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).     

Sintering of silica gel derived from the alkoxysilane solution containing N,N-dimethylformamide

The sintering of three kinds of dried gels with different microstructures prepared from the tetramethoxysilane (TMOS) solutions containing N,N-dimethylformamide (DMF) with varying [H₂O]/[TMOS] mole ratios and different catalysts was investigated by measuring the pore size distribution, the Vickers hardness, the linear shrinkage and the apparent density of the gels heated to various temperatures. The marked shrinkage of the pores started at temperatures above 900 °C. The temperature required for the conversion of a gel to a pore-free silica glass depended on the average size of the pores contained in the dried gels; the average pore sizes of about 50 Å, 75 Å and 100 Å corresponded to 1040 °C, 1060°C and 1080°C, respectively. The Vickers hardness, the linear shrinkage and the density of the gel showed a corresponding change with a change in heating temperature.     

DC conductivity and secondary structural relaxations in high-conducting Li glasses

In the course of investigating the mechanism of conductivity of fast-conducting, multicomponent lithium fluoroborate-type glasses, an isothermal drift in the conductivities has been found at temperatures so far below the normal glass transition ( 300°) that an origin in normal glass annealing effects appears improbable. Analysis of the 1st order kinetics of this relaxation in the temperature range 150–250°C yields an activation energy more than an order of magnitude less than that of the primary structural relaxation, suggesting the processunder observation is a secondary structural relaxation of the type frequently seen in polymer glasses.     

Surface morphology and lattice distortion of heteroepitaxial GaInP on InP

We examine the relation between surface morphology and lattice distortions of a number of Ga_xIn_1-xP epilayers (x ≈ 0.04) grown on InP (001) substrates using atomic force microscopy (AFM) and high resolution X-ray diffractometry (HRXRD). The heteroepitaxial layers (thickness ≈ μm) were grown by low pressure metalorganic vapour phase epitaxy (MOVPE) at temperatures from 600 to 660°C. AFM images over a scale of 75 μm by 75 μm show no surface corrugations for samples grown at 640°C. For other growth temperatures between 600 and 660°C, unidirectional corrugations aligned along the [1 0] direction or a cross-hatched pattern can be seen on the sample surfaces with a typical interline separation of 10 μm. Another feature revealed by AFM is the presence, for some samples, of weak parallel corrugations along the direction making a 54° angle with the [1 0] direction. HRXRD spectra recorded first with the [110] and then with the [1 0] direction in the plane of incidence reveal an asymmetric relaxation in the sample plane accompanied by a broadening of the epilayer X-ray peak for diffraction with the [110] direction in the plane of incidence. The lattice distortion changes from tetragonal to orthorhombic as the surface corrugations appear along the [1 0] direction. Proton microprobe scans indicate that these samples are compositionally uniform so that the asymmetry is not the result of lateral variations of the alloy composition. Finally, low temperature photoluminescence from these samples shows good agreement between observed and calculated peak positions on the basis of the strains obtained from HRXRD measurements.     

Analysis of precursor residues in lithium aluminosilicate gels using XPS and RGA

Lithium aluminosilicate gels of composition 15 mol% lithia-2 mol% alumina-83 mol% silica were prepared by adding nitrates to tetraethyl-orthosilicate (TEOS) and going through the sol-gel process. Samples were prepared in thin film and bulk form. Dried and outgassed samples were studied with X-ray photoelectron spectroscopy (XPS) d residual gas analysis (RGA). XPS spectra show similar species in both thin film and bulk samples, but different relative quantities of each species. Some oxidation of organic groups by nitrate is evident in bulk samples which were heated to 70°C during the drying process. In both thin film and bulk samples, the O 1s spectra indicate oxygens associated with a silicate network and higher binding energy species such as - -O-. The C 1s spectra of bulk gels heated to 140° and 350°C in vacuum show some organics are vaporized at low temperature, while the oxidized-organic residues are decomposed to CO₂ at high temperature. These species are also observed with residual gas analysis at the corresponding temperatures.     

Interfacial reactions between tellurite melts and silica during the production of microstructured optical devices

Interfacial reactions between silica glass and tellurite melts were studied under confined conditions in the temperature regime of 400-700 °C, applying two different sampling techniques: isothermal heat-treatment of a several micrometer thick tellurite film, confined in a silica/tellurite/silica sandwich, and capillary filling of tellurite melts into silica microcapillaries. The sandwich technique provides detailed ex situ insights on the interface chemistry, microstructure and diffusion after given treatment times and temperatures. Data on dynamic viscosity, surface tension, wetting behaviour and eventual scaling effects was obtained from the capillary filling technique. For temperatures > 500 °C, silica is completely wet by the considered tellurite melts. At T > 600 °C and for a treatment time of 20 min or longer, cationic diffusion of Na⁺ and Te⁴⁺ into the silica substrate occurs to a depth of several micrometers. At the same time, the tellurite melt attacks the silica surface, leading to the formation of a stationary silica-tellurite reaction layer and silica dissolution. Dissolved silica was observed to re-precipitate from the tellurite melt by liquid-liquid phase separation. In the early reaction stages, as a result of alkali diffusion into the silica substrate, β-quartz crystallizes at the interface (what can be avoided by using alkali-free filling glasses). Obtained data set the boundary conditions for the generation of tellurite-silica all-solid fiber waveguides by melt infiltration of silica photonic crystal fibers or microcapillaries.     

Annealing effects of highly homogeneous a-Si1−xCx:H

We report the effect of annealing on the properties of amorphous hydrogenated silicon carbide thin films. The samples were deposited onto different substrates by plasma enhanced chemical vapor deposition at temperatures between 300 and 350 °C. The gaseous mixture was formed by silane and methane, at the ‘silane starving plasma regime’, and diluted with hydrogen. Rutherford backscattering and Fourier transform infrared spectrometry were used, respectively, to determine the atomic composition and chemical bonds of the samples. The film’s structure was analyzed by means of X-ray absorption fine structure and X-ray diffraction. For temperatures higher than 600 °C, amorphous silicon carbide films annealed under inert atmosphere (Ar or N₂) clearly changed their structural and compositional properties due to carbon loss and oxidation, caused by the presence of some oxygen in the annealing system. At 1000 °C, crystallization of the films becomes evident but only stoichiometric films deposited on single crystalline Si[1 0 0] substrates presented epitaxial formation of SiC crystals, showing that the crystallization process is substrate dependent. Films annealed in high-vacuum also changed their structural properties for annealing temperatures higher than 600 °C, but no traces of oxidation were observed or variations in their silicon or carbon content. At 1200 °C the stoichiometric films are fully polycrystalline, showing the existence of only a SiC phase. The XANES signal of samples deposited onto different substrates and annealed under high-vacuum also show that crystallization is highly substrate dependent.     

Simulation of surface structural relaxation kinetics in silica glass accelerated by water vapor

It is known that surface structural relaxation of silica glass takes place more rapidly than bulk structural relaxation, especially in the presence of water vapor. The effect of water vapor pressure, heat-treatment temperature and initial fictive temperature on the surface structural relaxation kinetics in silica glasses was investigated by measuring the change of the surface fictive temperature determined from the IR reflection peak shift of silica structural bands. The superimposed component of bulk structural relaxation was subtracted from the measured surface structural relaxation data to isolate the true surface structural relaxation kinetics. The obtained surface structural relaxation data as a function of fictive temperature, heating temperature and water vapor pressure were simulated with a model based on the diffusion equation with time-dependent surface concentration. The simulation model was used to predict the surface structural relaxation kinetics of the optical fiber having a high fictive temperature of ～ 1650 °C at 950 °C under 355 torr of water vapor, and it was confirmed that the present model can simulate surface structural relaxation of the fiber reasonably well.     

Size effect on surface structural relaxation kinetics of silica glass sample

It is known that surface structural relaxation takes place more rapidly than bulk structural relaxation, especially in the presence of water vapor. The surface structural relaxation kinetics of the silica glass fiber and plates was compared at 950 °C and the surface structural relaxation kinetics of silica glass fiber was found to be faster than that of the silica glass plate, even though the composition and initial fictive temperatures of the samples were the same. The observed difference of the surface structural relaxation kinetics between silica glass fiber and silica glass plate can be accounted for using a diffusion equation with time-dependent surface concentration. The analysis indicates that there is a general size effect on the surface structural relaxation kinetics, with smaller sized samples exhibiting faster relaxation kinetics.     

Preparation of silica and soda-silica glasses by the sol-gel process

Samples of silica glass and soda-silica glasses (containing 1 wt% and 13 wt% Na₂O) were prepared by hydrolysis of alkoxides to form gels, followed by heating the gels at low temperatures, below the glass transformation. The water necessary for hydrolysis was either taken from the atmosphere or added to the starting solution to accelerate the process. In the latter case the gelation time exhibited a minimum for a certain quantity of water added. The heat treatment process to form a glass from a gel was studied by DTA and TGA. At low temperatures (below 100°C) an endothermic DTA peak was observed due to the loss of organic volatiles and water. A large exothermic peak was also found at 276°C for the silica and at 365–395°C for the soda-silica gels. At higher temperatures the 13Na₂O87SiO₂ (wt%) composition gave a DTA trace very similar to the same composition conventionally melted which had been rapidly quenched from well above the transformation range (i.e. with a high fictive temperature). The preparation of bulk samples of the glasses is discussed.     

A study of Raman spectroscopy and low-temperature specific heat in gel-synthesized amorphous silica

A comparative study of low-temperature specific heat (1.5–25 K), C_p, and low-frequency Raman scattering (<150 cm⁻¹) has been performed in amorphous silica samples synthesized by sol–gel method (xerogels) and thermally densified in a range of densities, from ρ=1250 kgm⁻³ to ρ=2100 kgm⁻³, close to the density of the melt quenched vitreous silica (v-SiO₂). The present analysis concerns the application of the low-energy vibrational dynamics as an appropriate tool for monitoring the progressive thermal densification of silica gels. By comparison with v-SiO₂, the Raman and thermal properties of xerogels with increasing thermal treatment temperature revealed the following important results: (i) the existence of a critical treatment temperature at about 870°C, where a homogeneous viscous sintering produces full densification of the samples. This effect is detected by the observations of the Boson peak in Raman spectra at about 45 cm⁻¹ and of a peak in C_p(T)/T³, very close to those observed in v-SiO₂; (ii) in silica xerogels treated at temperatures less than about 800°C, the low-frequency Raman scattering is greater, with a continuous decreasing unstructured shape, and the Boson peak is not detected in the spectra.     

The structural evolution of alkoxy-derived gel during drying

The effects of drying temperature on the structural evolution of alkoxy-derived silica gel prepared using various catalysts have been investigated. The dependence of specific surface area, S_g, reflecting the structure, on the temperature of drying was remarkable for a non-catalyzed xerogel. The effect of drying temperature on the S_g of an ammonia-catalyzed xerogel was also found but was not very large. The S_g of xerogels obtained by drying at 60°C was always higher by 50% than the gels dried at 30°C without regard to the aging temperature. The S_g of xerogels from HCl-catalyzed solution was of the order of several m²/g, however, the S_g of the aerogel obtained by hypercritical drying of the wet gel from a similar solution was about 800 m²/g. These phenomena were understood on the basis of SAXS measurements on both wet gels and aerogels.     

Hydrolytic resistance of Na2O---B2O3---SiO2 gels prepared by sol-gel process

Alkoxide derived gels were prepared in the system Na₂O---B₂O₃---SiO₂. The gel compositions were situated in the liquid-liquid immiscibility area of the phase equilibrium diagram.

Hydrolytic resistance tests were performed on the gels heat-treated at temperatures ranging between 120 to 850 °C. The Na₂O, B₂O₃ and SiO₂ extracted from the attack gels were analyzed. The experimental results indicate that the amount of B₂O₃ has a significant influence on the chemical durability of the heat-treated gels. At temperatures of 850 °C the greater the B₂O₃ mol% the greater are the amounts of Na₂O and B₂O₃ extracted. Different behaviour was observed for gels heat-treated at 600 °C where the amounts of B₂O₃ and Na₂O extracted slightly increases as the B₂O₃ mol% increases. Small amounts of extracted SiO₂ were always observed.

These results are complemented with other measurements so that an explanation of the controlling mechanism is given.     

Colloidal sol/gel processing of ultra-low expansion TiO2/SiO2 glasses

A series of titania-silica glasses with 0–9% TiO₂ were fabricated using a sol/gel process. The sol was prepared by dispersing colloidal silica fume in an aqueous solution of titania which was synthesized through the acid-catalyzed hydrolysis of titanium isopropoxide. The sols gelled in 2–4 days, and then were dried for 6–8 days. The dry gels were sintered at 1450–1500°C to produce clear, dense, microstructure-free glasses. The gels underwent a total shrinkage of 50% to yield glass rods about 50 mm long and 5 mm in diameter, or glass discs about 4 cm in diameter and 5 mm thick. The drying step was most critical in the production of crack-free specimens.

In the gel, the transmission electron microscope (TEM) revealed the presence of 1–5 nm rutile microcrystallites uniformly distributed within a network of colloidal silica particles. After sintering to 1450–1500°C, though, a dense, transparent, microstructure-free glass was created. Fourier transform infrared spectroscopy (FTIR) verified the formation of an amorphous solid-solution of titania and silica after sintering.

The thermal expansion of the glasses was measured using a differential dilatometer. The average linear coefficients of thermal expansion (CTE @ 25–675°C) varied between +5 × 10⁻⁷ and −0.2 × 10⁻⁷°C⁻¹ in the range 0 to 9% TiO₂. The glass with 7.2% TiO₂ exhibited a zero thermal expansion coefficient at 150–210°C. The hysteresis in CTE on heating and cooling was of the order of 0.01–0.02 ppm.     

Surface analysis of different oriented GaAs substrates annealed under bismuth flow

Several orientations of GaAs substrates, including (1 0 0), (4 1 1), (1 1 1) and (5 1 1) have been annealed in a metalorganic vapour phase epitaxy (MOVPE) horizontal reactor at different annealing temperatures and under different trimethyl-bismuth (TMBi) flux. Surface morphology of the annealed GaAs substrates was investigated by means of scanning electron microscopy (SEM) and atomic force microscopy (AFM). The results show islands formation on all the studied samples. The density and size of Bi islands vary greatly with annealing temperature and TMBi flow. For different substrate orientations, the activation energies were deduced from Arrhenius plot of island density. Except for (5 1 1) oriented GaAs, all the studied orientations show the same activation energy of 1.8 eV. For low annealing temperature 420 °C, and under different Bi flux, each oriented substrate shows a specific behaviour. For higher temperatures 700 °C and above Bi islands are totally removed and the substrates are smooth. Surface change of (1 0 0) oriented GaAs substrate was in situ monitored by laser reflectometry.     

Structural transformation of the TiO2---SiO2 system gel during heat-treatment

The gel formation of the (100-x)TiO₂·xSiO₂ (x = 0–10 mol%) system has been studied. The progressive elimination of residues was followed by DTA and TGA curves. DTA curves showed that the formation of anatase during heat treatment could be sensibly slowed down with the increase of SiO₂. The relationship between the gel composition and crystallization temperature of anatase has been systematically investigated. The X-ray diffraction spectra demonstrated that the crystallization temperature of anatase is 400°C for TiO₂ gel and 430°C for 90TiO₂ - 10SiO₂ gel. The infrared absorption spectra were used to follow the structural transformation of gels heat-treated at different temperatures. With the help of EPR it is evident that titanium ions exist only in tetravalence.     

Infrared and thermogravimetric study of high pressure consolidation in alkoxide silica gel powders

High density, transparent, crack-free and hard compacts of silica gel were produced by compaction under nearly hydrostatic environment at 4.5 GPa, at room temperature. The starting material was used three days after synthesis by hydrolysis of alkoxides without additional treatment. Fourier transform infrared spectroscopy (FTIR), using the KBr technique and a high vacuum cell at temperatures up to 450°C, and thermogravimetric analysis (TGA) up to 900°C was conducted. A reduction up to 60% in the adsorbed water content of the compacted silica gel was observed. Changes in the 3000 to 3800 cm⁻¹ region indicate that the surface silanol groups became bridged, which promotes condensation reactions of dehydroxylation. Those results suggest that the mechanism for consolidation under high-pressure, is ‘cold sintering’ process, where silanol groups at the surface of the nanoparticles condense to form siloxane bonds between the particles and water, resulting in a stiff body with closed nanopores containing trapped water.     

Effect of the inclusion of a water soluble high molecular weight oligomer on the surface characteristics of silica gel

The surface characteristics of pure and polyglycol 4000 containing silica gels were investigated by nitrogen adsorption at ? 195°C and water adsorption at 35°C. The addition of polyglycol 4000 during the precipitation of silica gel led to a remarkable increase in the surface area as well as the pore volume and on the other hand a sharp drop in c-BET constant.Thermal treatment led to a variation in the surface characteristics of both pure and polyglycol containing silica gels, but in different ways. Inclusion of polyglycol 4000 led to a retardation of the dehydroxylation process that occurs at elevated temperature. The decrease in surface area at 400°C was much higher in polyglycol 4000 containing silica gel than in the pure simple, an effect which correlated with shrinkage of the polymer at high temperature. Slight increase in surface area at 500°C may be attributed to the phase change from β to α form. For the high content polyglycol sample the phase change effect was counteracted by the shrinkage of the polyglycol.Water vapor adsoprtion indicated that polyglycol 4000 containing silica gel was more lyophobic in character than the pure gel at all temperatures except at 200°C.     

Rayleigh scattering in silica glass with heat treatment

We investigated Rayleigh scattering in silica glass with heat treatment under various conditions, including quenching and slow cooling, and its relation to fictive temperature. The Rayleigh scattering intensity varied according to the conditions of the heat treatment. The scattering intensity in slowly cooled samples is less than that in quenched samples after heating at the same temperature. We evaluated fictive temperatures based on measurements of infrared absorption and Raman scattering. The Rayleigh scattering intensity was related to the fictive temperature regardless of the heat treatment conditions, and a linear relation between them was obtained. In addition, we suggest that the decrease in scattering intensity in slowly cooled samples results from structural relaxation due to viscous flow during the cooling process.     

ESR and Mössbauer studies of dispersed states of Fe ions in silica gels

0.01Fe₂O₃---0.99SiO₂ gels were prepared by three different sol-gel methods and the effects of the heat-treatment temperature of the gels on the ESR and Mössbauer parameters were investigated to obtain information about the change in the state of iron ions in the gel. All the gels were found to be amorphous under 600 °C, and among these three methods, the best dispersed state of Fe³⁺ ions was obtained by the method where TEOS was partially hydrolyzed with the same mole content of water before the addition of Fe(OEt)₃. It was indicated that the reaction at the sol-stage plays an important role in the degree of dispersal of Fe³⁺ ions in this system.