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.