Showing papers on "Sol-gel published in 1990"

Doped sol-gel glasses as chemical sensors

Abstract: It is reported that by trapping suitable analytical reagents, porous sol-gel glasses can be used for the preparation of a wide variety of chemical-sensing materials. The key to the successful preparation of these materials is based on the observation that a substantial fraction of the trapped molecules is exposed to the liquid or gaseous intra-pore volume. The glass matrix thus serves as a solid support for analytical reagents which interact with analyzates present in the adjacent phase. The phenomenon is quite general, and has been demonstrated for a variety of color tests for metal cations, proton (pH indication), anions, and organic molecules.

Sol-gel condensation of rapidly hydrolyzed silicon alkoxides: A joint 29Si NMR and small-angle x-ray scattering study.

Abstract: We have studied by $^{29}\mathrm{Si}$ NMR the complete condensation kinetics in the conditions of rapid hydrolysis (acidic medium, water in excess) of three silicon alkoxides. The gelation of the tetravalent tetraethoxysilane (TEOS) takes several weeks, whereas the trivalent methyltriethoxysilane (MTEOS) and vinyltriethoxysilane (VTEOS) do not form gels. From a quantitative analysis of the data, we deduce that the first steps of the condensation proceed by progressive assembling of small organized units. This accounts for the very slow kinetics (logarithmic function of time), the occurrence of highly condensed agglomerates, and the absence of gelation in trivalent systems. For the tetravalent TEOS, this is followed by an aggregation phase, which has been studied both by NMR and small-angle x-ray scattering. The fractal dimension D=1.9 and the growing kinetics (cluster size increasing as a linear function of time) are consistent with reaction-limited cluster-cluster aggregation with preferential reactivity at the external cluster sites. Finally, we suggest that the progressive transformation of the sol phase into the gel phase after the gel time can be observed by comparing static and magic-angle-spinning NMR spectra.

Sol-gel synthesis of zirconia barrier coatings

Abstract: A method for applying zirconia barrier coatings using a sol-gel method is described. The coatings of 8 wt % yttria-stabilized zirconia are applied by spin coating a solution containing zirconium alkoxides and yttrium acetate on to stainless steel substrates. Crystallization of the films was observed for thermal treatments in the range 750 to 1050° C. Excellent adhesion at the interface, due to significant coating-substrate interfacial reactions, indicates that this sol-gel route is a feasible method for applying zirconia coatings.

Sol-gel thin film formation

Abstract: The overlap between the drying stage and the aggregation/gelation and aging stages of sol-gel film formation establishes a brief time for further condensation reactions to occur. For this reason, the structures of films are often considerably more compact than those of the corresponding bulk gels or xerogels prepared from identical precursors. Experimental techniques to study film formation and the structure of the deposited film in situ have been developed. These techniques include imaging ellipsometry, infrared microscopy and gas sorption on surface acoustic wave substrates.

The preparation and characterization of new polyether ketone-tetraethylorthosilicate hybrid glasses by the sol-gel method

Abstract: In recent years, tetraethyl orthosilicate (TEOS) has been used as a precursor in preparing inorganic glasses. The technique, known as the sol-gel process involves a two step poly(hydrolysis-condensation)reaction sequence. The work presented here is concerned with the utilization of a suitably functionalized oligomeric “glassy” organic polymeric component in this two step sequence. In particular, an amine terminated amorphous poly(arylene ether) ketone, (PEK), end capped with isocyantonropyl triethoxy silane was used to synthesize a hybrid network with TEOS. Low incorporation of the PEK oligomers into the network caused by early vitrification can be eliminated by carrying out reactions at elevated temperatures. A systematic change in mechanical and physical properties of the hybrid glass has been found with TEOS content and the annealing temperatures. A model for the network has been proposed based on structural and morphological evidence.

Aluminum sec-butoxide modified with ethylacetoacetate: An attractive precursor for the sol-gel synthesis of ceramics

Abstract: Aluminum sec-butoxide can be modified by chelating ligands such as ethylacetoacetate (etac). Al(OBus)2(etac) was characterized by IR, 27Al, 13C and 1H NMR: several oligomeric species containing 4, 5 or 6 coordinated Al atoms could be present. During hydrolysis, butoxy groups are rapidly removed, but the presence of less hydrolyzable ligands slows down the reactivity of the precursor towards water. Precipitation can be avoided. Thus, this precursor appears quite attractive for the preparation of multicomponent ceramics such as α-cordierite. Modified aluminum alkoxide, tetraethoxysilane and magnesium acetate were used to obtain monolithic gels that convert into α-cordierite at 1000°C. The conversion processes, from the solution to the gel and the ceramics, were followed by 27Al and 29Si NMR in both liquid and solid states.

Thermal evolution of gels derived from CH3Si(OC2H5)3 by the sol-gel method

Abstract: The structure change of CH3Si(OC2H5)3 (MTES)-derived gels on heating in N2 and NH3 to form C- and N-containing SiO2 glasses, respectively, was examined by IR spectroscopy and thermal analysis. The gel was considered to consist of siloxane polymers containing strained 4-fold siloxane ring structure. The SiCH3 and 4-fold rings were thermally cleaved around 700°C to form SiH, SiOH and carbonous matter in N2, and SiNH2 as well as SiH and SiOH bonds in NH3. The SiNH2 was condensed at higher temperatures, resulting in N-containing SiO2 glass.

Synthesis of Hydroxyapatite from Metal Alkoxides through Sot-Gel Technique

Abstract: Hydroxyapatite was synthesized through the sol-gel technique using Ca(OC2H5)2 and P(OC2H5)3 as starting metal alkoxides. It has been found that the pH of the starting solution is one of the most important factors affecting the formation of hydroxyapatite, The sol-gel reaction rate, the reaction products and the crystalline phases precipitated in heated gels have depended mainly on the pH range of the solution, that is, whether the solution is alkaline (pH≥9), neutral (8≥pH≥6) or acidic (ph≤5). Only hydroxyapatite crystals are precipitated and no additional phases are found when gels derived from the solution of 6 to 8 in pH and more than 60% in water content are calcined above 600°C. The resultant hydroxyapatite crystals obtained have a plate-like structure of about 1000A in size, resembling closely the hydroxyapatite found in living tissues.

Preparation of Small-Particle-Size, Semiconductor CdS-Doped Silica Glasses by the Sol–Gel Process

Abstract: The sol–gel process has been applied successfully to the preparation of small-particle-size CdS-doped silica glasses with a significant quantum size effect. Gels prepared through the hydrolysis of a complex solution of Si(OC2H5)4 and Cd(CH3COO)2·2H2O were heated at 500°C, then reacted with H2S gas to form fine, hexagonal, CdS-microcrystal-doped glasses. The optical absorption edge is blue shifted by ∼0.4 eV compared with the bulk absorption value of CdS crystal. This result is interpreted in terms of a quantum-confinement effect of small crystal size.

Metal oxide porous ceramic membranes with small pore sizes

Abstract: A method is disclosed for the production of metal oxide ceramic membranes of very small pore size. The process is particularly useful in the creation of titanium and other transition metal oxide membranes. The method utilizes a sol-gel process in which the rate of particle formation is controlled by substituting a relatively large alcohol in the metal alkoxide and by limiting the available water. Stable, transparent metal oxide ceramic membranes are created having a narrow distribution of pore size, with the pore diameter being manipulable in the range of 5 to 40 Angstroms.

Electrical conductivity of tin oxide films prepared by the sol-gel method

Abstract: A preliminary study concerning the electrical conductivity of tin oxide films obtained by a procedure of dipping sol-gel technique onto glass substrate is reported. The tin oxide-sol solution was prepared starting from stannous chloride. Crystallization of amorphous tin oxide was followed by means of XRD and DTA analyses. The morphology of the coatings was observed by SEM. By this method, it has been verified that tin oxide can be easily doped by means of zirconium and titanium oxides, giving films, in which electrical conductivity is strongly increased.

Synthesis of Glass‐like Cordierite from Metal Alkoxides and Characterization by 27Al and 29Si MASNMR

Abstract: A sol-gel process for the preparation of glass-like cordierite and ceramic oxide powder is described. Metal alkoxide precursors were dissolved in 2-methoxy ethanol and the metal cations were complexed using 2,4-pentanedione to overcome different hydrolysis rates of metal alkoxides which lead to microscopic inhomogeneities during gelation. Heating the gel in an aerated furnace resulted in ultrafine cordierite powder of stoechiometric composition at a relatively low temperature. Cordierite gel and glass were also prepared by other methods and compared with the above gel. The environments of aluminium and silicon in the glass and gels heated at various temperatures were studied using 27 Al and 29 Si magic angle spinning nuclear magnetic resonance (MASNMR) spectroscopy. Most of the heated gels and the glass showed resonance due to pentacoordinated aluminum along with 4- and 6-coordinated aluminum. The homogeneities of the resulting gels and glass are compared using the MASNMR data

Sol-gel processing and properties of lead magnesium niobate powders and thin layers

Abstract: Lead magnesium niobate powders and thin layers were formed from an alkoxide-based solution by sol-gel methods. The solution was synthesized by reacting a magnesium-niobium alkoxide solution with a lead acetate-based precursor solution. The effects of gelation conditions on the properties of the dried gel, and on the organic decomposition behaviour and crystalline phase development in gel-derived powders are reported. Gels prepared with greater molar ratios of water to alkoxide (3∶1) had the largest surface areas (130 m22 g−1) and required the lowest temperature (320 ° C) for organic removal. The perovskite phase first appeared at temperatures near 700 ° C, and developed at a faster rate in gels prepared with higher water contents. Approximately 95% developed after 1 h at 700 ° C, or 5 min at 775 ° C. Dielectric thin layers were prepared on platinum-coated silicon substrates by a multilayered spin-casting method. The perovskite phase formed most readily in the thin layers by a fast-firing treatment at 800 ° C. Microstructures and electrical properties are reported for the integrated thin layer dielectrics. Ferroelectric hysteresis loops were observed.

Processing and Properties of Cellular Silica Synthesized by Foaming Sol‐Gels

Abstract: A novel process for fabricating lightweight, cellular ceramics from sols is presented. The process utilizes the rapid viscosity change during gelation to stabilize the structure of a foamed silica sol. Manipulation of gel viscosity and foaming agent concentration resulted in a minimum cell size of 90 μm at 31% density and minimum density of 17% with average cell size of 400 μm. The flexural strength compared favorably with space shuttle tiles and sintered hollow glass spheres. From −50° to 150°C, the dielectric constant ranged from 1.51 to 1.55 for a 20% dense foam and was slightly dispersive, whereas the dielectric loss was comparable with fused silica.

Sol-gel preparation and transport properties of a tin oxide

Abstract: Much work has already been carried out concerning the preparation and study of tin (IV) oxide, alone and combined with other oxides, for applications related to electrical and catalytic properties. The methods of preparation have included: (i) coprecipitation of hydroxides or oxalates starting from diluted salts; (ii) solid state reactions, by decomposition of carbonates, hydroxides; (iii) deposits by vapour phase deposition; and (iv) oxidation of SnO (e) in SnO2 by epitaxial growth. It has been noticed that the properties related to this oxide depend on the texture of the material. For this reason the sol-gel procedure was used to prepare such a compound. Generally the gel is formed by peptisation (with nitric acid) of Sn(OH)4 obtained after action on tin chloride by ammonia [1-3]. The study of these ge][s has shown that there is a temperature dependent structural evolution [3, 4] with oxygen deficient oxide [5]. Within this scope we have prepared a pure gel using a new method, the hydrolysis of a tin (II) alkoxide, and compared its structure to other gels. We have studied this gel by the method of complex permittivities. In fact, for a long time, dielectric loss measurements have been the basis of various investigations on many chalcogenides and a few oxides [6, 7]. The results obtained have usually been interpreted by theoretical models using the concept of charge carriers hopping between localised sites. Experimental data of the a.c. conduction as a function of frequency have often exhibited the following behaviour [6-10]: % = + (1) where Odc is the conductivity in direct current; o-' (co), the real part of the polarisation conductivity, is related to the frequency, f, by the expression: cr'(co) = Ac0' (co = 2zcf) where A and s are constants at a given temperature. In this work the a.c. conductivity, as a function of frequency and temperature of the oxide was measured. The a.c. conductivity data were analysed using a numerical approach described previously [11]. The results showed that the a.c. behaviour of this poorly organised tin oxide could be described by a well-known theoretical model related to thermally activated electron hopping. The method used for the synthesis of the tin alkoxide

Control of pore size distribution of silica gel through sol-gel process using water soluble polymers as additives

Abstract: To control the pore size distribution of silica gel, several kinds of water soluble polymers were added in the sol-gel process starting from tetraethyl orthosilicate. The polymers employed here were classified into three groups. The first group comprised uncharged polymers such as polyvinyl alcohol and polyethylene glycol, which affected only the amount of micropores and small mesopores ( 20nm). The third group comprised proteins such as lipase and albumin and decreased the number of micropore and mesopores and greatly increased the number of macropores. The results could almost be explained in terms of the principal effects of the polymers on a process of sol particle growth.

Synthesis of Monodispersed Spherical β-Silicon Carbide Powder by a Sol-Gel Process

Abstract: Monodispersed spherical β-SiC powder was synthesized by heating spherical gel powder derived from the hydrolysis of a mixture of phenyltriethoxysilane and tetraethyl orthosilicate. The solution was prepared in a beaker and hydrolyzed by NH4OH without stirring. The monodispersed spherical gel powder of submicrometer size was obtained when the added amount of NH4OH was greater than 16 moles per mole of silane plus alkoxide, and it became monodispersed spherical β-SiC powder by heat-treating at 1500°C for 4 h in an Ar atmosphere. The SiC content of the powder was 92.6 wt%.

Novel poly(n-butyl methacrylate)/titanium oxide alloys produced by te sol–gel process for titanium alkoxides

Abstract: A method has been developed to cast novel organic/inorganic hybrid films from multicomponent solutions containing titanium alkoxides, poly(n-butyl methacrylate), water, and isopropanol of prescribed compositions. The sol–gel reactions and controlled drying procedure yielded materials of mechanical integrity for which the multistep thermal degradation profile of the organic polymer has been significantly modified. A crystallization exotherm, presumably due to anatase formation, is seen for tetraethyl titanate-derived films beyond the organic degradation temperature. The trend of mechanical tensile parameters with increasing Ti oxide content depicts progressive material strengthening. FT-IR as well as the thermal and mechanical studies of these films suggest a highly unconnected and heterogeneous Ti oxide phase.

Preparation of Nd-doped silica glasses by the sol-gel method

Abstract: Monolithic NdAl co-doped silica glasses were prepared by the sol-gel method with double metal alkoxide of neodymium and aluminum, and their fluorescence properties were evaluated and compared with those of Nd-doped silica glasses. The dispersibility of Nd 3+ in the NdAl co-doped silica glasses increases significantly because the structure of the double metal alkoxide is maintained until the final glassy state, with a higher Nd concentration of up to 4 mol% as Nd 2 O 3 . It was found that concentration quenching caused by clustering of Nd 3+ , as shown in the case of glasses with a relatively higher Nd concentration, was controlled. Aerogel prepared with a CO 2 extractor had the merit of removing the OH groups. Because of the large pore size and the resultant higher densification temperature, a relatively longer fluorescent lifetime was observed.

Preparation of inorganic/organic hybrid gels by the sol-gel process

Abstract: Preparation of inorganic/organic hybrid gels was carried out using tetraethoxysilane and organic polymers via the sol-gel process. Hydroxyl terminated siloxane polymers were reactive enough to be incorporated into the silica networks. However hydroxyl terminated organic polyethers failed to react; thus most of them were extracted by organic solvents. Induction of triethoxysilyl groups were found very effective to prepare truly hybrid gels by the sol-gel process.

Sol-gel processing of composite PbTiO3/PLZT thin films

Abstract: A novel sol-gel fabrication approach was developed for the fabrication of composite PbTiO3/PLZT thin films. The approach involves the deposition of a perovskite (PbTiO3) interlayer prior to deposition of the PLZT film. Composite PbTiO3/PLZT sol-gel thin films were prepared on (110) sapphire, platinum-coated sapphire, and platinum-coated silicon substrates. These films were highly crystalline and exhibited preferred crystallographic orientation. This perovskite interlayer approach is applicable to a wide variety ferroelectric thin film and substrate materials.

Preparation, processing, and fluorescence characteristics of neodymium-doped silica glass prepared by the sol-gel process

Abstract: For high-powered laser applications silica glass has favorable thermal and mechanical properties. However, the limited solubility of neodymium oxide in silica has prevented its use without the addition of other components, both to increase the solubility of the neodymium oxide and to reduce processing temperatures, minimizing impurities such as platinum inclusions. The preparation of up to 20 wt% Nd-doped silica has been reported recently using the sol-process, and the fluorescence spectrum of a one weight per cent composition was given (E.J.A. Pope and J.D. Mackenzie, J. Non-Cryst. Solids 106 (1988) 236). In this paper, the preparation and thermal processing of 3–5 wt% compositions, including BET, density, thermal and spectral analyses, are reported. The fluorescence spectrum of the material and its fluorescence lifetime are then correlated with TEM/EDS results.

Sol-gel processing by inroganic route to obtain a TiO2PbO xerogel as ceramic precursor

Abstract: The sol-gel process for ceramic preparation has been the focus of interest in the past few years With this kind of chemical process it is possible to enhance the purity and reactivity of the system and to ensure the control of the microstructure of the obtained ceramics Pure lead titanate has interesting properties as ferroelectric ceramic, but it has not been used because of its poor sinterability In this work, a sol-gel process by the inorganic route of the TiO 2 PbO system is presented With this process a xerogel is obtained which leads to pure PbTiO 3 ceramic materials after firing

Nitridation of the Sol–Gel-Derived Titanium Oxide Films by Heating in Ammonia Gas

Abstract: Dip-coated sol-gel derived TiO 2 films on an alumina substrate were converted to nonstoichiometric titanium nitride films by heating at approximately 1000°C in NH 3 gas. TiO 2 films made from TiO 2 sols prepared from Ti(O-i-C 3 H 7 ) 4 and stabilized by diethanolamine were more easily nitrided than those from sols containing HCl as a deflocculant reagent. This appears to be a result of the more porous structure of the former films.

Elastic Properties of Porous Silica Derived from Colloidal Gels

Abstract: The elastic properties were determined for gel-derived vitreous silica sintered from a relative density of 0.15 to full density. The gels were made from colloidal silica, potassium soluble silicate, and formamide. The soluble silicate was found to reinforce the gel network during gelation and its concentration relative to the colloidal silica affected Young's modulus for samples of relative density, 0.15 to 0.6 Young's modulus varied from 300 to 700 MPa at a relative density of 0.15 as a function of the soluble silicate concentration. Young's modulus at full density was measured to be 72.6 GPa. Poisson's ratio was found to be a function of relative density.

Layered sol-gel coatings

Abstract: A method of providing a substrate with a thin film coating of multiple layers by a single immersion of a substrate having active hydrogens on its surface into a coating solution including two or more dissimilar hydrolyzable and condensable organometallic compounds, water, a solvent, and a base, the solution having undergone substantially no polymer growth, for a time sufficient to form in situ the film on the substrate, removing and drying the coated substrate.

Electrical conduction in sol-gel derived glass-metal nanocomposites

Abstract: Glass-metal nanocomposites involving iron, nickel and copper in a silica glass matrix have been prepared as films on glass substrates by the sol-gel route. The metal phase is introduced as chloride and silicon tetraethoxide is used as the glass precursor. DC electrical resistivity of these nanocomposites has been measured over the temperature range 100 to 350 K. Resistivities varying from 10 Omega m to 106 Omega m have been obtained by controlling the volume fraction of the different metal phases within the silica glass. Low-temperature resistivities of the samples are controlled by a simple activation with energies in the range 0.01-0.1 eV. At temperatures above 150 K some of the nanocomposites exhibit another activated mechanism. The latter is believed to arise due to hopping of electrons between the localised states formed by the distributed metal atoms within the silica glass matrix.