Composition and structure control of ultralight graphene foam for high-performance microwave absorption

Abstract

Macroscopic lossy foam has been expected to be the most promising candidate for lightweight high-performance microwave absorption (MA). However, inferior MA behaviors of conventional foams reported previously are disappointing. The emerging graphene foam (GF) has broken this paradoxical state of affairs. Here, series of GFs with various chemical compositions and physical structures have been prepared via a facile and controllable method and their MA performance is investigated in 2–18 GHz. The in-depth analyses of the GF’s composition, structure and MA property demonstrate that the MA performance of the GF is strongly correlated with the C/O ratio, conjugated carbon domain size and graphene framework’s microstructure. A maximum absorption value of −34.0 dB as well as 14.3 GHz qualified bandwidth with reflection loss below −10 dB is achieved for the GF with an ultralow bulk density of 1.6 mg/cm³, of which the average absorption intensity and the specific MA efficiency are much higher than those of the best available MA materials in previous literature. The composition & structure–performance relationship of MA foams is revealed. The balance between small interfacial impedance gap and high loss characteristic has wide implications in improving the MA performance of the GF and other porous materials.

Introduction

With the rapid arising of information technology, microwave absorption materials are playing an increasingly significant role in electronic reliability, healthcare, and national defense security [1], [2], [3], [4]. For example, the microwave absorption (MA) materials applied in the emerging high-speed communication apparatus like satellites could improve the receiver’s signal quality by suppressing the noise [5]. Besides, MA materials in the radar station and the relay station could protect inside workers from overdose exposure to high-power microwave [6]. Most importantly, with the gradual maturation of novel advanced anti-stealth radars such as ultra wide band radar, phased array radar, multi-static radar and passive radar, high-performance counter-detection MA materials serve as a very efficient route in increasing the survivability of military units via reducing their radar cross-section [7]. The ideal MA materials are primarily required to establish an excellent double-win relationship between intense absorption ability and broad absorption bandwidth. In addition, MA materials with ultralight weight and thin thickness will be advantageous in the fields of aerospace, aviation, ground vehicles and fast-growing next-generation green miniature electronics [1], [8], [9].

The interfacial impedance gap and radiation energy loss characteristics are considered as the two core principles that determine the MA performance of a material [10], [11], [12], [13]. The microwave propagation for a typical homogenous material’s MA process depends on several factors, including dielectric permittivity ε, magnetic permeability μ and electrical conductivity δ, which are a comprehensive reflection of significant component and structural characteristics [4], [14], [15], [16].

For decades, researchers have made considerable efforts towards designing and fabricating various MA materials by adjusting the electrical conductivity, dielectric constant and magnetic permeability in the pursuit of low interfacial impedance gap as well as high loss ratio of incident microwave [10], [11], [13], [17], [18]. In most cases, separate solid particle absorbents, such as ferrites [19], [20], metal powders [20], [21], ceramics [22], carbon nano/micromaterials [23], [24] and their hybrids [2], [15], [25], [26], are extensively adopted as fillers into microwave-transparent organic or inorganic adhesives to fabricate MA composites. Besides mediocre MA performance, most of them have also been kept far from practical application for some shortcomings, such as high density, poor stability and large loading content [1], [12], [27]. It has been demonstrated, for instance, 70 wt% or more magnetic iron particles with a very high density of 8 g/cm³ are required in typical MA composites [17], [20].

Three-dimensional (3D) macroscopic porous lossy materials have been expected to be the most promising candidate for lightweight high-performance broadband MA application [28], [29], [30], [31], [32]. Compared with conventional uniform solid MA materials, the MA foam, with so many homogenously-dispersed internal pores, not only shows lower bulk density but also gives much smaller effective permittivity, which makes it less resistive to the detective incident microwave in a wide frequency range [16], [33]. Until now, considerable attentions have been paid to synthesis and application of porous bulk materials for microwave suppression, such as conductive polymer foam [28], [34], [35], silicon carbide foam [36], [37], carbon foam [7], [29], [38], [39] and carbon nanotube sponge [40]. However, for most MA foams reported previously, their MA behaviors could not be compared with those of traditional solid MA materials [7], [35], [36]. Furthermore, it is still a big challenge to reveal the composition & structure–performance relationship of MA foams due to their complicated irregular structures and preparation techniques, which severely hinders their practical application.

Recently, significant progress toward 3D macroscopic interconnected graphene networks has opened up a new route for the exploitation of porous bulk material for lightweight and broadband high-performance MA application [38], [41], [42], [43], [44], [45], [46], [47], [48], [49]. In the previous communication, we preliminarily proved the outstanding microwave absorbing performance of macroscopic GFs, which showed that GFs may have great potential in MA application [32]. However, there remains much uncertainty in the GF’s MA property dependence on its morphology and composition. Therefore, it is very significant to develop a facile and controllable method to prepare additive-free large-sized GFs and establish the relationship between the MA property and the GF’s intrinsic structure and component, which is essential in an in-depth understanding of its MA mechanism and more importantly developing a universal strategy to effectively enhance the MA property of bulk porous materials.

Herein, we demonstrate design and fabrication of various GFs with different internal morphologies and compositions and investigate their MA performance in 2–18 GHz, which is intensively occupied for satellite communications, remote sensing, radar detections and weapons guidance and tracking. The MA performance of the GF foam is found to be strongly correlated to the C/O ratio, sp2 carbon domain size and graphene framework microstructure. A maximum absorbing value of −34.0 dB as well as 14.3 GHz qualified bandwidth can be obtained for the GF with an ultralow density of 1.6 mg/cm³, which is close to the density of ambient air (1.2 mg/cm³) and much lower than those of the carbon foam (166 mg/cm³) [29] and the SiC foam (∼256 mg/cm³) [37]. More importantly, the GF presents the best average absorption intensity compared with other typical MA materials in 2–18 GHz.The specific MA efficiency is nearly two orders of magnitude higher than those of the best available MA materials ever reported. The mechanism for the MA performance dependence on the composition and structure of the GF is revealed. The well-matched interfacial impedance combined with high loss ability gives rise to the enhanced MA performance.