**3. Core-shell Ni@non-oxide composite as microwave absorbers**

#### **3.1 Ni@ZnS composites as microwave absorbers**

It is well known that ZnS, a wide band-gap semiconductor with the band-gap energy (Eg) of 3.6 eV, has been widely applied in displays, sensors, and lasers for many years [43, 44]. As far as I am concerning, the publications about the microwave absorption properties of ZnS and core/shell structured Ni/ZnS are not reported. Herein, we synthesized the core-shell structured composites with Ni cores and ZnS nanowall shells through a facile method. The microwave absorption properties of Ni, ZnS and Ni@ZnS composites are detailedly investigated in the frequency of 2–18 GHz.

#### *3.1.1 Preparation of core-shell Ni@ZnS composites*

ZnS nanowall-coated Ni composites were fabricated via a two-stage method [45]. First, Ni microspheres were synthesized based on our previous paper [15]. Second, Ni microspheres are coated by ZnS nanowalls to generate the core-shell structural composites. In the modified procedure, the as-obtained Ni microspheres (0.05 g) and Zn(CH3COO)2·2H2O (0.45 g) were added in a mixture solution of ethanol (30 mL) and distilled water (30 mL). Then, Na2S·9H2O (0.50 g) and ammonia solution (4 mL) were added into the mixture solution with intensely stirring for 20 min. Finally, the mixture was moved into a Teflon-lined stainless steel autoclave, and kept at 100°C for 15 h. In order to study the effect of core-shell structure on the microwave absorption properties of the Ni/ZnS composite, the pure ZnS particles were also prepared according to the above procedure without addition of Ni microspheres.

Inset of **Figure 8a** presents the XRD profiles of Ni microspheres, ZnS particles and Ni@ZnS composites. For the Ni microspheres, all the diffraction peaks can be well indexed to the face-centered cubic (fcc) structure of nickel (JCPDS No. 04-0850). For the ZnS particles, all diffraction peaks can be indexed to a typical zinc blende structured ZnS, which is consistent with the standard value for bulk ZnS (JCPDS Card No. 05-0566). The crystal structure of core/shell structured Ni/ZnS

**161**

**Figure 8.**

The thickness of ZnS nanowall is about 10 nm.

*3.1.2 EM properties of core-shell Ni@ZnS composites*

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

composites is also investigated by XRD measurements. Noticeably, we expectantly observed the diffraction peaks, which are in good accordance with Ni and ZnS, respectively. One can conclude that the as-obtained core/shell structural composites are made up of crystalline Ni and ZnS. **Figure 8a** presents the SEM image of the Ni microspheres. One can notice that the products have a relatively uniform spherical shape with the diameter of 0.7–1.0 μm. The pure ZnS products appear to have irregular shapes (**Figure 8b**). In raw ZnS particles, the formation of ZnS is via a two-step pathway. The fresh nanoparticles incline to aggregate for the sake of decreasing the surface energy. Therefore, we could obtain the irregular gathering ZnS particles. Whereas, as for the Ni/ZnS system, ZnS particles were generated via the template way (heterogeneous nucleation, raw Ni as template). Therefore, the variation of the shape and dimensions of ZnS particles in pure ZnS and core-shell Ni/ZnS composites could be seen. **Figure 8c, d** present the SEM images of core-shell Ni/ZnS. In comparison with pure Ni (**Figure 8a**), one significant distinction is clearly observed between the Ni/ZnS composites and the pure Ni particles. The distinction is that the Ni particles are absolutely wrapped by the ZnS nanowalls. The large-scale SEM image in **Figure 8d** suggests that the as-prepared Ni/ZnS composites show crinkled and rough textures, which are similar with the reduced graphene oxide sheets [46].

*SEM images of (a) Ni microspheres, (b) ZnS particles, and (c, d) the as-prepared Ni/ZnS composites. The inset in (b) is the magnified SEM image of ZnS particles. Inset in Figure 8a is the XRD patterns of Ni* 

*microspheres, ZnS particles and Ni/ZnS composites [45] (permission from RSC).*

The relative complex permittivity (ε′ and ε″) and permeability (μ′ and μ″) of the Ni/paraffin, ZnS/paraffin and Ni@ZnS/paraffin composite samples are measured over a frequency of 2–18 GHz. **Figure 9a–c** manifest the real part (ε′) and imaginary

*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 8.**

*Electromagnetic Materials and Devices*

Notably, the optimal RL peaks gradually shift toward lower frequencies with an increased absorber thickness, which can be described by quarter-wavelength cancelation model that the incident and reflected waves in the absorber are out of phase 180° and causing the reflected waves in the air-absorber interface are totally cancelled [40]. The enhanced microwave absorption property of core-shell Ni/CuO composites can be obtained by tuning the content of CuO. The rice-like CuO shell is expected to be helpful for the dissipation proprieties of the core/shell composites. The CuO shells are covered on the surface of Ni microspheres to produce the special core-shell structure, which brings metal-dielectric hetero-interfaces to cause interfacial polarization. It is supposed that the interfacial polarization taken place in heterostructures consisting of at least two constituents [17, 41, 42]. This type of polarization occurring at the interfaces results from the movement of charge carriers between different compositions, which accumulate the moving charge at these interfaces. When irradiated by alternating EM fields, the accumulated charge would redistribute periodically between Ni cores and CuO shells, which are favorable for the microwave dissipation. However, for the S-2 and S-3 samples, thanks to the high content of CuO, we cannot observe the synergistic effect between Ni cores and CuO

shells, which gives rise to inferior microwave absorption.

**3.1 Ni@ZnS composites as microwave absorbers**

*3.1.1 Preparation of core-shell Ni@ZnS composites*

frequency of 2–18 GHz.

**3. Core-shell Ni@non-oxide composite as microwave absorbers**

It is well known that ZnS, a wide band-gap semiconductor with the band-gap energy (Eg) of 3.6 eV, has been widely applied in displays, sensors, and lasers for many years [43, 44]. As far as I am concerning, the publications about the microwave absorption properties of ZnS and core/shell structured Ni/ZnS are not reported. Herein, we synthesized the core-shell structured composites with Ni cores and ZnS nanowall shells through a facile method. The microwave absorption properties of Ni, ZnS and Ni@ZnS composites are detailedly investigated in the

ZnS nanowall-coated Ni composites were fabricated via a two-stage method [45]. First, Ni microspheres were synthesized based on our previous paper [15]. Second, Ni microspheres are coated by ZnS nanowalls to generate the core-shell structural composites. In the modified procedure, the as-obtained Ni microspheres (0.05 g) and Zn(CH3COO)2·2H2O (0.45 g) were added in a mixture solution of ethanol (30 mL) and distilled water (30 mL). Then, Na2S·9H2O (0.50 g) and ammonia solution (4 mL) were added into the mixture solution with intensely stirring for 20 min. Finally, the mixture was moved into a Teflon-lined stainless steel autoclave, and kept at 100°C for 15 h. In order to study the effect of core-shell structure on the microwave absorption properties of the Ni/ZnS composite, the pure ZnS particles were also prepared according to the above procedure without addition of Ni

Inset of **Figure 8a** presents the XRD profiles of Ni microspheres, ZnS particles and Ni@ZnS composites. For the Ni microspheres, all the diffraction peaks can be well indexed to the face-centered cubic (fcc) structure of nickel (JCPDS No. 04-0850). For the ZnS particles, all diffraction peaks can be indexed to a typical zinc blende structured ZnS, which is consistent with the standard value for bulk ZnS (JCPDS Card No. 05-0566). The crystal structure of core/shell structured Ni/ZnS

**160**

microspheres.

*SEM images of (a) Ni microspheres, (b) ZnS particles, and (c, d) the as-prepared Ni/ZnS composites. The inset in (b) is the magnified SEM image of ZnS particles. Inset in Figure 8a is the XRD patterns of Ni microspheres, ZnS particles and Ni/ZnS composites [45] (permission from RSC).*

composites is also investigated by XRD measurements. Noticeably, we expectantly observed the diffraction peaks, which are in good accordance with Ni and ZnS, respectively. One can conclude that the as-obtained core/shell structural composites are made up of crystalline Ni and ZnS. **Figure 8a** presents the SEM image of the Ni microspheres. One can notice that the products have a relatively uniform spherical shape with the diameter of 0.7–1.0 μm. The pure ZnS products appear to have irregular shapes (**Figure 8b**). In raw ZnS particles, the formation of ZnS is via a two-step pathway. The fresh nanoparticles incline to aggregate for the sake of decreasing the surface energy. Therefore, we could obtain the irregular gathering ZnS particles. Whereas, as for the Ni/ZnS system, ZnS particles were generated via the template way (heterogeneous nucleation, raw Ni as template). Therefore, the variation of the shape and dimensions of ZnS particles in pure ZnS and core-shell Ni/ZnS composites could be seen. **Figure 8c, d** present the SEM images of core-shell Ni/ZnS. In comparison with pure Ni (**Figure 8a**), one significant distinction is clearly observed between the Ni/ZnS composites and the pure Ni particles. The distinction is that the Ni particles are absolutely wrapped by the ZnS nanowalls. The large-scale SEM image in **Figure 8d** suggests that the as-prepared Ni/ZnS composites show crinkled and rough textures, which are similar with the reduced graphene oxide sheets [46]. The thickness of ZnS nanowall is about 10 nm.

#### *3.1.2 EM properties of core-shell Ni@ZnS composites*

The relative complex permittivity (ε′ and ε″) and permeability (μ′ and μ″) of the Ni/paraffin, ZnS/paraffin and Ni@ZnS/paraffin composite samples are measured over a frequency of 2–18 GHz. **Figure 9a–c** manifest the real part (ε′) and imaginary part (ε″) of the complex permittivity as a function of frequency. The ε′ of Ni/paraffin composite shows a gradual decrease with frequency (**Figure 9a**).

However, the ε″ values are relative constant without significant change over the 2–18 GHz. The ε′ and ε″ of ZnS/paraffin composite presents constant value (4.5 and 0.5, respectively) in **Figure 9b**. The ε′ of the Ni/ZnS composite firstly reduces in the frequency of 2–15 GHz and then improves with increasing frequency (**Figure 9c**). Nevertheless, the ε″ exhibits the opposite tendency in the frequency of 2–18 GHz. One can note that the ε″ values of Ni/ZnS composite presents a peak in the 13–15 GHz range, which is originated from the natural resonance behavior of core-shell microstructure [21, 47]. Furthermore, it can be found that the ε″ values of Ni/paraffin composite are larger than those of ZnS/paraffin composite and Ni@ZnS/paraffin composite. Based on the free electron theory [17], ε″≈ 1/2<sup>0</sup> *f*, where*ρ*is the resistivity. The lower ε″ values of ZnS/paraffin composite and Ni@ZnS/paraffin composite indicate the higher electric resistivity. In general, a high electrical resistivity is favorable for improving the microwave absorption abilities [48].

**Figure 9** (d–f) present the curves of the real part (μ′) and imaginary part (μ″) of the complex permeability as a function of frequency for the Ni/paraffin, ZnS/ paraffin and Ni@ZnS/paraffin composites. The μ′ and μ″ of Ni/paraffin composite are 0.81–1.59 and 0.05–0.51, respectively (**Figure 9d**). Compared with the complex permittivity (**Figure 9a**), the values of complex permeability is relatively small, which lead to mismatch impedance. The impedance match is required that complex

#### **Figure 9.**

*Frequency dependence of the complex permittivity (εr = ε*′ *− jε*″*) of (a) Ni microspheres, (b) ZnS particles, and (c) Ni/ZnS composites; frequency dependence of the complex permeability (μr = μ*′ *− jμ*″*) of (d) Ni microspheres, (e) ZnS particles, and (f) Ni/ZnS composites [45] (permission from RSC).*

**163**

**Figure 10.**

*(permission from RSC).*

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

permittivity is close to the permeability, which can make microwaves enter into the materials as much as possible [49]. The higher permittivity of the absorber plays a negative role in the impedance match [50], thus gives rise to inferior microwave absorption. From the **Figure 9e**, **f**, it can be found that the complex permeability of ZnS/paraffin and Ni@ZnS/paraffin composite exhibit the similar tendency with an increased frequency. The μ′ values of 0.99–1.38 and 0.84–1.34 could be observed in ZnS/paraffin and Ni@ZnS/paraffin composites, respectively. The μ″ values are in the range of 0.02–0.24 and 0.03–0.34 for the ZnS/paraffin and Ni@ZnS/paraffin composites, respectively. Given the complex permittivity (**Figure 9b, c**), it can be found that the relation between permittivity and permeability is prone to be close (good impedance match). The good impedance match is beneficial for the microwave absorption. On the basis of the above results, one can deduce that the impedance match of ZnS/paraffin and Ni@ZnS/paraffin is superior to that of Ni/paraffin composite. Thus, the ZnS/paraffin and Ni@ZnS/paraffin composites may possess

It is widely accepted that the RL values could be utilized to evaluate the microwave absorption abilities of EM materials. **Figure 10a** exhibits the calculated RL values of Ni, ZnS and Ni/ZnS paraffin composites with 70 wt% amounts at the thickness of 2.5 mm in the frequency range of 2–18 GHz. Because a paraffin matrix is transparent to microwaves, these results are generally considered as the waveabsorption abilities of the filler itself. It is noting that the microwave absorption properties of Ni particles are weak and the optimal RL value is only −3.04 dB at 5.28 GHz, which is due to the mismatch impedance. Another possible factor is that the skin effect could be observed in Ni microspheres, which is harmful for microwave absorption [30]. Compared with Ni particles, ZnS particles and ZnS nanowall-coated Ni composite presents the superior microwave absorption abilities, which stems from good impedance match. It is worth pointing that, for Ni@ZnS composite, the minimal RL of −20.16 dB is observed at 13.92 GHz and RL values below −10 dB are seen in the 12–16.48 GHz rang. **Figure 10b** displays the relationship between RL and frequency for the paraffin wax composites with 70 wt% Ni/ZnS in various thicknesses. The optimal RL is −25.78 dB at 14.24 GHz with the corresponding thickness of 2.7 mm. The effective absorption (less than −10 dB) bandwidth reaches 4.72 GHz (11.52–16.24 GHz). Interestingly, with increasing the sample thickness, the location of minimal absorption peaks almost keeps the same at various thicknesses without moving to lower frequency, which has also been recorded by other' groups [51]. The location of absorption peaks is in accordance with the natural resonance, which means the resonance behavior in

*(a) RL curves of Ni, ZnS and Ni/ZnS paraffin composite with 70 wt% loadings at the thickness of 2.5 mm; (b) RL curves of 70 wt% Ni/ZnS wax-composite at various thicknesses in the frequency of 2–18 GHz [45]* 

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

better dissipation abilities of microwave energy.

permittivity influences the microwave absorption.

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

permittivity is close to the permeability, which can make microwaves enter into the materials as much as possible [49]. The higher permittivity of the absorber plays a negative role in the impedance match [50], thus gives rise to inferior microwave absorption. From the **Figure 9e**, **f**, it can be found that the complex permeability of ZnS/paraffin and Ni@ZnS/paraffin composite exhibit the similar tendency with an increased frequency. The μ′ values of 0.99–1.38 and 0.84–1.34 could be observed in ZnS/paraffin and Ni@ZnS/paraffin composites, respectively. The μ″ values are in the range of 0.02–0.24 and 0.03–0.34 for the ZnS/paraffin and Ni@ZnS/paraffin composites, respectively. Given the complex permittivity (**Figure 9b, c**), it can be found that the relation between permittivity and permeability is prone to be close (good impedance match). The good impedance match is beneficial for the microwave absorption. On the basis of the above results, one can deduce that the impedance match of ZnS/paraffin and Ni@ZnS/paraffin is superior to that of Ni/paraffin composite. Thus, the ZnS/paraffin and Ni@ZnS/paraffin composites may possess better dissipation abilities of microwave energy.

It is widely accepted that the RL values could be utilized to evaluate the microwave absorption abilities of EM materials. **Figure 10a** exhibits the calculated RL values of Ni, ZnS and Ni/ZnS paraffin composites with 70 wt% amounts at the thickness of 2.5 mm in the frequency range of 2–18 GHz. Because a paraffin matrix is transparent to microwaves, these results are generally considered as the waveabsorption abilities of the filler itself. It is noting that the microwave absorption properties of Ni particles are weak and the optimal RL value is only −3.04 dB at 5.28 GHz, which is due to the mismatch impedance. Another possible factor is that the skin effect could be observed in Ni microspheres, which is harmful for microwave absorption [30]. Compared with Ni particles, ZnS particles and ZnS nanowall-coated Ni composite presents the superior microwave absorption abilities, which stems from good impedance match. It is worth pointing that, for Ni@ZnS composite, the minimal RL of −20.16 dB is observed at 13.92 GHz and RL values below −10 dB are seen in the 12–16.48 GHz rang. **Figure 10b** displays the relationship between RL and frequency for the paraffin wax composites with 70 wt% Ni/ZnS in various thicknesses. The optimal RL is −25.78 dB at 14.24 GHz with the corresponding thickness of 2.7 mm. The effective absorption (less than −10 dB) bandwidth reaches 4.72 GHz (11.52–16.24 GHz). Interestingly, with increasing the sample thickness, the location of minimal absorption peaks almost keeps the same at various thicknesses without moving to lower frequency, which has also been recorded by other' groups [51]. The location of absorption peaks is in accordance with the natural resonance, which means the resonance behavior in permittivity influences the microwave absorption.

#### **Figure 10.**

*(a) RL curves of Ni, ZnS and Ni/ZnS paraffin composite with 70 wt% loadings at the thickness of 2.5 mm; (b) RL curves of 70 wt% Ni/ZnS wax-composite at various thicknesses in the frequency of 2–18 GHz [45] (permission from RSC).*

*Electromagnetic Materials and Devices*

part (ε″) of the complex permittivity as a function of frequency. The ε′ of Ni/paraffin

**Figure 9** (d–f) present the curves of the real part (μ′) and imaginary part (μ″) of the complex permeability as a function of frequency for the Ni/paraffin, ZnS/ paraffin and Ni@ZnS/paraffin composites. The μ′ and μ″ of Ni/paraffin composite are 0.81–1.59 and 0.05–0.51, respectively (**Figure 9d**). Compared with the complex permittivity (**Figure 9a**), the values of complex permeability is relatively small, which lead to mismatch impedance. The impedance match is required that complex

*Frequency dependence of the complex permittivity (εr = ε*′ *− jε*″*) of (a) Ni microspheres, (b) ZnS particles, and (c) Ni/ZnS composites; frequency dependence of the complex permeability (μr = μ*′ *− jμ*″*) of (d) Ni* 

*microspheres, (e) ZnS particles, and (f) Ni/ZnS composites [45] (permission from RSC).*

However, the ε″ values are relative constant without significant change over the 2–18 GHz. The ε′ and ε″ of ZnS/paraffin composite presents constant value (4.5 and 0.5, respectively) in **Figure 9b**. The ε′ of the Ni/ZnS composite firstly reduces in the frequency of 2–15 GHz and then improves with increasing frequency (**Figure 9c**). Nevertheless, the ε″ exhibits the opposite tendency in the frequency of 2–18 GHz. One can note that the ε″ values of Ni/ZnS composite presents a peak in the 13–15 GHz range, which is originated from the natural resonance behavior of core-shell microstructure [21, 47]. Furthermore, it can be found that the ε″ values of Ni/paraffin composite are larger than those of ZnS/paraffin composite and Ni@ZnS/paraffin composite. Based on the free electron theory [17], ε″≈ 1/2<sup>0</sup> *f*, where*ρ*is the resistivity. The lower ε″ values of ZnS/paraffin composite and Ni@ZnS/paraffin composite indicate the higher electric resistivity. In general, a high electrical resistivity is favor-

composite shows a gradual decrease with frequency (**Figure 9a**).

able for improving the microwave absorption abilities [48].

**162**

**Figure 9.**
