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

Carbon–metal oxide composites are widely utilized as lightweight materials in aerospace, automotive, and sports equipment. Since the discovery of carbon nanotubes (CNTs) in 1991, exploration of CNTs and their incorporation into composites has gained attention; however, large-scale, cost-effective production and property control remain challenges. This study reports the thermocatalytic synthesis of Al₂O₃–CNT, Al₂O₃/Fe₂O₃–CNT, Fe₂O₃–C, and ZnO–C composites using waste plastic as a carbon source and aluminum cans and scrap iron for metal oxides. Characterization via UV-vis spectroscopy, SEM, TEM, EDX, Raman spectroscopy, XRD, and nanoindentation test confirms the material composition and structure. TEM and Raman spectroscopy reveal successful CNT formation in Al₂O₃ and Al₂O₃/Fe₂O₃ systems, while Fe₂O₃ and ZnO form CNT-free carbon composites. The Al₂O₃/Fe₂O₃–CNT composite shows a better density-normalized elastic modulus value, i.e. 9.66 GPa g⁻¹ cm³, in comparison to other samples. The elastic modulus values of Al₂O₃–CNT, Fe₂O₃–C, Al₂O₃/Fe₂O₃–CNT, and ZnO–C are found to be in the range 10.19 GPa to 23.31 GPa, whereas hardness is in the range 0.42 GPa to 0.75 GPa, and elastic recovery is in the range 8.1% to 28.9%. The higher I _G/I _D ratio of the Al₂O₃/Fe₂O₃–CNT composite (2.05) compared to other composites (1.5, 1.41, 1.4) reflects a higher quality and responsible parameter for the higher elastic modulus of Al₂O₃/Fe₂O₃–CNT. This work highlights a low-cost, waste-derived route to high-performance, lightweight carbon–metal oxide composites.