Functionalized Graphene Nanoplatelets Research Articles

Experimental Convection Heat Transfer Analysis of a Nano-Enhanced Industrial Coolant.

Convection heat transfer coefficients and pressure drops of four functionalized graphene nanoplatelet nanofluids based on the commercial coolant Havoline® XLC Pre-mixed 50/50 were experimentally determined to assess its thermal performance. The potential heat transfer enhancement produced by nanofluids could play an important role in increasing the efficiency of cooling systems. Particularly in wind power, the increasing size of the wind turbines, up to 10 MW nowadays, requires sophisticated liquid cooling systems to keep the nominal temperature conditions and protect the components from temperature degradation and hazardous environment in off-shore wind parks. The effect of nanoadditive loading, temperature and Reynolds number in convection heat transfer coefficients and pressure drops is discussed. A dimensionless analysis of the results is carried out and empirical correlations for the Nusselt number and Darcy friction factor are proposed. A maximum enhancement in the convection heat transfer coefficient of 7% was found for the nanofluid with nanoadditive loading of 0.25 wt %. Contrarily, no enhancement was found for the nanofluids of higher functionalized graphene nanoplatelet mass fraction.

Functionalized graphene nanoplatelet nanofluids based on a commercial industrial antifreeze for the thermal performance enhancement of wind turbines

The overheating of mechanical and electrical components in generators of wind turbines considerably reduces their overall performance. Consequently, their cooling systems usually need to dissipate large amounts of heat, leading to high electrical energy consumptions. These systems habitually use commercial industrial coolants as working fluids in order to avoid freezing at low temperatures and corrosion of mechanisms. Dispersions of graphene nanoplatelets are expected to enhance the thermal conductivity of glycolated water-based fluids, but scarce studies were reported in the literature using commercial industrial antifreezes as base fluid.In this study a comprehensive thermophysical characterization of different loaded polycarboxylate chemically modified graphene nanoplatelet nanofluids (0.25, 0.50, 0.75 and 1.0 wt%) based on a commercial coolant, Havoline® XLC Premixed 50/50, extensively employed in cooling systems of wind turbines, was carried out. Firstly, with the purpose of achieving long-term stabilities, it was found the optimum sodium dodecyl benzene sulphonate concentration, 0.125 wt%, through zeta potential and dynamic light scattering measurements, finding no substantial alteration of the original pH value. Densities were measured by pycnometry, heat capacities by differential scanning calorimetry and thermal conductivities by transient hot wire technique, in the temperature range from (293.15 to 343.15) K. Moreover, rheological behaviour was experimentally determined by means of a rotational rheometer with cone-plate geometry at temperatures from 293.15 to 323.15 K. Thermal conductivity enhancements reaching 7.3% and dynamic viscosity increases up to 20% were found. Potential heat transfer performance capabilities and pumping power consumptions in both laminar and turbulent flow conditions were investigated through the analysis of equivalent ratios derived from the experimentally measured properties, optimal concentrations for both regimes being determined.

Nanoindentation and nanoscratch properties of graphene nanoplatelets reinforced spark plasma sintered aluminium-based nanocomposite

ABSTRACTAluminium matrix nanocomposites reinforced with 0.5 and 1wt% functionalized graphene nanoplatelets were fabricated via spark plasma sintering (SPS). Nanoindentation and nanoscratch tests were performed on the nanocomposites to evaluate the mechanical and tribological properties at small length scales. Al nanocomposite reinforced with 0.5wt% graphene nanaoplatelets (GNP) showed homogenous porosity free microstructure, whereas a significant amount of porosities were evident in Al-1.0wt% GNP nanocomposite. Indentations and scratches were performed at 2000 μN constant load using standard Berkovich indenter. Elastic modulus, nanohardness and wear volume were measured for the sintered samples. Al nanocomposite reinforced with 0.5wt% GNP showed high hardness and wear resistance as compared to that of monolithic aluminium. Improvement in the wear resistance was discussed with respect to strengthening effect by addition of GNPs.

Highly thermoconductive fluid with aqueous compatible graphene

Heat transfer of nanofluids is a complex phenomenon, where many factors like viscosity, surface charge, composition, and fluid stability of the dispersed nanoplatelets will affect the final thermal performance. Various layered nanoplatelets have been applied to fabricate composite nanofluids for applications on thermal management systems. Up to date, heat transfer fluids with high thermal and electrical properties have not yet been synthesized. In this study, functionalized graphene nanoplatelets (FGNs) with 1-5 atomic-thick were synthesized via a green steam exfoliated technique. Such the FGNs can generate the highly stable aqueous suspensions with high shelf life. The FGNs-water nanofluid exhibits an unprecedented thermal conductivity of 2.52 W mK−1 at 20 vol% loading of nanoplatelets, suggesting it has superb potential application on various thermal fields.

Effects of titania on tribological and thermal properties of polymer/graphene nanocomposites

In most engineering applications where fluid lubrication is practically impossible such as high temperature environment, solid lubrication becomes an alternative option. Polymers such as polytetrafluoroethylene are often used for solid lubrication due to their ability to provide low friction on interfacial sliding conditions. However, polymeric materials often show low wear resistance, which limits their applications. Therefore, there is need for high wear resistance polymers or polymer composites for such application. In this study, wear resistance of poly (vinylidene fluoride) (PVDF) was improved by incorporating hydroxylated titanium dioxide (TD-OH) and functionalized graphene nanoplatelets (fGNPs). The composites were fabricated by solution blending and further processed by melt compounding. Raman and X-ray diffractometer were used to characterize the particles, while morphological study and wear scars on the composite samples were examined using scanning electron microscope. From the results obtained, wear volume (WV) reduced from about 0.6255 mm3 for pure PVDF to 0.2439 mm3 for 3.34 wt% fGNPs composite and further reduced to 0.1473 mm3 with the addition of 10 wt% TD-OH to 3.34 wt% fGNPs composite. These are about 61% and 76% reduction respectively, compared to pure PVDF. It was noted that increase in TD-OH content up to 20 wt% in fGNPs binary composites increased the WV of the ternary composites. This indicates that ceramic nano-fillers at appropriate proportions in polymer/graphene composites can enhance the wear resistance of such composites. On the other hand, the ternary composites showed lower thermal stability compared to the binary composites, which was attributed to low thermal stability product(s) of chemical reaction between fGNPs and TD-OH in the PVDF matrix.

Surface functionality analysis by Boehm titration of graphene nanoplatelets functionalized via a solvent-free cycloaddition reaction.

In this work, the functionalization of graphene nanoplatelets (GNPs) performed by a solvent-free cycloaddition reaction on GNPs with iminodiacetic acid (IDA) and paraformaldehyde (PFA), and the functionality analysis of the resulting functionalized GNPs (f-GNPs) by Boehm titration are introduced. The f-GNPs synthesized at different temperatures were characterized by X-ray diffraction (XRD), Raman spectroscopy and scanning electron microscopy (SEM) for structural and morphological properties. Back titration of the f-GNPs selectively identified 3 types of functional groups on the f-GNP surface, carboxylic, lactonic and phenolic, and suggested that 200 °C gives the highest carboxylic group functionality. With the reaction temperature increasing from 180 to 220 °C, a decrease in the phenolic functionality and an increase in that of lactonic are observed. In the case of 250 °C reactions, it was found that the carboxylic functionality is greatly reduced, while the phenolic functionality showed a significant increase. The f-GNP samples were further characterized by thermogravimetric analysis (TGA) and X-ray photoelectron spectroscopy (XPS), the results of which showed a good agreement with the titration analysis.

The influence of covalent and non-covalent functionalization of GNP based nanofluids on its thermophysical, rheological and suspension stability properties.

Covalent functionalization (CF-GNPs) and non-covalent functionalization (NCF-GNPs) approaches were applied to prepare graphene nanoplatelets (GNPs). The impact of using four surfactants (SDS, CTAB, Tween-80, and Triton X-100) was studied with four test times (15, 30, 60, and 90 min) and four weight concentrations. The stable thermal conductivity and viscosity were measured as a function of temperature. Fourier transform infrared spectroscopy (FTIR), thermo-gravimetric analysis (TGA), X-ray diffraction (XRD) and Raman spectroscopy verified the fundamental efficient and stable CF. Several techniques, such as dispersion of particle size, FESEM, FETEM, EDX, zeta potential, and UV-vis spectrophotometry, were employed to characterize both the dispersion stability and morphology of functionalized materials. At ultrasonic test time, the highest stability of nanofluids was achieved at 60 min. As a result, the thermal conductivity displayed by CF-GNPs was higher than NCF-GNPs and distilled water. In conclusion, the improvement in thermal conductivity and stability displayed by CF-GNPs was higher than those of NCF-GNPs, while the lowest viscosity was 8% higher than distilled water, and the best thermal conductivity improvement was recorded at 29.2%.

A direct and sensitive electrochemical sensing platform based on ionic liquid functionalized graphene nanoplatelets for the detection of bisphenol A

A simple electrochemical sensor for bisphenol A (BPA) was developed based on a composite of graphene nanoplatelets (GNPs) and 1-butyl-2, 3-dimethylimidazolium tetrafluoroborate (ionic liquid, IL) as a modifier for glassy carbon paste electrodes (GCPEs). Scanning electron (SEM) and atomic force microscopy (AFM) were employed to characterize the morphology and surface modification. The electrochemical behavior of BPA on IL-GNP/GCPEs was investigated and the results showed that IL-GNP composites enhance the electrochemical signal toward BPA due to the synergetic effect of GNPs and IL. The experimental parameters including the amount of IL and GNPs, pH of solution, pulse potential, step potential, and scan rate were optimized. Under optimal conditions, the proposed sensor exhibited a linear relationship between signal and BPA concentrations ranging from 0.02–5.0μM, with detection and quantification limits of 6.4nM and 0.02μM respectively. Moreover, the electrochemical sensor showed good repeatability (RSD=3.3%, n=5 measurements), good reproducibility (RSD=3.8%, n=5 sensors), high accuracy of 95.3–104.5% recovery, acceptable selectivity, and stability. The sensor was successfully applied to the determination of BPA in water samples in contact with plastic materials. The results were satisfactory and in agreement with reference values from a standard HPLC method.

Preparation of Functionalized Graphene Nano-platelets and Use for Adsorption of Pb2+ from Solution

Functionalized graphene nano-platelets (FGN) were obtained via treating graphene nanoplatelets (GN) with HNO3, and served as adsorbent for the removal of Pb2+ from solutions. We investigated the FGN adsorption capacity for Pb2+ at different initial concentrations, varying pH, contact time and temperature. The characterization results of scanning electron microscopy (SEM), thermal analysis (TG/DTG), Fourier transform infrared spectroscopy (FT-IR) and Brunauer-Emmett-Teller (BET) method indicated that FGN layers were thin and possess large specific area with oxygen-containing functional groups grafted onto their surface. Meanwhile, the determined equilibrium adsorption capacity of FGN for Pb2+ was 57.765 mg/g and adsorption isotherms well confirmed to Langmuir isotherms models. The results reveals that the FGN has better effect of water treatment.

Rheological behaviour of functionalized graphene nanoplatelet nanofluids based on water and propylene glycol:water mixtures

Energy efficiency of systems stands out as a critical parameter to achieve sustainability. Heat transfer processes spend an important part of overall energy consumption, so the enhancement of its efficiency is an important research field. Nanofluids have received increasing attention to reach this goal because of its initial conception as thermal-improved working fluids. Nevertheless, the increase in thermal conductivity produced by the dispersion of nanoparticles should not be the only focus. In forced convection processes, viscosity plays an important role in achieving an improvement of the exchange of heat. In addition, the energy consumption needed to pump the new fluid is directly related to its viscosity. In this work, the rheological behaviour of different loaded functionalized graphene nanoplatelet dispersions (nanoparticle mass concentrations, wt%, of 0.25, 0.50, 0.75 and 1.0) in three based fluids (water, propylene glycol:water mixture at 30:70 wt% and propylene glycol:water mixture at 50:50 wt%) has been investigated by using a rotational rheometer. A double cone geometry was employed together with a cover specially appropriated to help thermostatization and avoid evaporation at high temperatures. The viscosity curves in the shear rate range from 10 to 1000 s−1, with 10 points per decade, and in the temperature range from 283.15 to 353.15 K, with 10 K step, were determined for the three base fluids and the twelve different nanofluids. The dynamic viscosities values of the base fluids were compared with those available in the literature, absolute average deviations being lower than the experimental uncertainty. The dynamic viscosities of the nanofluids were obtained for the Newtonian ranges and their dependences with temperature and nanoadditive concentration were analysed, as well as the influence of the base fluid on these trends. Furthermore, the fitting parameters of an equation that models temperature and nanoparticle concentration dependences are provided, which allow to describe viscosity data for each of the three sets of nanofluids with deviations lower than 1.8%. Finally, linear viscoelastic oscillatory experiments were performed for those samples that showed evidences of Non-Newtonian behaviour in the non-linear tests. These oscillatory tests were carried out in the deformation range from 0.1 to 1000% at constant frequency, 1 Hz, and at 283.15 and 293.15 K. Unlike the base fluids and other nanofluids sets, the nanofluids of the analysed propylene glycol:water 50:50 nanofluid set at 283.15 K and 293.15 K present a clear shear thinning (pseudoplastic) non-Newtonian behaviour, the Newtonian plateaus being easily identified as the concentration rises in the lowest deformations range.

Effect of employing a new biological nanofluid containing functionalized graphene nanoplatelets on thermal and hydraulic characteristics of a spiral heat exchanger

In this study, thermal and hydraulic attributes of an ecofriendly graphene nanofluid flowing within a countercurrent spiral heat exchanger are evaluated. The cold water flows in one side while the hot nanofluid or hot base fluid moves in the other side of heat exchanger. The heat transfer rate and overall heat transfer coefficient enhance with increasing either Reynolds number or concentration. The effect of adding nanoparticles becomes more important at higher Reynolds numbers. At great Reynolds number, the higher heat transfer occurs in the end sections of the heat exchanger whereas at low Reynolds number, the chief heat exchange happens at the initial sections. The results show that the value of effectiveness is much great (higher than 0.85) in all cases under investigation. Moreover, the effectiveness and number of transfer units decrease by increasing the Reynolds number. The pressure drop intensifies with the Reynolds number increment, and the nanofluid demonstrates a greater pressure drop than the base fluid especially at higher Reynolds numbers. Meanwhile, the cold fluid demonstrates a higher pressure drop compared with the hot fluid due to the greater viscosity. The performance index, i.e. the ratio of heat transfer rate to pressure drop, enhances with increase in either Reynolds number or concentration, which is a promising result. Thereby, the performance index for the nanofluid increases almost 142% by increasing the Reynolds number from 1000 to 3000.

An experimental evaluation on thermophysical properties of functionalized graphene nanoplatelets ionanofluids

Ionanofluids (ionic liquid-based nanofluids) as newly introduced types of nanofluids with promising potential for heat transfer and thermal storage applications are created through complex dispersion of ultrafine nanometer-sized particles in ionic liquids. As innovative agents for development of energy sustainability, ionanofluids are widely employed in some applications, i.e., solar panels, catalysts, heat insulators and so forth. Non-flammability and non-volatility features of ionic liquids, make them applicable as green working fluids for any chemical processes. In the present paper, an experimental investigation was conducted on some thermophysical properties (viscosity, electrical conductivity and surface tension) of graphene based ionanofluid as main effective parameters in performance analysis. For this purpose, ionanofluid was prepared at three levels of weight fractions (1%, 2% and 3%) by adding and dispersing polycarboxylate functionalized graphene nanoplatelets (GNPs) in BMIM-PF6 (1-Butyl-3-methylimidazolium hexafluorophosphate) with 98 + % purity as the base fluid. The experimental data was acquired within the temperature range of 293.15 to 333.15 K and at atmospheric pressure (~101 kPa). The results show that the viscosity of ionanofluid decreases with enhancement of temperature and nanoparticle concentration. On the other hand, electrical conductivity of ionanofluid augments as temperature and particle loading increase. For instance, EC at 1% wt. nanoparticles and 303.15 K enhances around 64% compared to the pure IL. Surface tension of the ionanofluids was also determined experimentally as a function of temperature for different mass loadings of nanoparticles. The results reveal that surface tension of ionanofluids decreases slightly as temperature enhances and it reduces with enhancement of nanoparticle fraction as well.

Oxygen plasma treatment for improving graphene distribution and mechanical properties of graphene/copper composites

Current graphene/metal composites are mostly limited to the cases of using graphene oxide (GO) which contains a high level of defects. In this work, the high-quality graphene nanoplatelets (GNPs) were used as starting graphene material to prepare the GNP/Cu composite. Oxygen plasma treatment was employed to conduct the surface functionalization of GNPs by grafting the considerable oxygen-functional groups but without noticeably damaging the graphene structure. It was found that the plasma-treated GNPs (P-GNPs) exhibited not only a good dispersability in ethanol/water, but also an enhanced electrostatic affinity with Cu powder, resulting in the P-GNP/Cu composite with a uniform GNP distribution and a good interfacial bonding. At 1 vol% GNP loading, the P-GNP/Cu composite presented a largely enhanced yield strength of 188 MPa with a relatively high failure elongation of 21%, significantly outperforming the composite with untreated GNPs (158 MPa, 12%). The enhanced strength could be explained by the load transfer mechanism and agreed approximately with the prediction of a modified shear-lag model. The good ductility was attributed to the prominent dislocation storage capability of P-GNP/Cu composite. Therefore, the oxygen plasma treatment provides a general and effective strategy to improve graphene distribution and mechanical properties of graphene/metal composites.

Enhanced properties of UPE/ESOA partially bio-nanocomposites reinforced with chitosan functionalized graphene nanoplatelets: an innovative approach

The current study deals with the successful development of chitosan-functionalized graphene nanoplatelets (CS/GNPs) and their dispersion in the unsaturated polyester (UPE)/epoxidized soybean oil acrylate (ESOA) (80:20 w/w) blend system in different compositions of 0.3, 0.5 and 0.7 wt%. The resulting nanocomposite mixture was achieved by simple sonication method and pressed into a mould for fabrication of nanocomposite. The whole functionalization and nanocomposite preparation procedure were successfully tracked by FTIR, SEM and TEM. Nanocomposite with 0.5 wt% of CS/GNPs nanofiller has demonstrated it as the better candidate due to its optimum properties. Again for better comparison, a nanocomposite with 0.5 wt% raw GNPs was also fabricated and its properties were studied in detail. This relative study reported lower mechanical, dynamic mechanical, thermal and electrical conductivity values for 0.5 wt% raw GNPs than the corresponding CS/GNPs nanofiller-filled nanocomposites. Nanocomposite with 0.5 wt% CS/GNPs showed dramatic enhancement in mechanical, dynamic mechanical, thermal and electrical properties as well as reduced corrosion and swelling performance owing to the homogeneous distribution of nanofiller in the blend. Again, the nanocomposite showed the highest thermal and electrical conductivities with the best dielectric strength. Thus, the prepared nanocomposites with optimum nanofiller content might serve as partially biodegradable nanomaterial for applications in nanotechnology engineering, thermal applications, such as circuit boards and electrical applications, such as electronic packaging components, electromechanical devices and electric energy storage devices. This nanocomposite can also find their applications in different corrosive- and solvent-based environments.

Comparing Selectivity of Functionalized Graphenes Used for Chemiresistive Hydrocarbon Vapor Detection

Portable or wearable sensors for volatile organic compounds (VOCs) such as benzene and naphthalene are important for occupational health monitoring of workers near refueling operations. Six commercially available, plasma-processed, functionalized-graphene nanoplatelet (fGNP) materials were dispersed between electrodes as disordered films to form chemiresistors. Putative functional groups included amino, carboxyl, fluoroalkyl, and hydroxyl. Sensor response trends were determined upon exposure to both nonpolar (fuel related alkanes and arenes) and polar compounds (alcohols, acetone, trichloroethylene, and water). The relative sensitivity to the fuel-related hydrocarbon compounds did not correlate with the functional group; but any functionalization significantly increased the sensor response compared to unfunctionalized graphene. Most notably, there is increased sensitivity to polar protic vapors. The results suggest that the dominant mechanism for sensor response is swelling caused by intercalation of vapo...

CFD modeling of turbulent convection heat transfer of nanofluids containing green functionalized graphene nanoplatelets flowing in a horizontal tube: Comparison with experimental data

In this research, a series of numerical simulations were conducted utilizing computational fluid dynamics (CFD) software in order to predict the heat transfer performance of queues nanofluids containing clove-treated graphene nanoplatelets (CGNPs) flowing in a horizontal stainless steel heated pipe. The GNPs were covalently functionalized with clove buds using free radical grafting reaction using an eco-friendly process. The advantage of this synthesis method was that it did not use hazardous acids, which are typically used in traditional treatment methods of carbon nanostructures. The thermo-physical properties of the aqueous nanofluids obtained experimentally were used as inputs for the CFD simulations for solving the governing equations of heat transfer and fluid motion. The shear stress transport (SST) k-ω turbulence model was also used in these simulations. The corresponding convective heat transfer coefficient and friction factor of aqueous nanofluids for nanoparticle weight concentrations of 0.025, 0.075, and 0.1% were evaluated. The simulation results for both heat transfer coefficient and friction factor were shown to be in agreement with the experimental data with an average relative deviation of about ±10%. The presented results confirmed the applicability of the numerical model for simulating the heat transfer performance of CGNPs aqueous nanofluids in turbulent flow regimes.

Functionalized graphene nanoplatelet-nanofluids for solar thermal collectors

This work reports on the characterization of new heat transfer fluids consisting in dispersions of functionalized graphene nanoplatelets (fGnP) in Havoline XLC Premixed 50/50 base fluid. Optical, stability and rheological properties have been considered, with the aim to assess the nano-suspension potential for volumetric direct solar absorbers. Nanoparticle addition leads to a considerably higher sunlight absorption with respect to the pure base fluid even at the investigated extremely low concentrations (0.005% and 0.05% wt). Rheology tests show a Newtonian behaviour in the shear rate range between 10 and 1000 s−1. Viscosity and thermal conductivities do not significantly increase with respect to the pure base fluid, showing that these new fluids will not require additional pumping powers to flow.

High sensitive damage sensors based on the use of functionalized graphene nanoplatelets coated fabrics as reinforcement in multiscale composite materials

Functionalized graphene nanoplatelets networks created through glass fiber fabrics were used to detect and locate damage in multiscale composite materials. The electrical behavior of multiscale composite materials was strongly influenced by microstructural features. Coated fabrics presented high sensitivity to breakage of fibers due to the preferential orientation of f-GNPs through fibers. This sensitivity was higher when damage was induced perpendicular to the fiber direction and the region where damage could be detected was bigger in the case of locating the measuring electrical channels perpendicular to fiber direction. These phenomena are related again to the morphology of the electrical network through the coating. Due to the insulating character through thickness of composites, detection and location was limited to layers of fabric of the composite. Nevertheless, self-sensors had the capacity of detecting and locating damage with high sensitivity by means of abrupt increases in electrical resistance induced by breakage.

(Invited) Edge-Selectively Functionalized Graphene Nanoplatelets As a Metal-Free Counter Electrode in DSSCs

Metal-free carbon-based electrocatalysts for dye-sensitized solar cells (DSSCs) are sufficiently active in Co(II)/Co(III) electrolytes but are not satisfactory in the most commonly used iodide/triiodide (I−/I3 −) electrolytes. Thus, developing active and stable metal-free electrocatalysts in both electrolytes is one of the most important issues in DSSC research. Edge-selectively heteroatoms-doped graphene nanoplatelets (XGnPs, X=N, halogens, Se, Te, Sb etc.) were prepared by a simple mechanochemical driven reaction between graphite and target materials. The edge-selective doping and the preservation of the pristine graphene basal plane in the XGnPs were confirmed by various analytical techniques, including atomic-resolution transmission electron microscopy (AR-TEM) and/or time of flight secondary ion mass spectrometry (TOF-SIMS). Here we report the synthesis of edge-selective graphene nanoplatelets (XGnPs) prepared by a simple mechanochemical reaction between graphite and target materials, and their application to counter electrodes (CE) for DSSCs in both Co(II)/Co(III) and I−/I3 − electrolytes. Specifically, tested as the DSSC CE in both Co(bpy)3 2+/3+ (bpy = 2,2′-bipyridine) and I−/I3 − electrolytes, among the XGnPs, the SeGnP-CEs exhibited outstanding electrocatalytic performance with ultimately high stability. The SeGnP-CE–based DSSCs displayed a higher photovoltaic performance than did the Pt-CE–based DSSCs in both SM315-sensitizer with Co(bpy)3 2+/3+ and N719-sensitizer with I−/I3 − electrolytes. Furthermore, the I3 − reduction mechanism, which has not been fully understood on carbon-based CE materials to date, was clarified by an electrochemical kinetics study combined with density functional theory (DFT) and nonequilibrium Green’s function calculations (NEGF). These new findings may pave the way for further development of more practical Pt-free CEs, which may help to accelerate the practical applications of DSSCs.

Potential heat transfer enhancement of functionalized graphene nanoplatelet dispersions in a propylene glycol-water mixture. Thermophysical profile

Abstract The thermal conductivity of the heat transfer fluids commonly used in industrial processes is a major concern in the continuous searching of their performance improvement. During last decades, dispersions of high thermal conductivity nanoparticles in conventional fluids, nanofluids, have received increasingly attention to achieve this aim. The thermophysical properties, not commonly available in the literature, are necessary to describe their thermo-fluid behaviour. Thus, characterization studies become necessary to evaluate the potential heat transfer enhancements. This work presents the thermophysical profile according to the properties with a noticeable influence on the heat transfer capability of different functionalized graphene nanoplatelet dispersions (0.25, 0.50, 0.75 and 1.0) wt% in a propylene glycol-water mixture at (30:70)% mass ratio, usually employed in thermal facilities. Initially, nanofluid stability was investigated optimizing the pH value by analysing zeta potential measurements. Then, densities, heat capacities, thermal conductivities, and dynamic viscosities were experimentally determined throughout the temperature range from (293.15 to 323.15) K. Density was studied by using a pycnometric technique and results were used to analyze the isobaric thermal expansivity behaviour. A differential scanning calorimeter was utilized to obtain heat capacities and thermal conductivities were measured for the nanopowder, base fluid and nanofluids through devices based on the guarded heat flow meter method and the transient hot wire technique. Furthermore, rheological behaviour was analysed by means of a rotational rheometer with cone-plate geometry. Conductivity enhancements up to 16% with respect to base fluid were achieved. Finally, potential enhancements of heat transfer performance in laminar and turbulent flow were investigated through an analysis of different figures of merit based on the experimental results.