Hierarchical CNT-Cu hybrid coatings for enhanced and durable pool boiling heat transfer: Mechanistic insights and CHF modelling

Abstract

Enhancing boiling heat transfer through scalable and sustainable surface engineering is critical for high heat-flux thermal management systems. This study investigates pool boiling performance on copper substrates functionalized with hierarchical carbon nanotube (CNT) and CNT-Cu hybrid coatings fabricated via an environmentally benign electrophoretic deposition (EPD) process. The approach integrates citric-acid conditioning, oxygen plasma activation, and chitosan-assisted CNT assembly to produce controlled micro/nano architectures without high-temperature processing or hazardous chemicals. Three surfaces (CNT-CS#1, CNT-Cu-CS#2, CNT-Cu-CS#3) were engineered with tailored pore structures (micropores 0.5-3 μm; nanopores 30-200 nm), surface roughness (R a = 1.6-3.1 μm), and wettability (contact angle reduced from 58° to <10°). The optimized CNT-Cu-CS#3 surface achieved a maximum heat transfer coefficient of 204 kW m −2 K −1 and a critical heat flux of 2004 kW m −2 , compared to 53.4 kW m −2 K −1 and 1073.5 kW m −2 for bare copper. High-speed visualization reveals that enhanced performance is associated with increased nucleation site density, reduced bubble departure diameter, and higher departure frequency. Mechanistically, nanoscale pores generate strong capillary-driven liquid replenishment, while interconnected micropores facilitate efficient vapor removal, sustaining microlayer evaporation and delaying dryout. A semi-empirical CHF framework is proposed to relate performance to porosity, wettability, pore size, and bubble dynamics. Long-term testing (300 h) shows negligible degradation, confirming structural robustness. The results establish hierarchical morphology and transport optimization as key design criteria for durable, high-performance boiling surfaces.