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We report an experimental investigation on the sintering process induced in fumed silica powders by isochronal thermal treatments at T=1270 K. Three types of fumed silica are considered, consisting of amorphous SiO2 (a-SiO2) particles with mean diameters 7, 14, and 40 nm. The study is performed by atomic force microscopy (AFM), to follow the morphological changes, and by Raman scattering, to obtain information on the concomitant structural modifications. The former method indicates that the sintering process proceeds by aggregation of single particles into larger grains, whose sizes increase with the thermal treatment duration. Furthermore, for each fumed silica type considered, the quantitative analysis of the AFM images shows that the grain growth process takes place approximately at constant rate for thermal treatment durations up to 290 h. Nevertheless, the value of the grain growth rate is sensitive to the system properties. In fact, it is found to increase with decreasing the particle mean diameter, giving a strong quantitative evidence of the size-dependence of the sintering process. On the other hand, Raman measurements indicate that the structure of the as-received fumed silica nanoparticles is significantly modified with respect to that of ordinary bulk a-SiO2, in agreement with previous experimental evidences. However, it rapidly relaxes upon thermal treatment at T=1270 K and its characterizing features are almost completely lost after treatment for about 80 h. Finally, the comparison of AFM and Raman data shows that the nanoparticles structure are completely relaxed (resembling that of bulk a-SiO2) when the grains formed by thermal treatment reach diameters greater than about 43 nm, indicating that it represents the characteristic size above which the effects of spatial confinement on the structure of the material become almost negligible. [ABSTRACT FROM AUTHOR]

We report an experimental investigation by electron paramagnetic resonance of the doublet of lines split by ∼1.3 mT and centered on the E′γ center resonance line in the spectrum of irradiated amorphous SiO2. Commercial and sol-gel materials, some of which subjected to hydrogen-deuterium exchange, were investigated. Exposure to γ or β rays at room temperature of the samples and subsequent thermal treatments were carried out to induce the defects and to study their thermal stability. In all the materials used the ratio between the signal of the E′γ centers and that of the 1.3 mT doublet is constant and independent of the OH and OD contents. Furthermore, the 1.3 mT doublet and the E′γ center feature similar thermal stability. These results support the attribution of the 1.3 mT doublet to the hyperfine interaction between the unpaired electron magnetic moment of the E′γ center and the nuclear magnetic moment of a second near neighboring 29Si atom. Our results also suggest that the E′γ site needs an appropriate surrounding of 29Si in back-bond configuration to experience this hyperfine interaction. [ABSTRACT FROM AUTHOR]

We report an experimental investigation by electron paramagnetic resonance of the doublet of lines split by ∼1.3 mT and centered on the E′γ center resonance line in the spectrum of irradiated amorphous SiO2. Commercial and sol-gel materials, some of which subjected to hydrogen-deuterium exchange, were investigated. Exposure to γ or β rays at room temperature of the samples and subsequent thermal treatments were carried out to induce the defects and to study their thermal stability. In all the materials used the ratio between the signal of the E′γ centers and that of the 1.3 mT doublet is constant and independent of the OH and OD contents. Furthermore, the 1.3 mT doublet and the E′γ center feature similar thermal stability. These results support the attribution of the 1.3 mT doublet to the hyperfine interaction between the unpaired electron magnetic moment of the E′γ center and the nuclear magnetic moment of a second near neighboring 29Si atom. Our results also suggest that the E′γ site needs an appropriate surrounding of 29Si in back-bond configuration to experience this hyperfine interaction. [ABSTRACT FROM AUTHOR]

The sintering process induced in fumed silica powders by isochronal thermal treatments is examined. The comparison of atomic force microscopy (AFM) and Raman data has shown that the nanoparticles structure are relaxed when the grains formed by thermal treatment reach diameters greater than 43 nm, which has represented the size above which the effects of spatial confinement on the structure of the material has become almost negligible.

Electron paramagnetic resonance of the doublet of lines split by 1.3 mT and centered on the [E.sub.[gamma].sup.t] center resonance line in the spectrum of irradiated amorphous Si[O.sub.2] is examined. The results have shown that the [E.sub.[gamma].sup.t] site requires an appropriate surrounding of [super 29]Si in back-bond configuration to experience the hyperfine interaction.