Microwave absorbing materials are very important for many applications in modern technology including electronics and telecommunications [1]. Microwave absorbing materials are designed to provide electromagnetic shielding and absorption that find their applications in commercial, civilian, and consuming electronic devices applications [2-9]. For example, circuitboard engineers often find that their well-devised circuit board does not operate properly when it is integrated with the shielded cavity or chassis. The resonance in shielded microwave cavities is the increasingly recognized cause of the problem. Microwave absorbing materials can be used as a viable method for dampening microwave cavity resonances. When exposed to an external electromagnetic field, the electrons in the microwave absorbing materials will generate an inductive current which then cause radiation attenuation and energy dissipation. Carbon nanotubes (CNTs) and CNT composites have been emerging as new perspective microwave absorbers due to their fascinating physico-chemical properties such as light-weight, resistance to corrosion, high mechanical strength, larger flexibility and superior electrical performance [6,10-13]. Theoretic and experimental studies have demonstrated that the unique one dimensional hollow tube structures of CNTs enable them to show outstanding electrical properties such as either ballistic transport or diffusive transport, along with long mean free paths [14]. The conjugate π electrons are restricted in the one-dimensional cylinder and thereby enable CNTs to show distinctive electronic response and offer CNTs the ability to serve as an emerging microwave absorbing material [15].

Multi-walled carbon nanotubes (MWCNTs)-polymer composites offer a large flexibility for design and control of microwave absorption behaviors, as the composites can be tailored through changes in loading fractions, matrix materials, complex permittivity and loss tangent of MWCNTs-polymer composites [1,16,17]. The microwave absorbing properties of the MWCNTs-polymer composites depend on the CNT loading fraction, which can affect the absorption bandwidth and resonance frequency. It was reported that the absorption peak of MWCNTsepoxy composites shifted downward with increasing the MWCNT loading [12]. It is observed that the absorption frequency ranges of single-walled carbon nanotubes (SWCNTs)-polyurethane composites broaden from 6.4 - 8.2 (1.8 GHz) to 7.5 - 10.1 (2.6 GHz) and to 12.0 - 15.1 GHz (3.1 GHz) as the SWCNTs loadings in the composites decreased from 10 to 5 and to 2 wt%, respectively [18]. Hence, by controlling the loading fraction of the CNT-composites, microwave absorbing response may be tuned in a broad range with strong absorption and wide absorption bandwidth. In addition, the complex permittivity and permeability of MWCNTs-polymer composites are the fundamental physical quantities in determining the microwave properties [19-23]. From the classic transmission line theory, an adjustment of complex permittivity and permeability satisfying the impedance match condition will offer the possibilities to produce light-weight and cost-effective microwave absorbing materials [24].

Among the broad polymer candidates suitable for embedding MWCNTs for preparing the microwave absorbing materials, epoxy is one of the most attractive polymers due to its commercial availability, resistance to corrosion, and relatively easy for processing. Thus, MWCNTsepoxy composites may be used as microwave absorbing as well as protective coating for potential applications. In this work, MWCNTs-epoxy composites with different MWCNTs weight loadings were fabricated by a mechanical stirring method. In the previous publications, we presented the microwave absorption properties of MWCNsepoxy composites at microwave frequencies around 9.968 GHz and 8.43 GHz, utilizing a microwave resonant cavity as a probe [25,26]. In this study, we focused on the microwave absorption performance of the MWCNTsepoxy composites in a continuous frequency range from 2 to 18 GHz.

MWCNTs with outer diameters (OD) less than 8 nm were obtained from a commercial company (Cheaptubes, USA). The purity of MWCNTs is better than 95%. The ash content in the MWCNTs powder is less than 1.5 wt%. The length distribution of MWCNTs is in the range of 10 - 50 µm. MWCNTs have much higher performance-toprice ratio (PPR) than that of SWNTs for large scale composite applications.

MWCNTs-epoxy composites were fabricated via mechanical mixture methods. The loading of MWCNTs in the composites was controlled from 1 to 10 wt%. Firstly, the epoxy resin (Aero Marine 300/21) and MWCNTs were mixed and stirred in a hotplate magnetic stirrer at 90˚C for 1 h, where the high temperature was used to reduce the viscosity of epoxy for well dispersion of the mixture. Then, a hardener agent was added into the mixture and stirred for 10 min. Care was taken to reduce bubbles in the mixture by controlling the stirring rate. After that, the mixture was injected into hollow cylinder molds and subsequently transferred to an oven for precuring and post-curing at 80˚C and 120˚C respectively.

Both curing processes lasted for 1 hour. The as-obtained O-ring shape samples (the inner and outer diameters of the samples are 1.5 mm and 3.5 mm, respectively) were carefully cut to yield a uniform thickness of 2 mm. In this experiment, a maximum loading amount of 10 wt% MWCNTs into the epoxy has been realized, beyond that MWCNTs cannot homogeneously dispersed in the epoxy.

Scanning electron microscopy (SEM) images of the samples were recorded on JSM-6390 (JEOL, Japan) with an accelerate voltage of 5 KV. All the samples were coated with a layer of gold for good conductivity. X-radiation Diffraction (XRD) analysis of the samples were performed on MiniFlex 600 (Rigaku, Japan) with 2θ scanning range from 10˚ to 80˚.

Both the relative complex dielectric permittivity ε = ε′ – jε″ and magnetic permeability μ = μ′ – μ″ were obtained by utilizing Agilent Vector Network Analyzer N5230C (Agilent Company, USA), a coaxial transmission method, and Agilent 85,071 material measurement software for a frequency range from 2 to 20 GHz.