Graphene-Based Functional Coatings for Pool Boiling Heat Transfer Enhancements

Abstract

Pool boiling heat transfer has proven to be the most effective ways to dissipate the large amount of heat fluxes and achieve the efficient cooling in many industrial applications including high-power electronics cooling, data center cooling, heat exchangers, batteries, refrigeration, and air conditioning. With the aggressive net-zero carbon footprint goals set up by the numerous industries across the globe, the need for development of innovative two-phase cooling solutions is of utmost importance. Graphene, being the highest thermal conductivity material, has been implemented in numerous studies for improving both the critical heat flux (maximum possible heat removed before thermal runaway of the heater surface) and a heat transfer coefficient (determines how efficiently the heat is removed) in pool boiling heat transfer. Initially, this chapter introduces various graphene-based nanomaterials and basics related to structure and characterization of graphene. Later, the highlights of some of the notable research work related to the graphene-based coatings for pool boiling enhancements are discussed. The responsible mechanism for such higher performance is summarized. Concluding remarks and industrial applicability of these techniques are also discussed in this section.

1. Introduction

Boiling heat transfer performance in any system primarily depends on the thermal conductivity of the heater surface as the heat is transferred from the heater surface to the fluid in contact. Higher thermal conductivity is desirable in achieving better heat removal efficiency. Various metals such as copper, gold, silver, and aluminum are commonly used in the industries as the heater surface materials due to their high thermal conductivity. After the discovery of world’s thinnest graphene material by Geim and Novoselov in 2004 [1], numerous research studies have focused on implementing the graphene on heater surface to increase the boiling heat transfer performance. Graphene is a 2D form of graphite and consists of a single layer of carbon atoms that are bonded together in a hexagonal lattice structure. Owing to its atomic layer thickness, a single layer of graphene possesses extremely high thermal conductivity, in the range of 4000–5300 W/m K. This chapter focuses on recent advancements in pool boiling heat transfer technique using graphene-based surfaces.

1.1 Graphene and its properties

1.1.1 Basics of sp²-hybridization

The extremely high in-plane thermal conductivity possessed by graphene is primarily due to the sp²-hybridized carbon atoms. In contrast, out-of-plane thermal conductivity is low because weak van der Waals interactions link the adjacent graphene planes within the multi-layer graphene.

A carbon atom consists of six electrons, and as per the energy states, two electrons are in 1 s state, and remaining four electrons occupy 2 s and 2p orbitals. In case of hexagonal structure of graphene as indicated in Figure 1, alternate carbon atoms are bonded via double bonds. Due to similar energies of 2 s and 2p orbitals, the electrons in these two orbitals arrange themselves such a way that one electron from 2 s orbital shifts in p orbit and contributes to forming three sp²-hybrid orbitals, while a remaining one electron in p orbital forms a pi-bond with the neighboring carbon atom. Three sp² hybridized carbon atoms are bonded via a strong covalent sigma bond to other carbon atoms. Figure 1b indicates the sp²-hybridization mechanism and the formation of 1 pi and three sigma bonds between two carbon atoms [2, 3].

Figure 1.

(a) Distribution of electrons in a carbon atom in a ground state, (b) sp²-hybridization of carbon atoms, and (c) structure of a single layer of carbon atoms forming a hexagonal lattice structure of graphene with sp² hybridized carbon atoms.

1.2 Graphene-based nanomaterials

Although a single layer of graphene has the highest thermal conductivity, manufacturing techniques to produce a single layer graphene are expensive and time-consuming. This has led the researchers to develop various derivatives of graphene that are easier to manufacture and are comparatively cheaper. Some of these alternatives are graphene oxide (GO), reduced graphene oxide (rGO), and graphene nano-platelets (GNP). These forms of graphene, however, have reduced thermal properties due to additional functional groups attached to the carbon atoms and a loss of pure carbon structure. For example, GO has an additional oxygen (-O), carbonyl (=O), hydroxyl (-OH), and carboxyl (-COOH) groups attached to the graphene’s hexagonal lattice of carbon atoms. Due to the presence of these groups, the thermal conductivity of GO is less than the pristine graphene, while the wettability of GO is higher. Graphene oxide can further be reduced to obtain a reduced graphene oxide (rGO) by eliminating the oxygen-based groups in GO. Defects in graphene structure also occur because of production methods that correspond to the breaking of the symmetry of honeycomb lattice carbon structure. Some of the defects include edge defect, grain boundaries defect, and defects associated with the change of hybridization of carbon from sp² into sp³. The amount and nature of defects strongly depend on the production method and can have a large influence on the properties of graphene.

Another derivative of graphene known as GNP comprises a few layers of graphene tightly packed together in a hexagonal lattice structure. And despite their multilayer structure, GNP can yield thermal conductivity in the range of 2500–3500 W/m K, that is, comparable to a single layer of graphene. In addition, GNP have higher wettability than pristine graphene due to the presence of oxygen-based chains attached to the carbon atoms.

Numerous research studies have shown that as compared to the plain surfaces, increased wettability or hydrophilic surfaces/coatings yield higher pool boiling performance compared to both plain and hydrophobic surfaces. Thus, having a hydrophilic nature for the graphene-based derivatives provide an additional advantage in case of boiling heat transfer. This will further be discussed in detail later in the chapter.

1.3 Characterization techniques of graphene

Due to its atomic layer thickness, in pool boiling heat transfer applications, graphene must be deposited on the heater surfaces through various deposition techniques. Determination of the quality of deposited graphene layers and quantification of the number of deposited layers is very important for establishing the enhancement mechanism and validation of the deposition technique used. Poor quality of deposited graphene can also substantially affect its thermal and mechanical properties. And with increased number of deposited layers, graphene structure tends to become like a 3D graphite structure that has lower thermal properties. Thus, it is important to understand the properties of the deposited graphene in any work that relies on properties of graphene for its superior performance. Most widely implemented, effective characterization techniques for graphene are discussed in this sub-section. These techniques are also of the utmost importance in determining the underlying enhancement mechanisms in pool boiling performance.

1.3.1 Raman spectroscopy

Raman spectroscopy is a light scattering technique in which molecules scatter incident light from a high-energy laser light source. Most of the scattered light have same frequency as the incident light; however, some fraction of light scatters at different frequency depending on the chemical structure such as benzene ring structure and bonds such as C〓C, C▬O, and C▬H. Each peak in the Raman spectra corresponds to vibration of a specific molecular bond. The wavelength of the Raman scattered light depends on the wavelength of the incident light, and thus, Raman scatter wavelength number becomes impractical for the comparison. Thus, Raman scatter position is converted to Raman shift which indicates the Raman shift away from excitation wavelength.

Graphene-based derivatives typically show D, G, and 2D peaks indicating the variation in peak intensity based on the quality and number of layers of deposited graphene. The distinct graphene peaks, G at ∼1580 cm⁻¹ and D at ∼1340 cm⁻¹, correlate to the in-plane vibration of sp²-hybridized carbon atoms and degree of disorder of sp³-hybridized carbon structure, respectively. Monolayer graphene is generally defect free and thus does not show D peak (as shown in Figure 2b). 2D peak is the second order D-peak that is observed at ∼2660 cm⁻¹ and is a connotation of D-peak. Depending on the color, intensity, and type of wavelength of the laser, a small Raman shift can be observed on the x-axis. The ratios of G and 2D peak intensities (I_G/I_2D) from Raman spectroscopy plot are used to find the number of deposited graphene layers. While the ratio of D and G peak intensities (I_D/I_G) represent the oxidation degree and defects on graphene sheets. It also represents the sp³/sp² carbon ratio. Generally, (I_D/I_G) ratio of less than 1 indicates the good quality and less defects on the graphene structure and thus has lesser impact on its thermal and mechanical properties.

Figure 2.

(a) Typical X-ray diffraction peaks for graphene, (b) typical Raman spectra of a monolayer and multilayer graphene.

1.3.2 X-ray diffraction (XRD)

X-ray diffraction is a technique used for determining the atomic and molecular structure of a crystal in which crystalline atoms cause a beam of X-rays to diffract in specific directions. When X-rays are incident on the sample, the incident beam gets separated into transmitted beam and diffracted beam. The diffraction pattern is recorded in terms of 2θ angle that indicates the crystalline phase of the material. The crystalline phases of graphene are typically investigated using an X-ray diffractometer (XRD) with Cu Kα radiation; wavelength 1.5418 Å. The spectra are recorded for 2θ ranges between 5° and 75° at a rate of 3°/min rate. The step size is 0.02° with an X-ray power of 40 kV and 35 mA. This range captures peaks from carbon and the underlying copper substrate in case of graphene deposition on copper. The location of characteristic peaks determines the presence of elements on the surface.

X-ray diffraction 2θ reflection peaks between 6° and 10° correspond to graphene. Peaks are typically either broader or sharp between 6° and 10°. Figure 2a shows the comparison of the XRD plot for monolayer and multilayer graphene coatings deposited on copper substrate. The peak intensity is on the y-axis, and 2θ reflections peaks are on the x-axis. The peaks confirm the presence of graphene along with copper. With the presence of large amount of carbon and more disordered structure, additional G peak at 20° can also be prominently observed for XRD plots.

2. Applications of graphene in pool boiling heat transfer

As discussed previously, owing to extremely advantageous thermal properties of graphene and graphene-based derivatives, numerous research studies have focused on using the graphene for enhancing the pool boiling heat transfer performance. Less complex, less hazardous, and less time-consuming deposition techniques are desired for environmental, economic, and industrial purposes. This section is divided into two main sub-topics: (1) Nanoscale graphene-based coatings and (2) Microscale graphene-based composite coatings. Various deposition techniques of graphene on the heater substrates are discussed along with their corresponding pool boiling heat transfer efficacies in the subsequent sections. The enhancement mechanisms responsible for such heat transfer improvements are also discussed. Amongst numerous research work available, the studies have been selected based on fulfillment of either one or all the following criteria: higher pool boiling performance, potential for applicability in industrial applications, and development of unique/innovative approach in creating graphene-based functional coatings.

2.1 Nanoscale graphene-based coatings

Nanoscale coatings, as the name suggests, have thickness in the range of nanometer and mainly consist of only graphene as the depositing material. Some of the widely implemented techniques to develop nanoscale graphene-based coatings are discussed here with the focus on their applications in pool boiling heat transfer enhancements.

2.1.1 Chemical vapor deposition (CVD)

Even though a variety of techniques exist focusing on production of high-quality single layer graphene, the challenges related to the transfer of graphene on the heater substrate after production still remain a significant obstacle for its industrial adoption. Chemical vapor deposition technique is a step toward the application of graphene in industries as a graphene layer can directly be formed on the heater substrate.

Generally, to generate the graphene film, an atmospheric pressure CVD (APCVD) technique is used. The polished copper substrate is loaded in the tubular furnace and vacuumed. The temperature is then ramped to 1333 K at the rate of 303 K/min. With a mixed gas of H₂/Ar (20/80 standard cubic centimeters per minute, (sccm)) at a constant pressure of 72 kPa. To flatten and to reduce the copper surface, an additional step of surface annealing is performed under a mixed gas of H₂/Ar (30/1000 sccm) at 101,325 kPa for 30 minutes. During the growth of graphene, the atmosphere is switched to CH₄/H₂/Ar (0.5/30/1000 sccm) at 101,325 kPa for 7 minutes. Followed by this, it is cooled down to room temperature under H₂/Ar (30/1000 sccm) flow to complete the process of deposition and to avoid the oxidation of the deposited film [4]. Bulk copper substrate in the CVD introduces the roughness effects due to thermal deformation. This provides additional morphological features and is beneficial for boiling heat transfer. A monolayer and multilayer graphene coated surfaces are developed using this APCVD technique. Raman spectroscopy confirms the quantification of the deposited layers. Compared to the plain copper surface (critical heat flux (CHF) = 1280 kW/m²) using distilled water as a working fluid, both monolayer and a multilayer (three layers) graphene coatings yielded higher pool boiling performance with CHF of 1490 kW/m² and 1570 kW/m², respectively. The improved performance is attributed to the altered liquid wettability along with the wrinkle induced roughness of the underlying copper that provides additional nucleation during the boiling [5].

Another study is performed by implementing APCVD to create graphene-based coatings on larger heater sizes (32 × 32 mm as against 10 × 10 mm) and HFE-7000 refrigerant as a working fluid instead of distilled water. A similar gas composition of CH₄/H₂/Ar are used to develop the coatings. Additional fluorinated-graphene coating is prepared by gas fluorination of CVD-grown graphene in a Teflon container with XeF₂ powder as a precursor (loading mass: 0.2 g per 0.4 L) followed by baking in the oven at 373 K for 12 hours. Compared to the plain copper surface, 2 times higher bubble density and 2.43 times higher bubble density is observed on CVD grown graphene and fluorinated-graphene coatings. Increased hydrophobicity of the coatings is mainly responsible for the increased bubble activity. This resulted in 80 and 20% higher heat transfer coefficient (HTC) for CVD grown graphene and fluorinated-graphene coatings as compared to the plain copper surface (HTC = 5.3 kW/m² K). However, due to the increased bubble density, the CHF for CVD grown fluorinated-graphene coating is 370 kW/m², which is lower than the plain copper surface (CHF = 380 kW/m²), while CVD graphene shows the highest CHF of 428 kW/m² [6].

Various other similar studies have been performed using APCVD methods to create graphene-based coatings for the improvement of pool boiling performance. One of the studies have created hybrid coated surfaces using graphene and carbon nanotubes for improvements in pool boiling. Similar improvements in pool boiling performance have been observed with these studies. However, longevity studies from the industrial applications perspective have been performed on CVD graphene coating methods for pool boiling heat transfer.

2.1.2 Nanofluids

To create graphene-based nanoscale coatings, researchers have utilized graphene (G)/graphene oxide (GO)/reduced graphene oxide (rGO) based nanofluids. These coatings are formed when the nanofluids are boiled on the substrate surfaces for a specific amount of time. This nanoscale coated surfaces are then utilized as the substrates on which pool boiling is performed. Typically, these developed coatings are self-assembled and have varied morphological features that assist in improving the pool boiling performance.

In one of the studies, a new concept of nucleation patterning surface along with rGO-coated pillars structures are developed. The micropillar structures are developed using UV lithography technique in which a pattern mask is created with the size of the pillars. Deep reactive ion etching is then performed to generate the micropillars. The diameter of 4 μm, a pitch of 20 μm, and a height of 20 μm are the dimensions of the micropillar structures. Additionally, to improve the nucleation performance during boiling, a micropillar free region of 200 × 200 μm was also kept. The prepared surfaces were then coated by boiling rGO using 0.0005 wt.% rGO nanofluid solution and stopped just before reaching the CHF. At this stage, the self-assembled structures of rGO on micropillars are obtained. Two different surfaces with pitch of 1 and 1.5 mm are developed to minimize the bubble coalescence. These coatings are implemented for pool boiling studies using de-ionized (DI) water as a working fluid. Compared to the CHF on a plain silicon chip (890 kW/m²), 1.5 pitch rGO-coated micropillar structure reached the CHF of 2700 kW/m² along with HTC of 89.7 kW/m² K (plain surface HTC = 20 kW/m² K). Plain silicon surface with rGO coating is also tested for the pool boiling which yields CHF of 1340 kW/m² and the HTC of 80 kW/m² K. The rGO-coated micropillar-free cavity on the surfaces facilitated bubble nucleation by providing cavities on the basis of bubble departure diameter which delayed the horizontal coalescence of the bubbles and increased the overall pool boiling performance. The micropillars provided the liquid paths to the nucleation cavities and rGO layer on top of the micropillars ensured the continuous capillary inflow [7, 8].

Another study is performed with varying the concentration of graphene in graphene-based nanofluid. The pool boiling is conducted on the copper substrates, and a maximum of 46% reduction in wall superheat is observed for 0.2% graphene nanofluid. Along with this, a maximum HTC improvement of 48.6% is observed as compared to the plain copper surface with DI water. This enhancement is attributed to the possible deposition of graphene at higher heat fluxes on the substrates and producing of additional nucleation sites that assist in boiling [9].

In one of the studies with the rGO, the effect of base-graphene layer, self-assembled foam like graphene layer and a thick-graphene layer are compared for their pool boiling performance on SiO₂ surfaces. These nanofluid-based coatings are generated by boiling the nanofluids on the boiling surface, followed by the actual boiling test. Different types of layer formation on the substrate depends mainly on applied heat flux and the concentration of the rGO colloid. The CHF is increased with reducing the concentration of rGO colloidal solution. A maximum CHF of 1600 kW/m² is achieved for 0.0001 wt.% of rGO. It is observed that during water boiling, the thick-graphene layer coating was completely detached and the portions of base-graphene layer and self-assembled foam like graphene layer remained and assisted in increasing heat spreading actions. Additionally, it is observed that even after reaching CHF, the substrate surface temperature increased very slowly, and the heat flux is maintained. It is hypothesized that this is due to the heat spreading action by base-graphene layer and porosity introduced due to the deposition [10].

The effect of rGO nanofluids is also studied on the copper flat plate heater. Different rGO wt.% of 0.2, 0.6, and 0.8 are considered, and since the thickness of the plate heater is 0.05 mm, initial step of developing a coating by boiling a nanofluid is not performed. The tests are performed till the CHF with different rGO concentrations and are compared with the plain DI water. The highest CHF of 945 kW/m² is attained for 0.2 wt. %. However, the HTC for 0.8 wt.% rGO is the highest due to increased nanoparticles in the DI water and their deposition to form a porous layer on the boiling surface that acts as secondary cavities. For the heat flux of 500 kW/m², 2.3 times higher HTC is obtained for 0.8 wt.% rGO than the boiling with DI water on copper plate heater [11]. Uncertainties of measured and calculated parameters are considered while reporting all the CHF and HTC values that have been mentioned throughout the chapter [12, 13, 14].

2.1.3 Concluding remarks

Many of similar studies have been performed to develop the nanoscale graphene-based coatings either by nanofluids or by chemical vapor deposition techniques [15, 16]. The enhancement mechanisms responsible for these improvements can be categorized into either one of the following or the combination of the following: Nanoscale coatings with graphene alter the wettability of surfaces, enhance the thermal conductivity, introduce additional surface roughness features, and increase the nucleation activity during boiling. Even though many innovative approaches have been introduced over the years, not many studies have focused on longevity of these surfaces for real-world industrial applications. Also, compared to microscale composite coatings, nanoscale coatings do not yield higher pool boiling performance and lack the ability to sustain continuous vigorous boiling over the longer periods. Considering the continuous usage and complications in the industrial applications, these nanoscale enhancement techniques appear to have limited scope.

2.2 Microscale graphene-based composite coatings

Pool boiling performance enhancements using deposition of composite coatings have been reported extensively over the years. Composite coatings represent the deposition of a material using more than one metal and/or non-metal on the heater substrate by means of various deposition techniques. And these composite coatings typically attain higher critical heat fluxes at lower wall superheat temperatures due to the formation of porosity which results in increased surface area and bubble nucleation sites available for boiling.

To be effective during the pool boiling, the bond strength of the porous coatings with the base substrate must be strong enough to sustain the vigorous boiling. Otherwise, coatings can peel off from the heater surface substrate and can result in thermal runaway. This bond strength and adhesion is observed higher in composite coatings than pure coatings since metal-to-metal diffusion bonds have higher mechanical properties. The process and mechanism of vapor bubble generation in the porous surfaces is as follows: When the heat is supplied to the substrate/heater surface, above a certain temperature, the nucleus of a bubble grows in the cavity. The cavities may be formed inside the porous matrix or may appear on the surface of the coating. When this bubble nucleates and grows, it carries heat with itself. As the bubble departs from a cavity after reaching its critical radius, the liquid in the vicinity of the void rushes into the cavity, ensuring a continuous supply of liquid for evaporation. Steady vapor formation takes place on the porous media, and the nucleation takes place within the porous matrix via the re-entrant cavities that are not susceptible to flooding by liquid. Higher the nucleation frequency, higher is the heat dissipated from the surface. And smaller the vapor bubble diameter, more is the bubble generation and thus higher is the heat removal. Further, the agitation caused by the bubble activity may increase the heat transfer rate between the surface and the liquid.

Compared to nanoscale coatings, microscale coatings have many advantages from industrial applicability perspective such as higher cohesive and adhesive bond strength, higher pool boiling performance, and ability to sustain repetitive boiling over a prolonged time. Electrodeposition, dip coating, and sintering are some of the simple and widely used deposition techniques to develop the microscale coatings not only for pool boiling enhancement applications but also for a variety of industrial applications, including nuclear power plants, water desalination, vapor chambers, electrodes of lithium-ion batteries, and condensation heat transfer. Detailed studies related to these coatings are discussed in this section.

2.2.1 Electrodeposition technique

Electrochemical deposition is the process of coating solids on the conductive base materials to modify the surface properties. It consists of an electrolyte solution containing positive and negative ions usually prepared from metal salts and the two working electrodes that can be of either conducting or semiconducting nature known as the cathode (on which the coating is desired) and an anode. The resultant electric current (rate of the motion of the electric charge) between the two electrodes under an external voltage is due to the migration and diffusion of the charged species. And since it is difficult to diffuse graphene and graphene-based nanomaterials in the metal base substrate, typically in pool boiling applications, a metal salt is also vital to deposit a composite mixture of the metal and graphene. This provides a very high bond strength with the substrate along with deposition of graphene that can both sustain the boiling for longer duration and maintain the higher heat transfer performance.

The principle behind electrodeposition is to use an electric current to strip the cations from a sacrificial material (anode) in a solution and coat that material in the form of a thin film onto a substrate that is conductive (cathode). The electrodes are positioned parallelly in the electrolyte solution containing both positively charged ions called cations and negatively charged ions called anions. When an external electric field is applied, the cations depart toward the cathode and get deposited as metal. According to which, the process follows Faraday’s law, and the amount of the deposited metal on the electrode is proportional to the applied current to the electrochemical cell.

$$Q=\frac{n\ast d\ast A\ast h\ast F}{M}$$E1

Where Q is the charge (C), n is number of electrons, d is the distance between cathode and anode (m), A is the coated surface area (m²), h is the thickness of the deposition (m), F is Faraday’s constant, and M is the atomic weight.

In this work, the duration of applied current is also considered.

$$Q=I\ast t$$E2

Where $Q$ is the charge applied (C), I is the amount of current supplied (mA), and t is the duration for which the current has been provided (seconds).

For the application of pool boiling heat transfer, a coating with defects and pores is desired due to its tremendous advantages that include pores acting as nucleation sites, increased wicking and wetting properties, and non-uniformity of the porous structures that serve as a perfect platform for very intense bubble formation and heat dissipation. Addition of graphene to such porous structures can further amplify the performance due to additional increased in-plane thermal conductivity of the coatings and formation of unique morphological structures of the coating.

A multi-step electrodeposition technique includes multiple steps of varying current and time durations while performing the electrodeposition process. Each step has specific values of controlling parameters. The electrodeposition technique can be implemented on the substrate of any shape, size, and material. And its applications are not only limited to boiling heat transfer substrates, but also include any electroplating and coating-based applications. Some of the target applications include abrasion and wear resistance protection, corrosion protection, decorative coatings, prolonged life of coating and surface, durability, to maintain the esthetics, integrated electronics, solar reactors, fabrications, and others.

2.2.1.1 Dynamic template-assisted electrodeposition technique

Template-assisted electrodeposition permits more size and shape-controlled deposition. The templates could either be dynamic, restrictive, or self-organized. In the case of an aqueous solution, where electrolysis of water takes place in one of the electrochemical reactions, the evolved hydrogen serves as the dynamic template that results in porous surface coatings. The electrochemical reaction takes place when direct current is supplied through the electrochemical cell. First, at a higher current density supply, hydrogen bubbles are formed on the cathode (substrate). Typically, the electrolysis of water in the electrolyte creates hydrogen gas. If the evolution of bubbles is continued, the copper ions start to grow within the interstitial spaces between hydrogen bubbles. The resultant hydrogen bubbles behave as dynamic templates around which copper particles deposit and grow. When the higher current density supply is stopped, the hydrogen gas bubbles collapse, leaving behind the porous open network of copper. The size of the pores is determined by the bubble behavior, and the morphology of the metal film is determined by the nucleation and growth mechanism of the metal on the substrate. Thus, two simultaneous reactions occur at the cathode, deposition of copper ions and the evolution of hydrogen bubbles.

The hydrogen evolution reaction rate depends on the applied current density which in turn is dependent on the depositing metal-hydrogen chemisorption energy or the dissociation of hydrogen ions from the electrode surface that further combines with protons to form hydrogen gas. This is dependent on the current exchange density, which is defined as the rate of hydrogen evolution per surface area at the electrode. Metals that have weak interaction energy with hydrogen do not abet in the passage of sufficient electrons, whereas metals that interact strongly with hydrogen result in greater adherence to the surface and not getting released in the solution instantly.

The applied current density and duration control the surface morphology characteristics such as porosity, thickness, wickability, wettability, contact angle, hydrophilicity, and hydrophobicity of the electrodeposited coatings. Bubble behavior dictates the size of the pores. A higher bubble generation rate leads to shorter residence times that further control the coalescence of the bubbles. Reduced coalescence results in the smaller pore sizes of the deposited materials. Effectively, higher current density results in the production of the higher amount of hydrogen ions that result in the formation of hydrogen bubbles which provide the anatomy for the porous network and subsequent higher amounts of metal ions produced in this step get deposited around these hydrogen bubbles.

According to Faraday’s law of electrolysis [17], the amount of deposited material is proportional to the duration and time of the deposition. The deposition is usually accomplished using two methods: (i) supplying the constant current during the deposition process and (ii) holding the constant potential during the electrodeposition. These methods are called as galvanostatic and potentiostatic modes of deposition, respectively. In the current study, all the depositions are performed using the galvanostatic method, i.e., the constant current is supplied for the fixed duration, and the voltage is varied accordingly. By controlling the current density and time required for the deposition, the morphological structure and porosity of the coating can be controlled using the electrodeposition technique.

2.2.1.1.1 G/GO-Cu composite coatings

Copper being highly thermally conductive metal, deposition of G/GO-Cu composites is performed using the copper block as anode and a plain copper substrate as the cathode with addition of G/GO colloidal solution by % by volume. Both the electrodes are held parallel by placing in a Poly-Tetra-Fluoro-Ethylene (PTFE) holder, and the entire assembly is placed in the electrolytic bath consisting of 5.85 gm of 0.8 Molar concentration of CuSO₄, 3.14 mL of 1.5 Molar concentrated H₂SO₄, 40 mL distilled water, and 0.5, 1, 1.5, and 2.5% vol./vol. G/GO solution. The working area on both the electrodes is delineated with Kapton® tape (Figure 3).

Figure 3.

(a) Typical setup for deposition using electrodeposition technique, (b) two-step electrodeposition technique schematic with copper and GNP.

To achieve a microporous coating on the heater surface, template assisted electrodeposition technique is adopted which includes supply of higher current density of 400 mA/cm² for 15 seconds that produce hydrogen gas bubbles as a result of electrolysis of water and the deposition occurs around these evolved bubbles. Lower current density supply step of 40 mA/cm² for 2500 seconds after high current density step deposits a small quantity of G/GO-Cu without evolution of hydrogen bubbles and strengthens the adhesion of the coating on the substrate. The formation and collapse of hydrogen bubbles during two-step electrodeposition technique ultimately yields highly microporous coating. Compared to the plain copper surface (CHF = 1280 kW/m²) using distilled water as a working fluid, all Cu-G/GO vol. % composite coatings yielded higher pool boiling performance, with 2.5% G/GO coating giving maximum CHF of 2200 kW/m² and the HTC of 155 kW/m² K (as compared to 55 kW/m² K for plain copper surface). The enhancement mechanism for achieving higher performance can be condensed to the combination of following multiple factors—increase in wetting and wicking properties, wicking through dendrite type copper structures and bubble nucleation on underlying deposited G/GO sheets, increased nucleation sites that become activated under suitable substrate temperature conditions and contribute toward decreasing the wall superheat, and enhanced evaporation through microlayer partitioning mechanisms that increase bubble growth rates and frequency [18].

2.2.1.1.2 GNP-Cu composite coatings

A similar electrodeposition technique as that of G/GO-Cu composite coatings is also implemented to develop GNP-Cu composite coatings considering higher thermal properties of GNP than G/GO. GNP powder is commercially available and is added by varying the wt. %, 0.25, 0.5, 1, 2, 2.5% in the electrolyte solution. All the electrochemical parameters and electrolyte solution composition are kept constant to compare the difference between G/GO and GNP. The electrodeposited coatings are then tested for pool boiling heat transfer with water as working fluid till the CHF condition.

Compared to the G/GO-Cu composite coatings, a very distinguished morphological features are developed for GNP-Cu composite coatings. Due to their multilayered structures, GNP sheets wrap and deposit around copper structures and the substrate. A wide range of porous network is developed for GNP-Cu coatings as against the G/GO-Cu coatings. Additionally, all the GNP-Cu electrodeposited coatings are superhydrophilic (0° contact angle) in nature (Figure 4c, d). For 2% GNP-Cu coating, a maximum wicking rate is attained, with 2 μL water droplet wicking within 12 ms. The wicking rate (unit - m/s) that is calculated by normalizing the wicked volume over the droplet contact area is 0.145 m/s, maximum for 2% GNP-Cu coating, while lowest rate of 0.018 m/s is obtained for 0.5% GNP-Cu coating. The pool boiling studies show an overall increase in both CHF and HTC for all GNP concentrations. Pool boiling and HTC plots are presented in Figure 4a, b. CHF of 2670, 2400, 2860, and 2750 kW/m² is achieved for 0.5, 1, 2, and 2.5% GNP/Cu coatings, while 0% GNP-Cu coating (only copper coating) achieved a CHF of 1560 kW/m². Heat transfer coefficients of 142, 194, 204, and 150 kW/m² K are rendered for 0.5, 1, 2, and 2.5% GNP-Cu coatings, while 0% GNP-Cu coating yielded an HTC of 60 kW/m² K. 2% GNP-Cu coating yielded maximum of ∼130% increment in CHF and 290% increment in HTC as compared to the plain copper surface.

Figure 4.

(a) Pool boiling curve, (b) heat transfer coefficients for GNP-Cu coatings using distilled water as a working fluid at an atmospheric pressure (size of the boiling surface 10 mm × 10 mm), (c) schematic of enhancement mechanism for GNP-Cu coatings, (d) porosity on the GNP-Cu electrodeposited coating, and e) stream of departing bubbles from nucleation cavity at 100 kW/m² for 2% GNP-Cu surface.

A unique shift in pool boiling curve toward the left (or lower wall superheat) with increase in heat flux is observed for GNP-Cu coatings as observed in Figure 4a. This shift is primarily due to the activation of additional nucleation sites for boiling at different wall superheat temperatures, and this phenomenon is termed as “boiling inversion”. The underlying mechanism for boiling inversion in porous surfaces is attributed to the presence of hierarchical pores that develop supplementary nucleation cavities at higher heat flux, and thermally induced gradients along the pores also dominate owing to varying thermal conductivity of the material.

The presence of hierarchical pores also improves the CHF and HTC due to added nucleation sites for generating additional vapor bubbles that improve the boiling efficacy. Superhydrophilic coatings further promote the nucleation and microlayer evaporation during the boiling. Visualization studies indicate very low bubble departure diameter of 0.68 mm for 2% GNP-Cu coating along with lower departure time of 5 ms as shown in Figure 4e. The combination of all these factors along with higher thermal conductivity of GNP forms the enhancement mechanism and yields the highest pool boiling performance ever recorded on plain copper surface with GNP [19].

2.2.2 Sintering technique

Sintering is the process of compacting a powdered material and forming a solid or porous coherent mass by heat or pressure without melting it to the point of liquefaction. It is a heat treatment process that is generally used to increase the strength and structural integrity of the material. The produced coating provides a higher surface area-to-volume ratio compared to its bulk counterpart. Sintering strengthens the particle contacts by means of the thermal mass transport process and provides the change in porosity and pore geometry. Sintering technique has many advantages over other coating processes—controlled deposition for tunable coating thickness; the ability to coat substrates of varying shapes and thickness; and cost-effectiveness.

The sintering process is an irreversible event that happens in different stages. The sintering temperature is high enough to promote neck formation at the point of contact between the adjacent metal particles. The process initiates with loose powders with a specific packing density if no compression is involved, as shown in Figure 5a. Initially, necks between the contacting particles grow to the point where the neck size is less than one-third of the particle size. During the next transitional stage, with the continuation of necking amongst the contacting particles, tubular pores start forming and connect to the external surface. Finally, the necking state is achieved to a point where only a small porosity is present in the material. The grain boundaries are developed at the neck regions. The porosity is an inherent property during a sintering process and can be altered by changing the sintering time. The schematic of the different stages is shown in Figure 5.

Figure 5.

Illustration of the sintering stages showing the change in porosity at each stage (a) loose powder, (b) initial stage, (c) intermediate stage, and (d) final stage.

The porosity of the sintered coating primarily depends on the sintering temperature, the ratio of powder to sintering oil, and sintering time. Effectively, a higher temperature can distort the shape of the particles. And a lower sintering temperature can result in a coating that cannot develop enough bonding between the particles due to low temperature. This can reflect in poor bond strength and removal of the coatings. Similarly, if the sintering time is less, the bond strength of the deposited coating is inadequate and fails to sustain vigorous forces. And if the sintering time is more, it can reduce the porosity drastically. Thus, with the help of sintering parameters, the surface morphological characteristics such as porosity and thickness and the subsequent surface properties such as wettability and wickability can be controlled.

2.2.2.1 G/GO-Cu composite coatings

Copper being highly thermally conductive metal, deposition of G/GO-Cu composites is performed using the copper block as anode and a plain copper substrate as the cathode with addition of G/GO colloidal solution by % by mass.

The produced coating provides higher surface area-to-volume ratio compared to its bulk counterpart. Formation of artificial nucleation cavities to promote vapor generation rate can be achieved using sintering technique. Here, the screen-printing paste is created by adding a commercially available screen-printing binder. The composite G/GO-Cu powder is mixed with the sintering oil with powder-to-oil ratio of 2:1. After screen-printing, the test surfaces are securely placed inside the sintering furnace with the inert helium atmosphere. During sintering, initially the sintering temperature is raised to 723 K for the duration of 30 min. to eliminate the binder from the coating and then is ramped up to 1073 K for a duration of 1 hour to develop a microporous sintered coating. The temperature is then ramped down to the room temperature via natural convection, and the sintered surfaces are removed from the furnace once the room temperature is attained. Results indicate the improvement in HTC for 0.2, 0.4, 0.6, and 1% G/GO % by mass than the plain copper surface. Critical heat fluxes obtained are similar to plain copper surface, with maximum CHF of 1420 kW/m² and the HTC of 194 kW/m² K attained for 1% G/GO-Cu coating. Small increments in CHF values are due to the poor wickability of the coatings and formation of hydrophobic coatings. Higher HTC is due to early nucleation activity on the surface and increased porosity [18].

Another study focused on the effect of foam thickness and graphene coating on pool boiling heat transfer of sintered porous surfaces. This study combined sintering technique for copper deposition and CVD process for monolayer graphene deposition on sintered copper deposits. Thinner foam thicknesses (0.5 and 1 mm) yielded better performance than 1.5 and 2 mm due to lower vapor resistance and efficient liquid supply. Heat transfer coefficients are observed 161% higher initially at lower heat fluxes, while this increment in HTC is reduced at higher heat fluxes [20].

2.2.2.2 GNP-Cu composite coatings

Traditionally implemented sintering techniques yield the coatings with uniform porous structure throughout the coating. However, the uniform spreading of graphene in the sintered coatings is not guaranteed and thus can affect the overall pool boiling performance of the coatings. In this work, to provide a homogeneous powdered mixture of copper and GNP, ball milling is performed prior to sintering. And this homogeneous composite powder is then used for sintering to achieve uniform spreading of powdered GNP. High-energy ball milling is selected due to its cost effectiveness in forming homogeneous powdered mixtures of composite materials and alloys. It is hypothesized that the ball milling enables draping of highly thermally conductive GNP around the copper particles and sintering with these GNP-draped-copper particles will result in microporous coatings with enhanced wetting and wicking properties, which will improve the pool boiling heat transfer performance.

During this ball milling process, a composite mixture is repeatedly cold welded, fractured, and re-welded to yield a homogenized powder. Before loading the composite particles mixture, to achieve a homogeneous mixing, GNP and copper particles are dispersed in the ethanol bath for 30 min. Along with stainless-steel balls, the entire solution is then transferred into the ball milling chamber, and the ethanol is used as a process control agent during the ball milling process. The ball-to-powder ratio of 40:1 is used to ensure the homogeneous distribution of GNP in the copper particles. The entire mixture is ball milled at 700 rpm for 1 hour. After every 15 minutes of ball milling, to avoid the overheating, the ball milling chamber is allowed to cool down for 1 hour. The resultant ball milled composite particles are then screen printed on the copper test surfaces using the sintering oil.

The collision of stainless-steel balls during the ball milling traps the GNP and copper particles (shown in Figure 6). The force of the impact flattens and plastically deforms the particles which lead to the work hardening and fracturing. Due to this, increment in surface-to-volume ratio of the particles is obtained. The repetitive ball-to-wall and ball-to-ball collisions during ball milling reduce particle size via fracturing and the cold welding, i.e., draping of GNP on copper particles. Annealing cycle of 1 hr. in between the ball milling relives the internal stresses and defects of GNP caused due to continuous collisions. In addition to particles size reduction, a uniform distribution of GNP around each copper particle facilitates, leading to increased wicking rates of the coatings [21].

Figure 6.

Typical process of developing the ball milled powder and sintered coating using the ball milled composite powder.

Amongst different copper particles sizes and GNP concentrations, the 2% GNP with 20 μm Cu particles size performed the best yielding CHF of 2390 kW/m² at wall superheat of 8.4 K. This performance with ball milling followed by sintering is higher than both plain copper surface and only sintered Cu-GNP composite. Combination of the following mechanisms yielded this high performance: superhydrophilic coatings leading to increased microlayer evaporation and improved liquid supply to nucleation cavities, homogeneous mixture formation of GNP-Cu powder due to ball milling, increased thermal conductivity of the coatings resulting from the usage of GNP [22].

Similar study is performed using aluminum powder particles and developing Al@GNPs coatings using sintering with ball milled powder of aluminum and GNP. Aluminum substrate is used as a heater surface with R-134a refrigerant as a working fluid. With increase in coating thickness, pool boiling performance is improved, with the highest improvement in HTC of 143% observed for 125 μm thick coating than a plain aluminum surface. This enhancement is primarily due to increased nucleation sites, coating thickness and porosity [23].

2.2.2.3 GNP-Cu composite coatings with salt-templated sintering

Even though the performance of ball milled sintered coatings is higher than the plain copper substrate, such sintering techniques are limited by their control over the resultant morphological features, such as porosity and pore diameter that play a crucial role in determining the overall pool boiling efficacies of the coatings. This also limits the consequential surface properties such as wettability and wicking behavior of the coatings. In this study, the focus is provided on increasing the control of various surface properties such as porosity, wettability, and wickability. During sintering process of ball milled powder, sodium carbonate (Na₂CO₃) salt pellets are used here as the templates to achieve wide range of porous network. And post sintering, the sintered surfaces are rinsed with distilled water to dissolve and remove the traces of these salts. This non-uniform highly microporous structure is then used for pool boiling studies.

Pool boiling performance is evaluated after characterization of the surfaces. A dramatic increment in CHF is observed for the combined ball milled and salt templated sintered surfaces. A maximum CHF of 2890 kW/m² is attained for 20 μm Cu-3% GNP surface, which translates to ∼131% enhancement in CHF than a plain copper surface. Besides, the wall superheat of just 2.2 K is achieved, representing ∼2390% improvement in HTC than a plain copper surface. These are the highest CHF and HTC values reported in the pool boiling literature for graphene-based and porous coatings coated on a plain copper surface. For 0, 2, and 5% GNP coatings, CHF of 1550, 2690, and 2670 kW/m² are achieved, respectively. The maximum HTC of 1314 kW/m² K is obtained for 20 μm Cu-3% GNP coating, while HTC of 227, 399, and 431 kW/m² K are achieved for 20 μm Cu-0, 2, and 5% GNP coatings, respectively [22].

Similar to ball milled and sintered coatings, here the combination of increased thermal conductivity of coatings due to GNP-Cu ball milling and formation of superhydrophilic coatings with increased microlayer evaporation and improved liquid supply to nucleation cavities assisted in achieving the drastic enhancements in pool boiling performance. Apart from this, the key factors responsible for the improvement of both CHF and HTC are as follows: Formation of hierarchical microporous structure creates a wide range of porosity ranging from ∼2 to ∼200 μm. This wide range of developed pores serve various functions: Different range of pores activate at different wall superheat/heater surface temperature and initiate bubble nucleation activity. While the cavities not in the range of nucleation activity (primarily formed via the salt templating) act as liquid reservoirs and provide continuous liquid supply as soon as the vapor bubble departs. This provides massive advantage and thus has attained the highest pool boiling performance for graphene-based coatings with lowest wall superheat temperature. Hsu’s model [24] supports in estimating these wide range of cavity sizes. The range of active nucleation cavity sizes is determined by the following equation:

$$\left[R_{c,\mathit{max}},R_{c,\mathit{min}}\right]=\frac{{\delta }_t{C}_2}{2C_1}\left[\frac{∆T_{\mathit{sat}}}{∆T_{\mathit{sat}}+∆T_{\mathit{sub}}}\right]\times$$

$$\left[1\pm \sqrt{\frac{1-8C_1\sigma T_{\mathit{sat}}\left(∆T_{\mathit{sat}}+∆T_{\mathit{sub}}\right)}{{\rho }_v{h}_{\mathit{fg}}{\delta }_t{\left(∆T_{\mathit{sat}}\right)}^2}}\right]$$E3

Where $C_1=1+cos{\theta }_r$ and $C_2=sin{\theta }_r$. $R_{c,\mathit{max}}\mathit{\text{and}}{R}_{c,\mathit{min}}$ are maximum and minimum radii of the nucleation cavities, ${\theta }_r$ is the receding contact angle, ${\delta }_t$ is thermal boundary layer thickness (m), $∆T_{\mathit{sat}}$ is the wall superheat temperature ($∆T_{\mathit{sat}}=T_{\mathit{\text{surface}}}-T_{\mathit{sat}}$) (K), $∆T_{\mathit{sub}}$ is the subcooled temperature (K), $\sigma$ represents the surface tension of water at saturation temperature (N/m), ${\rho }_v$ is the vapor density (kg/m³), and $h_{\mathit{fg}}$ represents the latent heat of vaporization (J/kg).

Plots in Figure 7a and b indicate that with increase in wall superheat temperature, smaller nucleation cavities become active, providing massive enhancement in HTC due to the amplified contribution from the rapid nucleation activity. Lowest cavity diameter ranges are (Figure 7 a,b) observed for 3% GNP coating, which suggests the presence of more liquid supply sites than 2% and 5% GNP coating. Thus, the highest HTC is attained for 3% GNP coating. Scanning electron microscopic images in Figure 7c, d, and e indicate the different size of pores developed as a result of salt templated sintering. Wide range of pore dimensions formed on the coating assist in boiling inversion as well, leading to increment in HTC of the heater surface.

Figure 7.

Range of active nucleation cavities for 2, 3, and 5% GNP-Cu coatings showing (a) minimum cavity diameters, (b) maximum cavity diameters as a function of wall superheat temperature using Hsu’s model, (c), (d), and (e) SEM images at 2kX magnification confirming the availability of wide range of porous network in the estimated range of diameters.

2.2.3 Dip coating technique

Dip coating, as the name suggests, is a simple deposition technique in which the heater surface is dipped in the G/GO colloidal solution for a certain duration and allowed to air dry in a controlled atmosphere after taking out from a dipping solution. Different morphological features as well the thicknesses can be generated on the substrate/heater surface by varying the dipping duration.

Various techniques can be used to create a colloidal graphene solution. One of the techniques for such G/GO colloidal solution is developed using the electrodeposition technique with non-electrolyte bath. This process is developed to avoid the production of graphene using highly toxic and harmful acids, thus allowing the solution to be directly implemented for the dip coating. The electrolyte bath consists of deionized water and carbon tetrachloride 10% by volume. A potential is applied between the copper cathode and graphite anode which introduces a current density of 300 mA/cm² with the gap of 1 mm between the cathode and the anode. Electrodeposition technique involves cleaving of graphite electrode and reduction of the cleaved graphene oxide (GO) to form G/GO colloidal solution. The copper test surface is then dipped in the G/GO colloidal solution for a specific period of time and then is dried in a controlled atmosphere. The longer dip coating duration creates coating with less voids and fill up the copper surface with graphene, while shorter duration coating creates more ridge type structure. The microscale coating for 2 min. Dip coating attained a CHF of 1820 kW/m², that is ∼45% higher than a plain copper surface. While 10- and 20-minutes dip coating surfaces showed a slight reduction in CHF than a plain copper surface. Increased microlayer evaporation and alteration of wettability are the responsible enhancement mechanisms for improvements in pool boiling performance for 2 min. Dip coating. Longer duration coatings created less voids and thus did not assist in improving the performance [25]. This suggests that the mere presence of graphene is highly unlikely to provide any benefits in improving the pool boiling heat transfer performance and thus has very limited scope in real-world applications.

2.2.4 Concluding remarks

Some of the microscale graphene-based coating studies that have attained the highest pool boiling performance have shown that graphene does indeed play a crucial role in efficiently removing the heat from heater surfaces. It has also been shown that addition of graphene is advantageous in improving the aging and repetitive performance of the coatings.

3. Conclusions and summary

This chapter has focused on recent advances and prominent research studies that have developed graphene-based functional surfaces for enhancing the pool boiling heat transfer efficiencies of the heater surfaces. Compared to nanoscale graphene coatings, composite graphene-based coatings have shown an immense potential in increasing the overall pool boiling performance along with the longevity and sustenance of the coatings. This is because along with thermal conductivity, additional surface properties of the coatings such as wettability, wickability, and porosity are equally important and play a key role in increasing the pool boiling performance. These are essential key factors to maximize the usage of properties of the graphene in pool boiling heat transfer applications. Wide range of porous graphene-based composite wicking structures are also ideal for enhancements in heat transfer.

Amongst different composite coating techniques, both electrodeposition and sintering have shown higher cohesive and adhesive bond strengths and thus have improved longevity in maintaining the pool boiling performance. Mechanical properties of deposited graphene further assist in increasing longevity. However, additional efforts are still essential to further extend the longevity of the graphene-based composite coatings and to implement these techniques in various industrial applications. Some of the approaches include the provision of additional corrosion protection layers for composite coatings along with enhancements in bond strength and adhesion.

Acknowledgments

I would like to acknowledge my PhD advisors Dr. Satish Kandlikar and Dr. Anju Gupta for their valuable guidance.

Conflict of interest

The authors declare no conflict of interest.

Nomenclature

cp, l	Specific heat of liquid, J/kg K
cp, v	Specific heat of vapor, J/kg K
F	Faraday’s constant
h	Heat transfer coefficient, W/m^2 K
hlv	Latent heat of vaporization, J/kg
k	Thermal conductivity, W/m K
kl	Thermal conductivity of liquid, W/m K
kv	Thermal conductivity of vapor, W/m K
M	Atomic weight
n	Number of electrons
Psat	Saturation pressure, kPa
q”	Heat flux per unit area, W/m^2
Q	Charge, C
$R_{c,\mathit{min}}$	Minimum radius of nucleation cavity, m
$R_{c,\mathit{max}}$	Maximum radius of nucleation cavity, m
Tsat	Saturation temperature, K
Tsurface	Heater surface temperature, K

Greek letters

${\delta }_t$	Thermal boundary layer, m
ΔTsat = Tsurface -Tsat	Wall superheat, K
ΔTsub	Subcooled temperature, K
μv	Dynamic viscosity of vapor, Pa·s
ρl	Density of liquid, kg/m^3
ρv	Density of vapor, kg/m^3
σ	Surface tension, N/m
θr	Receding contact angle

Acronyms and abbreviations

APCVD	Atmospheric Pressure Chemical Vapor Deposition
CHF	Critical Heat Flux
EDS	Energy Dispersive Spectroscopy
HTC	Heat Transfer Coefficient
G	Graphene
GO	Graphene Oxide
rGO	Reduced Graphene Oxide
GNP	Graphene NanoPlatelets
Sccm	Standard cubic centimeters per minute
SEM	Scanning Electron Microscopy
rpm	Revolutions per minute