46912-G7
Heat Transfer in Graphene-Oil NanoComposites: A Molecular Understanding to Overcome Practical Barriers
Graphene sheets, one-atom-thick layers of carbon atoms, are receiving enormous scientific attention because of extraordinary electronic and mechanical properties. These intrinsic properties will lead to innovative nano-composite materials that could be used to produce novel transistors and thermally-conductive polymeric materials. Such applications are currently hindered by the difficulty of producing large quantities of individual graphene sheets and by the propensity of these nanoparticles to agglomerate when dispersed in aqueous and/or organic matrixes.
In November 2006 we stated that ‘We are here interested in composites in which graphene sheets are dispersed within organic oils. Our goal is to control the composite thermal transport properties by manipulating the assembly of the graphene sheets. (…) We will assess the effective interactions between graphene sheets in oils, the structure of the fluid surrounding the graphene sheets, and the resistance to heat transfer from a graphene sheet to the surrounding matrix (Kapitza resistance)’. To reach those objectives one graduate student, Deepthi Konatham, has been supported by the ACS PRF grant since the summer of 2007, and will be supported until approximately mid 2009.
During the first year we employed all-atom molecular dynamics simulations to assess whether or not it is possible to produce stable dispersions of graphene sheets in oils. This work resulted in one paper, titled ‘Molecular design of stable graphene nano-sheets dispersions’, which is currently under review. Deepthi and I reported in that manuscript the results of molecular dynamics simulations for pristine and functionalized graphene sheets of 54 and 96 carbon atoms each dispersed in liquid organic linear alkanes (oils) at room conditions. For the first time, our results showed that, although pristine graphene sheets agglomerate in the oils considered, graphene sheets functionalized at their edges with short branched alkanes yield stable dispersions. We characterized the simulated systems by computing radial distribution functions between the graphene sheets centers of mass, pair potentials of mean force between the graphene sheets in solution, and site-site radial distribution functions. The latter were used to determine the preferential orientation between approaching graphene sheets and the packing of the organic oils on the graphene sheets. Our results are useful not only for designing practical recipes for stabilizing graphene sheets in organic systems, but also for comparing the molecular mechanisms responsible for the graphene sheets aggregation to those that stabilize graphene sheets – containing dispersions, and for controlling the coupling between organic oils and graphene sheets used as fillers. In particular, we demonstrated that excluded-volume effects, generated by the branched architecture of the functional groups grafted on the graphene sheets, are responsible for the stabilization of small graphene sheets in the organic systems considered here.
Now that we possess a practical strategy for producing stable dispersions of graphene sheets, albeit small ones, in oils, we are beginning to evaluate the barriers to heat transfer between the graphene sheets and the surrounding oils. We conducted preliminary non-equilibrium molecular dynamics simulations for such purposes. The simulations are performed as follows: the system composed by one graphene sheet immersed in n-octane molecules is first equilibrated at room conditions, and then the temperature of the graphene sheet is instantaneously increased to 500K. At this point the simulations are conducted in the NVE ensemble and the difference in temperature between the nanoparticle and the surrounding fluid, ΔT, is monitored as a function of time. As heat flows from the graphene sheet to the surrounding oil ΔT decreases to 0. The faster the decay, the lower the Kapitza resistance is. Our raw data indicate that heat transfer happens more quickly for graphene sheets of 54 or 96 carbon atoms than for carbon nanotubes (data from literature), suggesting a lower Kapitza resistance, and they also indicate that functionalization of some atoms at the graphene sheets edges decreases even further the Kapitza resistance.
In the near future we will conclude our preliminary simulations to quantify the graphene sheet – oil Kapitza resistance as a function of the size of the graphene sheet, of the degree of functionalization, and also as a function of the oil molecular weight. Continuing our recent studies for the stabilization of graphene sheets, we will simulate n-hexane, n-octane, and n-dodecane. We are increasing the concentration of graphene sheets in these oils, in search of a possible phase transition from a disordered to a nematic phase. If successful, our search may lead to fluids in which heat transfer is anisotropic. In addition, we are interested in the behavior, both thermodynamic and transport, of graphene sheets in polyethylene imine. Such simulations have not yet been initiated.
