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Microwave absorption properties of microporous [email protected](NiO-CoO) nanoparticles through dealloying
Journal of Magnetism and Magnetic Materials 503 (2020) 166631
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Research articles
Microwave absorption properties of microporous CoNi@(NiO-CoO) nanoparticles through dealloying
T
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Cui Nia,b,1, Dan Wua,1, XiuBo Xiea, , Baolei Wangc, Hongli Weia, Yuping Zhanga, Xiangjin Zhaob, ⁎ Li Liua, Bing Wangd, Wei Dua, a
School of Environmental and Material Engineering, Yantai University, Yantai, Shandong 264005, China College of Nuclear Equipment and Nuclear Engineering, Yantai University, No. 30 Qingquan Road, Shandong 264005, China c Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University, No.37 Xueyuan Road, Beijing 100191, China d Key Laboratory for Biomedical Effects of Nanomaterials and Nanosafety, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: CoNi alloy Dealloying method Microwave absorption performance Microporous structure
CoNi alloy is an important absorber and it can combine characteristics of transition metals Co and Ni with various advantages of strong magnetic loss, low cost and easy morphology control. The microporous CoNi@(CoO-NiO) nanoparticles (NPs) of about 45 nm are obtained by using hydrogen plasma metal reaction (HPMR) and dealloying methods. The Al element in Al9Co2, Al70Co15Ni15 and Al14Co3Ni3 phases can be dealloyed by NaOH at 323 K. The CoNi surface would oxidize to CoO and NiO, leading to the core@shell structure. The CoNi@(CoO-NiO)-paraffin wax composite displays high microwave absorption performance with minimum reflection loss (RL) of −38.1 dB at matching thickness of 5.0 mm. The effective absorption bandwidth (RL < −10 dB, at thickness of 2.0 mm) is 5.9 GHz. The RL values below −10 dB cover frequencies of 4.6–18.0 GHz with thickness ranging from 2.0 to 5.0 mm. The effective permittivity modification for a mixture of air and microporous CoNi host, and the large interfacial polarization relaxation of the CoNi@(CoO-NiO) structure contribute much to the enhanced microwave absorption performances.
1. Introduction The electromagnetic interference (EMI) problems [1–5] have been becoming potential threat for human health with the wide use of electron devices. Therefore, developing microwave absorbers with high absorption intensity and wide bandwidth is an important research issue for researchers and engineers from research institutions [6–11]. Over the past decade, absorbers of ferromagnetic materials (Co, Ni) and their oxides with different structures have been steadily studied [12–15]. However, the high density and poor chemical stability of pure Co and Ni metals hinders their application as microwave absorbers in commercial devices for military and civilian. As is known to us all, the good microwave absorption performances are dependent on the suitable impedance matching of the electromagnetic parameters. CoNi alloy as an important absorber possesses various advantages of strong magnetic loss, low cost and easy morphology control [16–21]. To tune the impedance matching of CoNi alloy, researchers have designed alloys with different morphologies
including flower-like [16], thistle-like [17], chain-like [18], hierarchical [19] and particle decorated grapheme [20,21]. Wang et al. [16] reported that RL value of the Ni0.5Co0.5(OH)2@PANI composite was −39.8 dB at 6.4 GHz and the absorption bandwidth (RL below −10 dB, at thickness of 2.5 mm) reached 3.1 GHz. The maximum RL value of the CoNi chains reported by Zhao et al. [18] was −33.4 dB, and the bandwidth (RL < −10 dB) varied from 4.6 GHz to 18.0 GHz for matching thickness of 1.0–3.0 mm. The effective absorbing bandwidth of the hierarchical CoNi@SiO2@C-3 composite [19] almost overlapped wavebands of C, X and Ku with thicknesses from 1.5 mm to 5 mm. Though the CoNi samples mentioned above exhibit considerable bandwidth, the microwave absorption intensity needs to be further improved. In our previous work, we systematically studied the positive influences of the microporous and core@shell structure on the microwave absorption modification for the Co and Ni NPs [22–24]. The antiferromagnetic CoO [23] and NiO [24] enhanced the interfacial polarization relaxation loss, while the microwave transmission and reflection can be hindered for the unique porous morphology. However,
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Corresponding authors. E-mail addresses: [email protected] (X. Xie), [email protected] (W. Du). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.jmmm.2020.166631 Received 22 October 2019; Received in revised form 11 February 2020; Accepted 17 February 2020 Available online 19 February 2020 0304-8853/ © 2020 Elsevier B.V. All rights reserved.
Journal of Magnetism and Magnetic Materials 503 (2020) 166631
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Al9Co2 (PDF#06-0699), Al14Co3Ni3 (PDF#48-1402) and Al70Co15Ni15 (PDF#46-1062) phases can be detected in the NPs, see Fig. 1. This is different from those of the CoAl and NiAl alloys. The NiAl alloys with Ni concentrations varying from 45 to 56 at% mainly contain Al3Ni2 and Al phases [24], while the Co50Al50 alloy mainly contains CoAl and Al13Co4 phases [23]. The main diffraction peak of Al centering at 38.7° (the standard intensity of main peak and second peak is about 2:1) indicates that the highest diffraction peak at about 44.5° (second peak of Al) is ascribed to other phases including Al. Although the main peaks of Al14Co3Ni3 and Al70Co15Ni15 phases at about 44.5° are coincided with the second diffraction peak of Al, the diffraction peaks at 42.4 and 43.0° belong to Al14Co3Ni3 phase, implying its existence in the particles. Since the intensities of diffraction peaks of Al at 78.2 and 82.4° are very low, the two peaks are mainly contributed for the Al70Co15Ni15 phase. The previous work proved that the Al elements in the Co13.3Ni16Al70.7, Co15Ni15Al70 and Co19Ni8.5Al72.5 phases of the CoxNi47.7−xAl52.5 alloys (x = 9,15.5,25) can be removed by NaOH [27]. This implies that the Al9Co2 and Al14Co3Ni3 can be dealloyed after reacting with sodium hydroxide, and the efficiency is determined later. Fig. 2a and b show the TEM images of the produced Co25Ni25Al50 NPs, and the NPs with particle size varying from 20 to 70 nm (average particle size: 45 nm) display a clear core@shell structure. The thin amorphous layer of 2 nm can be ascribed to the Al2O3 phase formed during the passivation process, and the shell can be dealloyed after reaction in sodium hydroxide solution, similar to our previous works. [23] The measured interplanar spacing of 2.037 Å corresponds to the Al70Co15Ni15 phase, and the lattice fringe spacings of 2.327, 2.226 and 2.045 Å at the edge of other two particles can be indexed to be the (1 1 1) plane of Al, (2 1 2) plane of Al9Co2 and certain plan of Al13Co3Ni3. Fig. 3a displays the XRD pattern of the Co25Ni25Al50 NPs after dealloying process, the nanocomposite mainly contains CoNi phase, similar to other reports. [27] According to the equation: Al + NaOH + H2O = Na(AlOH)4 + H2, the Al in the Co25Ni25Al50 NPs should react with NaOH, and the broad peak at about 45° belongs to CoNi alloy. The diffraction peak broadening effect may be caused by the decrease of grain size and crystallization for the removal of Al in the NPs. The detectable diffraction peaks of CoO and NiO are mainly caused by the partial oxidation of CoNi NPs in the following passivation process. CoO and NiO can also be detected after dealloying process in the microporous Co, [23] Ni [24] and CoNi [27] NPs of our previous work, and the NPs display a core@shell structure. The CoNi NPs show microporous structure and average particle size is calculated to be 45 nm, see Fig. 3b. Therefore, in this work, we suggest that the CoO and NiO are distributed uniformly on the surface of CoNi NPs and the sample is referred to CoNi@(CoO-NiO), hereafter. The hysteresis loop of the microporous CoNi@(CoO-NiO) NPs is detected under 300 K in this work, see Fig. 4. The Ms value of the microporous NPs is determined to be 50 emu g−1, much lower than those of bulk Co and Ni NPs [23,24]. The Ms values of the tremella-like NiCo/C are ranging from 62.8 to 103.8 emu g−1 for samples prepared at different carbonization temperatures [28]. We suggest that the microporous structure and existence of CoO, NiO contributed to the low Ms value in this work [21]. The Hc value of the sample is 596 Oe, much higher than those of the pure Co, Ni NPs and Co particles in micron-size (< 100 Oe) [23]. We suggest that the high Hc value is mainly ascribed to the larger magnetocrystalline anisotropy and surface anisotropy caused by the CoO, NiO and nanosize effects of CoNi NPs. According to the following equations: K = μ0MsHc/2; Ha = 4 K/3μ0Ms; 2πfr = γHa, where K is the magnetocrystalline anisotropy constant, γ represents the gyromagnetic ratio, Ha stands for the anisotropy energy, and fr is the natural resonance frequency [29]. The higher Hc value leads shift of the resonance frequency for imaginary part of the permeability (μ″) to a higher region, which can influence the microwave absorption performances of the absorber. The following equations [30] are used to calculate the RL value
the synergistic effect of the CoO, NiO and porous morphology on the properties of the CoNi NPs as microwave absorber needed to be further studied. In this work, we synthesized Co25Ni25Al50 NPs by hydrogen plasma metal reaction method (HPMR), and the microporous CoNi NPs were prepared by removing Al in the Co25Ni25Al50 NPs in the sodium hydroxide solution. The surface of CoNi NPs partially oxidized to CoO and NiO during dealloying process. The maximum RL value of CoNi@(CoONiO) NPs reached −38.1 dB, and absorption bandwidth (RL < −10 dB) was 5.9 GHz at thickness of 2.0 mm. Moreover, the measured electromagnetic parameters and possible microwave absorption mechanism of the NPs were discussed. 2. Experimental 2.1. Synthesis of the microporous CoNi NPs HPMR approach was applied to fabricate Co25Ni25Al50 NPs in this work [24–26]. The Co, Ni and Al ingots were arc melted under pure Ar for four times and the Co25Ni25Al50 NPs were obtained under a mixed gas of Ar and H2 (volume ratio of 1:1, pressure of 0.1 MPa). The reaction current was 180 A. Argon and air were simultaneously filled to the reaction chamber to prevent Co25Ni25Al50 precursor NPs from burning. Then the NPs were reacted with 20 wt% NaOH solution at 323 K for 10 min under strong ultrasonic stirring. 3. Characterizations X-ray diffraction (XRD) equipment was used to determine the phase composition of the Co25Ni25Al50 and microporous CoNi NPs. The morphology of the prepared NPs was observed by the transmission electron microscopy (TEM, JEOL-JSM-2100F, accelerating voltage of 200 kV). The as-prepared NPs-50 wt% PW composite was pressed to a cylindrical toroidal sample (outer diameter: 7 mm; inner diameter:3 mm and thickness:2 mm), and the electromagnetic measurement in 2–18 GHz were recorded by a vector network analyzer. The vibration sample magnetometer (VSM, LakeShore-7307) measurement was used for detecting the magnetic data. The transmission line theory was applied in this work to calculate the RL values of the sample. 4. Results and discussion To determine the phase composition of the obtained Co25Ni25Al50 NPs, Fig. 1 shows its measured XRD pattern. Except for the obvious diffraction peaks of Al phase (PDF#04-0787), diffraction peaks of
Fig. 1. XRD pattern of the as-prepared Co25Ni25Al50 NPs. 2
Journal of Magnetism and Magnetic Materials 503 (2020) 166631
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Fig. 2. (a)-(c) TEM images of the as-prepared Co25Ni25Al50 NPs with different magnifications, and (d) HRTEM image of the as-prepared Co25Ni25Al50 NPs.
−10 dB of the microporous CoNi@(CoO-NiO) NPs enlarges to 5.9 GHz at thickness of 2.0 mm, better than those of other CoNi particles [18,20,21]. The RL curve of the CoNi@(CoO-NiO) NPs shows two obvious peaks in the range of 2–18 GHz (at thickness of 5.0 mm), which are caused by multiple resonance of dielectric loss. The minimum RL value is surprisingly calculated to be −38.1 dB. The three-dimensional RL diagram for CoNi@(CoO-NiO) NPs in Fig. 5b clearly shows that the absorption area of RL values below −10 dB covers wide frequency range of 4.6–18 GHz. Generally, better absorption intensity and bandwidth can be obtained once magnetic contribution matched well with dielectric contribution. It is suggested that the porous structure gives a bridge for mixture of air and NPs to obtain a good impedance matching
based on transmission line theory:
Zin =
μr / εr tanh[j (2πfd/ c ) μr × εr ]
(1)
RL (dB ) = 20 log |(Z in − 1)/(Z in + 1)|
(2)
where c and f are the velocity, frequency of the magnetic wave, and d, Zin represent the absorber thickness and input impedance, respectively. The RL values of the CoNi@(CoO-NiO) NPs are investigated by calculating the electromagnetic parameters of the NPs mixed with 50 wt% PW. Given that the PW shows little impact on the microwaves propagation [31], the calculated RL value should be derived from the microporous CoNi@(CoO-NiO) NPs. Fig. 5a shows the frequency dependency of the reflection loss curve. The bandwidth of RL below
Fig. 3. (a) XRD pattern, (b) TEM image of the Co25Ni25Al50 NPs after dealloying. 3
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the pores would change the effective permittivity of the absorber and thus make another Debye relaxation process [24]. Fig. 6c shows frequency versus μ′ and μ″ curve of the CoNi@(CoO-NiO) NPs. The obtained μ′ value of the CoNi@(CoO-NiO) NPs gradually decreases with increase of frequency, and two resonance peaks can be clearly found at frequency of 4.3 and 15.5 GHz. The μ″ value displays a scattering peak between 4 and 14 GHz with a peak at about 6.5 GHz, coinciding with the analyses of Hc values in Fig. 4. Reports show that the eddy current loss would affect the magnetic loss once the value of μ″(μ′)−2f − 1 kept a constant between 2 and 18 GHz. However, the μ″(μ′)−2f − 1 value of the CoNi@(CoO-NiO) NPs in this work obviously varies with the frequency range, see Fig. 6d, indicating that the eddy current loss makes little contribution to the magnetic loss. For typical resonance behaviors of domain wall resonance, natural ferromagnetic resonance and exchange resonance contribute to the magnetic loss of the absorber [37,38], reports show that the domain wall resonance displays important effect in frequency range of 1–100 MHz [39]. Thus, we suggest that the magnetic loss of the CoNi@(CoO-NiO) NPs is mainly caused by the natural and exchange resonances in 2–18 GHz. The frequency dependencies of the tan δε = ε″/ε′ and tan δμ = μ″/ μ′ values of the CoNi@(CoO-NiO) NPs are shown in Fig. 7a and b. The tan δε and tan δμ values of the CoNi@(CoO-NiO) NPs display similar trend to that of ε″ and μ″ values in 2–18 GHz, see Fig. 7a and b. This implies that the dielectric loss together with magnetic loss improves the microwave absorption properties of CoNi@(CoO-NiO) NPs. The microwave dissipates rather than stores up once it is contacted with the CoNi@(CoO-NiO) NPs. On the basis of Eqs. (1) and (2), the impedance matching would influence the EM absorption properties of the CoNi@(CoO-NiO) NPs which are caused by: (1) Large interfacial polarization relaxation loss derived from the numerous interfaces of the CoO and NiO shell and the pores. (2) The effective impact of microporous air-CoNi mixture on the permittivity. The attenuation constant α is another important factor for analysis of the microwave absorption mechanism [40]:
Fig. 4. Magnetic hysteresis loop of the microporous CoNi@(CoO-NiO) NPs.
[23]. The CoO and NiO enlarge the surface area of absorber with enhanced interfacial polarization relaxation loss [23,24]. Therefore, the pore together with oxide shell improves the microwave absorption performances of CoNi@(CoO-NiO) NPs. The measured electromagnetic parameters are further analyzed for better understanding the possible mechanism of the CoNi@(CoO-NiO) absorber NPs. Fig. 6a displays the frequency versus complex permittivity (ε′ and ε″) curves of CoNi@(CoO-NiO) NPs. The ε′ values of the CoNi@(CoO-NiO) NPs decrease with the increase of frequency, and the curve shows three inconspicuous peaks at about 7.0, 12.3 and 17.0 GHz. The ε″ values of the CoNi@(CoO-NiO) NPs increase with the increase of frequency, and four peaks at about 4.2, 9.5, 14.8 and 17.5 GHz can also be found in the curve. It is well known that the electron, ion and dipole polarizations [32] dominate the resonance behavior of the permittivity. Since that the ion and electron polarizations are generally found in frequency range of THz and PHz [23]. Therefore, the resonance peak of permittivity of the CoNi@(CoO-NiO) NPs should be ascribed to the dipole polarization. Debye relaxation behaviors [33–35] described in Eq. (3) can be used to explain the dipole relaxation of the CoNi@(CoO-NiO) NPs:
⎛ε′ ⎝
εs + ε∞ ε + ε∞ 2 ⎞ + (ε′ ′)2 = ⎛ s ⎞ 2 ⎠ ⎝ 2 ⎠
α=
2 πf × c
(μ′ ′ε′ ′ − μ′ε′) +
(μ′ ′ε′ ′ − μ′ε′)2 + (μ′ε′ ′ + μ′ ′ε′)2
(4)
Generally, the α value reflects the attenuation ability of CoNi@(CoO-NiO) NPs. Fig. 6c shows that the attenuation constant increases from 8 to 117 with the increase of frequency, better than that of other CoNi particles [15,16], implying good attenuation attributes to the ε″ and μ″ values in Fig. 6a and b. The unique CoNi NPs with porous and core@shell structure may be beneficial for designing other microwave absorption materials.
(3)
The ε′ versus ε″ curve of the CoNi@(CoO-NiO) NPs shows four distorted Cole-Cole semicircles, see Fig. 6b, implying four Debye relaxation processes contribute to the polarization in the CoNi@(CoONiO) NPs. We suggest that the microporous structure of CoNi@(CoONiO) NPs and nanosize effects of the Co core lead to the orientational polarization [32]. The numerous interfaces between the porous CoNi and CoO-NiO shell also make one Cole-Cole semicircle (interfacial polarization relaxation) [36]. Moreover, the air-CoNi mixture caused by
5. Conclusions The microporous CoNi@(CoO-NiO) NPs of about 45 nm are obtained by HPMR and dealloying methods. The Al9Co3, Al70Co15Ni15 and Al14Co3Ni3 phases can react with sodium hydroxide at 323 K. The CoNi
Fig. 5. Frequency dependence and three-dimensional representation of the reflection loss curves of CoNi@(CoO-NiO) NPs. 4
Journal of Magnetism and Magnetic Materials 503 (2020) 166631
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Fig. 6. Frequency dependence of the real and imaginary parts of the complex permittivity (a), Cole-Cole semicircles (b), the real and imaginary parts of the complex permeability (c) and μ″(μ′)−2f − 1 value (d) for the CoNi@(CoO-NiO) NPs.
Fig. 7. Frequency dependences of the dielectric loss tangent (a), magnetic loss tangent (b) and attenuation constant α (c) of the CoNi@(CoO-NiO) NPs.
Declaration of Competing Interest
surface would oxidize to CoO and NiO, leading to the core@shell structure. The minimum RL value of CoNi@(CoO-NiO) NPs reaches −38.1 dB (at thickness of 5.0 mm), and absorption bandwidth (RL < 10 dB, at thickness of 2.0 mm) is as wide as 5.9 GHz. The RL value below −10 dB covers frequencies of 4.6–18 GHz with thickness ranging from 2.0 to 5.0 mm. The modified microwave absorption performances are caused by the effective permittivity modification of the mixture of air and microporous CoNi host, and the large interfacial polarization relaxation for the pores and CoO-NiO multiple interfaces.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors acknowledge the support of this work by the National Natural Science Foundation of China (No. 51871191), the Natural Science Foundation of Shandong Province (No. ZR2019MEM044), and research program of the Key Laboratory of Biomedical Effects of Nanomaterials and Nanosafety, Chinese Academy of Sciences (No: NSKF201908).
CRediT authorship contribution statement C. Ni and D. Wu performed the materials preparation and characterizations; Dr. X.B. Xie designed the experiment and wrote the paper; B. L. Wang, H.L. Wei and Dr. Y.P. Zhang provided help for data analysis; Prof. L. Liu, X.J. Zhao and B. Wang provided help for discussions of the data and checking the english expression of this work; Dr. X.B. Xie, Prof. B. Wang and W. Du provide financial support for the work.
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Research articles
Microwave absorption properties of microporous CoNi@(NiO-CoO) nanoparticles through dealloying
T
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Cui Nia,b,1, Dan Wua,1, XiuBo Xiea, , Baolei Wangc, Hongli Weia, Yuping Zhanga, Xiangjin Zhaob, ⁎ Li Liua, Bing Wangd, Wei Dua, a
School of Environmental and Material Engineering, Yantai University, Yantai, Shandong 264005, China College of Nuclear Equipment and Nuclear Engineering, Yantai University, No. 30 Qingquan Road, Shandong 264005, China c Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University, No.37 Xueyuan Road, Beijing 100191, China d Key Laboratory for Biomedical Effects of Nanomaterials and Nanosafety, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: CoNi alloy Dealloying method Microwave absorption performance Microporous structure
CoNi alloy is an important absorber and it can combine characteristics of transition metals Co and Ni with various advantages of strong magnetic loss, low cost and easy morphology control. The microporous CoNi@(CoO-NiO) nanoparticles (NPs) of about 45 nm are obtained by using hydrogen plasma metal reaction (HPMR) and dealloying methods. The Al element in Al9Co2, Al70Co15Ni15 and Al14Co3Ni3 phases can be dealloyed by NaOH at 323 K. The CoNi surface would oxidize to CoO and NiO, leading to the core@shell structure. The CoNi@(CoO-NiO)-paraffin wax composite displays high microwave absorption performance with minimum reflection loss (RL) of −38.1 dB at matching thickness of 5.0 mm. The effective absorption bandwidth (RL < −10 dB, at thickness of 2.0 mm) is 5.9 GHz. The RL values below −10 dB cover frequencies of 4.6–18.0 GHz with thickness ranging from 2.0 to 5.0 mm. The effective permittivity modification for a mixture of air and microporous CoNi host, and the large interfacial polarization relaxation of the CoNi@(CoO-NiO) structure contribute much to the enhanced microwave absorption performances.
1. Introduction The electromagnetic interference (EMI) problems [1–5] have been becoming potential threat for human health with the wide use of electron devices. Therefore, developing microwave absorbers with high absorption intensity and wide bandwidth is an important research issue for researchers and engineers from research institutions [6–11]. Over the past decade, absorbers of ferromagnetic materials (Co, Ni) and their oxides with different structures have been steadily studied [12–15]. However, the high density and poor chemical stability of pure Co and Ni metals hinders their application as microwave absorbers in commercial devices for military and civilian. As is known to us all, the good microwave absorption performances are dependent on the suitable impedance matching of the electromagnetic parameters. CoNi alloy as an important absorber possesses various advantages of strong magnetic loss, low cost and easy morphology control [16–21]. To tune the impedance matching of CoNi alloy, researchers have designed alloys with different morphologies
including flower-like [16], thistle-like [17], chain-like [18], hierarchical [19] and particle decorated grapheme [20,21]. Wang et al. [16] reported that RL value of the Ni0.5Co0.5(OH)2@PANI composite was −39.8 dB at 6.4 GHz and the absorption bandwidth (RL below −10 dB, at thickness of 2.5 mm) reached 3.1 GHz. The maximum RL value of the CoNi chains reported by Zhao et al. [18] was −33.4 dB, and the bandwidth (RL < −10 dB) varied from 4.6 GHz to 18.0 GHz for matching thickness of 1.0–3.0 mm. The effective absorbing bandwidth of the hierarchical CoNi@SiO2@C-3 composite [19] almost overlapped wavebands of C, X and Ku with thicknesses from 1.5 mm to 5 mm. Though the CoNi samples mentioned above exhibit considerable bandwidth, the microwave absorption intensity needs to be further improved. In our previous work, we systematically studied the positive influences of the microporous and core@shell structure on the microwave absorption modification for the Co and Ni NPs [22–24]. The antiferromagnetic CoO [23] and NiO [24] enhanced the interfacial polarization relaxation loss, while the microwave transmission and reflection can be hindered for the unique porous morphology. However,
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Corresponding authors. E-mail addresses: [email protected] (X. Xie), [email protected] (W. Du). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.jmmm.2020.166631 Received 22 October 2019; Received in revised form 11 February 2020; Accepted 17 February 2020 Available online 19 February 2020 0304-8853/ © 2020 Elsevier B.V. All rights reserved.
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Al9Co2 (PDF#06-0699), Al14Co3Ni3 (PDF#48-1402) and Al70Co15Ni15 (PDF#46-1062) phases can be detected in the NPs, see Fig. 1. This is different from those of the CoAl and NiAl alloys. The NiAl alloys with Ni concentrations varying from 45 to 56 at% mainly contain Al3Ni2 and Al phases [24], while the Co50Al50 alloy mainly contains CoAl and Al13Co4 phases [23]. The main diffraction peak of Al centering at 38.7° (the standard intensity of main peak and second peak is about 2:1) indicates that the highest diffraction peak at about 44.5° (second peak of Al) is ascribed to other phases including Al. Although the main peaks of Al14Co3Ni3 and Al70Co15Ni15 phases at about 44.5° are coincided with the second diffraction peak of Al, the diffraction peaks at 42.4 and 43.0° belong to Al14Co3Ni3 phase, implying its existence in the particles. Since the intensities of diffraction peaks of Al at 78.2 and 82.4° are very low, the two peaks are mainly contributed for the Al70Co15Ni15 phase. The previous work proved that the Al elements in the Co13.3Ni16Al70.7, Co15Ni15Al70 and Co19Ni8.5Al72.5 phases of the CoxNi47.7−xAl52.5 alloys (x = 9,15.5,25) can be removed by NaOH [27]. This implies that the Al9Co2 and Al14Co3Ni3 can be dealloyed after reacting with sodium hydroxide, and the efficiency is determined later. Fig. 2a and b show the TEM images of the produced Co25Ni25Al50 NPs, and the NPs with particle size varying from 20 to 70 nm (average particle size: 45 nm) display a clear core@shell structure. The thin amorphous layer of 2 nm can be ascribed to the Al2O3 phase formed during the passivation process, and the shell can be dealloyed after reaction in sodium hydroxide solution, similar to our previous works. [23] The measured interplanar spacing of 2.037 Å corresponds to the Al70Co15Ni15 phase, and the lattice fringe spacings of 2.327, 2.226 and 2.045 Å at the edge of other two particles can be indexed to be the (1 1 1) plane of Al, (2 1 2) plane of Al9Co2 and certain plan of Al13Co3Ni3. Fig. 3a displays the XRD pattern of the Co25Ni25Al50 NPs after dealloying process, the nanocomposite mainly contains CoNi phase, similar to other reports. [27] According to the equation: Al + NaOH + H2O = Na(AlOH)4 + H2, the Al in the Co25Ni25Al50 NPs should react with NaOH, and the broad peak at about 45° belongs to CoNi alloy. The diffraction peak broadening effect may be caused by the decrease of grain size and crystallization for the removal of Al in the NPs. The detectable diffraction peaks of CoO and NiO are mainly caused by the partial oxidation of CoNi NPs in the following passivation process. CoO and NiO can also be detected after dealloying process in the microporous Co, [23] Ni [24] and CoNi [27] NPs of our previous work, and the NPs display a core@shell structure. The CoNi NPs show microporous structure and average particle size is calculated to be 45 nm, see Fig. 3b. Therefore, in this work, we suggest that the CoO and NiO are distributed uniformly on the surface of CoNi NPs and the sample is referred to CoNi@(CoO-NiO), hereafter. The hysteresis loop of the microporous CoNi@(CoO-NiO) NPs is detected under 300 K in this work, see Fig. 4. The Ms value of the microporous NPs is determined to be 50 emu g−1, much lower than those of bulk Co and Ni NPs [23,24]. The Ms values of the tremella-like NiCo/C are ranging from 62.8 to 103.8 emu g−1 for samples prepared at different carbonization temperatures [28]. We suggest that the microporous structure and existence of CoO, NiO contributed to the low Ms value in this work [21]. The Hc value of the sample is 596 Oe, much higher than those of the pure Co, Ni NPs and Co particles in micron-size (< 100 Oe) [23]. We suggest that the high Hc value is mainly ascribed to the larger magnetocrystalline anisotropy and surface anisotropy caused by the CoO, NiO and nanosize effects of CoNi NPs. According to the following equations: K = μ0MsHc/2; Ha = 4 K/3μ0Ms; 2πfr = γHa, where K is the magnetocrystalline anisotropy constant, γ represents the gyromagnetic ratio, Ha stands for the anisotropy energy, and fr is the natural resonance frequency [29]. The higher Hc value leads shift of the resonance frequency for imaginary part of the permeability (μ″) to a higher region, which can influence the microwave absorption performances of the absorber. The following equations [30] are used to calculate the RL value
the synergistic effect of the CoO, NiO and porous morphology on the properties of the CoNi NPs as microwave absorber needed to be further studied. In this work, we synthesized Co25Ni25Al50 NPs by hydrogen plasma metal reaction method (HPMR), and the microporous CoNi NPs were prepared by removing Al in the Co25Ni25Al50 NPs in the sodium hydroxide solution. The surface of CoNi NPs partially oxidized to CoO and NiO during dealloying process. The maximum RL value of CoNi@(CoONiO) NPs reached −38.1 dB, and absorption bandwidth (RL < −10 dB) was 5.9 GHz at thickness of 2.0 mm. Moreover, the measured electromagnetic parameters and possible microwave absorption mechanism of the NPs were discussed. 2. Experimental 2.1. Synthesis of the microporous CoNi NPs HPMR approach was applied to fabricate Co25Ni25Al50 NPs in this work [24–26]. The Co, Ni and Al ingots were arc melted under pure Ar for four times and the Co25Ni25Al50 NPs were obtained under a mixed gas of Ar and H2 (volume ratio of 1:1, pressure of 0.1 MPa). The reaction current was 180 A. Argon and air were simultaneously filled to the reaction chamber to prevent Co25Ni25Al50 precursor NPs from burning. Then the NPs were reacted with 20 wt% NaOH solution at 323 K for 10 min under strong ultrasonic stirring. 3. Characterizations X-ray diffraction (XRD) equipment was used to determine the phase composition of the Co25Ni25Al50 and microporous CoNi NPs. The morphology of the prepared NPs was observed by the transmission electron microscopy (TEM, JEOL-JSM-2100F, accelerating voltage of 200 kV). The as-prepared NPs-50 wt% PW composite was pressed to a cylindrical toroidal sample (outer diameter: 7 mm; inner diameter:3 mm and thickness:2 mm), and the electromagnetic measurement in 2–18 GHz were recorded by a vector network analyzer. The vibration sample magnetometer (VSM, LakeShore-7307) measurement was used for detecting the magnetic data. The transmission line theory was applied in this work to calculate the RL values of the sample. 4. Results and discussion To determine the phase composition of the obtained Co25Ni25Al50 NPs, Fig. 1 shows its measured XRD pattern. Except for the obvious diffraction peaks of Al phase (PDF#04-0787), diffraction peaks of
Fig. 1. XRD pattern of the as-prepared Co25Ni25Al50 NPs. 2
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Fig. 2. (a)-(c) TEM images of the as-prepared Co25Ni25Al50 NPs with different magnifications, and (d) HRTEM image of the as-prepared Co25Ni25Al50 NPs.
−10 dB of the microporous CoNi@(CoO-NiO) NPs enlarges to 5.9 GHz at thickness of 2.0 mm, better than those of other CoNi particles [18,20,21]. The RL curve of the CoNi@(CoO-NiO) NPs shows two obvious peaks in the range of 2–18 GHz (at thickness of 5.0 mm), which are caused by multiple resonance of dielectric loss. The minimum RL value is surprisingly calculated to be −38.1 dB. The three-dimensional RL diagram for CoNi@(CoO-NiO) NPs in Fig. 5b clearly shows that the absorption area of RL values below −10 dB covers wide frequency range of 4.6–18 GHz. Generally, better absorption intensity and bandwidth can be obtained once magnetic contribution matched well with dielectric contribution. It is suggested that the porous structure gives a bridge for mixture of air and NPs to obtain a good impedance matching
based on transmission line theory:
Zin =
μr / εr tanh[j (2πfd/ c ) μr × εr ]
(1)
RL (dB ) = 20 log |(Z in − 1)/(Z in + 1)|
(2)
where c and f are the velocity, frequency of the magnetic wave, and d, Zin represent the absorber thickness and input impedance, respectively. The RL values of the CoNi@(CoO-NiO) NPs are investigated by calculating the electromagnetic parameters of the NPs mixed with 50 wt% PW. Given that the PW shows little impact on the microwaves propagation [31], the calculated RL value should be derived from the microporous CoNi@(CoO-NiO) NPs. Fig. 5a shows the frequency dependency of the reflection loss curve. The bandwidth of RL below
Fig. 3. (a) XRD pattern, (b) TEM image of the Co25Ni25Al50 NPs after dealloying. 3
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the pores would change the effective permittivity of the absorber and thus make another Debye relaxation process [24]. Fig. 6c shows frequency versus μ′ and μ″ curve of the CoNi@(CoO-NiO) NPs. The obtained μ′ value of the CoNi@(CoO-NiO) NPs gradually decreases with increase of frequency, and two resonance peaks can be clearly found at frequency of 4.3 and 15.5 GHz. The μ″ value displays a scattering peak between 4 and 14 GHz with a peak at about 6.5 GHz, coinciding with the analyses of Hc values in Fig. 4. Reports show that the eddy current loss would affect the magnetic loss once the value of μ″(μ′)−2f − 1 kept a constant between 2 and 18 GHz. However, the μ″(μ′)−2f − 1 value of the CoNi@(CoO-NiO) NPs in this work obviously varies with the frequency range, see Fig. 6d, indicating that the eddy current loss makes little contribution to the magnetic loss. For typical resonance behaviors of domain wall resonance, natural ferromagnetic resonance and exchange resonance contribute to the magnetic loss of the absorber [37,38], reports show that the domain wall resonance displays important effect in frequency range of 1–100 MHz [39]. Thus, we suggest that the magnetic loss of the CoNi@(CoO-NiO) NPs is mainly caused by the natural and exchange resonances in 2–18 GHz. The frequency dependencies of the tan δε = ε″/ε′ and tan δμ = μ″/ μ′ values of the CoNi@(CoO-NiO) NPs are shown in Fig. 7a and b. The tan δε and tan δμ values of the CoNi@(CoO-NiO) NPs display similar trend to that of ε″ and μ″ values in 2–18 GHz, see Fig. 7a and b. This implies that the dielectric loss together with magnetic loss improves the microwave absorption properties of CoNi@(CoO-NiO) NPs. The microwave dissipates rather than stores up once it is contacted with the CoNi@(CoO-NiO) NPs. On the basis of Eqs. (1) and (2), the impedance matching would influence the EM absorption properties of the CoNi@(CoO-NiO) NPs which are caused by: (1) Large interfacial polarization relaxation loss derived from the numerous interfaces of the CoO and NiO shell and the pores. (2) The effective impact of microporous air-CoNi mixture on the permittivity. The attenuation constant α is another important factor for analysis of the microwave absorption mechanism [40]:
Fig. 4. Magnetic hysteresis loop of the microporous CoNi@(CoO-NiO) NPs.
[23]. The CoO and NiO enlarge the surface area of absorber with enhanced interfacial polarization relaxation loss [23,24]. Therefore, the pore together with oxide shell improves the microwave absorption performances of CoNi@(CoO-NiO) NPs. The measured electromagnetic parameters are further analyzed for better understanding the possible mechanism of the CoNi@(CoO-NiO) absorber NPs. Fig. 6a displays the frequency versus complex permittivity (ε′ and ε″) curves of CoNi@(CoO-NiO) NPs. The ε′ values of the CoNi@(CoO-NiO) NPs decrease with the increase of frequency, and the curve shows three inconspicuous peaks at about 7.0, 12.3 and 17.0 GHz. The ε″ values of the CoNi@(CoO-NiO) NPs increase with the increase of frequency, and four peaks at about 4.2, 9.5, 14.8 and 17.5 GHz can also be found in the curve. It is well known that the electron, ion and dipole polarizations [32] dominate the resonance behavior of the permittivity. Since that the ion and electron polarizations are generally found in frequency range of THz and PHz [23]. Therefore, the resonance peak of permittivity of the CoNi@(CoO-NiO) NPs should be ascribed to the dipole polarization. Debye relaxation behaviors [33–35] described in Eq. (3) can be used to explain the dipole relaxation of the CoNi@(CoO-NiO) NPs:
⎛ε′ ⎝
εs + ε∞ ε + ε∞ 2 ⎞ + (ε′ ′)2 = ⎛ s ⎞ 2 ⎠ ⎝ 2 ⎠
α=
2 πf × c
(μ′ ′ε′ ′ − μ′ε′) +
(μ′ ′ε′ ′ − μ′ε′)2 + (μ′ε′ ′ + μ′ ′ε′)2
(4)
Generally, the α value reflects the attenuation ability of CoNi@(CoO-NiO) NPs. Fig. 6c shows that the attenuation constant increases from 8 to 117 with the increase of frequency, better than that of other CoNi particles [15,16], implying good attenuation attributes to the ε″ and μ″ values in Fig. 6a and b. The unique CoNi NPs with porous and core@shell structure may be beneficial for designing other microwave absorption materials.
(3)
The ε′ versus ε″ curve of the CoNi@(CoO-NiO) NPs shows four distorted Cole-Cole semicircles, see Fig. 6b, implying four Debye relaxation processes contribute to the polarization in the CoNi@(CoONiO) NPs. We suggest that the microporous structure of CoNi@(CoONiO) NPs and nanosize effects of the Co core lead to the orientational polarization [32]. The numerous interfaces between the porous CoNi and CoO-NiO shell also make one Cole-Cole semicircle (interfacial polarization relaxation) [36]. Moreover, the air-CoNi mixture caused by
5. Conclusions The microporous CoNi@(CoO-NiO) NPs of about 45 nm are obtained by HPMR and dealloying methods. The Al9Co3, Al70Co15Ni15 and Al14Co3Ni3 phases can react with sodium hydroxide at 323 K. The CoNi
Fig. 5. Frequency dependence and three-dimensional representation of the reflection loss curves of CoNi@(CoO-NiO) NPs. 4
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Fig. 6. Frequency dependence of the real and imaginary parts of the complex permittivity (a), Cole-Cole semicircles (b), the real and imaginary parts of the complex permeability (c) and μ″(μ′)−2f − 1 value (d) for the CoNi@(CoO-NiO) NPs.
Fig. 7. Frequency dependences of the dielectric loss tangent (a), magnetic loss tangent (b) and attenuation constant α (c) of the CoNi@(CoO-NiO) NPs.
Declaration of Competing Interest
surface would oxidize to CoO and NiO, leading to the core@shell structure. The minimum RL value of CoNi@(CoO-NiO) NPs reaches −38.1 dB (at thickness of 5.0 mm), and absorption bandwidth (RL < 10 dB, at thickness of 2.0 mm) is as wide as 5.9 GHz. The RL value below −10 dB covers frequencies of 4.6–18 GHz with thickness ranging from 2.0 to 5.0 mm. The modified microwave absorption performances are caused by the effective permittivity modification of the mixture of air and microporous CoNi host, and the large interfacial polarization relaxation for the pores and CoO-NiO multiple interfaces.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors acknowledge the support of this work by the National Natural Science Foundation of China (No. 51871191), the Natural Science Foundation of Shandong Province (No. ZR2019MEM044), and research program of the Key Laboratory of Biomedical Effects of Nanomaterials and Nanosafety, Chinese Academy of Sciences (No: NSKF201908).
CRediT authorship contribution statement C. Ni and D. Wu performed the materials preparation and characterizations; Dr. X.B. Xie designed the experiment and wrote the paper; B. L. Wang, H.L. Wei and Dr. Y.P. Zhang provided help for data analysis; Prof. L. Liu, X.J. Zhao and B. Wang provided help for discussions of the data and checking the english expression of this work; Dr. X.B. Xie, Prof. B. Wang and W. Du provide financial support for the work.
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