A review of pool and forced convective boiling of binary mixtures

Boiling of binary mixtures is characterized by a close linking between heat and mass transfer processes, with the evaporation rate usually being limited by the mass transfer process. This is significantly different from single-component systems where interfacial mass transfer rates are normally very high. Information on pool boiling of binary mixtures is widely available in the literature, whereas research on forced convective boiling of mixtures has become significant only over the last few years. This paper presents a brief review of experimental results obtained in pool and forced convective boiling of binary mixtures and upgrades the empirical or theoretical predictive tools for both situations.     

Synthesis of cobalt doped silica thin film for low temperature optical gas sensor

A modified sol–gel method was used to prepare cobalt doped silica thin film with a cobalt content of 10, 20 and 30 mol% (10Co, 20Co and 30Co). The prepared films were annealed at different temperatures in the range 400–1,000 °C, and their structural evolution examined. The mixed valence cobalt oxide, Co₃O₄, crystallizes only in the sample with the higher cobalt content, while cobalt silicate is the only crystalline phase detected in the sample 10Co and 20Co. Both the cobalt content and the temperature of heat treatment resulted to affect the nature of cobalt species dispersed in the silica matrix. The 30Co was selected for further investigations by FTIR spectroscopy to follow the structural evolution of 30Co film as function of the temperature and UV–Vis to get information on the cobalt valence state. The optical gas-sensing properties of 30Co films, containing Co₃O₄ as the major cobalt phase, were studied through the measuring of the film transmittance in dry air and in presence of dry air containing variable concentrations of polluting gases, CO and NO₂. The 30Co samples resulted to be highly sensitive to CO at room temperature. An explanation for the CO sensing characteristics, at low temperature, was proposed by referring to the physisorption-related mechanics of CO.     

Crystallization of hydrogenated amorphous silicon carbon films with laser and thermal annealing

The crystallization of silicon rich hydrogenated amorphous silicon carbon films prepared by Plasma Enhanced Chemical Vapor Deposition technique has been induced by excimer laser annealing as well as thermal annealing. The excimer laser energy density (E_d) and the annealing temperature were varied from 123 to 242 mJ/cm² and from 250 to 1200 °C respectively. The effects of the two crystallization processes on the structural properties and bonding configurations of the films have been studied. The main results are that for the laser annealed samples, cubic SiC crystallites are formed for E_d ≥ 188 mJ/cm², while for the thermal annealed samples, micro-crystallites SiC and polycrystalline hexagonal SiC are observed for the annealing temperature of 800 and 1200 °C respectively. The crystallinity degree has been found to improve with the increase in the laser energy density as well as with the increase in the annealing temperature.     

Polyglycerol-derived amphiphiles for the solubilization of single-walled carbon nanotubes in water: a structure-property study

A series of nonionic amphiphiles derived from polyglycerol dendrons were studied for their ability to solubilize and isolate single-walled carbon nanotubes. The amphiphiles possessed differently sized polar head groups, hydrophobic tail units, and various aromatic and non-aromatic groups between the head and tail groups. Absorbance analysis revealed that amphiphiles with anchor groups derived from pyrene were far inferior to those that possessed simple linear aliphatic tail groups. Absorbance and near-infrared fluorescence analyses revealed a weak dependence on the dendron size of the head group, but a strong positive trend in suspended nanotube density and fluorescence intensity for amphiphiles with longer tail units. Variations in the moieties linking the head and tail groups led to a range of effects on the suspensions, with linkers imparting flexibility and a bent shape that gave improved performance overall. This was illustrated most dramatically by a pair of benzamide-containing amphiphiles, the para isomer of which showed evidence in the fluorescence data of increased nanotube aggregate formation when compared with the meta isomer. In addition, statistical AFM was used to illustrate more directly the microscopic differences between amphiphiles that were effective at nanotube bundle disruption and those that were not.     

Carbon nanotubes as substrates for molecular spiropyran-based switches

We present a joint theory-experiment study investigating the excitonic absorption of spiropyran-functionalized carbon nanotubes. The functionalization is promising for engineering switches on a molecular level, since spiropyrans can be reversibly switched between two different conformations, inducing a distinguishable and measurable change of optical transition energies in the substrate nanotube. Here, we address the question of whether an optical read-out of such a molecular switch is possible. Combining density matrix and density functional theory, we first calculate the excitonic absorption of pristine and functionalized nanotubes. Depending on the switching state of the attached molecule, we observe a red-shift of transition energies by about 15?meV due to the coupling of excitons with the molecular dipole moment. Then we perform experiments measuring the absorption spectrum of functionalized carbon nanotubes for both conformations of the spiropyran molecule. We find good qualitative agreement between the theoretically predicted and experimentally measured red-shift, confirming the possibility for an optical read-out of the nanotube-based molecular switch.     

Selective interaction between nanotubes and perylene‐based surfactant

We recently reported a significant deviation in the photoluminescence intensities of HiPco nanotubes solubilized with a perylene‐based surfactant, C16 , compared to bile salt surfactants. For C16 , the photoluminescence emission of chiralities (9,5), (10,3), and (11,1) is enhanced by up to 430%. Resonant Raman spectroscopy is less sensitive to bundling state and also yields disparate chirality intensity ratios: The (11,0) zigzag tube signal increases by 100% compared to (8,6), (9,4), and (10,2) after suspension with C16 . We also report the change in photoluminescence intensity distribution after the swelling of bile salt suspended nanotubes with a series of small organic chromophores in DCM. These changes are attributed to chromophore‐induced surfactant reorganization, resulting in better nanotube individualization. The chirality for which the PL intensity is most enhanced is (11,1) for all chromophores, a chirality also increased by 300% in the C16 ‐solubilized sample. Conjointly, these measurements indicate that both preferential solubilization and enhanced debundling contribute to the PL intensity distribution in the C16 ‐solubilized nanotube sample. This has wide ranging implications for the design of chirally selective surfactants and the demonstration of their selectivity, which is typically only shown through luminescence measurements. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Tuning the interaction between carbon nanotubes and dipole switches: the influence of the change of the nanotube-spiropyran distance

The non-covalent functionalization of carbon nanotubes with spiropyran molecules, serving as optically addressable dipole switches, is reported. Two kinds of pyrene-spiropyran-based dyads with spacers of different lengths resulting in different switch-tube distances were investigated. While both surfactants were able to form stable carbon nanotube suspensions, the different distances between the switch and the tube affected both of the components' optical properties. In the case of the shorter spacer, the nanotubes' luminescence as well as the merocyanine absorption band were red-shifted and furthermore, the rate of the merocyanine?→?spiropyran thermal back isomerization was decreased.     

Microenvironment-switchable singlet oxygen generation by axially-coordinated hydrophilic ruthenium phthalocyanine dendrimers

A series of new metallodendrimers built around a ruthenium phthalocyanine core has been prepared. Employing a convergent synthetic strategy, pyridine-containing ligands were prepared and then assembled onto the ruthenium phthalocyanine through axial ligand coordination. The growing shell of oligoethylene glycol chains surrounding the lipophilic core allows solubilisation in water. Photophysical studies show that all the metallodendrimers are strongly phosphorescent and the deactivation pathway of their triplet state depends on the medium in which the compounds are dissolved. On one hand, quenching of the triplet state by the dendritic shell is observed and found to be substantially enhanced in aqueous media. On the other, the dendrimer shields the phthalocyanine from oxygen. This notwithstanding, the phthalocyanines are able to generate singlet oxygen in less polar environments such as in CHCl(3) or THF solution, while in water the generation of singlet oxygen is almost completely switched off.