From beverage waste to electrical withstand: crystal-phase-controlled recycled alumina nanofillers for surface charge regulation in HVDC epoxy spacers.

Epoxy spacers in gas-insulated substations (GIS) and transmission lines (GITL) accumulate surface charge under HVDC stress, distorting the electric field and lowering flashover voltage. Here, γ-rich alumina and α-Al2O3 nanofillers were synthesized from recycled aluminum beverage cans through acid dissolution, carbonate precipitation, and phase-selective calcination at 600 °C and 1200 °C, then functionalized with 3-aminopropyltriethoxysilane (APTES) and dispersed in epoxy at 1-7 wt%. Using a common recycled precursor provides a controlled basis for comparing the phase-dependent behavior of γ-rich alumina and α-Al2O3. Surface resistivity, surface potential mapping, isothermal surface potential decay (ISPD) trap analysis, DC flashover testing, and coupled electrothermal COMSOL simulation were performed. Among the investigated loadings, 5 wt% produced the highest surface resistivity for both alumina phases and was therefore selected for detailed electrical characterization. The γ-rich phase, with its defective spinel structure, gave the highest surface resistivity gain (68.1%) and mean flashover voltage (13.49 kV, +12.8% versus neat epoxy), whereas the corundum-structured α phase showed faster early charge dissipation (44.0% within 15 min) from a shallow-trap density more than double that of γ-rich alumina. Simulation showed γ-rich alumina cut accumulated charge by 61.8% and the peak triple-junction field by 23.1% after 120 s. The results demonstrate that calcination-induced structural modification of recycled alumina provides a controllable and sustainability-driven route for regulating HVDC surface-insulation performance.