Sustainable carbon-metal oxide composites from waste sources: synthesis, characterization, and mechanical properties

Abstract

Carbon-metal oxide composites are widely utilized as lightweight materials in aerospace, automotive, and sports equipment. Since the discovery of carbon nanotubes (CNTs) by Iijima in 1991, extensive research has focused on their synthesis and composite applications. However, large-scale, cost-effective production still remains a challenge, and the correlation between synthesis methods and mechanical properties is not well understood. This study explores the thermocatalytic synthesis of carbon-metal oxide composites Al₂O₃-CNT, Al₂O₃/Fe₂O₃-CNT, Fe₂O₃-C, and ZnO-C using low-cost waste materials and also explore their mechanical properties. Waste plastic served as the carbon source, while aluminum cans and scrap iron provided the source of Al and Fe, respectively. Characterization techniques, including UV-Vis spectroscopy, SEM, TEM, EDX, Raman spectroscopy, XRD, and nanoindentation, were used to characterize the materials and their mechanical properties. The synthesis mechanism for MO and MO-C composite have been proposed. EDX confirmed the purity of the composites, while TEM revealed CNT formation in the presence of Al₂O₃ and Al₂O₃/Fe₂O₃ catalysts. In contrast, Fe₂O₃ and ZnO did not facilitate CNT growth but instead formed Fe₂O₃-C and ZnO-C composites, as confirmed by Raman spectroscopy and TEM. The mechanical properties, including elastic modulus (E), hardness (H), and density-normalized elastic modulus (E/ρ), are reported and compared with previous studies. The elastic modulus ranged from 10.19 to 23.13 GPa, while the density-normalized values were 4.22 to 9.66 GPa. This work contributes to the development of low-cost, lightweight, and sustainable carbon-metal oxide composites, offering potential applications in structural and functional materials.