Lightweight and efficient microwave absorbing materials based on walnut shell-derived nano-porous carbon

Abstract

Lightweight microwave absorbing materials have drawn tremendous attention. Herein, nano-porous biomass carbon materials have been prepared by carbonization with a subsequent potassium hydroxide activation of walnut shells and the microwave absorption properties have also been investigated. The obtained samples have large specific surface areas with numerous micropores and nanopores. The sample activated at 600 °C with a specific surface area of 736.2 m g exhibits the most enhanced microwave absorption performance. It has the maximum reflection loss of -42.4 dB at 8.88 GHz and the effective absorption bandwidth (reflection loss below -10 dB) is 1.76 GHz (from 8.08 GHz to 9.84 GHz), corresponding to a thickness of 2 mm. Additionally, the effective absorption bandwidth can reach 2.24 GHz (from 10.48 GHz to 12.72 GHz) when the absorber thickness is 1.5 mm. Three-dimensional porous architecture, interfacial polarization relaxation loss, and the dipolar relaxation loss make a great contribution to the excellent microwave absorption performance. In contrast, the non-activated sample with lower specific surface area (435.3 m g) has poor microwave absorption performance due to a poor dielectric loss capacity. This comparison highlights the role of micropores and nanopores in improving the dielectric loss property of porous carbon materials. To sum up, porous biomass carbon has great potential to become lightweight microwave absorbers. Moreover, KOH is an efficient activation agent in the fabrication of carbonaceous materials.

Biomass-derived carbon materials with structural diversities and their applications in energy storage

“…In this progress, the biomass precursor is mixed with some activating agents such as KOH, NaOH, KHCO 3 , ZnCl 2 , H 3 PO 4 , H 2 SO 4 , then simultaneously carbonization and activation are carried out at 300-950°C. Among the activating agents, KOH has been explored for the activation of biomass-derived carbons in elm samara [91], honeysuckle [121], lignin [151], cabbage leaves [152], willow catkins [80] and so on [108,153]. For example, our group prepared a series of biomass-derived carbons by one-step activating cattail [154], willow catkin [81], bacterial cellulose [61], soybeans [130], and wheat flour [136] with KOH activation.…”

“…The as-prepared carbon has highly interconnected pores with the pore size of about 0.5-1 nm, high micropore volume ratio of 83% and large surface area of about 1069 m 2 g −1 . Moreover, the biomass-derived carbon materials with abundant micropores can also be prepared from ginkgo leaves [101], cotton stalk [102], pine needles [103], acacia gum [104], pomelo peel [105] popcorn [106], cornstalk [107], walnut shell [108]. Furthermore, microporous surface area and pore volume depend on the amount of KOH and heat treatment temperature [42,102,104].…”

Significant promotion of porous architecture and magnetic Fe3O4 NPs inside honeycomb-like carbonaceous composites for enhanced microwave absorption

“…Therefore, biochar-derived materials have been regarded as promising candidates for high-performance microwave absorbers. For instance, Qiu et al 25 reported that porous biomass carbon was a lightweight and effective microwave absorber, which was mainly due to its dielectric loss; however, the contribution of the magnetic loss mechanism that can be provided by magnetic components was not employed. Actually, many kinds of oxygencontaining functional groups on the surface of biomass-derived carbons are suitable for modifying with a magnetic material to improve impedance matching, which could distinctly improve the microwave absorption performance.…”

“…25 In detail, walnut shells were crushed into particles (about 10 mm) and then washed three times with deionized water (20 C) and dried at 80 C. The dried walnut shells (10 g)…”

Electromagnetic and microwave absorption characteristics of PMMA composites filled with a nanoporous resorcinol formaldehyde based carbon aerogel

“…3,8,24 Moreover, in addition to general absorption mechanisms, multiple reections of microwaves in CAs composites also lead to an additional loss mechanism of EM energy, which is based on the interference of reected waves inside CAs particles. 8,18,22,48,52 In comparison with the typical carbon-based absorbers, depends on synthesis procedure CAs possess pores with different sizes and shapes within their structure, which could provide additional pathway for the absorption of EM waves. 22 Hence, under the above circumstances, preparation of CAs composites with larger particle sizes results in better impedance matching condition under which the EM waves are retract and attenuate when penetrate through the carbon particles.…”