Microwave absorbing property and complex permittivity and permeability of epoxy composites containing Ni-coated and Ag filled carbon nanotubes

https://doi.org/10.1016/j.compscitech.2007.10.006Get rights and content

Abstract

Microwave absorbing property, complex permittivity, and permeability of epoxy composites containing Ni-coated and Ag nanowires filled carbon nanotubes (CNTs) have been investigated. CNTs were coated with nickel by an electroless plating technique, whereas CNTs were filled with Ag nanowires via a wet chemical method. Compared with the CNTs/epoxy composites, the real part (ε′) and imaginary part (ε″) of the complex permittivity as well as dielectric dissipation factor (tgδε = ε″/ε′) of the epoxy composites containing the Ni–P coated CNTs were much smaller, while the real part (μ′) and imaginary part (μ″) of the complex permeability and the magnetic dissipation factor (tgδμ = μ″/μ′) were greater. The microwave absorption enhancement of Ni-coated CNTs/epoxy composites resulted from dielectric and magnetic losses. In contrast, the microwave absorption of Ag nanowire-filled CNTs/epoxy composites was mainly attributed to the dielectric loss rather than magnetic loss. The absorption peak frequency of the CNTs/epoxy composites can be controlled by plating CNTs with nickel or by filling Ag nanowire into CNTs.

Introduction

Nano-materials have received increasingly more attention for their potential applications as practical structural and functional materials. Since the discovery in 1991 [1], carbon nanotubes (CNTs) have attracted more and more interest for their distinguished properties and promising future applications. The unique mechanical properties of CNTs, their high strength and stiffness and the enormous aspect ratio make them a potential structural elements for the improvement of mechanical properties. Further potential advantages, which promote CNTs as ultimate fillers for polymers, are their electrical and thermal conductivity together with a low density [2], [3], [4], [5], [6], [7]. Recently, synthesis and investigations of electrical properties of composites based on polymer matrices and CNTs are very popular topics in the field of materials science [8], [9]. However, the realization of CNTs reinforced polymer composites requires a homogenous dispersion and a strong interfacial interaction between CNTs and the polymer. A significant amount of different surface treatment and functionalization techniques have been devised for improving the dispersion and interfacial adhesion between the CNTs and polymer matrices [10]. These techniques include chemical treatments using high concentration acids [11], sheathing or wrapping the CNTs with polymer chains [12], [13], grafting the CNTs with a thin layer of polymer chains based on plasma [14], chemical functionalization with the ultra-violet/ozone (UV/O3) treatment and coupling agent [10], [15], and a combination of these. The acid treatments were aimed at creating defects and covalent sidewall derivations by an oxidative process with strong acids such as HNO3, H2SO4 or a mixture of them [16], or with oxidants such as KMnO4 [17]. As a result, the composites exhibited higher mechanical and other functional properties than those without treatment through the formation of stronger interfacial bonds and more homogeneous dispersion of CNTs [10].

Magnetic nano composites have potential applications in various areas such as magnetic recording, magnetic data storage devices, toners and inks for xerography, and magnetic resonance imaging. Therefore, studies on magnetic nano composites, especially on magnetic CNTs composites, are rapidly expanding. Many researchers have attempted to deposit metals or metal compounds onto the CNTs surface. The most straightforward route to achieving this should be to melt the elements on the tube surface. However, Dujardin et al. [18] have studied the wetting of CNTs in detail and reported that the determining factor for wetting was surface tension, with a cut-off limit between 100 and 200 mN/m. This limit implied that typical pure metals, such as: aluminum (surface tension of 865 mN/m), copper (1270 mN/m), iron (1700 mN/m), would not be easily wetted on the surface of CNTs [19]. This means that CNTs will be difficult to achieve high-strength interfacial adhesion between the CNTs and the metals, if their surfaces are not modified. Electroless plating with catalytic metals is an effective way to deposit metals onto the CNTs surface, after which the coated metal layers can serve as medium for adhesion and transferring loads in the metal–matrix composites. Continuous Ni-layer [19], [20], copper [20], cobalt [21], and Ni–P alloy [22], [23] have been coated onto the CNTs surface by electroless plating. Here we report an investigation of the possible use of Ni-coated CNTs prepared by electroless plating as microwave absorbing material.

Microwave absorbing materials have attracted significant interest because of their applications in commercial and military industries. The manufacture of microwave absorbing materials involves the use of compounds capable of generating dielectric and/or magnetic losses when impinged by an electromagnetic wave. CNTs as conductive fillers have been studied in microwave radiation absorbing materials due to their physical and chemical properties. Despite several studies on the microwave absorption and complex permittivity and permeability of CNTs and Fe-filled CNTs, the studies for Ni-coated CNTs and Ag nanowires filled CNTs are few. In this paper, we investigate the preparation of Ni-coated CNTs and Ag filled CNTs, and their microwave absorbing property and complex permittivity and permeability.

Section snippets

Experimental

The CNTs were prepared by catalytic decompose of benzene using floating transition method at 1100–1200 °C in our laboratory. Benzene was used as carbon source and iron as catalyst with sulfur. Ni was coated onto the CNTs by an electroless plating technique. The procedure of electroless plating can be divided into three steps as that of graphite [19]: sensitization, activation and plating. CNTs were suspended in an aqueous solution of 0.38 M of K2Cr2O7/4.5 M of H2SO4 and refluxed in a water bath

Morphology of Ni-coated CNTs and Ag nanowires filled CNTs

Fig. 1 shows the TEM image of CNTs. The TEM image reveals that the CNTs are straight with diameter 30–80 nm, internal diameter 10–50 nm and length 50–100 μm. Fig. 2 shows the morphologies of Ni–P and Ni–N coated CNTs. In the step of Ni electroless plating, Pd particles served as seeds for catalytic nucleating centers. Above the individual Pd catalytic centers deposited on the surface of CNTs nickel ions were reduced to neutral nickel atoms. Energy-dispersive X-ray spectroscopy (EDXS) analysis

Conclusions

Compared with the CNTs/epoxy composites, the ε′, ε″ and tgδε of the epoxy composites containing the Ni–P coated CNTs were much smaller, while the μ′, μ″ and tgδμ were greater. The increases of μ′, μ″ and tgδμ were mainly attributed to the nickel plate on the CNTs surface. The absorption peak frequency of the CNTs/epoxy composites can be manipulated easily by plating different nickel coatings onto the surface of CNTs or filling Ag nanowires into CNTs. The microwave absorptions of composites

Acknowledgement

This work was supported by the Natural Science Foundation of China (Grant No. 50672004).

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