**1. Introduction**

The discovery of electromagnetic (EM) waves boosted the development of transmission technology. It is well accepted that the various electromagnetic waves are widely applied in numerous areas and make our daily life convenient [1]. Extensive electronics devices, such as mobile phones, WiFi, Near Field Communication (NFC), and wireless charging, are developing and play an important role in modern life [2, 3]. With the extensive practical applications of electronic devices and densely packed systems, electromagnetic interference (EMI) becomes a more and more serious problem, which would lead to pernicious impacts on equipment performance, human health, as well as surrounding environment [4]. Furthermore, our individual mini device produces unwanted EM waves, which would influence other nearby devices [5]. Moreover, the global need for some EM waves, such as for military radar stealth, is also boosting, which produces plentiful concerns for human health. Therefore, the protection of electromagnetic radiation has been widely concerned by the whole society. Recently, development of high

performance microwave absorbing materials (MAMs) have attracted great interests to eliminate the electromagnetic pollution [6, 7].

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 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 structure may improve their microwave absorption capabilities.

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 inorganic and nonmagnetic shells.

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 attenuation theory in detail.

**153**

[Zn(OH)4]

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

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

For the ZnO nanostructural materials, due to the features of lightweight semiconductive properties and its easily mass synthesis, they are expected to be the potential applications in EM wave absorbing materials [22]. Therefore, when the Ni particles were covered by ZnO shell, the electromagnetic properties of Ni would be boosted, correspondingly. Moreover, it is well accepted that the EM absorption properties are closely related with their morphologies. Herein, the various morphologies of Ni/ZnO composites were synthesized by control of the amounts of NH3·H2O, and the microwave absorption properties of these Ni/ZnO composites have been investigated based on the complex permittivity and permeability.

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,

**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

**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 groups would link with the basic cells of crystalline growth

<sup>2</sup><sup>−</sup> easily by Coulomb force, which means that the positive hydrophilic

) and negative hydrophobic group (OH<sup>−</sup>). The

<sup>2</sup><sup>−</sup>; on the other hand, due to existence

+

**2.1 Core-shell Ni@ZnO composites as microwave absorbers**

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

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

SB and SC, respectively.

NH3·H2O content.

positive hydrophilic group (NH4

groups turn into the carriers of [Zn(OH)4]
