C. Reynaud

A benchmark study on the thermal conductivity of nanofluids

This article reports on the International Nanofluid Property Benchmark Exercise, or INPBE, in which the thermal conductivity of identical samples of colloidally stable dispersions of nanoparticles or “nanofluids,” was measured by over 30 organizations worldwide, using a variety of experimental approaches, including the transient hot wire method, steady-state methods, and optical methods. The nanofluids tested in the exercise were comprised of aqueous and nonaqueous basefluids, metal and metal oxide particles, near-spherical and elongated particles, at low and high particle concentrations. The data analysis reveals that the data from most organizations lie within a relatively narrow band (±10% or less) about the sample average with only few outliers. The thermal conductivity of the nanofluids was found to increase with particle concentration and aspect ratio, as expected from classical theory. There are (small) systematic differences in the absolute values of the nanofluid thermal conductivity among the various experimental approaches; however, such differences tend to disappear when the data are normalized to the measured thermal conductivity of the basefluid. The effective medium theory developed for dispersed particles by Maxwell in 1881 and recently generalized by Nan et al. [J. Appl. Phys. 81, 6692 (1997)], was found to be in good agreement with the experimental data, suggesting that no anomalous enhancement of thermal conductivity was achieved in the nanofluids tested in this exercise.

Size-, Composition- and Shape-Dependent Toxicological Impact of Metal Oxide Nanoparticles and Carbon Nanotubes toward Bacteria

Ecotoxicological effects of nanoparticles (NP) are still poorly documented while their commercialization for industrial and household applications increases. The aim of this study was to evaluate the influence of physicochemical characteristics on metal oxide NP and carbon nanotubes toxicological effects toward bacteria. Two strains of bacteria, Cupriavidus metallidurans CH34 and Escherichia coli MG1655 were exposed to TiO(2) or Al(2)O(3) NP or to multiwalled-carbon nanotubes (MWCNT). Particular attention was paid on optimizing NP dispersion to obtain nonagglomerated suspensions. Our results show that NP toxicity depends on their chemical composition, size, surface charge, and shape but not on their crystalline phase. MWCNT toxicity does not depend on their purity. Toxicity also depends on the bacterial strain: E. coli MG1655 is sensitive to NP, whereas C. metallidurans CH34 is not. Interestingly, NP are accumulated in both bacterial strains, and association between NP and bacteria is necessary for bacterial death to occur. NP may then represent a danger for the environment, causing the disappearance of some sensitive bacterial strains such as E. coli MG1655, but also being mobilized by nonsensitive strains such as C. metallidurans CH34 and transported through the whole ecosystem.

Photoluminescence of size-separated silicon nanocrystals: Confirmation of quantum confinement

Silicon nanocrystals with diameters between 2.5 and 8 nm were prepared by pulsed CO2 laser pyrolysis of silane in a gas flow reactor and expanded through a conical nozzle into a high vacuum. Using a fast-spinning molecular-beam chopper, they were size-selectively deposited on dedicated quartz substrates. Finally, the photoluminescence of the silicon nanocrystals and their yield were measured as a function of their size. It was found that the photoluminescence follows very closely the quantum-confinement model. The yield shows a pronounced maximum for sizes between 3 and 4 nm.

In vitro investigation of oxide nanoparticle and carbon nanotube toxicity and intracellular accumulation in A549 human pneumocytes

If released in the environment, nanomaterials might be inhaled by populations and cause damage to the deepest regions of the respiratory tract, i.e., the alveolar compartment. To model this situation, we studied the response of A549 human pneumocytes after exposure to aluminium oxide or titanium oxide nanoparticles, and to multi-walled carbon nanotubes. The influence of size, crystalline structure and chemical composition was investigated. After a detailed identification of nanomaterial physico-chemical characteristics, cells were exposed in vitro and viability and intracellular accumulation were assessed. In our conditions, carbon nanotubes were more toxic than metal oxide nanoparticles. Our results confirmed that both nanotubes and nanoparticles are able to rapidly enter into cells, and distribute in the cytoplasm and intracellular vesicles. Among nanoparticles, we demonstrate significant difference in biological response as a function of size, crystalline phase and chemical composition. Their toxicity was globally lower than nanotubes toxicity. Among nanotubes, the length did not influence cytotoxicity, neither the presence of metal catalyst impurities.

Diamond Infrared Emission Bands in Circumstellar Media

Since the discovery of small diamond grains in meteorites, the presence of such grains in the interstellar medium has been suspected. Here we report what we think to be the first unambiguous evidence of the presence of small crystallites of diamond in the dusty envelopes surrounding stars. Thanks to experimental results obtained in different laboratories on the diamond growth process, we identify two peculiar unidentified infrared emission bands at 3.43 and 3.53 μm as the vibrational modes of hydrogen-terminated crystalline facets of diamond. The intensities of these two strong features observed in the envelope of HD 97048 correspond to a mass of 10-10 to 10-9 M☉ of diamond dust at an equilibrium temperature of ~1000 K.

Carbon nanoparticles from laser pyrolysis

Carbon nanoparticles synthesised by laser pyrolysis of hydrocarbons in a flow reactor have been investigated as a function of laser power. Samples are cross-characterised by high resolution transmission electron microscopy (HRTEM) and infrared (IR) spectroscopy. Nanoparticles appear highly aromatic in character in all the experimental conditions explored here. As the flame temperature in the interaction zone increases, the nanoparticles evolve drastically from poorly organised, highly hydrogenated samples toward turbostratic concentric particles of carbon. The multiscale organisation of the samples and its evolution with the synthesis parameters are quantitatively determined and correlated to IR properties through an original development of HRTEM image analysis. The multiscale organisation does not reduce to the classical view of assemblies of basic structural units (BSU) of aromatic bricks. More refined models are proposed where non-stacked aromatic layers play a noticeable role and lead to a better understanding of the samples optical properties. Possible contribution to a better understanding of carbon cosmic dust is discussed from an astrophysical point of view.

Thermal and electrical conductivities of water-based nanofluids prepared with long multiwalled carbon nanotubes

Thermal and electrical conductivities of suspensions of multiwalled carbon nanotubes (MWCNT) in water were measured as a function of temperature, nanotube weight content, and nanotube length. Nanotubes were dispersed in water by using gum Arabic as surfactant. The thermal conductivity was measured by the steady-state method by using a coaxial-cylinder cell that allows the sample temperature to be varied from 15to75°C. Our measurements show that the thermal conductivity enhancement as compared to water linearly increases when the MWCNT weight content increases from 0.01to3wt%, reaching 64% for the MWCNT weight content of 3wt%. The thermal conductivity enhancement is found to be temperature independent up to MWCNT weight content of 2wt%. The average length of the nanotubes appears to be a very sensitive parameter. The thermal conductivity enhancement as compared to water increases by a factor of 3 when the nanotube average length increases in the 0.5–5μm range. Electrical conductivity measurements show that the electrical properties do not follow the same trend as a function of MWCNT weight content, as compared to thermal properties. The electrical conductivity is mainly constant in the studied range, but undergoes a drop when the weight content decreases to about 0.1wt%, which suggests that the MWCNT network in the base fluid might be percolating at this very low value. By comparison, the thermal conductivity does not show any percolation threshold.Thermal and electrical conductivities of suspensions of multiwalled carbon nanotubes (MWCNT) in water were measured as a function of temperature, nanotube weight content, and nanotube length. Nanotubes were dispersed in water by using gum Arabic as surfactant. The thermal conductivity was measured by the steady-state method by using a coaxial-cylinder cell that allows the sample temperature to be varied from 15to75°C. Our measurements show that the thermal conductivity enhancement as compared to water linearly increases when the MWCNT weight content increases from 0.01to3wt%, reaching 64% for the MWCNT weight content of 3wt%. The thermal conductivity enhancement is found to be temperature independent up to MWCNT weight content of 2wt%. The average length of the nanotubes appears to be a very sensitive parameter. The thermal conductivity enhancement as compared to water increases by a factor of 3 when the nanotube average length increases in the 0.5–5μm range. Electrical conductivity measurements show that...

In vitro evaluation of SiC nanoparticles impact on A549 pulmonary cells: Cyto-, genotoxicity and oxidative stress

Silicon carbide (SiC) is considered a highly biocompatible material, consequently SiC nanoparticles (NPs) have been proposed for potential applications in diverse areas of technology. Since no toxicological data are available for these NPs, the aim of this study was to draw their global toxicological profile on A549 lung epithelial cells, using a battery of classical in vitro assays. Five SiC-NPs, with varying diameters and Si/C ratios were used, and we show that these SiC-NPs are internalized in cells where they cause a significant, though limited, cytotoxic effect. Cell redox status is deeply disturbed: SiC-NP exposure cause reactive oxygen species production, glutathione depletion and inactivation of some antioxidant enzymes: glutathione reductase, superoxide dismutase, but not catalase. Finally, the alkaline comet assay shows that SiC-NPs are genotoxic. Taken together, these data prove that SiC-NPs biocompatibility should be revisited.

Grafting and growing mechanisms of polymerised organic films onto metallic surfaces

Abstract The build up of a homogeneous polyacrylonitrile film laid down by electropolymerisation on a metallic surface begins by the chemisorption of a monomer monolayer. As the growth of the conducing film proceeds (conduction by hopping) the number of electrons is constant. The electronic properties of the substrate and monolayer (conjugated molecule) are important.

Evidence of strong nanotube alignment and for iron preferential growth axis in multiwalled carbon nanotube carpets

This letter reports structural results obtained from x-ray scattering measurements performed on individual carbon nanotube carpets. Carpets of aligned multiwalled nanotubes are synthesized by the aerosol pyrolysis method using liquid hydrocarbon∕metallocene aerosols. Nanotube alignment of ±5.5° with respect to the average growth axis is obtained. In addition, nanotubes are partially filled with iron, and we demonstrate that the body of the carpet contains mainly one phase (γ‐Fe) encapsulated inside nanotubes which exhibits a preferential crystallographic ⟨110⟩ axis along the nanotubes.