Optimized Heat Transfer Rate in Cu/CNT Nano Composite Prepared by Electrodeposition Technique

The Cu/CNT nanocomposites are prepared by reinforcing Carbon Nanotubes (CNT) in a copper matrix through the electrodeposition technique. These nanocomposites are prepared by varying the diameter and concentrations of the CNT and are characterized by heat transfer rate. This study describes the overall heat transfer phenomena of Cu/CNT nanocomposite due to the preferential deposition of CNTs in the Cu matrix. The heat transfer rate is optimized and increased by 41.08% and 46.91% in natural and forced convection respectively compared to pure Cu coating. The reason is attributed to the better alignment, the optimum concentration of CNT in the composite, and the homogenously placed CNTs network in the composite.


Q
net heat (energy) transfer (Js −1 ) Δt time taken (s) ΔT difference in temperature between the cold and hot sides ( o C) Δx thickness of the material conducting heat (distance between hot and cold sides) (m) k thermal conductivity (W/(m⋅K)) A surface area of the surface emitting heat (m 2 ) U overall heat transfer coefficient (Jm −2 s −1 o C −1 ) ΔT mean temperature difference ( o C) Q total absorbed heat by water (J) q heat transfer rate (Js −1 ) t duration of heat transfer (s) 2][3] Many traditional materials have recently failed to achieve good mechanical, thermal, and electrical properties in a single material, prompting a transition to composite materials.CNTs are used as reinforcement in composites due to their unique structure and exceptional physical characteristics, which allow them to outperform traditional materials. 4,5Because of the strong interface connection between the CNTs and the metal matrix, incorporating CNTs into the metal matrix has resulted in composites with improved characteristics.][11][12] For new technologies, tuning thermal conductivity has become an important parameter, especially in state-of-the-art engineering applications.Because of CNTs have an extremely high axial thermal conductivity of 3000 Wm −1 K −1 , incorporating them into copper is expected to significantly improve thermal conductivity depending on CNT content and diameter. 13So the situation becomes a challenge to prepare quality thermal conductive materials for the above-said applications.This research is mainly focused on the investigation of heat transfer rates in boilers, heat exchangers, etc. in industries that can be replaced by Cu/CNT composite.Achieving components with a higher heat transfer rate while being lighter in weight which considered one of the most important industrial challenges.However, there are very few papers regarding thermal conductivity which reported results with very less enhancement in thermal conductivity.Recently, Chu et al. 14 prepared Cu/CNT composites successfully by spark plasma sintering and measured their thermal conductivity.They reported that there is no enhancement of thermal conductivity compared to pure copper.Other researchers also reported no change or almost negligible enhancement in thermal conductivity improved compared to the matrix by powder technology route and other methods. 15,16The thermal conductivities are dependent mainly on the material processing method.By electrodeposition technique, the thermal conductivities are increased compared to pure copper because of a reduction in interfacial resistance between CNTs and copper. 17,18he present work provides a comprehensive overview of heat rate variation study on Cu/CNT composites, with a focus on understanding the trends with geometrical parameters in the combination of axial and transverse directions.For this, the composites are prepared through the electrodeposition method.The thermal experiments were carried out for pure copper, SWCNT, MWCNT1, and MWCNT2.The effect of CNT concentration on heat transfer rate is also observed at ∼80 °C to room temperature.The surface topography and microstructure of the deposits are examined by Scanning electron microscope (SEM) and Field Emission scanning electron microscopy (FESEM).The X-ray Diffraction (XRD) analysis has been performed to validate the presence of CNTs in composites.MWCNT dispersion is investigated using Transmission electron microscopy (TEM) micrographs.

Experimental
Materials and methods.-Allcomposites were prepared with analytical-grade chemicals.The MWCNT1 with an outer diameter of 6-13 nm and MWCNT2 with an outer diameter of 110-170 nm were received from Sigma-Aldrich (Code: 698849 and 659258), and SWCNT with an outer diameter of 1.8 nm is received from PLATONIC (CAS No.308068-56-6).In 20 ml DI water, CNTs, z E-mail: rizwan@nitk.edu.in and CTAB got dispersed well in a ratio of 1:5 using a probe sonicator.The uniform dispersion of CNTs with CTAB and good suspension of CNTs are the key factors for homogeneous CNT distribution. 19The electrolyte bath is made by dissolving CNTs, CTAB, copper sulfate (CuSO 4. 5H 2 O, 0.7 M), and sulfuric acid (H 2 SO 4, 0.86 M).The electrolyte has a pH of ∼1.After obtaining a uniform electrolyte with magnetic stirring for 15-20 min, dispersed CNTs are added to the Cu electrolyte and sonicated for 5 min DC power analyzer (KEYSIGHT Model No.: N6761A) is used to prepare composites using the electrodeposition technique.Before electrodeposition, the Cu hollow pipe and Cu fins surfaces are polished with sand emery papers (300-2000 grade) to ensure smooth and parallel surface finish.The MWCNT along with the Cu atoms are electrodeposited on the Cu hollow pipe of 1 mm thickness and Cu fin of 3 mm diameter (cathode) by holding in a copper electrolyte between two anodes for 60 min to get the coating uniformly done throughout the surface of the Cu pipe as shown in Fig. 1.A protective Cu coating of few microns thickness is done on the Cu/ CNT coating surface to protect against loosely attached CNT on the surface of hollow Cu tube/Cu fin. Figure 1 shows the schematic diagrams of electrodeposition to prepare Cu-SWCNT/MWCNT1/ MWCNT2 nanocomposites on Cu fins and hollow Cu pipe as well as a cross-sectional view of the sample.Stainless steel plate is used as an anode and the current density is set to the range of 20-30 mAcm −2 for different diameters of CNTs using the hull cell method.
Characterization of Cu/CNT composites.-TEM(JEOL-2100, Japan) is used to examine the deagglomeration of CNT in presence of a sufficient amount of surfactant and also to measure the average diameter of CNT.XRD (JEOL X-ray diffractometer, DX-GE-2 P, Japan) is performed with Cu-Kα radiation of 0.154 nm wavelength and scanning rate at 2°per min.The homogeneous coating and surface morphology of Cu/CNT composite is characterized by SEM (JEOL model JSM 6380LA system, Japan).The cross-section of Cu/  CNT composites is observed using FESEM (GEMINI 300, Carl Zeiss, Germany) at various magnifications to measure the thickness of the coating as well as to study the distribution and morphology Cu and CNTs.
Thermal experiments.-Thethermal experiments are conducted with and without a fan on copper fins coated with SWCNT, MWCNT1, and MWCNT2 at 300 mgl −1 concentration.The experiments are also carried out on copper fins with a pure copper coating of the same thickness and the results are compared with Cu/CNT coated samples, as shown in Fig. 2a.The Cu/CNT composite with the best thermal conductivity is chosen from the above setup and measured on the copper pipe at different concentrations of CNTs (150 mgl −1 , 300 mgl −1 , 450 mgl −1 , 600 mgl −1 , and 750 mgl −1 ) with and without a fan to optimize.
To measure the heat transfer rate in axial and transverse directions, we prepared two identical acrylic boxes (7 cm × 7 cm × 5 cm) and covered outer surface with insulator to prevent heat loss.Hot water of temperature 80 °C, is poured into both acrylic boxes, then immediately fins coated with CNTs, and Cu coated fins/ pipes are inserted.After that, the insulator-covered lid is properly placed on the boxes to prevent heat loss.The heat transfer rate measurement system is designed so that heat can pass from inside to outside through the specimen coating in an axial direction, then cool in a transverse direction after reaching the top side, as shown by the arrows in the schematic diagram in Fig. 2b.Temperatures at various places are recorded using the KEITHLEY 2700 MULTIMETER instrument (accuracy of 0.001 °C).The heat transfer measurement system is depicted in Fig. 2 with schematic diagrams in both the transverse and axial directions.The temperature of the specimens are measured to a precision of 0.1 °C using a K-type thermocouple sensor probe.For forced convection, small mini fans were arranged to compare heat transfer rates with and without them.
The rate of heat flow in a material is the quantity of heat transferred per unit of time, and it is usually measured in watt.The flow of thermal energy caused by thermal non-equilibrium is known as heat.Fourier's Law of heat conduction is expressed by Rate of heat flow = −(heat transfer coefficient) × (area of the body) × (temperature difference)/(material thickness) Where, Q = net heat (energy) transfer, Δt = time taken, ΔT = difference in temperature between the cold and hot sides, Δx = thickness of the material conducting heat (distance between hot and cold sides), k = thermal conductivity, and A = surface area of the surface emitting heat.The total heat transferred per unit time is calculated using the basic overall heat transfer equation which is given by Where, U = overall heat transfer coefficient, ΔT = mean temperature difference, Q = total absorbed heat by water, q = heat transfer rate, and t = duration of heat transfer.The heat is transferred through the specimen, and Eqs. 2 and 3 are used to calculate the heat transfer rate (q) through the specimen.

Results and Discussion
Preparation of pure Cu coating and Cu/CNT nanocomposites.-TheCNT was dispersed uniformly in copper electrolyte and Cu/CNT coating was deposited on the surface of the Cu tube and Cu fins.The Cu tube of diameter 2.54 cm (1 inch) and 8 cm long, and the Cu fins of diameter 3 mm and 8 cm in length.The pure Cu coating and Cu/CNTs composite samples are prepared using the electrodeposition method by various concentrations of CNT.Table I listed the composition details and the deposition parameters of the samples.
Microstructural characterization.-Afterprobe sonication with a 1:5 surfactant ratio, TEM micrographs of MWCNT1 and MWCNT2 and FESEM micrographs of SWCNT confirmed the absence of agglomeration and the presence of minor metallic impurities in the CNTs, as shown in Fig. 3. Using ImageJ software the diameters of SWCNT, MWCNT1 and MWCNT2 were measured at different places along the length of CNT.The average diameter of SWCNT, MWCNT1 and MWCNT2 were measured as 6.06 nm, 17.85 nm, and 163.3 nm respectively.The Cu/CNT composites are prepared at a concentration of 300 mgl −1 , the same deposition parameters, and the same area of 1 cm × 1 cm.The FESEM images of pure Cu, Cu/SWCNT, Cu/MWCNT1, and Cu/MWCNT2 are shown in Figs.4a-d.The spherical copper deposits incorporating MWCNTs are observed.Arai et al. also observed similar kinds of microstructures. 20The sample has uniform deposition throughout the surface and the majority of the CNTs are observed on the plane with different orientations in the matrix and no axial direction CNTs were observed perpendicular to the plane.The SEM images in Fig. 5 show the morphology of Cu/MWCNT2 composites for 150 mgl −1 , 300 mgl −1 , 450 mgl −1 , 600 mgl −1 and 750 mgl −1 concentrations of CNT.More MWCNT2 was deposited at the constant area as the MWCNT2 concentration increased from 150 mgl −1 to 750 mgl −1 .Arai et al. reported that MWCNT content increased with increasing MWCNT concentration up to 2 g dm −3 in the electrolyte bath. 21he EDS analysis has been carried out to validate the presence and reveal the distribution of CNTs in samples.The EDS of Cu/    ECS Advances, 2023 2 011001 SWCNT, Cu/MWCNT1, and Cu/MWCNT2 coatings have been studied by using FESEM equipped with an EDS analyzer.SEM-EDS scan region of Cu/SWCNT, Cu/MWCNT1, and Cu/MWCNT2 coatings surface is smooth and distributed evenly with CNTs.The area of coatings was 2 mm × 2 mm selected at 5 different places of sample and performed EDS.The average mass fractions of chemical elements on the coating surface (wt%) and an average atomic fraction (at%) are shown individually in Table II and scan images are shown in Fig. 6.For the same deposition parameters, the weight % of C is observed from this EDS for Cu/SWCNT, Cu/MWCNT1, and Cu/MWCNT2 coatings as 8.4%, 7.58%, and 7.06% respectively.The rate of incorporation into the deposit is influenced by particle size.According to the findings, as the nanoparticle size becomes smaller, more nanoparticles can be incorporated into a metal deposit per unit volume. 22That means the Cu/SWCNT has the highest percentage/number of SWCNT in comparison with MWCNT1 and MWCNT2.The reason attributed to lighter weight CNTs deposited more in number.Because at the same current density, all the CNTs experience an equal amount of electromotive force.As a result, the lighter weight of CNTs or smaller diameter CNTs gets deposited more on the Cu surface, as suggested by EDS.
Figure 7 shows the cross-sectional micrographs of Cu/MWCNT1 (300 mgl −1 ) coating which is observed using FESEM.From the micrograph (Fig. 7a), coating thickness was observed as 75.78 μm for 60 min coating time.Also, CNTs are present between the Cu crystals randomly in a different direction.Since the cross-section of CNT is difficult to observe even at X100000 due to the high charging effect, most of the CNTs are observed lying in-plane to the Cu substrate in a different direction (Fig. 5) and very few are out of a plane as shown in Fig. 7b.The high electromotive force along the axis of the CNT due to more surface area leads to deposit in-plane axially, and low electromotive force on the cross-section of the CNT due to a low surface area lead to deposit less CNT perpendicular to the plane as illustrated in the schematic view of  8 because of slight changes in the diameters of the CNT which leads to the change in the crystallite size of Cu particles. 23,24at transfer rate experiments.-Experimentsusing Cu fins.-Heattransfer characteristics of prepared samples are tested for natural (without fan) as well as forced convection (with fan).The experiments are carried out on pure copper fins, and copper fins with a coating of SWCNT, MWCNT1, and MWCNT2 at 300    mgl −1 concentration with and without the fan.Among all these samples the MWCNT2 shows the highest cooling rates of 21.46 o C/hr and 28.83 o C/hr for natural and forced convection, respectively as shown in Fig. 9.The heat transfer rates for all the samples are shown in Table III.Due to the larger diameter, the number of MWCNT2 depositions is less compared to smaller diameter CNTs at the same operating conditions as observed in EDS results (Table II) as well as SEM images (Figs.3a-d).Due to the higher thermal resistance between tangled CNT-CNT in a transverse direction, due to this, the results showed less heat transfer for Cu/MWCNT1 and Cu/SWCNT as explained with detailed mechanism in Fig. 12.In aligned CNTs, Tengxiao et al. 25 also observed a high CNT-CNT contact thermal interface resistance.They observed that as the CNT density increases, the intercalation structure's heat transfer efficiency decreases due to the extensive CNT-CNT contact. 25,26Also due to the large diameter and less amount of CNTs deposited on the matrix, MWCNT2 shows a higher heat transfer rate compared to SWCNT and MWCNT1.Hence, MWCNT2 is chosen for further study of varying concentrations of CNT from 150 mgl −1 to 750 mgl −1 to comprehend the heat transfer behavior in axial as well as transverse directions.
Experiments using Cu pipe.-Theheat transfer rates are measured for Cu/MWCNT2 composite, prepared at different CNT concentrations as mentioned in Table I.These experiments are carried out with and without the fan, and values are compared with pure copper coating.The heat transfer rate graphs of Cu/MWCNT2 composites with pure copper plotted as shown in Fig. 10 same is presented in the form of ratio Q CNT /Q Cu in Fig. 11.The Q CNT /Q Cu values at 450 mgl −1 are observed to be maximum compared to all other concentrations considered in the study for natural as well as forced convection.At low concentrations (150 mgl −1 and 300 mgl −1 ), the composites show less conductivity due to a lack of a conductive network of CNTs (Fig. 12a).As the CNT concentration increases from 150 mgl −1 to 450 mgl −1 the Q CNT /Q Cu value increases from 1.12 to 1.41 in natural convection and 1.18 to 1.47 in forced convection, by increasing the concentration of CNT in the Cu matrix.However, from 450 mgl −1 to 750 mgl −1 the Q CNT /Q Cu value decreases from 1.41 to 1.08 in natural convection and 1.47 to 1.17 in forced convection.This shows the optimum number of CNT or better CNT network is required for achieving a high heat transfer in the above-prepared sample.As shown in Fig. 12b, the optimum number of the CNTs on the sample is deposited in the axial direction which results in better electrons and phonons contribution in high thermal conduction (Fig. 12b).As heating wall thickness can influence the heat transfer, 27 at the same thickness of the coating (76 μm), the Edward et al., 28 reported that CNT-Cu composite achieved higher heat transfer rate by 1.24 times of pure Cu coating (using cold spray process), whereas in present study we have achieved up to 1.41 and 1.47 times of pure Cu for natural and forced convection respectively.At 600 mgl −1 and 750 mgl −1 concentrations of CNT, the Q CNT /Q Cu values are less compared to 450 mgl −1 .As the number of CNTs deposited at higher concentrations is more, the resistance between CNT-CNT gets increased and CNT starts working as an insulator which is depicted in Fig. 12c. 29he CNTs act as an insulator across its transverse direction, so the heat transfer would be through the Cu atoms in the transverse direction of the CNTs (Fig. 12c).The fundamental theory of energy   transport states that a material's total thermal conductivity is determined by the energy carrier's electrons and phonons.Electrons dominate the energy transmission through copper, while phonons are dominant in CNTs.As a result, the total thermal conductivity of Cu/MWCNT2 composites enhances due to the effect of both electrons and phonons energy carriers.Because of the uniform dispersion of CNTs and the good interfacial bonding between Cu and CNTs, both electrons (Cu) and phonons (CNT) can contribute to the thermal conduction mechanism of a Cu/ MWCNT2 composite. 30,31The incredible effects of phonon conduction in the fabricated nanocomposites at 450 mgl −1 concentration could be attributed to the optimum number of CNTs align in the preferential direction (axial direction) of MWCNT, resulting in higher thermal conductivity. 31,32

Conclusions
The Cu/CNT nanocomposites were prepared successfully through the electrodeposition technique on Cu fins and Cu pipes.Because of the distribution of CNT in the matrix, this fact can be attributed to a good interfacial bond holding among Cu and CNT.The experiments with Cu fins reported that the Cu/MWCNT2 composites have relatively higher heat transfer rates compared to the pure Cu, Cu/SWCNT, and Cu/MWCNT1 composites.At 450 mgl −1 concentration of CNT, the results of Cu/MWCNT2 coating on Cu pipe reported a maximum heat transfer rate of 1.47 times higher compared to pure Cu coating.The effect of electrons in Cu and phonons in connected CNTs network causes a high heat transfer rate in Cu/MWCNT2 composite.As the concentration of CNT increases above 450 mgl −1 , the number of CNTs deposited is more, and the resistance between CNT-CNT got increased (transverse directions), resulting decrease in heat transfer rate.SEM micrographic microstructures also carry this signature and confirm the presence of most of the CNTs along the axial direction and very few are in perpendicular (transverse) to the surface in the Cu matrix.

Figure 1 .
Figure 1.Schematic diagram of electrodeposition technique setup and the cross sectional view of Cu fins and Cu pipe samples.

Figure 2 .
Figure 2. Schematic diagrams of setup of the Heat transfer using (a) Cu fins (b) Cu pipe in axial & transverse direction along with original images.
Fig. 7c.XRD analysis.-XRD is used to investigate the pure Cu coating and Cu/MWCNT2 composites of two CNT concentrations of 150 mgl −1 , 300 mgl −1 and 450 mgl −1 .The XRD patterns confirm the presence of MWCNT in the Cu matrix as shown in Fig. 5.The peaks are observed at 2θ of 43.51°and 50.18°for pure Cu coating, and it matches with the JCPDS file no.00-003-1018.The extra peaks were observed on the reinforcements of MWCNT at 2θ values of 25.38°, 25.66°, and 25.72°which are corresponding to the carbon plane of (111) and match with JCPDS file no.41-1487.A shift of 2θ values is also observed from the XRD pattern in Fig.

Figure 7 .
Figure 7. FESEM images of cross section view of Cu/MWCNT1 coating (a) coating thickness (b) CNTs alignment in Cu matrix (c) Schematic view of CNT deposition.

Figure 9 .
Figure 9. Heat transfer rate curves of Cu, SWCNT, MWCNT1 and MWCNT2 coated on copper pipe (a) natural convection and (b) forced convection.

Figure 11 .
Figure 11.The Q CNT /Q Cu values of Cu/MWCNT2 coatings at various concentrations of CNT in natural and forced convection.

Table I .
Detailed deposition parameters of pure Cu and Cu/CNT composites.

Table III .
The heat transfer rate values of Pure Cu, SWCNT, MWCNT1 and MWCNT2 composites.