Showing papers on "Sol-gel published in 1988"

Preparation of Pb(Zr,Ti)O3 thin films by sol gel processing: Electrical, optical, and electro‐optic properties

Abstract: Crack‐free transparent ferroelectric polycrystalline Pb(Zr,Ti)O3 thin films were prepared by spin‐coating solutions of complex alkoxides. The preparation of stock solution, firing, and annealing of films was described. The coating of the intermediate layer of Al2O3 increased the adhesion between Pb(Zr,Ti)O3 thin films and glass substrates. The crystalline phases of films with varying Zr/Ti ratios were investigated. The dielectric constants were about 260. The remanent polarization and coercive field were 6.6 μC/cm2 and 26.7 kV/cm, respectively. The refractive index of the perovskite Pb(Zr,Ti)O3 films was 2.6 at 6328 A, and the absorption edge was at 3400 A. The quadratic and linear electro‐optic effects were measured with respect to the Zr/Ti ratio from 40/60 to 60/40 for films grown on glass substrates. The quadratic and linear electro‐optic coefficients were about 1×10−18 m2 /V2 and 2.4×10−11 m/V at 6328 A, respectively.

Precursors for sol-gel preparations

Abstract: Precursors used for the synthesis of oxide systems by the sol-gel method are reviewed and their role in the various stages of the process is discussed. Emphasis is given to alkoxide precursors and to their physical and chemical properties. In particular, the following topics are discussed: degree of oligomerization, volatility, viscosity, reactions with alcohol, molecular association between alkoxides, hydrolysis, stabilization against hydrolysis. Some information about preparation methods, commercial products and availability is also given. Among non-alkoxide precursors, nitrates, carboxylates, acetylacetonates, chlorides and other inorganics are described from the point of view of their use in sol-gel preparations. They are compared, when possible, with the corresponding alkoxides.

Sol-gel antireflective coating on plastics

Abstract: An antireflection film made from a reliquified sol-gel hydrolyzation, condensation polymeric reaction product of a silicon, alkoxides and/or metal alkoxides, or mixtures thereof The film is particularly useful for coating plastics

Structure property behavior of polydimethylsiloxane and poly(tetramethylene oxide) modified TEOS based sol-gel materials V. Effect of titaniumisopropoxide incorporation

Abstract: Titanium has been successfully incorporated into polydimethylsiloxane and poly(tetramethylene oxide) modified TEOS based materials using sol-gel chemistry. Incorporation of titanium into the oligomer modified TEOS based materials altered the physical properties, however, the gross structural features remained the same.

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

Abstract: 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.

Nd-doped silica glass I: Structural evolution in the sol-gel state

Abstract: Silica glass containing neodymium is a potentially important laser glass because of its low thermal expansion coefficient and high temperature stability. The sol-gel method is highly advantageous in that it permits the uniform distribution of relatively large concentrations of neodymium in a silica host with respect to conventional processing techniques. In this paper, the evolution of the Nd-doped silica gel as a function of heat treatment temperature, from the liquid state to the glassy state, is discussed. The fluorescence spectra for sol-gel derived neodymia-silica glass is presented. A tentative structural model of the local environment around the neodymium ions in silica glass is presented.

Formation of mullite and other alumina-based ceramics via hydrolytic polycondensation of alkoxides and resultant ultra- and microstructural effects

Abstract: This manuscript discusses the formation of alumina-based ceramic materials from alkoxide starting materials. Particular attention is given to Al2O3-SiO2 systems, especially mullite. When the oxide network of these materials is formed via chemical polymerization of alkoxides, the ultrastructure of the resultant condensates is fundamentally affected by the nature and kinetics of the condensation reactions. Crystallization of mullite from an amorphous Al6Si2O13 at ∼ 980°C appears to be related to the intimacy and homogeneity of the aluminium-silicon bonding in the precursor networks. Network inhomogeneities due to the different hydrolysis rates of mixed alkoxides diminish by reduction of the chemical encounter rates with water during the hydrolytic polycondensation. Mullite produced from homogeneous and stoichiometric condensate shows a large spontaneous densification at ∼ 980° C. The microstructure of this material shows no indication of a liquid phase at the grain boundaries.

Characterization of B2O3-SiO2 glasses prepared via sol-gel

Abstract: B2O3-SiO2 glasses were prepared by the sol-gel method from boron and silicon alkoxides. The gels were densified by several heat-treatments at temperatures above 800° C. The gel-glass transition was studied with the data obtained from differential thermal analysis, dilatometry and thermal evolution of density. The structural features of the glassy materials prepared were analysed by means of infrared (IR) and near-infrared (NIR) spectroscopy. The IR spectra indicated the existence of mixed Si-O-B bonds, preferential located at the outside of the material particles. The NIR spectra, recorded from gels heat-treated at low temperatures, demonstrated the existence of a great number of OH− groups varying in nature, as well as their evolution and their positional changes on the silica particle surface as a function of the heat-treatment.

Bulk and thick films of the superconducting phase YBa2Cu3O7−y made by controlled precipitation and sol‐gel processes

Abstract: The synthesis of high‐temperature superconducting oxides using solution chemistry has been investigated and physical properties are compared to ceramics obtained by conventional solid‐state reactions. We report on controlled precipitation and sol‐gel processes, both of which produce materials with particle sizes smaller than 5 μm. We find that the superconducting properties of the high Tc ceramics are affected by their manner of preparation, such that the transitions are slightly lower in temperature, but sharper, for samples made by solution rather than solid‐state chemistry. The ability to prepare stable viscous gels provides an opportunity for obtaining large areas of superconducting coatings. For thick films on alumina or silicon substrates, contamination from the substrate is shown to be a problem. Finally, we observe that the sol‐gel process lowers the synthesis temperature by 100 °C.

AMINOSILS: New solid state protonic materials by the sol-gel process

Abstract: A new family of ORganically MOdified SILicates (ORMOSILs) has been developed using the sol-gel process. Monolithic transparent films are obtained from a solution containing an aminated organotrisiloxane, an acid (HX) and water. The alkylamine chain R, acts as a network modifier and, as a base, interacts with the acid, to yield final material represented by the formula SiO 3 2 R-(HX) x , O ≤ x ≤ 0,5 . We propose to call these materials AMINOSILs. The dried products are hard and non-porous, with densities ranging around 1.3. They exhibit a glass transition around −30°C and are thermally stable in air up to 180°C. Conductivity studies show a non-Arrhenius behaviour characteristic of a liquid-like state, and a high conductivity ( ⋍10 −5 Σ −1 cm −1 ) at room temperature.

Sol-gel processed BaTiO3: Structural evolution from the gel to the crystalline powder

Abstract: An amorphous solid, precursor to BaTiO3, has been prepared by the sol-gel route and its structural organization studied by LAXS as a function of temperature. In the low-temperature solid, the barium atoms keep the same environment as in barium acetate. The titanium alkoxide reacts with acetic acid to give an hexanuclear molecule.

Fine-patterning on glass substrates by the sol-gel method

Abstract: A novel process has been developed to form fine patterns on glass substrates. The process consists of the following steps: (1) coating of gel film on a substrate, (2) patterning on the gel film, and (3) heat-treatment. The key point in this process is the control of hardening of the gel films by the addition of organic polymers such as polyethylene glycol (PEG). The addition of PEG increased the viscosity of the coating solutions and delayed the gelation time, but had little effect on the optical properties of the resultant heat-treated glass films. The newly developed process is expected to be applicable to the formation of the optical devices such as diffraction gratings and pre-groovesof optical memory disks.

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

Abstract: 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(OC 2 H 5 ) 3 F, is titrated into SiO 2 sol solutions. The fluorine content in silica glass depends on: the fluorine concentration in the gel, the specific surface area of SiO 2 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.

Sol gel formation of polysilicate, titania, and alumina interlayers for enhanced adhesion of metal films on substrates

Abstract: A metal-coated substrate, e.g., of glass, ceramic, or a hydroxy-functionalized material, wherein the improvement comprises a polysilicate, titania, or alumina interlayer between the substrate and the metal coating. The interlayer may have a porous microstructure, e.g., a polysilicate interlayer with an average pore size on the order of 50-150 Angstroms. Such articles, e.g., in the form of metal-coated fibers, may suitably be employed as reinforcing media in material composites having utility in structural applications, such as EMI shielding elements. Also disclosed is a corresponding method for forming a metal coating on a substrate by the provision of an interlayer of the above type. The interlayer may suitably be formed by applying to the substrate a sol gel dispersion of the polysilicate, titania, or alumina material, followed by drying of the applied dispersion.

ZrO2-SiO2 materials prepared by sol-gel

Abstract: Gels with composition xZrO2−(100−x)SiO2, x = 10−55, were prepared in different conditions using zirconium acetylacetonate and TEOS as precursors. Gels treated at different temperatures up to 1100°C were characterized by X-ray diffraction, IR spectroscopy and TEM. Preparation conditions determined the subsequent development of crystalline phases following thermal treatment. Monoclinic zirconia segregation dispersed in a silica matrix occurred when the gels were prepared in a strong hydrocloric acid medium. Preparation with a lower acid content favours instead the formation of very small crystals of tetragonal zirconia.

Superconducting Oxide Thin Films Prepared by Sol–Gel Technique Using Metal Alkoxides

Abstract: Superconducting oxide thin films of the YBa2Cu3Ox system have been prepared on zirconia (YSZ) substrates by sol–gel technique using metal alkoxide solution. Yttrium butoxide, barium methoxide and copper methoxide in a given atomic ratio were dissolved in a triethanolamine–methanol solution. Coating of the resultant solution, heating at 800 °C in air and annealing in an oxygen atmosphere resulted in superconducting oxide thin films. The onset of superconductivity was seen at 98 K and the resistance rapidly decreased with decreasing temperature, reaching substantially zero at 56 K.

Preparation and Characterization of Machinable Mica Glass-Ceramics by the Sol-Gel Process

Abstract: The preparation and characterization of machinable mica glass-ceramics by the sol-gel process are described. The thermal behavior of the precursor and the optimum conditions for calcination, molding, and sintering are given. Physical, chemical, thermal, electrical, and mechanical properties are investigated, and the machinability of the ceramics is discussed.

Superconducting Oxides by the Sol-Gel Method Using Alkoxides

Abstract: The superconducting oxide Y-Ba-Cu-O has successfully been fabricated by the sol-gel method using Y, Ba, and Cu alkoxides as starting materials. Powders with specific surface area of 15 m2/g are prepared by this method. An X-ray diffraction pattern attributed to a 1-2-3 structure appears at 750°C and a bulk sample with Tc of 90 K is made by 900°C sintering in an oxygen atmosphere. A film of Tc=80 K with c-axis orientation is also fabricated using the sol-gel paste on an yttrium-stabilized ZrO2 substrate.

Preparation of CeO2-TiO2 coatings by the sol-gel process

Abstract: Yellow coatings have been prepared from titanium tetraisopropoxide and cerium chloride by the sol-gel dip-coating process, and the transmittance, chromaticity, thickness and acidic durability of the coating films produced on glass or aluminum were studied. The effects of the pH of the solution on the preparation of the transparent and homogeneous coating films were studied by adding the alkali or acid, and as a result transparent and homogeneous yellow coating films were prepared by dipping the substrate in a solution containing acetic acidic solution. The acidic durability of the aluminum foil was improved by dip-coating the brilliant yellow CeO 2 -TiO 2 coating three times.

Multilayer SiO2 and TiO2 coatings on glasses by the sol-gel process

Abstract: Glass films of pure SiO2 and TiO2 have been prepared on sodalime silica flat slide glasses by the sol-gel process using the dip-coating technique from TEOS and Ti(OC3H7)4 solutions. The various parameters such as chemicals concentrations, viscosity, type of catalyst, withdrawal speed and temperature of densification leading to the obtention of good and adherent coatings with definite film thicknesses are reported. The same technique has been used for the depositon of layers of colored films SiO2MxOy (M = Co, Mn, Nd and Cr). Brilliant yellow coatings have been obtained with TiO2CeO2.

Silica glass tubes by new sol-gel method

Abstract: Silica glass tubes were prepared by following a new sol-gel method. The pH of the sol containing hydrolyzed Si(OC2H5)4 with HCl and colloidal silica was adjusted by adding ammonia solution. Sol was poured into a cylindrical tube and gelled while rotating cylinder at 1000 rpm to form a tube-shaped gel. Then the gel was dried carefully to be tube dry-gel and finally sintered to become a clear silica glass tube. The effects were examined of the speed of rotation before and after gelation, to form the tubular gel, upon the shape and appearance of the derived tubes. The best speed of rotation was about 1000 rpm, and an adequate long period of rotation after gelation was necessary to maintain the tube shape. One of the large silica tubes had dimensions of 26 mm outside diameter, 13 mm internal diameter and 1000 mm length, and the ovality of the silica glass tube was 0.02%. The derived tube can be applied in a silica glass tube for an optical fiber fabrication process or in a furnace tube for IC production.

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

Abstract: 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.

Al2O3-TiO2 and Al2TiO5 ceramic materials by the sol-gel process

Abstract: Ceramic materials with a very low thermal expansion coefficient are synthesized by the sol-gel process. The binary gel is obtained by hydrolysis and polycondensation reactions of organometallic compounds of aluminium and titanium. The thermal evolution of the amorphous powder is followed by DTA and TGA measurements. Structural evolution is followed using X-ray diffraction. The crystallization of the TiO2 rutile and Al2O3 corindon starts at 700 and 900°C respectively. The transformation of Al2O3 and TiO2 into Al2TiO5 appears between 1200 and 1300°C. The densification of the powder is performed by the hot pressing process. The shrinkage of the powder was previously followed by dilatometric measurements. The physical properties of the final material are studied as a function of pressing parameters.