Figure 5. Thermal conductivity of the graphene in purified water at different temperatures for which the material consists of stacks of graphene sheets having a platelet shape.
The effect of nanoparticle volume fraction on the thermal conductivity enhancement ratio is illustrated in Figure 6 for the nanofluid with suspended graphene nanoparticles. Improved thermal conductivity enhancement of 4.8 percent or more can be achieved with only a 0.008 percent volume fraction of graphene nanoparticles. Despite those extraordinary promising thermal properties exhibited by graphene suspensions, it remains to be a serious technical challenge to effectively and efficiently disperse graphene into aqueous or organic mediums to produce a nanoparticle suspension with a sustainable stability and consistent thermal properties. Due to hydrophobic natures of graphitic structure, graphene is not soluble in any known solvent. They also have a very high tendency to form aggregates and extended structures of linked nanoparticles, thus leading to phase separation, poor dispersion within a matrix, and poor adhesion to the host. However, stability of the nanoparticle suspension is especially essential for practical industrial applications. Otherwise, the thermal properties of a nanofluid, such as thermal conductivity, will constantly change as the solid nanoparticles gradually separate from the fluid. Accordingly, there is a great need for the development of an effective formulation which can be used to efficiently disperse different forms of graphene into a desired heat transfer fluid and produce a nanofluid with a sustainable stability and consistent thermal properties. Therefore, the nanofluid may comprise a conventional heat transfer fluid, graphene nanoparticles, metal oxide nanoparticles and a surfactant. The metal oxide nanoparticles in combination with the surfactant are used to facilitate the dispersion of the graphene nanoparticles and to increase the stability of the nanofluid. There is a charge attraction between the nonpolar region of the surfactant molecules and the graphene nanoparticles. This interaction forms a shell around the graphene nanoparticles, with the charged head region of the surfactant molecules oriented towards the outside. This facilitates the dispersion of the graphene nanoparticles in the fluid thus preventing precipitation from the fluid, which in turn enhances thermal conductivity. To further enhance thermal conductivity, metal oxide nanoparticles are also added to the thermal transfer fluid. These positively charged metal oxide nanoparticles repel one another and further enhance stability and thermal conductivity of the nanofluid. The term surfactant refers to a molecule having surface activity, including wetting agents, dispersants, emulsifiers, detergents, and foaming agents. A variety of surfactants may alternatively be included in the present study as a dispersant to facilitate uniform dispersion of nanoparticles in a desired thermal transfer fluid, and to enhance stabilization of such a dispersion as well. Typically, the surfactants used in the present study contain a lipophilic nonpolar hydrocarbon group and a polar functional hydrophilic group. The polar functional group may be a carboxylate, ester, amine, amide, imide, hydroxyl, ether, nitrile, phosphate, sulfate, or sulfonate. The surfactants that are useful in the present study may be used alone or in combination. Accordingly, any combination of surfactants may include anionic, cationic, nonionic, zwitterionic, amphoteric and ampholytic surfactants, so long as there is a net positive charge in the head regions of the population of surfactant molecules. In most instances, a single negatively charged surfactant is used in the preparation of the nanofluids of the present study.