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.     

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 large-size silica glass produced by a new sol-gel process

A large-size silica glass was fabricated with a new sol-gel process involving the following procedures: (a) hydrolysis of Si(OC₂H₅)₄ with HCl, (b) addition of silica powders and their high dispersion, (c) adjustment of the pH value by adding ammonia solution, (d) gellation, (e) drying to dry-gel, and (f) sintering to silica glass. With this process a dry-gel plate as large as 520 x 360 mm² was obtained, which was sintered to a 420 x 290 mm² sized silica glass. The effect of pH on the gelation time of the sol, and the correlation between the weight percentage of silica powder and porosity of dry-gel were examined. In addition, fiber preforms were produced by this sol-gel process.     

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.     

Fabrication of fluorine-doped silica glasses by the sol-gel method

Fluorine-doped silica glasses are produced by the sol-gel method for optical fiber preforms. In order to dope fluorine into silica glass, fluorinated silicon alkoxide, Si(OC₂H₅)₃F, is titrated into SiO₂ sol solutions. The fluorine content in silica glass depends on: the fluorine concentration in the gel, the specific surface area of SiO₂ particles and the heating rate in the sintering process. Fluorine-doped silica glass with a maximum relative refractive index difference of −0.93% is obtained. Using this technique, optical fibers with a triangular refractive index profile are fabricated with a minimum optical loss of 1.6 dB/km at 1.69 μm wavelength.     

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.     

Gradient-index glass rods of PbO---K2O---B2O3---SiO2 system prepared by the sol-gel process

A glass rod of the PbO---K₂O---B₂O₃---SiO₂ system having a radial gradient of refractive index has been prepared by the sol-gel process using aqueous solutions of lead acetate and potassium nitrate as the sources for index-modifying cations. A gel prepared by gelatinizing a sol from the mixture of tetramethoxysilane, tetraethoxysilane, and the aqueous solutions of boric acid and lead acetate was placed in an aqueous solution of potassium nitrate to form the concentration gradient of modifier cations by diffusion through the micropores of the gel. The glass rod of about 7 mm in diameter obtained by drying and sintering the gel had a refractive index of parabolic profile changing from the center toward the perimeter with a maximum difference of about 0.04.     

Influence of the dipping coating procedure on the mechanical strength of soda-lime glass rods

In order to investigate the effect of sol-gel coatings on the mechanical behavior of a high strength glass substrate, HF etched glass rods were used.

Coatings with different compositions (SiO₂, ZrO₂ and 20%ZrO₂---80%SiO₂) and different thicknesses were produced to study the influence of these parameters on strength.

Functions of integral probability versus strength were used to analyze data.

Coated glass rods exhibit higher strength in comparison with uncoated control rods that have undergone the same treatment. It seems evident that there is an effect of composition on strengthening. SiO₂ coated rods displayed higher strength.

A lowering in strength at thickness higher than 0.2 μ was observed, independently of composition.

The effect of a saturated Ca(OH)₂ solution was also studied.     

Glass formation, properties and structure of glasses in the P2O5---WO3---K2O---Al2O3 system

Glass formation in the P₂O₅---WO₃---K₂O---Al₂O₃ system was investigated and the glass-forming regions are presented.

the properties of the glasses in the P₂O₅---WO₃---K₂O---Al₂O₃ system (Al₂O₃ 8 mol.%) are reported.

The colouration of glass was studied. It was found that W⁵⁺ ions make glass blue.

Infrared spectra were measured by means of making KBr pills. Results of the investigation suggest that P---O---P, P---O---W, and W---O---W bonds form a continuous network in the phosphate glasses. So we suggest that tungsten trioxide is a glass former.     

Infrared study of SiO2 sol to gel evolution and gel aging

The evolution of the gelation of silica gel was studied by means of Fourier transform infrared spectroscopy. The gel was prepared by hydrolysis and polycondensation of tetraethyl orthosilicate in the presence of water with HCl, and with formamide (DCCA) in methanol either added or not. For both systems the gelation process was followed by the time evolution of the ν-Si---O(H) and ν-Si---O(Si) absorption bands. In the systems which used formamide, the ν-Si---O(H) peak shifts from ≈ν = 950 cm⁻¹ for τ = 0 to ≈ν = 968 cm⁻¹ (t = t_gel) and tends to shifts to 975 cm⁻¹ over a long period of time (t = 100t_gel), and a slow rate evolution between 0.1 and 0.4 t_gel is observed. In the absence of formamide the same evolution is observed without the slow rate plateau. For ν-Si-O(Si) absorption bands, the band near 1075 cm⁻¹ remains practically unchanged in both systems during the experiment time, but the second absorption band at 1133 cm⁻¹ is split into two bands each having its own specific evolution, depending on composition and temperature.     

The corrosion of zircon and zirconia refractories by molten glasses

The corrosion of ZrO₂-containing refractories by molten glasses has been investigated experimentally in relation to the compositions of the glasses. For a zircon refractory, zircon crystals contacting the glass decomposed into ZrO₂ crystals and glassy phase at higher temperatures. The decomposition temperature was changed with the glass compositions. Alkali components in the glasses were confirmed to be most corrosive for the zircon refractory. For a ZrO₂ refractory, compositions of the glass-refractory interfaces were nearly saturated with ZrO₂. The corrosion rate of the ZrO₂ refractory was considered to be controlled by the transport rate of zirconium away from the interface.

The concentration of CaO in glasses, as well as that of alkali components, was found to be very effective for increasing the corrosion rate of ZrO₂ refractories.     

Sol → gel → glass: I. Gelation and gel structure

The mechanisms of gel formation in silicate systems derived from metal alkoxides were reviewed. There is compelling experimental evidence proving, that under many conditions employed in silica gel preparation, the resulting polysilicate species formed prior to gelation is not a dense colloidal particle of anhydrous silica but instead a solvated polymeric chain or cluster. The skeletal gel phase which results during desiccation is, therefore, expected to be less highly crosslinked than the corresponding melted glass, and perhaps to contain additional excess free volume. It is proposed that, during gel densification, the desiccated gel will change to become more highly crosslinked while reducing its surface area and free volume. Thus, it is necessary to consider both the macroscopic physical structure and the local chemical structure of gels in order to explain the gel to glass conversion.     

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.     

Growth parameters for large diameter float zone silicon crystals

Factors that directly affect the ability to grow dislocation free float zone silicon crystals up to 80 mm in diameter have been experimentally determined. The highest yield is obtained for 80 mm diameter crystals by starting with 68 mm to 74 mm diameter poly crystal rod stock. Lower transport speeds for crystal growth of (111) orientation crystals were 3 to 4 mm/min and for (100), 2 to 3 mm/min. Rotation rates of both upper and lower shafts were found to have an effect on growth at the solid-liquid interface. Rates established for lower shaft were 6 to 8 rpm for the (111) crystals and 3 to 4 rpm for (100), counter-clock-wise. Upper rotation rates were 2 rpm on (111) crystals and 3 to 5 rpm on (100), clockwise. Seed orientation, which is critical, was held to within plus or minus $\text{1}\text{2}\text{°}$ of perfect orientation. The minimum seed growth length was 50 to 70 mm. To assist in reducing the side lobes on (111) dislocation free crystals, a cooling ring with a flow or argon was used. For best (100) growth the shape of the lower side of a one turn copper rf work coil was made conical. Six to ten dislocation free crystals in each orientation group were produced using these parameters.     

Changes in the porous structure of silica gels during the gel-to-glass conversion

The changes in porous structure during the conversion from the gel to the SiO₂ glass have been studied through the xerogel and aerogel routes. For both gels, selective collapse of relatively large pores was observed during the first contraction stage (ΔL/L₀ = 0−38%). In the xerogel route, the size distribution of smaller pores less than 100 Å in diameter also changed slightly, but there was no difference in the pores in the aerogel route. In the second stage, both the large pores which remained and the smaller pores collapsed at the same time. However, it was characteristic that once the aerogel was converted to the porous structure it contained uniformly sized pores (150 Å in diameter) during the densification. This phenomenon requires spherical particles (500–1000 Å in diameter) which we observed using a SEM arranged in a near close-packed structure. The results indicate that the contraction of the aerogel occurs by the rearrangement of the particles which move individually, while the particles in the xerogel route behave like a cluster.     

Vibrational spectroscopy of LaBSiO5 glass and glass–crystal composites

Infrared (IR) reflectivity and Raman scattering spectra of LaBSiO₅ glass and glass–crystal composites were studied in the temperature range 25–260 °C. Using an analogy with the LaBGeO₅ crystal it was possible to assign the main spectral features of the LaBSiO₅ glass and glass–crystal composites. The BO₄ chain arrangement and the bending vibrations of SiO₄ are influenced by the loss of the long-range order in the glass whereas the stretching vibrations of the SiO₄ groups are practically unaffected. The structural disorder in LaBSiO₅ crystallites is caused by rotation of BO₄ tetrahedra.     

GdAlO3∶Tb3+-Al2O3有序共晶生长及其微结构研究

两相有序共晶材料由于具有不同折射率的两晶相呈现有序排列,可降低荧光在共晶材料内部的散射而实现导光功能,可被应用于高分辨探测成像器件中。本工作根据定向凝固原理,用微下降法生长技术制备得到了直径为3 mm的GdAlO₃∶Tb³⁺-Al₂O₃两相有序共晶。通过SEM和元素分析,探究了GdAlO₃∶Tb³⁺-Al₂O₃共晶内部的微结构,结果显示,所得共晶中GdAlO₃∶Tb³⁺晶相均匀有序地分布于基质Al₂O₃晶相中,GdAlO₃∶Tb³⁺晶相直径的大小受生长速率的影响,速率越快,直径越小。所制备得到的GdAlO₃∶Tb³⁺-Al₂O₃有序共晶在X射线辐照下发射出明亮的绿色荧光,并在GdAlO₃∶Tb³⁺晶相中定向传播,有望被用作X射线探测成像材料,提高探测器的空间分辨率。     

The growth kinetics of (SmY)3(FeGa)5O12 epitaxial layers in the 920–980°C range

The growth kinetics of (SmY)₃(FeGa)₅O₁₂ garnets obtained by liquid phase epitaxy on Gd₃Ga₅O₁₂ substrates are discussed in the 920 to 980°C. range. The experiments have been carried out with the substrates in a horizontal plane and with unidirectional rotation ranging from 30 to 300 rotations per minute (rpm). A sharp increase in the growth velocity at temperatures higher than 960°C is interpreted on the basis of a diffusion-reaction theory. It is found that the diffusion constant increases steeper at higher temperatures and that the linear law of the growth rate as a function of the square root of the rotation rate holds to much higher rotation rates at these temperatures. The influence of the surface incorporation is too small to be detectable. The results are compared with published data concerning other compositions.     

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 Raman spectra of defects in neutron bombarded and Ge-rich vitreous GeO2

This paper reports the polarized Raman spectra of three forms of vitreous GeO₂: the pure glass, neutron irradiated pure glass and unirradiated Ge-rich glass of composition Ge_1.1O₂. The data reveals that the line seen at 520 cm⁻¹ in the pure glass is due to a network defect that is not a Ge---Ge bond and very probably also not an O---O bond. Comparison with spectra of fused silica suggests that the 606 cm⁻¹ defect line seen in v-SiO₂ is not due to Si---Si or O---O bonds.