Showing papers on "Sol-gel published in 1989"

Rigidochromism as a probe of gelation and densification of silicon and mixed aluminum-silicon alkoxides

Abstract: Etude par spectrometrie luminescence, de la gelification du melange aluminosilicate, tetraethoxysilane

Synthesis and Microstructure of Highly Oriented Lead Titanate Thin Films Prepared by a Sol-Gel Method

Abstract: Thin films of PbTiO3 were deposited on fused silica, resistor-grade alumina, and single-crystal (100) MgO by a sol–gel processing method. Whereas the films deposited on silica and alumina substrates were randomly oriented and polycrystalline, highly {100} oriented PbTiO3 films were grown on the MgO single crystals. The perovskite-type structure was observed with films deposited on the single-crystal MgO and annealed at temperatures as low as 470°C, whereas a pyrochlore-type strcuture was observed with films on fused silica and alumina processed in a similar manner. All films heat-treated at temperatures in excess of 570°C showed significant formation of a second PbTi3O7 phase. The films were characterized by electron microscopy and glancing-incidence-angle X-ray diffraction.

Sol‐gel preparation of high‐Tc Bi‐Ca‐Sr‐Cu‐O and Y‐Ba‐Ca‐O superconductors

Abstract: High‐Tc superconducting oxides have been prepared by a liquid‐mix technique using etylene‐diamine‐tetra‐acetic acid (EDTA) as a complexing agent. Bi‐Sr‐Ca‐Cu oxides and Y‐Ba‐Cu oxides were made using this technique giving a better compositional homogeneity, a more precise control of the cation stoichiometry and a decreased firing temperature as compared to conventional material produced by a solid‐state reaction. This technique extends the amorphous citrate process to those systems where no citrate complex exists. EDTA binds with most metallic elements of the periodic table, making this technique a versatile tool in the production and study of these new ceramic materials. Therefore, the method is easily adapted to the preparation of new superconducting oxides.

A silicon-29 NMR study of the structural intermediates in low pH sol-gel reactions

Abstract: Silicon-29 NMR is used to study the structural intermediates in tetramethyl and tetraethyl orthosilicate (TMOS and TEOS) low pH sol-gel reactions. Differences in the number of compact ring structures are related to differences in gel times. Reactions are followed at various silicon: water: acid molar ratios.

Hybrid Gels for Homoepitactic Nucleation of Mullite

Abstract: Hybrid gels, defined as gels from mixtures of polymerically and colloidally derived sols, offer many opportunities for crystalline microstructure development upon heating. In this study, hybrid mullite gels are formed by mixing a colloidal boehmite—silica sol with a polymeric aluminum nitrate—tetraethoxysilane-derived sol. The polymeric gel crystallizes in situ to form mullite that acts as seed crystals for homoepitactic nucleation during the subsequent transformation of the colloidal component of the hybrid gel. Compared with the entirely colloidal gel, the introduction of a 30 wt% polymeric gel fraction results in an increase in apparent nucleation frequency from ∼5x1011 to ∼1x1014 nuclei / cm3 at 1375°C, a reduction in high-temperature grain size from 1.4 to 0.4 μm at 1550°C, and an increase in the degree of microstructural homogeneity, as evidenced by intragranular pore removal.

Fabrication of hollow silica aerogel spheres by a droplet generation method and sol--gel processing

Abstract: A system was designed to produce hollow silica aerogel spheres by combining a droplet generation method with sol–gel processing. Simultaneous passage of an alkoxide solution and a fill gas, through a dual‐nozzle system, formed a continuous stream of hollow droplets in the fluid state. The spherical droplets were levitated in a basic atmosphere containing ammonia which converted the sol into a rigid gel state. Supercritical drying was used for the elimination of residual solvents without shrinkage and cracking.

Preparation of cordierite glass by the sol-gel process

Abstract: Cordierite glass was prepared by the sol-gel process from magnesium, Al(OC4H9)3 and Si(OC2H5)4. The solution having cordierite-like structure was formed by refluxing the raw materials, followed by gelling and drying to 150° C in the saturated water vapour to hydrolyse completely. On heating the gel, AlO6 groups transformed into AlO4 groups, in which aluminium ions were incorporated in SiO4 tetrahedra units to form a Si-O-Al network structure. The gel was converted into the transparent dense glass by heating above 850° C. The glass transition temperature, 820° C, thermal expansion coefficient, 3.1×10−6° C−1, Vickers hardness, 6.22 GPa and density, 2.59 g cm−3 were almost the same as those of the conventional glass.

Low‐Temperature Sintering of Seeded Sol—Gel‐Derived, ZrO2‐Toughened Al2O3 Composites

Abstract: Seeding a mixture of boehmite (AIOOH) and colloidal ZrO2 with α-alumina particles and sintering at 1400°C for 100 min results in 98% density. The low sintering temperature, relative to conventional powder processing, is a result of the small alumina particle size (∼0.3 μm) obtained during the θ-to α-alumina transformation, homogeneous mixing, and the uniform structure of the sol-gel system. Complete retention of pure ZrO2 in the tetragonal phase was obtained to 14 vol% ZTA because of the low-temperature sintering. The critical grain size for tetragonal ZrO2 was determined to be ∼0.4 μm for the 14 vol% ZrO2—Al2O3 composite. From these results it is proposed that seeded boehmite gels offer significant advantages for process control and alumina matrix composite fabrication.

Preparation of Stoichiometric Crystalline Lithium Niobate Fibers by Sol‐Gel Processing with Metal Alkoxides

Abstract: Homogeneous and stoichiometric crystalline LiNbO3 fibers were successfully prepared above 450°C by sol-gel processing with metal alkoxides. Controlling the concentration of alkoxide solution and the amount of water used for partial hydrolysis made it possible to draw gel fibers having uniform diameters (10 to 100 μm) from the metal alkoxide solution. The gel fibers withdrawn crystallized directly to single-phase LiNbO3 fibers by heating from 450° to 600°C. The density of the crystalline LiNbO3 fibers was higher than 4.22 g/cm3, and the dielectric constant at room temperature was about 10 at 10 MHz.

Sol-gel method of making ceramics

Abstract: The invention concerns sol-gel techniques for making PZT-type ceramics having a perovskite-type structure and ferrite ceramics having a spinel-type structure. Colloidal hydrated oxide sols are mixed with metal salt solutions in the required concentration and the mixed sols dehydrated to form homogeneous gels which are fired at 250° C.-650° C., pressed and sintered at 900°-1300° C. The resulting ceramics include PZT4, PZT5 and lithium ferrites. These have excellent homogeneity, small grain size, high density and improved electrical properties, particularly low dielectric loss, compared with known ceramics of similar composition.

Preparation of Pb(Mg1/3Nb2/3)O3 Thin Film by Sol-Gel Method

Abstract: A thin film of Pb(Mg1/3Nb2/3)O3 was prepared by the sol-gel method using metal alkoxides. The X-ray analysis indicated that the perovskite phase was formed by sintering at over 600°C. The orientation and the crystallization rate were greatly influenced by the type of substrate and sintering conditions.

Preparation and characterization of zirconium oxide coatings prepared from ZrCl2(OC2H5)2 by the sol-gel process

Abstract: Transparent single-component ZrO 2 coatings on soda-lime glass substrates have been successfully prepared by the sol-gel route using ZrCl 2 (OC 2 H 5 ) 2 as the starting material. Detailed studies were carried out of the whole process including the synthesis of the organometallic compounds, the hydrolysis-polymerization and the sintering process. The results showed that zirconium dichloride ethoxide is preferred over other zirconium alkoxides for its enhanced stability on storage and for the feasibility of controlling the hydrolysis, i.e. the hydrolysis temperature changes the ZrCl 2 (OC 2 H 5 ) 2 precursor at a molecular level therefore modifying the whole hydrolysis-polymerization process. Infrared multiple internal reflection spectroscopy was used to monitor structural variation of the coatings during heat treatment and the results suggested the formation of SiOZr bonds between the coating films and the substrate. The results of X-ray diffraction and scanning electron microscopy showed that the ZrO 2 coatings remained in the amorphous state up to 400° C, but on increasing the heat treatment temperature to around 500° C oriented tetragonal ZrO 2 monocrystals were precipitated. It is shown that such coatings have excellent properties and are therefore of high technological interest.

Dye polymer/sol-gel composites

Abstract: Dye-containing polymers wherein the dyes are organic in nature are incorporated into glasses produced by a sol-gel technique. The glasses may be inorganic or organic-modified metal oxide heteropolycondensates. The dye-containing polymers are covalently bonded to the glass through a linking group. The products of this invention can be used to make optically clear colored films which can be employed in the imaging, optical, solar heat energy and related arts.

Preparation of Amorphous Al2O3 Thin Films from Stabilized Al-Alkoxides by the Sol-Gel Method

Abstract: Highly transparent amorphous thin films of Al2O3 were prepared by adding some chelating agents such as β-diketones to the Al-alkoxide/alcohol starting solutions. Among the β-diketones examined, ethylacetoacetate was the most effective in stabilizing the alkoxides against water, because of the formation of an Alchelate complex. The Al2O3 films obtained were amorphous and transparent below 700°C, whereas they crystallized in γ-Al2O3 at 900°C, which transformed to α-Al2O3 at 1000°C, accompanying the loss of transparency.

Ferroelectric Thin Films by sol-gel Processing

Abstract: The sol-gel processing of ferroelectric thin films is being investigated at Battelle. The ferroelectric materials included in this study are PbTiO3, Pb(Zr, Ti)O3 (PZT), and KNbO3. The sol-gel processing and crystallization of these films on fused silica, silicon, alumina, and single crystal SrTiO3 substrates is described. Sol-gel derived PbTiO3 thin films crystallized into the expected tetragonal perovskite structure when heated to 500 C and above. However, the crystallization of sol-gel PZT (20/80) thin films was found to be substratedependent. The heat-treated PZT films were amorphous when deposited on silica and silicon substrates. Crystalline perovskite PZT films were produced on alumina substrates, and epitaxial PZT films were produced on single-crystal SrTiO3. Heat treatment of sol-gel KNbO3 films on silicon and alumina substrates resulted in the crystallization of a variety of non-perovskite phases, but epitaxial growth of KNbO3 was observed on single crystal SrTiO3.

A two-step silica sol-gel process investigated with static and dynamic light-scattering measurements

Abstract: The hydrolysis-condensation reactions in mixtures of tetraethoxysilane, ethanol and water were followed in the basic step of a two-step silica sol-gel process with dynamic and static light-scattering techniques. The radii of the particles formed were found to be about 1.5 nm and in agreement with the results found with specific surface area measurements on the corresponding dried silica gels. The relatively high scattering found for mixtures having incomplete hydrolysis in the acid step was ascribed to the formation of large particles caused by prolonged hydrolysis-condensation in the basic step. The decreased light transmission as a function of the decreasing density of dried silica gels was assigned to Rayleigh scattering by pores.

Continuous silica glass fiber produced by sol-gel process

Abstract: We have developed a new industrial technology related to the sol-gel process for manufacturing continuous silica glass fiber. This process has remarkable features such that the fiber can be made at temperatures below 1000° C. The sol-gel derived fiber contains very few impurities, and both shrinkage and drop in strength at high temperatures are much lower than those of fused silica glass fiber. With these features the fiber and its products can be used as electronic materials and thermal insulation at high temperatures.

Hydrolysis-condensation reactions of silica gels during autoclave drying

Abstract: The effect of the drying temperature, drying time and the presence of water vapour during the autoclave process on the specific surface area ( A ) of gels, prepared by a two-step sol-gel process was investigated for a variety of silica gels. Up to about 300°C a large decrease of A was found only for silica gels, in which an excess of water was present. This decrease was attributed to hydrolysis-condensation reactions at the surface of the silica particles. The decrease at higher temperatures was assigned to organic cracking processes. If an excess of water was added to the autoclave a large decrease of A was found for all the compositions used, depending on the relative amount of water in the autoclave and on the autoclave drying time. Depending on these conditions gels were found consisting of large strongly necked silica particles, as shown in TEM micrographs.

Study of the formation of silicoaluminates by the sol-gel method, by means of IR, DTA and TGA

Abstract: Silicoaluminates were synthesized from aluminum and silicon alkoxides with varying water content. The obtained solids were studied by infrared spectroscopy (IR), Thermal Differential Analysis (DTA) and Thermogravimetric Analysis (TGA). The results show a positive water effect in the rate of polymerization and in the number of Si tetrahedral functional groups (OR) substituted by the aluminum. This type of material has high surface area, suitable to act as a catalyst support.

Magnetic, electrical, and microstructural properties of YBa 2 Cu 3 O 7 : A comparison of sol-gel, co-precipitated, and solid state processing routes

Abstract: Samples of YBa{sub 2}Cu{sub 3}O{sub 7} were prepared by sol-gel, co-precipitation, and solid state processes. Sol-gel samples were prepared from a solution of yttrium, barium, and copper nitrates dissolved in ethylene glycol, co-precipitated samples were made by the amorphous citrate method, and solid state samples were prepared by conventional high temperature reaction of the appropriate metal oxides and carbonates. The sol-gel process was shown to yield superconducting samples of superior Meissner effect, critical current, and critical field. The co-precipitated samples contain impurities that affect the critical properties. The sol-gel and co-precipitated processes yield materials with well-formed, plate-like particles with a fairly uniform size of about 10 {mu}m. The grains in the solid state sample are smaller but have a much wider distribution of sizes than the samples prepared by solution methods.

Functional hybrid compounds and thin films by sol-gel process

Abstract: Materials and a process for producing the materials are useful in electrophotographic imaging members. The materials are produced through a sol-gel process wherein a hybrid material, which may be obtained through hydrolysis and condensation, includes an inorganic glassy network, a flexible organic subunit and a functional subunit. The latter two subunits may be unitary. The materials can be easily coated for fabricating electrophotographic imaging members and the like.

Synthesis of Uvarovite Garnet

Abstract: A garnet with interesting optical properties is synthesized by means of the ceramic method and the formation of gels. Colloidal silica or tetraethoxysilane (TEOS) is used as a starting reagent in the latter method of synthesis, together with nitrate and chloride salts of the remaining components. Use is made of ultraviolet-visible and infrared spectroscopy techniques and X-ray diffraction in studying the evolution of the system. Formation of the garnet phase at lower temperatures is observed when using the gel methods, as well as the absence of chromates during the process in the samples which contained chlorides.

Comparative studies on the silica gels doped with polyamine copper(II) complexes by ion-exchange and sol-gel techniques

Abstract: Silica gels doped with polyamine copper(II) complexes were prepared by hydrolysis of tetraethyl orthosilicate solution containing the complexes (sol-gel technique) and by ion-exchange of the cations of those complexes for silanol protons on the silica surface (ion-exchange technique). The copper species trapped within silica gels by the ion-exchange technique were attached to two neighbouring silanol oxygens, well-dispersed in gel bodies and little eluted with water, whereas silica gels prepared by the sol-gel technique contained the copper(II) complexes as aggregates and most of the complexes were leached with water. The copper(II) complexes on silica gels thermally decomposed to copper(II) oxide and copper metal particles under atmospheres of air and nitrogen, respectively. The formation temperatures of these particles in the ion-exchange silica glasses were 200 ∼ 300 °C higher than those in the sol-gel glasses. The particle sizes of the oxide and metal of copper in the ion-exchange silica glasses were about 200 A and considerably smaller than those in the sol-gel glasses.

Preparation of Bi-Pb-Sr-Ca-Cu-O superconducting fibers by the sol-gel method

Abstract: Bi-Sr-Ca-Cu-O superconducting fibers were prepared by the sol-get method. Gel fibers of 0.01–1 mm diameter were directly drawn from a homogeneous viscous sol which was prepared from an aqueous solution of metal acetates containing acetic acid and tartaric acid. Ceramic fibers with the high-Tc phase as the main phase were obtained by heating the gel fibers derived from the sol of molar composition of Bi:Pb:Sr:Ca:Cu=1.85:0.35:1.9:2.0:3.1 to 835°C at a slow rate of 0.33°C/min and keeping them there for 40 h. The fiber showed diamagnetism below 110 K and zero electrical resistance at 98 K.

Use of polymeric catalyst in synthesis of sol-gel derived ceramic materials

Abstract: Polymeric catalysts (e.g., polymeric acid catalysts such as poly(styrene)sulfonic acid) can be used in the synthesis of sol-gel derived ceramic materials from metal alkoxides, for example, organic/inorganic hybrid materials formed by reaction of a metal alkoxide and a reactive endcapped polymeric modifier. The use of a polymeric catalyst, rather than a conventional monomeric (inorganic or organic) acid catalyst, has a non-deleterious effect on the aging behavior and offers potential for control of rheological properties (spinnability, coatability, etc.) of the hybrid materials.