Low temperature synthesis of monolithic silica glass from the system Si(OC2H5)4---H2O---HCl---HOCH2CH2OH by the sol-gel method

To avoid breaking of the gel during drying any organic solvent such as ethanol has not been used. When the amount of HCl is increased and the mixture is stirred vigorously, a transparent silica sol can be prepared immediately with only Si(OC₂H₅)₄ and H₂O. By adjusting the amount of HCl and the temperature, the sol may gel at any time, even in 5 min at room temperature. Such a gel may be dried without cracking under completely open conditions. The addition of glycol as a complex agent can apparently increase the pore size of the gel and decrease its capillary force and surface tension. So, large dried gels can be made in a rather short time. For example, a dried gel of diameter 68 mm and thickness 13 mm can be made in only 4 days. Because of the large pores, the dried gels can be sintered to monolithic silica glasses of practical size with a rather fast heating rate. The mechanisms in the whole process have been explored.     

TiO2-SiO2 monolithic glass formation from sol-gel

This study demonstrates that hydrolysis should be carried out in a step manner in gel synthesis. The key to the increase in the amount of water added is the control of the hydrolysis rate of Ti(OC₄H₉)₄. The hydrolysis of Si(OC₂H₅)₄ can be carried out at about 75°C. The amount of added water (γ_WI), which varied with TiO₂ content (in mol%), was about 64–88% of the total amount of added water. The hydrolysis reaction should be performed at room temperature while Ti(OC₄H₉)₄ is added. The total amount of added water (γ_W) is related to the amount of solvent (R). For example, if TiO₂ is 40mol%, γ_W will vary from 3.2 to 8.0 when R varies from 0.8 to 2.0. The amount of added water was affected by the distribution of solvent in the metal alkoxides. The amount of added water can be increased when R_Si(OC2H5)4 = 1, R_Ti(OC4H9)4 > 1. The rate of rise in temperature of the thermolysis of the dried gel should be less than 10°C per hour, and the heat treatment temperatyre is related to the TiO₂ content (in mol%). Gel glasses without devitrification can only be obtained by thermolysis at 600°C from the gel with no less than 20 mol% TiO₂.     

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.     

Mechanochemical synthesis of non-crystalline ZrTiO4 precursor from inhomogeneous mixed gels

The effects of vibration ball-milling of a mixture of two gels, prepared from mixing ZrO₂ and TiO₂ sols, on crystallization processes during heating are studied in order to determine enhanced mechanochemical reactions with the aid of hydroxyl groups. Data of differential thermal analysis (DTA) and X-ray diffractometry show that TiO₂ (anatase) and ZrO₂ (tetragonal) crystallized at 698 and 712 K, respectively, when a powdered gel mixture was heated to 1023 K to 10 K min⁻¹ in air. No reactions between the two phases were detected. After vibro-milling a powdered gel mixture at room temperature for 3 to 30 h, a new exothermic DTA peak appeared around 950 K and ZrTiO₄ (orthorhombic) was detected on heating to 1023 K. After vibro-milling, the compositional micro-homogeneity of non-heated gels was estimated from the fluctuations of elemental compositions in local areas using energy dispersive spectrometer with a 25 nm electron beam. The fraction of ZrTiO₄ crystallized on heating the gels to 1023 K, determined by Raman spectroscopy, increased linearly with increasing the micro-homogeneity for non-heated gels. It is suggested that atomic movement and short-range ordering of ZrTiO₄ rather than crystallization of individual oxides occur when applying mechanical stress to a powdered mixture of ZrO₂ and TiO₂ gels.     

Sol–gel synthesis and optical properties of size controlled Indium Arsenide nanocrystals embedded in silica glasses

III–V semiconductor Indium Arsenide (InAs) nanocrystals embedded in silica glasses was synthesized by combining the sol–gel process and heat treatment in H₂ gas. The size of InAs nanocrystals can be easily controlled via changing the In and As content in the starting materials and the heating temperature in a H₂ gas atmosphere. Absorption measurements indicate a blue shift in energy with a reduction on the In and As content in the SiO₂ gel glasses as a result of quantum confinement effects. A near-infrared photoluminescence with peak at 3.40 μm was observed at 6 K under 514.5 nm Ar⁺ laser excitation from InAs nanocrystals embedded in the silica gel glasses.     

Synthesis of fluorine doped silica gel and its properties at high temperature

Fluorine doped silica gels were synthesized by using the sol-gel processes of (A) SiF₄(g) and H₂O(1) and (B) the mixed solution of Si(OC₂H₅)₄, C₂H₅OH, H₂O and H₂SiF₆. By the former process we obtained a gel of relatively high fluorine content (8–12 at.%F), while we could synthesize the gel of 0–12 at.% F by adjusting the F/Si ratio of the starting solution mixtures by the latter process.

The defluorination behavior and the structural change of these gels at high temperature were studied by heating-mass spectrometry, IR and ESR measurements. The results revealed the following: (1) defluorination by liberation of SiF₄(g) was admitted from temperatures at about 400°C and was controlled by the diffusion of fluorine in the gel bulk. (2) The peak separation analysis for the IR band of 1300-900 cm⁻¹, where the stretching vibrations of Si---O and Si---F appear, showed that the change of the band shape resulted from the increase or the decrease of the Si---F bonds and the change of the bond angle of Si---O---Si as well as the change of the force constant accompanied by fluorination or defluorination. (3) The defects of the Si E′ center were induced by X-ray irradiation depending on the degree of the defluorination, and were reduced by the heat treatment. However, with the heat treatment at temperatures higher than 1000°C, the E′ center increased again. The IR spectra suggest that this behavior might correspond to the gel-glass trasition.     

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).     

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.     

Energetics of high surface area silicas

The energetics and structure of high surface area, amorphous silicas prepared by low pressure chemical vapor deposition (LPCVD), flame hydrolysis and sol-gel were studied by high temperature transposed temperature drop calorimetry and solution calorimetry. Utilizing appropriate thermodynamic cycles, the total stored energy (measured as ‘fast’ energy release during drop experiments and as ‘slow’ energy release during solution experiments) of impurity free amorphous silicas relative to fused silica glass was determined. The ‘fast’ energy release involves the healing of point defects, reduction of surface area, release of strain, rearrangement of 2- and 3-fold rings by pore collapse or annealing of 2-fold rings (in conjunction with an appropriate concentration of 3- and 4-fold rings). The ‘slow’ energy release differences in the distribution of 3-fold and higher rings in annealed silica relative to fused silica glass.

LPCVD film silicas had been deposited at 0.4 Torr pressure by the reaction of SiH₄ and excess O₂ and 523, 643 and 703 K. The total stored energy of 22 to 44 kJ/mol is mainly due to the presence of 2- and 3-fold rings, consistent with Raman and infrared spectra of films and diffraction studies on related ‘snows’. The metastability of the LPCVD films decreases with increasing temperature of deposition due to the increased capacity to anneal metastable siloxane bonds. This trend continues to higher temperatures. An amorphous silica prepared by flame hydrolysis at 1073 K by the combustion of SiCl₄ in O₂ shows little or no stored energy and is energetically almost identical to fused silica glass.

Acid- (pH 1) and base- (pH 11) catalyzed dry silica gels were prepared by mixing TEOS : ethanol: water in molar proportion 1 : 4 : 4, then aged at 333 K for 24 h and dried at 423 K for 2–3 days. ‘Fast’ energy release accounts for most of the total stored energy of 7.3 kJ/mol for acid-catalyzed and 66.2 kJ/mol for base-catalyzed dry silica gel. It is unlikely that high concentrations of 2- and 3-fold rings percontact with the aqueous medium during the sol-gel process. Therefore, the total stored energy arises predominantly from structural relaxation and rearrangement in the base-catalyzed gel and rearrangement of surface siloxane by pore collapse during volatile loss in the acid-catalyzed gel. The creation of metastable siloxanes from the rapid condensation of monomers (present due to the high solubility of silica in the basic solution) during the drying of the base-catalyzed gel may be the source of its extremely large metastability.     

Kinetics of strain relaxation in metallic glasses

Torque relaxation and strain recovery measurements under linear heating for three Fe-, Ni- and Co-based metallic glasses are presented. Torque relaxation is determined by the pre-annealing temperature, T_a: at T < T_a, the degree of relaxation is small and high-rate torque relaxation starts near T_a. Analogously, high-rate strain recovery starts near the temperature of preliminary homogeneous deformation. The temperatures where high-rate stress relaxation and strain recovery begins are calculated in the framework of an activation energy spectrum model and are consistent with experiment. A homogeneous-flow model is developed which qualitatively explains macroscopically reversible behaviour as a result of directed irreversible microscopic relaxation events with distributed activation energies.     

SAXS investigations of structure of formation in alcoholic ZrO2---SiO2 solutions

Small-angle X-ray scattering was used to examine in situ formation of ZrO₂---SiO₂ structures in alcoholic solution of tetraethoxysilan (TEOS) as a function of the ratio of ZrO₂ to SiO₂. For the moment of the first measurement (15 min after the preparation) primary particles with R_g ≈ 1.5 nm exist in all investigated mixed gels. These particles aggregated to secondary clusters. The resulting clusters can be described by means of fractal theory, where the determining mechanism of formation is cluster-cluster aggregation (diffusion or chemical limited). The time of gelation is a function of the ZrO₂ concentration. The higher the ZrO₂ concentration in the solution, the faster is the aggregation to secondary clusters. Gelation times were between 170 and 970 h.     

Electrolytic growth and dielectric properties of magnesium hydrogen phosphate single crystals

The growth of MgHPO₄, magnesium hydrogen phosphate, (MHP) single crystals in silica gels under the influence of an externally applied uniform electric field is described. A controlled reaction between magnesium chloride and orthophosphoric acid by a diffusion process in the gel medium is employed. The bright, transparent MHP crystals upto 4 × 2 × 1 mm³ in sizes are grown at room temperature. The dielectric measurements are carried out in the temperature range between 300 to 650 K. The effect of the applied frequency on the dielectric constants and the dielectric losses are investigated. It is observed that the mechanism of the dielectric behaviour is different in the lower and higher frequency and temperature ranges. The activation energy of the oscillators are calculated and found to be 1.105 eV. Attempt is made to draw some qualitative conclusions, taking in view the existing theories of various kinds of polarization, and implications are discussed.     

The gel to glass transition: Chemical and microstructural evolution

The chemical and microstructural changes occurring in the conversion of a died gel to fully dense glass are reviewed. The main emphasis is on gels prepared from alkoxide precursors, including silica and more complex silicate compositions. The gel to glass conversion in these systems is contrasted with that in colloidal systems. The processes occurring in the conversion are crucially dependent on the composition of the starting solution and the chemistry of the sol to gel transformation. Shrinkage is governed by four processes operating at different stages during the gel to glass transition: capillary contraction, condensation-polymerization, structural relaxation and viscous sintering. A variety of techniques have recently been applied to study the changes in the porous gel as a result of heat treatment, including dilatometry, gas adsorption, DTA, TGA, TEM, infra-red spectroscopy (to monitor OH content, in particular), Raman spectroscopy, resonance techniques and SAXS. The conversion of dried gels into monolithic glass samples using the slow drying and firing method is discussed, including removal of hydroxyl content and prevention of bloating. Other processing routes are also briefly reviewed including hypercritical drying and sintering, the use of drying control additives prior to sintering, and colloidal techniques.     

Proposals for the Optimization of Crystal Growth in Gels

Crystal growth in gels is viewed under the aspect of diffusion and factors governing this. From simple diffusion experiments proposals for the modification of growth apparatus and growth regime are made to minimize quality decreasing effects. The application of diffusion measurement results on crystal growth in gels is shown by optaining NaHC₂O₄ × × H₂O crystals in silica gel (sodium metasilica gel respectively).     

Influence of oxygen, hydrogen, helium, argon and vacuum on the surface behavior of molten InSb, other semiconductors, and metals on silica

Sessile drop experiments were performed on molten indium antimonide on clean quartz (fused silica) surfaces. A cell was constructed through which argon, helium, oxygen, hydrogen or a mixture of these was flowed at 600 °C. Some of the InSb was doped with 0.1% Ga. The surface tension σ of oxide-free molten InSb was smaller in Ar than in He, may have increased with increasing O₂ in the gas, and was not influenced by Ga or H₂. The contact angle θ on silica was higher in the presence of Ar, was lowered by O₂, and was not influenced by H₂ or Ga. The work of adhesion W and the surface energy σ_sv of the silica were higher in He than in Ar. The surface remained free of solid oxide only in flowing gas containing 0.8 ppm O₂. This behavior is attributed to reaction of O₂ at the surface of the melt to form In₂O gas. When solid oxide formed on Ga-doped material, it was strongly enriched in Ga, with the Ga/In ratio increasing with the concentration of O₂ in the gas.

Examination of published sessile-drop results for liquid metals and semiconductors on silica revealed that W and σ_sv were highest for reactive melts, in which SiO₂ dissolves. For non-reactive melts, W and σ_sv were lower and θ higher in a gas than in a vacuum, regardless of whether the experiments had been carried out in sealed ampoules, a flowing gas, or dynamic vacuum. The implication is that the surface of silica was different in a vacuum than in a gas at 1 bar.     

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.     

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.     

Crystallization of gels and glasses made from hot-pressed gels

Silica gels were prepared by two different methods: (1) destabilization of a silica hydrosol (gel 1); (2) hydrolysis and polycondensation of a tetra-methoxysilane (gel 2). The crystallization of the gels was then studied as a function of the temperature by means of X-ray diffraction. It was evident that the crystallization was strongly influenced by the amount of alkali oxides present in the gel. It is effectively the concentration of akali impurities which could explain the lower temperature of crystallization necessary for gel 1 compared with that of gel 2. During the crystallization of the gel containing Na₂O the crystalline phase of silica which appears first is the cristobalite; with Li₂O it is quartz. The effect of additives such as boric anhydride was studied. This oxide was found to reduce the tendency of the gels to crystallize. The glasses of the system SiO₂B₂O₃ obtained by the hot-pressing of the gels confirmed this phenomenon. Above 10 mol% B₂O₃ it was impossible to crystallize the gels and the glasses of this system under two hours.     

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.     

Effect of dehydration on the viscosity of sol-gel processed silica

For short times and low temperatures, the dehydration of silica gels is limited by the diffusion of water species. For long times and temperatures 600°C and higher, dehydration shows first order kinetics. That is dehydration rates decrease with increasing temperature where silica gels have less surface area and consequently fewer hydroxyls.

For comparison with samples treated isothermally, samples were heated to 600, 700, 800 and 850°C at 100°C intervals and held 4 h at each temperature starting with 500°C. Comparison of isothermal and multistep treatments shows the contributions of structural relaxation and dehydration to the viscosity. At a given temperature, two gels of different water content may show the same viscosity, because the difference in water is matched by the difference in degree of structural relaxation. More complete dehydration is accomplished by constant rate heating to a single isothermal treatment rather than heating in steps.