Robust carbon metacomposite foams from wasted orange peel for electromagnetic interference shielding and thermal insulation

Robust carbon foams were obtained by freeze-casting of slurries prepared by grinding fresh orange peel waste in water containing sucrose and boric acid, followed by freeze-drying, annealing at 150°C and carbonisation at 900°C. The shear thinning flow behaviour of the slurries enables casting, and their high yield stress avoids settling of orange peel particles before freezing. Annealing at 150°C facilitates the caramelisation of sucrose present in the body, which prevents deformation during subsequent carbonisation. Boric acid cross-links the sucrose molecules and promotes the caramelisation by providing H + ions to catalyse the –OH condensation reactions, enhances the resistance to oxidation of the carbon foams. The diametric shrinkage of freeze-dried specimens after annealing (1.2 to 10.7%) and carbonisation (29.7 to 24.5%), alongside the density (0.09 to 0.41 g.cm -3 ), compressive strength (0.09 to 4.19 MPa), and Young's modulus (1.18 to 116 MPa) of the carbon foams were observed to be dependent on the sucrose to orange peel weight ratio (0 to 0.8). The closed pore structure, large population of phonon-scattering defects and interfaces and amorphous nature of the carbon result in low thermal conductivity (0.068 to 0.212 W m -1 K -1 ) values. The as-prepared carbon foams function like a metacomposite and exhibit Drude model permittivity behaviour. The negative real permittivity also plays a role in the high EMI shielding effectiveness (32.7 to 77.2 dB). Similarly, the multiple internal reflections of the penetrated EM waves, as well as Ohmic and polarisation losses, contribute to electromagnetic wave attenuation.