Bare Copper Surface Research Articles

Inherent scatter in pool boiling critical heat flux on reference surfaces

Evaluating boiling heat transfer enhancement depends on reliable reference values in the form of boiling curves and critical heat flux (CHF) values. Typically, the evaluation is performed in pool boiling conditions with water at atmospheric pressure. Literature includes a wide scatter in reference values, prompting this study to comprehensively evaluate boiling performance and CHF on reference surfaces to investigate the scatter's origin and set a definitive reference value. The study recorded 125 boiling curves and CHF values on nominally identical bare copper surfaces, establishing an average boiling curve and mean CHF value. Despite consistent experimental conditions, the recorded CHF values displayed significant variability with a mean CHF of 1112 ± 102 kW m−2 and a scatter from 902 kW m−2 (−19 % of average CHF) to 1339 kW m−2 (+25 % of average CHF). Using Rohsenow's correlation on the average boiling curve, a Csf factor of 0.0151 was obtained. The acquired CHF data is proposed to serve as a foundational benchmark for future research in enhancing pool boiling heat transfer.

Phase Transition Heat Transfer Enhancement of a Graphene-Coated Microporous Copper Surface Using Two-Step Electrodeposition Method

Abstract Owing to their exceptionally high thermal conductivity, there is a growing demand for graphene nanoparticles in phase transition heat transfer applications. This research delves into the exploration of various critical phenomena within the realm of surface science, specifically focusing on interactions at solid-liquid and liquid-liquid interfaces. In this work, graphene nanoparticles at varying concentrations are subject to electrochemical deposition on a microporous copper substrate to form graphene coated over microporous copper (GCOMC). The study encompasses a comprehensive analysis of surface characteristics, such as porosity, roughness, and wettability. Furthermore, the study involves the calculation of two key heat transfer metrics, the critical heat flux (CHF) and the boiling heat transfer coefficient (BHTC), through the execution of pool boiling experiments. The findings of this research underscore the remarkable superiority of GCOMC surfaces over their uncoated copper counterparts in terms of boiling performance. Particularly, the GCOMC surface showcases an impressive 87.5% enhancement in CHF and a 233% increase in BHTC compared to the bare copper surface. Furthermore, this investigation delves into a detailed quantitative analysis of bubble behavior, encompassing parameters such as bubble departure diameter, bubble departure frequency, and nucleation site density, employing high-speed camera techniques to comprehensively understand the underlying processes.

Experimental comparison of pool boiling characteristics between CNT, GO, and CNT + GO‐coated copper substrate

AbstractThe present investigation aims to study the comparative analysis of CNT, GO, and CNT + GO nanoparticle coatings on copper surfaces under a pool boiling situation. Coating on copper surfaces is performed using the dip coating method. Pool boiling experiments are conducted for all the coated surfaces at atmospheric pressure. Water is used as a working fluid. A comparison was made based on the coated surface characteristics and pool boiling behavior of the samples. The results show that CNT + GO coating on copper surfaces is better in terms of critical heat flux (CHF) and boiling heat transfer coefficient (BHTC). It is also observed that CHF and BHTC of the CNT + GO‐coated surface are 129.19% and 194.75% higher, respectively, compared to the bare copper surface. Bubble dynamics studies were also performed for all the samples.

Investigation of droplet boiling on superhydrophilic CuO multiscale hierarchical structured surfaces

This work presents the experimental investigation of droplet boiling characteristics on superhydrophilic surfaces. These surfaces are prepared on a copper substrate, and the boiling characteristics are studied and compared with the bare copper surface. Two types of superhydrophilic surfaces exhibiting needle-like (CuO-1) and sheet-like (CuO-2) nanostructures are prepared by the chemical etching oxidation technique. The hierarchical surface structures are characterized by carrying out SEM and XRD analysis. We employ high-speed imaging along with temperature measurement to characterize the heat transfer phenomenon. The surface temperature varies from 100 °C to 190 °C, and the Weber number is maintained constant at 2.78, corresponding to the gentle droplet impact regime. The effectiveness in the heat transfer performance of the different surfaces is characterized by measuring the critical heat flux (CHF) and the Leidenfrost point (LFP), and it is correlated with the surface properties viz. roughness, wettability, and porosity. The superhydrophilic surface alteration reduces the contact angle by about 99.7% compared to the bare surface. This reduction in contact angle corresponds to an increase in Critical Heat Flux (CHF) by 69.95% for CuO-1 and 47.17% for CuO-2, respectively, compared to bare copper. It is also seen that the LFP is delayed on the superhydrophilic surfaces, with an increase in the LFP temperature by 16.67% and 14.28% for CuO-1 and CuO-2 coatings, respectively. The effect of structures on the atomization and formation of secondary droplets is also presented. We report the contact coefficient (kw) for superhydrophilic surfaces for the first time, which is helpful in the theoretical prediction of CHF. These results show that significant improvement in heat transfer can be obtained in spray cooling and hot spot cooling applications without resorting to sophisticated microfabrication techniques.

Nucleate boiling enhancement on hybrid graphene-nanoplatelets/carbon nanotubes coatings for light-emitting diode cooling

This study explores the remarkable enhancement of light-emitting diode (LED) cooling through nucleate boiling using hybrid coatings of graphene-nanoplatelets (GNP) and carbon nanotubes (CNT). The introduction of oxygenated functional groups in the cured GNP and CNT coatings fosters improved hydrophilicity, promoting water intercalation within the nanostructures. The hybrid cured GNP/CNT coating demonstrates superior nucleate boiling performance compared to individual cured GNP and CNT coatings, owing to complementary mechanisms. Cured CNT facilitate efficient water permeation by enabling water molecules to infiltrate through their inner cores and intertwining tube spaces, while cured GNP offer an escape route for trapped water molecules and absorbed latent heat due to their layer-by-layer arrangement of graphene sheets with broadened interlayers. As compared to the bare copper surface, the LED cooled using the hybrid cured GNP/CNT coating exhibits a maximum temperature drop of 20.7 °C and an enhancement up to 1212 % in heat transfer coefficient, surpassing the cured GNP and CNT coatings. Moreover, the hybrid cured GNP/CNT coating improves the LED illuminance by 16.6 %, validating its potency in improving cooling performance. These findings highlight the potential of GNP/CNT hybrid coatings in advancing nucleate boiling cooling technologies and offer valuable insights into optimizing LED performance for high-quality lighting solutions.

Pool Boiling Performance on Cu-TiO2 Nanoparticle-Coated Copper Surfaces Prepared Through Hybrid Method

In large-scale, high-heat industrial applications, surfaces obtained using efficient and cost-effective methods are highly desirable. In this paper, the surfaces are modified through two-step processes, i.e., a chemical etching process to grow CuO thin film on bare copper (Cu) surfaces, and then a Cu-TiO2 composite nanoparticle is deposited over the CuO thin-film copper surface through electrochemical deposition. The experiment is conducted on both the prepared surfaces and the bare Cu surface using deionized water as a boiling fluid. Results reveal that the prepared surfaces exhibit better pool boiling performance than the bare Cu surface due to improved properties like porosity and roughness. The critical heat flux and boiling heat transfer coefficient are found to have 61 and 72% enhancement, respectively, compared to bare copper surfaces. Bubble behavior during boiling is also captured and analyzed for all surfaces through a high-speed camera.

Enhanced nucleate pool boiling heat transfer on CNTs-Cu nanoparticles-coated surfaces: Effects of sintering temperature and CNTs composition on pool boiling behavior

The present study involves the experimental investigation of pool boiling heat transfer characteristics on CNTs-Cu nanoparticles-coated heating surfaces. The advantages of coating the mixture of infinitesimal CNTs with Cu particles on the copper surface are to generate the active nucleation site density by porous structures and enhance the thermal conductivity of the heating surfaces. To explore the effects of sintering temperatures and CNTs composition on pool boiling behavior, we fabricated the boiling test sections under the sintering temperatures of 700, 750, and 800 °C for four different CNTs compositions of 0.0CNTs-100Cu, 0.2CNTs-99.8Cu, 0.3CNTs-99.7Cu, and 0.5CNTs-99.5Cu in wt.%. We first analyzed the surface morphologies, that is, the average pore size, porosity, and pore density, to characterize each tested sample. Pool boiling experiments were later conducted under atmospheric pressure using water to obtain the pool boiling curves and bubble dynamics for different sintering temperatures and CNTs composition. Based on the experimental analysis, nucleate boiling heat transfer enhancement was interpreted in relation to the sintering temperature with surface morphologies and CNTs composition in relation to improved pore density. Compared to the bare copper surface, the achieved maximum enhancement for boiling heat transfer coefficient and critical heat flux were 3.86 and 1.49, respectively, when a 0.5CNTs-Cu-coated surface sintered at 800 °C was applied.

Graphene coating reduces the heat transfer performance of water vapor condensation on copper surfaces: A molecular simulation study

Water vapor condensation is a key process for many applications. Existing studies in water vapor heterogeneous condensation have used different methods to modify the hydrophobicity of the water vapor condensation surface. One way to modify the surface properties is to use graphene coating. On a macroscale, surface modification using graphene coating can improve water vapor condensation heat transfer of various substrates. However, on the molecular scale, its effect is poorly understood. This work investigated the effect of graphene coating on water vapor condensation using molecular dynamics simulations (MDS). We examined the water vapor condensation on bare copper surfaces considering the effects of initial temperature difference, water model, and surface size, parameters that have not been investigated by previous studies employing MDS for the same water-surface configuration. One water model was then used to simulate the condensation on a copper surface with and without graphene coating. We then investigated the effect of graphene defect, the energy and vibration of graphene atoms, and the interaction between graphene and copper layers. The surface size notably influenced the condensation rate and heat transfer performance. The condensation rate and heat transfer performance were significantly reduced when the copper surface was coated by graphene. The number of water molecules condensed was 1253 molecules/ns on the bare copper surface, compared to 587 molecules/ns on the graphene-coated copper surface. Moreover, the water molecules condensed on the graphene-coated copper surface tended to return to the bulk vapor phase. Other important results are also provided. This study gives an insight into the water vapor condensation on graphene-coated copper surfaces, useful to pursuit the design and optimization of graphene-coated copper surfaces for applications that need efficient water vapor condensation, such as for industrial applications, like thermal, chemical, and nuclear.

Pool-boiling enhancement on periodic micro/nano ripple-structured surfaces fabricated by femtosecond laser

A study was conducted on whether periodic micro/nano ripple structures generated on metal surfaces using femtosecond laser processing could improve pool boiling heat transfer. Depending on the laser irradiation conditions on the metal surface, a surface structure with different geometrical characteristics is created. Changes in sample surface morphology caused by the laser irradiation intensity are examined using scanning electron microscopy and confocal laser microscopy. Pool boiling tests are conducted to compare three laser irradiation conditions fabricated samples and the bare copper surface. The working fluid is deionized water. The pool boiling curves of each sample surface were compared with each other. Visualization of bubble nucleation is performed using a high-speed camera. The superheat, bubble departure diameter, and bubble growth period of each sample are measured and compared at the onset of nucleate boiling (ONB). The most efficient periodic micro/nano ripple-structured surface reduces the superheat at the ONB by 6.13 °C in comparison with that at the bare Cu surface. The enhanced surface structure reduced the bubble departure diameter and renewal period by 60.5% and 55.6% in comparison with that of the smooth surface. Further, the structured surface showed 178.5% enhancement in heat transfer coefficient and 39.1% in critical heat flux, respectively. Thus, the combination of increased nucleate site density, high nucleation site activation, and a wicking effect in the periodic micro/nano ripple structure significantly improves the pool boiling heat transfer.

Analysis of freezing of a sessile water droplet on surfaces over a range of wettability

HypothesisNonwetting surfaces, by virtue of their water-repelling trait, offer desirable anti-icing characteristics. Surface roughness, type and wettability are important interfacial characteristics that affect the icing dynamics that can be tailored to achieve desired anti-icing designs. Experiments and SimulationsThe present study systematically explores the effect of surface roughness on the freezing behaviour of water droplets on surfaces ranging in their wettability. Surfaces with tailored textures and wettability were fabricated using chemical etching and electrodeposition by varying the voltage. The surfaces studied include bare copper, five different dry nonwetting copper surfaces, and five different lubricant-infused copper surfaces that ranged in surface texture fractal dimension from nearly 1.0 to 1.92 and wettability measures of average water contact angle from 91° to 162° and sliding angle from less than 3° to greater than 50°. A computational model is developed to simulate the freezing dynamics on the surfaces studied. FindingsWith increasing roughness features, the freezing time increased due to the dual effects of increased contact angle and poor interfacial conductance caused by trapped air or infused liquid within the asperity textures. In general, the nonwetting surfaces increased the freezing time by a factor of at least 1.33 and up to about 3.2 compared to freezing on bare copper surfaces. The computational model shows close agreement with experimental measurements on the freeze front progression as well as freeze time. Design guidelines on the suitability of the different nonwetting surfaces for anti-icing purposes are derived from the systematic study, with the overall design recommendation favoring lubricant infused surfaces.

Enhancement in pool boiling performance of GNP/Cu-Al2O3 nano-composite coated copper microporous surface

An experimental analysis of pool boiling is conducted on the GNP/Cu-Al2O3 (GNP-graphene nano particle) nanocomposite coated copper surfaces at atmospheric pressure using DI (distilled water) as boiling fluid. A two-step electro-deposition technique is used for fabrication of these micro/nano porous coated surfaces. A modified surface structure is formed due to this deposition and various surface morphology parameters such as surface structure, roughness, wettability are analysed. Results show the enhancement in BHTC (boiling heat transfer coefficient) and CHF (Critical heat flux) in nano composite coated surfaces compare to the bare copper surface. The maximum CHF of 2415 (kW/m2) and BHTC of 171.28 (kW/m2K) is found in the NCCS (nano-composite coated superhydrophilic) surfaces which are 122% and 239% more than the bare copper surfaces. The boiling performance is augmented mainly because of the improvement in roughness, wettability and also for the high thermal conductivity of the GNP/Cu- Al2O3 nano coating. Quantitative study of the bubble dynamics like active nucleation site density, bubble emission frequency, departure diameter of bubbleand their impact on heat transfer performance in the case of NCCS surfaces is performed with high-speed visualization.

Effects of silver epoxy reinforcement on graphene nanoplatelets functionalized surface in pool boiling heat transfer enhancement

Silver Epoxy reinforcement is pivotal to enhance the adhesive strength of Graphene nanoplatelets (GNP) on the substrate. This experimental study seeks to scrutinize the effects of Silver Epoxy reinforced GNP functionalized surface on the heat transfer of pool boiling. Three different GNP-based coatings are analyzed: simple GNP, uncured GNP reinforced with Silver Epoxy, and cured GNP reinforced with Silver Epoxy, all applied to copper substrate. For each coating, the surface morphology, chemical bonding, and wettability are examined. Under a heat flux of 550 kW/m2, the experimentally derived heat transfer coefficient improves by 194%, 36%, and 77% respectively on pristine GNP, uncured GNP/Epoxy, and cured GNP/Epoxy compared to the bare copper surface. The epoxy reinforcement weakens the exceptional boiling improvement achieved by a GNP functionalized surface. Although the thermal curing process turns the GNP/epoxy coating from hydrophobic to hydrophilic and recovers partly the level of boiling enhancement, the biphilicity of GNP tends to dominate boiling heat transfer enhancement by enhancing the bubble dynamics. The hydrophilicity has a subordinate effect by re-wetting the dry spots and preventing the vapour blanket. Silver Epoxy reinforcement significantly enhances the adhesive strength of GNP on a substrate but at the expense of boiling heat transfer enhancement.

Thermal reliability of Cu sintering joints for high-temperature die attach

In this study, the thermal reliabilities of sintered Cu joints were investigated under aging and in thermal shock environments. Sintered Cu joints were maintained for 2000 h and 2000 cycles in high-temperature (200 °C) and thermal shock (from −45 to 175 °C) environments, respectively. Subsequently, the microstructural changes, Cu-oxide formation, and bonding strengths of the joints were evaluated. The initial shear strengths of the chips with bare copper and ENIG surfaces were 40.84 and 36.44 MPa, respectively, and after a 2000-h aging test, the strengths decreased to as 39.84 and 18.58 MPa, respectively. With an increase in the aging time under an aging environment of 200 °C, the internal porosity decreased owing to the formation of Cu2O and additional diffusion between the particles. Therefore, the shear strength of the chips with bare copper surfaces was maintained throughout the aging tests, whereas that of the electroless nickel immersion gold chip decreased owing to the formation of Kirkendall voids at the interface, which were attributed to the different diffusion rates of Cu and Au. In addition, Au3Cu, AuCu, and AuCu3 interfacial intermetallic compounds were formed between the Au and Cu layers after the aging test was performed at 200 °C for 2000 h. Copper oxide formed during the thermal shock test caused delamination at the interface because the coefficient of thermal expansion mismatched with that of pure Cu. Understanding this intrinsic Cu oxidation mechanism will widen the prospects for the application of Cu sintering in various devices.

Boiling heat transfer study of Micro-Diamond coated surface using underwater explosion shock wave method

Pool boiling heat transfer experiments are performed on micro-diamond particles coated heating surface (DPC heating surface) and no-coated smooth surface on copper substrates. Here DPC heating surface is fabricated with the aid of underwater explosion shock wave. Experiments are conducted on the substrate surface area of 15 × 15 mm2 using distilled water as the working fluid. The substrate is fixed tightly on the heater block, which is embedded with ceramic cartridge heater of 1500 W. The influence of pool boiling heat transfer is experimentally studied over a range of surface roughness values with particles coated heating surface at atmospheric pressure. The obtained results of DPC surfaces are compared with that of the smooth bare copper surface. A remarkable enhancement of the critical heat flux (CHF) at lower wall superheat is observed on the DPC surfaces. The DPC surface is found to be effective in increasing the nucleate boiling and CHF

Efficient defrosting on hybrid surfaces with heterogeneous wettability

Frosting behavior and dynamic defrosting characteristics of horizontally oriented hybrid surfaces with heterogeneous wettability were experimentally investigated. The developed hybrid surfaces with superhydrophobic patterns built on superhydrophilic background integrated the properties of frost retardation under frosting conditions and fast surface cleaning of residual water upon defrosting. Four types of hybrid surfaces with different pattern spacings were tested in this work. Hybrid surfaces exhibited superior anti-frosting performance compared with that of a bare copper surface but lower than a complete superhydrophobic surface. During defrosting process, surface energy gradient and Laplace pressure gradient enabled by heterogeneous wettability on hybrid surfaces could drive the mobile slush composites on superhydrophobic regions move to superhydrophilic areas to achieve quick cleaning even in the absence of gravity force. The proportion of surface clean time of one full cycle duration on hybrid surfaces was as low as 32.2%–41.5%. Compared to complete superhydrophobic and bare copper surfaces, hybrid surfaces exhibited a significant enhancement on defrost efficiency due to larger three phase contact line length on evaporation-mediated surface cleaning performance, especially the SSH-3 surface, achieving the largest defrost efficiency within the present frost mass range.

Experimental Investigation of Tube-in-Tube Nanocomposite Coated Heat Exchanger

In this paper, heat transfer performance is investigated for plain and nanoparticle coated tube-in-tube heat exchangers. Four types of tubes, i.e. bare copper tube, bare aluminium tube, Cu-Al2O3 nanoparticle coated tube and anodized aluminium tubes are used for performing the experimental investigation. The coating thickness of Cu-Al2O3 nanocomposite surface and anodized aluminium tube varies from 10, 25 and 30 micrometres to 15, 20 and 30 micrometres respectively. The surface is found to be hydrophilic in nature as the contact angle changes from 79.82deg to 55.47deg. The prepared surfaces are characterized by FESEM, EDS and FTIR. By adjusting the hot and cold fluids relative mass flow rates, one may calculate not only the overall heat transfer coefficient but also the efficiency of the tube-in-tube heat exchanger. The Cu-Al2O3 nanocomposite coated surface has the highest overall heat transfer coefficient and efficiency, followed by an anodized aluminium surface, bare copper surface and bare aluminium surface. The aluminium anodic oxide (AAO) surface also exhibits an increased heat transfer coefficient, but to a lesser extent than the nanocomposite coating.

Protective coatings on copper using as-deposited sol-gel TiO2 - SiO2 films

Copper has many benefits with its extensive electrical properties, but is prone to corrosion in oxide, sulfate, and sulfide environments. Based on our previous studies of as-deposited TiO2-SiO2 films which acted as protective coatings against sulfide and sulfate on silver, we extended our research to the same films coated on copper and checked the corrosion rates using Tafel plots. The dip-coated sol–gel TiO2-SiO2 films reduced the corrosion rate four times compared to the bare copper surface due to Ti-O-Si bonds formed in the mixed metal oxide films which enhances the adherence to the substrate, observed in the Raman spectroscopy studies.

Evaluation of graphene transparent coating on copper for color and oxidation control at low temperatures

In this paper, the ability to preserve copper color was investigated by using graphene thin film coating against oxidation at 200 °C. For this purpose, graphene nano-coating was applied to the copper substrate by using chemical vapor deposition (CVD) process. Raman spectroscopy was used for the detection of the coatings. Structural investigation of the coating was performed before and after oxidation tests using optical and scanning electron microscopes. Color changes due to the formation of copper oxides were calculated using the CIE–L*a*b* color space defined by the Commission Internationale de l'Éclairage (CIE). Furthermore, the reflectance was evaluated in the UV–visible spectrum range. The results revealed that the graphene coated copper had a higher L* than the untreated copper due to the removal of oxide layer during graphene synthesis. The color change (∆E) caused by oxidation at 200 °C, in the graphene coated copper was negligible (∆E = 2.88) after 2 h, and its value was below 5 after 4 h. However, the color change on the bare copper surface was significant (∆E = 26.87) after 15 min of oxidation. The results of the present research show that the graphene coating is effective in protecting copper color. Moreover, since graphene oxidation extends from defects and grain boundaries, synthesis technique plays an important role in protecting copper against oxidation and thus preserving its color.

Hook-shaped structures to improve pool boiling heat transfer

Boiling performance while using raised hook-shaped metal features, manufactured from copper by a skiving process, was investigated and compared with the boiling performance of a bare copper surface. Deionized water at the saturated conditions at 1 atm was used as the working medium. These fins can potentially improve the performance of immersion cooling spreaders in power electronics applications. The advantages of this technology compared with other methods presented in the literature include its scalability, low cost, resistance to erosion and clogging, and the industrialization of the manufacturing method. Four GRIPMetal hooked surfaces were investigated, each with different profiles (i.e., metal features, heights, and spacing) induced by the manufacturing process. The results show that the surface with hooks had lower wall superheat at the onset of nucleate boiling (ONB) compared with the bare surface. This appears to be due to the surface cavities—i.e., the negative potions of the raised hooks created by the skiving process, which helped initiate bubbles earlier. Moreover, the heat transfer coefficient (HTC) was superior for all surfaces with hooks at low heat fluxes because of increased surface area and enhanced bubble dynamics offered by the arrangement of the hooks. The hooked surface achieved a maximum HTC of 8.9 W/cm2K, which represents a 96 % enhancement ratio over the bare surface. The surface area increase (At/Abare) offered by the proposed hooks and their corresponding grooves was measured using the photogrammetry technique and was found to range from 1.35 to 1.8. Lastly, the critical heat flux (CHF) was significantly enhanced, reaching a 67 % increase over the bare surface; this is attributed to the available bare inter-fin area that can supply the liquid to the nucleation sites and the assisted suction flow through the front of the grooves.

The effect of air solubility on the Kapitza resistance of the copper-water interface

We perform a calibration of the force fields between copper, water, and air based on experimental parameters, such as the value of the water contact angle on a bare copper surface, the bulk density and thermal conductivity of water and air. Besides, the air solubility in water is estimated at different pressures and temperatures. Using nonequilibrium molecular dynamics (NEMD) simulations, we investigate the effect of the air on Kapitza resistance at high pressure in a copper-water-air system. The results show that a thick air layer at the copper–water interface reduces the ordering of the water layers adjacent to the copper surface, and is responsible for an increase in the Kapitza resistance. Moreover, we find that high pressure can induce highly-ordered water layers that significantly promote the phonon transport across the solid-liquid-gas interface, which leads to the reduction of the Kapitza resistance. Furthermore, we analyze the normalized copper–water Kapitza resistance RK,eff/R0 for different gas solubilities and pressures. Here, RK,eff denotes the effective Kapitza resistance between copper and water, where the temperature jump is approximated by the temperature difference across the air layer. R0 is the Kapitza resistance at ambient pressure in the copper–water system without air. The normalized Kapitza resistance suggests that higher pressures can reduce the contribution of the thickness of the air layer on the increase of Kapitza resistance. We propose a simple model for the effective Kapitza resistance taking into account a thin air layer at the solid–liquid interface. At ambient conditions, the model predicts a 3-fold increase in the Kapitza resistance for a 1nm thick air layer.