Phase transformation and carbon precipitation of coal fly ash magnetospheres during a CVD process for microwave adsorption

Abstract

Abstract Coal fly ash is industrial waste produced by coal power plants, containing large amounts of magnetospheres (MSs), where Fe3O4 is abundant and magnetite is the major composition. However, the functional reuses of MSs are always restricted by the impurities of aluminosilicate and other forms of iron oxide including maghemite, magnesioferrite, as well as hematite. In this paper, a chemical vapor deposition (CVD) process was proposed to convert MSs into a magnetic core@ carbon shell structure, which would found its application in microwave adsorption field. The prepared samples were characterized by using SEM, HRTEM, EDX, BET, XRD, Raman spectroscopy and TGA analysis. Results showed that MSs burst during the CVD process and transformed into a loose agglomeration of submicron-sized particles with a typical core@ shell structure. Crucially, instead of single sphere of MSs, the ultrafine grains of ferrospinel within MSs served as the core. The mechanism of carbon deposition of graphite shells and phase transformation of ferrospinel cores during the CVD process were revealed. Magnetism and microwave adsorption properties were tested, and the structural dependence relationship between these properties and the core@ shell structure was clearly identified. For a sample time of 60 min, the minimum absorption peak was observed as −32.4 dB at 12 GHz, with the minimum frequency bandwidth below −10 dB at 4.3 GHz. We propose a low-cost, one-step preparation of cementite core@ graphite shell composites with high-performing microwave adsorption. Our method provides an alternative option for the practical reuse of the industrial waste.