Strong Electromagnetic Wave Response Derived from the

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Strong Electromagnetic Wave Response Derived from the Construction of Dielectric/Magnetic Media Heterostructure and Multiple Interfaces Bin Quan,† Xiaohui Liang,† Guangbin Ji,*,† Jianna Ma,† Peiyi Ouyang,† He Gong,† Guoyue Xu,† and Youwei Du‡ †

College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 211100, P. R. China Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, P. R. China

‡

S Supporting Information *

ABSTRACT: A novel yolk−shell structure of cobalt nanoparticle embedded nanoporous carbon@carbonyl iron (Co/NPC@Void@CI) was synthesized via metal organic chemical vapor deposition (MOCVD) and subsequent calcination treatment. The in situ generation of void layer, which originated from the shrink of a Co-based zeolitic imidazolate framework (ZIF-67) during carbonization, embodies distinct advantage compared to the conventional template method. Thanks to the introduction of customdesigned dielectric/magnetic media heterostructure and multiple interfaces, the composites filled with 40 wt % of Co/NPC@Void@CI samples in paraffin exhibit a maximum reflection loss of −49.2 dB at 2.2 mm; importantly, a broad absorption bandwidth (RL < −10 dB) of 6.72 GHz can be obtained, which covers more than onethird of the whole frequency region from 10.56 to 17.28 GHz. This study not only develops the application of carbonyl iron as a high-efficiency light absorber but also initiates a fire-new avenue for artificially designed heterostructures with target functionalities. KEYWORDS: carbonyl iron, heterostructure, multiple interfaces, yolk−shell, electromagnetic wave absorbing

1. INTRODUCTION An urgent demand for effective solutions to challenges in high standards of pursuance to performances and totipotential materials equipped with advantages from different aspects has stimulated research on the design of multilayer heterogeneous structures.1−3 Combining the merits of various ingredients in one integrated material is still an assignment for which researchers currently have no universal solutions. Hence, the custom-designed heterostructures have aroused many interests.4−8 The high saturation magnetization (Ms) and relatively high permeability (μr) at radar wave frequency band of soft magnetic carbonyl iron (CI) has drawn extensive attention in scientific and industrial researches, but the narrow absorption bandwidth and high density limit its applications.9−11 There is thereby an urgent need, but it is still a significant challenge to designing heterostructures integrating CI with appropriate candidate. The top priority issue comes from the synthesis technique. On the one hand, commercial CI powder cannot be well dispersed in conventional solvent on account of the prominent polarity comparison between the solvent and particles.12,13 Although extra additive could shorten the gap to a certain extent, the binding force between two members is still not strong enough. On the other hand, the considerable difference of density between CI and liquid medium makes it hard to contact each other uniformly under high stirring rate. Given these challenges, © 2017 American Chemical Society

a novel synthesis pathway employing metal−organic chemical vapor deposition (MOCVD) was developed.14 Highly purified Fe(CO)5 was used as raw material, and CI was obtained during the thermal decomposition process in the inert atmosphere. Not only the problems mentioned above can be solved efficiently, but also the output of CI could be altered by just changing the addition of Fe(CO)5. With the preparation method settled, it is necessary to figure out a suitable partner to remedy the shortcoming of CI.15,16 The predominant electric/magnetic characteristics of CI decay dramatically as the reduction of its filling content, which astrict the application of CI as a light absorber. Light materials are generally based on porous or hollow structures, among which porous-carbon-based or yolk−shell nanostructures are ideal candidates.17,18 In addition, the selected substance should possess nice electromagnetic behaviors even at low addition loading to neutralize the low electromagnetic parameters of CI. In view of this, Co-based zeolitic imidazolate framework material (ZIF-67),19,20 a well-known member of metal−organic frameworks (MOFs),21,22 could satisfy the above demands. ZIF-67 is composed through the coordination interaction between imidazole derivatives and metal irons Co2+. As we all Received: December 9, 2016 Accepted: March 1, 2017 Published: March 1, 2017 9964

DOI: 10.1021/acsami.6b15788 ACS Appl. Mater. Interfaces 2017, 9, 9964−9974

Research Article

ACS Applied Materials & Interfaces Scheme 1. Synthetic Scheme for the Preparation of Co/NPC@Void@CI

Figure 1. TEM images of ZIF-67 (a) and Co/NPC@Void@CI (b). (c) XRD patterns of ZIF-67, Co/NPC, and Co/NPC@Void@CI. Inset shows the Raman spectra of Co/NPC and Co/NPC@Void@CI. (d) EDS line scans of Co/NPC@Void@CI.

know, the introduction of some magnetic metal (Fe, Co, and Ni) can improve the electrical conductivity of carbon due to their catalysis on the graphitization of amorphous carbon.23,24 Obviously, the nanoporous graphite carbon matrix with uniform distribution of Co nanoparticles (Co/NPC) can be obtained by calcination treatment of ZIF-67.25 What is more, there exists drastic shrink phenomenon of ZIF-67 during the calcination process, which is convenient for the construction of hollow structures without any template or additive. Herein, we set out to prepare the Co/NPC@Void@CI yolk−shell structure by thermal decomposition of Fe(CO)5 in N2 atmosphere with as-prepared ZIF-67 dissolved in the Fe(CO)5−kerosene solvent system. Importantly, the in situ generation of void layer, which was derived from the shrink of ZIF-67 during the calcination process, embodies absolute advantage compared to the traditional template method. With the construction of dielectric/magnetic media heterostructure as well as the generation of multiple interface polarization derived from carbon/Co, carbon/void, Co/void, and CI/void interfaces, the electromagnetic behaviors of CI can be improved to a quite appropriate levels at low filler content. The asobtained 40 wt % Co/NPC/wax showed remarkable microwave absorption properties. For example, the minimal RL of −49.2 dB at 2.2 nm and a broad effective bandwidth of 6.72 GHz can

be achieved. This study not only opens up the application of CI as a light absorber but also initiates a fire-new avenue for artificially designed heterostructures with target functionalities.

2. EXPERIMENTAL SECTION 2.1. Materials. Cobalt chloride (CoCl2) was purchased from Sinopharm Chemical Reagent Co. 2-Methylimidazole (MeIm, purity 99%) and methanol were obtained from Nanjing Chemical Reagent Co. Iron pentacarbonyl (Fe(CO)5) was purchased from Beijing Xin Ding Teng Fei Co. and kerosene from Chengdu Cheng Tai Co. All the chemicals were used without further purification. 2.2. Preparation of ZIF-67. In the typical preparation of ZIF-67,26 a methanolic solution (80 mL) of MeIm (2630 mg), PVP (600 mg), and cobalt chloride (519 mg) were mixed under stirring for 5 h. Then the mixture was aged at room temperature for 24 h. The purple ZIF-67 powder was collected after washing with absolute ethanol and distilled water for three times and drying at 60 °C for 24 h in a vacuum. 2.3. Preparation of Co/NPC@Void@CI. The outer coating layer of carbonyl iron was prepared by the thermal decomposition of Fe(CO)5 based on a previous study.27 As shown in Scheme 1, 380 mg of as-prepared ZIF-67 powder was dissolved in a four-necked flask containing 120 mL of kerosene under ultrasonic treatment for 0.5 h to gain well-dispersed ZIF-67. The flask was equipped with mechanical agitator, reflux condensation device, temperature gauge, and airway tube with continuous N2 flow (5 mL min−1). The carbonyl iron slowly generated as the equation Fe(CO)5 = Fe + 5CO, and the ZIF-67@CI 9965


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ACS Applied Materials & Interfaces

Figure 2. XPS survey scan of Co/NPC@Void@CI (a), Co/NPC (b), and ZIF-67 (c). High-resolution Co 2p spectra of (d) Co/NPC and (e) ZIF67.

Fe(CO)5−kerosene solvent system, accompanying with a calcination treatment. It is worth mentioning that the in situ generation of void layer, which was derived from the shrink of ZIF-67 during the carbonization process, embodies evident advantage compared to the traditional template method. The composition and structure play significant roles in the absorption of electromagnetic waves. The TEM image in Figure 1a reveals that the ZIF-67 samples were synthesized with typical rhombic dodecahedra feature, which is in accord with the XRD pattern (Figure 1c) marked in dark gray. Figure 1b exhibits the in situ generation of yolk−shell structure of Co/ NPC@Void@CI, which was obtained through the calcination treatment of ZIF-67@CI. The Co/NPC@Void@CI basically maintains the original configuration of ZIF-67, and obvious void space appears with the separation of Co/NPC and carbonyl iron. To further demonstrate the successfully obtained yolk−shell structure, the EDS line scans were also carried out. As seen from Figure 1d, the spatial variations of the concentration of the specific element (Fe, Co, C) are revealed. The line scan intensity can reflect the relative amount of these three kinds of elements. C has a larger intensity difference than Co and Fe due to the vast existence of carbon in the core part. In addition, the centralized distribution of Co in the center region shows that cobalt nanoparticles are uniformly embedded in the carbon matrix. The intensity of Fe mainly distributed in both sides of the scanning area, which is exactly corresponding to the yolk−shell structure. However, one cannot observed the diffraction peaks of Co phase from the XRD patterns of Co/ NPC@Void@CI, given that the relatively high diffraction intensity of carbonyl iron in the outermost layer covers up the diffraction peaks of cobalt. In addition, the Raman spectra of Co/NPC and Co/NPC@Void@CI are shown in the inset of Figure 1c. Clearly, the graphitization extents of Co/NPC are much higher than that of Co/NPC@Void@CI. As is wellknown, magnetic metal like Fe, Co, and Ni can accelerate the graphitization of amorphous carbon.26,28 However, the result

of core−shell structure eventually formed after 8 h of heating and reflux at 140 °C in the oxygen-free condition. The formed stuff was collected by magnetic separation with 5 times of ethanol washing and dried in the vacuum oven at 60 °C for 20 h. Finally, the obtained powder was annealed under a N2 atmosphere at 800 °C for 3 h with a heating rate of 1 °C min−1. The ZIF-67 transformed into nanoporous carbon with Co nanoparticles (Co/NPC) accompanied by some degree of shrinkage during the calcination process; therefore, the core−shell structure of ZIF-67@carbonyl iron ultimately turned into yolk−shell structure of Co/NPC@carbonyl iron, which is denoted as Co/NPC@Void@CI. In contrast, the obtained ZIF-67 powder was also directly calcined under the same condition; the product was denoted as Co/NPC. 2.4. Characterization. Structural properties of the samples were carried out using X-ray diffraction with a Bruker D8 Advanced X-ray diffractometer equipped with Cu Kα (λ = 1.5405 Å) radiation. The Xray machine was operated at 45 kV, and the scanning range was between 10° and 90°. The morphology was determined through highresolution transmission electron microscopy (TEM, JEOL JSM-2010). XPS spectra were detected in a PHI 5000 VersaProbe systems with an Al Kα X-ray source at 150 W. Raman spectra were recorded on a Raman spectrometer (Renishaw InVia). BET specific surface areas of the samples were determined by a high-speed automated area and pore size analyzer (ASAP 2010). Vibrating sample magnetometer (VSM, Lakeshore, Model 7400 series) was used to test the magnetic properties at an applied magnetic field of 10 kOe. The electromagnetic parameters of complex magnetic permeability and complex permittivity were recorded at 2−18 GHz by an Agilent PNA N5224A vector network analyzer via the coaxial-line method. The mixtures were prepared by homogeneously mixing 40 wt % samples with paraffin. The mixture was compacted and cut into toroidal-shaped samples (Φout: 7.0 mm; Φin: 3.04 mm). In addition, all the characterizations and measurements about CI in this work are based on the commercial carbonyl iron.

3. RESULTS AND DISCUSSION As shown in Scheme 1, the Co/NPC@Void@CI yolk−shell structure was acquired via thermal decomposition of Fe(CO)5 in N2 atmosphere with as-prepared ZIF-67 dissolved in the 9966


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ACS Applied Materials & Interfaces indicates that pyknotic multiple bed of carbonyl iron hinders the catalysis for graphitization. Further evidence for the chemical composition of the obtained samples was detected by the X-ray photoelectron spectrum (XPS) measurements, as revealed in Figure 2. The survey scan of the samples Co/NPC@Void@CI, Co/NPC, and ZIF-67 are shown in Figures 2a, 2b, and 2c, respectively. Evident characteristic peaks of Co29 and C elements are observed from the three Co/C-based materials. N (Figure 2c) and Fe30 (Figure 2a) have also been detected from the surveys scan of ZIF-67 and Co/NPC@Void@CI, which are mainly derived from the 2-methylimidazole precursor and coated carbonyl iron, respectively. Moreover, in order to demonstrate the valence variation of Co element before and after calcination, the Co 2p spectra for ZIF-67 and Co/NPC were investigated. The binding energy shifts of 2p1/2 from 796.68 to 794.18 eV and 2p3/2 from 781.08 to 778.58 eV prove the transformation of Co-based zelitic imidazolate framework (Co2+) to nanoporous carbon with magnetic Co nanoparticles (Co0), which is in good agreement with analogous works.31 To evaluate the variation of the specific surface areas and porosity of samples, nitrogen adsorption−desorption measurements were carried out, as shown in Figure 3. Theoretically, the specific surface areas of ZIF-67 would decrease dramatically because of a certain degree of bulk shrink as well as the collapse of well-defined porous structure after the carbonization treatment. The Co/NPC@Void@CI ought to possess the minimum SBET due to the coating of carbonyl iron. As shown in Table 1, our deduction is verified by the results that the Brunauer−Emmett−Teller surface areas of ZIF-67, Co/NPC, and Co/NPC@Void@CI are 1814.23, 72.65, and 60.69 m2/g, respectively, and the Langmuir surface areas keep the same trend. In addition, it can be observed from the pore size distributions in the inset of Figure 3a that the specimen ZIF-67 mainly possesses microspores below 2 nm, while the pore size distributions of both carbonized ZIF-67 (Co/NPC) and the final product Co/NPC@Void@CI locate at the mesoporous size range of 2−50 nm, as revealed in the insets of Figure 3b,c. Carbonyl iron powders have been widely applied at radar wave frequency with the high saturation magnetization and relative high permeability. However, the heavy density limits its application as light absorbers. It can be obviously seen from Figure 4a−d that the complex permittivity and complex permeability exhibit different levels of decrease as the increasing wax addition. The superior characteristics of electromagnetic parameters, especially the high permeability, basically disappear with the reduction of filling of carbonyl iron. In addition, the corresponding reflection loss properties of CI/paraffin composites with 30, 40, 60, and 70 wt % of CI filling are presented in Figure 4e,f. Drastic decaying of the microwave absorbing abilities manifests that there exists severe drawback of the carbonyl iron powders when used as a light absorber. Therefore, a suitable material is needed to combine with carbonyl iron as an effective light absorbing material. MOFs has attracted increasing interest for its promising application as templates or precursors to synthesize nanoporous materials via thermal decomposition,32,33 and Co-base ZIF-67 is a suitable candidate for the relative high dielectric constant and a certain magnetism after being calcinated. In order to demonstrate the advantage of this kind of composite in microwave absorbing compared to the pristine CI and Co/ NPC, the reflection loss values and the corresponding maps of CI, Co/NPC, and Co/NPC@Void@CI samples dispersed in

Figure 3. Nitrogen adsorption−desorption isotherms of ZIF-67 (a), Co/NPC (b), and Co/NPC@Void@CI (c). Inset: the corresponding BJH pore-size distribution of the three samples calculated from the desorption branch of the N2 isotherm.

Table 1. Specific Surface Areas and Total Pore Volumes of ZIF-67, Co/NPC, and Co/NPC@Void@CI sample

SBET (m2/g)

SLangmuir (m2/g)

VPore (cm3/g)

ZIF-67 Co/NPC Co/NPC@Void@CI

1814.23 72.65 60.69

2253.15 86.4498 72.90

0.0803 0.120 0.0973

paraffin with 40% mass fraction are shown in Figure 5, which were evaluated by the following equations: RL = 20 log|(Z in − Z0)/(Z in + Z0)| 9967

(1)


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Figure 4. Measured frequency dependence of commercial carbonyl iron (CI)/paraffin composites real (a) and imaginary (b) parts of complex permittivity and real (c) and imaginary (d) parts of complex permeability. The legend represents various mass ratios between CI and paraffin. Microwave absorbing properties of CI/paraffin composites with 30, (e) 40, (f) 60, (g) and 70 wt % (h) of CI filling.

Z in = Z0

⎛ 2πfd ⎞ tanh⎜j με r⎟ r ⎝ c ⎠ εr μr

permeability values, d is the absorber thickness, and c is the velocity of light. Co/NPC@Void@CI shows notably elevated microwave absorbing ability than the pristine CI and Co/NPC, as can be seen in Figure 5. The minimal RL of Co/NPC@ Void@CI is −49.2 dB at 13.68 GHz with a low thickness of 2.2

(2)

where Zin is the input characteristic impedance, Z0 is the impedance of free space, εr and μr are complex permittivity and 9968


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Figure 5. Microwave reflection loss data and maps of CI (a d), Co/NPC (b, e), and Co/NPC@Void@CI (c, f).

properties of CI at low filler content. It is worth mentioning that there appear typical Debye relaxation characteristics in the region of 13−15 GHz (marked with rectangle) for the complex permittivity of CI and Co/NPC. By contrast, Co/NPC displays the highest complex permittivity, which results from the higher graphitization degree of Co/NPC than that of Co/NPC@ Void@CI, as shown in the inset of Figure 1c. On the basis of the free electron theory,36,37 ε″ ≈ 1/πε0ρf, where ε0, ρ, and f are the permittivity of free space, the resistivity, and the frequency, respectively, it can be deduced that high conductivity of Co/ NPC is in favor of enhancing the ε″. In addition, numerous reports have manifested a significant phenomenon that ε′ also keeps pace with the variation of conductivity and there exists the same change rules about ε′ and ε″,38,39 though there is no specific quantitative relationship between the real parts of complex permittivity and conductivity as well as the imaginary parts of complex permittivity. For Co/NPC@Void@CI, the complex permittivity lies in the modest level of all, and typical frequency dispersion phenomena are observed where the complex permittivity decreases with increasing frequency. At gigahertz frequency, the rotational motion of polar molecules is

nm. It is worth mentioning that the minimal reflection loss appears at the same frequency as pristine CI, which possesses a typical maximum absorbing value at 13.68 GHz when its filler content is 40 wt %. Therefore, we can conclude that CI act as a significant role in the microwave absorption, and the absorbility and the effective frequency bandwidth (RL < −10 dB) are enhanced by a long way. With a broad effective bandwidth of 6.72 GHz at low thickness, the Co/NPC@Void@CI sample can serve as a potential candidate of a high-performance light absorber. The possible mechanism of the promotional electromagnetic wave absorbing ability can be explained by the variation of the complex permittivity (εr = ε′ − jε″) and complex permeability (μr = μ′ − μ″) as well as the dielectric/magnetic loss (tan δε/ tan δμ). Generally, the real parts of permittivity (ε′) and permeability (μ′) represent the storage ability of electric and magnetic energy, and the imaginary parts (ε″, μ″) represent dissipation capability of electric and magnetic energy.34,35 As shown in Figure 6a,b, CI has the lowest complex permittivity among three samples, where its ε′ value is basically around 4 and the ε″ value is rather weak, indicating the bad dielectric 9969


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Figure 6. Frequency dependence of (a) real and (b) imaginary parts of relative complex permittivity and (c) real and (d) imaginary parts of relative complex permeability of CI, Co/NPC, and Co/NPC@Void@CI. The dielectric (e) and magnetic (f) loss factor of the three samples versus frequency.

standard for all the three samples, illustrating that magnetic dissipation are not the primary absorption mechanisms. In order to excavate the relationship between permeability and magnetization, the hysteresis loops of CI, Co/NPC, and Co/ NPC@Void@CI have been presented in Figure S1. In addition, the frequency dependence of dielectric/magnetic loss tangent (tan δε/tan δμ) is shown in Figures 6e and 6f. The tan δε values of Co/NPC and Co/NPC@Void@CI are much higher than CI, but no evident advantages are observed for the tan δμ, indicating the dominant role of dielectric loss in the microwave dissipation. Moreover, the dielectric loss peak for tan δε, which is often called α-relaxation and related to the Debye relaxation process, appears at the corresponding frequency as shown in the ε′−f and ε″−f spectra. The frequency dependence of ε′, ε″

not rapid enough to obtain equilibrium with the applied electric field, leading to reduced dielectric constant as the increasing of frequency.40,41 Theoretically, the complex permittivity of Co/ NPC@Void@CI should be low for its bad degree of graphitization (inset of Figure 1d). However, as shown in Figure 6a,b, Co/NPC@Void@CI exhibits relatively fine dissipation capability of microwave, which is mainly attributed to the multiple shell scattering between Co/NPC and CI as well as a variety of interfacial polarization and electric dipole polarization.42,43 As shown in Figure 6c,d, the magnetic properties of CI also get poor at low filler content, just like the change of dielectric constant. The magnetic energy storage ability of Co/NPC@Void@CI is better than CI, while the dissipation capability of magnetic energy keeps the approximate 9970


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ACS Applied Materials & Interfaces and tan δ in the Debye relaxation process can be seen from Figure 6e that the α-relaxation of carbonyl iron jumps to higher value at higher frequency when compared with Co/NPC. The dipoles in carbonyl iron can follow faster alternations of applied electric fields, resulting in the higher value at higher frequency.41 To the best of our knowledge, the ultimate electromagnetic wave dissipation derives from the comprehensive effect of dielectric and magnetic loss. In general, the integral losses ability is evaluated by the attenuation constant α, as expressed in eq 3: α= ×

2 πf c (μ″ε″ − μ′ε′) +

(μ″ε″ − μ′ε′)2 + (μ′ε″ + μ″ε′)2 (3) Figure 8. Frequency dependence of RL values, attenuation constant α, and the modulus of normalized input impedance (|Zin/Z0|) for Co/ NPC@Void@CI (40 wt %) with 2.2 mm thickness.

As shown in Figure 7, the attenuation capacity of Co/NPC@ Void@CI is much higher than CI, indicating the enhanced

while its dissipation ability is not the most outstanding. It is the impedance matching that acts a critical role for the effective absorbing of microwave.25,45 If the impedance matching is poor, strong attenuation loss ability will make no sense for little entered electromagnetic wave.35,46 Meanwhile, the results offer a significant reference for the design desired for an ideal microwave absorber. We should give consideration to both impedance matching and attenuation loss ability.47,48 Based on the above analysis, the possible mechanisms for the enhanced absorption capacity include the following aspects. The electromagnetic parameters of carbonyl iron decline dramatically with the addition amount of CI in the CI/paraffin composites decreasing. In fact, the variation makes for the promotion of impedance matching because of the big gap between εr and μr; however, the poor attenuation ability makes it no sense. Therefore, the application as a light absorber of CI has been restricted. The introduction of ZIF-67 changes the actuality commendably. As shown in Scheme 2, the moderate

Figure 7. Frequency dependence of attenuation constant (α) of CI, Co/NPC, and Co/NPC@Void@CI.

microwave wastage performance in terms of the electromagnetic wave entering into the interior of the absorbers. However, from an overall perspective, the attenuation ability of sample Co/NPC is stronger than the sample Co/NPC@ Void@CI which exhibits optimal microwave absorption ability. Therefore, another essential factor (impedance matching) determining the microwave absorbing capacity should be taken into account. Here, we select Co/NPC@Void@CI with a thickness of 2.2 mm as an example to illustrate the significance of impedance matching on the enhanced microwave absorbing ability of Co/ NPC@Void@CI. The value of Z = |Zin/Z0|44 was obtained by means of eq 2, where the completely impedance matching will be gained when Z = 1. Figure 8 clearly demonstrates the frequency dependence of RL values, attenuation constant α and the modulus of normalized input impedance (|Zin/Z0|) for Co/ NPC@Void@CI (40 wt %) with the thickness of 2.2 mm. When the attenuation constant reaches the maximum value at 16.84 GHz, the minimum RL can not be obtained and corresponding Z is about 0.63. The minimum RL appears when Z is close to 1 while the relevant attenuation loss value is only 118 (the maximum attenuation constant is 191). The result gives a reasonable explanation why Co/NPC@Void@CI possesses optimal electromagnetic wave absorbing capacity

Scheme 2. Schematic Illustration of the Absorption Mechanism of Co/NPC@Void@CI

impedance matching condition after the integration let much microwave in which is the primary step for an efficient absorption. When the electromagnetic wave enters into the absorber, multiple loss mechanisms make most ingoing microwave scatters and disappear within the materials. (i) Both carbonyl iron and graphitized carbon matrix are rich in electrons. The accumulation of free charges at multiple 9971



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interfaces of Co/carbon, void/Co/NPC, and CI/void gives rise to intense space-charge polarization. In addition, electronic polarization also occupies certain proportion in the electronabundant materials. From a macroscopic view, microcurrent comes into being when the absorber is placed in alternating electromagnetic fields. A large part of electromagnetic energy would consume away in the form of thermal energy. (ii) Generally, magnetic loss mainly includes three aspects: hysteresis loss, eddy current loss,49,50 and residual loss. There is no doubt that hysteresis loss plays a significant role in magnetic loss process for either ferrite or metal. Eddy current is derived from the variation of magnetic flux density in alternating electromagnetic fields, and the joule heat is also the form of lost energy.51,52 However, eddy current loss can be regarded as a secondary dissipation form for Co/NPC@Void@CI, which is analyzed in Figure S2. Residual loss comes from the relaxation process during magnetization. At the high frequency region of gigahertz, the resonance peaks appearing in ε″ mainly result from dimension resonance, domain wall resonance, nature resonance, exchange resonance, and so on. In general, the domain wall resonance is just existed in the low-frequency range (

Strong Electromagnetic Wave Response Derived from the

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