Eco-Friendly Electrophoretic Carbon Nanotube Coatings on Copper for Enhanced Pool Boiling: Mechanistic Insights and Long-Term Stability

Abstract

Abstract. This study investigates pool-boiling heat transfer on copper substrates coated with carbon nanotube (CNT) films fabricated via a sustainable electrophoretic deposition (EPD) process. Unlike conventional routes requiring aggressive chemicals or high-temperature oxidation, the present method employs citric acid activation, oxygen plasma functionalization, and chitosan-stabilized CNT inks, offering a green and scalable fabrication pathway. The resulting hierarchical CNT networks exhibit tunable porosity, roughness, and wettability, enabling systematic correlation between surface morphology and boiling performance. Boiling experiments showed that all CNT-coated surfaces outperformed bare copper [boiling heat transfer coefficient (BHTC) = 53.4 kW/m2/K, critical heat flux (CHF) = 1073.5 kW/m2. The ED-CNT-270 surface achieved the best performance, with a BHTC of 205.53 kW/m2/K (+285%) and CHF of 2012 kW/m2 (+87%), while ED-CNT-90 gave the lowest enhancements (+101% BHTC, +39% CHF). Progressive deposition increased roughness from 1.45 to 3.20 µm, expanded nanoporosity from 55 to 155 nm and microcavities from 1.5 to 4.7 µm, raised porosity from 20% to 52%, and reduced contact angle from 28 deg to 5 deg. These synergistic modifications enhanced nucleation site density, capillary liquid supply, and bubble departure frequency, thereby dictating observed performance. Long-term tests over nine 20-hour cycles confirmed negligible CHF degradation (<1%), demonstrating robust operational stability. By quantitatively linking morphology to percentage improvements in boiling heat transfer coefficient and critical heat flux, this work advances beyond empirical correlations. The eco-friendly CNT coatings thus represent a durable, application-ready solution for high-flux thermal management in electronics, electric vehicles, aerospace systems, and compact heat exchangers.