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

*Electromagnetic Materials and Devices*

*<sup>α</sup>* <sup>=</sup> <sup>√</sup> \_\_ <sup>2</sup> *<sup>f</sup>* \_\_\_ *<sup>c</sup>* ×√

microwaves [30].

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

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

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

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

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

\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_

) +√ \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ (*<sup>μ</sup>* ″ <sup>ε</sup>″ <sup>−</sup> *<sup>μ</sup>*′

ε″ + *μ*″ ε′

) 2

(3)

ε′ ) 2 + (*μ*′

attenuation constant *α*, which can be expressed as [31, 32]:

(*μ*″ ε″ − *μ*′

improvement of dielectric loss and magnetic loss.

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

ε′

**156**

**Figure 4.**

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

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, S-2 and S-2, respectively.

**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

#### **Figure 5.**

*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).*

clearly observed from **Figure 5a, b**. The inset SAED pattern of the CuO particles indicated that CuO particles are polycrystalline (**Figure 5a**). The HRTEM image (inset of **Figure 5b**) displays that the lattice spacing is 0.276 nm, which is in good agreement with the (110) lattice spacing of CuO. Based on the SEM and TEM results, it can be concluded that the CuO is deposited on the surface of Ni, the coreshell composites are obtained under this procedure.

**Figure 6** exhibits the morphologies of the obtained products with different molar ratio of the CuCl2·2H2O to Ni microspheres. Noticeably, the surfaces of all samples turns coarser in comparison with the pure Ni microspheres, which indicates the successful coating of the CuO nanoparticles on the pristine Ni surfaces. Furthermore, the shape and coverage density of CuO materials could be controlled by tuning the content of precursor (Cu2+). When the molar ratio of the CuCl2·2H2O to Ni microspheres in the precursor solution is 1: 0.85 (S-1), one can find (**Figure 6a, b**) that the Ni microspheres are coated by a large number of CuO nanorices. But, due to the low content of precursor (Cu2+), we just could obtain thin CuO shell. If the

#### **Figure 6.**

*FESEM images of hierarchical Ni/CuO core-shell heterostructures with different molar ratio: (a, b) S-1; (c, d) S-2; and (e, f) S-3 [37] (permission from RSC).*

**159**

**Figure 7.**

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

molar ratio of precursor is enhanced to 2: 0.85 (S-2), one can see that the aggregation occurs and CuO nanorices are compactly covered on the smooth surfaces of Ni microspheres to produce coarser thick CuO shells (**Figure 6c, d**). If the molar ratio is improved continuously to 3:0.85 (S-3), a thick layer of compact CuO nanoflakes coated on Ni microspheres could be observed (**Figure 6e, f**). Based on above results, the microstructures and coverage density of CuO shells can be effectively monitored

To compare and assess the EM wave absorption properties of Ni, Ni/CuO coreshell composites, and CuO nanoflakes, the paraffin (30 wt%, which is transparent to microwave) are mixed with as-obtained products, and pressed into a ring shape with an outer diameter of 7.00 mm and an inner diameter of 3.04 mm. The microwave absorption abilities of these as-fabricated products could be evaluated by the RL values, which could be simulated on the basis of the complex permeability and permittivity with the measured frequency and given layer thickness [38, 39]. As presented in **Figure 7a**, the three Ni/CuO composites show the superior microwave-absorption properties to those of the pure Ni microspheres and the CuO nanoflakes. Taking an example, when the thickness is 2 mm, the S-1 sample exhibits the enhanced EM-wave absorption with the minimal RL value of −15.6 dB at 11.9 GHz among the five samples. From Eqs. (1) and (2), one can find that the thickness of the absorber is one important factor, which would affect the position of minimal RL value and the absorption bandwidth. Therefore, the RL values of Ni/CuO samples with different thicknesses are also calculated. Compared with S-2 (**Figure 7c**) and S-3 (**Figure 7d**) samples, the S-1 (**Figure 7b**) displays the outstanding microwave absorption performances. The lowest RL of the S-1 sample is −62.2 dB at 13.8 GHz with the only thickness of 1.7 mm. The effective absorption (below −10 dB) bandwidth can be tuned between 6.4 GHz and 18.0 GHz by adjusting thickness in 1.3–3.0 mm.

*(a) Comparison of RL of the five as-obtained samples with a thickness of 2.0 mm. The RL values of (b) S-1,* 

*(c) S-2, and (d) S-3 samples with various thicknesses [37] (permission from RSC).*

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

by selecting a suitable content of Cu2+.

*2.2.2 EM properties of core-shell Ni@CuO composites*

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

molar ratio of precursor is enhanced to 2: 0.85 (S-2), one can see that the aggregation occurs and CuO nanorices are compactly covered on the smooth surfaces of Ni microspheres to produce coarser thick CuO shells (**Figure 6c, d**). If the molar ratio is improved continuously to 3:0.85 (S-3), a thick layer of compact CuO nanoflakes coated on Ni microspheres could be observed (**Figure 6e, f**). Based on above results, the microstructures and coverage density of CuO shells can be effectively monitored by selecting a suitable content of Cu2+.
