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Construction of MnO nanoparticles anchored on SiC whiskers for superior electromagnetic wave absorption
Journal of Colloid and Interface Science 559 (2020) 186–196
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Construction of MnO nanoparticles anchored on SiC whiskers for superior electromagnetic wave absorption Shun Dong a,⇑, Yang Lyu b, Xiutao Li c, Jingmao Chen c, Xinghong Zhang a,⇑, Jiecai Han a, Ping Hu a a
National Key Laboratory of Science and Technology for National Defence on Advanced Composites in Special Environments, Harbin Institute of Technology, Harbin 150080, China School of Materials Science and Engineering, Harbin University of Science and Technology, Harbin 150001, China c China Academy of Launch Vehicle Technology, Beijing 100076, China b
g r a p h i c a l a b s t r a c t A multiscale MnO nanoparticles/SiC whiskers hybrid heterostructures with significantly enhanced reflection loss and broadened effective absorption bandwidth.
a r t i c l e
i n f o
Article history: Received 25 July 2019 Revised 24 September 2019 Accepted 8 October 2019 Available online 9 October 2019 Keywords: SiC whiskers MnO nanoparticles Electromagnetic wave absorption
a b s t r a c t MnO nanoparticles (MnONP) decorated SiC whiskers (SiCw) with superior electromagnetic (EM) wave absorption performances were successfully synthesized by combining a hydrothermal and thermal annealing process. The microstructural feature and the content of MnONP of these MnONP/SiCw composites could be effectively controlled by the hydrothermal temperature, resulting in the adjustable EM wave absorption capacity. Compared with the poor EM wave absorption property of pristine SiCw (10.48 dB), the MnONP/SiCw heterostructures achieve substantially enhanced microwave absorption performances, attributing to the suitable impedance matching and improved loss ability arose from the synergetic effect between MnO and SiC, whose minimum reflection loss (RLmin) is improved to 15.84 dB for MnONP/SiCw obtained at 80 °C (S-80), to 15.17 dB for MnONP/SiCw obtained at 100 °C (S-100), and to 55.10 dB for MnONP/SiCw obtained at 120 °C (S-120), respectively. The MnONP/SiCw composite not only exhibits enhanced microwave absorption property, but also presents wider effective absorption bandwidth
⇑ Corresponding authors. E-mail addresses: [email protected] (S. Dong), [email protected] (X. Zhang). https://doi.org/10.1016/j.jcis.2019.10.026 0021-9797/Ó 2019 Elsevier Inc. All rights reserved.
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(EAB), reaching up to 5.4 GHz for S-80, 3.6 GHz for S-100 and 5.2 GHz for S-120 in comparison with pristine SiC (1.5 GHz). This work is expected to provide an effective approach to enhance EM wave absorption property of dielectric materials by incorporation of MnO and the MnONP/SiCw composites could be as a promising candidate for EM wave absorption applications. Ó 2019 Elsevier Inc. All rights reserved.
1. Introduction An ideal microwave absorber with high efficiency and broad frequency is always pursed by researchers due to its promising applications in dual-use fields, including military radar, stealth aircraft, information communication and healthcare, and numerous types materials have been developed widely and their electromagnetic (EM) wave absorption properties are upgrading ongoing so far [1–5]. In general, EM absorbers could be classified as dielectric materials with high dielectric constant (graphene, carbon nanotubes, ZnO, SiC, etc.), and magnetic materials with high permeability, such as Ni, Fe, and various metal oxides [6–11]. Notably, to achieve excellent EM wave absorption performance, the impedance matching between complex permeability and complex permittivity should be suitable, and thus tremendous efforts have been made to construct hybrid structures [12–15]. Owing to its superior integrated performance (low density, excellent thermal and chemical stability at room and high temperatures, high strength, and adjustable electric conductivity), SiC has been considered as one of promising candidates for microwave absorbers with high absorption capacity and broad absorption bandwidth [16–18]. In addition, recent literature has confirmed that SiC structures with low dimensionality exhibit better microwave absorption abilities than that of bulk SiC materials [19–21]. However, it is worth noting that the single SiC nano- or microcomposition could not satisfy the demand of excellent EM absorbers, resulting from the single polarization mechanism and low conductivity and restricting their practical application [22–25]. To date, numbers of methods have been conducted to enhance EM wave absorption performance of SiC-based composites, including doping, defects regulation and surface modification [26–31]. Wang et al. fabricated SiC/Co hybrid nanowires (NWs) by a facile electroless plating method, decorating the SiC NWs with monodisperse magnetic Co nanocrystals, and the hybrid NWs exhibit a high capability for EM absorption compared to a simple physical mixture of SiC and Co [13]. Wang and coworkers also synthesized heterostructured SiC/C NWs by combining an interfacial in situ polymer encapsulation and carbonization process, and the resulting core-shell SiC/C nanostructures present excellent EM wave absorption properties with a minimum reflection loss (RLmin) value of 50 dB [3]. According to the above results, it is reasonable to believe that modified SiC with other component through in-situ decorating method might be an effective approach to achieve outstanding EM wave absorption performance compared with the single SiC material. Recently, manganese oxides have drawn significant research attention mainly in energy storage and catalysis applications due to their unique properties with abundant resource reserves, lowcost and environmental-friendly [32,33]. Among them, MnO is similar to SiC, as a wide-band semiconductor with low conductivity, while it might suppress eddy current and coordinate the EM parameter, resulting in better impedance matching, since its small permittivity and weak attenuation ability [34,35]. Li et al. prepared peapod-like MnO/C core-shell NWs via a simple method, which reveals superior EM microwave absorption property with a RLmin value of 55 dB at 13.6 GHz [34]. Thus, we suppose that construct a hybrid structure through in-situ encapsulating SiC nano- or
micro- structure with MnO might enhance the EM wave absorption performance of the signal SiC nano- or micro- structure, and the incorporation of MnO nanoparticles (MnONP) on SiC nano- or micro- structure has never been reported for EM wave absorption applications according to our survey. Herein, we report a feasible strategy following hydrothermal and thermal annealing processes for the in-situ synthesis of hierarchical MnONP/SiC whiskers (MnONP/SiCw) for the first time. Taking the SiCw as the substrate material, well-constructed MnONP/ SiCw hybrids possessing superior EM wave absorption capacities were successfully conducted. The content of MnONP could be modulated easily by changing the hydrothermal temperature, and the EM wave absorption properties of structured materials could be tailored by controlling the content of MnONP via the hydrothermal temperature. Meanwhile, the developed method could be generally applied to improve the other EM absorbers by coated with MnONP. The results indicated that the as-obtained MnONP/SiCw heterostructures exhibit significantly enhanced EM wave absorption capacity comparing with previously reported SiC-based absorbers, and the microwave absorption mechanisms were disclosed from the point of view of the synergy between multiple components as well as the structural features. 2. Experimental section 2.1. Materials All regents are AR grade without further purification. The commercially SiC whiskers with about 1.0 and 10 lm average diameter and length (purity >99.0%, Aesar (China) Chemicals Co. Ltd.) were used as raw materials. Potassium permanganate (KMnO4), ethanol and deionized water were purchased from Harbin Kecheng Chem. Co. 2.2. In-situ synthesis of MnONP/SiCw heterostructures In a typical process, a certain amount of 12 mM KMnO4 aqueous solution was first prepared and then 0.2 g SiCw were added into the above solution at room temperature under vigorous stirring for 2 h to form a uniform state. The mixture solution was transferred into a Teflon-lined stainless-steel autoclave and sealed to heat at different hydrothermal temperatures for 12 h. After cooled down to room temperature, the mixture was filtered and washed with deionized water and ethanol for at least 5 times, and then dried at 50 °C for 24 h. To obtain MnONP/SiCw heterostructures, the dried mixture was put into a ceramic crucible (60 mm 30 mm 30 mm) to thermal treatment in a horizontally tubular furnace at 900 °C for 2 h under a flowing N2 atmosphere at 50 mL/min. The products were denoted as pristine and S-T, where the pristine sample was the raw material SiCw, and T = 80, 100 and 120 presented the hydrothermal temperature, respectively. 2.3. Characterization X-ray diffraction (XRD, X’PERT PRO MPD, Holland) under Cu Ka radiation (40 kV, 30 mA) and Fourier transform infrared spectroscopy (FT-IR) collected from a PerkinElmer 2000 spectrometer
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(USA) were employed to analyze the phase and chemical compositions of the products. A field-emission scanning electron microscope (SEM, HELIOS NanoLab 600i, USA), transmission electron microscopy and high-resolution transmission electron microscopy (TEM and HRTEM, Tecnai G2-F30, USA) were used to observe and confirm the morphology and microstructure of the products. The elemental area scanning under SEM was also conducted to confirm the composition of the products. 2.4. EM wave absorption properties To achieve the EM wave absorption properties, 50 wt% asprepared products were mixed uniformly with paraffin wax used as the binder at 85 °C and pressed into a ring shape (the size: Dout = 7.0 mm and Din = 3.0 mm), and then the sample was tested by a Vector Network Analyzer (Agilent N5230A, USA) in the frequency range of 2.0–18.0 GHz to obtain the complex permittivity (er, er = e0 - je00 ) and permeability (lr, lr = l0 - jl00 ), using the simulation program of Reflection/Transmission Nicolson–Ross model [36–38]. According to the transmit line theory, the EM wave absorption performance could be reflected by the value of reflection loss (RL) calculated by the following formula
Z in 1 RLðdBÞ ¼ 20lg Z in þ 1 rffiffiffiffiffi Z in ¼
ð1Þ
lr 2pfd pffiffiffiffiffiffiffiffiffi lr er tanh j c er
ð2Þ
where Zin is the input impedance, f refers to the microwave frequency, d represents the thickness of the sample and c stands for the velocity of light, respectively [36–38]. 3. Results and discussion The multiscale hierarchical architecture of MnONP/SiCw composites were successfully synthesized by the hydrothermal method and subsequent thermal annealing process. Typically, the manganese oxide would first grow on SiCw after the hydrothermal treatment of a solution containing KMnO4 and SiCw, and then the as-prepared composites would be thermally treated at 900 °C for 2 h under flowing N2 atmosphere to complete the transformation of manganese oxide into MnO. Notably, the MnONP content of MnONP/SiCw heterostructures could be effectively controlled by changing the hydrothermal temperature, achieving the desirable EM wave absorption performance.
The crystal structures and phase compositions of the assynthesized materials were analyzed by XRD. In Fig. S1, the diffraction peaks around 35.6°, 41.4°, 60.0°, 71.8° and 75.5° could be assigned to the (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) planes of the crystalline of b-SiC (JCPDS NO. 29–1129) [21,23,24,28]. The small diffraction peak at about 33.7°, marked as SF, could be attributed to the stacking fault of SiCw [21,23,24,28]. Fig. 1a reveals the XRD patterns of MnONP/SiCw heterostructures obtained at different hydrothermal temperature, which are denoted as S-80, S-100 and S-120, respectively. From Fig. 1, five new diffraction peaks located at about 35.0°, 40.5°, 58.7°, 70.2° and 73.7° could be indexed to the diffraction of (1 1 1), (2 0 0), (2 2 0), (3 1 1), and (2 2 2) planes of standard cubic MnO (JCPDS NO. 07-0230) after the modification through the hydrothermal and thermal annealing processes, demonstrating the successful introduction of MnO, while the characteristic diffraction peaks of 3C-SiC or even the SF peak remained unchanged, indicating the high purity of MnONP/SiCw composites [34,35]. In addition, the MnO relative diffraction peaks become more distinct at elevated the hydrothermal temperature, suggesting that the coverage or content of MnO on the SiCw might increase just by adjusting the hydrothermal temperature. Furthermore, FT-IR spectra shown in Fig. 1b were employed to confirm the successful formation of MnO on the SiCw. All the samples show similar absorption peaks at different wave number regions, where the two strong absorption peaks centered at nearly 792 and 943 cm1 could be ascribed to the transverse optical (TO) photon and longitudinal optic (LO) phonon vibration modes of the SiAC bonds [39]. Apart from the presence of SiC, there are three new absorption peaks centered at 480, 565 and 858 cm1 in the composites, and all these absorption peaks could be attributed to the vibrations of MnAO bonds according to previous literature [40,41]. Therefore, combined with the above analysis, it is reasonable to believe that the facile method combined with the hydrothermal method and subsequent thermal annealing processes could be considered as an effective way to introduce MnO into the composites. The morphology and microstructure of the as-obtained samples were further characterized by the SEM and TEM. From Fig. 2a, 2c and 2e, the average diameter and length of SiCw are 1 lm and 10 lm, respectively. The pristine SiCw with a smooth surface could be observed in the right inset of Fig. 2b before the introduction of MnO, while the MnONP/SiCw heterostructures exhibit a rough surface after the growth of MnO, and lots of protuberances could be found on the surface of SiCw, namely MnO particles. The size of
Fig. 1. (a) XRD patterns and (b) FT-IR spectra of pristine SiCw and the MnONP/SiCw heterostructures.
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Fig. 2. SEM images of pristine SiCw and the MnONP/SiCw heterostructures. (a)–(b) S–80 sample, (c)–(d) S–100 sample, and (e)–(f) S–120 sample. The inset SEM image in (b) is a single pristine SiC whisker with smooth surface.
MnO particles is about 100 nm, and the MnONP are homogeneously anchored to the SiCw. Noteworthily, the coverage or content of MnONP on the SiCw increases as the hydrothermal temperature rising from the Fig. 2b, 2d and 2f, suggesting that the content of MnONP could be controlled just by changing the hydrothermal temperature. In addition, the elemental area scanning image of a single MnONP/SiCw with corresponding elemental distribution in Fig. S2 demonstrates that Si, C, Mn and O without other elements homogeneously distributed in the MnONP/SiCw heterostructures, which is also in good agreement with the result of XRD analysis. To obtain more detailed information about the microstructure of the MnONP/SiCw heterostructures, typical TEM and HRTEM images were obtained, as revealed in Fig. 3. From Fig. 3a-b, the smooth surface of pristine SiCw and the rough surface of MnONP/SiCw composite could also be confirmed, and the average size of single SiCw is 1 lm, which is consistent with the result of SEM images. The MnONP were observed separately, and the size
of single MnO nanoparticle is about 100 nm. Moreover, the HRTEM image in Fig. 3d taken from the edge of a single MnO nanoparticle suggests that the lattice space of the product is about 0.26 nm, which is in accordance with the distance of (1 1 1) plane of the cubic MnO [34]. According to the above analysis, the MnONP with about 100 nm were successfully incorporated onto SiCw to form a multiscale hierarchical architecture, which might achieve superior EM wave absorption performance. Theoretically, an ideal microwave absorber with highperformance EM wave absorption should own efficient complementarities between complex permittivity and permeability, and the single component materials might exhibit weak EM absorption properties due to the poor impedance matching. Herein, the asprepared MnONP/SiCw heterostructures are likely to achieve outstanding EM absorption capabilities due to the multicomponent with the well matching. To evaluate the EM absorption performance of pristine SiCw and as-obtained MnONP/SiCw samples,
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Fig. 3. TEM images of (a) pristine SiCw, (b) the MnONP/SiCw heterostructures and (c) MnONP. (d) The HRTEM image of a single MnO nanoparticle.
the complex permittivity and permeability were calculated through the real and imaginary parts of complex permittivity and permeability, namely e0 , e00 , l0 and l00 . Noticeably, the e0 and l0 refer to the storage abilities of electric and magnetic energy, and e00 and l00 stand for the dissipation abilities of electric and magnetic energy [34,42]. The EM parameters of pristine SiCw and hierarchically MnONP/ SiCw composites are shown in Fig. 4, including e0 , e00 , l0 and l00 . Apart from some high frequency regions, the values of e0 and e00 of pristine SiCw are both the minimum in the measurement frequency region, suggesting the poor storage and dissipation abilities of electric, and the values of e0 and e00 of the MnONP/SiCw samples increase as the hydrothermal temperature after the incorporation of MnONP, signifying the enhancement of storage and dissipation abilities of electric energy, which also confirmed that the MnONP could improve the dielectric loss of the composites. The values of e0 for all samples decrease with increasing frequency with several fluctuation, as shown in Fig. 4a, and this phenomenon could be attributed to the following factors. Generally, e0 is strongly dependent on the various polarizations, such as electronic, ion and intrinsic electric dipole polarizations, and the electrons possess enough time to respond to the alternating external EM field when the EM field frequency is relatively low, while the interval of alternating external EM field would be shorter and shorter as the frequency increases, and the reduced time would lead to that part of the above polarizations of these electrons could not catch up since electrons packed would be occurred at the interfaces due to the different electrical conductivity between SiC and MnO, resulting in the decrease of e0 value [11,43]. The above analysis could be further confirmed by the following Cole-Cole semicircles (Fig. S3),
representing the presence of Debye relaxation processes [43,44]. Meanwhile, the value of e0 increases significantly with the increasing the hydrothermal temperature, except for the inconspicuous difference between S-100 and S-120. Similarly, apart from S-120 sample, the of e00 value all increases obviously with the increasing the hydrothermal temperature, which is almost larger than that of pristine SiCw. From Fig. 4b, the value of e00 increases with the enhancement of electrical conductivity based on the free electron theory, which indicates the improved electron transmission capacity of the MnONP/SiCw heterostructures along with the increase of the content of MnONP adjusted by the hydrothermal temperature, resulting in the enhanced electrical conductivity and the value of e00 . In addition, there some peaks around 4–6, 6–8, 10, 12–14 and 16–18 GHz should be ascribed to the resonance behaviors [11,43]. Fig. 4c-d reveal the values of l0 and l00 of pristine SiCw and as-prepared MnONP/SiCw composites in the frequency range of 2–18 GHz, respectively. In general, MnO is a typical antiferromagnetic phase, and the l0 and l00 values of pristine SiCw are close to 1 and 0, respectively. Compared with pristine SiCw in Fig. 4c, the maximum values of l0 for MnONP/SiCw composites increase with the hydrothermal temperature, while the average values of l0 for MnONP/SiCw composites are all close to 1. Likewise, from Fig. 4d, the l00 values of S-80 and S-100 samples are about 0, which is similar to that of pristine SiCw. However, the S-120 sample owns the largest l00 value, demonstrating the strongest the dissipation ability of magnetic energy. Fig. 5a-b display the tangential dielectric loss (tande = e00 /e0 ) and the tangential magnetic loss (tandl = l00 /l0 ), respectively. Obviously, the tande values of S-80 and S-100 are slightly higher than that of pristine SiCw aside from some small frequency regions,
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Fig. 4. (a) The real (e0 ) and (b) imaginary (e00 ) parts of the complex permittivity, and (c) the real (l0 ) and (d) imaginary (l00 ) parts of the complex permeability of pristine SiCw and the MnONP/SiCw heterostructures.
indicating the enhanced dielectric losses after the introduction of MnONP, and the similar result exists in the magnetic loss for S80 and S-100 composites, in accordance with the above analysis of l00 value. Although the S-120 composite presents the lowest average tande value and the largest tandl value, the sample might have a better matched degree between the dielectric loss and the magnetic loss. According to previous theory, the magnetic loss of materials mainly arises from the natural, exchange and domain wall resonances, the magnetic hysteresis loss and eddy current effect [43]. The contribution of the natural resonance to the magnetic loss could be evaluated by the C0 (l00 /{(l0 )2f}, f refers to frequency) value, and a highly frequency-dependent C0 values means the natural resonance plays an effective positive role in enhancing the magnetic loss [43,45]. After plotting C0 curves, as shown in Fig. 5c, it can be seen that the C0 values, especially for the S-120 composite, always keep changing in the frequency range of 2–12 GHz and several resonance peaks could be found, suggesting that the natural resonance and eddy current effect are together responsible for the contribution of magnetic loss. Compared with that of pristine SiCw, there several peaks could be observed at about 4, 6, 8–10, 10–12 and 14–16 GHz in the curves of tangential magnetic loss, as shown in Fig. 5b. In addition, the exchange resonance usually appears at a high resonance frequency compared with that of the natural resonance based on the theory of Aharoni [43,46]. Therefore, the resonance peaks at about 4, 6, 8–10, and 10–12 GHz should be induced by the natural resonance and eddy current effect, and the other resonance peaks centered at about
14–16 GHz should be caused by the exchange resonance and eddy current effect. In general, the domain wall resonance just exists in multidomain materials within 1–100 MHz frequency range, and the magnetic hysteresis loss could be negligible in a weak applied EM field originating from the irreversible magnetization, and then the domain wall resonance and the magnetic hysteresis loss should be excluded to explain the enhancement of magnetic loss [43,45]. Therefore, the enhanced magnetic loss of the MnONP/SiCw heterostructures should be ascribed to the natural and exchange resonances, and the eddy current effect. As for the origin of slightly enhanced magnetic loss, it might be attributed to the introduction of MnONP after high-temperature treatment under N2, bringing the nano-size effect simultaneously. Fig. 6 reveals three-dimensional RL maps of pristine SiCw and MnONP/SiCw composites in the frequency range of 2–18 GHz with various thicknesses from 1.00 to 5.00 mm, and the typical RL curves at some certain thicknesses are recorded in Fig. S4. As shown in Fig. 6a, pristine SiCw performs a RLmin value of 10.48 dB at 7.6 GHz with a thickness of 5 mm, and the absorption bandwidth below 10 dB just ranges from 12.9 to 14.4 GHz with a thickness of 3.0 mm. For S-80 sample shown in Fig. 6b, its RLmin value is 15.84 dB at a high frequency 14.2 GHz with a thin thickness of 2.48 mm, and the corresponding EAB could cover 5.4 GHz from 11.8 to 17.2 GHz. In Fig. 6c, the S-100 composite shows the similar EM wave absorption ability of S-80 sample, whose RLmin value is 15.17 dB at 6.0 GHz with 5.0 mm thickness and the maximum EAB value is 3.6 GHz ranging from 11.9 to 15.5 GHz with an
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Fig. 5. (a) Tangential dielectric loss and (b) tangential magnetic loss values of pristine SiCw and the MnONP/SiCw heterostructures, and (c) values of l00 /{(l0 )2f} vs. frequency for ristine SiCw and the MnONP/SiCw heterostructures.
absorber thickness of 2.5 mm. Surprisingly, the S-120 composite exhibits a remarkable EM wave absorption performance shown in Fig. 6d, and its RLmin value could be as low as 55.10 dB at a relatively lower frequency 8.8 GHz with a thickness of 3.99 mm, and the maximum EAB value could reach 5.2 GHz with 4.41 mm thickness. From the results, the microwave absorption performance of MnONP/SiCw composite is further enhanced with the incorporation of MnO, resulting from the unique microstructure and synergistic effect between SiC and MnO, and the MnONP/SiCw heterostructures could be potential candidates as EM wave absorbers with outstanding absorption of EM waves. Combined with above results, it can be concluded that the incorporation of MnONP could be an effective way to enhance the EM wave absorbing absorption property of SiCw, which should be ascribed to the synergistic effects between the enhanced impedance matching and the improved loss ability. A good impedance matching could be achieved when it is equal or close to that of free space, leading to zero reflection at the front surface of the materials, which is strongly dependent on the relationship between the complex permittivity and permeability, and most of incident EM waves would be reflected off when the complex permittivity is much larger than the complex permeability [11,47]. Meanwhile, the impedance matching degree could be evaluated by the deltafunction method through the following equation [11,47]:
2 jDj ¼ sinh ðKfdÞ M
ð3Þ
where K and M could also be calculated by the following equations [11,47]:
K¼
pffiffiffiffiffiffiffiffi m 4p l0 e0 sin de þd 2 ccosde cosdm
M¼
4l0 cosde e0 cosdm 2 ðl0 cosde e0 cosdm Þ2 þ tan d2m d2e ðl0 cosde þ e0 cosdm Þ2
ð4Þ
ð5Þ According to previous literature, a small delta value and a large area, namely the area is close to zero, suggesting a good impedance matching [11,47]. Fig. 7 reveals the calculated delta value maps of pristine SiCw and the MnONP/SiCw heterostructures. Obviously, it can be seen that the MnONP/SiCw heterostructures always possess larger area close to zero than that of pristine SiCw, indicating that the introduction of MnONP could improve the impedance matching of the composites. It is worth noting that the S-120 sample owns the correspondingly minimum area close to zero compared with those of S-80 and S-100 samples, and this consequence could be responsible for its relatively small EAB value in comparison with S-80, which could be ascribed to the excessive MnO content. In addition, the S-80 and S-100 samples exhibit almost same area close to zero, and the results are in good agreement with those of EM wave absorption performances with the similar RL value. Moreover, the other crucial parameter for the microwave absorber is the EM wave attenuation, and the attenuation constant (a) could be calculated by the following equation [11,47,48]:
a¼
pffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi12 2pf ðl00 e00 l0 e0 Þ þ ðl00 e00 l0 e0 Þ2 þ ðl0 e00 þ l00 e0 Þ2 c
ð6Þ
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Fig. 6. The three–dimensional RL maps of pristine SiCw and the MnONP/SiCw heterostructures. (a) Pristine SiCw, (b) S–80 sample, (c) S–100 sample, and (d) S–120 sample.
Fig. 7. The calculated delta value maps of pristine SiCw and the MnONP/SiCw heterostructures. (a) Pristine SiCw, (b) S–80 sample, (c) S–100 sample, and (d) S–120 sample.
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Fig. 8. Attenuation heterostructures.
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constants
of
pristine
SiCw
and
the
MnONP/SiCw
Generally speaking, a large a value means a good attenuation capacity, revealing the great dissipation properties of materials [11,47,48]. As recorded in Fig. 8, the attenuation value of the S120 composite is highest among the tested samples, especially at the relatively low frequency (<9.0 GHz), which could be explained that why the S-120 sample shows the least RLmin value among the MnONP/SiCw composites. The S-80 sample exhibits the largest average a value at the relatively high frequency (>9.0 GHz), resulting in the widest EAB among the MnONP/SiCw composites. Clearly, the MnONP/SiCw heterostructures all present larger a value than that of pristine SiCw, leading to better EM wave absorption performances compared with that of pristine SiCw. All these above results manifest that the incorporation of MnONP endows the MnONP/SiCw heterostructures as novel EM wave absorbers with outstanding EM wave absorbing property through the matched impedance and the enhanced attenuation capacity. According to the above results and analysis, the following factors are responsible for the glorious EM wave absorption performances of the MnONP/SiCw heterostructures [5,49–55]. Firstly, the dispersed MnONP cases avoid agglomeration due to the presence of SiCw, as shown in Fig. 2. The heterogeneous interfaces between MnONP and SiCw cause interfacial polarization, making contribute to the improvement of EM wave absorption performance. Secondly, the introduction of MnONP improve the conductive interconnections in the absorber, which would promote the formation of more induced micro electric currents under alternating EM field and then result in the conductive loss, leading to enhanced EM wave absorption. In addition, the defects and stacking faults in SiCw might also act an important role in enhancing the EM wave absorption properties of the composites. The charged defects in SiCw along with unpaired electrons would response to the external electric field, and the stacking faults in SiCw would be acted as the polarized centers, synergistically causing the space charge polarization and relaxation. In a word, the outstanding EM wave absorption properties should be mainly attributed to the matched impedance and improved loss ability arose from the synergetic effect between MnONP and SiCw as well as the unique hierarchical structure. To further assess the EM wave absorption performance of the MnONP/SiCw heterostructures, the RLmin and EAB values of typical EM absorbers reported by recent literature are summarized in Fig. 9 and Table S1. Combined with the results shown in Fig. 9, Tables 1 and S1, our products, namely the MnONP/SiCw composites exhibit superior EM wave absorption properties both with high efficiency and broad frequency compared with most of these
Fig. 9. Comparison of the RLmin values between present work and recent reported EM wave absorbers.
absorbers. From the point of view of these excellent advantages, it is believed that the MnONP/SiCw heterostructures could be regarded as a potential candidate for the oncoming generation of EM wave absorbing materials, and this strategy of construction of multicomponent composites with introduction of MnO is a promising method to prepare high-performance EM wave absorbers in practical applications. 4. Conclusions In conclusion, novel MnONP/SiCw hybrid composites with highperformance EM wave absorption were successfully synthesized via hydrothermal and heat treatment afterward. The content of MnONP could be effectively controlled by the hydrothermal temperature. The MnONP/SiCw hybrid whiskers obtained at 120 °C exhibit the optimal EM absorption ability with RLmin value of 55.10 dB at 8.8 GHz with a thickness of 3.99 mm, which is fivetimes higher than that of pristine SiCw. As the content of MnONP increases with the hydrothermal temperature, the absorption bandwidth of the MnONP/SiCw heterostructures becomes wider, and the maximum EAB value reaches 5.4 GHz. The outstanding EM wave attenuation performances could be attributed to the synergetic effect between the MnONP and SiCw, resulting in the suitable impedance matching and enhanced loss ability. The broad wideband microwave absorption of the MnONP/SiCw heterostructures highlights their potential applications in functional EM absorption and shielding devices, and this study also opens a new door for enhancing EM wave absorption property of the material by incorporation of MnO. Acknowledgements The authors gratefully acknowledge the financial supports provided by the National Natural Science Foundation of China (Grant Nos. 51372047, 51202048, 11421091, 11402252, 91216301 and 51902067), the China National Funds for Distinguished Young Scientists (No. 51525201), the National Key Laboratory of Science and Technology for National Defence on Advanced Composites in Special Environments (KL.PYJH.2016.001), and China Postdoctoral Science Foundation (No. 2019M651282). Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2019.10.026.
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Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis
Construction of MnO nanoparticles anchored on SiC whiskers for superior electromagnetic wave absorption Shun Dong a,⇑, Yang Lyu b, Xiutao Li c, Jingmao Chen c, Xinghong Zhang a,⇑, Jiecai Han a, Ping Hu a a
National Key Laboratory of Science and Technology for National Defence on Advanced Composites in Special Environments, Harbin Institute of Technology, Harbin 150080, China School of Materials Science and Engineering, Harbin University of Science and Technology, Harbin 150001, China c China Academy of Launch Vehicle Technology, Beijing 100076, China b
g r a p h i c a l a b s t r a c t A multiscale MnO nanoparticles/SiC whiskers hybrid heterostructures with significantly enhanced reflection loss and broadened effective absorption bandwidth.
a r t i c l e
i n f o
Article history: Received 25 July 2019 Revised 24 September 2019 Accepted 8 October 2019 Available online 9 October 2019 Keywords: SiC whiskers MnO nanoparticles Electromagnetic wave absorption
a b s t r a c t MnO nanoparticles (MnONP) decorated SiC whiskers (SiCw) with superior electromagnetic (EM) wave absorption performances were successfully synthesized by combining a hydrothermal and thermal annealing process. The microstructural feature and the content of MnONP of these MnONP/SiCw composites could be effectively controlled by the hydrothermal temperature, resulting in the adjustable EM wave absorption capacity. Compared with the poor EM wave absorption property of pristine SiCw (10.48 dB), the MnONP/SiCw heterostructures achieve substantially enhanced microwave absorption performances, attributing to the suitable impedance matching and improved loss ability arose from the synergetic effect between MnO and SiC, whose minimum reflection loss (RLmin) is improved to 15.84 dB for MnONP/SiCw obtained at 80 °C (S-80), to 15.17 dB for MnONP/SiCw obtained at 100 °C (S-100), and to 55.10 dB for MnONP/SiCw obtained at 120 °C (S-120), respectively. The MnONP/SiCw composite not only exhibits enhanced microwave absorption property, but also presents wider effective absorption bandwidth
⇑ Corresponding authors. E-mail addresses: [email protected] (S. Dong), [email protected] (X. Zhang). https://doi.org/10.1016/j.jcis.2019.10.026 0021-9797/Ó 2019 Elsevier Inc. All rights reserved.
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(EAB), reaching up to 5.4 GHz for S-80, 3.6 GHz for S-100 and 5.2 GHz for S-120 in comparison with pristine SiC (1.5 GHz). This work is expected to provide an effective approach to enhance EM wave absorption property of dielectric materials by incorporation of MnO and the MnONP/SiCw composites could be as a promising candidate for EM wave absorption applications. Ó 2019 Elsevier Inc. All rights reserved.
1. Introduction An ideal microwave absorber with high efficiency and broad frequency is always pursed by researchers due to its promising applications in dual-use fields, including military radar, stealth aircraft, information communication and healthcare, and numerous types materials have been developed widely and their electromagnetic (EM) wave absorption properties are upgrading ongoing so far [1–5]. In general, EM absorbers could be classified as dielectric materials with high dielectric constant (graphene, carbon nanotubes, ZnO, SiC, etc.), and magnetic materials with high permeability, such as Ni, Fe, and various metal oxides [6–11]. Notably, to achieve excellent EM wave absorption performance, the impedance matching between complex permeability and complex permittivity should be suitable, and thus tremendous efforts have been made to construct hybrid structures [12–15]. Owing to its superior integrated performance (low density, excellent thermal and chemical stability at room and high temperatures, high strength, and adjustable electric conductivity), SiC has been considered as one of promising candidates for microwave absorbers with high absorption capacity and broad absorption bandwidth [16–18]. In addition, recent literature has confirmed that SiC structures with low dimensionality exhibit better microwave absorption abilities than that of bulk SiC materials [19–21]. However, it is worth noting that the single SiC nano- or microcomposition could not satisfy the demand of excellent EM absorbers, resulting from the single polarization mechanism and low conductivity and restricting their practical application [22–25]. To date, numbers of methods have been conducted to enhance EM wave absorption performance of SiC-based composites, including doping, defects regulation and surface modification [26–31]. Wang et al. fabricated SiC/Co hybrid nanowires (NWs) by a facile electroless plating method, decorating the SiC NWs with monodisperse magnetic Co nanocrystals, and the hybrid NWs exhibit a high capability for EM absorption compared to a simple physical mixture of SiC and Co [13]. Wang and coworkers also synthesized heterostructured SiC/C NWs by combining an interfacial in situ polymer encapsulation and carbonization process, and the resulting core-shell SiC/C nanostructures present excellent EM wave absorption properties with a minimum reflection loss (RLmin) value of 50 dB [3]. According to the above results, it is reasonable to believe that modified SiC with other component through in-situ decorating method might be an effective approach to achieve outstanding EM wave absorption performance compared with the single SiC material. Recently, manganese oxides have drawn significant research attention mainly in energy storage and catalysis applications due to their unique properties with abundant resource reserves, lowcost and environmental-friendly [32,33]. Among them, MnO is similar to SiC, as a wide-band semiconductor with low conductivity, while it might suppress eddy current and coordinate the EM parameter, resulting in better impedance matching, since its small permittivity and weak attenuation ability [34,35]. Li et al. prepared peapod-like MnO/C core-shell NWs via a simple method, which reveals superior EM microwave absorption property with a RLmin value of 55 dB at 13.6 GHz [34]. Thus, we suppose that construct a hybrid structure through in-situ encapsulating SiC nano- or
micro- structure with MnO might enhance the EM wave absorption performance of the signal SiC nano- or micro- structure, and the incorporation of MnO nanoparticles (MnONP) on SiC nano- or micro- structure has never been reported for EM wave absorption applications according to our survey. Herein, we report a feasible strategy following hydrothermal and thermal annealing processes for the in-situ synthesis of hierarchical MnONP/SiC whiskers (MnONP/SiCw) for the first time. Taking the SiCw as the substrate material, well-constructed MnONP/ SiCw hybrids possessing superior EM wave absorption capacities were successfully conducted. The content of MnONP could be modulated easily by changing the hydrothermal temperature, and the EM wave absorption properties of structured materials could be tailored by controlling the content of MnONP via the hydrothermal temperature. Meanwhile, the developed method could be generally applied to improve the other EM absorbers by coated with MnONP. The results indicated that the as-obtained MnONP/SiCw heterostructures exhibit significantly enhanced EM wave absorption capacity comparing with previously reported SiC-based absorbers, and the microwave absorption mechanisms were disclosed from the point of view of the synergy between multiple components as well as the structural features. 2. Experimental section 2.1. Materials All regents are AR grade without further purification. The commercially SiC whiskers with about 1.0 and 10 lm average diameter and length (purity >99.0%, Aesar (China) Chemicals Co. Ltd.) were used as raw materials. Potassium permanganate (KMnO4), ethanol and deionized water were purchased from Harbin Kecheng Chem. Co. 2.2. In-situ synthesis of MnONP/SiCw heterostructures In a typical process, a certain amount of 12 mM KMnO4 aqueous solution was first prepared and then 0.2 g SiCw were added into the above solution at room temperature under vigorous stirring for 2 h to form a uniform state. The mixture solution was transferred into a Teflon-lined stainless-steel autoclave and sealed to heat at different hydrothermal temperatures for 12 h. After cooled down to room temperature, the mixture was filtered and washed with deionized water and ethanol for at least 5 times, and then dried at 50 °C for 24 h. To obtain MnONP/SiCw heterostructures, the dried mixture was put into a ceramic crucible (60 mm 30 mm 30 mm) to thermal treatment in a horizontally tubular furnace at 900 °C for 2 h under a flowing N2 atmosphere at 50 mL/min. The products were denoted as pristine and S-T, where the pristine sample was the raw material SiCw, and T = 80, 100 and 120 presented the hydrothermal temperature, respectively. 2.3. Characterization X-ray diffraction (XRD, X’PERT PRO MPD, Holland) under Cu Ka radiation (40 kV, 30 mA) and Fourier transform infrared spectroscopy (FT-IR) collected from a PerkinElmer 2000 spectrometer
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(USA) were employed to analyze the phase and chemical compositions of the products. A field-emission scanning electron microscope (SEM, HELIOS NanoLab 600i, USA), transmission electron microscopy and high-resolution transmission electron microscopy (TEM and HRTEM, Tecnai G2-F30, USA) were used to observe and confirm the morphology and microstructure of the products. The elemental area scanning under SEM was also conducted to confirm the composition of the products. 2.4. EM wave absorption properties To achieve the EM wave absorption properties, 50 wt% asprepared products were mixed uniformly with paraffin wax used as the binder at 85 °C and pressed into a ring shape (the size: Dout = 7.0 mm and Din = 3.0 mm), and then the sample was tested by a Vector Network Analyzer (Agilent N5230A, USA) in the frequency range of 2.0–18.0 GHz to obtain the complex permittivity (er, er = e0 - je00 ) and permeability (lr, lr = l0 - jl00 ), using the simulation program of Reflection/Transmission Nicolson–Ross model [36–38]. According to the transmit line theory, the EM wave absorption performance could be reflected by the value of reflection loss (RL) calculated by the following formula
Z in 1 RLðdBÞ ¼ 20lg Z in þ 1 rffiffiffiffiffi Z in ¼
ð1Þ
lr 2pfd pffiffiffiffiffiffiffiffiffi lr er tanh j c er
ð2Þ
where Zin is the input impedance, f refers to the microwave frequency, d represents the thickness of the sample and c stands for the velocity of light, respectively [36–38]. 3. Results and discussion The multiscale hierarchical architecture of MnONP/SiCw composites were successfully synthesized by the hydrothermal method and subsequent thermal annealing process. Typically, the manganese oxide would first grow on SiCw after the hydrothermal treatment of a solution containing KMnO4 and SiCw, and then the as-prepared composites would be thermally treated at 900 °C for 2 h under flowing N2 atmosphere to complete the transformation of manganese oxide into MnO. Notably, the MnONP content of MnONP/SiCw heterostructures could be effectively controlled by changing the hydrothermal temperature, achieving the desirable EM wave absorption performance.
The crystal structures and phase compositions of the assynthesized materials were analyzed by XRD. In Fig. S1, the diffraction peaks around 35.6°, 41.4°, 60.0°, 71.8° and 75.5° could be assigned to the (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) planes of the crystalline of b-SiC (JCPDS NO. 29–1129) [21,23,24,28]. The small diffraction peak at about 33.7°, marked as SF, could be attributed to the stacking fault of SiCw [21,23,24,28]. Fig. 1a reveals the XRD patterns of MnONP/SiCw heterostructures obtained at different hydrothermal temperature, which are denoted as S-80, S-100 and S-120, respectively. From Fig. 1, five new diffraction peaks located at about 35.0°, 40.5°, 58.7°, 70.2° and 73.7° could be indexed to the diffraction of (1 1 1), (2 0 0), (2 2 0), (3 1 1), and (2 2 2) planes of standard cubic MnO (JCPDS NO. 07-0230) after the modification through the hydrothermal and thermal annealing processes, demonstrating the successful introduction of MnO, while the characteristic diffraction peaks of 3C-SiC or even the SF peak remained unchanged, indicating the high purity of MnONP/SiCw composites [34,35]. In addition, the MnO relative diffraction peaks become more distinct at elevated the hydrothermal temperature, suggesting that the coverage or content of MnO on the SiCw might increase just by adjusting the hydrothermal temperature. Furthermore, FT-IR spectra shown in Fig. 1b were employed to confirm the successful formation of MnO on the SiCw. All the samples show similar absorption peaks at different wave number regions, where the two strong absorption peaks centered at nearly 792 and 943 cm1 could be ascribed to the transverse optical (TO) photon and longitudinal optic (LO) phonon vibration modes of the SiAC bonds [39]. Apart from the presence of SiC, there are three new absorption peaks centered at 480, 565 and 858 cm1 in the composites, and all these absorption peaks could be attributed to the vibrations of MnAO bonds according to previous literature [40,41]. Therefore, combined with the above analysis, it is reasonable to believe that the facile method combined with the hydrothermal method and subsequent thermal annealing processes could be considered as an effective way to introduce MnO into the composites. The morphology and microstructure of the as-obtained samples were further characterized by the SEM and TEM. From Fig. 2a, 2c and 2e, the average diameter and length of SiCw are 1 lm and 10 lm, respectively. The pristine SiCw with a smooth surface could be observed in the right inset of Fig. 2b before the introduction of MnO, while the MnONP/SiCw heterostructures exhibit a rough surface after the growth of MnO, and lots of protuberances could be found on the surface of SiCw, namely MnO particles. The size of
Fig. 1. (a) XRD patterns and (b) FT-IR spectra of pristine SiCw and the MnONP/SiCw heterostructures.
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Fig. 2. SEM images of pristine SiCw and the MnONP/SiCw heterostructures. (a)–(b) S–80 sample, (c)–(d) S–100 sample, and (e)–(f) S–120 sample. The inset SEM image in (b) is a single pristine SiC whisker with smooth surface.
MnO particles is about 100 nm, and the MnONP are homogeneously anchored to the SiCw. Noteworthily, the coverage or content of MnONP on the SiCw increases as the hydrothermal temperature rising from the Fig. 2b, 2d and 2f, suggesting that the content of MnONP could be controlled just by changing the hydrothermal temperature. In addition, the elemental area scanning image of a single MnONP/SiCw with corresponding elemental distribution in Fig. S2 demonstrates that Si, C, Mn and O without other elements homogeneously distributed in the MnONP/SiCw heterostructures, which is also in good agreement with the result of XRD analysis. To obtain more detailed information about the microstructure of the MnONP/SiCw heterostructures, typical TEM and HRTEM images were obtained, as revealed in Fig. 3. From Fig. 3a-b, the smooth surface of pristine SiCw and the rough surface of MnONP/SiCw composite could also be confirmed, and the average size of single SiCw is 1 lm, which is consistent with the result of SEM images. The MnONP were observed separately, and the size
of single MnO nanoparticle is about 100 nm. Moreover, the HRTEM image in Fig. 3d taken from the edge of a single MnO nanoparticle suggests that the lattice space of the product is about 0.26 nm, which is in accordance with the distance of (1 1 1) plane of the cubic MnO [34]. According to the above analysis, the MnONP with about 100 nm were successfully incorporated onto SiCw to form a multiscale hierarchical architecture, which might achieve superior EM wave absorption performance. Theoretically, an ideal microwave absorber with highperformance EM wave absorption should own efficient complementarities between complex permittivity and permeability, and the single component materials might exhibit weak EM absorption properties due to the poor impedance matching. Herein, the asprepared MnONP/SiCw heterostructures are likely to achieve outstanding EM absorption capabilities due to the multicomponent with the well matching. To evaluate the EM absorption performance of pristine SiCw and as-obtained MnONP/SiCw samples,
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Fig. 3. TEM images of (a) pristine SiCw, (b) the MnONP/SiCw heterostructures and (c) MnONP. (d) The HRTEM image of a single MnO nanoparticle.
the complex permittivity and permeability were calculated through the real and imaginary parts of complex permittivity and permeability, namely e0 , e00 , l0 and l00 . Noticeably, the e0 and l0 refer to the storage abilities of electric and magnetic energy, and e00 and l00 stand for the dissipation abilities of electric and magnetic energy [34,42]. The EM parameters of pristine SiCw and hierarchically MnONP/ SiCw composites are shown in Fig. 4, including e0 , e00 , l0 and l00 . Apart from some high frequency regions, the values of e0 and e00 of pristine SiCw are both the minimum in the measurement frequency region, suggesting the poor storage and dissipation abilities of electric, and the values of e0 and e00 of the MnONP/SiCw samples increase as the hydrothermal temperature after the incorporation of MnONP, signifying the enhancement of storage and dissipation abilities of electric energy, which also confirmed that the MnONP could improve the dielectric loss of the composites. The values of e0 for all samples decrease with increasing frequency with several fluctuation, as shown in Fig. 4a, and this phenomenon could be attributed to the following factors. Generally, e0 is strongly dependent on the various polarizations, such as electronic, ion and intrinsic electric dipole polarizations, and the electrons possess enough time to respond to the alternating external EM field when the EM field frequency is relatively low, while the interval of alternating external EM field would be shorter and shorter as the frequency increases, and the reduced time would lead to that part of the above polarizations of these electrons could not catch up since electrons packed would be occurred at the interfaces due to the different electrical conductivity between SiC and MnO, resulting in the decrease of e0 value [11,43]. The above analysis could be further confirmed by the following Cole-Cole semicircles (Fig. S3),
representing the presence of Debye relaxation processes [43,44]. Meanwhile, the value of e0 increases significantly with the increasing the hydrothermal temperature, except for the inconspicuous difference between S-100 and S-120. Similarly, apart from S-120 sample, the of e00 value all increases obviously with the increasing the hydrothermal temperature, which is almost larger than that of pristine SiCw. From Fig. 4b, the value of e00 increases with the enhancement of electrical conductivity based on the free electron theory, which indicates the improved electron transmission capacity of the MnONP/SiCw heterostructures along with the increase of the content of MnONP adjusted by the hydrothermal temperature, resulting in the enhanced electrical conductivity and the value of e00 . In addition, there some peaks around 4–6, 6–8, 10, 12–14 and 16–18 GHz should be ascribed to the resonance behaviors [11,43]. Fig. 4c-d reveal the values of l0 and l00 of pristine SiCw and as-prepared MnONP/SiCw composites in the frequency range of 2–18 GHz, respectively. In general, MnO is a typical antiferromagnetic phase, and the l0 and l00 values of pristine SiCw are close to 1 and 0, respectively. Compared with pristine SiCw in Fig. 4c, the maximum values of l0 for MnONP/SiCw composites increase with the hydrothermal temperature, while the average values of l0 for MnONP/SiCw composites are all close to 1. Likewise, from Fig. 4d, the l00 values of S-80 and S-100 samples are about 0, which is similar to that of pristine SiCw. However, the S-120 sample owns the largest l00 value, demonstrating the strongest the dissipation ability of magnetic energy. Fig. 5a-b display the tangential dielectric loss (tande = e00 /e0 ) and the tangential magnetic loss (tandl = l00 /l0 ), respectively. Obviously, the tande values of S-80 and S-100 are slightly higher than that of pristine SiCw aside from some small frequency regions,
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Fig. 4. (a) The real (e0 ) and (b) imaginary (e00 ) parts of the complex permittivity, and (c) the real (l0 ) and (d) imaginary (l00 ) parts of the complex permeability of pristine SiCw and the MnONP/SiCw heterostructures.
indicating the enhanced dielectric losses after the introduction of MnONP, and the similar result exists in the magnetic loss for S80 and S-100 composites, in accordance with the above analysis of l00 value. Although the S-120 composite presents the lowest average tande value and the largest tandl value, the sample might have a better matched degree between the dielectric loss and the magnetic loss. According to previous theory, the magnetic loss of materials mainly arises from the natural, exchange and domain wall resonances, the magnetic hysteresis loss and eddy current effect [43]. The contribution of the natural resonance to the magnetic loss could be evaluated by the C0 (l00 /{(l0 )2f}, f refers to frequency) value, and a highly frequency-dependent C0 values means the natural resonance plays an effective positive role in enhancing the magnetic loss [43,45]. After plotting C0 curves, as shown in Fig. 5c, it can be seen that the C0 values, especially for the S-120 composite, always keep changing in the frequency range of 2–12 GHz and several resonance peaks could be found, suggesting that the natural resonance and eddy current effect are together responsible for the contribution of magnetic loss. Compared with that of pristine SiCw, there several peaks could be observed at about 4, 6, 8–10, 10–12 and 14–16 GHz in the curves of tangential magnetic loss, as shown in Fig. 5b. In addition, the exchange resonance usually appears at a high resonance frequency compared with that of the natural resonance based on the theory of Aharoni [43,46]. Therefore, the resonance peaks at about 4, 6, 8–10, and 10–12 GHz should be induced by the natural resonance and eddy current effect, and the other resonance peaks centered at about
14–16 GHz should be caused by the exchange resonance and eddy current effect. In general, the domain wall resonance just exists in multidomain materials within 1–100 MHz frequency range, and the magnetic hysteresis loss could be negligible in a weak applied EM field originating from the irreversible magnetization, and then the domain wall resonance and the magnetic hysteresis loss should be excluded to explain the enhancement of magnetic loss [43,45]. Therefore, the enhanced magnetic loss of the MnONP/SiCw heterostructures should be ascribed to the natural and exchange resonances, and the eddy current effect. As for the origin of slightly enhanced magnetic loss, it might be attributed to the introduction of MnONP after high-temperature treatment under N2, bringing the nano-size effect simultaneously. Fig. 6 reveals three-dimensional RL maps of pristine SiCw and MnONP/SiCw composites in the frequency range of 2–18 GHz with various thicknesses from 1.00 to 5.00 mm, and the typical RL curves at some certain thicknesses are recorded in Fig. S4. As shown in Fig. 6a, pristine SiCw performs a RLmin value of 10.48 dB at 7.6 GHz with a thickness of 5 mm, and the absorption bandwidth below 10 dB just ranges from 12.9 to 14.4 GHz with a thickness of 3.0 mm. For S-80 sample shown in Fig. 6b, its RLmin value is 15.84 dB at a high frequency 14.2 GHz with a thin thickness of 2.48 mm, and the corresponding EAB could cover 5.4 GHz from 11.8 to 17.2 GHz. In Fig. 6c, the S-100 composite shows the similar EM wave absorption ability of S-80 sample, whose RLmin value is 15.17 dB at 6.0 GHz with 5.0 mm thickness and the maximum EAB value is 3.6 GHz ranging from 11.9 to 15.5 GHz with an
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Fig. 5. (a) Tangential dielectric loss and (b) tangential magnetic loss values of pristine SiCw and the MnONP/SiCw heterostructures, and (c) values of l00 /{(l0 )2f} vs. frequency for ristine SiCw and the MnONP/SiCw heterostructures.
absorber thickness of 2.5 mm. Surprisingly, the S-120 composite exhibits a remarkable EM wave absorption performance shown in Fig. 6d, and its RLmin value could be as low as 55.10 dB at a relatively lower frequency 8.8 GHz with a thickness of 3.99 mm, and the maximum EAB value could reach 5.2 GHz with 4.41 mm thickness. From the results, the microwave absorption performance of MnONP/SiCw composite is further enhanced with the incorporation of MnO, resulting from the unique microstructure and synergistic effect between SiC and MnO, and the MnONP/SiCw heterostructures could be potential candidates as EM wave absorbers with outstanding absorption of EM waves. Combined with above results, it can be concluded that the incorporation of MnONP could be an effective way to enhance the EM wave absorbing absorption property of SiCw, which should be ascribed to the synergistic effects between the enhanced impedance matching and the improved loss ability. A good impedance matching could be achieved when it is equal or close to that of free space, leading to zero reflection at the front surface of the materials, which is strongly dependent on the relationship between the complex permittivity and permeability, and most of incident EM waves would be reflected off when the complex permittivity is much larger than the complex permeability [11,47]. Meanwhile, the impedance matching degree could be evaluated by the deltafunction method through the following equation [11,47]:
2 jDj ¼ sinh ðKfdÞ M
ð3Þ
where K and M could also be calculated by the following equations [11,47]:
K¼
pffiffiffiffiffiffiffiffi m 4p l0 e0 sin de þd 2 ccosde cosdm
M¼
4l0 cosde e0 cosdm 2 ðl0 cosde e0 cosdm Þ2 þ tan d2m d2e ðl0 cosde þ e0 cosdm Þ2
ð4Þ
ð5Þ According to previous literature, a small delta value and a large area, namely the area is close to zero, suggesting a good impedance matching [11,47]. Fig. 7 reveals the calculated delta value maps of pristine SiCw and the MnONP/SiCw heterostructures. Obviously, it can be seen that the MnONP/SiCw heterostructures always possess larger area close to zero than that of pristine SiCw, indicating that the introduction of MnONP could improve the impedance matching of the composites. It is worth noting that the S-120 sample owns the correspondingly minimum area close to zero compared with those of S-80 and S-100 samples, and this consequence could be responsible for its relatively small EAB value in comparison with S-80, which could be ascribed to the excessive MnO content. In addition, the S-80 and S-100 samples exhibit almost same area close to zero, and the results are in good agreement with those of EM wave absorption performances with the similar RL value. Moreover, the other crucial parameter for the microwave absorber is the EM wave attenuation, and the attenuation constant (a) could be calculated by the following equation [11,47,48]:
a¼
pffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi12 2pf ðl00 e00 l0 e0 Þ þ ðl00 e00 l0 e0 Þ2 þ ðl0 e00 þ l00 e0 Þ2 c
ð6Þ
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Fig. 6. The three–dimensional RL maps of pristine SiCw and the MnONP/SiCw heterostructures. (a) Pristine SiCw, (b) S–80 sample, (c) S–100 sample, and (d) S–120 sample.
Fig. 7. The calculated delta value maps of pristine SiCw and the MnONP/SiCw heterostructures. (a) Pristine SiCw, (b) S–80 sample, (c) S–100 sample, and (d) S–120 sample.
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Fig. 8. Attenuation heterostructures.
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constants
of
pristine
SiCw
and
the
MnONP/SiCw
Generally speaking, a large a value means a good attenuation capacity, revealing the great dissipation properties of materials [11,47,48]. As recorded in Fig. 8, the attenuation value of the S120 composite is highest among the tested samples, especially at the relatively low frequency (<9.0 GHz), which could be explained that why the S-120 sample shows the least RLmin value among the MnONP/SiCw composites. The S-80 sample exhibits the largest average a value at the relatively high frequency (>9.0 GHz), resulting in the widest EAB among the MnONP/SiCw composites. Clearly, the MnONP/SiCw heterostructures all present larger a value than that of pristine SiCw, leading to better EM wave absorption performances compared with that of pristine SiCw. All these above results manifest that the incorporation of MnONP endows the MnONP/SiCw heterostructures as novel EM wave absorbers with outstanding EM wave absorbing property through the matched impedance and the enhanced attenuation capacity. According to the above results and analysis, the following factors are responsible for the glorious EM wave absorption performances of the MnONP/SiCw heterostructures [5,49–55]. Firstly, the dispersed MnONP cases avoid agglomeration due to the presence of SiCw, as shown in Fig. 2. The heterogeneous interfaces between MnONP and SiCw cause interfacial polarization, making contribute to the improvement of EM wave absorption performance. Secondly, the introduction of MnONP improve the conductive interconnections in the absorber, which would promote the formation of more induced micro electric currents under alternating EM field and then result in the conductive loss, leading to enhanced EM wave absorption. In addition, the defects and stacking faults in SiCw might also act an important role in enhancing the EM wave absorption properties of the composites. The charged defects in SiCw along with unpaired electrons would response to the external electric field, and the stacking faults in SiCw would be acted as the polarized centers, synergistically causing the space charge polarization and relaxation. In a word, the outstanding EM wave absorption properties should be mainly attributed to the matched impedance and improved loss ability arose from the synergetic effect between MnONP and SiCw as well as the unique hierarchical structure. To further assess the EM wave absorption performance of the MnONP/SiCw heterostructures, the RLmin and EAB values of typical EM absorbers reported by recent literature are summarized in Fig. 9 and Table S1. Combined with the results shown in Fig. 9, Tables 1 and S1, our products, namely the MnONP/SiCw composites exhibit superior EM wave absorption properties both with high efficiency and broad frequency compared with most of these
Fig. 9. Comparison of the RLmin values between present work and recent reported EM wave absorbers.
absorbers. From the point of view of these excellent advantages, it is believed that the MnONP/SiCw heterostructures could be regarded as a potential candidate for the oncoming generation of EM wave absorbing materials, and this strategy of construction of multicomponent composites with introduction of MnO is a promising method to prepare high-performance EM wave absorbers in practical applications. 4. Conclusions In conclusion, novel MnONP/SiCw hybrid composites with highperformance EM wave absorption were successfully synthesized via hydrothermal and heat treatment afterward. The content of MnONP could be effectively controlled by the hydrothermal temperature. The MnONP/SiCw hybrid whiskers obtained at 120 °C exhibit the optimal EM absorption ability with RLmin value of 55.10 dB at 8.8 GHz with a thickness of 3.99 mm, which is fivetimes higher than that of pristine SiCw. As the content of MnONP increases with the hydrothermal temperature, the absorption bandwidth of the MnONP/SiCw heterostructures becomes wider, and the maximum EAB value reaches 5.4 GHz. The outstanding EM wave attenuation performances could be attributed to the synergetic effect between the MnONP and SiCw, resulting in the suitable impedance matching and enhanced loss ability. The broad wideband microwave absorption of the MnONP/SiCw heterostructures highlights their potential applications in functional EM absorption and shielding devices, and this study also opens a new door for enhancing EM wave absorption property of the material by incorporation of MnO. Acknowledgements The authors gratefully acknowledge the financial supports provided by the National Natural Science Foundation of China (Grant Nos. 51372047, 51202048, 11421091, 11402252, 91216301 and 51902067), the China National Funds for Distinguished Young Scientists (No. 51525201), the National Key Laboratory of Science and Technology for National Defence on Advanced Composites in Special Environments (KL.PYJH.2016.001), and China Postdoctoral Science Foundation (No. 2019M651282). Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2019.10.026.
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