This study presents new Nano-enhanced Phase Change Materials, NePCMs, formulated as dispersions of functionalized graphene nanoplatelets in a poly(ethylene glycol) with a mass-average molecular mass of 400 g·mol−1 for possible use in Thermal Energy Storage. Morphology, functionalization, purity, molecular mass and thermal stability of the graphene nanomaterial and/or the poly(ethylene glycol) were characterized. Design parameters of NePCMs were defined on the basis of a temporal stability study of nanoplatelet dispersions using dynamic light scattering. Influence of graphene loading on solid-liquid phase change transition temperature, latent heat of fusion, isobaric heat capacity, thermal conductivity, density, isobaric thermal expansivity, thermal diffusivity and dynamic viscosity were also investigated for designed dispersions. Graphene nanoplatelet loading leads to thermal conductivity enhancements up to 23% while the crystallization temperature reduces up to in 4 K. Finally, the heat storage capacities of base fluid and new designed NePCMs were examined by means of the thermophysical properties through Stefan and Rayleigh numbers. Functionalized graphene nanoplatelets leads to a slight increase in the Stefan number.

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Dispersions based on hexagonal boron nitride, h-BN, nanoparticles, at 0.50, 0.75 and 1.0 wt.% mass concentrations, in an ester base oil composed mainly of trimethylolpropane trioleate, were investigated as potential nanolubricants. The stability of the dispersions was assessed to determine the reliability of the tribological, thermophysical and rheological measurements. Density and viscosity were measured from 278.15 to 373.15 K, while rheological behavior was analyzed at shear rates from 1 to 1000 s −1 at 283.15 K. Newtonian behavior was exhibited by all nanolubricants at the explored conditions, with the exception of the highest concentration at the lowest shear rates, where possible non-Newtonian behavior was observed. Tribological tests were performed under a normal load of 2.5 N. Wear was evaluated by means of a 3D profiler, scanning electron microscopy and confocal Raman microscopy. The best tribological performance was achieved by the 0.75 wt.% nanolubricant, with reductions of 25% in the friction coefficient, 9% in the scar width, 14% in the scar depth, and 22% of the transversal area, all with respect to the neat oil. It was observed that physical protective tribofilms are created between rubbing surfaces.Coatings 2019, 9, 509 2 of 17 relatively weak van der Waals interactions between loosely stacked h-BN sheets that provide excellent lubrication characteristics [9]. h-BN, the softest and most lubricious polymorph of BN, has both high thermal stability and thermal conductivity as well as numerous industrial applications, especially in metalworking processes where cleanliness of the working environment is required [10][11][12]. Additionally, h-BN is an environmentally-friendly material [13], which is being studied as a nanoadditive for liquid lubricants [14-22] because, compared with solid lubricants, liquid lubricant additives present advantages such as faster entrance and replenishment of the sliding interface, and easier removal of wear debris from the contact zone [23].With respect to the tribological behavior of nanolubricants containing h-BN nanoparticles, Mosleh et al. [14] found that when nanoparticles of h-BN, with irregular flake-like shapes and an average size of 70 nm, were dispersed in commercial sheet metal forming fluids, the friction coefficient for titanium-steel pairs showed little or no improvement; nevertheless, the wear reduction using the 1 wt.% h-BN nanodispersion was 55%. Çelik et al. [15] found that an engine oil (Society of Automotive Engineers (SAE) 10 W) containing h-BN nanoparticles of disk-like shape with an average diameter of 114 nm and thickness of 30-70 nm, led to a 14.4% improvement in the friction coefficient and a 65% decrease in the wear rate with respect to the neat oil for an AISI 4140 steel material. Wan et al. [16] studied nanolubricants composed of a formulated oil 15W-40, a dispersant, and h-BN nanoparticles (disks with average diameter of 120 nm and a single layer thickness of around 30 nm), finding that the nanolubricants with low nanoparticle concentrations s...

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