Electromagnetic Property and Tunable Microwave Absorption of 3D

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Electromagnetic Property and Tunable Microwave Absorption of 3D Nets from Nickel Chains at Elevated Temperature Jia Liu, Maosheng Cao, Qiang Luo, Honglong Shi, Wenzhong Wang, and Jie Yuan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b05480 • Publication Date (Web): 10 Aug 2016 Downloaded from http://pubs.acs.org on August 11, 2016

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ACS Applied Materials & Interfaces

Electromagnetic Property and Tunable Microwave Absorption of 3D Nets from Nickel Chains at Elevated Temperature Jia Liu†, Mao-Sheng Cao*, †, Qiang Luo†, Hong-Long Shi‡, Wen-Zhong Wang‡, Jie Yuan‡ †

School of Material Science and Engineering, Beijing Institute of Technology, Beijing 100081, China

‡

School of Science, Minzu University of China, Beijing 100081, China

ABSTRACT We fabricated the nickel chains by a facile wet chemical method. The morphology of nickel chains were tailored by adjusting the amount of PVP during the synthesis process. Both the complex permittivity and permeability of the three-dimensional (3D) nets constructed by nickel chains present strong dependences on temperature in the frequency range of 8.2-12.4 GHz and temperature range of 323-573 K. The peaks in imaginary component of permittivity and permeability mainly derive from interfacial polarizations and resonances, devoting to dielectric and magnetic loss, respectively. The effect from both dielectric and magnetism contribute to enhancing the microwave absorption. The maximum absorption value of the 3D nickel chain nets is ~ -50 dB at 8.8 GHz and 373 K with a thickness of 1.8 mm, and the bandwidth less than -10 dB almost covers the whole investigated frequency band. These are encouraging findings, which provide the potential advantages of magnetic transition metal-based materials for microwave absorption application at elevated temperature.

KEYWORDS: nickel chains, electromagnetic property, polarization, resonance, microwave absorption

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INTRODUCTION Microwave absorbers have aroused intensive interest for their applications in various fields, including electric safety, information counterwork, environment defense and health care.1-6 In recent years, plenty of materials are researched as microwave absorbers, such as carbon nanomaterials7-16, zinc oxide17-22, silicon carbide23-26, ferrite27-30 etc. They have been investigated extensively for electromagnetic characteristics and microwave absorption, which display potential applications in the field of microwave device and absorber. Furthermore, microwave absorbers are sought after with light weight and high efficiency to meet the requirements of service objects. According to recent observations, transition metal magnetic materials, including Fe, Co, Ni and their alloy & compounds, are drawing researchers’ particular attention for applying in microwave absorption.31-46 Cobalt nanochains were fabricated and presented dual dielectric resonance and two strong absorption peaks.47-49 Iron submicron cubes show good electromagnetic wave absorbing characteristics and their minimum reflection loss values are less than -20 dB.50 CoO 3D nano-flowers exhibit significantly enhanced microwave absorption properties.51 However, practical microwave absorbers require light weight, high efficiency, and especially, stability in harsh-thermal environment. Unfortunately, the temperature dependences of electromagnetic characteristics and microwave absorption are reported rarely. Investigating the electromagnetic property and tuning the absorption capacity at elevated temperature are still a challenge. In this work, 3D nets constructed by nickel chains are fabricated to characterize the electromagnetic properties in the temperature range of 323-573 K and in X band (8.2-12.4 GHz). The complex permittivity (ε', ε") and permeability (µ', µ") presents obvious temperature dependences. The peaks of ε" drive from interfacial polarizations in nickel chains. Meanwhile the peaks of µ" come from natural resonance. The maximum microwave absorption value of


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the 3D chain nets reaches up to -49 dB. In contrast to other magnetic materials investigated at elevated temperature, such as substituted ferrites,52-54 carbonyl iron,55 Iron-Alumina,56 Fe3O4@ZrO2,57 and Fe3O4-MWCNTs,4 the 3D nickel chain nets possess superior absorption capacity. Hence, these results indicate that the 3D nickel chain nets are promising absorbers and can provide a strategy for designing excellent dielectric-magnetic absorbers applied at elevated temperature. EXPERIMENTAL SECTION Materials Nickel chloride hexahydrate(NiCl2·6H2O), hydrazine monohydrate (N2H4·H2O), and polyvinyl pyrrolidone (PVP) were purchased from Fuchen Chemical Reagent Co., Ltd. (Tianjin, China). Ethylene glycol (EG) was purchased from Beijing Chemical Factory (Beijing, China). All the chemicals were analytical reagent grade and used without further purication. Fabrication of nickel chains In a typical procedure, 0.19g NiCl2·6H2O and 0.444g PVP were added to 100ml EG with intensive stirring for 1h to obtain a transparent grass green solution. Then, 0.8ml N2H4·H2O was added to the as prepared solution dropwise, with continuing stirring for another 1h. Subsequently, the mixture was transferred to a three-necked flask and refluxed at 197 ℃ for 2h. After refluxing, the obtained precipitate was washed with deionized water and ethanol for several times, and then dried at 313 K in vacuum oven for 12h. In order to investigate the effect of PVP on the morphology of products, the ratio of PVP to nickel precursors is varied. Different weights of PVP, 0.0888g, 0.2644g, 0.444g, were added to the reaction solution, respectively. The three kinds of obtained nickel samples were marked as S1, S2, and S3. Samples The as-prepared nickel chains powder (30, 40, 50wt% loading) was mixed with SiO2



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nanopowder, respectively. A certain amount of mixture powder was cold-pressed into a rectangular sample with dimensions of 22.86 mm×10.16 mm× ~1.6 mm for electromagnetism measurement. After dispersing in SiO2 nanopowder, the 3D nickel chain nets are constructed. Characterization and measurement X-ray diffraction (XRD) characterization was carried out on an XPert PRO system (Cu-Kα). Magnetic properties were measured by a vibrating sample magnetometer (VSM Lakeshore 7407). The morphology was carried out on a scanning electron microscope system (SEM Hitachi S4800). Transmission electron microscopy (TEM) images were obtained on a JEM-2100 system, coupled with copper grids. Electromagnetic parameter measurement The complex permittivity and permeability were measured using wave-guide method at six temperatures between 323 K and 573 K in X band (8.2–12.4 GHz) on a vector network analyzer (VNA Anritsu 37269D) coupled with a high-temperature testing chamber, as reported in our previous work.58, 59 The as-prepared sample sheet was positioned vertically in the center of the testing chamber and heated by an inner heater at the heating rate of 20 K/min. In order to ensure accuracy of measurement, the system holds for 20 minutes when temperature reaches the set point. After the complex permittivity and permeability obtained at this temperature, the next set point was followed. RESULTS AND DISCUSSION The SEM images of as-prepared nickel samples are shown in Figure 1. The sample S1 consists of nickel spheres, which aggregate together disorderly (Figure 1a,b). For the sample S2, there are a few of nickel chains and plenty of aggregated spheres (Figure 1c,d). Whereas the SEM images of sample S3 demonstrate that the nickel spheres contacting with each other orderly form the chain-like morphology with the length of tens of micrometers (Figure 1e,f). Compared with sample S1 and S2, the nickel spheres in sample S3 have smoother surfaces


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and the nickel chains are longer and more uniform. The results indicate that the amount of PVP has tailored effect on the formation of nickel chains. With a suitable molar ratio of PVP: Ni2+ = 5:1, the uniform and long nickel chains were fabricated, which are further demonstrated in Figure 2a,b. As shown in Figure 2c, the size distribution of nickel spheres in Figure 2b is counted, which is relatively concentrated with mean size of ~245 nm. The mean size of joints between nickel spheres shown in Figure 2d is about 130 nm. Figure 3 is the probable formation schematic illustration for nickel chains and 3D chain nets. In the beginning, the Ni2+ reacted with hydrazine to form a stable complex at ambient temperature. With the increase of temperature to the boiling point of EG, the Ni2+ was reduced by the excessive hydrazine to form nickel nucleus. Due to the attractive effect from magnetic dipole-dipole and template effect of PVP, the small nuclei aggregated to form large nanoparticles and grew further. Subsequently, under the minimization of interfacial energy and magnetic dipole-dipole attraction, large nickel nanoparticles assembled into chains.47 The nickel chains then cross linked with each other to form nets, further aggregating to 3D nickel chain nets. The TEM images of nickel chains in sample S3 under different magnifications are presented in Figure 4a,b. The microspheres contact to each other tightly and their diameters are about 200-300 nm. As shown in Figure 4c, the lattice spacing of nickel chains is about 0.18 nm, which corresponds to (200) crystal plane of cubic nickel. Their SAED pattern in Figure 4d exhibits that three rings can be assigned to the crystal planes of cubic phase of nickel. Also, the structure of the as-prepared sample is identified by XRD pattern in Figure 4e, all the diffraction peaks can be indexed to cubic structure of nickel, and no impurity phase is detected. Figure 4f is the diffraction profile generated by the SAED pattern in Figure 4d. These results appear to be consistent with the XRD result, further indicate the cubic structure of nickel chains.



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As shown in Figure 5, the static magnetic property of nickel chains is characterized with hysteresis loop, which exhibits typical ferromagnetic hysteresis curves. The inset indicates that nickel chains dispersed in deionized water can be separated by external magnetic field in 10 seconds. The saturation magnetization (Ms) of nickel chains is 42 emu/g, which is lower than that of bulk nickel (ca. 55.15 emu/g). Generally, the variation of Ms is related to surface antiferromagnetic oxidation, specific morphology, nanoeffects, crystallinity, surface spin disorder, etc.47, 60 In this work, the decreased Ms may originate from the specific chain-like structure and surface spin disorder of the nanoscale material. The remanent magnetization (Mr) of nickel chains is 6.7 emu/g, higher than those of bulk nickel (2.7 emu/g) and nickel nanoparticles (5.0 emu/g). This may originate from the differences in microstructure. Meanwhile, the coercivity (Hc) of nickel chains is 120 Oe, which is significantly higher than those of bulk nickel (100 Oe) and nickel nanoparticles (40 Oe).61 The coercivity of magnetic materials generally derived from various kinds of anisotropy such as, crystal, stress, externally induced, shape. Herein, the higher Hc of nickel chains can likely be attributed to the reduced size and the chain-like structure.60 The electromagnetic properties of the samples loading with 30wt %, 40wt % and 50wt % 3D nickel chain nets were investigated in the frequency range of 8.2-12.4 GHz and temperature range of 323-573 K. The ε', ε", µ', and µ" depending on frequency and temperature are illustrated in Figure 6 and 7, respectively. Note that both the ε' and ε" display a decrease trend with the increase of frequency, and monotonically increase with the increase of chain nets loading contents, changing from 6.1 to 21.2 and 0.2 to 5.9, respectively (Figure 6a,c,e). Furthermore, the relaxation peaks can be observed in ε" from different chain nets loadings. This phenomenon indicates the existence of relaxation behaviors in nickel chains, which can be proved by the Cole-Cole plots in the Supporting Information (Figures S1-S3). These relaxations are probably derived from the interfacial polarizations in nickel chains, as


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schematically shown in Figure 8. Typically, the activation energy of dipole decreases with the increase of temperature, resulting in the weakening of relaxation. In Figure 6b,d,f, the µ' and µ" decrease with increasing frequency, and increase slightly with increasing loading of chain nets. Meanwhile, multiple resonance peaks are observed in µ" of all the samples, indicating the existence of ferromagnetic resonance behavior. Figure 7 displays the temperature dependences of ε', ε", µ', and µ" in the investigated temperature range of 323-573 K. Significantly, both the ε' and ε" of the three samples present a rapid decrease in 323-373 K and a slight decrease in 373-573 K. This decrease of ε" in the temperature range results from the decreasing conductivity of nickel chain nets, which is based on the Debye theory. Meanwhile, the µ" of all the samples decrease with the increase of temperature, which can be attributed to the weakened damping of magnetic moment precessing.54 In general, the magnetic loss mainly comes from magnetic hysteresis, domain wall resonance, eddy current loss, natural resonance and exchange resonance.62 In microwave frequency band, only eddy current loss, natural resonance and exchange resonance can be considered.60,63 The eddy current coefficient (µ"(µ')-2f -1) of 3D nickel chain nets depending on frequency can be clearly shown in Supporting Information (Figure S4). As we known, if magnetic loss only originates from eddy current loss, the value of µ"(µ')-2f -1 is constant when frequency changes.64 Thus, Figure S4 demonstrates that the magnetic loss in chain nets is not only contributed from eddy current loss, which is schematically illustrated in Figure 8. In addition, the natural resonance in bulk nickel usually occurs at lower frequency around megahertz. However, with the decrease of particle size, the resonance frequency shifts towards higher frequency due to the high surface anisotropy affected by the small size effect.64-66 Hence, the resonance behavior in nickel chains is probably derived from natural resonance, namely the magnetization vectors (M) precess around the direction of the effective



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field,28 as schematically shown in Figure 8. In addition, the nickel chains dispersing in waveguide sample joint with each other and form 3D nets as shown in Figure 8. In the 3D nets, the distributed micro-current is generated when electromagnetic wave travel into the sample,17,67 which also contribute to microwave absorption. Furthermore, the 3D nickel chain nets increase the travel paths of electromagnetic wave in the sample, inducing the formation of multiple internal reflections, as well as the multiple scattering. Hence, they significantly enhance attenuation capacity, and then benefit microwave absorption performance.68 Typically, the eddy current loss, natural resonance, multiple internal reflections, multiple scattering, and dielectric relaxations contribute together to enhance the attenuation capacity of 3D nickel chain nets. The attenuation constant α of the three samples with 30wt%, 40wt% and 50wt% chain nets loadings is exhibited in Figure S5, which determines the attenuation properties of materials.20,65 With the increase of loading, the values of α clearly increases. The 50wt% loading sample presents the biggest attenuation constant α in the investigated frequency band, which confirms its superior absorption performance. The reflection loss (RL) of the samples loading with 30wt %, 40wt % and 50wt % 3D nickel chain nets is presented in Figure 9, which is calculated based on transmit line theory using the experimentally determined complex permittivity and permeability.4, 58 Figure 9a-c shows the RL values versus frequency at different temperatures. Obviously, the 50wt % sample demonstrates the maximum absorption of -49 dB at 8.8 GHz and 373 K with a thickness of 1.8 mm, which is superior to those of 30wt % and 40wt % loading samples. Figure 9d-f shows the RL values of three samples versus temperature at different frequencies. With the increase of temperature, the microwave absorption capacities of samples with 30wt % and 40wt % loadings demonstrate a decrease trend, while the 50wt % loading sample presents the best absorption capacity at 373 K.


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Moreover, thickness matching is also a factor for excellent microwave absorption. The contour plots of RL versus frequency and thickness for the samples with 30wt %, 40wt % and 50wt % chain nets loadings are shown in Supporting Information (Figure S6 a-c). The corresponding frequency dependences of quarter-wavelength (λ/4) which calculated using complex permittivity and permeability are shown in Supporting Information (Figure S6 d-f). Note that the regions with maximum absorption move towards lower frequency with the increase of thickness. The best absorption regions of the three samples all correspond to λ/4 thickness, conforming to quarter-wavelength matching model.65, 69 Figure 10a-c is the 3D plots of RL values versus frequency and thickness for the samples with 30wt %, 40wt % and 50wt % chain nets loadings. Compared with 30wt % and 40wt % loading samples, the 50wt % loading sample has the best microwave absorption performance with a thinnest thickness. The RL value can reach ~ -50 dB by tuning temperature, and the absorption region (RL≤-10 dB) is expanded obviously. To evaluate the absorption capacity of the three samples at elevated temperature, Figure 10d demonstrates the 3D bar plot of their maximum absorption in the temperature range of 323-573 K and X band with the thickness range of 1-5mm. It is observed that the maximum absorption of 50wt % loading sample is much better than others, especially at 373 K. These results reveal the 3D nickel chain nets have a promising microwave absorption application at elevated temperature.

CONCLUSIONS In summary, the nickel chains are fabricated by a facile wet chemical method, and the 3D nets are constructed by dispersing nickel chains in sample. It was found that their electromagnetic characteristics and microwave absorption properties show strong temperature dependences in 8.2-12.4 GHz and 323-573 K. The sample with 50wt % nickel chain nets presents best microwave absorption capacity of ~-50dB even at relatively high temperature of 373 K. This



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enhanced absorption property is related to both dielectric and magnetic loss, which are benefited from interfacial polarizations and resonances. Significantly, the excellent microwave absorption performance demonstrates that the 3D nickel chain nets provide a promising route to design microwave absorbers for many application fields in thermal-harsh environment.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation of China (Grants 51132002 and 51372282)

Supporting Information The Cole-Cole plots of the samples with 30wt% , 40wt% and 50wt% 3D nickel chain nets; frequency dependence of µ"(µ')-2f -1 and attenuation constant for samples with 30wt%, 40wt% and 50wt% 3D nickel chain nets; the contour plots of reflection loss versus frequency and thickness; the calculated λ/4 thickness versus frequency for 3D nickel chain nets samples.


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Figure captions Figure 1. SEM images of (a, b) sample S1; (c, d) sample S2; (e, f) sample S3 under different magnifications. Figure 2. (a, b) SEM images of nickel chains in sample S3; (c) diameter size distributions of nickel spheres in (b); (d) joint size distributions between nickel spheres in (b); Figure 3. Schematic illustration of the formation of nickel chains and 3D nickel nets. Figure 4. (a, b) TEM images of nickel chains under different magnifications; (c) HRTEM image of nickel chain; (d) SAED pattern of nickel chain; (e) XRD pattern of nickel chain; (f) diffraction profile generated by SAED pattern in (d). Figure 5. Hysteresis loop of nickel chain; left bottom inset is the experiment for magnetism of nickel chain; right bottom inset magnified from red dashed circle. Figure 6. Complex permittivity and permeability at different temperatures of samples with (a, b) 30wt%, (c, d) 40wt%, and (e, f) 50wt% 3D nickel chain nets loadings versus frequency. Figure 7. Complex permittivity and permeability at different frequencies of samples with (a, b) 30wt%, (c, d) 40wt%, and (e, f) 50wt% 3D nickel chain nets loadings versus temperature. Figure 8. Schematic illustration of micro-current, micro eddy current, natural resonance, dielectric polarizations and microwave propagation in 3D nickel chain nets. Figure 9. Reflection loss of samples with 30wt%, 40wt% and 50wt% 3D nickel chain nets loadings versus (a-c) frequency and (d-f) temperature. Figure 10. (a-c) The 3D plots of reflection loss versus frequency and thickness for samples with 30wt%, 40wt% and 50wt% 3D nickel chain nets loadings, respectively; (d) the 3D plot of minimum reflection loss evaluation for all the samples.


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Graphic Abstract 75x40mm (300 x 300 DPI)



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Figure 1. SEM images of (a, b) sample S1; (c, d) sample S2; (e, f) sample S3 under different magnifications. 79x104mm (300 x 300 DPI)


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Figure 2. (a, b) SEM images of nickel chains in sample S3; (c) diameter size distributions of nickel spheres in (b); (d) joint size distributions between nickel spheres in (b). 85x68mm (300 x 300 DPI)



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Figure 3. Schematic illustration of the formation of nickel chains and 3D nickel nets. 83x111mm (300 x 300 DPI)


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Figure 4. (a, b) TEM images of nickel chains under different magnifications; (c) HRTEM image of nickel chain; (d) SAED pattern of nickel chain; (e) XRD pattern of nickel chain; (f) diffraction profile generated by SAED pattern in (d). 81x120mm (300 x 300 DPI)



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Figure 5. Hysteresis loop of nickel chain; left bottom inset is the experiment for magnetism of nickel chain; right bottom inset magnified from red dashed circle. 85x70mm (300 x 300 DPI)


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Figure 6. Complex permittivity and permeability at different temperatures of samples with (a, b) 30wt%, (c, d) 40wt%, and (e, f) 50wt% 3D nickel chain nets loadings versus frequency. 85x105mm (300 x 300 DPI)



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Figure 7. Complex permittivity and permeability at different frequencies of samples with (a, b) 30wt%, (c, d) 40wt%, and (e, f) 50wt% 3D nickel chain nets loadings versus temperature. 85x107mm (300 x 300 DPI)


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Figure 8. Schematic illustration of micro-current, micro eddy current, natural resonance, dielectric polarizations and microwave propagation in 3D nickel chain nets. 129x80mm (300 x 300 DPI)



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Figure 9. Reflection loss of samples with 30wt%, 40wt% and 50wt% 3D nickel chain nets loadings versus (a-c) frequency and (d-f) temperature. 129x75mm (300 x 300 DPI)


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Figure 10. (a-c) The 3D plots of reflection loss versus frequency and thickness for samples with 30wt%, 40wt% and 50wt% 3D nickel chain nets loadings, respectively; (d) the 3D plot of minimum reflection loss evaluation for all the samples. 85x70mm (300 x 300 DPI)


Electromagnetic Property and Tunable Microwave Absorption of 3D

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