Repetitive Pool boiling runs: A Controlled Process to Form Reduced

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Materials and Interfaces

Repetitive Pool boiling runs: A Controlled Process to Form Reduced GO Surfaces from GO with Tunable Surface Chemistry and Morphology Aniket Rishi, Satish G. Kandlikar, and Anju Gupta Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b06062 • Publication Date (Web): 09 Apr 2019 Downloaded from http://pubs.acs.org on April 10, 2019

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Repetitive Pool boiling runs: A Controlled Process to Form Reduced GO Surfaces from GO with Tunable Surface Chemistry and Morphology Aniket M. Rishi1, Satish G. Kandlikar1, 2 and Anju Gupta1, 3, * 1Microsystems

Engineering, Rochester Institute of Technology, 76 Lomb Memorial Drive,

Rochester, NY 14623. E-mail: [email protected] 2

Mechanical Engineering, Rochester Institute of Technology, 76 Lomb Memorial Drive,

Rochester, NY 14623. E-mail: [email protected] 3Chemical

Engineering, Rochester Institute of Technology, 160 Lomb Memorial Drive,

Rochester, NY 14623. E-mail: [email protected] *Corresponding author: Anju Gupta E-mail: [email protected] , [email protected] Chemical Engineering, Rochester Institute of Technology, 160 Lomb Memorial Drive, Rochester, NY 14623. Telephone: +1-(585)-475-4093


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ABSTRACT Wettability and wickability of a surface are the key properties that govern a variety of industrial applications. Understanding the effect of aging on the durability of these surfaces and subsequent changes occurring in their molecular structure is vital to maintain functionality. In this work, we used repetitive pool boiling to test the durability of graphene oxide-copper (GO-Cu) coated surfaces and studied subsequent changes in surface chemistry and morphology. We observed the transformation of GO-Cu to reduced GO-copper (rGO-Cu) coating that led to improved boiling performance. This improvement is attributed to a) transition of hydrophilic GO to hydrophobic rGO due to the repetitive pool boiling, and b) increased surface roughness as a result of hydrophobic surfaces. The durability of the GO-Cu coatings was found to be higher than pristinecopper and just-copper coated surfaces. A higher performance demonstrated by GO-Cu coatings indicates its increased sustainability and practicability in numerous engineering applications. KEYWORDS: Repetitive pool boiling, Reduced graphene oxide, Electrodeposition


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1.

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Introduction Pool boiling is a phase change cooling technique which involves transfer of heat from a heater

surface to liquid and extraction of heat via vapor bubbles formed as a result of heat supply. These vapor bubbles can be increased by adding more nucleating pores on the surface, which in turn alters the wettability of the surface. To achieve higher pool boiling performance, an ideal surface would possess high thermal conductivity and improved surface properties such as porosity and wettability 1. Highly wettability surfaces induce continuous supply of liquid in the pores that plays a crucial role in governing high pool boiling performance of the surface. Wettability of the surface can also be tuned by altering the molecular structure of the surface, for example, adding hydroxyl groups can increase the wettability or the hydrophilicity of the surface. A number of studies have reported surface modifications that led to improved pool boiling performance by altering the surface morphology and resultant surface wettability 2-6. In addition to pool boiling, wettability of the surfaces are also desirable in corrosion resistance 7,8, biomedical systems 9, anti-fogging 10,11, condensation and evaporation

12,13,

desalination membrane applications

oil water separation systems, and water purification and 14,15.

Therefore, a deeper understanding of alternation in

molecular structure on the surface and wettability is essential. To achieve a desired wettability for a surface, molecular structure on the surface and surface geometry are the two major parameters that play vital role

16.

This wettability modification is generally achieved using microscale or

nanoscale coatings which either introduce surface roughness or increase the wettability of the surface. Graphene based coatings have gained a lot of attention in the pool boiling community owing to their enhanced thermal conductivity, mechanical

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and structural properties imparted by their

unique structure comprising of a single layer of carbon atoms. Various micro and nanoscale


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graphene-based coatings such as graphene

18,

graphene oxide (GO)

19,

reduced graphene oxide

(rGO) 20, and nanofluids containing graphene/graphene oxide have been exploited to improve the pool boiling heat transfer performance 21, 22. Production of a single layer of graphene is very costly and is not suitable for the large-scale production. On the other hand, graphene oxide (GO) is a graphene-based material which is cheaper and easier to produce. The most common approach for the production of GO is by using strong oxidizing agents to exfoliate graphite. Hummer’s method is widely used in which the oxidation of graphite is performed by a treatment with KMnO4 and NaNO3 in conc. H2SO4 23. Researchers have also implemented electrolyte free electrochemical processes for producing GO to avoid the generation of toxic gases 24. GO consists to acquire a large number of oxygen-based groups, which not only expand the interlayer distance between the carbon atoms but also increase the hydrophilicity of graphene 25. Graphene oxide can further be reduced to form rGO by eliminating the oxygen-based groups. The route of reduction process impacts the quality of rGO generated and determines how close structure of the resultant rGO is compared to that of the pristine graphene 25.

A brief literature on synthesis of rGO is provided. The most popular method of obtaining rGO

is the Hummers method that involves treating GO with hydrazine hydrate while maintaining the solution at 80°C for 24 hours 26. To avoid the toxicity of hydrazine, different reductants such as sodium borohydride, pyrogallol, and vitamin C have been exploited 27. Many reducing agents such as hydrogen sulphide 28, hydroquinone 29, aluminum 30, 31, and metallocene reductants 32 have been exploited to reduce GO to rGO. Exposing GO through thermal annealing in ammonia to obtain bulk quantities of N-doped reduced graphene oxide sheets has also been explored

33.

Feng and

coworkers reported producing large quantity of rGO by using sodium-ammonia (Na-NH3) system in dry ice-acetone bath 34. Additionally, techniques such as plasma reduction 35, thermal reduction


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36-38

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exposing GO to strong pulse light produced by xenon flashtube, heating GO in distilled water

at varying degrees for different lengths of time 25, microwave reduction 39, chemical reduction 40, 41, 42

have also been successfully employed to reduce GO to rGO.

In our previous publications, we demonstrated exceptional pool boiling performance quantified by higher critical heat fluxes at lower wall superheats by exploiting a variety of coating techniques such as screen printing 43, electrodeposition 43, dip coating 44, and chemical vapor deposition 45 to generate graphene/graphene oxide, graphene/copper coatings surfaces with hierarchical surface porosity and controlled morphologies. Microlayer evaporation, increased nucleation, altered wettability and wickability are some of the mechanisms to support the improved pool boiling performance. Sustainable and durable coating is the most important factor in real-world applications, yet, a very few studies have focused on pool boiling aging performance of different coatings 46, 47. In our previous work 47, we demonstrated the aging effects of subsequent boiling tests on porous copper surfaces. We developed a new multi-step electrodeposition technique to enhance the substrate bond strength of the coatings which led to higher durability and the boiling performance of the coatings. In this study, we present a) aging study of GO-Cu coated surface with repetitive pool boiling and its effect on changes in molecular structure and wettability of the coating, b) a combination of two mechanisms responsible for the transition of GO-Cu coating from hydrophilic to hydrophobic nature, and c) discovery that led to the formation of rGO-Cu coating after repetitive pool boiling. We also propose the pool boiling process as a controlled or systematic technique to form rGO coatings from GO coatings that may allow the formation of tunable rGO surfaces with more control


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over surface chemistry and morphology. Since the robustness and reliability of these graphenebased coatings was found to be higher, these coatings can also be realized for real-world applications. 2. Experimental Section 2.1 Electrodeposition technique for graphene oxide-copper (GO-Cu) composite coatings The schematic of electrochemical cell used for the electrodeposition process to form graphene oxide-copper (GO-Cu) is shown in Figure 1. The pristine copper surface (cathode), and the copper block (anode) were thoroughly cleaned with IPA and washed under the reflux of distilled water before using for electrodeposition. The electrodes were placed in a Polytetrafluoroethylene (PTFE) holder to maintain parallelism and the entire assembly was placed in the electrolytic bath consisting of 5.85 gm of 0.8M CuSO4, 3.14 mL of 1.5M conc. H2SO4, 40mL distilled water, and 2.5% v/v G/GO solution 43. The working area on both the electrodes was delineated with Kapton® tape. A Galvanostatic method, which involves maintaining a constant current between the electrodes, was used in this study. To form a porous structure of GO-Cu composite, a two-step electrodeposition technique was implemented reported by us previously. Step 1 involved depositing of GO/Cu on the surface under a current density of 400 mA/cm² for 15 s, followed by application of a current density of 40 mA/cm² for 2500 s (step 2) which resulting in bonding of the electrodeposited GO and Cu 48.


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Figure 1. Schematic representation of experimental setup of electrochemical cell 2.2 Characterization of the GO-Cu coatings The morphology and element-composition of the electrodeposited surface was investigated using both a TESCAN Field Emission Mira III LMU (Czech Republic) and JSM-6400 V scanning electron microscope (SEM), JEOL, Ltd., Tokyo, Japan. The energy dispersive x-ray spectroscopy (EDS) measurements were done on Bruker Quantax EDS with XFLASH 5010 detector attached to a field emission scanning electron microscope MIRA II LMH. To confirm the successful deposition of graphene oxide-copper composite and to have a deeper understanding of the chemical composition on copper surface different characterization techniques were employed. A multi-wavelength Jobin Yvon Horriba LabRAM HR Raman Spectroscope using He-Ne Laser (λ = 632.8 nm) was used to identify the graphene with 20 second acquisition time. To produce repeatable and reliable data, a total of 10 scans were taken by observing the characteristic D and G peaks of graphene. The crystalline phases of the electrodeposited surface were investigated using a Rigaku DMAZ-IIB X-Ray Diffractometer (XRD) with (Cu Kα radiation; wavelength 1.5418 ˚ A). The data was recorded for 2θ ranges between 5° and 80° at a rate of 3°/min rate. The step size was 0.01° with an X-ray power of 40 kV and 35 mA. This range of 2θ angles is expected to capture peaks from carbon and the copper from the coating. Analytical spectra of GO-copper


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composite were taken using a Fourier transform infrared (FTIR) spectroscopy (Shimadzu, IR Prestige 21, Kyoto, Japan) for the frequency range of 4000–600 cm−1, a scan number of 100 times, and a resolution of 4 cm−1. Additionally, static wettability and dynamic wicking of GO-Cu coatings were measured for the surfaces using a VCA Optima Goniometer. For dynamic wicking, a distilled water droplet of 2 µL was steadily brought into contact with the electrodeposited surfaces using a static sessile drop method and liquid propagation was recorded using a high-speed camera. 2.3 Pool boiling setup Pool boiling studies were carried out to investigate the heat transfer performance of the surfaces. Figure 2 shows the schematic of the pool boiling setup used for performing the pool boiling experiments. The pool boiling setup consisted three main components, a test section, a heater block and a water reservoir. The copper surface is positioned in a ceramic chip holder with its top surface exposed for boiling with the working fluid as distilled water, and the bottom part of the surface is kept in contact with the heater block to supply the heat through 120-VDC, 4 × 200 W cartridge heaters. This ensures the 1-D heat conduction from the heater to the surface. A glass water bath is assembled over the surface to hold the boiling liquid secured by a rubber gasket. To record the temperatures at the different location of the surface, a National Instruments cDaq-9172 data acquisition system with an NI-9211 thermocouple input module was used. This work focused on determining the two important performance parameters viz. critical heat flux (CHF) and heat transfer coefficient (HTC). The distilled water was degassed to eliminate the influence of dissolved gases during the pool boiling.


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Figure 2. Schematic of the pool boiling system 3. Results and discussions To investigate the longevity of the 2.5% GO-Cu surface, an aging study consisting of repetitive pool boiling runs was performed. For comparison, aging study was also accomplished on pristine copper surface. For each repetitive pool boiling run, heat flux was raised to ~80 W/cm² and back to zero. Total 20 repetitive pol boiling runs were performed on each surface and the effect of repetitive pool boiling was inspected through various characterization techniques including Fourier transform infrared, scanning electron microscope, X-ray diffraction and Raman spectroscopy.


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3.1 Morphological changes on the surface due to repetitive pool boiling To investigate the transition of GO-Cu electrodeposited surface from hydrophilic to hydrophobic, the morphological changes were observed using scanning electron microscope (SEM). Figure 3 shows the comparison of SEM images of the GO-Cu surface before and after repetitive pool boiling, showing Figure 3(a) and (b) at 70° tilt of carbon mapping of the surface before and after respectively, and Figure 3(c) indicating the corresponding energy dispersive spectroscopy plot of both before and after pool boiling runs. After 20 repetitive pool boiling runs on the same surface, the bitter gourd morphological structure underwent subtle changes. It was observed that the structure became less dense and the average width of the bitter gourd structure was reduced from 10-12 µm to 6-8 µm after 20 repetitive pool boiling runs. Overall, despite the continuous pool boiling runs, the morphological structure of the aged surface in general remained the same. This indicates that the multi-step electrodeposition process renders improved metallic bond strength that supports structural integrity and thus can sustain repetitive boiling.


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Figure 3. Scanning electron microscope images of GO-Cu surface, a) before and b) after 20 pool boiling runs, c) Energy dispersive x-ray spectroscopy (EDS) analysis of GO-Cu surface, and comparison of Laser confocal images of GO-Cu surface d) before and e) after 20 pool boiling runs Energy dispersive spectroscopy (EDS) mapping of the surface before and after 20 repetitive pool boiling runs is shown in Figure 3(c). EDS data not only confirms the presence of copper and carbon on the GO-Cu surface before and after 20 repetitive pool boiling runs, but also shows the reduction in oxygen peak intensity after 20 repetitive pool boiling runs, indicating the reduction of oxygenbased groups from GO. SEM images indicated that due to higher substrate bond strength, the bitter gourd structure after 20 repetitive pool boiling runs stayed similar to that of before performing any pool boiling run. However, the surface roughness of the GO-Cu coating after 20 repetitive pool boiling runs was increased (Ra = 12.371 µm, compared to Ra = 8.493 µm of GO-Cu coating before the run). The average surface roughness of these surfaces was measured using Laser Confocal Microscope (LCM) at a magnification of 10X. Figures 3(d) and 3(e) show the 3D profile of GO-Cu coating before and after 20 repetitive pool boiling runs. Increased surface roughness influences the surface topography and alters the water to surface contact. Similar phenomena of increased surface roughness and hydrophobicity of the surfaces for graphene-based coatings has been reported by other researchers. 3.2 Chemical changes on GO-Cu surface as a result of repetitive pool boiling Raman spectroscopy is widely used and a very effective technique for studying different graphene-based materials. To understand the effect of repetitive pool boiling on structural changes of graphene and to investigate the defects on graphitic sheets Raman spectroscopy was performed


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on GO-Cu surface before and after 20 repetitive pool boiling runs. Raman spectra on Figure 4 shows D and G band with intensity ratio ID/IG correlating to degree of disordered carbon as expressed by the sp3/sp2 carbon ratio 50. The ratio of intensities of G and 2D peaks determines the number graphene layers deposited on the surface 51. For GO-Cu surface, before pool boiling run, the absorption peaks for D and G bands were seen at ~ 1340 cm-1 and ~ 1583 cm-1 respectively, while for the Raman spectroscopy of GO-Cu surface after 20 repetitive pool boiling runs, D band became more prominent than G band without shifting its position, while G band broadened and shifted to the right side. These changes on D and G band indicate the self-healing and successful reduction of GO to rGO with removal of oxygen groups and allowing the restacking of graphene layers. Additionally, the increase in the intensity ratio of ID/IG is a very common phenomenon which further confirms a successful reduction of GO to rGO 52-55.

Figure 4. Raman spectroscopy of GO-Cu surface before and after 20 pool boiling runs


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The 2D peak for the GO-Cu surface before pool boiling was observed at ~2690 cm-1. However, the widening and the shift in peak for rGO after 20 repetitive pool boiling runs is associated with D+D’ or D+G mode. These peaks are observed for the samples with sufficient defects. This again confirms the successful reduction of GO to rGO 52. Figure 5(a) shows the comparison of x-ray diffraction spectroscopy of GO-Cu surface before pool boiling, after first boiling run, and of the aged surface after 20 repetitive pool boiling runs. The schematic of G/GO and rGO shown in the Figure 5 indicates the presence and absence of oxygen-based groups from graphene 56. The peaks between 6°-10° indicate presence of graphene (G) and graphene oxide (GO) on surface. G and GO peaks were observed on surface before pool boiling and on the aged surface after 20 repetitive pool boiling runs. Copper oxide peak was also noticed for the aged coatings indicating oxidation of the copper due to repetitive pool boiling runs. G and GO peaks on aged surface indicate the higher substrate bond strength of GO-Cu composite coating. However, the small peak around 2θ ~ 20° indicate that some of the GO has converted to rGO 57 58. Interestingly, XRD of GO-Cu surface after first pool boiling run showed a small bump around 2θ ~ 20° indicating that some of GO has reduced to rGO. The continuous repetitive pool boiling further converts more of GO to rGO. To further validate the reduction of GO to rGO, Fourier transform infrared spectroscopy was performed on the aged GO-Cu surface.


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Figure 5. a) Comparison of x-ray diffraction spectroscopy of GO-Cu surface before, after 1st and after 20th pool boiling run, b) Comparison of Fourier transform infrared spectroscopy (FTIR) of GO-Cu surface before and after 20th pool boiling run Figure 5(b) shows the comparison of Fourier transform infrared spectroscopy of GO-Cu electrodeposited surface before and after 20 repetitive pool boiling runs. The FTIR spectra of GO sample show the functional groups of carbon, oxygen, and hydrogen. The characteristic peaks of C=O including stretching vibration of carboxyl group at 1726 cm-1 and C-O-C stretching vibration at 1037 cm-1 indicate FTIR spectra of GO. The peak at 1570 cm-1 corresponding to the presence of aromatic rings C=C and peak at 3120 cm-1 corresponding to H2O/OH was also realized. The combination of C=C, C=O, C-O peaks confirm the presence of G and GO on the surface. However, compared to fresh surface (before pool boiling run), the peak intensity of aged GO-Cu surface was marginally reduced and functional groups of hydrogen and oxygen through carbon bonding were absent. Only C=C peak was observed indicating the reduction of GO to rGO 57, 58.


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Reduced Graphene Oxide (rGO) is the state of GO with fraction of oxygen groups removed from GO. Typically, when water molecules bond with other molecules such as hydroxyl groups, attains higher surface energy i.e. higher hydrophilicity. For the GO-Cu surface before any pool boiling run, the presence of hydroxyl groups on GO allowed to form a network with better hydrophilicity and lower contact angles. However, after repetitive pool boiling runs, due to conversion of GO to rGO, hydroxyl groups were not present on the surface which caused water molecules to adsorb on other water molecule layers via hydrogen bonding to achieve a lower energy state i.e. higher hydrophobicity. The main reason for change in contact angle is believed to be through the combined effect of change in nanostructure and change in chemical properties of the surface. The reduction in peak intensity of oxygen observed from energy dispersive spectroscopy (EDS) plot (Fig. 3(c)), shows removal of oxygen groups. Additionally, C=O. C-O-C, and H2O/OH peaks are vanished for aged GO-Cu surface which are observed from Fourier Transform Infrared spectroscopy plot (Fig. 5(b)). In contrast, the “Bitter gourd” shaped morphological structure has not changed drastically after 20 repetitive pool boiling studies (Fig. 3(a) and 3(b)), which further confirms that nano-structure does not play a major role in altering the contact angle of the coating. Thus, although the change in nano-structure is responsible for increment in contact angle, the removal of oxygen group plays a major role in increasing the contact angle. Figure 6 shows the schematic of a GO-Cu coated surface with higher hydrophilicity due to the presence of oxygen-based groups and a rGO-Cu surface formed due to repetitive pool boiling runs showing higher hydrophobicity due to absence of oxygen-based groups.


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Figure 6. Schematics showing effect of conversion of GO to rGO and its effect on contact angles Additionally, wickability of the rGO-Cu coating was negligible (Figure 5(b)) as water molecules did not get wicked in due to the absence of hydroxyl groups on the surface. Thus, the combination of two different mechanisms, increased surface roughness and chemical changes on the coating (playing a major role) is responsible for the transition of the surface from its hydrophilic to hydrophobic nature. 3.3 Effect on wettability and wickability due to chemical and morphological changes Boiling involves a chaotic motion of bubbles on the surface. Various morphological changes are observed on the surfaces due to continuous boiling which in turn affect the wetting properties of


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the surface. Surface energy i.e. wettability and wicking properties of the surface are dependent on various factors such as morphology of the surface, availability of pores, surface roughness and presence of chemical groups on the surface. Based on the chemical and morphological changes, the effect on wettability is observed and is shown below. Table 1 shows the comparison of wettability of surfaces in terms of static, advancing, and receding contact angles. Compared to GO-Cu surface after 20 repetitive pool boiling runs, GO-Cu surface before pool boiling show smaller values of contact angles indicating more hydrophilicity of the coated surface. Table 1. Comparison of static and dynamic contact angles of GO-Cu surface before and after 20 repetitive pool boiling runs Surface

Pool boiling run

Contact angle (°) Static

Electrodeposited 2.5% GO-Cu

Before pool boiling After 20 runs (aged surface)

Advancing

Receding

Hysteresis

48.5

42.6

13.5

29.1

140

145

38.6

106.4

Figure 7(a) shows the change in water contact angle of the GO-Cu surface before and after 20 repetitive pool boiling runs. Repetitive boiling runs not only increase the hydrophobicity of the surface but also reduce the change in contact angle over time.


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Figure 7. a) Comparison of change in contact angle with time, and b) Comparison of change in droplet volume of GO-Cu surface before and after 20 pool boiling runs The wicked volume (µL/s) for the GO-Cu surface before and after pool boiling was estimated as the droplet volume change over time using a VCA Optima goniometer. Figure 7(b) shows the change of droplet volume over a period of 80 seconds. The change in droplet volume can also be considered as the wickability of the coated surface. As seen from Figure 7(b), GO-Cu surface indicates a very high wicking rate before performing any pool boiling run as compared to that after 20 repetitive pool boiling runs. 3.4 Effect of aging on pool boiling performance To determine aging effect of repetitive pool boiling on pristine copper surface and electrodeposited 2.5% GO-Cu surface, an aging study similar to that of the previous work was conducted 47. The repetitive pool boiling runs were performed on the same surface by applying a heat flux of up to ~70% of CHF of a pristine copper surface for each pool boiling run. The heat flux was raised to ~80 W/cm² and then was reduced back to zero. The duration for each pool boiling run was 3 hours and the interval between the two successive runs was 24 hours. Total 20 repetitive pool boiling runs were performed on both pristine copper surface and 2.5% GO-Cu


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surface. After 20 repetitive boiling cycles, each surface was finally tested till it reached the CHF. The CHF performance of both the surface under the same number of repetitive cycles was then compared. To avoid the confusion and clustering of the data points, comparison of wall superheat temperature during the repetitive pool boiling for GO-Cu surface is shown in two different plots. Figure 8(a) and 8(b) show a comparison of the wall superheat for GO-Cu surface during the first, fifth, ninth, and thirteenth repetitive boiling runs, R1, R5, R9, and R13, respectively. The wall superheat temperature was observed to remain unchanged beyond the 17th repetitive run, although 20 repetitive runs were conducted. This assured that the surfaces reached their asymptotic value during the aging study. Figure 9(a) and 9(b) show the overall summary of repetitive pool boiling including first, eleventh and twentieth repetitive pool boiling runs, R1, R11, and R20 respectively. The reduction in wall superheat was observed for each successive pool boiling run between first and thirteenth repetitive run, indicating a very less degradation and a higher bond strength of the GO-Cu surface. A small increment in the wall superheat temperature was noticed between eleventh and twentieth repetitive pool boiling runs. For each repetitive run, lower wall superheat temperature for the GO-Cu surface indicated the higher heat transfer efficiency for a particular heat flux. For example, heat transfer coefficient of 86 kW/m²-°C was achieved for the eleventh repetitive run, while for the corresponding repetitive run of pristine copper surface, heat transfer coefficient was of 46 kW/m²-°C. Moreover, aging performance of GO-Cu coated surface was also better than that of the copper on copper coated surface 47.


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Figure 8. a) Comparison of wall superheat temperature and b) heat transfer coefficient for GO-Cu surface, first, fifth, ninth, and thirteenth (R1, R5, R9, R13 respectively) repetitive pool boiling runs

Figure 9. a) Comparison of wall superheat temperature and b) heat transfer coefficient for GO-Cu surface, summary of repetitive pool boiling runs indicating first, eleventh, and twentieth (R1, R11, and R20) repetitive runs


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3.5 Effect of aging on Critical Heat Flux (CHF) and Heat Transfer Coefficient (HTC): Figures 10(a) and 10(b) show the pool boiling curves and HTC plots for aged pristine copper surface, aged 2.5% GO-Cu surface and their comparison with pristine copper surface and 2.5% GO-Cu surface. Compared to pristine and aged copper surface, 2.5% GO-Cu gave higher CHF and HTC value. As observed from Fig. 10(a), a shift in pool boiling curve towards the left or lower wall superheats for 2.5% GO-Cu surface is observed. This shift is a function of active nucleation sites available for boiling and these nucleation sites become active at different wall superheat temperatures for surface 43. This phenomena is termed as “pool boiling inversion” and the primary reason for this inversion is believed to be activation of more nucleation cavities, or establishment of a more structured two-phase flow patterns of liquid and vapor bubbles near the heater surface. The effect of the contact angle on CHF was established by Kandlikar in his pool boiling CHF paper

59.

Initially, when surface is hydrophilic (48.5°), the CHF is high (as observed from Fig.

10(a)). However, for higher contact angle (140°), i.e. hydrophobic surface, the HTC is higher. This enhancement for rGO-Cu coating is primarily due to increased nucleation activity. The range of cavity sizes available for nucleation is recognized as the main contributing mechanism for the increase in HTC. However, on the other hand, compared to GO-Cu coating, the wicking rate of rGO-Cu coating is extremely less (Fig. 7(b)), indicating that liquid retention in the wicked structure did not contribute in the CHF enhancement. This is observed from the lower CHF value of rGOCu coating compared to GO-Cu coating (Fig. 10(a)).


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Figure 10. Summary of heat transfer performance of pristine and aged surfaces a) Comparison of pool boiling performance, and b) Comparison of heat transfer coefficient The aged copper surface showed increased HTC in an extension of the pristine copper surface HTC, while the 2.5% GO-Cu surface steadily showed higher HTC values. The improved CHF of aged pristine copper surface was attributed to oxidation of copper during the repetitive pool boiling. Compared to the pristine copper surface, ~18% higher CHF and ~104% higher HTC was achieved for the aged 2.5% GO-Cu surface. Thus, implementation of graphene-based coatings can improve the durability of the coatings and assist in enhancing the performance. Another approach for obtaining the high performing rGO structures would be to develop coatings using rGO in the electrolyte directly to form rGO-Cu composite coatings. The addition of rGO can be optimized further to achieve higher pool boiling performance. 4. Conclusions In this work, we have proposed a combination of two mechanisms responsible for the transition of the GO-Cu based composite coatings from their hydrophilic to hydrophobic nature (static


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contact angle from 48° to 140°). Repetitive pool boiling was the primary reason for the changes in wettability and wickability of the electrodeposited coatings. Repetitive pool boiling increased the surface roughness of GO-Cu coating and converted GO to rGO. Partial removal of oxygen groups from GO yielded rGO which caused water molecules to adsorb on other water molecule layers via hydrogen bonding to achieve a lower energy state. These factors increased the hydrophobicity and drastically reduced the wickability of the coating. The repetitive pool boiling performance of the GO-Cu coating demonstrated higher efficiency and pool boiling performance compared to pristine copper surfaces and copper on copper coated surfaces. Owing to their higher durability, compared to other surfaces, graphene-based coated surfaces are found to be advantageous and preferred for the commercial applications. 5. Acknowledgements This work was conducted in the Thermal Analysis, Microfluidics and Fuel Cell Laboratory in the Mechanical Engineering Department at Rochester Institute of Technology, Rochester, NY. The authors would like to thank Dr. Surendra Gupta for his assistance with the XRD equipment. The authors would also like to thank Andrew Layman for his assistance in Raman spectroscopy. The authors gratefully acknowledge the financial support provided by the National Science Foundation under CBET Award No. 1335927. 6. Abbreviation and nomenclature G, graphene; GO, graphene oxide; rGO, reduced graphene oxide; GO-Cu, graphene oxidecopper; rGO-Cu, reduced graphene oxide-copper; CHF, critical heat flux (W/cm²); HTC, heat transfer coefficient (kW/cm²-°C); q’’, heat flux (W/cm²); Twall, surface temperature of the copper chip (°C);


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7. References (1)

Li C., Peterson G. P., Parametric Study of Pool Boiling on Horizontal Highly Conductive

Microporous Coated Surfaces. J. Heat Transfer 2007, 129, 1465. (2)

Rahman, M. M.; Ölçeroğlu, E.; McCarthy, M. Role of Wickability on the Critical Heat

Flux of Structured Superhydrophilic Surfaces. Langmuir 2014, 30, 11225. (3)

Rishi, A. M.; Gupta, A.; Kandlikar, S. G. Electrodeposition of Selective Anions with Non-

Electrolyte Bath for Enhancing the Pool Boiling Performance. Sri Lanka 2017, 7. (4)

Li, Q.; Wang, W.; Oshman, C.; Latour, B.; Li, C.; Bright, V. M.; Lee, Y.-C.; Yang, R.

Enhanced Pool Boiling Performance on Micro-, Nano-, and Hybrid-Structured Surfaces. In Volume 10: Heat and Mass Transport Processes, Parts A and B; ASME: Denver, Colorado, USA, 2011, 633. (5)

Kumar, R.; Gupta, A.; Rohatgi, N. Boiling Heat Transfer on Wire-Mesh-Wrapped

Extended Tube Surfaces. Industrial & Eng. Chem. Research 2006, 45, 9156. (6)

Cai, Y.; Liu, M.; Hui, L. Observations and Mechanism of CaSO 4 Fouling on Hydrophobic

Surfaces. Industrial & Eng. Chem. Research 2014, 53, 3509. (7)

Liu, W.; Xu, Q.; Han, J.; Chen, X.; Min, Y. A Novel Combination Approach for the

Preparation of Superhydrophobic Surface on Copper and the Consequent Corrosion Resistance. Corros. Sci. 2016, 110, 105. (8)

Isimjan, T. T.; Wang, T.; Rohani, S. A Novel Method to Prepare Superhydrophobic, UV

Resistance and Anti-Corrosion Steel Surface. Chem. Eng. J. 2012, 210, 182.


24


(9)

Page 26 of 32

Shin, S.; Seo, J.; Han, H.; Kang, S.; Kim, H.; Lee, T. Bio-Inspired Extreme Wetting

Surfaces for Biomedical Applications. Materials 2016, 9, 116. (10) Zhang, L.; Zhong, Y.; Cha, D.; Wang, P. A Self-Cleaning Underwater Superoleophobic Mesh for Oil-Water Separation. Scientific Reports 2013, 3. (11) Feng, X. J.; Jiang, L. Design and Creation of Superwetting/Antiwetting Surfaces. Adv. Materials 2006, 18, 3063. (12) Edalatpour, M.; Liu, L.; Jacobi, A. M.; Eid, K. F.; Sommers, A. D. Managing Water on Heat Transfer Surfaces: A Critical Review of Techniques to Modify Surface Wettability for Applications with Condensation or Evaporation. Appl. Energ. 2018, 967. (13) Peng, B.; Ma, X.; Lan, Z.; Xu, W.; Wen, R. Experimental Investigation on Steam Condensation Heat Transfer Enhancement with Vertically Patterned Hydrophobic–Hydrophilic Hybrid Surfaces. Int. J. Heat and Mass Transf. 2015, 27. (14) Le Guéhennec, L.; Soueidan, A.; Layrolle, P.; Amouriq, Y. Surface Treatments of Titanium Dental Implants for Rapid Osseointegration. Dent. Mater. 2007, 23, 844. (15) Wang, Z.; Elimelech, M.; Lin, S. Environmental Applications of Interfacial Materials with Special Wettability. Environ. Sci. & Technology 2016, 50, 2132. (16) Crick, C. R.; Bear, J. C.; Kafizas, A.; Parkin, I. P. Superhydrophobic Photocatalytic Surfaces through Direct Incorporation of Titania Nanoparticles into a Polymer Matrix by Aerosol Assisted Chemical Vapor Deposition. Adv. Materials 2012, 24, 3505. (17) Bunch, J. S. Mechanical and Electrical Properties of Graphene Sheets; Cornell University Ithaca, NY, 2008.


25

Page 27 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(18) Park, S. D.; Won Lee, S.; Kang, S.; Bang, I. C.; Kim, J. H.; Shin, H. S.; Lee, D. W.; Won Lee, D. Effects of Nanofluids Containing Graphene/Graphene-Oxide Nanosheets on Critical Heat Flux. Appl. Phys. Lett. 2010, 97, 023103. (19) Gupta, A.; Jaikumar, A.; Kandlikar, S. G.; Rishi, A.; Layman, A. A Multiscale Morphological Insight into Graphene Based Coatings for Pool Boiling Applications. Heat Transf. Eng.2017, 1. (20) Ahn, H. S.; Kim, J. M.; Kaviany, M.; Kim, M. H. Pool Boiling Experiments in Reduced Graphene Oxide Colloids. Part I – Boiling Characteristics. Inter. J. Heat and Mass Transf. 2014, 74, 501. (21) Akbari, A.; Alavi Fazel, S. A.; Maghsoodi, S.; Kootenaei, A. S. Pool Boiling Heat Transfer Characteristics of Graphene-Based Aqueous Nanofluids. J. Therm. Analysis and Calorimetry 2018. (22) Park, S. D.; Won Lee, S.; Kang, S.; Bang, I. C.; Kim, J. H.; Shin, H. S.; Lee, D. W.; Won Lee, D. Effects of Nanofluids Containing Graphene/Graphene-Oxide Nanosheets on Critical Heat Flux. Appl. Phys. Lett. 2010, 97, 023103. (23) Hummers, W. S.; Offeman, R. E. Preparation of Graphitic Oxide. J. Am. Chem. Soc. 1958, 80, 1339. (24) Santhanam, K. S. V.; Kandlikar, S. G.; Mejia, V.; Yue, Y. Electrochemical Process for Producing

Graphene,

Graphene

Oxide,

Metal

Composites

and

Coated

Substrates.

WO2016011180A1, January 21, 2016. (25) Ray, S. C. Applications of Graphene and Graphene-Oxide Based Nanomaterials. 2015, 41.


26


Page 28 of 32

(26) Cao, N.; Zhang, Y. Study of Reduced Graphene Oxide Preparation by Hummers’ Method and Related Characterization. J. Nanomaterials 2015, 1. (27) Fernández-Merino, M. J.; Guardia, L.; Paredes, J. I.; Villar-Rodil, S.; Solís-Fernández, P.; Martínez-Alonso, A.; Tascón, J. M. D. Vitamin C Is an Ideal Substitute for Hydrazine in the Reduction of Graphene Oxide Suspensions. J. Phys. Chem. C 2010, 114, 6426. (28) Zhang, C.; Lv, W.; Zhang, W.; Zheng, X.; Wu, M.-B.; Wei, W.; Tao, Y.; Li, Z.; Yang, Q.H. Reduction of Graphene Oxide by Hydrogen Sulfide: A Promising Strategy for Pollutant Control and as an Electrode for Li-S Batteries. Adv. E. Materials 2014, 4, 1301565. (29) Wang, G.; Yang, J.; Park, J.; Gou, X.; Wang, B.; Liu, H.; Yao, J. Facile Synthesis and Characterization of Graphene Nanosheets. J. Phys. Chem. C 2008, 112, 8192. (30) Fan, Z.; Wang, K.; Wei, T.; Yan, J.; Song, L.; Shao, B. An Environmentally Friendly and Efficient Route for the Reduction of Graphene Oxide by Aluminum Powder. Carbon 2010, 48, 1686. (31) Wan, D.; Yang, C.; Lin, T.; Tang, Y.; Zhou, M.; Zhong, Y.; Huang, F.; Lin, J. LowTemperature Aluminum Reduction of Graphene Oxide, Electrical Properties, Surface Wettability, and Energy Storage Applications. ACS Nano 2012, 6, 9068. (32) MacInnes, M. M.; Hlynchuk, S.; Acharya, S.; Lehnert, N.; Maldonado, S. Reduction of Graphene Oxide Thin Films by Cobaltocene and Decamethylcobaltocene. ACS Appl. Mater. & Interfaces 2018, 10, 2004. (33) Li, X.; Wang, H.; Robinson, J. T.; Sanchez, H.; Diankov, G.; Dai, H. Simultaneous Nitrogen Doping and Reduction of Graphene Oxide. J. Am. Chem. Soc. 2009, 131, 15939.


27

Page 29 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(34) Feng, H.; Cheng, R.; Zhao, X.; Duan, X.; Li, J. A Low-Temperature Method to Produce Highly Reduced Graphene Oxide. Nat. Commun. 2013, 4. (35) Lee, S. W.; Mattevi, C.; Chhowalla, M.; Sankaran, R. M. Plasma-Assisted Reduction of Graphene Oxide at Low Temperature and Atmospheric Pressure for Flexible Conductor Applications. J. Phys. Chem. Lett. 2012, 3, 772. (36) Larciprete, R.; Fabris, S.; Sun, T.; Lacovig, P.; Baraldi, A.; Lizzit, S. Dual Path Mechanism in the Thermal Reduction of Graphene Oxide. J. Am. Chem. Soc. 2011, 133, 17315. (37) Gao, X.; Jang, J.; Nagase, S. Hydrazine and Thermal Reduction of Graphene Oxide: Reaction Mechanisms, Product Structures, and Reaction Design. J. Phys. Chem. C 2010, 114, 832. (38) Du, D.; Li, P.; Ouyang, J. Nitrogen-Doped Reduced Graphene Oxide Prepared by Simultaneous Thermal Reduction and Nitrogen Doping of Graphene Oxide in Air and Its Application as an Electrocatalyst. ACS Appl. Mater. Interfaces 2015, 7, 26952. (39) Voiry, D.; Yang, J.; Kupferberg, J.; Fullon, R.; Lee, C.; Jeong, H. Y.; Shin, H. S.; Chhowalla, M. High-Quality Graphene via Microwave Reduction of Solution-Exfoliated Graphene Oxide. Science 2016, 353, 1413. (40) Pei, S.; Cheng, H.-M. The Reduction of Graphene Oxide. Carbon 2012, 50, 3210. (41) Wang, J.; Salihi, E. C.; Šiller, L. Green Reduction of Graphene Oxide Using Alanine. Mat. Sci. and Eng.: C 2017, 72, 1. (42) Guex, L. G.; Sacchi, B.; Peuvot, K. F.; Andersson, R. L.; Pourrahimi, A. M.; Ström, V.; Farris, S.; Olsson, R. T. Experimental Review: Chemical Reduction of Graphene Oxide (GO) to Reduced Graphene Oxide (RGO) by Aqueous Chemistry. Nanoscale 2017, 9, 9562.


28


Page 30 of 32

(43) Jaikumar, A.; Rishi, A.; Gupta, A.; Kandlikar, S. G. Microscale Morphology Effects of Copper–Graphene Oxide Coatings on Pool Boiling Characteristics. J. Heat Transf. 2017, 139, 111509. (44) Jaikumar, A.; Kandlikar, S. G.; Gupta, A. Pool Boiling Enhancement through Graphene and Graphene Oxide Coatings. Heat Transf. Eng. 2017, 38, 1274. (45) Gupta, A.; Jaikumar, A.; Kandlikar, S. G.; Rishi, A.; Layman, A. A Multiscale Morphological Insight into Graphene Based Coatings for Pool Boiling Applications. Heat Transf. Eng. 2018, 39, 1331. (46) Ross, T. K. Corrosion and Heat Transfer — A Review*. Br. Corros. J. 1967, 2, 131. (47) Rishi, A. M.; Gupta, A.; Kandlikar, S. G. Improving Aging Performance of Electrodeposited Copper Coatings during Pool Boiling. Appl Therm. Eng. 2018, 140, 406. (48) Patil, C. M.; Kandlikar, S. G. Pool Boiling Enhancement through Microporous Coatings Selectively Electrodeposited on Fin Tops of Open Microchannels. Int. J. Heat and Mass Transf. 2014, 79, 816. (49) Rishi, A. M.; Gupta, A.; Kandlikar, S. G. Improving Liquid Supply Pathways on Graphene Oxide Coated Surfaces for Enhanced Pool Boiling Heat Transfer Performance. 2018, 51197, V001T03A003. (50) Graf, D.; Molitor, F.; Ensslin, K.; Stampfer, C.; Jungen, A.; Hierold, C.; Wirtz, L. Spatially Resolved Raman Spectroscopy of Single- and Few-Layer Graphene. Nano Lett. 2007, 7, 238.


29

Page 31 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(51) Ferrari, A. C.; Meyer, J. C.; Scardaci, V.; Casiraghi, C.; Lazzeri, M.; Mauri, F.; Piscanec, S.; Jiang, D.; Novoselov, K. S.; Roth, S.; et al. Raman Spectrum of Graphene and Graphene Layers. Phys. Rev. Lett. 2006, 97. (52) Kaniyoor, A.; Ramaprabhu, S. A Raman Spectroscopic Investigation of Graphite Oxide Derived Graphene. AIP Advances 2012, 2, 032183. (53) Khan, Q. A.; Shaur, A.; Khan, T. A.; Joya, Y. F.; Awan, M. S. Characterization of Reduced Graphene Oxide Produced through a Modified Hoffman Method. Cogent Chem. 2017, 3. (54) Ferrari, A. C.; Basko, D. M. Raman Spectroscopy as a Versatile Tool for Studying the Properties of Graphene. Nat. Nanotech. 2013, 8, 235. (55) Moon, I. K.; Lee, J.; Ruoff, R. S.; Lee, H. Reduced Graphene Oxide by Chemical Graphitization. Nat. Commun. 2010, 1, 1. (56) Pei, S.; Cheng, H.-M. The Reduction of Graphene Oxide. Carbon 2012, 50, 3210. (57) Husnah, M.; Fakhri, H. A.; Rohman, F.; Aimon, A. H.; Iskandar, F. A Modified Marcano Method for Improving Electrical Properties of Reduced Graphene Oxide (RGO). Mate. Res. Exp. 2017, 4, 064001. (58) Manickam, S.; Muthoosamy, K.; Geetha Bai, R.; Babangida Abubakar, I.; Mudavasseril Sudheer, S.; Hongngee, L.; Hwei-San, L.; Nayming, H.; Ch, C. Exceedingly Biocompatible and Thin-Layered Reduced Graphene Oxide Nanosheets Using an Eco-Friendly Mushroom Extract Strategy. Int. J. Nanomedicine 2015, 1505. (59) Kandlikar, S. G. A Theoretical Model to Predict Pool Boiling CHF Incorporating Effects of Contact Angle and Orientation. J. Heat Transf. 2001, 123, 1071.


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8. Abstract Graphics


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Repetitive Pool boiling runs: A Controlled Process to Form Reduced

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