Valorizing waste-derived carbon precursors and steel pickling liquor-derived catalyst for the synthesis of multiwalled carbon nanotubes via CVD for EMI shielding

Lightweight, high-performance electromagnetic interference (EMI) shields are ideal for high-frequency communication systems, such as 5G/6 G platforms, and aerospace applications. In this work, steel pickling waste liquor (SPWL), an iron-loaded industrial effluent, was employed as a catalyst source for the chemical vapor deposition (CVD) synthesis of multi-walled carbon nanotubes (MWCNTs) using three waste-derived carbon precursors, namely cooking-oil waste (COW), bioethanol residue, and sludge palm oil (SPO), under identical growth conditions. In contrast to conventional CNT synthesis routes that depend on refined carbon feedstocks, this approach uses low-cost industrial and agro-industrial waste streams, thereby reducing raw material costs and mitigating waste disposal burdens. Structural characterization revealed an increase in graphitic ordering from bioethanol-derived MWCNTs to SPO-derived MWCNTs, as evidenced by a reduction in the Raman ID/IG ratio from 0.40 to 0.08, sharpening of the XRD (002) reflection, and enhanced thermal stability with residual mass increasing from 45 to 70% at 950 °C in TGA. Epoxy composites containing 60 wt% MWCNT showed clear structure-property relationships, with the SPO-derived MWCNT composite exhibiting a minimum reflection loss of -18.5 dB at 17.1 GHz and a total shielding effectiveness of 21 dB at 18 GHz, corresponding to a 25% increase relative to the bioethanol-derived MWCNT composite. EMI attenuation was primarily governed by dielectric loss (ε″ =5.4) arising from improved interfacial polarization and sp² carbon ordering. Optical performance showed transmittance <10% at 550 nm, confirming functional opacity. Thus, this integrated waste-valorization strategy, based on the synergistic use of waste oils and steel-processing effluent, offers a sustainable route for producing waste-derived MWCNTs with promising EMI-shielding functionality, multifunctional CNT/epoxy composites with tunable Ku-band EMI shielding and optical attenuation performance, with potential relevance to circular-economy-driven materials manufacturing.

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