*3.2.2 EM properties of urchin-like ZnS/Ni3S2@Ni composites*

With the purpose of revealing the electromagnetic wave absorption properties, the representation RL values of the core-shell ZnS/Ni3S2@Ni paraffin-composites with different sample thicknesses are simulated. **Figure 14** reveals the RL values of the core-shell ZnS/Ni3S2@Ni composites prepared at different temperatures with thickness varies from 0.8 to 2.5 mm in the frequency range of 1–18 GHz. It can be found that urchin-like ZnS/Ni3S2@Ni composite exhibits outstanding electromagnetic absorption. The minimal RL is down to −27.6 dB at 5.2 GHz as the thickness is 2.5 mm. Notably, the reflection loss of urchin-like ZnS/Ni3S2@Ni is −21.6 dB at 13.3 GHz with the absorber thickness of 1.0 mm, and the valuable bandwidth (RL below −10 dB) could reach 2.5 GHz (12.2–14.7 GHz), which is better than those of the literatures about microwave absorption performances of dielectric/magnetic composites, such as Fe@SnO2 (>−10 dB) [63], and Co/CoO(−14.5 dB) [64]. Meanwhile, the absorption peaks would move to lower frequencies and dual RL peaks could be obtained with an increased absorber thickness above 2.5 mm. This phenomenon could be described by the quarter-wavelength cancelation model [65].

According to above results, we propose a possible electromagnetic wave absorption mechanism for the core-shell ZnS/Ni3S2@Ni heterogeneous system When the

**167**

netic absorption [72].

**4. Conclusion**

**Figure 14.**

*(permission from RSC).*

*Electromagnetic Wave Absorption Properties of Core-Shell Ni-Based Composites*

ZnS/Ni3S2@Ni composite is subjected to EM wave radiation, the Ni3S2 thorns grown on the surfaces of Ni microspheres are expected as antenna receiver to allow electromagnetic waves penetrate into interior of absorber as much as possible, namely called good impedance match [66, 67]. Moreover, EM absorption is also understood from the viewpoint of Ohmic heating induced by an alternating magnetic field [68], in which ZnS/Ni3S2@Ni composite with relatively high electric conductivity. Thirdly, due to the heterogeneous systems of core-shell ZnS/Ni3S2@Ni composites, the multi-interfaces between the ZnS, Ni3S2, and Ni are favorable for improvement of electromagnetic wave absorption thanks to the interaction between electromagnetic radiation and charged multipoles at the interfaces [69]. The interfacial polarizations [42, 70] occurring at the interfaces are resulted from the migration of charge carriers through different conductivity properties of the composite material. During the irradiated by alternating electromagnetic field, an additional interfacial relaxation is produced, which is beneficial for the electromagnetic absorption [71]. On the other hand, the cooperation effect between dielectric loss at high frequency and magnetic loss at low frequency contribute to the improvement of electromag-

*The calculated RL of core-shell ZnS/Ni3S2@Ni composites prepared at various temperatures: (a) 60°C, (b) 80°C, (c) 100°C, and (d) 120°C with different thickness in the frequency range of 1–18 GHz [61]* 

Various morphologies of Ni/ZnO composites are successfully fabricated by a hydrothermal method. The shapes of ZnO nanostructures could be effectively monitored by tuning the contents of NH3·H2O. One can find that the morphology of ZnO plays an important role on the microwave absorption capabilities. The coreshell structural Ni/polyhedron ZnO presents relatively high dielectric loss, magnetic loss and microwave dissipation abilities compared with the other Ni/ZnO samples.

*DOI: http://dx.doi.org/10.5772/intechopen.82301*

*Electromagnetic Wave Absorption Properties of Core-Shell Ni-Based Composites DOI: http://dx.doi.org/10.5772/intechopen.82301*

**Figure 14.**

*Electromagnetic Materials and Devices*

microspheres were coated by wrinkle ZnS products. Ni3S2 nanoparticles are insitu formed by depletion of Ni particles and then covered on the surface of left Ni products. With elevating the reaction temperatures to high temperatures (80°C), one can notice that some protuberant prickles were grown on the surfaces of core-shell composites (**Figure 13b**). When the reaction temperature is increased to 100°C, the presence of more and strong protuberant stabs on the crumple surfaces of composite can be obviously observed (**Figure 13c**). With further enhancing the temperature to 120°C, the target urchin-like core-shell structural ZnS/Ni3S2@Ni composites are formed with numerous of thorns or rods grown on the rugged surfaces (**Figure 13d**).

*FESEM images of core-shell ZnS/Ni3S2@Ni composites prepared at various reaction temperatures: (a) 60°C,* 

With the purpose of revealing the electromagnetic wave absorption properties, the representation RL values of the core-shell ZnS/Ni3S2@Ni paraffin-composites with different sample thicknesses are simulated. **Figure 14** reveals the RL values of the core-shell ZnS/Ni3S2@Ni composites prepared at different temperatures with thickness varies from 0.8 to 2.5 mm in the frequency range of 1–18 GHz. It can be found that urchin-like ZnS/Ni3S2@Ni composite exhibits outstanding electromagnetic absorption. The minimal RL is down to −27.6 dB at 5.2 GHz as the thickness is 2.5 mm. Notably, the reflection loss of urchin-like ZnS/Ni3S2@Ni is −21.6 dB at 13.3 GHz with the absorber thickness of 1.0 mm, and the valuable bandwidth (RL below −10 dB) could reach 2.5 GHz (12.2–14.7 GHz), which is better than those of the literatures about microwave absorption performances of dielectric/magnetic composites, such as Fe@SnO2 (>−10 dB) [63], and Co/CoO(−14.5 dB) [64]. Meanwhile, the absorption peaks would move to lower frequencies and dual RL peaks could be obtained with an increased absorber thickness above 2.5 mm. This phenomenon could be described by

According to above results, we propose a possible electromagnetic wave absorption mechanism for the core-shell ZnS/Ni3S2@Ni heterogeneous system When the

*3.2.2 EM properties of urchin-like ZnS/Ni3S2@Ni composites*

*(b) 80°C, (c) 100°C, and (d) 120°C [61] (permission from RSC).*

the quarter-wavelength cancelation model [65].

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**Figure 13.**

*The calculated RL of core-shell ZnS/Ni3S2@Ni composites prepared at various temperatures: (a) 60°C, (b) 80°C, (c) 100°C, and (d) 120°C with different thickness in the frequency range of 1–18 GHz [61] (permission from RSC).*

ZnS/Ni3S2@Ni composite is subjected to EM wave radiation, the Ni3S2 thorns grown on the surfaces of Ni microspheres are expected as antenna receiver to allow electromagnetic waves penetrate into interior of absorber as much as possible, namely called good impedance match [66, 67]. Moreover, EM absorption is also understood from the viewpoint of Ohmic heating induced by an alternating magnetic field [68], in which ZnS/Ni3S2@Ni composite with relatively high electric conductivity. Thirdly, due to the heterogeneous systems of core-shell ZnS/Ni3S2@Ni composites, the multi-interfaces between the ZnS, Ni3S2, and Ni are favorable for improvement of electromagnetic wave absorption thanks to the interaction between electromagnetic radiation and charged multipoles at the interfaces [69]. The interfacial polarizations [42, 70] occurring at the interfaces are resulted from the migration of charge carriers through different conductivity properties of the composite material. During the irradiated by alternating electromagnetic field, an additional interfacial relaxation is produced, which is beneficial for the electromagnetic absorption [71]. On the other hand, the cooperation effect between dielectric loss at high frequency and magnetic loss at low frequency contribute to the improvement of electromagnetic absorption [72].

#### **4. Conclusion**

Various morphologies of Ni/ZnO composites are successfully fabricated by a hydrothermal method. The shapes of ZnO nanostructures could be effectively monitored by tuning the contents of NH3·H2O. One can find that the morphology of ZnO plays an important role on the microwave absorption capabilities. The coreshell structural Ni/polyhedron ZnO presents relatively high dielectric loss, magnetic loss and microwave dissipation abilities compared with the other Ni/ZnO samples.

The minimal RL could reach −48.6 dB at 13.4 GHz in the absorber thickness of 2.0 mm. The absorption band with RL below −10 dB could reach 6.0 GHz between 10.5 and 16.5 GHz. The enhanced microwave dissipation abilities are attributed to the synergetic effect between dielectric loss and magnetic loss, strong dissipation ability, as well as the multiple polarization of the core/shell interfaces.

The hierarchical Ni-CuO heterostructures have been successfully synthesized by a two-step process. The as-prepared Ni-CuO products display a rice-like coating composite. Moreover, by tuning the molar ratio of CuCl2·2H2O to the Ni microspheres, different shapes and coverage densities of CuO coating are obtained. The effects of CuO amounts on the Ni microspheres for microwave absorption properties have been investigated. The thin CuO-coated Ni composites (S-1) exhibit the enhanced electromagnetic absorption properties. The optimal RL is −62.2 dB at 13.8 GHz with only thickness of 1.7 mm. The outstanding microwave absorption properties result from the strongest attenuation constant, interfacial polarization of and the synergetic effect between the dielectric loss and magnetic loss.

ZnS nanowall-covered Ni composite is fabricated via a hydrothermal template method. The as-obtained Ni/ZnS composites display the crumble and rough features and the thickness of ZnS nanowall is about 10 nm. In comparison with raw Ni and ZnS particles, the Ni@ZnS composites show superior microwave dissipation abilities. The optimal RL of −25.78 dB could be obtained at 14.24 GHz and the valuable (less than −10 dB) band could reach 4.72 GHz (11.52–16.24 GHz) in the thickness of 2.7 mm. Moreover, the location of absorption peaks is almost similar at various thicknesses without moving to low frequency, which originates from natural resonance in permittivity.

Novel and interesting urchin-like ZnS/Ni3S2@Ni composites are synthesized through a two-step process including solution reduction and subsequently a template method. Crucially, the morphologies of the core-shell ZnS/Ni3S2@Ni composites are determined by the reaction temperature. Different ZnS/Ni3S2@ Ni composites prepared at different temperatures show variable electromagnetic absorption responses, for which the urchin-like ZnS/Ni3S2@Ni obtained at 120°C show the enhanced EM absorption properties thanks to its promising dielectric loss and magnetic loss, good impedance match, as well as its unique urchin-like structure. Multiple dielectric resonances stemming from effective accumulation of different polarizations in the urchin-like structure are regarded to make a contribution to the enhancement of electromagnetic wave absorption. It is believed that *in situ* synthesis of core-shell ZnS/Ni3S2@Ni composites may open up a new avenue for the design and preparation of novel microwave absorbers with promising application potential.

In a word, the core-shell configuration is proved to be a promising pathway to design high-efficiency EM absorption properties of Ni based composites.

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**Author details**

Biao Zhao1,2\* and Rui Zhang1,3

Zhengzhou, Henan, China

Toronto, Ontario, Canada

Henan, China

provided the original work is properly cited.

\*Address all correspondence to: zhaobiao1813@163.com

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

1 Henan Key Laboratory of Aeronautical Materials and Application Technology, School of Material Science and Engineering, Zhengzhou University of Aeronautics,

2 Department of Mechanical and Industrial Engineering, University of Toronto,

3 School of Material Science and Engineering, Zhengzhou University, Zhengzhou,

*Electromagnetic Wave Absorption Properties of Core-Shell Ni-Based Composites*

*DOI: http://dx.doi.org/10.5772/intechopen.82301*
