Nitric acid oxidation treatment promoting microwave absorption performance of carbonized melamine foam

Abstract

Carbonized melamine foam has been recognized as a promising material for microwave absorption due to its exceptional thermal stability, lightweight, and remarkable dielectric properties. In this study, we investigated the impact of nitric acid oxidation on the surface of carbonized melamine foam and its microwave absorption properties. The treated foam exhibits optimal reflection loss of −21.51 dB at 13.20 GHz, with an effective absorption bandwidth of 7.04 GHz. The enhanced absorption properties are primarily attributed to the strengthened dielectric loss, improved impedance matching, and increased polarization losses resulting from the oxidized surfaces. This research demonstrates a promising new approach for research into surface treatments to improve the performances of microwave absorbers.

1. Introduction

Microwave absorbing materials have attracted growing research interest for electromagnetic interference (EMI) shielding applications in aviation, civilian, and military fields [1−3]. To mitigate harmful electromagnetic radiation, materials with strong microwave absorption properties are required [4, 5]. Over the past decades, various microwave absorbers have been developed with excellent absorption performances, including carbon materials [6], ferrites [7] and ceramics [8]. Carbon-based absorbers, including carbon nanotubes [9, 10], nanofibers [11, 12] and graphene [13, 14], have emerged as promising candidates owing to their high thermal stability, low density, and tunable electromagnetic properties. Among them, carbon foam, possessing open-cell structure with ultrahigh surface area, has gained much interest as lightweight and highly effective microwave absorbers. For example, the multifunctional Ni/MXene-MF with the microcapacitor structure and 3D porous structure was reported to enable good impedance matching and strong attenuation efficiency, showing minimum reflection loss (RLmin) of −62.7 dB and effective absorption bandwidth (EAB) of 6.24 GHz at thickness of 2 mm [15]. 3D graphene-coated foam structure with embedded Co3O4 particles was developed to achieve RLmin of −31.88 dB at 11.54 GHz and EAB of 3.4 GHz [16]. The enhanced performance was attributed to the synergistic effects of impedance matching, interfacial polarization, and polarization relaxation. In addition, porous carbonated melamine foam (CMF) with layers of FeNi alloy and SiO2 (CMF/FeNi15-SiO2) exhibited RLmin of −53 dB at 3.38 GHz and EAB of 0.93 GHz at thickness of 4.1 mm [17]. It is worthy to be mentioned that the 3D foam structure, carbon skeleton, FeNi alloy, and SiO2 coating shell synergistically contributed to the excellent microwave absorption in the 2−6 GHz frequency band. However, pure carbon-based materials alone as microwave absorbers remain a major challenge caused by high reflectivity and poor impedance mismatch [18−20].

As we know, microwave absorption ability relies on converting incident electromagnetic waves into thermal energy through dielectric and magnetic loss. Generally, modifying the surface, interface and inner chemical structure of carbon materials can optimize dielectric properties and the impedance match, promoting the electromagnetic waves absorption performance [21−24]. For example, HU et al [24] studied the microwave absorption properties of pristine multi-walled carbon nanotube (MWCNT)-P, MWCNT-OH and MWCNT-COOH, and proposed to tune the microwave absorption properties of multi-walled carbon nanotubes by surface functional groups. The absorption efficiencies of MWCNT-P, MWCNT-OH and MWCNT-COOH at 8−18 GHz are >33%, >50% and >45%, respectively. Compared with MWCNT-P, 17% absorption efficiency of MWCNT-OH is significantly enhanced. DAI et al [25] designed 2D structure of Ti3C2Tx MXenes/nano-carbon-sphere hybrids, which exhibited superior microwave absorption performance due to formation of heterogeneous interface structure. The optimal reflection loss (RL) was −54.67 dB at 3.97 GHz with an absorber thickness of 4.8 mm. SONG et al [26] incorporated the dielectric polarization effect induced by atomic-scale structural defects to improve the electromagnetic absorption performances of materials.