Graphene-coated alumina nano/microfibers as filler for composites

The present work is focused on the preparation and characterization of composite nano/microfibers based on alumina applicable as fillers in ceramic-matrix composites. The graphene-coated Al2O3 microfibers have been prepared in the form of continuous alumina/graphene fibers from electrospun precursors using needle-less electrospinning, calcination, and catalyst-free chemical vapor deposition of graphene layers on the surface of the microfibers. The phase composition and morphology of the composite fibers were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, scanning, and transmission electron microscopy (SEM and TEM). The porosity of the final fibers was evaluated by low-temperature nitrogen adsorption. The surface microstructures of composite fibers were rough and their diameters were in the range from 300 nm to 2 μm. The phase transformation at 1100 °C led to the formation of a mixture of γ-, δ-, and α-Al2O3. TEM results confirmed that the alumina fibers were covered by the 3–4 layers of graphene-based structure. The specific surface area of pure alumina fibers after carbon deposition slightly increased from the initial 40 m2/g to 45 m2/g, due to a slightly wavy surface of graphene layers. Raman spectroscopy and XPS analysis results showed that the surface layer of fibers is formed as graphene oxide. Between the graphene oxide and alumina grains a chemical bonding C–O–Al, C–O–O–Al was discovered as the nucleation of the graphene oxide sheet. The growth mechanism of graphene oxide layers on the surface edges of Al2O3 grains was described.