Pool boiling enhancement by nanotextured surface of hierarchically structured electroplated Ni nanocones
Introduction
Boiling heat transfer is one of the most effective heat transfer methods and has been widely utilized in modern industry as a key technique in various applications, such as refrigeration devices, distillation units, boilers, nuclear reactors, and immersion cooling systems [1], [2], [3], [4], [5], [6]. In addition, the performance requirements for hardware in information and communications technologies (ICTs) have dramatically increased in recent years; therefore, various cooling techniques, such as pool boiling, flow boiling, and spray cooling, have attracted considerable attention because they can alleviate the accumulation of heat in ICT devices that would otherwise cause significant degradation in device performance [7], [8], [9], [10]. A recent study demonstrated that 200 W of heat generated by computer chips could be removed by the cooling technique of liquid forced convection with microchannels [1].
To improve the cooling performance, enhancements in the critical heat flux (CHF) and effective heat transfer coefficient (heff) are necessary. In initial studies, the CHF was considered a simple phenomenon associated with hydrodynamic instability in the boiling process, and the corresponding effects thereof on the properties of the boiling surface received little attention until the mid-20th century. In the 21st century, several studies showed that modification of the surface properties (roughness, wettability, porosity, and surface area ratio) considerably affects the rate at which heat is transferred by way of boiling [11], [12], [13], [14]. During this time, micro- and nanostructures designed to improve the properties of the boiling surface, such as microporous surfaces [12,[15], [16], [17]], nanowires or nanorods [18,19], and nanocavity structures [20,21], were developed using various fabrication techniques that include chemical vapor deposition (CVD) [22,23], sputtering and etching [24,25], electrochemical deposition [26,27], chemical bath deposition [28], and supersonic spraying [29], [30], [31]. Studies on surface texturing have contributed to significant advances in the efficacy of cooling techniques by showing that texturing can increase the density of boiling nucleation sites and manipulate the wettability of boiling surfaces, which can improve heat removal and liquid permeation during boiling [32], [33], [34]. However, CVD, which can uniformly deposit high-purity materials with high adhesion, limits the selection of materials for both the coating and the substrate because of the high temperatures used in this process [35]. Although physical deposition techniques, such as sputtering and spray coating, can be conducted at lower temperatures than those needed for CVD, their deposition rates and degrees of adhesion are low [36]. Even though etching can form various patterns on flat substrates [37], it is unsuitable for forming nanostructures on the interior surfaces of heat pipes.
Compared to the aforementioned techniques, the electrochemical deposition technique of electroplating is not only facile and scalable, but also capable of achieving good mechanochemical properties, such as high uniformity and adhesion to the substrate, on the deposited surface. Although the electrospinning process is of the duration of a few minutes, various materials and morphologies can be used and constructed by the electroplating method [38], [39], [40]. In our study, the electroplating method was used to deposit hierarchically structured Ni nanocones (NiNCs) on the Cu substrate. The unique nanostructure of the NiNCs was expected to significantly improve the resulting values of CHF and heff because of the increase in the number of nucleation sites and the liquid permeation rate of the boiling surface.
In addition, the correlation between the heat transfer performance and the novel dimensionless relative roughness r* was experimentally investigated; this has not been attempted in previous studies on boiling heat transfer [9,11,41,42]. The morphology of the NiNCs and the resulting surface properties could be controlled by varying the electroplating time (tep). Pool boiling experiments were performed at atmospheric pressure using the coolant HFE-7000. The reliability of the developed setup was investigated by additionally conducting pool boiling experiments with the bare substrate.
Section snippets
Materials
The electroplating solution for fabricating the hierarchical NiNCs on the Cu substrate was prepared as follows: 1 mol⋅L−1 NiCl2⋅H2O (Sigma-Aldrich, USA), 0.5 mol⋅L−1 H3BO4 (Sigma-Aldrich, USA), and 1.5 mol⋅L−1 ethylenediamine dihydrochloride (Sigma-Aldrich, USA) dissolved in distilled water were first subjected to magnetic stirring for 1 d at 40 °C. Then, HCl (0.1 M, Sigma-Aldrich, USA) or NaOH (1 M, Sigma-Aldrich, USA) was added to the solution to adjust the pH. HFE-7000 (3M Novec Engineering
Hierarchical NiNCs
Figs. 2a and 2b show photographic images and the corresponding top-view SEM images of the specimens with tep of 0, 2, 5, 10, 20, and 30 min. As the tep was increased from 2 to 30 min, the surface luster of the Cu substrate gradually diminished and the surface became silver in color because of the formation of NiNCs (Fig. 2a). In the case of the lowest tep (2 min), the hierarchical NiNCs were not fully formed (Fig. 2b), indicating that tep of 2 min was insufficient to form the complete NiNC
Conclusions
The pool boiling performance of a material can be enhanced by employing hierarchically structured NiNCs on its surface. The electroplating time for NiNC formation was varied from tep = 2 to 30 min to yield surfaces with different nanotextures. Although the size of each NiNC increased as tep increased from 2 to 30 min, the dimensionless relative roughness (r*) of the nanotextured surface was highest at tep = 5 min. The variations in the values of CHF, heff, Ra, Nu, and the superheating effect
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. NRF-2020R1A5A1018153).
Authorship Statement
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Authorship contributions
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These authors have contributed equally.