Synthesis and Microwave Absorption Enhancement of CoNi@ SiO2

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Applied Chemistry

Synthesis and Microwave Absorption Enhancement of CoNi@SiO2@C Hierarchical Structures Suhua Zhou, Ying Huang, Xudong Liu, Jing Yan, and Xuansheng Feng Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00997 • Publication Date (Web): 03 Apr 2018 Downloaded from http://pubs.acs.org on April 3, 2018

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Industrial & Engineering Chemistry Research

Synthesis and Microwave Absorption Enhancement of CoNi@SiO2@C Hierarchical Structures Suhua Zhou, Ying Huang*, Xudong Liu, Jing Yan, Xuansheng Feng MOE Key Laboratory of Material Physics and Chemistry under Extraordinary Conditions, Ministry of Education, School of Science, Northwestern Polytechnical University, Xi’an 710072, People’s Republic of China Abstract: A series of hierarchical CoNi@SiO2@C composites were successfully fabricated through a workable procedure and the content of carbon can be adjusted by modulating the additive dose of phenolic resin precursors. The compositions, microstructure and magnetic characters of the obtained products were characterized via XRD, SEM, XPS and VSM detailedly. The results manifested that the urchin-like CoNi alloy particles were successively encapsulated with SiO2 and carbon coatings. The exploration into microwave absorbing performances demonstrate that wax-based hybrids containing 50 wt% CoNi@SiO2@C-3 are endowed with improved absorbing capability possessing the maximum RL value of -46.0 dB at 10.8 GHz with an effective absorption band width of 5.6 GHz (from 7.1 to 12.7 GHz) for a layer thickness of only 2.2 mm. Moreover, in the wake of layer thicknesses varying from 1.5 to 5 mm, the effective absorbing bandwidth almost overlaps the whole C, X and Ku wavebands. The remarkable microwave absorption performances are attributed to the joint outcome of combination of dielectric and magnetic loss, the numerous interfaces polarizations and dipolar relaxations provided by their hierarchically multicomponent structure and the good impedance matching. Keywords: CoNi alloys, magnetic loss, carbon, microwave absorption performances. 1 Introduction With the extensive utilization of electronic equipment all over the world, the pollution originating from electromagnetic (EM) waves causing a latent and invisible hazard has become an increasingly severe problem, which threatens people’s health and incurs the malfunction and degradation of other electronics1-5. Therefore, high-performance EM wave absorbers have been paid widespread attentions to deal with this issue.

Corresponding author: Ying Huang E-mail: [email protected]

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Microwave absorbing materials can transform electromagnetic energy into other energy forms or make the EM waves dissipated via interference6-8. What is universally accepted is that ideal microwave absorption materials are generally expected to possess these features: strong absorption, wide absorption band, thin thickness and lightweight9-13. It is a truism that the microwave absorbing performance of absorbing materials is effected by complex permittivity, complex permeability and impendence matching, which can be influenced by their component, microstructure, size and so on14-15. According to recent researches, an incorporation of dielectric and magnetic materials can help to realize better impedance matching, arousing the synergic effect of magnetic and dielectric loss. All the above mentioned will contribute to improve the absorption capacity. Hence, considerable efforts have been dedicated to fabricate elaborately designed dielectric-magnetic hierarchical composite materials16-17. For example, the double-shelled Fe3O4@SnO2 composites were prepared through combining the so-gel approach with hydrothermal shell-by-shell deposition process, and the maximum RL value reached -36.5 dB at 7 GHz for a thickness of 2 mm, owing to the synergic effect between magnetic Fe3O4 cores and dielectric SnO2 double shells18. Wei et al.19 reported with -43.0 dB of the maximum RL when the thickness was 3.4 mm, which was much higher than that of C@NiCo2O4 or pure NiCo2O4. Compared with ferrite and monometallic materials, metal alloy materials come into sight as widely-applied microwave absorbers owing to high Curie temperature, high permeability and saturation magnetization20-22. Magnetic CoNi alloy has aroused extensive interest on account of its low cost and strong magnetic loss23-24. However, a single-component metal absorber system usually suffers a lot from intrinsic shortcomings, for instance, easy oxidation, large density, magnetic aggregation, which limit the actual applications25. Microwave absorbers with multi-component structures always display a wider spectrum range along with stronger reflection loss by comparison with single-component absorbers. For example, Chen et al.26 synthesized a suite of Co20Ni80@TiO2 core-shell particles with different core sizes, and the core size of 500 nm demonstrated a prominent wide-band absorbing property of up to 6.2 GHz (＜-10 dB) in range of 2-18 GHz. Besides, Zhao27 prepared unique yolk-shell Ni@void@SnO2 composites showing the RLmax of -50.2 dB at 17.4 GHz when thickness was 1.5 mm. Carbon-based materials are potential candidates for EM absorbers by reasons of their ease synthesis, high dielectric loss, chemical resistance and light weigh28-29. In fact, they have been


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regarded as the hottest topics in the latest investigation advance 30-33. Liu et al.34 demonstrated that Co/C nanoparticles (45 nm) exhibited excellent MA performance, and the maximum RL could reach -43.4 dB with 2.3 mm of absorber thickness. Moreover, a kind of yolk-shell C@C microparticles synthesized by a “coating-coating-etching” method showed improved MA performance with a RLmax value of -39.4 dB at 16.2 GHz for a thickness of 1.85 mm28. Yang et al.35 prepared NiCo2/GNS nanohybrids and a supreme RL of -30 dB was obtained at 11.7 GHz with a thickness of 1.6 mm. In another aspect, silicon dioxide (SiO2) usually acts as a composite coating material for it is a nice semiconductor36. There have been reported that the microwave absorption performances were enhanced by a mix of SiO2 particles7, 37-38. Nevertheless, there are limited literatures of fabrication and microwave absorption research for CoNi alloy-based composites combining with both silica and carbon. Herein, we report the successful fabrication of a series of ternary CoNi@SiO2@C composites by modulating the additive dose of phenolic resin precursors. In this multi-component system, the inner magnetic CoNi cores intrinsically devote to the active magnetic loss, while the SiO2 and carbon layers support the dielectric loss, resulting in enhancement of microwave absorption characters. It is interesting microwave absorbing characters of the CoNi@SiO2@carbon composites can be easily regulated by altering dose of phenolic resin precursors. The results state that the hierarchical CoNi@SiO2@C-3 hybrids attain a strong absorption capability with the RLmax of -46.0 dB at 10.8 GHz for the layer thickness of only 2.2 mm, along with the effective bandwidth of 5.6 GHz (from 7.1 to 12.7 GHz). 2. Experimental section 2.1 Materials. Nickel(II) acetate tetrahydrate (Co(CH3COO)2·4H2O), cobalt(II) acetate tetrahydrate (Ni(CH3COO)2·4H2O), ethylene glycol (EG), tetraethyl orthosilicate (TEOS), sodium hydroxide (NaOH), resorcinol (C6H4(OH)2), formaldehyde solution (CH2O, 38 wt %) and ammonia aqueous solution (NH3·H2O, 25-28 wt%) were of analytical grade (Sinopharm Chemical Reagent Co., Ltd.) and applied without further purification. 2.2 Synthesis of CoNi and CoNi@SiO2 microparticles. CoNi and CoNi@SiO2 microparticles were prepared using facile solvothermal strategy and classical Stöber method with some modification23. Typically, Co(CH3COO)2·4H2O (0.1mmol),



Ni(CH3COO)2·4H2O (0.4mmol) and NaOH (0.06mol) were dissolved in an EG solution (80mL), then a solvothermal treatment was carried out at 200 ℃ for 12 h. The resulting black particles were washed with distilled water and absolute ethanol, respectively, and then dried under a vacuum at 60 °C. The 0.1g of as-prepared particles were redispersed in deionized water and absolute ethanol through ultrasonic treatment. Whereafter, NH3·H2O (2 mL) and TEOS (0.6 mL) were successively added drop by drop. The experiment was carried out under mechanical stirring for 8h at RT. The subsequent treatment was similar to that of CoNi alloy. 2.3 Synthesis of CoNi@SiO2@C Composites. The CoNi@SiO2@C microparticles were fabricated via in-situ polymerization and high-temperature calcination. Briefly, 0.2g of CoNi@SiO2 were dispersed into a stock solution containing 50mL of water, 20mL of absolute ethanol and 1mL of ammonia by ultrasonication. Then, a certain amount of resorcinol and formaldehyde solution were separately added into the aforementioned mixture under mechanical stirring. The reaction was carried out under continuous mechanical agitation at 30℃ for 24h. The obtained CoNi@SiO2@phenolic resin composites were separated via magnetic decantation and washed with deionized water and ethanol. The above process was repeated several times, and then the products were dried under vacuum at 60°C. At last, the CoNi@SiO2@phenolic resin (PR) particles were carbonized in inert gas atmosphere at 650℃ for 3 h at a ramp rate of 1℃/min. In the course of experiments, the amounts of resorcinol were 0.2, 0.3, and 0.4g, respectively, while the molar ratio of formaldehyde to resorcinol was set at 2:1. The corresponding specimens were labeled as CoNi@SiO2@C-1, CoNi@SiO2@C-2 and CoNi@SiO2@C-3, respectively. 2.4 Characterization. The phase structures were analyzed via X-ray diffractometer (XRD, Rigaku) with Cu Ka radiation (1.5406 Å). The microstructures and morphologies were observed by a field-emission scanning electron microscope (FE-SEM, Quanta 600FEG). The surface components of specimens were characterized on a Thermal Scientific K-Alpha X-ray photoelectron spectroscope (XPS). The magnetic characters of the samples were measured through vibrating sample magnetometer (VSM, Riken Denshi, BHV-525) at RT. 2.5 Electromagnetic measurements. The electromagnetic parameters were measured with a vector network analyzer (HP8753D)


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via the coaxial-line method within 2-18 GHz. Before the test, a homogeneous mix of the sample and paraffin at mass ratio of 1:1 was pressed into a certain toroidal-shaped ring (Φouter = 7.00 mm and Φinner = 3.04 mm). Then, software on the Agilent PNA can figure the values of electromagnetic parameters. At last, the RL values of diverse thickness can be calculated based on the transmission line theory. 3. Results and discussion The phase and crystal structures of products are characterized via XRD. As shown in Fig. 1, the sharp diffraction peaks of CoNi at 44.5°, 51.9° and 76.4° can be indexed to (111), (200), and (220) crystal planes with face centered cubic structure (PDF#04-0850 and PDF#15-0806), respectively39. There are no typical peaks of cobalt/nickel hydroxides and oxides can be detected, demonstrating the phase purity of samples. By a clear contrast, it is obvious to observe that the characteristic peaks of CoNi@SiO2 and CoNi@SiO2@C-1/C-2/C-3 have no evident difference and all the peaks are also centered at 44.5°, 51.9° and 76.4°, implying the amorphous nature of silica and carbon in the above composites. Besides, a comparison of the aforementioned patterns also explains that the presence of SiO2 and C has no influence on the CoNi alloy crystal. Meanwhile, other impurity diffraction characters can scarcely be detected from the XRD patterns, accounting for high purity of all products.

Fig. 1. XRD patterns of CoNi, CoNi@SiO2, CoNi@SiO2@C-1, CoNi@SiO2@C-2 and



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CoNi@SiO2@C-3. Fig. 2 records the morphology evolution from CoNi alloy to final CoNi@SiO2@carbon composites observed by SEM. It is clearly indicated that CoNi alloys are urchin-like particles having an average diameter of approximately 1 µm (Fig. 2a and b). The magnified SEM image of CoNi apparently shows that the surface of individual cobalt-nickel alloy microparticles is fairly rough, and it displays a fine hierarchical structure. Many irregular thorns are observed to be radiated from the core as well. Fig. 2c reveals that the surface of CoNi@SiO2 turns into relatively smooth which is responsible for the successful coating of silica. Treating the as-synthesized CoNi@SiO2 microparticles as cores, carbon coating can be achieved on these microparticles by in-situ

polymerization

of

PR.

Figure

2d-f

exhibit

the

morphology

images

of

CoNi@SiO2@C-1/C-2/C-3 with different carbon content, respectively. It is apparent that the surfaces of ternary composites CoNi@SiO2@C-1 (Fig. 2d), CoNi@SiO2@C-2 (Fig. 2e) and CoNi@SiO2@C-3 (Fig. 2f-g) become rough again and still remain the urchin-like microstructures even though the polymerization of RF precursors and following carbonization process were conducted. Besides, the energy-dispersive X-ray spectroscopy result of CoNi@SiO2@C-3 composites is shown in Fig. 2h, which demonstrates the existence of Co, Ni, O, Si and C elements in this sample.


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Fig. 2. SEM images of pure CoNi (a-b), CoNi@SiO2 (c), CoNi@SiO2@C-1 (d), CoNi@SiO2@C-2 (e) and CoNi@SiO2@C-3 (f-g); EDX map of CoNi@SiO2@C-3 (h). To further confirm the chemical compositions and valence states of elements of composites, the XPS was utilized and the corresponding figures are presented in Fig. 3. Because of all the ternary composites were fabricated by the same method apart from the additive amount of RF precursors, the representative XPS spectra of CoNi@SiO2@C-3 are displayed, as follows. Fig. 3a demonstrates the wide scan spectrum of CoNi@SiO2@C-3 composites, displaying that Si, C, O, Co and Ni exist in these hybrids. In Fig. 3(b), the deconvolutions of C 1s spectrum with three different peaks are located at 284.5, 285.9 and 288.3 eV in accord with graphitic carbon, C−OH or C−O−C groups, and carboxyl or ester groups40-42, respectively. The existence of peaks at 285.9 and 288.3 eV accounts for that phenolic resin precursors are not completely converted into graphitic carbon43. Two obvious peaks at 780 and 795.7 eV in Fig. 3c (left) can be assigned to Co 2p3/2 and Co 2p1/2. Furthermore, the high-resolution spectrum of Ni 2p (Fig. 3c (right)) is comprised of two spine-orbit doublet peaks of Ni 2p3/2 (855.2 eV for the core level and 860.8 eV for the satellite feature) and Ni 2p1/2 (872.5 eV for the core level and 879.1 eV for the satellite feature)39. The Si 2p spectrum shows a main peak centered at 102.9 eV, which further ensures the



presence of SiO2 in the sample44. All of the above consequences reflect that the CoNi@SiO2@C composites are formed, which is in accordance with the above analyses.

Fig. 3. XPS spectra of CoNi@SiO2@C-3: survey scan (a), C 1s spectrum (b), Co 2p and Ni 2p spectrum (c), and Si 2p spectrum (d). The magnetic characters of all products were checked via VSM with an applied field of -13.0-13.0 kOe at RT, as shown in Fig. 4. The saturation magnetization (Ms) values of the CoNi alloy, CoNi@SiO2, CoNi@SiO2@C-1, CoNi@SiO2@C-2 and CoNi@SiO2@C-3 hybrids are determined to be 64.5, 57.3, 52.7, 50.4 and 46.1 emu g-1, respectively. The saturation magnetization values apparently decrease owing to the presence of nonmagnetic silica and carbon layers. From CoNi@SiO2@C-1 to C-3 samples, the values of Ms gradually decline owing to the increasing content of nonmagnetic carbon layers. As shown in the inset picture in Fig. 4, it is apparent that the composite CoNi@SiO2@C-3 microparticles can be redispersed in water by vigorous shaking to form a black suspension. While a magnet is placed, the quick aggregation of microparticles is observed within a few seconds, which leaves a transparent solution.


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Fig. 4. Magnetization curves of CoNi, CoNi@SiO2 and CoNi@SiO2@carbon composites at RT. It is commonly believed that the microwave absorption performances of an absorber can be judged from the reflection loss (RL) curves. On the basis of transmission line theory, the RL values of samples can be deduced from Eq.(1) and (2). Microwave absorption properties of an absorber are closely connected with its complex permittivity (εr=ε’-jε’’) and complex permeability (µr=µ’-jµ’’), where the real parts stand for the storage ability of electric and magnetic energy, and imaginary parts mean the loss capability of electric and magnetic energy45-46. 20

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/ ! "#

(

√ *

(1) (2)

The microwave absorption behaviors of CoNi alloy and CoNi@SiO2@C hybrids were researched within 2-18 GHz. Fig. 5 presents the frequency-dependence electromagnetic parameters of CoNi alloy, CoNi@SiO2@C-1, CoNi@SiO2@C-2 and CoNi@SiO2@C-3. Fig. 5a displays that the ε′ values of CoNi alloy and CoNi@SiO2@C-1/C-1/C-3 showing some fluctuation in the range of 6.0-5.3, 5.8-4.7, 6.3-4.9 and 7.1-5.8, respectively. Meanwhile, the values of ε” (Fig. 5b) vary from 0.64-0.13, 1.34-0.29, 1.98-0.88 and 2.50-0.93, respectively. It can be discovered that there are remarkable frequency-dependent variations for ε′ and ε’’. Clearly, after the introduction of the silica and carbon layers, both the values of ε′ and ε″ for ternary hybrids exceed that of pure CoNi alloy, while the CoNi@SiO2@C-3 composites possess much higher ε' and ε''



than that of CoNi@SiO2@C-1 and CoNi@SiO2@C-2 over the most of frequency. In terms of Fig. 5c, the µ′ values of all show the sharp decreasing trend within range of 2-8GHz and then approximately remain stable in the rest range. As for µ″, the values of the above specimens behave complicated fluctuation over the whole range and emerge a strong resonance peak at 6.7-7.9 GHz, which is due to the small size effect, surface effect, and spin wave excitations47. From Fig. 5e, the values of tan δε for CoNi@SiO2@C-1/C-2/C-3 hybrids change in range of 0.05-0.26, 0.16-0.40 and 0.15-0.38, respectively, both higher than that of the CoNi alloy (0.03-0.10), indicating that the existence of dielectric components (silica and carbon) promotes the dielectric loss to some extent. Moreover, for CoNi@SiO2@C-1/C-2/C-3 hybrids, the tanδµ values behave the same variation tendency broadly similar to the pure CoNi, as shown in Fig. 5f. According to the above analysis, the ternary CoNi@SiO2@carbon composites inherit magnetic loss from metallic CoNi along with the derived dielectric loss from SiO2 and C layers, which can be expected to endow effective improvement in reflection loss of microwaves.


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Fig. 5. Electromagnetic parameters of CoNi, CoNi@SiO2@C-1, CoNi@SiO2@C-2 and CoNi@SiO2@C-3 composites in 2-18 GHz: the relative complex permittivity (a,b), the relative complex permeability (c,d), tan δε (e) and tan δµ (f). The RL characters of CoNi alloy and CoNi@SiO2@C hybrids are calculated and analyzed, based on the measured data of εr and µr. Fig. 6 displays the RL curves of paraffin-based composites

containing

50

wt%

CoNi,

CoNi@SiO2@C-1,

CoNi@SiO2@C-2

and

CoNi@SiO2@C-3 with different thicknesses from 1.5 to 5 mm. It is apparent that the absorbers thickness has an important influence on the microwave absorption performances. Fig. 6a shows



the maximum RL of bare CoNi alloy reaches -30.4 dB at 16.1 GHz when the thickness is 4.5mm, and the absorption bandwidth exceeding -10 dB (reflection loss more than 90%) attains 1.9 GHz (15.2-17.1 GHz). With the coating of silica and carbon, the CoNi@SiO2@C-1 behaves the RLmax value of -24.4 dB at 6.7 GHz and the effective bandwidth ranged from 5.4 to 8.3 GHz with a thickness of 2.5 mm. When the thickness is 2.5 mm, the maximum RL value of CoNi@SiO2@C-2 attains a high of -34.8 dB at 7.6 GHz with a bandwidth of 3.4 GHz (6.1-9.5 GHz). By contrast with the above two composites, CoNi@SiO2@C-3 behaves remarkable microwave absorption performances as shown in Fig. 6d. It can be discovered that the optimum RL up to -46.0 dB at 10.8 GHz for a layer thickness of only 2.2 mm, along with a quite wide bandwidth of 5.6 GHz (7.1-12.7 GHz). Besides, the RL peaks gradually shift to a lower frequency with the increasing of thickness. What is more noteworthy is that the ternary CoNi@SiO2@C-3 hybrids (Fig. 6d) exhibit enhanced absorption capacity not only in the intensity of reflection loss, but, by tailoring the layer thicknesses varying from 1.5 to 5mm, the effective absorbing bandwidth almost cover the whole C, X, and Ku wavebands.


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Fig. 6. Calculated RL curves and 3D presentations versus frequency and thickness of CoNi (a, b), CoNi@SiO2@C-1 (c, d), CoNi@SiO2@C-2 (e, f) and CoNi@SiO2@C-3 (g, h) with filler loading



of 50 wt%. Subsequently, to further demonstrate the advantages of CoNi@SiO2@C-3 composite microparticles, we have also studied the microwave absorption performances of specimens with different filler loadings. Fig. 7 exhibits RL values versus different CoNi@SiO2@C-3 contents in paraffin wax with the 20 wt % and 35 wt % loading at various coating thicknesses within 2-18 GHz. Fig. 7a demonstrates the maximum RL of CoNi@SiO2@C-3 with a 20 wt % filler loading reaches to -32.4 dB at 3.9 GHz when the thickness is modulated to 4.5 mm. Besides, the bandwidth that RL values exceed -10 dB is 9.3 GHz (3.2-12.5 GHz) as the absorber thickness varies from 1.5 to 5 mm. As depicted in Fig. 7b, when the loading is 35 wt%, the maximum RL value is as high as -35.3 dB at 6.6 GHz with the thickness of 3.6 mm, and the effective absorbing bandwidth is 4.3 GHz (5.7-10.0 GHz). It can be readily found that the bandwidth (RL＜-10dB) exceeds 11 GHz almost covering the C, X and Ku wavebands. The superior EM wave absorbing performances and wider absorption bandwidths have evidenced the great potentials of ternary hierarchical CoNi@SiO2@C-3 composites once again.

Fig. 7. Calculated RL curves of CoNi@SiO2@C-3 with a filler loading of (a) 20 wt% and (b) 35 wt% in different thicknesses. The enhanced microwave absorption capability of CoNi@SiO2@C-3 composites can be interpreted by the following factors. First, the prominent microwave absorption performances are connected with the hetero structure of CoNi@SiO2@C-3 hybrids encapsulating the magnetic cores with double dielectric components. As we know, unilateral dielectric or magnetic loss mechanism is difficult to fulfill the impedance match condition; and the pronounced microwave absorption capability is the joint outcome of dielectric and magnetic loss48. Second, hierarchical structure


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providing numerous interfaces, leads to various interface polarizations occurring in the multicomponent CoNi@SiO2@C-3 composites, for example, the interface polarizations between the irregularly urchin-like magnetic CoNi and silica coatings; silica and carbon coatings; carbon shells and air/paraffin, which associate with enhanced dielectric relaxation32. For one thing, during the carbonization of phenolic resin precursors, the doping of oxygen atoms and generation of some defects are conducive to introduce dipole polarizations. Furthermore, some literatures illuminated that Debye dipolar relaxations were beneficial to improve the performances of microwave absorption as well. According to Debye theory, the relationship of ε′ and ε″ can be deduced as follows49-51: + ,

-. -/ $ $

-. -/ $ $

0 ++ $

(3)

In this case, the plot of ε″ vs. ε′ would be a single semicircle, commonly denoted as the Cole-Cole semicircle, indicating a Debye relaxation process52. The plot of ε′ vs. ε″ is displayed in Fig. 8. There are at least three semicircles observed in the whole curve, implying that hierarchical structure endows these composites with multiple dielectric relaxation processes, which also account for the dielectric loss.

Fig. 8. Typical Cole-Cole semicircles (ε″ vs ε′) of 50 wt% loaded-CoNi@SiO2@C-3 composites. Third, the quarter-wavelength matching model is presented to interpret the possible mechanism causing the improved microwave absorption capability of CoNi@SiO2@C-3



composites. In this criteria, the relationship between a matching thickness (tm) and the corresponding peak frequency (fm) can be expressed by the following equation53-54:

t 2 nc/462 | || |)

(n=1, 3, 5…)

(4)

Under the condition of the thickness meets Eq.(4), the phase difference between air-sample interface and the sample-metal interface is 180°, causing the extinction of the EM energy; and thus the absorbing materials tend to behave remarkable EM wave absorbing performances55. As shown in Fig. 9b, the tm vs. fm curve (λ/4 wavelength) of CoNi@SiO2@C-3 with 50 wt% loading is described by the blue line, while the matching thicknesses corresponding to the RL curves in Fig. 9a are denoted as orange squares. It is clear that the experimental orange squares are ideally located around the blue line (λ/4 curve), revealing the relation between tm and fm for the CoNi@SiO2@C-3 hybrids is in agreement with the quarter-wavelength matching model, which account for the enhanced microwave absorption performances of CoNi@SiO2@C-3. Besides, the reflection loss is greatly connected with the normalized impedance matching characteristics (Z=|Zin/Z0|). When Z is near to 1, most of the EM waves could enter into the absorbers and then would be transformed into thermal energy or dissipated by interference, thus denoting the better impedance53, 56-57. In Fig. 9c, the corresponding Z of CoNi@SiO2@C-3 ternary composites is deduced. The Z value most approaching to 1 at 10.8 GHz occurs in a place where the thickness is 2.2 mm, and even is consistent with the black dash line (denoted as Z=1) in a certain frequency band, which display their superiority in both strong microwave absorption capability (-46 dB) and wide absorbing bandwidth (5.6 GHz). Consequently, the well-matched impedance characteristic is conducive to the microwave absorption properties of CoNi@SiO2@C-3 as well.


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Fig. 9. Reflection loss at various thicknesses (a); tm vs. fm under the λ/4 (b); the impedance matching characteristics vs. the RL peak frequency (c) of 50 wt% loaded-CoNi@SiO2@C-3 composites.

4. Conclusions In summary, a series of ternary CoNi@SiO2@C composites were successfully prepared via a rational approach. The content of carbon coating can be controlled by modulating the additive dose of phenolic resin precursors. When the matching thickness is only 2.2 mm, wax-based composites containing 50 wt% CoNi@SiO2@C-3 show the maximum RL value is as strong as -46.0 dB at 10.8 GHz, and the effective bandwidth exceeding -10 dB is 5.6 GHz (7.1-12.7 GHz). Compared with single CoNi alloy, the CoNi@SiO2@C composites behave distinctly enhanced microwave absorption performances, by reason of the combination of dielectric and magnetic loss, introduction of interfacial loss and improved matching impedance after silica and carbon coatings. This result indicates that these absorbers will find their extensive utilization in microwave absorbing field. Acknowledgements This work was sponsored by the National Natural Science Foundation of China (51672222), the



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45. Wang, L; Huang, Y.; Sun, X.; Huang, H.; Liu, P.; Zong, M.; Wang, Y. Synthesis and microwave absorption

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