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Absorption and scattering properties of carbon nanohorn-based nanofluids for direct sunlight absorbers.

In the present work, we investigated the scattering and spectrally resolved absorption properties of nanofluids consisting in aqueous and glycol suspensions of single-wall carbon nanohorns. The characteristics of these nanofluids were evaluated in view of their use as sunlight absorber fluids in a solar device. The observed nanoparticle-induced differences in optical properties appeared promising, leading to a considerably higher sunlight absorption with respect to the pure base fluids. Scattered light was found to be not more than about 5% with respect to the total attenuation of light. Both these effects, together with the possible chemical functionalization of carbon nanohorns, make this new kind of nanofluids very interesting for increasing the overall efficiency of the sunlight exploiting device.PACS78.40.Ri, 78.35.+c, 78.67.Bf, 88.40.fh, 88.40.fr, 81.05.U.

Tribological Properties of Engine Oil with Carbon Nano-horns as Nano-additives

AbstractnThe work illustrates the preparation of stable suspensions of single-walled carbon nano-horns (SWCNHs) in engine oil and shows their tribological behavior at different temperatures. The suspension stability was verified by dynamic light scattering, and the viscosity of suspensions was measured. Stribeck test was used to evaluate the tribological performances of nano-lubricants in the 25–80xa0°C temperature range. The coefficient of friction was reduced with the addition of SWCNHs at all temperatures and concentrations. In particular, at Txa0=xa025xa0°C, the coefficient friction was reduced up to about 12xa0% in boundary regime even with a very low SWCNH concentration (0.01xa0vol%). Wear tests were carried out to verify results obtained from Stribeck characterization. A decrease in mean wear rate was observed at each temperature, between 25 and 30xa0%. In particular, at 25xa0°C, a decrease of about 30xa0% in wear rate was estimated. Friction coefficient decrease was also observed during all wear tests, probably due to the ball-bearing effect played by SWCNHs spherical aggregates. Therefore, SWCNHs revealed the potential to be promising candidates as additives, to develop a new class of lubricants that are suitable and effective in different operating environments.

Tuning the thermal diffusivity of silver based nanofluids by controlling nanoparticle aggregation

With the aim of preparing stable nanofluids for heat exchange applications and to study the effect of surfactant on the aggregation of nanoparticles and thermal diffusivity, stable silver colloids were synthesized in water by a green method, reducing AgNO₃ with fructose in the presence of poly-vinylpyrollidone (PVP) of various molecular weights. A silver nanopowder was precipitated from the colloids and re-dispersed at 4 vol% in deionized water. The Ag colloids were characterized by UV-visible spectroscopy, combined dynamic light scattering and ζ-potential measurements, and laser flash thermal diffusivity. The Ag nanopowders were characterized by scanning electron microscopy and thermal gravimetric analysis. It was found that the molecular weight of PVP strongly affects the ζ-potential and the aggregation of nanoparticles, thereby affecting the thermal diffusivity of the obtained colloids. In particular, it was observed that on increasing the molecular weight of PVP the absolute value of the ζ-potential is reduced, leading to increased aggregation of nanoparticles. A clear relation was identified between thermal diffusivity and aggregation, showing higher thermal diffusivity for nanofluids having higher aggregation. A maximum improvement of thermal diffusivity by about 12% was found for nanofluids prepared with PVP having higher molecular weight.

Optical characterisation of Carbon-Nanohorn based nanofluids for solar energy and life science applications

Single Wall Carbon Nanohorns (SWCNHs) are innovative carbon nanostructures belonging to the thriving family of carbon nanotubes and fullerenes. They have being studied up to now mainly as non-cytotoxic drug carriers [1], but they are interesting for a huge number of applications such as electronics [2], hydrogen and methane storage [3] and renewable energies. In this paper we investigate their optical properties in view of their use as direct sunlight absorbers in solar thermal devices and as reference standard in Biological Tissue Optics.

Surface oxidation of single wall carbon nanohorns for the production of surfactant free water-based colloids

In this work, powders of Single Wall Carbon Nanohorns (SWCNHs), a typical hydrophobic material, were oxidized with concentrated HNO3 with the aim of surface carboxylation and consequent improved hydrophilicity and dispersibility in polar solvents. Dynamic Light Scattering and ζ-potential measurements demonstrated that very stable colloidal suspensions of SWCNH in water were obtained in total absence of stabilizers. By properly optimizing the reaction parameters, the suspensions achieved stability even higher than colloids with similar composition but prepared with the use of surfactants. Surface damage and oxidation degree of SWCNHs were evaluated by SEM microscopy, Thermogravimetric Analysis, Residual Gas Analysis, XPS and UV-visible spectroscopy.