged. Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.jallcom.2018.08.069. References  X.G. Liu, C.Y. Cui, J.Y. Yu, Y.P. Sun, A.L. Xia, Ag3PO4 sub-microcubic/SrFe12O19 hexagon nanoﬂake heterostructure for broadband electromagnetic absorber at GHz frequency, Mater. Lett. 225 (2018) 1e4.  Z.Y. Huang, H.H. Chen, Y. Huang, Z. Ge, Y. Zhou, Y. 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