Metal–organic-frameworks derived porous carbon-wrapped Ni composites with optimized impedance matching as excellent lightweight electromagnetic wave absorber

Highlights

• Unique rod-like porous Ni/C nanocomposites can be facially obtained.

• Electromagnetic parameters can be tuned by carbonization temperature.

• Impedance match is optimized to achieve reflection loss performance.

• RL value of −23.4 dB and effective frequency bandwidth of 4.68 GHz can be achieved at 1.9 mm.

Abstract

In recent years, metal–organic-frameworks derived composites, especially magnetic nanoparticles embedded in porous carbon matrix have emerged as promising candidate for lightweight electromagnetic wave absorber. Nevertheless, investigation on the optimization of impedance matching in the microwave absorption properties is insufficient. In this work, impedance matching is optimized to achieve strongest absorption intensity and broaden the effective frequency bandwidth. In detail, electromagnetic parameters have been controlled through changing the carbonization temperature to fulfill optimized impedance matching. RL value of −51.8 dB and an effective frequency bandwidth (f_e) of 3.48 GHz with a thickness of 2.6 mm can be achieved by sample prepared at 500 °C and RL value of −15.0 dB and a f_e of 4.72 GHz with a thin thickness of 1.8 mm can be reached by sample synthesized at 600 °C. Comparative studies of each sample directly display the effect of impedance matching on the RL performance and possible attenuation mechanisms are also discussed. This work may deepen the understanding of the impedance matching and pave the way for the synthesis of high performance lightweight microwave absorber.

Introduction

As the rapid development of information technology, especially the explosive progress in the radar technology, tremendous efforts have been made to obtain high performance microwave absorption materials [1], [2], [3]. Generally, strong absorption intensities, broad frequency bandwidth, thin thickness and lightweight are desired particularly in the fields of aeronautics and microelectronics. Nevertheless, traditional fillers including magnetic metal or alloys, ferrites, ceramics and their composites can hardly meet all the requirements due to their high density, poor stability as well as high weight ratio [4], [5].

As alternatives, carbon materials have attracted a lot of attention owing to their unique properties including strong dielectric loss capabilities, low density, tunable electrical conductivity, high thermal and chemical stability and easy preparation from natural world [6], [7], [8]. Several kinds of carbon materials such as carbon fibers [3], carbon nanotubes [9], carbon nanocoils [10] and graphene [11] have been explored thoroughly and proved to possess good microwave absorption performance. Although excellent microwave absorption performance can be achieved by several carbon materials, more attempts should be made to overcome their drawbacks.

On one hand, it is believed that impedance mismatching between carbon and air may hinder the incident electromagnetic wave enter into the microwave absorbing materials and result in enhanced reflection and poor absorption [12]. Generally, there are two methods to achieve impedance match. One way is decreasing the complex permittivity. For instance, Wang et al. managed to prepare carbon foam from coal liquefaction residue and impedance matching was realized through changing carbonization temperature [13]. Qiang et al. illustrated that carbon sphere with york-shell structure possesses lower complex permittivity than solid carbon spheres, thus excellent dielectric loss can be gained [6]. Insulators and semiconductors with low complex permittivity are often introduced to achieve better impedance matching. SiO₂ shell was employed not only to protect Fe₃O₄-MWCNT composites, but also improve the impedance matching [14]. The complex permittivity of graphene was also successfully decreased by in-situ growing ZnO nanocrystals [15]. The other way is increasing the complex permeability. Magnetic metal and ferrites can not only increase magnetic loss, but also strengthen dielectric loss through catalytic graphitization of carbon. Fe₃O₄ was combined with graphene capsules to improve impedance matching and resulted in good microwave absorption performance [8]. Carbon foam has been synthesized from Ni²⁺/alginate foam in which Ni not only improve graphitization but also enhance complex permeability [7].

On the other hand, limited loss mechanisms would restrict the further application of single component carbon materials [16]. Generally, loss mechanism for pure carbon is mainly conduction loss which is harmful for stronger attenuation abilities. Interface polarization is emphasized recent years with systemic research [17]. Several techniques have been developed to enhance the interface polarization of carbon material. For example, Kong et al. aimed to deposit gamma-Fe₂O₃ on graphene and found that interface polarization was enhanced with better microwave absorption performance [18]. Che’s group pointed out that intersectional angel of neighboring CNTs films has great influence on the interfacial polarization and reflection loss (RL) performance [9]. Besides, the synthetic routes for some carbon material are complicated, time-consuming, low-yield with expensive equipment [19].

Although excellent work has been done, more attention can be paid on the optimization of impedance matching as well as metal–organic-frameworks-derived (MOFs-derived) carbon composites [20], [21], [22], because they may possess better impedance matching and RL performance related with their unique composition and highly porous structures. To summarize, MOFs-derived carbon composites are designable duo to that MOFs can be designed to get desirable structure and composition [23]. Moreover, facial solid-state carbonization technique has been developed to obtain MOFs-derived porous metal/carbon composites. They can be classified as Fe/C [24], Co/C [25], Ni/C [26], etc.

As for Fe/C composites, excellent work has been done [24]. However, it should be mentioned that Fe can be hardly obtained by carbothermal reduction under low temperature. Meanwhile, the yield can hardly meet the practical requests considering the large amount of polyvinylpyrrolidone and water. Moreover, although the effect of temperature has been studied, the change and effect of impedance match deserves more investigation. As regard to Co/C composites, great research has also been done [27]. Nevertheless, Co²⁺ is also hard to be reduced to Co during carbonization process and the existence of CoO may weak the magnetic loss abilities. The use of poisonous methanol and low-yield may also restrict its further application. Moreover, the study about effect of temperature on impedance matching is incomplete. It is generally accepted that more positive reduction potential stands for easier reduction of metal ions [28]. Compared with Fe²⁺ (−0.44 V) and Co²⁺ (−0.28 V), Ni²⁺ possessed more positive reduction potential (−0.25 V). Thus, metallic Ni can be easily obtained from Ni-based MOFs by carbothermal reduction under lower temperature. However, to the best of knowledge, very little report about MOFs-derived Ni/C composites for microwave absorption can be found.

Inspired by previous work, porous Ni/C composites were gained from a Ni-based MOFs, Ni(bdc)(ted)_0.5 via solid-state carbonization. Ni(bdc)(ted)_0.5 is one of members in the group of microporous metal organic frameworks M(bdc)(ted)_0.5 (M = Cu, Zn, Ni, Co; bdc = 1, 4-benzenedicarboxylate; ted = triethylenediamine). Its crystal belongs to the tetragonal crystal system with space group of P4/ncc. And the frameworks contains secondary building unit of Ni₂(COO)₄ which is interconnected via bdc and ted. In detail, four oxygen atoms from bdc and one nitrogen atom from ted coordinate with one Ni atom [29]. It can be prepared solvothermally in large scale without poisonous reagent and Ni/C composites can be obtained under a rather low temperature (500 °C). Effect of temperature on impedance matching is studied systematically which is achieved by carefully tuning the complex permittivity and permeability [30], [31]. Besides, the highly porous nature of the MOFs-derived carbon-based composites gives the lightweight feature. The experiments may shed light on the synthesis of high-performance lightweight microwave absorbing materials.