An experimental study on thermal conductivity enhancement of DI water-EG based ZnO(CuO)/graphene wrapped carbon nanotubes nanofluids
Introduction
The performance of instruments and machines decreases with the increase in operating temperature. For the conservation of desired performance and dissipation of heat in sectors like industry, infrastructure, transportation, and defense, cooling is crucial. The surrounding air is very poor coolant due to poor thermal conduction. Liquid coolant removes heat more rapidly than air-coolant. However, efficiency of liquid coolant such as de-ionized (DI) water or ethylene glycol (EG) is limited due to its low thermal conductivity. Thus, the suspension of solid particles having higher thermal conductivity can be used for improving the efficiency of cooling when used as coolant [1]. Nanofluids are the stable and homogeneous suspension of solid nanoparticles in base fluids established about a decade ago by Choi for improving the thermal conductivity of conventional heat-transfer fluids [2]. Use of large micron size particles leads to the problems like fast settlement in base fluids, abrasion and clogging [3]. In cooling application, these problems are not desirable and are overcome by the suspension of nanometer-sized particles in fluids. Nanoparticles stay suspended much longer in base fluids owing to their high surface area to volume ratio as compared to microparticles. The surface area to volume ratio of nanoparticles is 1000 times larger than that for microparticles. The high surface area of nanoparticles enhances the stability, and reduce the sedimentation problem in base fluid [3] which further increase the heat conduction of nanofluids since heat transfer occurs on the surface of these particles. For preparing nanofluids, several types of nanoparticles have been used, however carbon nanomaterials have grabbed the attention due their high intrinsic thermal conductivities [4], superior strength and high aspect ratio [5]. Among all carbon nanomaterials, carbon nanotubes (CNTs) and graphene have gained interest due to its unique properties like high thermal conductivity (∼3000 W/m/K for CNTs [6], ∼5500 W/m/K for graphene [7,8]), high surface to volume ratio [9,10]as well as high mechanical strength [[11], [12], [13]] in the area of nanofluids research. Theoretical and experimental studies on the heat transfer properties of graphene and CNTs have become an interesting topic of Physics. Several investigations on thermal properties of CNTs based nanofluids and attempts for enhancement in the thermal conductivity of CNTs based nanofluids have been reported [[14], [15], [16], [17]]. Wei Yu has reported significant enhancement (86%) in thermal conductivity for graphene based nanofluids (5 vol.%) in ethylene glycol base fluids [18]. Remarkable thermal conductivity enhancement of 15% has been reported by Fuxian for surfactant-free ionic liquid based nanofluids at a very low loading of graphene (0.06 vol.%) [19].
As-grown carbon nanomaterials are difficult to disperse uniformly in polar solvents directly due to their hydrophobic nature [9,20,21]. This problem is resolved by functionalization, in which the attachment of hydrophilic functional groups on the basal planes of graphene and side walls of CNTs is done. However, strong acid functionalization reduces the inherent high thermal conductivity of graphene as well as CNTs. Moreover, the functionalization can result in restacking of graphene layers, which also has an adverse effect on its thermal properties and stability in nanofluids. Hence, the functionalization process needs to be done judiciously, as the performance of resulting nanofluids depends on the delicate morphological organization, uniform dispersion and ease of processing [22]. Recently, graphene and CNTs based hybrid nanocomposites have gained attention due to the synergistic effects between these two different carbon nanomaterials [23,24]. Fan et al. [25] has reported synthesis of three dimensional composite material comprising two graphite oxide (GO) layers in between CNTs pillars were grown using chemical vapor deposition (CVD). There are many research reports depicting the carbon nanocomposite consisting of intercalated carbon nanotubes between the graphene sheets which prevent the restacking of graphene sheets. However, the synthesis procedure is multistep and time consuming [21,26,27]. Like a carbon based nanomaterials, it has been reported that metal and metal oxide nanomaterials are suitable for improving heat transfer properties of nanofluids [[28], [29], [30], [31]].
Present work reports the integration of 1D CNTs and 2D graphene (graphene wrapped carbon nanotubes (GC)) with metal oxides (CuO and ZnO) abbreviated as CuO/GC and ZnO/GC. Further, zeta potential and thermal conductivity studies have been carried out on GC, CuO/GC and ZnO/GC dispersed nanofluids. The hybrid structure of GC consisting of 1D CNTs and 2D graphene is achieved by simple chemical vapor deposition (CVD) technique using GO and MmNi3 (1:1) mixture as catalyst. Afterwards, amorphous carbon and catalytic impurities were removed by air oxidation and acid treatment (using conc. HNO3, at 80 °C, for 6 h). Here, during acid treatment, the functional groups were attached to rough surface of GC. Thus, the separate process of harsh acid functionalization was prohibited to get stable dispersion. In the literature, a number of reviews on metal oxide/CNTs, metal oxides/graphene and their application in different fields have been reported. However, to the best of our knowledge, there is no comprehensive literature on the nanocomposite consisting of 1D CNTs, 2D graphene, and metal oxides based composites and their application in nanofluids. In this work, CuO and ZnO were chosen because of their better thermal conductivity as compared to other metal oxides. The nanocomposite dispersed well in base fluids without any harsh chemical functionalization and surfactant. The present work demonstrates significant improvement in effective thermal conductivity at a very low volume fraction of GC, CuO/GC and ZnO/GC dispersed in DI water and EG.
Section snippets
Synthesis of GC, ZnO/GC and CuO/GC
GC was synthesized by CVD technique using graphene oxide (GO) and MmNi3 in 1:1 ratio as precursor. Here, GO was prepared by Hummers method [32]. Initially, GO and MmNi3 were grinded well to obtain homogeneous mixture. Subsequently, this mixture was loaded into the middle of the furnace and argon gas was flushed to create an inert atmosphere. After 10 min, the temperature of the furnace was raised to 200 °C and hydrogen gas was allowed for 45 min. At 200 °C, GO was exfoliated to graphene and MmNi
Material characterizations
The formation of GC, Zn(OH)2/GC, ZnO/GC and CuO/GC samples was confirmed through X-ray diffraction (XRD) patterns, shjown in Fig. 1. XRD pattern of purified GC exhibits an intense peak at ∼26.5° corresponding to C (002) plane with an interlayer d-spacing of 0.34 nm. The broadening in peak confirms the presence of 2D graphene in GC nanocomposite.
The XRD pattern of Zn(OH)2/GC shows the major peaks between 10° to 80° along with the (002) plane peak of GC which can be indexed to an orthorhombic
Conclusion
Chemical vapor deposition technique (CVD) is truly effective for preparing nanostructures of various morphologies. Graphene wrapped carbon nanotubes (GC), metal oxide dispersed graphene wrapped carbon nanotubes (ZnO/GC and CuO/GC) were successfully synthesized and characterized by different techniques. Thermal conductivity of GC, ZnO and CuO nanoparticles and GC composites (ZnO/GC, CuO/GC) based nanofluids in two different types of polar solvents, DI water and EG has been demonstrated. The
Acknowledgement
Authors would like to thank Indian Institute of Technology Madras, Chennai, India for financial support.
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