A SAXS study of kinetics of aggregation of silica sols

SAXS in situ experiments on the evolution of TMOS solutions during hydrolysis and polycondensation lead to power laws with scaling exponents ≈ 2. It is suggested that this could be the result of the polydispersity of the samples and that only an apparent fractal dimension can be obtained in this way. Kinetic studies tend to indicate that agglomeration in the sol is the result of a diffusion-controlled process.     

Rheology of the gelation of fluorine-doped silica sols

Dynamic oscillatory shear measurements are used to probe the gelation kinetics of aqueous sols composed of either particulate silica or silicon alkoxide solution. Unlike steady shear measurements, these dynamic tests do not alter the structure and kinetics of the sol-gel process. The dynamic storage moduli of both systems show sharp transitions at the onset of gelation. However, the gelation kinetics of the two systems are very different; the modulus of alkoxide system remains unchanged until the gel point is reached, whereas that of the particulate system increases with time even below the gel point as the colloid cluster grow. Unlike the alkoxide gel, the particulate gel reverses to a sol upon shearing. The gelation kinetics of a resheared sample is slower than that of a fresh sample. The structures associated with the sol-gel transition for this particulate system have been monitored using freeze fracture microscopy and correlate with rheological observations. The gelation times for both sols are varied by changing the fluoride ion content.     

Infiltration under isostatic pressure of porous silica glasses with silica sols

In this work a SiO₂ matrix with more than 50% porosity was developed and infiltrated with a pure silica sol under isostatic pressure, as a prior step to the immobilization of radioactive waste using this technique. The silica glass was prepared through the acidic leaching of phase-separated and partially-sintered sodium–borosilicate glass powder compacts. Phase separation was promoted at different stages of the sintering process to obtain different total porosity or pore size distributions, which in all cases showed macro, meso and micropores. Infiltration leads to a significant increase in weight, reflecting the initial porosity of the substrates. Porosimetry techniques (Hg porosimetry and N₂ adsorption isotherms) show that the silica sol fills practically all the pores with diameters over 3 nm. Preliminary sintering tests show that the infiltration technique lowers the sintering temperature by more than 150 °C.     

Comparative study on hydrophobic anti-reflective films from three kinds of methyl-modified silica sols

Methyl-modified silica sols were prepared with the polymer of methyltriethoxysilane (MTES), the polymer of dimethyldiethoxysiloxane (DDS), and hexamethydisizane (HMDS) as mono-, di-, and tri-methyl modifiers respectively. By comparing the size and the shape of clusters in three different methyl-modified silica sols, the special nature of the sol was found to be the key to the property of films. Different modal modification of methyl to silica particles led to different cluster fractal structures that influenced the morphology, the porosity of films and consequently the anti-reflection characteristics. The contribution of methyls in or on clusters directly determined the hydrophobicity. Using mono- or di-methyl-modified silica sol, the film/water contact angles were less than 120°. But it could reach 165° when using tri-methyl-modified silica sol. The lowest reflectivity of film could reach 0.0% for all the three modified sols. As a result of the comparative study, tri-methyl-modified silica sol was more suitable to prepare hydrophobic anti-reflective film with required high optical performance.     

High refractive index contrast in fused silica waveguides by tightly focused,high-repetition rate femtosecond laser

A new domain of optical waveguide writing with record high refractive index contrast (0.022) is reported in fused silica by strong focusing of a 522 nm wavelength, 500 kHz repetition rate femtosecond laser with oil-immersion optics. The strongly confining waveguide supports a mode of only 7 μm mode field diameter at 1550 nm wavelength, opening the door for higher density integration in photonic circuits formed by femtosecond lasers. It is found that green and fundamental wavelengths have similar absorption in femtosecond laser waveguide writing in fused silica and that the advantage of the second harmonic is simply from an increased fluence through a smaller focal volume.     

Fabrication of buried waveguides in planar silica films using a direct CW laser writing technique

A CW CO₂ laser ablation technique is used to form buried waveguides in planar silica films. It is shown that the refractive index of a silica thin film is reduced sufficiently adjacent to the laser processed region to allow the fabrication of low loss waveguides. The refractive index distribution of these structures is measured using the reflectance of a focussed spot from the surface of the films. The change in refractive index is measured to be of the order of the core cladding refractive index difference of typical single mode waveguides. The spatial resolution of the reflectance technique is 1.3 μm with a refractive index resolution of ±5 × 10^?4. Devices such as 1 × 2 and 1 × 4 multi-mode interference (MMI) splitters have also been demonstrated and shown to exhibit low transmission losses.     

Shrinkage behavior of silica gels during drying

The shrinkage behavior during drying has been studied for silica gels prepared from a mixture of TEOS, water and ethanol. Significant differences in the shrinkage were noted between base- and acid-catalyzed gels after that the volumes shrank to about 40% of the initial value. The volume shrinkage was also affected by the alcohol/water ratio of the pore liquid. The gel had a tendency to shrink still more with increasing water concentration in the liquid. On the other hand, the higher the alcohol content of the pore liquid, the lower the bulk density of the dried gel. SEM pictures illustrate that both base- and acid-catalyzed gels have granular structures consisting of particles of similar size initially and, when dried, the packing state of the gel particles are different, depending on catalysis for the hydrolysis-polymerization and alcohol/water ratio of the pore liquid.     

On the glass transition in vitreous silica by differential thermal analysis measurements

Scanning calorimetry measurements of the glass transition in vitreous SiO₂ (about 120 wt ppm. OH groups) are reported. Data were obtained upon heating after controlled cooling through the glass transition, and after annealing at temperatures between 990 and 1292 K. The onset of the glass transition is at 1247 K, and the supercooled liquid state is reached at 1475 K. The step in the specific heat is (2.9 ± 0.7) J mol⁻¹ K⁻¹. This value, lower than the results of drop calorimetry experiments, agrees with the calculated value from viscosity data. The glass transition is nearly twice as wide as expected from the temperature dependence of the viscosity. Annealing reduces the enthalpy of glasses as usually expected, and the corresponding entropy decrease is in agreement with results for other network glasses. In vitreous silica, depending on the annealing temperature, both exothermic and endothermic processes take place. Based on Davis’ and Tomozawa’s results, endothermic processes upon annealing are attributed to the diffusion of the OH groups.     

Generation and annealing of defects in virgin fused silica (type III) upon ArF laser irradiation: Transmission measurements and kinetic model

The transmission of ArF laser pulses in virgin fused silica (type III) samples changes during N = 10⁶ pulses at an incoming fluence H_in = 5 mJ cm⁻² pulse⁻¹. The related absorption is determined by the pulse energy absorption coefficient α(N, H_in) using a modified Beer’s law, yielding initial values α_ini around 0.005 cm⁻¹, a maximum α_max ? 0.02 cm⁻¹ at N = 10³-10⁴ and stationary values 0.0045 cm⁻¹ ? α_end ? 0.0094 cm⁻¹ after N ≈ 6 × 10⁵ pulses. The development α(N, H_in = const.) is simulated by a rate equation model assuming a pulse number dependent E′ center density E′(N). E′(N) is determined by a dynamic equilibrium between E′ center generation and annealing. Generation occurs photolytically from the precursors ODC II and unstable SiH structures upon single photon absorption and from strained SiO bonds via two-photon excitation. Annealing in the dark periods between the laser pulses is dominated by the reaction of E′ with H₂ present in the SiO₂ network. The development α(N, H_in = const.) is observed for the very first sample irradiation only (virgin state). The values α_end are not accessible by simple spectrophotometer measurements.     

Probing the erbium ion distribution in silica optical fibers with fluorescence based measurements

Accurate determination of the rare earth dopant distribution in optical fibers enhances our understanding of the fiber manufacture process and enables further improvement in the design of fiber based products such as optical fiber lasers and amplifiers. Here a simple theoretical model consisting of an ensemble of rate equation systems, characteristic of the most likely electronic transitions that take place in the vicinity of erbium (Er³⁺) doped silica glasses, is developed and solved. Through this theoretical study it is established that information about the relative Er³⁺ ion distribution in fibers can be inferred by simply monitoring the backscattered fluorescence signal originating from the de-excitation of specific energy levels in the investigated samples. Following these theoretical studies a fluorescence intensity confocal optical microscopy (FICOM) scheme was employed to investigate the Er³⁺ ion distribution profiles in a range of silica optical fibers. The validity of the proposed theoretical model was confirmed through a comparison of the Er³⁺ ion distribution profiles acquired using the FICOM technique and those obtained from the application of a powerful analytical ion probe.     

Water diffusion coefficient measurements in deposited silica coatings by the substrate curvature method

The diffusion of water in silica coatings deposited by evaporation and physical vapor deposition (sputtering) is studied using the substrate curvature measurement technique. The diffusion of water into the coatings induced a swelling, which in turn caused bending (curvature) of the silicon substrate. The curvature change was measured in situ during a humidity increase from 0% to 95% at room temperature. The diffusivity of water in the sputtered silica coating was measured to be 10 × 10^?12 cm²/s and achieved equilibrium in about 10 min. The diffusion of water in the evaporated silica coating achieved equilibrium in about 2 min. Because the coatings exhibited very short equilibration times, the impact of a non-instantaneous humidity change on the calculated diffusion coefficients was also examined.     

Rapid lifetime testing of fused silica for DUV laser applications

Lifetime testing of fused silica's absorption degradation upon 193 nm is shortened by enhancing the two-photon absorption (TPA) induced generation of E′ and NBOH defect centers per laser pulse. Increasing the irradiation fluence from typical marathon test values H < 1 mJ/cm² to H = 10 mJ/cm² gives a more efficient TPA process. In addition, the sample's temperature is lowered to ?180 °C during irradiation yielding an increased breaking efficiency of weak Si–O bonds per pulse. A small sample length of 10 mm combined with the laser induced deflection (LID) technique for direct absorption measurements prevents microchannel (MC) formation, a common break-down criterion in marathon tests.For a UV grade fused silica sample (type III) the end of absorption degradation is found after about 1.2 * 10⁷ laser pulses. Absorption measurements between 3 and 25 mJ/cm² before and after lifetime testing reveal that the laser induced absorption change decreases with decreasing fluence. The experimental results are in good agreement with a real marathon test at a fixed fluence.     

Densification kinetics of porous colloidal P2O5-doped silica gels using constant heating rate measurements

Commercially available fumed colloidal silica was gelled in an aqueous solution containing PO₄H₃. Instantaneous gelation was obtained by adding several drops of HF (48 wt%). The objective of this paper was to study the gel-to-glass conversion of these colloidal gels using constant heating rate (CHR) experiments.

Gel densification was measured at temperatures ranging from 50 to 1550°C at different heating rates (1 to 10°C/min) using a dilatometer.

Shrinkage and shrinkage rate as a function of temperature were measured and CHR equations were used to derive information on the densification mechanisms.

The experimental results show that small additions of PO₄H₃ into pure silica dispersions give rise to gels which densify to high silica glass at much lower temperatures than pure colloidal silica gels. The bloating effects produced by pure colloidal silica gels at temperatures above 1280°C were also eliminated. For these P₂O₅-doped silica gels maximum shrinkage rates were found at temperatures between 1050 and 1150°C according to the heating rate used.

The CHR analysis showed that several different mechanisms seem to operate in a complex interdependence in the whole range of temperature studied. This CHR analysis was compared with those results obtained from isothermal shrinkage experiments in the range where viscous sintering is the predominant shrinkage-controlling mechanism (between 1000 and 1100°C).     

Effects induced by 4.7 eV UV laser irradiation on pure silica core multimode optical fibers investigated by in situ optical absorption measurements

We investigated by in situ optical absorption measurements the effects induced by 4.7 eV UV laser irradiation on pure silica core optical fibers. Laser irradiation with 100 MW cm^? 2 laser intensity generates in the fiber E′ centers which partially decay after irradiation due to their reaction with diffusing H₂. An absorption band peaked at 5.3 eV is observed to grow in the post-irradiation stage with a kinetics anti-correlated to the decay of the 5.8 eV band of the E′ centers. The defect absorbing at 5.3 eV is proposed to be formed by trapping on pre-existing precursors of hydrogen atoms made available by breaking of H₂ on E′. We also show by repeated irradiation experiments that the 5.3 eV-absorbing center is photochemically destroyed by 4.7 eV laser light, and we estimate the cross section of this process. Possible structural models for this defect are discussed.     

Analysis of waveguide silica glasses using Raman microscopy

A spectroscopic method to determine dopant concentrations in silicas used in silica on silicon planar waveguides has been developed. Raman spectroscopic measurements in the range 740 cm⁻¹–1370 cm⁻¹ of cross-sections of the glass layers identified correlations between simple, rapidly calculated, spectral features related separately to each of the three dopants, boron, phosphorous and germanium, and the wt% analyses results for these dopants from inductively coupled plasma mass spectrometry (ICP-MS) measurements on fragments from the respective wafers. The calibration wafers comprised a set of monitor wafers with dopant concentrations spanning the ranges used in devices. The Raman-based analyses were able to determine boron and phosphorous wt% s in boro-phosphosilicate cladding glasses with accuracies of ≈0.1 wt% and germanium wt% s in core glasses with an accuracy of at least ≈0.3 wt% (small batch size). The method, which performed successfully in blind tests, provides a spatially resolving and rapid alternative to ICP-MS analyses of monitor wafers. Exploratory face-on measurements were performed on device wafers. Spectra of the cladding, core and underlayer were obtained from AWG samples. The effects of the confocal volume’s finite size and refractive index differences were observed. Exploratory measurements using UV Raman excitation showed potential advantages for cladding glass analyses.     

Structural evolutions of hydrothermally prepared mesostructured MCM-48 silica using differently manufactured amorphous silica powders

The hydrothermal synthesis of Si-MCM-48 mesoporous molecular sieves was carried out using a ternary SiO₂:CTAOH:H₂O system wherein differently manufactured amorphous silica powders such as fumed silica (FMDS), spray dried precipitated silica (SDPS) and flash dried precipitated silica (FDPS) were used as silica source materials. The changes in structural/textural properties were evaluated using powder XRD, N₂ adsorption-desorption and scanning electron microscopy techniques. Studies on the progressive development of MCM-48 mesophases revealed that, the reactivity of the silica source follow the trend: FMDS > SDPS > FDPS. MCM-48 synthesized using low cost FDPS has exhibited thicker pore walls but poorer orderness, while MCM-48 prepared from relatively expensive FMDS has thinner pore walls and more ordered structure. Moreover, the extent of contraction caused by calcinations, agglomerate size and structural stability were found to depend on the reactivity of the silica source used.     

Indentation crack initiation behavior of vitreous silica glasses containing different hydroxyl concentration

The indentation crack initiation behavior of eight vitreous silica specimens containing bulk OH concentrations ranging from 0.2 to 754 wt ppm was investigated. A recording microindentation instrument equipped with optical observation and acoustic emission detection was used to study, in situ, the cracking behavior from indentation with a Vickers diamond. No significant differences in the threshold loads for various types of cracking behavior of the specimens were found. In addition, the polishing medium was found to have little influence on the cracking behavior. The lengths of median-radial cracks around indentations varied little between specimens. The Vickers hardness of the specimens measured at 0.98 N ranged from 6.6 ± 0.3 GPa to 7.5 ± 0.7 GPa, and no trend with the OH concentration was apparent. In addition, the Vickers hardness of the specimens measured while under a 9.81 N maximum load (LVH_max), showed little variation, and no apparent trend with the OH concentration.     

Elastic anomalous behavior of silica glass under high-pressure: In-situ Raman study

We study the changes in the Raman optical vibrations of pure silica glass under high-pressure up to 4.3 GPa and room temperature, namely in the elastic domain. Several mechanical anomalies, as the decrease of bulk modulus between 0 and 2 GPa, have been revealed many years ago (P.W. Bridgmann, Am. J. Sci 10 (1925) 359), but no physical experiments have explained what happens at the atomic scale. Our experiments show that gradual structural reversible rearrangement from 0 to 2 GPa leads to a more flexible material, in good agreement with molecular dynamics (MD) simulations (L. Huang, J. Kieffer, Phys. Rev. B 69 (2004) 224203). Above 2 GPa, a fast homogenization occurs.     

On the limitation of quantitative measurements using transmission electron microscopy

This work discussed the limitation of (scanning) transmission electron microscopy (TEM/STEM) techniques in quantitative measurements in electron-beam-sensitive silicate glasses and glass ceramics. Electron beam induced damages in the silicate glasses containing Na and the glass ceramics containing fluorite nanocrystals were demonstrated. The damages were mainly caused by preferentially removing Na and decomposing CaF₂ into Ca. All the damage phenomena were observed under electron beam intensities, which were much weaker than the intensities used in the conventional high-resolution electron microscopy (HREM) and microanalysis in STEM. Therefore, although the advanced TEM/STEM techniques are very promising in the precise measurement of local composition at ultra-high spatial resolution in some materials, they may not be applicable to Na-containing silicate glasses and glass ceramics containing fluorite nanocrystals.     

Compaction versus expansion behavior related to the OH-content of synthetic fused silica under prolonged UV-laser irradiation

The behavior of different types of synthetic fused silica under prolonged 193 nm excimer laser irradiation at very low energy densities was investigated. Irradiation experiments at energy densities below 50 μJ/cm² for tens of billions of laser pulses were performed. Effects of changes in the density in the irradiated region of the samples were measured at 633 nm, both with an interferometer as an optical path difference and as the distribution of stress induced birefringence. Samples of synthetic fused silica of type III and IIIa with different levels of hydroxyl-content and different levels of molecular hydrogen content were manufactured. Depending on the composition of the fused silica samples, compaction as well as rarefaction was observed. The OH-content in the sample is found to be the dominating factor in the expansion-versus-compaction behavior. A qualitative model is postulated in order to describe the dependence on the OH-content. The induced optical path difference as a function of energy density and number of laser pulses is described with a phenomenological model function.