Densification Of Silica Research Articles

Water‐assisted sintering of silica: Densification mechanisms and their possible implications in biomineralization

AbstractTaking silica as an exemplary material system, we studied water‐assisted densification behaviors of different crystallinities (quartz, glass, and vitreous silica). To avoid the complexity in data interpretation, we adopted a simple procedure similar to those used for pressing salt pellets for IR: compressing silica powders in a mold with pure water under ambient conditions. It is discovered that crystalline silica is compacted through liquid lubrication, while amorphous silica's densification behaviors contradict the widely regarded dissolution‐reprecipitation mechanism. Another mechanism is thus proposed: stress‐driven water incorporation into the solid structures produces hydrated silica of considerable plasticity for deformation and fusion. Inspired by this water‐assisted mechanism, a more effective sintering method is developed via repetitive stressing/destressing treatments at room temperature, enabling dramatically boosted densities (e.g., over 90% with transparent appearance for silica glass) and enhanced mechanical performance. This generic strategy may apply to a wide range of materials. Furthermore, the hydration‐enabled deformation/sintering mechanism proposed in this work offers fresh insights into the biomineralization puzzles, particularly those on how life accomplishes some of the most challenging tasks faced by humans in modern ceramic technology, for example, to fuse, mend or reshape the rigid brittle ceramic objects in aqueous environments under ambient conditions. This purely inorganic biomineralization mechanism may be particularly important for life at its early stage of evolution on earth.

Permanent densification of compressed silica glass: a Raman-density calibration curve

Raman scattering experiments have been carried out to study persistent densification in SiO2 glass following hydrostatic compression at room temperature. A new relationship linking selective Raman parameters to the degree of densification in the glass has been developed here. This approach will allow quantification of the residual densification in silica following microindentation experiments, with the goal being the development of a constitutive law for amorphous silica.

In situ growth of silver nanoparticles in mesoporous silica by spray pyrolysis

Silver-dispersed mesoporous silica was in situ synthesized in aerosol phase. The solidification of silica was catalyzed in the presence of the silver, which increased the order and d values of the mesophase at given reactor temperature. Silver nanoparticles grew confined in the pores when the atomic % of silver to Si was below 10 and the pore wall turned to be impermeable above 400 °C. Silver permeated through the pore wall below 400 °C to grow freely in the carrier gas. The mesophase deteriorated using spray pyrolysis above 800 °C due to the further densification of silica, or above 10 at.% of silver due to the excessive growth within the phase. The highest dispersion of silver ~4 nm in diameter with the highest order of the mesoporosity was obtained at 600 °C with 5 at.% silver. Calcination following the spray pyrolysis further densified the silica phase to freeze the growth of silver particles as well as lower the d value of the mesophase.

Water vapor corrosion behavior of porous silicon carbide membrane support

High-temperature water vapor corrosion of porous silicon carbide with and without additive alumina and its microstructural feature were investigated to examine the corrosion resistance of porous membrane supports for hydrogen production by steam modification of methane. Corrosion test was performed under similar condition as hydrogen production reaction occurred at 600 and 1000 °C, 4 atm (0.4 MPa) and 3/1 = H 2O/N 2 where nitrogen gas was substituted for methane. In the corrosion at 600 °C, the alumina-doped support showed weight gain of 1.3 mg/cm 2, while the undoped support showed weight gain of 0.7 mg/cm 2. In the alumina-doped support, pore growth was observed because of the coalescence among oxidized fine particles. In contrast, the pore size of the supports without alumina was slightly reduced, due to thin silica layer formed on the SiC particle. In the corrosion at 1000 °C, the almost complete conversion to silica and the densification of silica were found. The densification of silica was due to the viscous flow sintering of silica under water vapor.

Transformation pathways of silica under high pressure

Network-forming oxides with rigid polyhedral building blocks often possess significant capacity for densification under pressure owing to their open structures. The high-pressure behaviour of these oxides is key to the mechanical properties of engineering materials and geological processes in the Earth's interior. Concurrent molecular-dynamics simulations and first-principles calculations reveal that this densification follows a ubiquitous two-stage mechanism. First, a compact high-symmetry anion sublattice forms, as controlled by strong repulsion between the large oxygen anions, and second, cations redistribute onto the newly created interstices. The same mechanism is observed for two different polymorphs of silica, and in the particular case of cristobalite, is corroborated by the experimental finding of a previously unidentified metastable phase. Our simulations not only clarify the nature of this phase, but also identify its occurrence as key evidence in support of this densification mechanism.

Direct measurement of 248- and 193-nm excimer-induced densification in silica–germania waveguide blanks: reply to comment

The criticisms raised by Poumellec and Niay in their Comment on our paper are primarily based on their misreading the original paper. We respond to their concerns and reiterate that Bragg gratings are not formed primarily by compaction from 248-nm exposure. New calculations are presented that corroborate our previous analysis.

Direct measurement of 248- and 193-nm excimer-induced densification in silica–germania waveguide blanks: comment

We argue that the conclusion of Borrelli [J. Opt. Soc. Am. B16, 1672–1679 (1999)] in favor of a negligible contribution of the UV-induced densification to the refractive index change raises questions for the following reasons: (i) The use of a relation between index change and inelastic strains is inadequate to the problem of UV exposure, (ii) the authors claim to describe the process in fiber but forget to take into account the mechanical reaction of the cladding, and (iii) the authors transfer results obtained in oxidized bulk samples to reduced fiber glass.

Direct measurement of 248- and 193-nm excimer-induced densification in silica–germania waveguide blanks

The densification of a waveguide blank of composition 14GeO2:86SiO2 produced by 193- and 248-nm excimer exposure was measured by an interferometric technique. With the aid of a finite-element model, we derive the unconstrained densification from the experimentally determined optical phase shift. The densification versus exposure can be fitted to a power law and is compared with the behavior of fused silica. Again, with the finite-element model, an estimate is made of the refractive-index change that would be produced in a single-mode fiber. For the exposure levels typically used for the 248-nm exposure, the densification contribution to the optical index change is found to be negligible for this composition. On the other hand, for the high 193-nm exposure, the densification contribution can be dominant.

Alternative modification methods for sol–gel coatings of silica, titania and silica–titania using ultraviolet irradiation and water vapor

We investigated relatively new techniques for densification and crystallization of sol–gel silica, titania and silica–titania coatings at relatively low temperatures. Irradiation of ultraviolet photons above the band-gap energy was found to lead shrinkage of the sol–gel films through electronic excitations. Crystallization of sol–gel titania coatings was achieved by irradiation of ultraviolet laser (4.7 eV) beam. Exposure to water vapor was shown to be effective in rearrangement of sol–gel films causing densification of silica, crystallization of titania and phase separation of silica–titania around 100°C. These techniques are useful for preparation of hard silica coatings and crystalline titania films on low-heat-resistant substrates, such as polymer plates.

The kinetics and mechanism of oxide layer formation from porous silicon formed on p-Si substrates

The formation of oxide layers from porous silicon layers formed on P substrates which present different and well-controlled porous textures is studied in detail. It is shown that the overall process leading to oxides with properties similar to standard thermal oxides consists of two steps: the oxidation of silicon in the porous layer and the densification of silica. The first step does not depend upon the initial texture of the layer, but is mostly determined by the porosity of the sample and the temperature of the oxidation sequence. The second step, oxide densification, which takes place at temperatures higher than 1000 °C, is very sensitive to the porous texture of the layer, the densification times increasing sharply with the average pore size. The densification process has a high activation energy, which can be related to the activation energy for the viscous flow of silica.