Construction and microwave absorbing properties of Fe_xO_y/C composites based on corncobs

As a by-product of corn production and processing, corn cobs have the characteristics of high yield, wide source and low cost, but the potential value of waste corn cobs is often not fully utilized, and the random discarding or incineration has caused serious waste and environmental pollution. In order to realize the high-value utilization of waste corn cobs and the low-cost preparation of carbon-based composite electromagnetic wave absorbing materials (EMWAMs), the waste corn cobs were recycled as the carrier to prepare Fe_xO_y/C composite EMWAMs by using the simple wet chemical impregnation method and carbon thermal reduction route. The effects of different magnetic components and their loadings on the properties of the composites were investigated by optimizing the sintering temperature and the concentration of precursor solution. Various characterization methods including XRD, Raman, SEM, BET, VNA were used to analyze the phase composition evolution, microscopic morphology, and microwave absorbing properties of the composites. It was founded that the microwave absorbing properties of the Fe_xO_y/C composites could be effectively improved when introducing the moderate amount of magnetic components, under which the Fe/C composite still maintained the natural three-dimensional pore structure of corncobs, with a specific surface area of up to 126.97 m²/g and a pore diameter of 5.40 mm, which benefited for the multiple reflections and losses of incident electromagnetic waves. In the case of excessive loading, it would lead to the impedance mismatch of the composites, which is not conducive to the optimization of the wave-absorbing ability. Among them, FCC-0.3-700 and FCC-0.3-900 exhibited excellent microwave absorption performance, and the minimum reflection loss (RL_min) value reached −41.4 dB at a coating thickness of 2.0 mm for FCC-0.3-700, with an effective absorption bandwidth (EAB) of 5.6 GHz. While the RL_min value was −41.7 dB at 8.4 GHz and the EAB was 4.5 GHz under the coating thickness of 1.5 mm for FCC-0.3-900.The outstanding absorption performance of Fe_xO_y/C composites was attributed to the fact that the defects created by the partial consumption of fixed carbon in the corn cores enhanced the multiple scattering and reflection of the incident electromagnetic wave, and these defects could also provide dipole sites for generating dipole polarization, which was beneficial to the attenuation of electromagnetic wave. In addition, Fe₃O₄ and Fe particles were uniformly embedded in the apertures of the waste corn cobs or dispersed among the fragments, forming a large number of effective interfaces and enhancing the interfacial polarization. At the same time, the conductive network formed by the graphitic carbon and Fe₃O₄/Fe particles in the Fe_xO_y/C composites greatly promoted the conductive loss. The Fe_xO_y/C composites prepared in this work are not only accord with the characteristics “thinner thickness, lighter mass, wider absorption bandwidth and stronger absorption intensity” of EMWAMs, but also open the new approach for the high value and functional utilization of waste corn cobs.