Microstructures and Structural Properties of Sol−Gel Silica Foams

Abstract

Silica xerogels were synthesized and annealed at 1000 degrees C for different durations to yield stable silica materials. The samples were prepared through base-catalyzed hydrolysis and condensation of tetramethyl orthosilicate in methanol. After aging and drying steps, clear and solid xerogels exhibiting a narrow pore size distribution were achieved. The annealing treatment of these xerogels was performed at 1000 degrees C and proved in the present study to lead to a monolithic glass when a progressive heat-treatment procedure was employed to attain 1000 degrees C. In addition to the expected glass, silica foams and ordered phases were observed when the samples were instantaneously heat-treated at 1000 degrees C. Raman spectra of the foamed materials exhibit the classical features of amorphous silica, whereas transmission electronic microscopy pictures reveal the presence of crystallized domains within the vitreous matrix. These crystallites are prone to nucleation and growth processes, which jeopardize the believed stability of the silica foam. The assessment of the hydroxyl content by IR spectroscopy reveals the role played by the latter polycondensation of silanols. The occurrence of foaming process was thus found to result from two competitive phenomena occurring at 1000 degrees C: evacuation of water-related species and viscous sintering.

Er photoluminescence enhancement in Ag-doped sol–gel planar waveguides

“…Note that, for such heat treatment, silica-titania waveguides may not be completely densified [17]. On the other hand, a faster densification of the sol-gel derived materials has been claimed to occur in the presence of silver [18], which may also have contributed to the PL enhancement. Extending the heat treatment from 2 to 6 min at 700°C, the PL signal ratio, between waveguides with and without silver, was reduced from $57 to $3 times only.…”

Optical properties and crystallization behaviour of some MnO and/or Fe2O3-containing silicate glasses and glass-ceramics

“…The SEM micrographs of the samples heat treated at 1000°C reveal that the silicate matrix undergo a profound structural change reminding a silica foam (Fig. 10) [20].…”

Structure determination of molecular nanocomposites by combining pair distribution function analysis and solid-state NMR

“…Given the high sensitivity of the 23 Na nuclei, it is relatively easy to deconvolute the central resonance lines in order to extract the relative ratio between the crystalline and mobile phases (Fig. 7).…”

“…7). Taking into account the 23 Na magnetisation distributed in the satellite transitions, we nd that, for the sample studied, 25% of the SNP complexes are in a 'liquid like' state. Since we know from our previous study that water molecules can easily exchange between the matrix pores and the atmosphere, we also decided to study the effect of hydration on the conned nanoparticles.…”