Enhanced microwave electromagnetic characteristics of porous ZnO/Ni/ZnxNiyFe3−x−yO4 hybrid micro-hexahedra

https://doi.org/10.1016/j.matchemphys.2015.06.012Get rights and content

Highlights

  • The selective, mass preparation of porous ZnO/Ni/ZnxNiyFe3−x−yO4 hybrid micro-hexahedra.

  • Modulation over composition, grain size, and SBET modulation.

  • Revealing relationships between structure and properties.

  • Insight into the absorption mechanism of hybrid materials.

Abstract

A green versatile glucose-engineered precipitation/sintering process was developed for the selective, large-scale growth of porous ZnO/Ni/ZnxNiyFe3−x−yO4 micro-polyhedra. Modulation over composition, grain size, and specific surface area can be expediently achieved by changing the Fe3+/Ni2+/Zn2+ molar ratio (γ) and sintering temperature (Ts). Relationships between structure and properties were investigated. High Ts and the appropriate addition of Zn2+ improved the microwave electromagnetic properties of the materials. The ZnO/Ni/ZnxNiyFe3−x−yO4 hybrid micro-hexahedra obtained at 700 °C and γ = 2:0.5:0.5 showed a minimum reflection loss (RL) of −36.52 dB at 10.0 GHz with absorbing frequency (RL ≤ −20 dB) from 4.59 GHz to 18.0 GHz, corresponding to 1.7 mm–5.5 mm coating thickness. The enhanced absorption performances were attributed to the additional multiphase interface, multiresonance, and good matching and absorbing properties of the hybrid materials. The obtained magnetic hybrid materials exhibited promising applications in magnetic devices, catalysis, solar energy conversion, and electromagnetic wave-absorbing materials.

Introduction

With the extensive use of electronic devices and radio technology in modern society, electromagnetic interference (EMI) is given significant research interest in relation to electromagnetic (EM) wave absorbing and shielding materials. Microwave absorbers (a kind of functional material) are used to absorb EM waves effectively by converting EM energy into thermal energy or dissipating EM waves by interference. Complex permittivity and permeability of microwave absorbers are generally considered the key factors in determining microwave absorption properties. Significant effort has been focused on modulating the EM parameters and on the search for new EM-wave absorbing mechanism [1], [2], [3]. Owing to low impedance matching characteristics of absorbing materials with unilateral dielectric loss or magnetic loss, such materials fail to meet several desirable characteristics of microwave absorbers, such as light weight, minimal thickness, broad band, as well as strong absorption and antioxidation properties. As a result, various nanostructured hybrid materials consisting of two or more components have been developed in the recent years [4], [5]. Up to date, the hybrid nanostructures are classified into two types, namely, disordered hybrid nanostructures and ordered hybrid nanostructures, which include spherical core–shell structures [6], yolk-shell structures [7], coaxial nanocables [8], multilayer films [9], and flower-like heterostructures [10], [11]. The design of various hybrid nanomaterials mainly depends on the internal and external interfacing capabilities and the physical/chemical compatibility of their individual components. Furthermore, many characteristics of the absorbent, including intrinsic physical properties, microstructure, morphology, grain size, and congregating state, are important in enhancing the attenuation behavior and broadening the effective bandwidth.

Spinel ferrites are one of the most versatile soft magnetic materials and widely utilized in many fields, such as high-density magnetic memories, sensitive elements, absorption, catalysis, and biology [12], [13]. As absorbing materials, spinel ferrites are particularly suitable for high-frequency applications because of their low magnetic coercivity, high electrical resistivity, low cost, and low eddy current loss [14]. However, some inevitable disadvantages (e.g., high density and big coating thickness) limit their practical application in broad band and light weight absorption coatings. Thus, several approaches have been used to improve the microwave absorbing characteristics of ferrites. One approach is to dope ferrites with different divalent and trivalent cations, which impart distinct electrical and magnetic properties characterized by shift of ions between two sub-lattices. Another approach is to form nanostructured hybrid materials with other materials (i.e., carbon, metal, and polymers.). In addition, modulating grain size, texture, and shape can also control the resulting properties. ZnO nanomaterials are considered promising microwave absorption materials in civilian and military applications because of their light weight, low cost, simple synthesis, light color, and strong absorption ability [15], [16]. Ni exhibits both magnetic loss and dielectric loss with high electrical conductivity [17].

Based on the characteristics of the above materials, we propose a versatile glucose-engineered precipitation/sintering process for the selective and mass preparation of porous ZnO/Ni/ZnxNiyFe3−x−yO4 hybrid micro-polyhedra. The hybridization of the dielectric ZnO, ferromagnetic Ni, and ferromagnetic ZnxNiyFe3−x−yO4 into porous heterostructure micro-hexahedra can realize their objectives through three ways. (1) High specific surface area and additional multiphase interfaces between the dielectric and magnetic materials could generate high dielectric constant and loss, which are advantageous for matching complex permittivity and permeability by adjusting the component and the morphology. (2) Metal dopants can be used to modify the static magnetic and EM properties of ZnxNiyFe3−x−yO4 ferrites by increasing its resistivity and permeability. (3) The use of ZnO and ZnxNiyFe3−x−yO4 to depress the eddy current loss from Ni may improve the high-frequency absorption properties. Additionally, porous structure has usually low density, which is expected to solve the problem of overweight.

The polysaccharide-templating method has been widely used in synthesizing metal or metal oxide sponges [18]; to best of our knowledge, only some irregularly shaped particles with sponge-like structure have been obtained. Few studies have been conducted on the preparation and microwave absorbing properties of ZnO/Ni/ZnxNiyFe3−x−yO4 hybrid micro-polyhedra. Compared with a previous study [18], glucose used in the current study functions as reductive, protecting agents, structure-directing agents, and sacrificial template. Glucose can not only guide the self-assembly of sheet-like nuclei into regular precursor polyhedra, but it can also serve as sacrificial template in forming porous structures. The current method possesses significant benefits over existing technologies because of its simplicity, low cost, benign environment, and mass preparation. The homogeneous porous ferrite micro-polyhedra prepared through this method exhibit composition- and size-dependent changes in the properties. The effects of Fe3+/Ni2+/Zn2+ molar ratio (γ) and sintering temperature (Ts) on structure and microwave EM properties of ZnO/Ni/ZnxNiyFe3−x−yO4 hybrid materials were systematically investigated.

Section snippets

Synthesis of samples

All the chemicals used in this study were of analytical grade and used without further purification. In a typical procedure, 0.15 mol d-(+)-glucose and 0.05 mol metal nitrate with a Fe3+/Ni2+/Zn2+ molar ratio (γ) of 2:0.5:0.5 were dissolved in 100 mL of deionized water. The above solution was vigorously stirred at 70 °C until the water evaporated and yellow-green sediment appeared. The precursors were collected by filtration, washed several times with deionized water, and dried in air at 80 °C

Structural characterization of the sintered products

The oxalate precursors were obtained by the d-(+)-glucose-engineered co-precipitation method [13], [19]. The results of the XRD and IR analyses indicated that the typical precursors were a mixture of ZnC2O4·2H2O, NiC2O4·2H2O, and FeC2O4·2H2O (Supporting Information Fig. S1), according to the standard data (JCPDS Card Nos. 25-1029, 25-0581, and 22-0635). The carbothermic reduction method is an inexpensive and general technology used in controlling phase, grain size, and texture of sintered

Conclusions

In summary, we reported the selective and mass preparation of porous ZnO/Ni/ZnxNiyFe3−x−yO4 hybrid micro-hexahedra through a green versatile glucose-engineered precipitation/sintering process. The as-obtained ZnO/Ni/ZnxNiyFe3−x−yO4 micro-hexahedra possessed numerous inter-particle mesopores (1 μm–4 μm long and aspect ratios of 1.5–2). Composition, grain size, and SBET modulation can be achieved by altering sintering temperature Ts and Fe3+/Ni2+/Zn2+ molar ratio γ, allowing the control of the

Acknowledgments

This work was financially supported by the National Natural Scientific Foundation of China (51102215), Natural Scientific Foundation of Zhejiang Province (Y4100022 and LY14B010001), and National Innovation and Entrepreneurship Training Program of Undergraduates (201310345016 and 201410345014).

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