*2.1.1 Preparation of core-shell Ni@ZnO composites*

Ni microspheres were prepared based on our previous publication [15]. Ni/ZnO composites were synthesized through a facile hydrothermal method [23]. Typically, 0.05 g of the as-obtained Ni microspheres was distributed in 60 mL distilled water. Then 0.45 g of Zn(CH3COO)2·2H2O and a certain amounts of ammonia solution were added into the mixture solution. The mixture was transferred into a Teflonlined stainless steel autoclave, and maintained at 120°C for 15 h. The precipitates were collected by centrifugation, washed several times with distilled water and absolute ethanol, respectively. For the convenience of discussion, the Ni/ZnO prepared at 1 mL NH3·H2O, 2 mL NH3·H2O and 3 mL NH3·H2O were denoted as SA, SB and SC, respectively.

**Figure 1a** displays the representative FESEM image of the Ni particles, which possesses uniformly spherical shape and the diameter is about 1.0–1.2 μm. The SEM images of the Ni/ZnO obtained at different contents of NH3·H2O are displayed in **Figure 1b**–**d**. **Figure 1b** exhibits that the as-prepared Ni/ZnO product is composed of plentiful ZnO polyhedrons with the diameter of 0.2–0.5 μm covered on the surface of Ni particles to generate special core-shell structure if small amount of NH3·H2O (1 mL) was added. If the content of NH3·H2O is lifted to 2.0 mL, the football-like Ni/ZnO samples with the size of 4–5 μm could be observed (**Figure 1c**). One can infer that the thickness of ZnO polyhedron is about 2-3 μm, which is larger than that of Ni microsphere. Therefore, the Ni microspheres are completely coated by ZnO, thus, we could not see the existence if individual Ni microspheres. When the content of NH3·H2O is further improved to 3 mL, ZnO rods and Ni microsphere coexist in the final products (**Figure 1d**), separately. These results indicate that the morphology of Ni/ZnO composite can be effectively adjusted by controlling the NH3·H2O content.

**Figure 2** depicts the schematic diagram of the generation for various shapes of Ni/ZnO composite. First, the distributed Ni microspheres are fabricated through a chemical reduction method. Following, the different shapes of Ni/ZnO composites are fabricated by the addition of various NH3·H2O contents. The ZnO nuclei is prone to plant along special crystal planes and finally generate polyhedron-like or rod-like ZnO products. It is accepted that ZnO is a polar crystal with a polar c-axis ([0001] direction) [24]. In the solution system, the NH3·H2O consists of the positive hydrophilic group (NH4 + ) and negative hydrophobic group (OH<sup>−</sup>). The positive hydrophilic groups would link with the basic cells of crystalline growth [Zn(OH)4] <sup>2</sup><sup>−</sup> easily by Coulomb force, which means that the positive hydrophilic groups turn into the carriers of [Zn(OH)4] <sup>2</sup><sup>−</sup>; on the other hand, due to existence

*Electromagnetic Materials and Devices*

to eliminate the electromagnetic pollution [6, 7].

performance microwave absorbing materials (MAMs) have attracted great interests

composites, is conductive to the enhancement of the magnetic loss. The dielectric materials considering as shells, which are supposed to play the roles not only as a center of polarization but also as an insulating medium between the magnetic particles, lead to the increased dielectric loss and good impedance match. The high-efficiency microwave absorption properties resulted from the enhanced magnetic loss, dielectric loss, reduced eddy current loss and impedance match [8]. Thus, the traditional microwave absorbing materials holding a core-shell

It is well known Ni is regarded as a typical magnetic metal material, which are supposed to have numerous applications in many fields such as magnetic recording devices, clinical medicine, catalysis and so on [13, 14]. It is worth pointing that Ni was also proved to be as a competitive candidate for high-efficiency electromagnetic absorption materials to address the electromagnetic interference and pollution problems because Ni can provide more beneficial features, such as high saturation magnetization, distinguishable permeability, and compatible dielectric loss ability in the gigahertz range when compared with those nonmagnetic EM absorption materials. However, single-component electromagnetic absorption materials easily suffer from mismatched characteristic impedance and poor microwave absorption performance. Moreover, Ni would generate eddy current induced by microwave in GHz range because of high conductivity. The eddy current effect may cause impedance mismatching between the absorbing materials and air space, which would make microwave reflection rather than absorption. This issue is a challenge to handle for scientists. Thus, for the sake of getting superior microwave absorption ability, a promising pathway is to compound the Ni products with an inorganic or nonmagnetic constituent to produce a core@shell configuration. Numerous literatures have been carried out to cover the magnetic Ni with inorganic or nonmagnetic shells. For example, Ni/SnO2 core-shell composite [15], carboncoated Ni [16], Ni/ZnO [17], Al/AlOx-coated Ni [18], Ni@Ni2O3 core-shell particles [19], Ni/polyaniline [20], and CuO/Cu2O-coated Ni [21] show the better microwave absorption performance than the pure core or shell materials. Thus, the EM wave absorption abilities of Ni particles can be obviously enhanced after coating inor-

Herein, we report the microwave absorption properties of core-shell structured Ni based composited and discuss how does core-shell ameliorate the electromagnetic wave absorption properties and also investigate the related electromagnetic

structure may improve their microwave absorption capabilities.

Recently, considerable research attention has been focused on core-shell structure for innovative electromagnetic absorption due to the potential to combine the individual properties of each component or achieve enhanced performances through cooperation between the components [8–10]. Liu and co-workers have fabricated core-shell structured Fe3O4@TiO2 with Fe3O4 as cores and hierarchical TiO2 as shells and the Fe3O4@TiO2 core-shell composites displayed the enhanced microwave absorption properties than pure Fe3O4 [11]. Chen and co-workers have successfully synthesized core/shell Fe3O4/TiO2 composite nanotubes with superior microwave attenuation properties [12]. Combining Fe3O4 and TiO2 can take advantage of both the unique magnetic properties of Fe3O4 and strong dielectric characteristics of TiO2 as well as coreshell structure, and therefore offer an avenue to achieve excellent microwaveabsorption performance. In this kind of core/shell configurations, the magnetic materials regarding as cores, which could improve the permeability of the

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ganic and nonmagnetic shells.

attenuation theory in detail.

#### **Figure 1.**

*(a) SEM image of pure Ni microspheres and (b–d) SEM images of as-prepared Ni/ZnO samples prepared at various concentration of NH3·H2O: (b) 1 mL, (c) 2 mL, and (d) 3 mL [23] (permission from Elsevier).*

#### **Figure 2.**

*Schematic illustration of formation of various morphologies of Ni/ZnO composites [23] (permission from Elsevier).*

of van der Waals force, the connection between the negative hydrophobic groups and the non-polar lateral surfaces of ZnO would take place, indicating that the occurrence of hydrophobic films thanks to the negative hydrophobic groups on the non-polar lateral surfaces [25]. The basic cells of growing [Zn(OH)4] 2− attracted by the positive hydrophilic groups would move to the polar axial surface (0001) easily to integrate together and remain at the suitable lattice locations but difficultly reach the non-polar lateral remains due to the presence of the hydrophobic films. It indicates that the positive polar surface (0001) grows quicker than that of non-polar lateral surfaces in a fixed content of NH3·H2O. As a result, more OH<sup>−</sup> ions assimilate and hamper the growth on the positively charged (0001) surface, forcing a shape transition [26]. At the high content of NH3·H2O, long ZnO nanorods could be produced because of the fast growth rate along the [0001] direction [27].

**155**

**Figure 3.**

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

To reveal the electromagnetic wave absorption properties of SA, SB and SC paraffin composites, the reflection loss (RL) values of the Ni/ZnO samples are

*RL* = 20log10|(*Zin* − *Z*0)/(*Zin* + *Z*0)| (1)

2*fd*√ \_\_\_\_ \_\_\_\_\_\_\_ *μr* <sup>ε</sup>*<sup>r</sup>*

*<sup>c</sup>* ) (2)

\_\_ \_\_ *μr* <sup>ε</sup>*<sup>r</sup>* tanh( *j*

Herein *Z*0 is the impedance of free space, Zin is the input impedance of the material, ƒ is the frequency of the microwave, *c* is the velocity of microwave in free space, μr and εr are, respectively, the relative complex permeability and permittivity, and *d* is the thickness of the absorber. The RL values of the three samples with a thickness of 2.0 mm are displayed in **Figure 3a**. The SA sample holds the outstanding EM wave absorption performances. A strong peak (−48.6 dB) could be seen at 13.4 GHz. The RL less than −10 dB (90% absorption) reaches 6.0 GHz (10.5–16.5 GHz). Furthermore, the RL less than −20 dB (99% microwave dissipation) is also obtained in the range of 11.5–14.2 GHz. But, for the SB and SC samples, they present inferior microwave dissipation capabilities. **Figure 3b** depicts the simulated RL of SA paraffin-composite with various thicknesses in the frequency of 1–18 GHz. Clearly, one can notice that the optimal RL shifts into lower frequency range along with an increased thickness, indicating that we could adjust the absorption bandwidth by tuning absorber thickness. From above analysis, one can note that the minimal RL of −48.6 dB could be observed at 13.4 GHz with a layer thickness of 2.0 mm. The effective absorption (below −10 dB) bandwidth could be monitored in the frequency of 9.0–18.0 GHz by control of the absorber thickness between 1.5 mm and 2.5 mm. Furthermore, the frequency with RL below −20 dB could be observed at 11.1–16.2 GHz with thickness of 1.8–2.2 mm. For the SA sample, the enhanced microwave absorption properties are stemmed from the good impedance match, synergistic effect between dielectric loss and magnetic loss, and special core-shell microstructures, which could induce the interference of microwave multiple reflection [29]. In addition, the compact polyhedron ZnO coating brings the metal/dielectric interfaces, in which the interface polarization boosts the microwave dissipation. For the football-like Ni/ZnO (SB), the size of ZnO is so big that the Ni microspheres could not interact with incident microwave, which

*(a) Frequency dependences of RL with the thickness of 2.0 mm for the three samples SA, SB, SC; (b) RL of Ni/ZnO* 

*(SA) paraffin composite of varying thicknesses [23] (permission from Elsevier).*

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

*2.1.2 EM properties of core-shell Ni@ZnO composites*

calculated based on following equations [28]:

*Zin* <sup>=</sup> *<sup>Z</sup>*<sup>0</sup> <sup>√</sup>

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

#### *2.1.2 EM properties of core-shell Ni@ZnO composites*

*Electromagnetic Materials and Devices*

**154**

**Figure 1.**

**Figure 2.**

*Elsevier).*

[0001] direction [27].

of van der Waals force, the connection between the negative hydrophobic groups and the non-polar lateral surfaces of ZnO would take place, indicating that the occurrence of hydrophobic films thanks to the negative hydrophobic groups on the non-polar lateral surfaces [25]. The basic cells of growing [Zn(OH)4]

*Schematic illustration of formation of various morphologies of Ni/ZnO composites [23] (permission from* 

*(a) SEM image of pure Ni microspheres and (b–d) SEM images of as-prepared Ni/ZnO samples prepared at various concentration of NH3·H2O: (b) 1 mL, (c) 2 mL, and (d) 3 mL [23] (permission from Elsevier).*

attracted by the positive hydrophilic groups would move to the polar axial surface (0001) easily to integrate together and remain at the suitable lattice locations but difficultly reach the non-polar lateral remains due to the presence of the hydrophobic films. It indicates that the positive polar surface (0001) grows quicker than that of non-polar lateral surfaces in a fixed content of NH3·H2O. As a result, more OH<sup>−</sup> ions assimilate and hamper the growth on the positively charged (0001) surface, forcing a shape transition [26]. At the high content of NH3·H2O, long ZnO nanorods could be produced because of the fast growth rate along the

2−

To reveal the electromagnetic wave absorption properties of SA, SB and SC paraffin composites, the reflection loss (RL) values of the Ni/ZnO samples are calculated based on following equations [28]:

$$RL = \text{2O} \log\_{10} | \langle Z\_{\text{in}} - Z\_0 \rangle / \langle Z\_{\text{in}} + Z\_0 \rangle | \tag{1}$$

$$Z\_{in} = Z\_0 \sqrt{\frac{\mu\_r}{\varepsilon\_r}} \tanh\left(j \frac{2\pi fd\sqrt{\mu\_r \varepsilon\_r}}{c}\right) \tag{2}$$

Herein *Z*0 is the impedance of free space, Zin is the input impedance of the material, ƒ is the frequency of the microwave, *c* is the velocity of microwave in free space, μr and εr are, respectively, the relative complex permeability and permittivity, and *d* is the thickness of the absorber. The RL values of the three samples with a thickness of 2.0 mm are displayed in **Figure 3a**. The SA sample holds the outstanding EM wave absorption performances. A strong peak (−48.6 dB) could be seen at 13.4 GHz. The RL less than −10 dB (90% absorption) reaches 6.0 GHz (10.5–16.5 GHz). Furthermore, the RL less than −20 dB (99% microwave dissipation) is also obtained in the range of 11.5–14.2 GHz. But, for the SB and SC samples, they present inferior microwave dissipation capabilities. **Figure 3b** depicts the simulated RL of SA paraffin-composite with various thicknesses in the frequency of 1–18 GHz. Clearly, one can notice that the optimal RL shifts into lower frequency range along with an increased thickness, indicating that we could adjust the absorption bandwidth by tuning absorber thickness. From above analysis, one can note that the minimal RL of −48.6 dB could be observed at 13.4 GHz with a layer thickness of 2.0 mm. The effective absorption (below −10 dB) bandwidth could be monitored in the frequency of 9.0–18.0 GHz by control of the absorber thickness between 1.5 mm and 2.5 mm. Furthermore, the frequency with RL below −20 dB could be observed at 11.1–16.2 GHz with thickness of 1.8–2.2 mm. For the SA sample, the enhanced microwave absorption properties are stemmed from the good impedance match, synergistic effect between dielectric loss and magnetic loss, and special core-shell microstructures, which could induce the interference of microwave multiple reflection [29]. In addition, the compact polyhedron ZnO coating brings the metal/dielectric interfaces, in which the interface polarization boosts the microwave dissipation. For the football-like Ni/ZnO (SB), the size of ZnO is so big that the Ni microspheres could not interact with incident microwave, which

#### **Figure 3.**

*(a) Frequency dependences of RL with the thickness of 2.0 mm for the three samples SA, SB, SC; (b) RL of Ni/ZnO (SA) paraffin composite of varying thicknesses [23] (permission from Elsevier).*

gives rise to the mismatch between the magnetic loss and dielectric loss, leading to inferior microwave absorption. For the SC, due to the presence of uncover Ni, these uncoated Ni microspheres play a negative in the wave-absorption of materials thanks to the occurrence of a significant skin effect when its surface is irradiated by microwaves [30].

On the basis of transmission line theory, the suitable microwave absorption properties are determined by two key factors. One factor is the impedance match, which need the complex permittivity is close to the complex permeability, and the other one is the EM attenuation ability, which dissipates the microwave energy through dielectric loss or magnetic loss. The EM attenuation was determined by the attenuation constant *α*, which can be expressed as [31, 32]:

##  $\alpha$ sısı o падвель  $\alpha$ sı. псе шаспаланон чаз честиплиса  $\mathfrak{h}$  а  $\alpha$  соль  $a$ , which can be expressed as [31, 32]:

$$\alpha = \frac{\sqrt{2}\,\pi f}{c} \times \sqrt{\left(\mu^{'}\,\text{e}^{'} - \mu^{'}\,\text{e}^{'}\right) + \sqrt{\left(\mu^{'}\,\text{e}^{'} - \mu^{'}\,\text{e}^{'}\right)^{2} + \left(\mu^{'}\,\text{e}^{'} + \mu^{'}\,\text{e}^{'}\right)^{2}}} \tag{3}$$

where *f* is the frequency of the microwave and *c* is the velocity of light. **Figure 4** displays the frequency dependence of the attenuation constant. The SA possesses the biggest *α* in all measured frequency ranges, meaning the outstanding attenuation. Moreover, based on the above equation, one can notice that the attenuation constant is closely related to the values of ε″ and μ″. The highest ε″ and μ″ values are the responsible for the highest *α* in Ni@ZnO (SA) core/shell structures, which is related to interface polarization and relaxation. As a result, the enhancement of the microwave absorption properties for the dielectric coating originates from the improvement of dielectric loss and magnetic loss.

#### **2.2 Core-shell Ni@CuO composites as microwave absorbers**

Nowadays, CuO is well accepted as an important p-type semiconductor, which holds the unique features of narrow band gap (Eg = 1.2 eV), and has captured more and more interests. This material has proved to exhibit widespread potential applications in optical switches, anode materials, field emitters, catalyst, gas sensors, photoelectrode and high-temperature micro-conductors [33, 34]. Recently, CuO has been realized as an efficient material for the preparation of microwave absorbing materials [21, 35, 36]. Herein, we fabricated the core-shell structural composites with Ni cores and rice-like CuO shells via a facile method. The microwave absorption properties of Ni, CuO and Ni/CuO composites are studied in term of complex

**Figure 4.** *Attenuation constant of Ni/ZnO samples-paraffin composites versus frequency [23] (permission from Elsevier).*

**157**

**Figure 5.**

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

and microwave absorption properties of Ni/CuO composites in detail.

Teflon-lined autoclave and heated hydrothermally at 150°C for 15 h.

permittivity and permeability. In comparison with pristine Ni and CuO, rice-like CuO-coated Ni composites displayed the enhanced microwave absorption properties. Furthermore, we also studied the effects of CuO amounts on microstructures

The Ni microspheres were prepared by a solvothermal method, which was described in our pervious literature [15]. Synthesis of CuO nanoflakes: CuCl2·2H2O (0.36 g) was dissolved in a mixture of distilled water (60 mL) and ammonia (2 mL) under continuously stirring (30 min); The final mixture was transferred into a

Synthesis of CuO rice-coated Ni core/shell composites [37]: the as-prepared Ni microspheres (0.05 g) and CuCl2·2H2O (0.36 g) were both added in distilled water (60 mL). Then, the ammonia (2 mL) was introduced into the mixture. Finally, the prepared mixture was moved into a Teflon-lined autoclave. The Teflon-lined autoclave was sealed and kept at 150°C for 15 h. The Ni/CuO composites prepared at 0.18 g CuCl2·2H2O, 0.36 g CuCl2·2H2O and 0.54 g CuCl2·2H2O were denoted as S-1,

**Figure 5c, d** exhibits FESEM micrographs of Ni@CuO composites with different magnification after hydrothermal treatment at 150°C for 15 h. It can be observed that the products are composed of CuO rices-coated smooth Ni microspheres heterostructures with the diameter of 1.0–1.2 μm. One can notice that rice-like CuO/Ni composites hold rough surfaces, which results from compactly aggregated panicle-shape CuO nanostructures. In order to get more information about microstructure of Ni/CuO composite, TEM and HR-TEM images of Ni/CuO composites are carried out. The core-shell structure of Ni/CuO composite can be

*Typical TEM and HRTEM images of the as-prepared Ni/CuO structures: (a) low magnification, inset of (a) shows the SAED pattern; (b) high magnification; inset of (b) shows the HRTEM. (c, d) FESEM images of the* 

*Ni microsphere-CuO rice core-shell structures [37] (permission from RSC).*

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

*2.2.1 Preparation of core-shell Ni@CuO composites*

S-2 and S-2, respectively.

permittivity and permeability. In comparison with pristine Ni and CuO, rice-like CuO-coated Ni composites displayed the enhanced microwave absorption properties. Furthermore, we also studied the effects of CuO amounts on microstructures and microwave absorption properties of Ni/CuO composites in detail.
