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NiCo2O4 constructed by different dimensions of building blocks with superior electromagnetic wave absorption performance
Composites Part B xxx (xxxx) xxx
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NiCo2O4 constructed by different dimensions of building blocks with superior electromagnetic wave absorption performance Hongjing Wu *, 1, Ming Qin 1, Limin Zhang MOE Key Laboratory of Material Physics and Chemistry Under Extraordinary Conditions, Shaanxi Key Laboratory for Condensed Matter Structure and Properties, Department of Applied Physics, School of Science, Northwestern Polytechnical University, Xi’an, 710072, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Electromagnetic wave absorption NiCo2O4 Precipitation-hydrothermal Building cornerstones
A series of NiCo2O4 absorbers constructed by different building cornerstones were successfully fabricated through precipitation-hydrothermal method. By adjusting the precipitants from NaHCO3, urea, and NaOH to Na2CO3, the NiCo2O4 absorbers assembled through zero-dimensional (0D) nanoparticles, one-dimensional (1D) nanorods and two-dimensional (2D) micro/nanoplates could be obtained. We found that NiCo2O4 absorbers formed by two-dimensional building blocks displayed high dielectric loss capacity but rather poor magnetic loss, resulting in inferior electromagnetic (EM) wave absorption performance. On the contrary, the sphere-like and urchin-like NiCo2O4 EM wave absorbing materials assembled by zero-dimensional nanoparticles and onedimensional nanorods possess multiple magnetic loss mechanisms, which can achieve a balance with dielec tric loss, leading to remarkably promoted EM wave attenuation performance. The effective absorption bandwidth for urchin-like and sphere-like NiCo2O4 is up to 5.84 GHz and 6.08 GHz at thickness of 1.88 mm and 2.06 mm, respectively. Moreover, the minimum reflection loss (RLmin) of sphere-like NiCo2O4 also reaches to 42.8 dB as well. The thin thickness, strong absorption capacity and wide effective absorption bandwidth (fe), which is the widest among the previously reported NiCo2O4-based absorbers so far, is prone to be a competitive candidate as the materials for EM wave absorption devices.
1. Introduction Electromagnetic pollution accompanied by the extensively utilizing of electronic devices has aroused widespread attention due to their harm to operation of electronic equipment and human health [1,2]. Under this circumstance, electromagnetic wave absorbing materials, which can consume the EM energy and convert it into other forms of energy, have become the research hotspot [3–6]. Great efforts have been devoted on searching for thin-thickness, light-weight and high-performance ab sorbers. Up to now, dielectric loss absorbers, including carbonaceous absorbing materials [7–9] and conducting polymers [10,11], magnetic loss materials, such as ferrites [12,13], and dual loss materials by inte grating dielectric loss components and magnetic loss components [14–16] have been reported. Among the reported absorbers, Co-based spinel oxide absorbing materials, whose chemical formula is MCo2O4, are attractive candidates for EM wave consumption due to their corrosion resistance, synergistic effects between the metal species and strong dielectric loss capacity
[17–21]. For instance, Che’s group [17] reported the preparation of cabbage-like ZnCo2O4 absorbers with fe of 5.1 GHz accomplished at thickness of 2.5 mm. In addition, our group successfully synthesized chrysanthemum-like CuCo2O4–CuO [18] and ellipsoid-like MgCo2O4/ Co3O4 [19] by one-pot hydrothermal method. The materials displayed excellent EM wave absorption capacity and fe was up to 4.02 GHz and 5.16 GHz, respectively. Apart from the above mentioned Co-based spinel oxides, the NiCo2O4 is the most promising EM wave absorbing material owing to its high polarization and loss ability as well as large shape anisotropy. When applied as EM wave absorbing materials, it is a common strategy to coupling NiCo2O4 with other components to satisfy the requirement of ideal absorbers [22–25]. Our group [22] reported the NiCo2O4–CoNiO2 composite displayed strong absorption capacity with RLmin of 42.13 dB at thin thickness of 1.55 mm. Similarly, the coupling of Co3O4/NiO [23] and MnO2 [24] with NiCo2O4 is also proved to be an effective approach to fulfill wide absorption bandwidth and strong ab sorption capacity, respectively. As a promising research point,
* Corresponding author. E-mail address: [email protected] (H. Wu). 1 Contributed to this work equally. https://doi.org/10.1016/j.compositesb.2019.107620 Received 30 October 2019; Received in revised form 21 November 2019; Accepted 23 November 2019 Available online 23 November 2019 1359-8368/© 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Hongjing Wu, Composites Part B, https://doi.org/10.1016/j.compositesb.2019.107620
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investigation of pristine NiCo2O4 could provide us simplified model for the study of EM wave absorption mechanisms. However, opposite to the composites containing NiCo2O4, researches on the absorbers with monocomponent of NiCo2O4 remain few [26,27]. Though some pro cesses have been achieved in manufacturing single-component NiCo2O4 absorbers, problems such as the narrow effective absorption bandwidth or the imbalance of dielectric loss with their magnetic loss remain to be tackled. Numerous researches have verified that the solution to alleviate the poor EM wave attenuation is to design absorbers with tunable morphology. By providing different geometrical morphologies, the properties that related to the EM wave dissipation can be elevated, thus promote their EM wave absorption performance. For example, onedimensional absorbers possess shape anisotropy, which could endow them the ability to acquire rapid charge transportation rate [28,29]. In addition, the two-dimensional materials have high specific surface area and thin thickness in certain dimension, and these features allow the absorbers generate abundant interfaces and shortened conductive paths, leading to enhanced interfacial polarization and conductive loss [30, 31]. Moreover, three-dimensional absorbing materials assembled by these primary structural units also exhibit satisfactory performance. Xu et al. [32] discovered that EM wave consumption performance of cactus-like Co/N-decorated carbon absorbers constructed by both of one-dimensional and two-dimensional hierarchical building blocks was promoted due to the enhanced multiple reflections and optimized impedance matching. Che and his coworkers [17] also reported the flake units on the ZnCo2O4 absorbers determined its absorption behavior. Therefore, it is prospective to synthesize high-performance pristine NiCo2O4 absorbers by regulating its morphologies through tuning their primary structural units. Inspired by this thought, we put forward the precipitationhydrothermal method to fabricate NiCo2O4 materials. Influenced by the different properties of the precipitants as well as the hydrothermal reaction, the final products with different morphologies formed through self-assembly of diverse building blocks can be obtained. Compared with the NiCo2O4 constructed by two-dimensional flakes, urchin-like and sphere-like NiCo2O4 formed through 1D nanorods and 0D nanoparticles displayed remarkably enhanced EM wave absorption capacity. The effective absorption bandwidth (fe) of urchin-like NiCo2O4 is up to 5.84 GHz at thickness of 1.88 mm. For sphere-like NiCo2O4, the fe is even promoted to 6.08 GHz from 11.92 GHz to 18 GHz at 2.06 mm, covering the whole Ku band. The corresponding minimum reflection loss (RL) reaches to 42.8 dB as well. Compared with NiCo2O4 absorbers con structed by two-dimensional flakes, the elevated EM wave attenuation behavior of urchin-like and sphere-like NiCo2O4 is ascribed to the presence of multiple magnetic loss mechanisms, which can achieve a balance with their dielectric loss ability. Our work proposed a precipitation-hydrothermal method for the synthesis of highperformance and single-component NiCo2O4 absorbers. The wide ab sorption bandwidth (exceeding 6 GHz and covering the whole Ku band) and strong absorption capacity (RLmin ¼ 42.8 dB) make the NiCo2O4 material a valid candidate as EM wave absorbers.
min. Then, resultant mixtures were poured into 100 mL of PTFE-lined stainless steel autoclave and heated to 160 � C, kept for 24 h. The products were collected by filtration and washed by deionized water and alcohol to eliminate the residual impurity. Finally, heating treatment, which was accomplished from 20 � C to 500 � C and heating rate of 2 � C/ min in air atmosphere for 3 h, was conducted to attain the final products. The samples obtained with NaOH, urea, NaHCO3 and Na2CO3 were labeled as S1, S2, S3 and S4, respectively. The arrangement of the samples is based on the molecular weight of the precipitants. 3. Results and discussion The formation process of NiCo2O4 samples is illustrated in Fig. 1. Firstly, when different precipitants were added into the solutions, nucleation formed due to the precipitation between the metal ions and anions (except for the urea precipitant). Owing to the varied properties of anions, nucleation with different structures was attained, which also reflected on the varied color of the precipitations. The S4 was selected as the example to explain the formation process. During the hydrothermal reaction, the original sediments will gradually dissolve under the high temperature and pressure, and recrystallize due to the existence of CO23 . As the reaction proceeding, the new formed nanoparticles will aggregate into triangle sheet structure to reduce the surface energy. In the following process, the triangle sheet structure underwent selfassembly with the aid of CO23 , leading to the formation of compactly stacked hierarchical structures. After the calcination in the air, pre cursors were successfully converted into the desired NiCo2O4 products. The morphologies of NiCo2O4 samples are shown in Fig. 2. It can be seen the precipitant is dominant factor for the morphologies of final products. For S1, the field-emission SEM images prove the products are composed of numerous heterogeneous nanosheets, the diameters of which range from ~100 nm to ~500 nm. However, the nanosheets severely aggregated with each other, leading to the irregular shapes of products. As for S2, well-defined sea urchin structure with diameter ranging from 2 to 5 μm is observed. The uniform microsphere is con structed by radially-grown nanorods, diameter and length of which are 200 nm and 1 μm, respectively. With respect to S3, sphere-like structure was attained after the hydrothermal reaction. The diameters of the spheres range from 2 to 5 μm. The field-emission SEM clearly discloses that the sphere is formed by the closely packed nanoparticles. When the precipitant changed into Na2CO3, NiCo2O4 with peculiar shape can be obtained. The samples are formed by the several packed triangle microplates, which possess plenty of cracks and pores on the surface. In addition, the EDS mapping images of all the samples clearly demonstrate the uniform distribution of Ni, Co and O. Therefore, by adjusting the precipitants, the NiCo2O4 assembled through 0D nanoparticles, 1D nanorods and 2D nano/microplates can be fabricated. In consequence, the chemical and physical properties of NiCo2O4 would be influenced, and reflected on their EM wave attenuation properties. The as-prepared NiCo2O4 samples display the same XRD patterns (see Fig. 3a). All the diffraction peaks in the patterns are well matched with the cubic spinel structure NiCo2O4. In addition to the diffraction peaks of NiCo2O4, no other peak for the impurities, such as Ni(OH)2, Co (OH)2, NiCO3, CoCO3 can be detected, implying the successful trans formation from precipitates to NiCo2O4 material. This result confirms the high purity of the NiCo2O4 products. Fig. 3b displays the Raman spectra of the NiCo2O4 samples. Two distinctive peaks are observed in the curves. The peak located around 526 cm 1 is indexed to the vibra tion of the tetrahedral oxygen ions in tetrahedral site while the other one at ~660 cm 1 is assigned to the octahedral-site Co–O bond vibrations [33]. Obviously, the locations of the latter one gradually shift to lower wavenumber with following order: S3>S1>S2>S4, suggesting that lat tice defects are existed in the spinel structure [34,35]. Based on previous studies, the lattice defects will induce the generation of dipole polari zation, beneficial to dielectric loss and promoted EM wave absorption capacity [22].
2. Experimental section 2.1. Synthesis of NiCo2O4 with different morphologies The NiCo2O4 absorbers were synthesized through precipitatehydrothermal reaction and subsequent calcination process. The syn thetic procedure was described as follow. Firstly, 2 mmol of NiSO4⋅6H2O and 4 mmol of CoCl2⋅6H2O were together dissolved in 60 mL of deion ized water. Thereafter, 0.2 g of PVP (K30) was added into the mixture under agitated stirring to form a homogeneous solution. After being stirred for 10 min, 8 mmol of four kinds of precipitants, i.e., NaOH, urea, NaHCO3 and Na2CO3 were severally added into the above solutions. The precursor solutions were attained with a further magnetic stirring for 30 2
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Fig. 1. The formation schematic diagram of NiCo2O4 samples assembled by building blocks with different dimensions.
Fig. 2. The SEM images of a-c) S1, d-f) S2, g-i) S3 and j-l) S4 and the corresponding EDS mapping of d), h), l), p) for S1–S4, where the red, cyan and magenta are assigned to O, Co and Ni for all the samples, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
The chemical states of each element in the samples are revealed by XPS spectra. For the C 1s XPS spectrum, three kinds of carbon species can be detected. The peaks located at 284.8 eV, 286.1 eV and 288.6 eV can be assigned to the C–C, C–O–C and O¼C–O functional groups. The Ni 2p XPS spectrum is well fitted with two spin-orbit doublets character istic of bivalent Ni ions and trivalent Ni ions. and two shakeup satellites (labeled as “sat”). Likewise, multiple valence states of Co also are in the presence of Co 2p XPS spectra. It can be learned that multiple states Ni and Co ions are coexisted in the samples, which is in accord with pre vious researches [5]. With regard to O 1s XPS spectrum, it consists of four kinds of components, including the metal-O band (529.5 eV for Ni–O and 530.4 eV for Co–O), oxygen vacancies (531.8 eV and labeled in blue area in Fig. 4d) and adsorbed water on the surface (533.1 eV). The oxygen vacancies in the as-prepared NiCo2O4 absorbers can lead to the
asymmetric distribution of charges. In consequence, the corresponding dipole polarization process can be induced. Thus, the high-concentration oxygen vacancies in the samples will induce strong dipole polarization effect, which is beneficial to the dissipation of EM waves [17,20,36,37]. The result is in good accordance with the analysis of Raman spectra. Among the NiCo2O4, the S4 possesses the lowest ox ygen vacancies, which may weaken its dipole polarization effect. The static magnetic properties of the NiCo2O4 were test at 25 � C and shown in Fig. 5. The sample which displayed typical ferrimagnetic behavior and hysteresis loops is observed. The magnetization of samples are not saturated at the external field, which are ascribed to antiferro magnetic (AFM) phases and magnetic surface disorder effects arising from reduced dimensions. The maximum magnetization of S1 and S4 are 1.86 emu/g and 3.60 emu/g, lower than S2 and S3, which are 4.14 emu/ 3
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Fig. 3. The XRD patterns and Raman spectra of the NiCo2O4 samples.
Fig. 4. The high-resolution XPS spectra of a) Ni, b) Co, c) C and d) O of NiCo2O4 samples.
g and 3.89 emu/g, respectively. The exchange bias phenomenon is in the presence of the as-obtained samples, which may originate from inter facial exchange coupling between FM and AFM layers [38]. The coer civity values of the samples are 69.15 Oe, 70.85 Oe, 69.75 Oe and 68.1 Oe from S1 to S4. The slightly higher coercivity values of S2 and S3 may be resulted from the magnetocrystalline anisotropy provided by their 0D and 1D structural units. It has been reported that the higher coercivity can promote the EM wave absorption performance of the samples [21, 39]. Thus, S2 and S3 are expected to reveal better magnetic loss capacity
than that of S1 and S4. It is acknowledged that the complex permittivity and permeability are key parameters that closely related the EM wave absorption per formance. Hence, the parameters of these NiCo2O4 absorbers with different morphologies were tested and presented in Fig. S1. For the real part of complex permittivity (ε0 ) of S1 and S4, typical frequency dispersion behavior is in the presence in the curves. The value of ε0 decreases rapidly from 20.3 to 8.3 and 18.4 to 4.5 along the increasing of frequency for S1 and S4, respectively. With respect to S2 and S3, the 4
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Fig. 5. The a) magnetization curve and b) corresponding maximum magnetization as well as the coercivity values of the samples.
values of ε0 barely change in frequency range of 2–11 GHz, and then reach to peaks at 13.4 GHz and 14.2 GHz, respectively. As the frequency further increased, the values dramatically decrease. For the imaginary part (ε00 ) of all samples, distinguishable resonance peaks are observed in the plots. This suggests that the presence of multiple Debye dielectric relaxation process. It should be noted that the S1 and S4 possess higher ε00 compared with S2 and S3. This may be ascribed to the building blocks of S1 and S4 are two-dimensional sheet structure, which can increase the conductive paths thus enhance the conductivity. Since the ε" ¼ 1/2ε0πρf, the improved conductivity results in the elevated ε00 value. As a conse quence, the ε" values of S1 and S4 are promoted compared with that of S2 and S3. These results confirm the species of the precipitant play a vital role on the dielectric loss capacity of the absorbers by changing
their microstructures through nanoscale primary building blocks. The dielectric loss tangent (ε00 /ε0 ) values of the samples constructed by 2D flakes are also higher than those constructed by 0D nanoparticles and 1D nanorods. The high tanδ values of the absorbers imply that they possess strong dielectric loss capacity. Typically, the dielectric loss is consisted of interfacial polarization, dipole polarization as well as the conduction loss [40–42]. There exist abundant oxygen vacancies and lattice defects in the NiCo2O4 samples, which serve as polarization centers under the alternating EM field and induce orientation polarization relaxation, thus boost the dipole polar ization effect. In addition, more interfaces are generated on the ab sorbers arising from their hierarchical nanostructure and the coarse surface, resulting in strong interfacial polarization effect. The
Fig. 6. The Cole-Cole semicircles of S1–S4. 5
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abovementioned polarizations can be further confirmed by the Cole-Cole semicircles (see Fig. 6). Apparently, the disordered semicircles can be observed in the curves, indicating the existence of other dielectric loss, such as conductive loss and electron polarization [5]. The real part and imaginary part of complex permeability, which stand for the storage capacity and loss capacity of magnetic field energy, are depicted in Fig. S2. It is obvious that the μ00 values of S2 and S3 are higher than that of S1 and S4, suggesting the strengthened magnetic loss capacity. It should be noted that the negative μ00 values can be observed for all the absorbers when the frequency range is higher than 8 GHz. This phenomenon may be caused by the eddy current induced under alter nating electromagnetic field. Due to the existence of eddy current, the
extra magnetic field can be generated. As a consequence, the inherent magnetic field is cancelled, leading to the negative permeability [5,43]. Simultaneously, multiple resonance peaks are in the presence of the curves, demonstrating the existence of ferromagnetic resonance behavior. The magnetic loss tangent (μ00 /μ0 ) of the absorbers is presented in Fig. S2c. As can be observed, resonance peaks are located at ~4 GHz for S1, S2, S3 and 15.2 GHz for all the absorbers. Generally, the reso nance peak below 10 GHz is ascribed to natural resonance whereas the peak above 10 GHz results from exchange resonance [33]. Thus, it can be deduced the existence of natural resonance in S1, S2, S3 and ex change resonance for all samples. Apart from above mentioned mech anism, magnetic loss is also comprised of eddy current loss, which can be
Fig. 7. RL values vs. frequency at different thicknesses for a-d) S1–S4, e-h) 3D plots and i-l) 2D counter maps of the corresponding absorbers. 6
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expressed as follow formula: 00
’
2
μ ðμ Þ f
1
precipitant, though the RLmin value is only 17.5 dB, the fe still can reach to 4.24 GHz at a relative thin thickness of 1.5 mm. As the pre cipitant replaced by urea and NaHCO3, the EM wave dissipation behavior of the NiCo2O4 absorbers is dramatically improved. For the urchin-like NiCo2O4, the fe is up to 5.84 GHz (from 12.16 GHz to 18 GHz) at thickness of only 1.88 mm. With regard to S3, the fe ranges from 11.92 GHz to 18 GHz, covering the whole Ku band at 2.06 mm. Besides, the RLmin value at corresponding thickness is as high as 42.8 dB. Both of the strong absorption capacity and wide fe of S2 and S3 make them competitive candidates for their practical application as effective EM wave absorbers. Nevertheless, the inferior EM wave attenuation behavior is observed on S4 sample. The fe value decreases to 2.72 GHz, which is much narrower than the abovementioned absorbers. The detail data are depicted in Fig. 7. To have a deeper insight into the cause of the superior EM wave dissipation behavior of the NiCo2O4 samples, the impedance matching characteristic (Z ¼ |Zin/Z0|) of the absorbers are calculated and pre sented in Fig. 8. In general, when the value of |Zin/Z0| is closer to 1, the input impedance of the absorbers can be well matched with impedance of the free space. Under this circumstance, the incident EM wave can permeate in absorbers as much as possible with less reflection. It is obvious that both of the Z values of urchin-like and sphere-like NiCo2O4 samples equal to 1 under suitable thickness. Meanwhile, Z values of S1 and S4 are 0.8 and 0.7, respectively, which is a little away from the ideal value. This result indicates the deteriorative impedance matching characteristic for S1 and S4. Though they possess high potential for EM wave dissipation in view of their higher dielectric loss, the practical EM wave attenuation performance is inhibited by their poor impedance
(1)
2
¼ 2πμ0 d σ
where f, μ0, and σ are the frequency, the vacuum permeability, and the electric conductivity, respectively. One can see from the curves that the values fluctuate on the investigated frequency for S1 and S4. Never theless, the values for S2 and S3 are maintained unchanged in frequency range of 6–14 GHz, demonstrating the magnetic loss caused by eddy current loss. Based on above analysis, the magnetic loss capacity of S2 and S3 is convinced to be better than that of S1 and S4 due to the multiple magnetic loss mechanisms, including natural resonance, ex change resonance and eddy current loss. To understand the EM wave dissipation behavior of NiCo2O4 ab sorbers, the reflection loss (RL) values were calculated based on trans mission line theory. The specific equations are described below: � � �Zin Z0 � � RL ¼ 20 log�� (2) Zin Z0 � rffiffiffiffi Zin ¼
�
μr 2πfd pffiffiffiffiffiffiffiffi μr εr tan h j c εr
� (3)
where d is the thickness of absorbers, f is the frequency of EM wave and c is the velocity of light in vacuum. The RL values of these samples were calculated and plotted in Fig. 7 in the form of 2D and 3D figures. Generally, an effective absorption performance requires the RL values lower than 10 dB. The frequency range that the RL values lower than 10 dB is labeled as fe. For the NiCo2O4 sample prepared by NaOH
Fig. 8. RL values vs. frequency at different thicknesses for a) S2 and d) S3, the simulations of the absorber thickness vs. peak frequency for b) S2 and e) S3 under l/4 conditions, the impedance matching characteristic of c) S2 and f) S3, the impedance matching characteristic of g) the as-prepared NiCo2O4 absorbers under the optimized condition, and h) the RL value at thickness of 2.06 mm with corresponding impedance matching characteristic and the attenuation constants by taking S3 as an example. 7
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matching. The quarter wavelength matching model for S2 and S3 is also investigated to study the relationship between the thickness of absorbers and their RL values. The RLmin value can be achieved at specific fre quency when thickness of absorber obeys following equation: tm ¼
nλ nc ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffi ðn ¼ 1; 3; 5…::Þ 4 4fm jεr jjμr j
oxygen vacancies and functional groups in the as-prepared NiCo2O4 absorbers can lead to the asymmetric distribution of charges. In conse quence, the corresponding dipole polarization process can be induced. This process is prone to improve the EM wave consumption of absorber. Secondly, the excellent EM wave attenuation is ascribed to the unique structure of sphere-like NiCo2O4 absorber assembled by numerous 0 D nanoparticles. The EM wave entered into the EM wave absorbing ma terial will undergo multiple reflections and scattering on the surface of sphere-like NiCo2O4, thus extend the propagation path and helpful for the energy dissipation. Thirdly, plenty of NiCo2O4–NiCo2O4 interfaces can be formed since the sphere-like NiCo2O4 absorber is constructed by plenty of interconnected nanoparticles, undoubtedly will enhance the interfacial polarization relaxation. Finally, due to the electrical con ductivity and ferromagnetism of the absorber, the magnetic loss including natural resonance and exchange resonance, and the dielectric loss including conductive loss also contribute to consumption of EM wave energy. To figure out the EM wave absorbing capacity of sphere-like NiCo2O4, we collect the related excellent NiCo2O4-based absorbers as well as their absorption performance and list in Fig. 10 and Table 1. According to the results, the NiCo2O4 absorber possesses the widest effective absorption bandwidth (fe) among the NiCo2O4-based absorber so far, which exceeds 6 GHz and covers the whole Ku band. More importantly, the superior performance can be achieved under thin
(4)
where tm and fm are the thickness of the absorber and the frequency corresponding to the RLmin value, the other parameters are the same as described earlier. The rhombic symbols stand for the matching thickness vs. the peak frequency collected from Fig. 8. The locations of these data are exactly around the 1/4 λ, implying the relationship between matching thickness and peak frequency can satisfy the quarter wave length matching model. Therefore, two reflected EM wave will dissipate of each other in the interface of air-absorber due to the phase difference of 180� , resulting in maximized the EM wave attenuation capacity. Simultaneously, the attenuation constant is calculated to inspect its impact on the EM wave absorption behavior. Specific data are obtained based on the following formula: ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiq pffiffi 2π f � ðμ00ε00 μ’ε’Þ þ ðμ00ε00 μ’ε’Þ2 þ ðμ’ ε00 þ μ00ε’Þ2 (5) α¼ c We choose S3 as an illustration. Specifically, there are two Z values that reach to 1 in the curves, which is marked as A and B. As it can be seen, the better attenuation behavior is accomplished at B instead of A. This can be ascribed to the fact that the value of attenuation constant is monotonically increased as a function of frequency. The enlarged attenuation constant of point B is favorable for strengthened absorption capacity. Based on this, it can be concluded that higher attenuation constant is conductive to elevated absorption performance under the suitable impedance matching characteristic. According to above analysis, the superior EM wave dissipation of S3 sample, which possesses the widest fe of 6.08 GHz among the NiCo2O4 absorbers, originates from multiple loss mechanisms. Before the ab sorption process occurred, the well matched impedance endows the NiCo2O4 absorbers to receive more incident EM wave rather than reflecting it on the surface. In addition, the quarter-wavelength inter ference can also ensure the cancellation between the EM waves reflected from the different interfaces (air-absorber and absorber-metal), benefi cial to elevated attenuation performance. When the incident EM wave entered into the absorbers, the absorption is ascribed to following mechanisms, which is depicted in Fig. 9. Firstly, there are many oxygen vacancies and functional groups existed on the NiCo2O4 absorbers. The
Fig. 10. The comparison of EM wave attenuation behavior between the sam ples and previously reported spinel absorbers.
Fig. 9. EM wave absorption mechanisms of the sphere-like NiCo2O4 absorbers. 8
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Table 1 Some typical NiCo2O4 based EM wave absorbers. Absorber NiCo2O4 CuCo2O4–CuO MgCo2O4/Co3O4 NiCo2O4/CoNiO2 NiCo2O4/Co3O4/NiO MnO2@NiCo2O4 C@NiCo2O4@Fe3O4 NiCo2O4 NiCo2O4 (S1) NiCo2O4 (S2) NiCo2O4 (S3)
RLmin (dB) 35.7 23 48.54 42.13 28.6 58.4 43.0 25.5 17.6 26.1 42.8
Matching thickness d (mm)
Percentage (wt. %)
Effective bandwidth fe (GHz) RL �
1.5 2.8 2.3 1.55 1.64 4 3.4 4.0 1.5 1.88 2.06
50 70 50 50 30 60 60 50 50 50 50
4.18 4.02 5.16 3.92 4.72 2.70 2.10 3.20 4.24 5.84 6.08
10 dB
Refs. [26] [18] [19] [22] [23] [24] [25] [27] This work
matched thickness of 2.06 mm. In view of the facile preparation process, high EM wave absorption capacity, the pristine sphere-like NiCo2O4 is prone to be a valid alternative for high-performance pristine ferrite EM wave absorbers.
Appendix A. Supplementary data
4. Conclusion
References
In this work, NiCo2O4 absorbers constructed by different dimensions of primary building blocks were successfully fabricated by precipitationhydrothermal method and subsequent calcination. The EM wave attenuation performance was found to be greatly affected by the struc tural units of the NiCo2O4 absorbers. Though the NiCo2O4 absorbers assembled by 2D nanosheet structure (i.e., S1 and S4) possess high dielectric loss capacity, the poor magnetic loss is unable to achieve a balance with the exaggerated dielectric loss capacity, resulting in deteriorated EM wave attenuation performance. On the contrary, the NiCo2O4 absorbing materials constructed by 0D nanoparticles and 1D nanorods displayed superior absorption behavior, mainly originate from their multiple magnetic loss, including natural resonance, exchange resonance and eddy current loss, which can reach balance with their dielectric loss capacity. Under the optimized parameters, the fe of urchin-like NiCo2O4 is up to 5.84 GHz at thickness of 1.88 mm. In particular, the fe of sphere-like NiCo2O4 is even as wide as 6.08 GHz ranging from 11.92 GHz to 18 GHz, covering the whole Ku band. By this approach, we have obtained the NiCo2O4 absorber with the widest fe among the NiCo2O4-based absorber so far. The facile synthetic route and the idea of assembling absorbers with varied primary building blocks may offer a new approach for the preparation of high-performance pristine ferrite EM wave absorbers.
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Declaration of competing interest We declare that we do not have any commercial or associative in terest that represents a conflict of interest in connection with the work submitted. CRediT authorship contribution statement Hongjing Wu: Conceptualization, Writing - original draft, Writing review & editing, Supervision, Funding acquisition. Ming Qin: Meth odology, Validation, Data curation, Writing - original draft. Limin Zhang: Visualization, Investigation, Resources, Funding acquisition. Acknowledgments Financial support was provided by the National Science Foundation of China (Grants nos. 21806129, 51872238 and 51704242), the Fundamental Research Funds for the Central Universities (Nos. 3102018zy045, 3102019AX11 and 310201911cx019) and the Natural Science Basic Research Plan in Shaanxi Province of China (Nos. 2017JQ5116 and 2018JM5094). 9
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Contents lists available at ScienceDirect
Composites Part B journal homepage: www.elsevier.com/locate/compositesb
NiCo2O4 constructed by different dimensions of building blocks with superior electromagnetic wave absorption performance Hongjing Wu *, 1, Ming Qin 1, Limin Zhang MOE Key Laboratory of Material Physics and Chemistry Under Extraordinary Conditions, Shaanxi Key Laboratory for Condensed Matter Structure and Properties, Department of Applied Physics, School of Science, Northwestern Polytechnical University, Xi’an, 710072, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Electromagnetic wave absorption NiCo2O4 Precipitation-hydrothermal Building cornerstones
A series of NiCo2O4 absorbers constructed by different building cornerstones were successfully fabricated through precipitation-hydrothermal method. By adjusting the precipitants from NaHCO3, urea, and NaOH to Na2CO3, the NiCo2O4 absorbers assembled through zero-dimensional (0D) nanoparticles, one-dimensional (1D) nanorods and two-dimensional (2D) micro/nanoplates could be obtained. We found that NiCo2O4 absorbers formed by two-dimensional building blocks displayed high dielectric loss capacity but rather poor magnetic loss, resulting in inferior electromagnetic (EM) wave absorption performance. On the contrary, the sphere-like and urchin-like NiCo2O4 EM wave absorbing materials assembled by zero-dimensional nanoparticles and onedimensional nanorods possess multiple magnetic loss mechanisms, which can achieve a balance with dielec tric loss, leading to remarkably promoted EM wave attenuation performance. The effective absorption bandwidth for urchin-like and sphere-like NiCo2O4 is up to 5.84 GHz and 6.08 GHz at thickness of 1.88 mm and 2.06 mm, respectively. Moreover, the minimum reflection loss (RLmin) of sphere-like NiCo2O4 also reaches to 42.8 dB as well. The thin thickness, strong absorption capacity and wide effective absorption bandwidth (fe), which is the widest among the previously reported NiCo2O4-based absorbers so far, is prone to be a competitive candidate as the materials for EM wave absorption devices.
1. Introduction Electromagnetic pollution accompanied by the extensively utilizing of electronic devices has aroused widespread attention due to their harm to operation of electronic equipment and human health [1,2]. Under this circumstance, electromagnetic wave absorbing materials, which can consume the EM energy and convert it into other forms of energy, have become the research hotspot [3–6]. Great efforts have been devoted on searching for thin-thickness, light-weight and high-performance ab sorbers. Up to now, dielectric loss absorbers, including carbonaceous absorbing materials [7–9] and conducting polymers [10,11], magnetic loss materials, such as ferrites [12,13], and dual loss materials by inte grating dielectric loss components and magnetic loss components [14–16] have been reported. Among the reported absorbers, Co-based spinel oxide absorbing materials, whose chemical formula is MCo2O4, are attractive candidates for EM wave consumption due to their corrosion resistance, synergistic effects between the metal species and strong dielectric loss capacity
[17–21]. For instance, Che’s group [17] reported the preparation of cabbage-like ZnCo2O4 absorbers with fe of 5.1 GHz accomplished at thickness of 2.5 mm. In addition, our group successfully synthesized chrysanthemum-like CuCo2O4–CuO [18] and ellipsoid-like MgCo2O4/ Co3O4 [19] by one-pot hydrothermal method. The materials displayed excellent EM wave absorption capacity and fe was up to 4.02 GHz and 5.16 GHz, respectively. Apart from the above mentioned Co-based spinel oxides, the NiCo2O4 is the most promising EM wave absorbing material owing to its high polarization and loss ability as well as large shape anisotropy. When applied as EM wave absorbing materials, it is a common strategy to coupling NiCo2O4 with other components to satisfy the requirement of ideal absorbers [22–25]. Our group [22] reported the NiCo2O4–CoNiO2 composite displayed strong absorption capacity with RLmin of 42.13 dB at thin thickness of 1.55 mm. Similarly, the coupling of Co3O4/NiO [23] and MnO2 [24] with NiCo2O4 is also proved to be an effective approach to fulfill wide absorption bandwidth and strong ab sorption capacity, respectively. As a promising research point,
* Corresponding author. E-mail address: [email protected] (H. Wu). 1 Contributed to this work equally. https://doi.org/10.1016/j.compositesb.2019.107620 Received 30 October 2019; Received in revised form 21 November 2019; Accepted 23 November 2019 Available online 23 November 2019 1359-8368/© 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Hongjing Wu, Composites Part B, https://doi.org/10.1016/j.compositesb.2019.107620
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investigation of pristine NiCo2O4 could provide us simplified model for the study of EM wave absorption mechanisms. However, opposite to the composites containing NiCo2O4, researches on the absorbers with monocomponent of NiCo2O4 remain few [26,27]. Though some pro cesses have been achieved in manufacturing single-component NiCo2O4 absorbers, problems such as the narrow effective absorption bandwidth or the imbalance of dielectric loss with their magnetic loss remain to be tackled. Numerous researches have verified that the solution to alleviate the poor EM wave attenuation is to design absorbers with tunable morphology. By providing different geometrical morphologies, the properties that related to the EM wave dissipation can be elevated, thus promote their EM wave absorption performance. For example, onedimensional absorbers possess shape anisotropy, which could endow them the ability to acquire rapid charge transportation rate [28,29]. In addition, the two-dimensional materials have high specific surface area and thin thickness in certain dimension, and these features allow the absorbers generate abundant interfaces and shortened conductive paths, leading to enhanced interfacial polarization and conductive loss [30, 31]. Moreover, three-dimensional absorbing materials assembled by these primary structural units also exhibit satisfactory performance. Xu et al. [32] discovered that EM wave consumption performance of cactus-like Co/N-decorated carbon absorbers constructed by both of one-dimensional and two-dimensional hierarchical building blocks was promoted due to the enhanced multiple reflections and optimized impedance matching. Che and his coworkers [17] also reported the flake units on the ZnCo2O4 absorbers determined its absorption behavior. Therefore, it is prospective to synthesize high-performance pristine NiCo2O4 absorbers by regulating its morphologies through tuning their primary structural units. Inspired by this thought, we put forward the precipitationhydrothermal method to fabricate NiCo2O4 materials. Influenced by the different properties of the precipitants as well as the hydrothermal reaction, the final products with different morphologies formed through self-assembly of diverse building blocks can be obtained. Compared with the NiCo2O4 constructed by two-dimensional flakes, urchin-like and sphere-like NiCo2O4 formed through 1D nanorods and 0D nanoparticles displayed remarkably enhanced EM wave absorption capacity. The effective absorption bandwidth (fe) of urchin-like NiCo2O4 is up to 5.84 GHz at thickness of 1.88 mm. For sphere-like NiCo2O4, the fe is even promoted to 6.08 GHz from 11.92 GHz to 18 GHz at 2.06 mm, covering the whole Ku band. The corresponding minimum reflection loss (RL) reaches to 42.8 dB as well. Compared with NiCo2O4 absorbers con structed by two-dimensional flakes, the elevated EM wave attenuation behavior of urchin-like and sphere-like NiCo2O4 is ascribed to the presence of multiple magnetic loss mechanisms, which can achieve a balance with their dielectric loss ability. Our work proposed a precipitation-hydrothermal method for the synthesis of highperformance and single-component NiCo2O4 absorbers. The wide ab sorption bandwidth (exceeding 6 GHz and covering the whole Ku band) and strong absorption capacity (RLmin ¼ 42.8 dB) make the NiCo2O4 material a valid candidate as EM wave absorbers.
min. Then, resultant mixtures were poured into 100 mL of PTFE-lined stainless steel autoclave and heated to 160 � C, kept for 24 h. The products were collected by filtration and washed by deionized water and alcohol to eliminate the residual impurity. Finally, heating treatment, which was accomplished from 20 � C to 500 � C and heating rate of 2 � C/ min in air atmosphere for 3 h, was conducted to attain the final products. The samples obtained with NaOH, urea, NaHCO3 and Na2CO3 were labeled as S1, S2, S3 and S4, respectively. The arrangement of the samples is based on the molecular weight of the precipitants. 3. Results and discussion The formation process of NiCo2O4 samples is illustrated in Fig. 1. Firstly, when different precipitants were added into the solutions, nucleation formed due to the precipitation between the metal ions and anions (except for the urea precipitant). Owing to the varied properties of anions, nucleation with different structures was attained, which also reflected on the varied color of the precipitations. The S4 was selected as the example to explain the formation process. During the hydrothermal reaction, the original sediments will gradually dissolve under the high temperature and pressure, and recrystallize due to the existence of CO23 . As the reaction proceeding, the new formed nanoparticles will aggregate into triangle sheet structure to reduce the surface energy. In the following process, the triangle sheet structure underwent selfassembly with the aid of CO23 , leading to the formation of compactly stacked hierarchical structures. After the calcination in the air, pre cursors were successfully converted into the desired NiCo2O4 products. The morphologies of NiCo2O4 samples are shown in Fig. 2. It can be seen the precipitant is dominant factor for the morphologies of final products. For S1, the field-emission SEM images prove the products are composed of numerous heterogeneous nanosheets, the diameters of which range from ~100 nm to ~500 nm. However, the nanosheets severely aggregated with each other, leading to the irregular shapes of products. As for S2, well-defined sea urchin structure with diameter ranging from 2 to 5 μm is observed. The uniform microsphere is con structed by radially-grown nanorods, diameter and length of which are 200 nm and 1 μm, respectively. With respect to S3, sphere-like structure was attained after the hydrothermal reaction. The diameters of the spheres range from 2 to 5 μm. The field-emission SEM clearly discloses that the sphere is formed by the closely packed nanoparticles. When the precipitant changed into Na2CO3, NiCo2O4 with peculiar shape can be obtained. The samples are formed by the several packed triangle microplates, which possess plenty of cracks and pores on the surface. In addition, the EDS mapping images of all the samples clearly demonstrate the uniform distribution of Ni, Co and O. Therefore, by adjusting the precipitants, the NiCo2O4 assembled through 0D nanoparticles, 1D nanorods and 2D nano/microplates can be fabricated. In consequence, the chemical and physical properties of NiCo2O4 would be influenced, and reflected on their EM wave attenuation properties. The as-prepared NiCo2O4 samples display the same XRD patterns (see Fig. 3a). All the diffraction peaks in the patterns are well matched with the cubic spinel structure NiCo2O4. In addition to the diffraction peaks of NiCo2O4, no other peak for the impurities, such as Ni(OH)2, Co (OH)2, NiCO3, CoCO3 can be detected, implying the successful trans formation from precipitates to NiCo2O4 material. This result confirms the high purity of the NiCo2O4 products. Fig. 3b displays the Raman spectra of the NiCo2O4 samples. Two distinctive peaks are observed in the curves. The peak located around 526 cm 1 is indexed to the vibra tion of the tetrahedral oxygen ions in tetrahedral site while the other one at ~660 cm 1 is assigned to the octahedral-site Co–O bond vibrations [33]. Obviously, the locations of the latter one gradually shift to lower wavenumber with following order: S3>S1>S2>S4, suggesting that lat tice defects are existed in the spinel structure [34,35]. Based on previous studies, the lattice defects will induce the generation of dipole polari zation, beneficial to dielectric loss and promoted EM wave absorption capacity [22].
2. Experimental section 2.1. Synthesis of NiCo2O4 with different morphologies The NiCo2O4 absorbers were synthesized through precipitatehydrothermal reaction and subsequent calcination process. The syn thetic procedure was described as follow. Firstly, 2 mmol of NiSO4⋅6H2O and 4 mmol of CoCl2⋅6H2O were together dissolved in 60 mL of deion ized water. Thereafter, 0.2 g of PVP (K30) was added into the mixture under agitated stirring to form a homogeneous solution. After being stirred for 10 min, 8 mmol of four kinds of precipitants, i.e., NaOH, urea, NaHCO3 and Na2CO3 were severally added into the above solutions. The precursor solutions were attained with a further magnetic stirring for 30 2
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Fig. 1. The formation schematic diagram of NiCo2O4 samples assembled by building blocks with different dimensions.
Fig. 2. The SEM images of a-c) S1, d-f) S2, g-i) S3 and j-l) S4 and the corresponding EDS mapping of d), h), l), p) for S1–S4, where the red, cyan and magenta are assigned to O, Co and Ni for all the samples, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
The chemical states of each element in the samples are revealed by XPS spectra. For the C 1s XPS spectrum, three kinds of carbon species can be detected. The peaks located at 284.8 eV, 286.1 eV and 288.6 eV can be assigned to the C–C, C–O–C and O¼C–O functional groups. The Ni 2p XPS spectrum is well fitted with two spin-orbit doublets character istic of bivalent Ni ions and trivalent Ni ions. and two shakeup satellites (labeled as “sat”). Likewise, multiple valence states of Co also are in the presence of Co 2p XPS spectra. It can be learned that multiple states Ni and Co ions are coexisted in the samples, which is in accord with pre vious researches [5]. With regard to O 1s XPS spectrum, it consists of four kinds of components, including the metal-O band (529.5 eV for Ni–O and 530.4 eV for Co–O), oxygen vacancies (531.8 eV and labeled in blue area in Fig. 4d) and adsorbed water on the surface (533.1 eV). The oxygen vacancies in the as-prepared NiCo2O4 absorbers can lead to the
asymmetric distribution of charges. In consequence, the corresponding dipole polarization process can be induced. Thus, the high-concentration oxygen vacancies in the samples will induce strong dipole polarization effect, which is beneficial to the dissipation of EM waves [17,20,36,37]. The result is in good accordance with the analysis of Raman spectra. Among the NiCo2O4, the S4 possesses the lowest ox ygen vacancies, which may weaken its dipole polarization effect. The static magnetic properties of the NiCo2O4 were test at 25 � C and shown in Fig. 5. The sample which displayed typical ferrimagnetic behavior and hysteresis loops is observed. The magnetization of samples are not saturated at the external field, which are ascribed to antiferro magnetic (AFM) phases and magnetic surface disorder effects arising from reduced dimensions. The maximum magnetization of S1 and S4 are 1.86 emu/g and 3.60 emu/g, lower than S2 and S3, which are 4.14 emu/ 3
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Fig. 3. The XRD patterns and Raman spectra of the NiCo2O4 samples.
Fig. 4. The high-resolution XPS spectra of a) Ni, b) Co, c) C and d) O of NiCo2O4 samples.
g and 3.89 emu/g, respectively. The exchange bias phenomenon is in the presence of the as-obtained samples, which may originate from inter facial exchange coupling between FM and AFM layers [38]. The coer civity values of the samples are 69.15 Oe, 70.85 Oe, 69.75 Oe and 68.1 Oe from S1 to S4. The slightly higher coercivity values of S2 and S3 may be resulted from the magnetocrystalline anisotropy provided by their 0D and 1D structural units. It has been reported that the higher coercivity can promote the EM wave absorption performance of the samples [21, 39]. Thus, S2 and S3 are expected to reveal better magnetic loss capacity
than that of S1 and S4. It is acknowledged that the complex permittivity and permeability are key parameters that closely related the EM wave absorption per formance. Hence, the parameters of these NiCo2O4 absorbers with different morphologies were tested and presented in Fig. S1. For the real part of complex permittivity (ε0 ) of S1 and S4, typical frequency dispersion behavior is in the presence in the curves. The value of ε0 decreases rapidly from 20.3 to 8.3 and 18.4 to 4.5 along the increasing of frequency for S1 and S4, respectively. With respect to S2 and S3, the 4
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Fig. 5. The a) magnetization curve and b) corresponding maximum magnetization as well as the coercivity values of the samples.
values of ε0 barely change in frequency range of 2–11 GHz, and then reach to peaks at 13.4 GHz and 14.2 GHz, respectively. As the frequency further increased, the values dramatically decrease. For the imaginary part (ε00 ) of all samples, distinguishable resonance peaks are observed in the plots. This suggests that the presence of multiple Debye dielectric relaxation process. It should be noted that the S1 and S4 possess higher ε00 compared with S2 and S3. This may be ascribed to the building blocks of S1 and S4 are two-dimensional sheet structure, which can increase the conductive paths thus enhance the conductivity. Since the ε" ¼ 1/2ε0πρf, the improved conductivity results in the elevated ε00 value. As a conse quence, the ε" values of S1 and S4 are promoted compared with that of S2 and S3. These results confirm the species of the precipitant play a vital role on the dielectric loss capacity of the absorbers by changing
their microstructures through nanoscale primary building blocks. The dielectric loss tangent (ε00 /ε0 ) values of the samples constructed by 2D flakes are also higher than those constructed by 0D nanoparticles and 1D nanorods. The high tanδ values of the absorbers imply that they possess strong dielectric loss capacity. Typically, the dielectric loss is consisted of interfacial polarization, dipole polarization as well as the conduction loss [40–42]. There exist abundant oxygen vacancies and lattice defects in the NiCo2O4 samples, which serve as polarization centers under the alternating EM field and induce orientation polarization relaxation, thus boost the dipole polar ization effect. In addition, more interfaces are generated on the ab sorbers arising from their hierarchical nanostructure and the coarse surface, resulting in strong interfacial polarization effect. The
Fig. 6. The Cole-Cole semicircles of S1–S4. 5
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abovementioned polarizations can be further confirmed by the Cole-Cole semicircles (see Fig. 6). Apparently, the disordered semicircles can be observed in the curves, indicating the existence of other dielectric loss, such as conductive loss and electron polarization [5]. The real part and imaginary part of complex permeability, which stand for the storage capacity and loss capacity of magnetic field energy, are depicted in Fig. S2. It is obvious that the μ00 values of S2 and S3 are higher than that of S1 and S4, suggesting the strengthened magnetic loss capacity. It should be noted that the negative μ00 values can be observed for all the absorbers when the frequency range is higher than 8 GHz. This phenomenon may be caused by the eddy current induced under alter nating electromagnetic field. Due to the existence of eddy current, the
extra magnetic field can be generated. As a consequence, the inherent magnetic field is cancelled, leading to the negative permeability [5,43]. Simultaneously, multiple resonance peaks are in the presence of the curves, demonstrating the existence of ferromagnetic resonance behavior. The magnetic loss tangent (μ00 /μ0 ) of the absorbers is presented in Fig. S2c. As can be observed, resonance peaks are located at ~4 GHz for S1, S2, S3 and 15.2 GHz for all the absorbers. Generally, the reso nance peak below 10 GHz is ascribed to natural resonance whereas the peak above 10 GHz results from exchange resonance [33]. Thus, it can be deduced the existence of natural resonance in S1, S2, S3 and ex change resonance for all samples. Apart from above mentioned mech anism, magnetic loss is also comprised of eddy current loss, which can be
Fig. 7. RL values vs. frequency at different thicknesses for a-d) S1–S4, e-h) 3D plots and i-l) 2D counter maps of the corresponding absorbers. 6
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expressed as follow formula: 00
’
2
μ ðμ Þ f
1
precipitant, though the RLmin value is only 17.5 dB, the fe still can reach to 4.24 GHz at a relative thin thickness of 1.5 mm. As the pre cipitant replaced by urea and NaHCO3, the EM wave dissipation behavior of the NiCo2O4 absorbers is dramatically improved. For the urchin-like NiCo2O4, the fe is up to 5.84 GHz (from 12.16 GHz to 18 GHz) at thickness of only 1.88 mm. With regard to S3, the fe ranges from 11.92 GHz to 18 GHz, covering the whole Ku band at 2.06 mm. Besides, the RLmin value at corresponding thickness is as high as 42.8 dB. Both of the strong absorption capacity and wide fe of S2 and S3 make them competitive candidates for their practical application as effective EM wave absorbers. Nevertheless, the inferior EM wave attenuation behavior is observed on S4 sample. The fe value decreases to 2.72 GHz, which is much narrower than the abovementioned absorbers. The detail data are depicted in Fig. 7. To have a deeper insight into the cause of the superior EM wave dissipation behavior of the NiCo2O4 samples, the impedance matching characteristic (Z ¼ |Zin/Z0|) of the absorbers are calculated and pre sented in Fig. 8. In general, when the value of |Zin/Z0| is closer to 1, the input impedance of the absorbers can be well matched with impedance of the free space. Under this circumstance, the incident EM wave can permeate in absorbers as much as possible with less reflection. It is obvious that both of the Z values of urchin-like and sphere-like NiCo2O4 samples equal to 1 under suitable thickness. Meanwhile, Z values of S1 and S4 are 0.8 and 0.7, respectively, which is a little away from the ideal value. This result indicates the deteriorative impedance matching characteristic for S1 and S4. Though they possess high potential for EM wave dissipation in view of their higher dielectric loss, the practical EM wave attenuation performance is inhibited by their poor impedance
(1)
2
¼ 2πμ0 d σ
where f, μ0, and σ are the frequency, the vacuum permeability, and the electric conductivity, respectively. One can see from the curves that the values fluctuate on the investigated frequency for S1 and S4. Never theless, the values for S2 and S3 are maintained unchanged in frequency range of 6–14 GHz, demonstrating the magnetic loss caused by eddy current loss. Based on above analysis, the magnetic loss capacity of S2 and S3 is convinced to be better than that of S1 and S4 due to the multiple magnetic loss mechanisms, including natural resonance, ex change resonance and eddy current loss. To understand the EM wave dissipation behavior of NiCo2O4 ab sorbers, the reflection loss (RL) values were calculated based on trans mission line theory. The specific equations are described below: � � �Zin Z0 � � RL ¼ 20 log�� (2) Zin Z0 � rffiffiffiffi Zin ¼
�
μr 2πfd pffiffiffiffiffiffiffiffi μr εr tan h j c εr
� (3)
where d is the thickness of absorbers, f is the frequency of EM wave and c is the velocity of light in vacuum. The RL values of these samples were calculated and plotted in Fig. 7 in the form of 2D and 3D figures. Generally, an effective absorption performance requires the RL values lower than 10 dB. The frequency range that the RL values lower than 10 dB is labeled as fe. For the NiCo2O4 sample prepared by NaOH
Fig. 8. RL values vs. frequency at different thicknesses for a) S2 and d) S3, the simulations of the absorber thickness vs. peak frequency for b) S2 and e) S3 under l/4 conditions, the impedance matching characteristic of c) S2 and f) S3, the impedance matching characteristic of g) the as-prepared NiCo2O4 absorbers under the optimized condition, and h) the RL value at thickness of 2.06 mm with corresponding impedance matching characteristic and the attenuation constants by taking S3 as an example. 7
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matching. The quarter wavelength matching model for S2 and S3 is also investigated to study the relationship between the thickness of absorbers and their RL values. The RLmin value can be achieved at specific fre quency when thickness of absorber obeys following equation: tm ¼
nλ nc ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffi ðn ¼ 1; 3; 5…::Þ 4 4fm jεr jjμr j
oxygen vacancies and functional groups in the as-prepared NiCo2O4 absorbers can lead to the asymmetric distribution of charges. In conse quence, the corresponding dipole polarization process can be induced. This process is prone to improve the EM wave consumption of absorber. Secondly, the excellent EM wave attenuation is ascribed to the unique structure of sphere-like NiCo2O4 absorber assembled by numerous 0 D nanoparticles. The EM wave entered into the EM wave absorbing ma terial will undergo multiple reflections and scattering on the surface of sphere-like NiCo2O4, thus extend the propagation path and helpful for the energy dissipation. Thirdly, plenty of NiCo2O4–NiCo2O4 interfaces can be formed since the sphere-like NiCo2O4 absorber is constructed by plenty of interconnected nanoparticles, undoubtedly will enhance the interfacial polarization relaxation. Finally, due to the electrical con ductivity and ferromagnetism of the absorber, the magnetic loss including natural resonance and exchange resonance, and the dielectric loss including conductive loss also contribute to consumption of EM wave energy. To figure out the EM wave absorbing capacity of sphere-like NiCo2O4, we collect the related excellent NiCo2O4-based absorbers as well as their absorption performance and list in Fig. 10 and Table 1. According to the results, the NiCo2O4 absorber possesses the widest effective absorption bandwidth (fe) among the NiCo2O4-based absorber so far, which exceeds 6 GHz and covers the whole Ku band. More importantly, the superior performance can be achieved under thin
(4)
where tm and fm are the thickness of the absorber and the frequency corresponding to the RLmin value, the other parameters are the same as described earlier. The rhombic symbols stand for the matching thickness vs. the peak frequency collected from Fig. 8. The locations of these data are exactly around the 1/4 λ, implying the relationship between matching thickness and peak frequency can satisfy the quarter wave length matching model. Therefore, two reflected EM wave will dissipate of each other in the interface of air-absorber due to the phase difference of 180� , resulting in maximized the EM wave attenuation capacity. Simultaneously, the attenuation constant is calculated to inspect its impact on the EM wave absorption behavior. Specific data are obtained based on the following formula: ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiq pffiffi 2π f � ðμ00ε00 μ’ε’Þ þ ðμ00ε00 μ’ε’Þ2 þ ðμ’ ε00 þ μ00ε’Þ2 (5) α¼ c We choose S3 as an illustration. Specifically, there are two Z values that reach to 1 in the curves, which is marked as A and B. As it can be seen, the better attenuation behavior is accomplished at B instead of A. This can be ascribed to the fact that the value of attenuation constant is monotonically increased as a function of frequency. The enlarged attenuation constant of point B is favorable for strengthened absorption capacity. Based on this, it can be concluded that higher attenuation constant is conductive to elevated absorption performance under the suitable impedance matching characteristic. According to above analysis, the superior EM wave dissipation of S3 sample, which possesses the widest fe of 6.08 GHz among the NiCo2O4 absorbers, originates from multiple loss mechanisms. Before the ab sorption process occurred, the well matched impedance endows the NiCo2O4 absorbers to receive more incident EM wave rather than reflecting it on the surface. In addition, the quarter-wavelength inter ference can also ensure the cancellation between the EM waves reflected from the different interfaces (air-absorber and absorber-metal), benefi cial to elevated attenuation performance. When the incident EM wave entered into the absorbers, the absorption is ascribed to following mechanisms, which is depicted in Fig. 9. Firstly, there are many oxygen vacancies and functional groups existed on the NiCo2O4 absorbers. The
Fig. 10. The comparison of EM wave attenuation behavior between the sam ples and previously reported spinel absorbers.
Fig. 9. EM wave absorption mechanisms of the sphere-like NiCo2O4 absorbers. 8
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Table 1 Some typical NiCo2O4 based EM wave absorbers. Absorber NiCo2O4 CuCo2O4–CuO MgCo2O4/Co3O4 NiCo2O4/CoNiO2 NiCo2O4/Co3O4/NiO MnO2@NiCo2O4 C@NiCo2O4@Fe3O4 NiCo2O4 NiCo2O4 (S1) NiCo2O4 (S2) NiCo2O4 (S3)
RLmin (dB) 35.7 23 48.54 42.13 28.6 58.4 43.0 25.5 17.6 26.1 42.8
Matching thickness d (mm)
Percentage (wt. %)
Effective bandwidth fe (GHz) RL �
1.5 2.8 2.3 1.55 1.64 4 3.4 4.0 1.5 1.88 2.06
50 70 50 50 30 60 60 50 50 50 50
4.18 4.02 5.16 3.92 4.72 2.70 2.10 3.20 4.24 5.84 6.08
10 dB
Refs. [26] [18] [19] [22] [23] [24] [25] [27] This work
matched thickness of 2.06 mm. In view of the facile preparation process, high EM wave absorption capacity, the pristine sphere-like NiCo2O4 is prone to be a valid alternative for high-performance pristine ferrite EM wave absorbers.
Appendix A. Supplementary data
4. Conclusion
References
In this work, NiCo2O4 absorbers constructed by different dimensions of primary building blocks were successfully fabricated by precipitationhydrothermal method and subsequent calcination. The EM wave attenuation performance was found to be greatly affected by the struc tural units of the NiCo2O4 absorbers. Though the NiCo2O4 absorbers assembled by 2D nanosheet structure (i.e., S1 and S4) possess high dielectric loss capacity, the poor magnetic loss is unable to achieve a balance with the exaggerated dielectric loss capacity, resulting in deteriorated EM wave attenuation performance. On the contrary, the NiCo2O4 absorbing materials constructed by 0D nanoparticles and 1D nanorods displayed superior absorption behavior, mainly originate from their multiple magnetic loss, including natural resonance, exchange resonance and eddy current loss, which can reach balance with their dielectric loss capacity. Under the optimized parameters, the fe of urchin-like NiCo2O4 is up to 5.84 GHz at thickness of 1.88 mm. In particular, the fe of sphere-like NiCo2O4 is even as wide as 6.08 GHz ranging from 11.92 GHz to 18 GHz, covering the whole Ku band. By this approach, we have obtained the NiCo2O4 absorber with the widest fe among the NiCo2O4-based absorber so far. The facile synthetic route and the idea of assembling absorbers with varied primary building blocks may offer a new approach for the preparation of high-performance pristine ferrite EM wave absorbers.
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Supplementary data to this article can be found online at https://doi. org/10.1016/j.compositesb.2019.107620.
Declaration of competing interest We declare that we do not have any commercial or associative in terest that represents a conflict of interest in connection with the work submitted. CRediT authorship contribution statement Hongjing Wu: Conceptualization, Writing - original draft, Writing review & editing, Supervision, Funding acquisition. Ming Qin: Meth odology, Validation, Data curation, Writing - original draft. Limin Zhang: Visualization, Investigation, Resources, Funding acquisition. Acknowledgments Financial support was provided by the National Science Foundation of China (Grants nos. 21806129, 51872238 and 51704242), the Fundamental Research Funds for the Central Universities (Nos. 3102018zy045, 3102019AX11 and 310201911cx019) and the Natural Science Basic Research Plan in Shaanxi Province of China (Nos. 2017JQ5116 and 2018JM5094). 9
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