Glass ceramics of composition 95SiO2-5SnO2 doped with 0.4mol% Eu3+ have been prepared by thermal treatment of sol-gel glasses. The segregated SnO2 nanocrystals present a mean size comparable to the bulk exciton Bohr radius (about 2.4nm), corresponding to a wide band-gap quantum-dot system in an insulator SiO2 glass. A fraction of the Eu3+ ions is incorporated to the SnO2 nanocrystals in the process. In these strong confinement conditions, the energy gap presents a high dependence on the nanocrystal size. Taking advantage of this effect, it has been possible to excite selectively the Eu3+ ions located in the SnO2 nanocrystals, by energy transfer from the host, obtaining emission spectra that depend on the nanocrystal size. The Eu3+ ions environment in small nanocrystals (radius under 2nm) are very distorted, meanwhile they are like crystalline for nanocrystals with a radius of some nanometers.

Nanostructured silica based ceramic samples of composition (100 − x)SiO 2 -xSnO 2 , doped with 0.4 mol% of Eu 3+ and with x from 1 to 10, have been developed after a thermal treatment of precursor sol-gel glasses. A structural analysis has been performed by x-ray diffraction and high resolution transmission electron microscopy. The mean radius of the obtained SnO 2 nanocrystals, ranging from 2 to 5 nm, is comparable to the exciton Bohr radius, corresponding to wide bandgap semiconductor quantum dots in an insulator SiO 2 glass. A spectroscopy study in terms of emission and excitation spectra has been carried out as a function of SnO 2 concentration. Moreover, time-resolved fluorescence measurements have also been performed in order to discern the emission of ions in glassy and nanocrystalline environments. The nanocrystal sizes have been obtained and compared by using the Brus and Scherrer equations.

The time-dependent thermal conversion of vacuum residue deasphalted oil was studied at 280, 320, 360, and 400 °C. The vacuum residue deasphalted oil was an industrial product produced by vacuum distillation of Athabasca bitumen followed by solvent deasphalting using n-pentane. This type of visbreaking process was of interest for partial upgrading of bitumen to facilitate pipeline transport. Practically useful cracking conversion and viscosity reduction for upgrading were found only at 360 and 400 °C. The viscosity measured at 40 °C could be reduced by 3 orders of magnitude from 3720 Pa s in the feed to 2–5 Pa s in the product. The density of the product was not reduced by much, despite vacuum residue cracking conversions of 34% at 360 °C and 45–47% at 400 °C before the onset of coking. The liquid yield was 88–89%. A heavier product fraction was formed during thermal conversion. The heavy material was not necessarily asphaltenes, but an increase in n-pentane-insoluble material was also found that appeared correlated with the aromatic hydrogen content of the product. The limited change in density was at least partly explained by the increase in both heavy material and aromatic nature of the product. Vacuum residue conversion at 360 °C increased linearly with time, which indicated zero-order kinetics. Vacuum residue conversion at 400 °C was non-zero-order. This study showed that thermal cracking at 360–400 °C is better described by a rate equation with two terms. Temperature-dependent differences in the maximum conversion before the onset of coking and the kinetic description of vacuum residue conversion indicated that the equivalent residence time description of visbreaking was an inadequate approximation of thermal conversion at 400 °C and below.

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Transparent glass‐ceramics with composition of 95SiO2–5LaF3 doped with 0.1 mol% of Ho3+ or co‐doped with 0.1 mol% of Ho3+ and 0.3 mol% of Yb3+ were synthesized by thermal treatment of precursor sol–gel glasses. Segregation of LaF3 nanocrystals in the matrix was confirmed from a structural analysis by means of X‐ray diffraction. Blue, green and red efficient up‐conversion emissions were observed under 980 nm excitation at room temperature. These results could be attributed to the partition of a fraction of Ho3+ ions into the precipitated LaF3 nanocrystals. Moreover, near infrared down‐conversion at 1.2 μm is also observed. The mechanisms involved in the up‐conversion emissions could be ascribed to a two‐ and three‐ photon process. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)