Showing papers on "Sol-gel published in 1995"

Structural Characterization and High‐Temperature Behavior of Silicon Oxycarbide Glasses Prepared from Sol‐Gel Precursors Containing Si‐H Bonds

Abstract: Silicon oxycarbide glasses have been synthesized by inert atmosphere pyrolysis at 1000°C of gel precursors obtained by cohydrolysis of triethoxysilane, HSi(OEt)3, and methyl-diethoxysilane, HMeSi(OEt)2. The oxycarbide structures have been carefully characterized by means of different techniques such as 29Si magic angle spinning nuclear magnetic resonance (MAS-NMR) and Raman spectroscopies, X-ray diffraction (XRD), and chemical analysis. Experimental results clearly indicate that, depending on the composition of the starting gels, the resulting oxycarbide glass either is formed by a pure oxycarbide phase or contains an extra carbon or silicon phase. By increasing the temperature up to 1500°C, the oxycarbide glasses display compositional and weight stability; however, the amorphous network undergoes structural rearrangements that lead to the precipitation of nano-sized β-SiC crystallites into amorphous silica. Crystallization of metallic silicon is also clearly observed at 1500°C for the samples in which the presence of Si-Si bonds was postulated at 1000°C.

Effect of hydrolysis catalyst on the Ti deficiency and crystallite size of sol-gel-TiO2 crystalline phases

Abstract: We prepared sol-gel titania by using different hydrolysis catalysts, and characterized it by x-ray powder diffraction. The structure of the crystalline phases—brookite, anatase, and rutile—in the samples annealed between 70 and 900 °C was refined by using the Rietveld technique. From the refinement we obtained the structure parameters, the concentration of each phase, and their average crystallite size. These quantities and their evolution with temperature depended on the hydrolysis catalyst. Anatase and rutile were deficient in Ti, suggesting that their crystalline structure contained hydrogen atoms, forming OH− ions inside. In anatase this deficiency depended on its crystallite size, but it was constant in rutile. When anatase was annealed, it dehydroxylized, producing either crystallitc growing up or its conversion into rutile. From the analysis we also found the conditions for obtaining single-phase samples that could be used as precursors for making up titania single-phase thin films.

Microstructural evolution of a silicon oxide phase in a perfluorosulfonic acid ionomer by an in situ sol-gel reaction

Abstract: Nanocomposites were produced via sol–gel reactions for tetraethylorthosilicate within the cluster morphology of perfluorosulfonic acid films. Small-angle x-ray scattering revealed that the polar/nonpolar nanophase-separated morphological template persists despite invasion by the silicon oxide phase. Scanning electron microscopy (ESEM–EDAX) studies have indicated that the greatest silicon oxide concentration occurs near the surface and decreases to a minimum in the middle. Optical and ESEM micrographs revealed a brittle, surface-attached silica layer at high silicon oxide contents. © 1995 John Wiley & Sons, Inc.

Thick ceramic coatings using a sol gel based ceramic-ceramic 0–3 composite

Abstract: A sol gel based coating technology has been developed for making high quality, thick ceramic coatings. The films are ceramic-ceramic 0–3 composites made by dispersing ceramic powders in a sol gel solution. The resulting system has all the benefits of sol gel, i.e. ease of fabrication, ability to coat complex geometries and relative cost effectiveness, but is not limited in film thickness. Lead zirconate titanate (PZT), yttria- and ceria-stabilized zirconia, titania, silica and alumina films have been fabricated in the 5–200 μm thickness range. The electrical properties of the PZT films are comparable with those of the bulk ceramic, and the zirconia and alumina films exhibit excellent potential as corrosion- and wear-resistant coatings as well as thermal barrier and performance enhancing coatings.

Sol-gel processing of optical and electrooptical materials

Abstract: ~'I as it has become known, is used by a high and increasing number of materials science researchers because its versatility allows the preparation of a large number of materials. However, it is by no means new. Its origin goes back to early experiments on thin dielectric coatings at Schotti8] in Germany in the early 1970s or the work carried out at the Oak Ridge National Laboratory, USA, and Harwell. Great Britain, with the aim of preparing nuclear ceramic fuels.['- "I This work resulted in the development of the sol-gel process by the end of the 1970s for the processing of advanced ceramic materials and glasses for technological applications and interest has accelerated ever since. The fundamental idea was replacing the classical and ancient high-temperature techniques by a room temperature process based on the possibility of forming a glassy network by polymerization of suitable compounds (monomers) at low teniperaturc. Ideal monomers are molecules which form M-0-M units (M = metal) upon polymerization, and where the reaction byproducts can easily leave the system. Metal alkoxides, M(OR), , fulfil these requirements, where R is an alkyl radical. A popular choice of precursor solution is based on the hydrolysis and polycondensation reactions of metal alkoxides.". 21 Tetramethoxysilane, Si(OCH,),, or tetraethoxysilane, Si(OC,H,),, known as TMOS and TEOS, respectively, are currently used in the preparation of silicabased gels. Since, however, these compounds and water are not miscible, a common solvent (generally, methyl or ethylalcohol) have to be added to obtain an initially homogeneous liquid. The chemical reactions involved are complicated but they can be summarized as shown in Equations 1 and 2.

Synthesis and processing of barium titanate ceramics from alkoxide solutions and monolithic gels

Abstract: We report the sol-gel processing of barium titanate (BaTiO{sub 3}) ceramics from methoxyethoxide precursors. The effects of heat treatment on phase and microstructure development for solution- and gel-derived ceramics are reported. Drying of monolithic alcogels yielded the low-temperature (T < 125 {degrees}C) crystallization of BaTiO{sub 3}. Crystallization was associated with gel syneresis. Nanocrystalline BaTiO{sub 3} gels which crystallized before the removal of solvent were used as monolithic precursors for the powderless processing of fine-grain dielectrics. Lower levels of hydrolysis content for solution-derived ceramics led to higher crystallization temperatures for the perovskite phase. In these cases, BaTiO{sub 3} crystallized via an intermediary oxycarbonate phase. The efficiency with which carbonaceous species were eliminated during calcination of powders derived from sols or gels was investigated by thermal analysis, infrared spectroscopy, and helium gas pycnometry. Dielectric properties were determined for dense ceramics derived form monolithic xerogels. Micrographs illustrate the development of structure for ceramics derived form the xerogel precursors. 27 refs., 9 figs.

Room temperature persistent spectra hole burning in Sm2+‐doped silicate glasses prepared by the sol‐gel process

Abstract: Persistent spectra hole burning is observed at room temperature in the excitation spectrum for the 7F0→5D0 transition of the Sm2+ ions that are doped in aluminosilicate glass. Glass having the composition of Sm2+‐doped Al2O3⋅9SiO2 has been prepared by the sol‐gel processing of metal alkoxides and the reaction with H2 gas at 800 °C. Sol‐gel derived glass enables Sm to be doped in the Sm2+ state of which the 4f6 (7F0)→4f55d band does not overlap with the 5D0→7F0 line. The hole width and depth, which are burned by the DCM dye laser, are ∼15 cm−1 and ∼10% of the total intensity, respectively, at 20 °C.

17O and 29Si Solid State NMR Study of Atomic Scale Structure in Sol-Gel-Prepared TiO2-SiO2 Materials

Abstract: Solid state O-17 and Si-29 magic angle spinning (MAS) NMR spectroscopy has been applied to the study of the structure of three Ti-Si mixed oxides and their gel precursors. Gels were prepared by the hydrolysis of tetraethyl orthosilicate (TEOS) and titanium isopropoxide (Ti(OPrn)4). Both one-stage (addition of the Ti(OPrn)4 to the TEOS) and two-stage (partial hydrolysis of the TEOS before addition of the Ti(OPrn)4) procedures were used. Si-29 MAS NMR spectra hardly show any differences between the one- and two-stage gels. All consist of Q(2), Q(3), and Q(4) SiO4 species becoming Q(4) after heat treatment at 600 degrees C. This study shows that O-17 MAS NMR is a valuable tool in the characterization of amorphous oxides and that it is much more informative than Si-29 MAS NMR. O-17 MAS NMR shows that titanium is incorporated only as Ti-O-Si bonds in a two-stage gel, whilst samples that show phase separation alsb have Ti-O-Ti bonds. Ti-O-Si linkages persist until 600 degrees C and in conjunction with the Si-29 NMR data indicate that titanium enters the silica network. The assignment of the Ti-O-Si resonance is confirmed by studying the mineral fresnoite (Ba2TiSi2O8).

Composites of Electronically Conductive Polyaniline with Polyacrylate-Silica Hybrid Sol-Gel Materials

Abstract: A series of new composite materials that consist of electronically conducting polyaniline and polyacrylate-silica hybrid sol-gel network have been prepared via the sol-gel reactions of the precursor polymer, poly[methyl methacrylate-co-3-(trimethoxysilyl)propyl methacrylate], in the presence of camphorsulfonic acid-doped polyaniline in m-cresol solution. These materials as coatings can be tailored by varying the composition to have an excellent adhesion to inorganic substrates such as silica glass and indium-tin oxide and a reasonably high conductivity. The adhesion and durability of the new composites as the electroactive coatings on indium-tin oxide electrode are much superior to those of the conventional polyaniline system under repeated electric potential cycling. conditions. The modulus of the comDosite films was also found to be relatively greater

In Situ Fluorescence Probing of the Chemical Changes during Sol–Gel Thin Film Formation

Abstract: Pyranine (8-hydroxy-1,3,6-trisulfonated pyrene) was used as an in situ fluorescence probe to monitor the chemical evolution during sol-gel thin film deposition of silica by the dip-coating process. The sensitivity of pyranine luminescence to protonation/deprotonation effects was used to quantify changes in the water/alcohol ratio in real time as the substrate was withdrawn from the sol reservoir. The spatially resolved spectral results showed that preferential evaporation of alcohol occurred, and that the solvent composition in the vicinity of the drying line reached values in excess of 80 vol% water. Correlation of the luminescence results with the interference pattern of the depositing film allowed the solvent composition to be mapped as a function of film thickness. HE sol-gel process is a method of preparing inorganic oxide T glasses and ceramics from polymeric or colloidal sols at temperatures well below those used in traditional processing technique^.'-^ Interest in sol-gel thin films has grown in recent years as a number of applications have emerged in areas such as protective and optical coatings, high and low dielectric constant films, electrochromics, waveguides, ferroelectric thin films, sensors, and membrane^.^ The films can be produced readily by several different methods including spin coating and dip coating.5 Dip coating is perhaps more important technologically since a uniform coating can be deposited onto substrates of large dimensions and complex geometries. In the dip-coating process the substrate is immersed into the coating sol and then withdrawn at a constant rate. Brinker, Hurd, and co-workers have carried out extensive work, both experimental and theoretical, and have established how various physical and chemical parameters involved in dip coating affect the structure and properties of the final film.5 In the dip-coating process, a substrate is withdrawn slowly at constant speed (typically 10-20 cm/min) from a sol which contains polymeric or colloidal species in suspension (see

Formation of Cagelike Intermediates from Nonrandom Cyclization during Acid-Catalyzed Sol-Gel Polymerization of Tetraethyl Orthosilicate

Abstract: In the sol-gel synthesis of silica by the acid-catalyzed hydrolysis and condensation of tetraethyl orthosilicate (TEOS), the fractal dimension, density, and homogeneity of the gel depend strongly on reaction conditions such as the acid and water concentrations. However, 29 Si nuclear magnetic resonance shows that gel conversions of at least 80% are observed over a wide range of initial acid (10 -5 -10 -1 mol/L), water (4-22 mol/L), and TEOS (1.3-2.5 mol/L) concentrations. This is very high compared to the value predicted by random branching theory even under the worst possible negative first-shell substitution effect. Moreover, nonrandom cyclization is commonly evidenced in these systems by 29 Si NMR, Raman spectroscopy, and gas chromatography with mass spectroscopy. The gel conversions suggest that nonrandom cyclization reactions are strongly favored, and the development of conversions with time is consistent with the formation of compact three-dimensional cagelike intermediates. These are probably the same nanometer-scale objects shown by recent scattering and microscopy observations.

Synthesis of silicon carbide through the sol-gel process from different precursors

Abstract: Silicon carbide (SiC) was synthesised from silicon alkoxides and various carbon sources. Tetraethoxysilane (TEOS), methyltriethoxysilane (MTES) and a mixture of TEOS and MTES were hydrolysed in the presence of phenolic resin, ethylcellulose, polyacrylonitrile (PAN) and starch to incorporate the gel into the carbon source in the silica network. The gel thus obtained was carbonized at 800°C in an argon atmosphere to obtain the mixture of silica and carbon which when heated to 1550°C in argon yielded silicon carbide. The characterization of the product by X-ray, FTIR and SEM showed it to be β SiC with different crystallite and grain sizes. The difference in the crystallite and grain sizes is attributed to the nature of the carbon source. The density of the SiC obtained by the sol—gel process was found to be lower than the values reported for SiC and this is ascribed to the porous nature of the products generally obtained by this process.

Abrasion resistant inorganic/organic coating materials prepared by the sol-gel method

Abstract: Novel abrasion resistant coating materials prepared by the sol-gel method have been developed and applied on the polymeric substrates bisphenol-A polycarbonate and diallyl diglycol carbonate resin (CR-39). These coatings are inorganic/organic hybrid network materials synthesized from 3-isocyanatopropyltriethoxysilane functionalized organics and metal alkoxide. The organic components are 3,3′-iminobispropylamine (IMPA), resorcinol (RSOL), diethylenetriamine (DETA), poly(ethyleneimine) (PEI), glycerol and a series of diols. The metal alkoxides are tetraethoxysilane (TEOS) and tetramethoxysilane (TMOS). These materials are spin coated onto bisphenol-A polycarbonate and CR-39 sheets and thermally cured to obtain a transparent coating of a few microns in thickness. Following the curing, the abrasion resistance is measured and compared with an uncoated control. It was found that the abrasion resistance of inorganic/organic hybrid coatings in the neat form or containing metal alkoxide can be very effective to improve the abrasion resistance of polymeric substrates. The adhesion tests show that the adhesion between coating and substrate can be greatly improved by treating the polymeric substrate surface with a primer solution of isopropanol containing 3-aminopropyltriethoxysilane (3-APS). The interaction between 3-APS and the polycarbonate surface was investigated by a molecular dynamics simulation. The results strongly suggest that the hydrogen bonding between the amino group of the 3-APS and ester group in the polycarbonate backbone are sufficiently strong to influence the orientation of the primer molecules. The abrasion resistance of these new coating systems is discussed in light of the structure of the organic components. All of these results show that these coating materials have excellent abrasion resistance and have potential applications as coating materials for lenses and other polymeric products.

Polymeric-silica-based sols for membrane modification applications: sol-gel synthesis and characterization with SAXS

Abstract: Polymeric SiO2 and binary SiO2/TiO2, SiO2/ZrO2 and SiO2/Al2O3 sols, for ceramic membrane modification applications, have been prepared by acid-catalyzed hydrolysis and condensation of alkoxides in alcohol. The sols were characterized with small angle X-ray scattering, using synchrotron radiation. Directly after synthesis, the sols were found to consist of weakly branched polymeric structures with typical fractal dimensions of around 1.5 and radii of gyration of ? 2 nm. The aggregation for silica sols obeys the tip-to-tip cluster-cluster aggregation model in the initial stages. Prehydrolysis of TEOS was found to be the best method to synthesize polymeric binary systems. Based on an analysis of film formation from sols consisting of weakly branched polymers, it is expected that consolidation of these polymers will result in microporous materials.

Effects of H2O on structure of acid-catalysed SiO2 sol-gel films

Abstract: Thin silica films were deposited on silicon wafers by the sol-gel technique, using spin coating. The sols were prepared by HCl catalysis of tetraethylorthosilicate (TEOS) diluted in ethanol, using different molar ratios, R, of H2O:TEOS. The films were then baked at various temperatures, and characterised using ellipsometry, profilometry, optical scattering and infrared spectroscopy. It was found that the thickness, shrinkage, porosity and pore sizes all decrease with increasing R. It was also found that high water levels yield films of higher homogeneity and finer texture, and less tensile stress.

Sol–gel transition of colloidal suspensions of anisotropic particles of laponite

Abstract: The thermodynamical properties, structure and rheology of laponite dispersions have been studied as a function of solid fraction and ionic strength. The dispersions undergo a transition from Newtonian liquid to viscoelastic solid without phase separation. The increase of the ionic strength shifts the transition to lower solid fraction and a phase diagram has been constructed for the onset of viscoelastic solid like behaviour. This sol–gel transition line coincides with the appearance of a singularity in the equation of state as determined by osmotic pressure measurements. Analysis of the dispersions by transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS) shows correlated but well separated particles. In order to elucidate the nature of the observed sol–gel transition and the particle organization, we discuss the role of particle anisotropy coupled with diffuse-layer repulsion.

Novel thin films of titanium dioxide particles synthesized by a sol-gel process

Abstract: Novel thin films of ultrafine titanium dioxide particles dispersed in a matrix of hydroxypropylcellulose (HPC) polymer have been made on quartz and silicon substrates. The titanium dioxide particles were made by the hydrolysis and condensation of titanium tetraethoxide (TEOT) in solutions of HPC in a mixture of ethanol and water. HPC controlled the particle size by adsorbing at the particle surface during the growth process and generating repulsive steric forces. The TiO2/HPC composite films were transparent in the visible region and completely blocked ultraviolet radiation at 300 nm. These films were crack-free and uniform in composition and thickness. Transparent films of amorphous TiO2 were made by burning out the HPC at 500 °C. These films were highly uniform and had no macroscopic cracks. X-ray diffraction revealed a transition to the anatase form upon sintering at 600 °C. A film sintered at 700 °C had a porosity of 38%. The crystalline films remained transparent until they densified at 800 °C.

Sol-gel TiO2-SiO2 films as protective coatings against corrosion of 316L stainless steel in H2SO4 solutions

Abstract: Sol-gel TiO2-SiO2 films were deposited on 316L stainless steel by dip coating process from a sono-catalysed sol of composition 30TiO2-70SiO2 prepared from a mixture of Ti(OC2H5)4 and Si(OC2H5)4, absolute ethanol C2H5OH and glacial acetic acid CH3COOH as precursors and solvents. The films, densified at 800° C in air for 2 h, are composed of small orthorhombic titania (anatase) crystallites embedded in a SiO2 amorphous matrix as identified by X-ray diffraction. The temperature dependence of the film morphology was observed using scanning electron microscopy (SEM) and the content was determined by FTIR reflection spectroscopy. The corrosion behaviour of 316L stainless steel samples coated with densified 30TiO2-70SiO2 films was studied in 15% H2SO4 by potentiodynamic polarization curves at 25, 40 and 50°C. The measured corrosion rates show a considerable decrease for the protected steel samples in comparison to the bare substrate. The effect of time of heat treatment of the films on the corrosion parameters is also reported.

Solid phase epitaxial growth of sol‐gel derived Pb(Zr,Ti)O3 thin films on SrTiO3 and MgO

Abstract: Pb(Zr0.52Ti0.48)O3 (PZT) thin films were crystallized on SrTiO3 (100) and MgO (100) substrates by a sol‐gel process using nonhydrolyzed metal methoxyethoxide precursors, spin coating, and rapid thermal annealing. Solid phase epitaxial growth of PZT on SrTiO3 was observed directly from the amorphous phase even at 425 °C. The PZT had a single (001) orientation and rocking curve full width at half maximum (FWHM) less than 0.1°. High‐temperature annealing of MgO substrates improved orientation of PZT thin films. Epitaxial crystallization of PZT with a single (001) orientation on the annealed MgO was observed at temperature above 550 °C after the formation of the pyrochlore phase.