Enhanced electromagnetic wave absorption properties of NiCo2 nanoparticles interspersed with carbon nanotubes

Accepted Manuscript Enhanced electromagnetic wave absorption properties of NiCo2 nanoparticles interspersed with carbon nanotubes Bochong Wang, Can Zhang, Congpu Mu, Ruilong Yang, Jianyong Xiang, Jiefang Song, Fusheng Wen, Zhongyuan Liu PII: DOI: Reference:

S0304-8853(18)31227-7 https://doi.org/10.1016/j.jmmm.2018.09.090 MAGMA 64372

To appear in:

Journal of Magnetism and Magnetic Materials

Received Date: Revised Date: Accepted Date:

23 April 2018 23 September 2018 23 September 2018

Please cite this article as: B. Wang, C. Zhang, C. Mu, R. Yang, J. Xiang, J. Song, F. Wen, Z. Liu, Enhanced electromagnetic wave absorption properties of NiCo2 nanoparticles interspersed with carbon nanotubes, Journal of Magnetism and Magnetic Materials (2018), doi: https://doi.org/10.1016/j.jmmm.2018.09.090

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Enhanced electromagnetic wave absorption properties of NiCo2 nanoparticles interspersed with carbon nanotubes Bochong Wanga*, Can Zhanga，Congpu Mua, Ruilong Yanga, Jianyong Xianga*, Jiefang Songa, Fusheng Wena and Zhongyuan Liua a

State Key Laboratory of Metastable Materials Science & Technology and Key

Laboratory for Microstructural Material Physics of Hebei Province, Yanshan University, Qinhuangdao 066004, People’s Republic of China *Correspondence:[email protected]; [email protected]

Abstract The NiCo2 nanoparticles and NiCo2/carbon nanotubes (CNTs) nanohybrids were prepared by a single-model microwave heating and annealing treatment. It was found that the NiCo2 nanoparticles were evenly distributed around with CNTs. As a result, the complex permittivity of NiCo2/CNTs nanohybrids could be adjusted effectively, and achieved a well impedance matching for the microwave absorption. The dielectric loss, magnetic loss mechanism and the impedance matching were also investigated systematically. As a result, the reflection loss (RL) less than -20 dB (99% absorption) were observed at 3.7-16.5 GHz with the thickness of 1.2-4.0 mm; moreover, the maximum bandwidth of the RL values less than -10 dB (90% absorption) was 4.4 GHz (12.5-16.9 GHz), and a minimum RL value of -25.5 dB could be observed at 14.7 GHz with the thickness of 1.3 mm. These performances revealed that the NiCo2/10%CNTs nanohybrids have a great potential to be used as the lightweight microwave absorption material. Keywords: NiCo2; carbon nanotubes; complex permittivity; microwave absorption

1. Introduction Recently, the influence of electromagnetic interference phenomenon is huge in daily life, not only for the disturbance on electronic facilities but also for the hazards on human health [1-3]. In order to solve these problems, electromagnetic wave

absorption materials, especial in microwave range, have been studied intensively. Comparing with the electromagnetic wave shielding materials [4], microwave absorption materials can eliminate the microwave by converting the microwave energy into thermal energy or dissipating it through interferences. Based on the microwave loss mechanisms, microwave absorption materials are divided into two types: the magnetic loss materials, such as Fe, Ni, Co, FeNi3, FeCo, Fe3O4 [5-11], which consume the microwave energy by natural resonance and eddy current loss; and the dielectric loss materials, such as SiC, Al2O3, TiO2, LaMnO3, SiO2 [12-16], which attenuate the microwave energy by conductivity and polarization relaxation. To realize the high efficiency of microwave absorption, an appropriate combination of magnetic loss and dielectric loss materials is the preferential research topic in the field of microwave absorption. In the recent researches, carbon-based materials are studied for being highperformance microwave absorption composites [17-24]. Their excellent electrical conductivity property, large surface area and high aspect ratio are benefit for enhancing the dielectric loss and preventing the agglomeration. As one of the important members, carbon nanotubes (CNTs) is always the hot topic and the CNTs filled with magnetic nanoparticles have been investigated numerously due to their controllable complex permittivity, such as Fe3O4/CNTs, Fe/CNTs, FeNi/MWCNTs [25-27]. On the other hand, with the choiceness complex permeability, magnetic metals and magnetic metal oxide nanomaterials show great magnetic loss, such as Fe, Co, Co3O4 and CoFe2O4 [5,7,28,29]. As a member of magnetic materials, NiCo alloy has been demonstrated high magnetization and large complex permeability as microwave absorption material [30,31]. However, NiCo alloy has a high density that is an obstacle to fabricate the lightweight microwave absorber, and the nano-sized NiCo particles are not fully discussed. Therefore, the assembly of NiCo nanoparticles with weightless CNTs could be one of the solutions; moreover, compounding different components of CNTs are expected to have a tunable permittivity which may lead to an improvement microwave absorption performance. In this work, CNTs wrapped NiCo2 nanoparticles (NiCo2/CNTs nanohybrids)

were manufactured by single-mode microwave-assisted hydrothermal and annealing method. The NiCo2 nanoparticles uniformly dispersed with CNTs. The microwave absorption performance of NiCo2/CNTs nanohybrids were investigated systematically and it could be improved by changing the CNTs proportions due to the tunable permittivity. The exciting microwave absorption performance of NiCo2/CNTs nanohybrids provided an effective way to solve the electromagnetic interference problems.

2. Experiments 2.1. Sample Preparation The CNTs were purchased from Beijing DK nano technology Co., Ltd., China. The geometrical parameters are listed as follows: the length is 10-30 μm, the mean diameter is 10-20 nm，the number of walls is around 7-11 and the purity exceeds 98%. The SEM and TEM images of CNTs are shown in Figure S1 in the Supporting Information. CNTs were ultrasonic treated by the mixed acid of vitriol and nitric acid (3:1, Vol %) for 5 h to wipe off microimpurity. Then, the CNTs were washed by the deionized water and freeze-dried for 12 h. The precursors of CO(NH2)2 (120 mg), Co(NO3)2·6H2O (194 mg) and Ni(NO3)2·6H2O (97 mg) were added into 10 mL mixed solution (deionized water and anhydrous ethanol, 1:1, Vol %). Then, the treated CNTs (5 and 10 wt %) were dispersed in the solution, respectively. The mixed solution was heated to 95 °C by the single-model microwave heating and maintained for 24 h. The single-model microwave heating method can prevent the sample agglomeration effectively [30]. When the reaction finished, the NiCo2/CNTs precursors were obtained by centrifugation. After washing via deionized water and anhydrous ethanol, they were dried at 60 °C in a vacuum oven overnight. Finally, the NiCo2/CNTs precursors were annealed at 600 °C for 2 h under Ar/6%H2 atmosphere to obtain the NiCo2/CNTs nanohybrids (as shown in Fig. 1).

Fig. 1. Schematic illustration of fabricating the NiCo2/CNTs nanohybrids. 2.2. Characterization The crystal structures were examined by X-ray diffraction (XRD, Rigaku, Japan) using Cu Kα (λ= 1.5406 Å) radiation. Raman spectroscopy was tested by a Renishaw InVia Raman Microscope (λ = 532 nm) to detect the structural deformation. The morphologies and structures were characterized by a scanning electron microscope (SEM, Hitachi, Japan) and a transmission electron microscope (TEM, JEOL, Japan). Samples for microwave absorption tests were prepared by mixing paraffin with 50 wt% NiCo2/CNTs nanohybrids, and the mixtures were pressed into toroidal shape ( 𝜑𝑜𝑢𝑡 = 7.0 mm, 𝜑𝑖𝑛 = 3.04 mm). The scatting parameters (S11, S21) were measured under 1-18 GHz by a network analyzer (VNA, AV3629D), and the relative complex permittivity (𝜀𝑟 = 𝜀𝑟' ‒ 𝑗𝜀𝑟'') and complex permeability (𝜇𝑟 = 𝜇𝑟' ‒ 𝑗𝜇𝑟'') values could be determined with scatting parameters. Then, the reflection loss (RL) of the samples was calculated by using the complex permittivity and complex permeability.

3. Results and discussion The XRD patterns of NiCo2/CNTs nanohybrids with different CNTs proportions are show in Fig. 2a. Three diffraction peaks appear and corresponds to (111), (200) and (220) lattice planes of fcc Ni and Co, locating at 2θ = 44.3 °, 51.7 ° and 76.1 °, respectively. The sharp and strong diffraction peaks confirm the high crystallinity degree of NiCo2 nanoparticles. There is no carbon diffraction peak because the peak intensity of CNTs is too weak and covered up by the NiCo2 diffraction peaks. To

confirm the existence of CNTs, Raman spectra of composite materials were measured. Raman measurement is an efficiency tool to detect the degree of structural deformation of carbon-based materials. As shown in Fig. 2b, two characteristic peaks of carbon are detected in NiCo2/CNTs nanohybrids. One peak locates around 1340 cm-1 (D band) relating to the vibration of sp3 defects or lattice distortion; another peak locates around 1578 cm-1(G band) indicating the presence of a graphitic structure and the vibration of in-plane sp2 hybridization [32,33]. Moreover, the intensity ratio of two peaks (ID/IG) describes the disorder degrees and defects of carbon materials. From the Raman results, the ratios equal 1.24, 1.37 and 1.45 for the pure CNTs, NiCo2/10%CNTs and NiCo2/5%CNTs, respectively. It suggests that more defects and disorders are lead into the low CNTs proportion samples. The defects and disorders in CNTs can act as polarization relaxation under altering microwave field and are beneficial to absorb the microwave energy.

Fig. 2. (a) XRD patterns of NiCo2 nanoparticles and NiCo2/CNTs nanohybrids with different CNTs proportions; (b) Raman spectra of CNTs and NiCo2/CNTs nanohybrids with different CNTs proportions. The morphologies and sizes of the samples were studied by SEM and TEM measurement. Fig. 3a and 3b show the SEM and TEM images of NiCo2 nanoparticles, respectively. The NiCo2 nanoparticles show elliptical shape and the diameter of nanoparticles is about 100−200 nm. The SEM and TEM images of NiCo2/5%CNTs nanohybrids are show in Fig. 3d and 3e. The addition of CNTs reduces the size of

NiCo2 particles that have a diameter of about 10-100 nm, and the CNTs are separated by NiCo2 nanoparticles due to the low concentration. The ratio between large-sized and small-sized NiCo2 nanoparticles is almost the same. By increasing the proportion of CNTs, shown in Fig. 3g and 3h, the number of large-sized NiCo2 particles is reduced significantly and the CNTs connect to each other which are beneficial to increase the permittivity of nanohybrids. From these analyses, we can predict that the dielectric property of NiCo2/CNTs nanohybrids can be tuned by changing the CNTs proportions. In the HRTEM image as shown Fig. 3c, 3f and 3i, the interplanar spacing of NiCo2 nanoparticle is 0.22 nm corresponding to the (111) crystal plane, which in line with the XRD results in Fig. 2a. In addition, the elemental mapping and EDS analysis of the NiCo2 nanoparticles are provided in Fig. S2 and Table S1, respectively. These results confirm that the NiCo2 nanoparticles with the element ratio of 1:2 are successfully prepared.

Fig. 3. SEM images of (a) NiCo2 nanoparticles, (d) NiCo2/5%CNTs and (g) NiCo2/10%CNTs nanohybrids; TEM images of (b) NiCo2 nanoparticles, (e)

NiCo2/5%CNTs and (h) NiCo2/10%CNTs nanohybrids; The HRTEM images of (c) NiCo2 nanoparticles, (f) NiCo2/5%CNTs and (i) NiCo2/10%CNTs nanohybrids. Fig.4 shows the frequency dependence of complex permittivity (𝜀𝑟) and complex permeability (𝜇𝑟) of the samples. From Fig. 4a and 4b, the values of ε′ and ε″ for the pure NiCo2 nanoparticles are low (ε′ = 8.5 and ε″ = 0.8) and almost keep constant through 1-18 GHz range, which is attributed to its low conductivity. By increasing the CNTs proportions, the permittivity values increase due to the outstanding electrical conductivity property of CNTs. For the NiCo2/10% CNTs nanohybrids, the ε″ values are about 4 times larger than the one of pure NiCo2 nanoparticles, indicating a much enhanced dielectric loss ability. On the other hand, the permeability values are only determined by the contents of magnetic component. In this experiment, the contents of NiCo2 nanoparticles are theoretical constant in all samples. Therefore, the values of μ′ and μ″ are almost the same, shown in Fig. 4c and 4d. The little fluctuation may come from the experimental error during the fabrication process. There is a small peak around 4.5 GHz in the spectra of μ″ which can be attributed to the natural resonance of NiCo2 magnetic nanoparticles.

Fig. 4. Frequency dependence of (a) real part, (b) imaginary part of permittivity and (c) real part, (d) imaginary part of permeability of the NiCo2 nanoparticles and the NiCo2/CNTs nanohybrids with different CNTs proportions. The dielectric loss tangent (tan𝛿𝜀 = 𝜀𝑟''/𝜀𝑟' ) and magnetic loss tangent ( tan𝛿𝜇 = 𝜇𝑟'' /𝜇𝑟' ) of NiCo2 nanoparticles and NiCo2/CNTs nanohybrids with different CNTs proportions are shown in Fig. 5a and 5b. Clearly, the tan𝛿𝜀 of NiCo2/10% CNTs nanohybrids is 3 times larger than others, suggesting a stronger dielectric loss. The improvement of tan𝛿𝜀 can be contributed to the interfacial polarization effects and the dipole effects of NiCo2/CNTs nanohybrids, and also the defects and disorders in CNTs. The magnetic loss tangents are almost the same because of the similar results of the permeability. In general, the magnetic loss in microwave absorption is mainly due to the magnetic hysteresis loss, domain wall resonance, eddy-current loss, natural resonance and exchange resonance of the magnetic materials. The magnetic hysteresis loss in weak magnetic field can be neglected and the domain wall resonance occurs only in low frequency range (below 100 MHz). Hence, these two mechanisms cannot be the magnetic loss contributor in this study. The eddy-current loss of magnetic material can be expressed as 2

𝜇𝑟''(𝜇𝑟') ‒ 2𝑓 ‒ 1 = 3𝜋𝜇0𝑑2𝜎, (1) where 𝜇0 is the permeability of vacuum, d is the thickness that is less than the skin depth, and 𝜎 is the electric conductivity [34,35]. From Eq. 1, if the magnetic loss only comes from the eddy-current loss, the product of 𝜇𝑟''(𝜇𝑟') ‒ 2𝑓 ‒ 1 should be independent of frequency, which means constant in the measurement frequency range. The calculation results are shown in Fig. 5c. Take NiCo2 for example, the values decrease from 0.067 to 0.012 during 1-18 GHz. Therefore, the eddy-current loss can be precluded from the dominant magnetic loss mechanism in these samples. Based on the analyses, it can be suggested that the natural resonance and exchange resonance

should be the main contributor for the magnetic loss in this experiment. As an important factor to evaluate the microwave absorption property, attenuation constant α reflects the damping property of the material, and it can be expressed by α=

2𝜋𝑓 × 𝑐

2

(𝜇𝑟''𝜀𝑟'' ‒ 𝜇𝑟'𝜀𝑟') + (𝜇𝑟''𝜀𝑟'' ‒ 𝜇𝑟'𝜀𝑟')2 + (𝜇𝑟' 𝜀𝑟'' + 𝜇𝑟''𝜀𝑟') , (2)

where f is the microwave frequency, and c is the velocity of light [34]. Fig. 5d shows the attenuation constant of NiCo2 nanoparticles and NiCo2/CNTs nanohybrids with different CNTs proportions. In whole frequency range, NiCo2/10%CNTs nanohybrids have much larger values than other samples. Thus, it can be expected that the NiCo2/10%CNTs nanohybrids exhibit better microwave absorption performance than others.

Fig.5. Frequency dependence of the (a) dielectric loss tangent and (b) magnetic loss tangent, (c)

𝜇𝑟''(𝜇𝑟') ‒ 2𝑓 ‒ 1

values and (d) attenuation constants of NiCo2

nanoparticles and NiCo2/CNTs nanohybrids with different CNTs proportions. In order to show the microwave absorption properties of NiCo2 nanoparticles and NiCo2/CNTs nanohybrids, the reflection loss (RL (dB)) can be calculated by the

following equations from the transmission line theory [36-38]: 12

() RL = 20log | 𝑍𝑖𝑛 = 𝑍0

𝜇𝑟 𝜀𝑟

[ ( )(𝜇 𝜀 ) ], (3) |, (4)

tanh 𝑗 𝑍𝑖𝑛 ‒ 𝑍0

2𝜋𝑓𝑑 𝑐

𝑟 𝑟

12

𝑍𝑖𝑛 + 𝑍0

where Zin is the input impedance of microwave absorber, Z0 is the air impedance, 𝜇𝑟 is the complex permeability, 𝜀𝑟 is the complex permittivity, and d is the thickness of microwave absorber. The frequency (f), thickness (t) and reflection loss (RL), which are criteria to evaluate the microwave absorption material, are plotted in Fig. 6a-c using 2D color maps for the NiCo2 nanoparticles and NiCo2/CNTs nanohybrids. A typical value of RL = -20 dB corresponds to 99% energy consumption, which is our goal for the high performance microwave absorption material. For the pure NiCo2 nanoparticles, only 10 dB can be obtained in a wide range of thickness and frequency. By introducing a few of CNTs (5%), the microwave absorption performance is improved, but there are just some discrete points reaching -20 dB. Obviously, further increasing the proportion of CNTs, the absorption performance of NiCo2/10%CNTs nanohybrids is tremendously enhanced. In order to more intuitive display the superiority of the NiCo2/10%CNTs nanohybrids, the reflection loss of the samples under the thicknesses of 1.3 mm and 2.5 mm are shown in Fig. 6d. Both RL intensity and bandwidth of the NiCo2/10%CNTs nanohybrids are much better than other two samples. Then, the RL values of NiCo2/10%CNTs nanohybrids at different thicknesses varies with the frequency are as shown in Fig. 6e. The RL less than -20 dB are observed at 3.7-16.5 GHz with the thickness of 1.2-4.0 mm; moreover, the maximum bandwidth of the RL values less than -10 dB is 4.4 GHz (12.5-16.9 GHz) and a minimum RL value of -25.5 dB can be observed at 14.7 GHz with the thickness of 1.3 mm. Furthermore, the relationship between the minimum RL peak frequency and the thickness can be explained by the quarter-wavelength (𝜆 4) matching microwave absorption mechanism [39] that is shown in Fig. S3. At last, the microwave absorption properties of some CNTs-based composites

are listed in Table 1. Compared with the listed CNTs-based materials, NiCo2/10%CNTs nanohybrids not only have a high reflection loss performance but also have a wide bandwidth under thinner thickness. Therefore, it is believed that the NiCo2/CNTs nanohybrids can be applied with high efficiency, board frequency range and light weight for the microwave absorption.

Fig.6. 2D color map of RL values of (a) NiCo2 nanoparticles, (b) NiCo2/5%CNTs nanohybrids and (c) NiCo2/10%CNTs nanohybrids; (d) the reflection loss of the samples under the thicknesses of 1.3 mm and 2.5 mm; (e) the frequency dependence of RL at various thicknesses. Table 1. Microwave absorption properties of some CNTs-based composites

Material

Ratio

Thickness

(wt %)

(mm)

Matrix

RL˂-10 dB bandwidth (GHz)

RLmin

ref

(dB)

Fe3O4/5wt%CNTs

wax

50

4.4

3.9

-51.3

11

CNT@Fe@SiO2

wax

50

2.3

3.5

-22.3

40

Fe-filled CNTs

epoxy

50

1.0

2.9

−31.71

41

H-Fe3O4/Ppy/CNT

epoxy

20

3.0

4.5

-25.9

42

wax

20

1.9

3.2

-54.43

43

1

2.5

4.8

-20.5

44

50

1.3

4.4

-25.5

CNTs/Fe3O4/rGO/C

PS-grafted MWCNTs SiO2 NiCo2/10%CNTs

wax

This work

4. Conclusions In conclusion, NiCo2 nanoparticles and NiCo2/CNTs nanohybrids were successfully prepared by the single-model microwave heating and annealing treatment. It was found that the NiCo2 nanoparticles were uniformly dispersed with the CNTs. And the CNTs connected each other for high concentration which were beneficial to increase the permittivity of nanohybrids. From the electromagnetic parameters analysis, the dielectric loss was much enhanced in NiCo2/10%CNTs nanohybrids due to the interfacial polarization effects and dipole effects of the nanohybrids. The magnetic loss was mainly attributed to the natural resonance and exchange resonance. As a result, the complex permittivity of NiCo2/CNTs nanohybrids could be adjusted effectively and achieved a well impedance matching for the microwave absorption. The reflection loss (RL) less than -20 dB (99% absorption) were observed at 3.7-16.5 GHz with the thickness of 1.2-4.0 mm; moreover, the maximum bandwidth of the RL values less than -10 dB (90% absorption) was 4.4 GHz (12.5-16.9 GHz), and a minimum RL value of -25.5 dB could be observed at 14.7 GHz for the thickness of 1.3 mm. Through the numerical calculation, it could be confirmed that the minimum RL peak frequency obey the quarter-wavelength (𝜆 4) matching microwave

absorption mechanism. These results revealed that the NiCo2/10%CNTs nanohybrids have an excellent performance as the lightweight microwave absorbing material.

Acknowledgments This work is supported by the National Natural Science Foundation of China (Grant No. 51672240, 51571172, 51571171), Natural Science Foundation for Distinguished Young Scholars of Hebei Province (Grant No. E2017203095), Natural Science Foundation for Excellent Young Scholars of Hebei Province (Grant No. E2018203380).

Appendix A. Supplementary data Supplementary data related to this article can be found at the website.

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Highlight 1. NiCo2/CNTs nanohybrids were prepared by single-model microwave heating and annealing treatment. 2. The NiCo2 nanoparticles were uniformly dispersed with the CNTs. 3. The complex permittivity of NiCo2/CNTs nanohybrids could be adjusted effectively 4. The reflection loss less than -20 dB were observed at 3.7-16.5 GHz with the thickness of 1.2-4.0 mm 5. A minimum RL value of -25.5 dB could be observed at 14.7 GHz for the thickness of 1.3 mm.