High-Efficiency Boiling Heat Transfer Interfaces Composed of Electroplated Copper Nanocone Cores and Low-Thermal-Conductivity Nickel Nanocone Coverings

High-Efficiency Condensation Heat Transfer Interfaces Based on Superwetting Copper Microgroove/Nanocone Structure

Utilizing superhydrophobic micro/nanostructures to enhance condensation heat transfer (CHT) of copper surfaces has attracted intensive interest in recent years due to its significance in multiple industrial fields including nuclear power generation, thermal management, water harvesting, and desalination. However, superhydrophobic surfaces have instability risk caused by microcavity defect-induced vapor penetration and/or hydrophobic chemistry destruction. Here, we report a superwetting copper hierarchical microgroove/nanocone (MGNC) structure strategy that can realize high-efficiency CHT over a whole range of surface subcooling. By regulating groove width, fin width, groove depth, and nanostructure growth time, we obtain the optimal MGNC structure, where the CHT coefficient is 121% and 107% higher than that of hydrophilic flat surfaces at surface subcooling of 2 and 15 K, respectively. Such remarkable enhancement can be ascribed to the synergy of three interface effects: more nucleation sites for phase-change energy exchanging, thinner condensate films for reducing thermal resistance, and parallel microchannels for timely drainage. Compared with superhydrophobic strategies, our strategy not only can be mass-producible but also has other inherent advantages: no microcavity-induced performance failure risk as well as being free of chemistry modification, which makes the fabrication process simpler and more economic. Hierarchical micropillar/nanocone structure is also fabricated as the contrast sample for highlighting the superiority of the superwetting MGNC structure in enhancing CHT. This work not only enriches research systems of superwettability surfaces but also helps develop high-performance chips' cooling devices and explore more potential applications.

Effects of surface nanotexturing on the wickability of microtextured metal surfaces

HYPOTHESIS

Achieving spontaneous, rapid, and long-distance liquid transport is crucial for many practical applications such as phase change heat transfer and reactions at solid-liquid interfaces. Surface nanotexturing has been widely reported to improve the wickability of microtextured metal surfaces. Although surface nanotextures show high capillary pressure, the high fluid flow resistance through nanotextures prevents fluid transport. The underlying mechanisms responsible for the enhanced wickability of nanotextured surfaces are still unclear.

EXPERIMENTS

Herein, we prepared a variety of microtextures and nanotextures on copper surfaces by femtosecond laser micromachining and chemical oxidation, respectively. The wickability of these textured surfaces was quantitively compared by measuring wicking coefficient and capillary rise speed. We designed experiments to eliminate any possible effects of surface oxidation and metal composition on wickability. A theoretical model describing the vertical and horizontal capillary flow in V-shaped microgrooves was proposed and utilized to analyze the experimental results. The effects of time-dependent wettability on wickability were also examined.

FINDINGS

Surface nanotexturing can enhance surface wettability while altering the micrometer-scale structural characteristics. The greatly enhanced wickability of nanotextured surfaces can only be observed when the surface microtextures have a very small aspect ratio. Otherwise, for metal surfaces with fine microgrooves, the latter effect is more pronounced, and thus the surface wickability may deteriorate after preparing surface nanotextures; for surfaces with wide microgrooves, both effects are minimal, and the surface wickability enhances only marginally after surface nanotexturing. Furthermore, the wickability of microtextured surfaces will decay rapidly due to the adsorption of airborne organics, whereas adding surface nanotextures can significantly inhibit this degradation. The anti-contamination capability of surface nanotextures is considered likely to be a potential mechanism responsible for the greatly enhanced wickability of nanotextured surfaces noted in some studies.

Enhanced Phase-Change Heat Transfer by Surface Wettability Control

The phase-change heat-transfer coefficient can be improved by several orders of magnitude through the design of micro-nanostructures on typical surfaces. However, with the rapid development of intelligent and integrated devices, there is an increasing desire to regulate the heat exchange form of the surface to adapt to various environmental requirements. This study concerns the design of a carbon nanotube array-based phase-change heat-transfer surface, which can switch its wettability between superhydrophobicity and superhydrophilicity. By installing this surface on a device that integrates boiling heat transfer and condensation heat transfer, the device can independently adjust the surface wettability for different heat-transfer requirements. As a result, this surface can enhance condensation heat-transfer coefficient over 90 % in the superhydrophobic state and enhance the boiling heat-transfer coefficient over 41 % in the superhydrophilic state. Surfaces with controllable wettability can aid development of a new generation of smart control technologies to actively regulate system and device temperatures.

Evaluation of Surfactants on Graphene Dispersion and Thermal Performance for Heat Dissipation Coating

With the development of thin and high-power electronic devices, heat dissipation has become an important and urgent issue in thermal management. In this study, a water-based epoxy was used as a polymer matrix to prepare heat dissipation coatings utilizing low volatile organic compounds, which were environmentally friendly and had a high heat-dissipating performance. Graphene flakes, multi-walled carbon nanotubes and aluminum oxide particles were used as fillers for preparing the heat dissipation coating. The graphene flakes and multi-walled carbon nanotubes were dispersed in a water-based epoxy by adding sodium dihexyl sulfosuccinate and poly (dimethyldiallylammonium chloride). These two surfactants were combined as a dispersant to improve the dispersibility of the carbon nanomaterials in the water-based epoxy. The synergistic effect of the well-dispersed fillers improved the heat-dissipating performance. The experimental results show that the infrared emissivity of the heat dissipation film was 0.96 after filling 30 wt% aluminum oxide particles, 2 wt% graphene flakes and 2 wt% multi-walled carbon nanotubes into a water-based epoxy. The heat dissipation film reduced the thermal equilibrium temperature of the bare copper panel by 17.8 °C under a heating power of 10 W. The film was applied in a heat dissipation test on a 15 W LED bulb, and the thermal equilibrium temperature was reduced by 21.3 °C. The results demonstrate that the carbon nanomaterial-based heat dissipation coating with a water-based epoxy could significantly reduce the thermal equilibrium temperature, giving a high potential for the application of thermal management.

Hydrophilic and Hydrophobic Nanostructured Copper Surfaces for Efficient Pool Boiling Heat Transfer with Water, Water/Butanol Mixtures and Novec 649

Increasing heat dissipation requirements of small and miniature devices demands advanced cooling methods, such as application of immersion cooling via boiling heat transfer. In this study, functionalized copper surfaces for enhanced heat transfer are developed and evaluated. Samples are functionalized using a chemical oxidation treatment with subsequent hydrophobization of selected surfaces with a fluorinated silane. Pool boiling tests with water, water/1-butanol mixture with self-rewetting properties and a novel dielectric fluid with low GWP (Novec™ 649) are conducted to evaluate the boiling performance of individual surfaces. The results show that hydrophobized functionalized surfaces covered by microcavities with diameters between 40 nm and 2 µm exhibit increased heat transfer coefficient (HTC; enhancements up to 120%) and critical heat flux (CHF; enhancements up to 64%) values in comparison with the untreated reference surface, complemented by favorable fabrication repeatability. Positive surface stability is observed in contact with water, while both the self-rewetting fluids and Novec™ 649 gradually degrade the boiling performance and in some cases also the surface itself. The use of water/1-butanol mixtures in particular results in surface chemistry and morphology changes, as observed using SEM imaging and Raman spectroscopy. This seems to be neglected in the available literature and should be focused on in further studies.

Liquid film-induced critical heat flux enhancement on structured surfaces

Enhancing critical heat flux (CHF) during boiling with structured surfaces has received much attention because of its important implications for two-phase flow. The role of surface structures on bubble evolution and CHF enhancement remains unclear because of the lack of direct visualization of the liquid- and solid-vapor interfaces. Here, we use high-magnification in-liquid endoscopy to directly probe bubble behavior during boiling. We report the previously unidentified coexistence of two distinct three-phase contact lines underneath growing bubbles on structured surfaces, resulting in retention of a thin liquid film within the structures between the two contact lines due to their disparate advancing velocities. This finding sheds light on a previously unidentified mechanism governing bubble evolution on structured surfaces, which has notable implications for a variety of real systems using bubble formation, such as thermal management, microfluidics, and electrochemical reactors.

Nanoparticle-Assisted Pool Boiling Heat Transfer on Micro-Pin-Fin Surfaces

Boiling heat transfer intensification is of significant relevance to energy conversion and various cooling processes. This study aimed to enhance the saturated pool boiling of FC-72 (a dielectric liquid) by surface modifications and explore mechanisms of the enhancement. Specifically, circular and square micro pin fins were fabricated on silicon surfaces by dry etching and then copper nanoparticles were deposited on the micro-pin-fin surfaces by electrostatic deposition. Experimental results indicated that compared with a smooth surface, the micro pin fins increased the heat transfer coefficient and the critical heat flux by more than 200 and 65-83%, respectively, which were further enhanced by the nanoparticles up to 24% and more than 20%, respectively. Correspondingly, the enhancement mechanism was carefully explored by high-speed bubble visualizations, surface wickability measurements, and model analysis. It was quantitatively found that small bubble departure diameters with high bubble departure frequencies promoted high heat transfer coefficients. The wickability, which characterizes the ability of a liquid to rewet a surface, played an important role in determining the critical heat flux, but further analyses indicated that evaporation beneath bubbles was also essential and competition between the wicking and the evaporation finally triggered the critical heat flux.