Total exfoliation of graphite in molten salts.

The exfoliation of graphite to graphene nanoplatelets (GnP) in a molten salt medium is investigated in this study. It is shown that this mechanical force-free process yielded a large-sized GnP product (>15 microns) with a low defect density. The effect of the surface tension of the molten salt on graphite exfoliation efficiency was investigated for a series of alkali chloride salts (CsCl, KCl, NaCl and eutectic NaCl-KCl) at 850 °C. It was demonstrated that the produced GnP could be completely and easily separated from the salt. Molten salt with the lowest value of surface tension (CsCl) displayed the highest wettability of the graphitic layers and hence facilitated total exfoliation of the graphite to GnP. The exfoliation of graphite in molten salts is applicable in the thermal energy storage field, as well as in exfoliation of other layered materials. Herein, it is demonstrated that the thermal conductivity of the GnP-CsCl composite is enhanced by ∼300% compared to the neat salt.

Sensing Exposure Time to Oxygen by Applying a Percolation-Induced Principle.

The determination of food freshness along manufacturer-to-consumer transportation lines is a challenging problem that calls for cheap, simple, reliable, and nontoxic sensors inside food packaging. We present a novel approach for oxygen sensing in which the exposure time to oxygen-rather than the oxygen concentration per se-is monitored. We developed a nontoxic hybrid composite-based sensor consisting of graphite powder (conductive filler), clay (viscosity control filler) and linseed oil (the matrix). Upon exposure to oxygen, the insulating linseed oil is oxidized, leading to polymerization and shrinkage of the matrix and hence to an increase in the concentration of the electrically conductive graphite powder up to percolation, which serves as an indicator of food spoilage. In the developed sensor, the exposure time to oxygen (days to weeks) is obtained by measuring the electrical conductivity though the sensor. The sensor functionality could be tuned by changing the oil viscosity, the aspect ratio of the conductive filler, and/or the concentration of the clay, thereby adapting the sensor to monitoring the quality of food products with different sensitivities to oxygen exposure time (e.g., fish vs grain).

Thermally Conductive Molten Salt for Thermal Energy Storage: Synergistic Effect of a Hybrid Graphite-Graphene Nanoplatelet Filler.

Renewable energy technologies depend, to a large extent, on the efficiency of thermal energy storage (TES) devices. In such storage applications, molten salts constitute an attractive platform due to their thermal and environmentally friendly properties. However, the low thermal conductivity (TC) of these salts (<1 W m-1 K-1) downgrades the storage kinetics. A commonly used method to enhance TC is the addition of highly conductive carbon-based fillers that form a composite material with molten salt. However, even that enhancement is rather limited (<9 W m-1 K-1). In this study, the partial exfoliation of graphite to graphene nanoplatelets (GnP) in a molten salt matrix is explored as a means to address this problem. A novel approach of hybrid filler formation directly in the molten salt is used to produce graphite-GnP-salt hybrid composite material. The good dispersion quality of the fillers in the salt matrix facilitates bridging between large graphite particles by the smaller GnP particles, resulting in the formation of a thermally conductive network. The thermal conductivity of the hybrid composite (up to 44 W m-1 K-1) is thus enhanced by two orders of magnitude versus that of the pristine salt (0.64 W m-1 K-1).

Filler dimensionality effect on the performance of paraffin-based phase change materials.

HYPOTHESIS: Phase change materials have the potential for use in high-density thermal energy storage. However, their low thermal conductivity and the need for shape stabilization restrict their performances and implementation in various fields. The inclusion of thermally conductive nanomaterial as a single or hybrid filling is expected to form 3D network that enhances the thermal performances of phase change materials. The encapsulation of the colloidal composites in a polymer matrix stabilizes the phase change material. EXPERIMENTS: A paraffin matrix was loaded with carbon-based fillers of various dimensionalities, namely, 1D-carbon nanotubes, 2D-graphene nanoplatelets, and 3D-graphite flakes. The thermal conductivity of the colloidal composite was measured by transient plane source and the latent heat capacity by differential scanning calorimetry techniques. Modeling the thermal conductivity by the effective medium approach predicts the experimental results. FINDINGS: The thermal conductivity of the phase change material loaded with fillers is enhanced from 0.2 to 11 W (m K)-1 (×55) compared with a filler-free paraffin matrix. We attribute this enhancement to the synergetic effect of the hybrid fillers (8 vol% graphite flakes and 12 vol% graphene nanoplatelets) and consequent compression (25 bar) of the colloidal composite. Moreover, the obtained phase change material is completely stable during charging and discharging cycles.