Trapped and Alone: Clay-Assisted Aqueous Graphene Dispersions

Abstract

Dispersing graphene sheets in liquids, in particular water, could enhance the transport properties (like thermal conductivity) of the dispersion. Yet, such dispersions are difficult to achieve since graphene sheets are prone to aggregate and subsequently precipitate due to their strong van der Waals interactions. Conventional dispersion approaches, such as surface treatment of the sheets either by surfactant adsorption or by chemical modification, may prevent aggregation. Unfortunately, surfactant-assisted graphene dispersions are typically of low concentration (<0.2 wt %) with relatively small sheets (<1 μm lateral size) while chemical modification is punished by increased defect density within the sheets. We investigate here a new approach in which the concentration of dispersed graphene in water is enhanced by the addition of a fibrous clay mineral, sepiolite. As we demonstrate, the clay particles in water form a kinetically arrested particle network within which the graphene sheets are effectively trapped. This mechanism keeps graphene sheets of high lateral size (∼4 μm) dispersed at high concentrations (∼1 wt %). We demonstrate the application of such dispersions as cooling liquids for thermal management solutions, where a 26% enhancement in the thermal conductivity is achieved as compared to that in a filler-free fluid.

Down the Dimensionality Lane: Thermal Conductivity Enhancement in Carbon-Based Liquid Dispersions

“…Therefore, we dispersed all fillers via the "kinetically arrested" approach in which charged fibers, sepiolite clay in our case, "arrest" the fillers and prevent their precipitation, as shown by TEM imaging (Figure 2a). The sepiolite fibers (length of 1.3 ± 0.6 μm and diameter of 30 ± 10 nm) 4 turn the aqueous dispersion viscoelastic. Above 0.2 wt %, the sepiolite fibers induce a kinetically arrested phase in which the elastic modulus is higher than the viscous modulus (i.e., G′ > G′′, respectively), and a sepiolite network is formed (Figure S8, Supporting Information).…”

“…Above 0.2 wt %, the sepiolite fibers induce a kinetically arrested phase in which the elastic modulus is higher than the viscous modulus (i.e., G′ > G′′, respectively), and a sepiolite network is formed (Figure S8, Supporting Information). 4 Exploiting the kinetically arrested phase allows the carbon-based particles we wish to disperse to be "trapped" within the sepiolite network, thus preventing their aggregation and precipitation. An example of a trapped GNP H-15 can be observed in Figure 2b.…”

“…This phenomenon is likely due to the salt-based buffer introduced into the water− EG mixture (see Experimental Section), which leads to partial electrostatic screening, thus weakening the sepiolite network and its ability to trap particles. 4,32 3.3. Intrinsic Properties Effect of the Filler on the TC Enhancement in GNP-Based Dispersions.…”

“…A network is formed above a threshold concentration of these charged particles, allowing the trapping of the large particles that we wish to disperse. 4,20 This work explores the potential of kinetically arrested dispersions by employing sepiolite, a fibrous and charged magnesium silicate-based clay mineral. We wish to go beyond the dispersion limit of the conventional methods mentioned above (<0.2 wt %) and evaluate the effect of size and dimensionality of the carbon-based fillers on the TC enhancement in aqueous-based fluids (water or water− ethylene glycol mixture, a commonly used coolant or antifreeze for thermal management applications).…”

“…The TC error bars are smaller than the size of the markers and thus not shown. The TC measurements of H-5 GNP dispersions in panel (a) were extracted from Cullari et al4…”