Showing papers on "Sol-gel published in 1987"

NMR study of the sol/gel polymerization

Abstract: Kinetics and mechanisms of the hydrolysis-condensation reactions, starting with tetraethyl orthosilicate (TEOS), have been studied by 29 Si NMR. A standard experimental procedure for the sol/gel process was used: acid-catalyzed reactions and different concentration ratios of alkoxide and water. The inorganic polymerization was influenced by the following experimenta) parameters: — The starting alkoxides which may exhibit weakly reactive “Si-OR groups”. — The use of gel precursors with different chain lengths which lead to parasite ester exchange reactions. — The variations of pH which modify hydrolysis and condensation rates. — The H 2 O content where an excess of water was required to fully separate hydrolysis and condensation domains, and to eliminate “OR groups” in the gel.

Ceramics by the solution-sol-gel route

Abstract: The use of solution mixing followed by gelation to make ultrahomogeneous glasses and ceramics of various oxide compositions for laboratory experimentation, with first organic and later principally colloidal inorganic, precursors was developed over 30 years ago. Major technologies that use the solution-sol-gel route have been developed to make nuclear fuel pellets, ceramic fibers, thin coatings, and abrasive grain. This article reviews the early history and summarizes present research in this field, particularly new processes aimed at achieving xerogel precursors of maximum heterogeneity with respect either to composition or to structure. Such nanocomposites provide major advantages in lowering sintering temperatures, refining microstructure, and controlling morphology and final phase composition.

Spectroscopic investigations of borosiloxane bond formation in the sol-gel process

Abstract: The formation of borosilicate glass by the sol-gel process has been monitored during all stages, from the initial alkoxide solution to the final heat-treated product, using nuclear magnetic resonance and infrared spectroscopy as well as chemical analysis. The reactions of boric acid and trialkyl borates with silicon alkoxides and their hydrolysis mixtures have been studied for various compositions and catalysts. For all samples it was found that there exists only a small number of borosiloxane (-B-O-Si-) bonds at the time of gelation, the majority of boron being in the form of boric acid. This boric acid is condensed to borosiloxane bonds only upon heat treatment of the dried gel. For compositions having higher boron contents a small number of -B-O-B- bonds are also formed, which do not convert to -B-O-Si- upon heating to 500 °C.

Alcohol is an unnecessary additive in the silicon alkoxide sol-gel process

Abstract: It is unnecessary to add alcohol to a polymerizing silicon tetramethoxide-H2O sol-gel system as a co-solvent. The alcohol released during the reaction is sufficient to convert the initially biphasic system into a homogeneous one, resulting in the usual highly transparent, porous xerogel. We suggest that the alcohol-free sol-gel system may replace in many cases the traditional H2O-alcohol-alkoxide system

Properties and microstructure of thin LiNbO3 films prepared by a sol-gel process

Abstract: Thin LiNbO3 films were prepared from polymerized sol-gel precursor solutions having various concentrations and water:alkoxide ratios in an effort to investigate the effects of these and other processing variables on the resultant film properties and microstructure. Films deposted on silicon substrates were mostly amorphous when pyrolyzed at 435°C for 30 min. Randomly oriented polycrystalline films having distinctive microstnietures were produced using longer heating times or higher temperatures. All of the films exhibited low refractive indices due to porosity, which was attributed to the low level of hydrolysis water required to produce stable polymeric precursor solutions. When single-crystal LiNbO4 was used as the substrate, epitaxial growth of the film resulted. This ideal case establishes the feasibility of producing epitaxial films via sol-gel processing. All films were characterized by transmission electron microscopy (TEM) and thin-film x-ray diffraction patterns.

The chemistry of and physics with porous sol‐gel glasses

Abstract: Porous media with well‐defined pore diameters can be employed as hosts within which one may study a wealth of physical phenomena in well‐characterized confining geometries. In this paper we describe the procedures for forming variable pore size glasses via the sol‐gel process. Two general methods were used to form the gels from which the porous glasses were formed; the hydrolysis of the alkoxides and the gelling of colloidal silica, e.g., DuPont LUDOX. By controlling the pH of the gelling solution, the water/silica ratio and the firing temperature, glasses with pore diameters ranging from less than 2–12 nm were formed from the alkoxide hydrolysis. Larger pore diameters were obtained from colloidal silica gels, the largest being in the 45‐nm range in glasses made from leached alkali silicate gels. Extensive high resolution transmission electron microscopy analysis showed both ‘‘colloidal’’ and ‘‘polymeric’’ type glasses, in addition to an intraparticle structure in the 1–3 nm range. An example is given where subpicosecond optical birefringence techniques have been used to directly probe the rotational dynamics of different molecular liquids in several glasses having different pore sizes. Thus, a systematic evaluation of the liquid behavior as a function of its geometrical confinement is obtained and the results discussed.

Deposition of ceramic coatings using sol-gel processing with application of a thermal gradient

Abstract: A method of ceramic coating a substrate by thermodeposition from a sol made by hydrolyzing and peptizing organometallic compounds in water. The sol contains colloidal particles having a size between about 0.0005 micron and about 10 microns and the deposited coatings have good packing characteristics as well as smoothness. Composite coatings are also deposited which contain reinforcing materials such as SiC embedded in the coated matrix. These coatings afford the substrate onto which they are coated excellent wear, anti-corrosion and insulating characteristics.

Chemical processing of (Ba, Sr)(Ti, Zr)O3 perovskite ceramics

Abstract: Stoichiometric high-purity submicrometre-size perovskite powders have long been of interest for the preparation of dense ferroelectric bodies, thin film electronic components piezoelectrics and electro-optic materials. In recent years, numerous methods of preparing fine perovskite powders have been developed, such as the thermal decomposition of oxalates, citrates and carbonates [1-3], the organic sol-gel technique from metal alkoxides [4-6] and more recently the metallo-organic decomposition [7, 8]. The organic techniques which lead to homogeneity, stoichiometry and high purity in fine particles, offer a great flexibility because of the large variety of organic precursors that are available. However, concerning the very interesting barium or strontium perovskites, the use of the expensive alkaline earth alkoxides is generally very inconvenient because of their high reactivity to moisture and of the high molecular complexity which decreases their solubility in alcohols. We have studied the synthesis of alkaline earth perovskite ceramics from a sol-gel route, previously proposed by Kiss et al. [9] for barium titanate, in which titanium and zirconium alkoxides are hydrolysed by an aqueous solution of barium and/or strontium hydroxides. The method first makes possible the synthesis of a cubic perovskite phase from 100 ° C [10, 11], second, a low sintering temperature (1150°C in the barium titanate) of the powder compact to near theoretical density, and finally the preparation of uniform grain-size bodies. Fig. 1 shows the diagram of preparation of perovskites by the alkoxide-hydroxide method. The organic mixture containing titanium and/or zirconium normal butoxides and 2-butanol in the equimolar proportions is hydrolysed, at about 80 ° C, in a carbon dioxide-free atmosphere, by an aqueous solution containing barium and/or strontium hydroxides with a concentration of about 2.5moll-L For all the compositions, doping elements (Nb, Sb, La, etc.) can be easily introduced by mixing alkoxides in inorganic salts with the organic solution of butoxides. Reactions which take place during this method have been previously described by Flashen for the barium titanate [10]. When a pH of 11 to 14 is maintained, in the presence of hydroxyl ions, four molecules of water and two hydroxyl groups attach through their oxygen atoms to the titanium of Ti(OR)4 in a nucleophilic process and, following an electronic rearrangement, four molecules of alcohol are expelled (Fig. 2). The result of the initial step is an anion of the formula Ti(OH)~ or Zr(OH)~ which is neutralized by the Ba 2+ or Sr 2+ ions in the solution following Equation 1.

Electronic properties of sol‐gel‐derived oxides on silicon

Abstract: Sol‐gel‐derived SiO2, borosilicate, and aluminosilicate thin films deposited on silicon and heated for 5 min at temperatures of 1000 °C or lower exhibit dielectric strength as great as 5 MV/cm and interface state densities as low as ∼1×1011/cm2 eV. These values represent significant improvements over previous sol‐gel‐derived oxides on semiconductors and indicate that sol‐gel processing can provide device quality oxides in situations where native oxides are unavailable or exhibit poor dielectric behavior, e.g., amorphous, hydrogenated silicon or III‐V compound semiconductors.

Coating Films of 20B2O3·80SiO2 by the Sol‐Gel Method

Abstract: Alkali-free borosilicate films of 20B2O3. 80SiO2 (in mol%) have been prepared by the sol-gel method. The transparency of these films was found to be very sensitive to the humidity of the atmosphere during the coating process. Transparent films were obtained only in a dry atmosphere. Measurements of IR spectra and load at scratch showed that these films were densified above 600°C.

The gel route to Cr3+-doped TiO2, an ESR study

Abstract: Cr 3+ ions have been incorporated as Cr(acac) 3 to the titanium alkoxide precursor in order to follow hydrolysis and polycondensation by ESR. X-band and Q-band spectra have been recorded and analyzed all the way from the molecular precursors to the rutile TiO 2 , giving detailed information about the different steps involved in the sol-gel process. Our experiments show that a mixed Cr-Ti alkoxide is obtained upon mixing and heating the molecular solution. They also point out the role of acetate as ligands chemically involved in the process. No modification is observed during the gel-xerogel transformation and short range order in the amorphous xerogel appears to be completely different than in TiO 2 crystalline phases. Upon heating anatase and then rutile are successively obtained.

Fundamentals of sol-gel thin film formation

Abstract: It is shown that the structure of sol-gel-derived thin films depends on the precursor structure and deposition conditions. Weakly branched precursors (reduced D) applied under reaction-limited conditions result in the densest films, whereas branching or aggregation (increased D) combined with transport-limited deposition conditions result in the most porous films. A complete spectrum of microstructures is achievable by independent control of both precursor size and structure and the condensation rate or sticking probability. Application of shear during deposition is shown to restructure tenous, fractal structures and align weakly branched precursors.

Sol-gel preparation and dielectric properties of lead iron niobate thin films

Abstract: We have developed a new method to prepare lead iron niobate, Pb(Fe1/2Nb1/2)O3, by using organometallic precursors. The metal-containing sol was prepared by hydrolysis of the metal alkoxides. Homogeneous mixing of the constituents is achieved at the molecular level and densification of the ceramic occurs at temperatures considerably lower than that prepared by the conventional oxide power method. Due to the homogeneous mixing, the lead iron niobate gel forms a pyrochlore phase at temperatures as low as 300°C and ≥90% of it is converted to a perovskite phase at 700°C, and these conversion temperatures are about 100-200°C lower than those observed in samples prepared by conventional methods. In addition, we have prepared lead iron niobate capacitors on silicon wafers. This was achieved by spinning a prepared sol onto the silicon wafers, followed by gelation and pyrolysis of the precursor gels to give monolithic oxide films on the wafers. The maximum dielectric constant obtainable is ∼ 81, which is more than ...

Fabrication of homogenous sol-gel derived multiphase materials using gelling control agents

Abstract: A method of preparing a composite of a metal oxide matrix with reinforcement particles distributed therein in gel form, by preparing a sol of a metal oxide of Al2 O3, SiO2, MgO, ZrO2, Y2 O3 and mixtures thereof, bringing the sol near the point of gelation, adding reinforcement material consisting of Al2 O3, SiC, SiO2, MgO, ZrO2, Y2 O3 and mixtures thereof in particulate form in an amount from about 5% by volume to about 90% by volume, and adding a control gelling agent in the form of an aqueous solution of a metal salt consisting of aluminum nitrate, magnesium nitrate, zirconium nitrate, chromium nitrate, silicon nitrate, neodymium nitrate and mixtures thereof in an amount corresponding to from about 1% to about 5% by weight of the metal salt based on the metal oxide in the sol to promote controlled gelation.

Method for fabricating superconductors by sol-gel process

Abstract: Superconducting oxides in the A, B, Cu oxide ternary system, where A is yttrium or a rare earth and B is an alkaline earth, are formed as thin films on a substrate by a sol-gel technique.

Sol-gel method for making gradient-index glass

Abstract: A sol-gel method for making gradient-index glass and a novel composition therefor, comprising at least a ternary system of metal alkoxides.

AFM Study of Multilayer Sol-gel BaxSr1-xTiO3 Thin Films

Abstract: Multilayer Barium Strontium Titanate (BST) sol-gel films with molar formula BaxSr1-xTiO3 (x = 0.5, 0.7 and 0.8) are deposited on SiO2/Si substrates. The surface morphology and grain size are characterized via Atomic Force Microscope (AFM), showing the films to be uniform and crack-free

Fiber Drawing from Silicon Alkoxide Solutions in the Sol–Gel Process

Abstract: The occurrence of the spinnability in the sols prepared by the hydrolysis and polycondensation of tetramethoxysilane, tetraethoxysilane, tetraisopropoxysilane and tetranormalbutoxysilane has been investigated. It has been found that the spinnability appears at high viscosities in all of these solutions when the H2O/Si(OR)4 mole ratio of the starting solution is 2 and an acid is used in the hydrolysis reaction. It has also been shown that the spinnable solutions show the Newtonian behavior at high viscosities.

Preparation of thin Film YBa2CU3O6+X Ceramic Superconductors by the Sol-Gel Process

Abstract: High Tc superconducting powders and thin films on silicon have been fabricated by the sol-gel process. Both powder and film preparation involved the synthesis of metal alkoxides, followed by the complexation of yttrium, barium, and copper alkoxides in a common solvent. Bulk gels were then vacuum dried and fired at 700 and 950 °C in flowing oxygen. Thin films on (100) silicon were fired at 700 °C in flowing oxygen. Susceptibility vs temperature measurements showed that the thin films on silicon had a Tc of 40 K while the bulk gels fired under the same conditions had a Tc of 75 K. The sample fired at 950 °C was superconducting at 90 K.

Preparation of oxide glasses from metal alkoxides by sol-gel method

Abstract: An investigation is carried out on the types of siloxane polymers produced in the course of the hydrolysis of silicon tetraethoxide, as well as the preparation of oxide glasses from metal alkoxides by the sol-gel method.

Design of Microstructures in Sol-Gel Processed Silicates

Abstract: More and more, oxides and glasses are being prepared by chemical routes (1) In some cases, chemical routes are used to prepare ideal oxide powder compacts (2,3) These compacts, in turn, densify to defect free polycrystalline ceramics (4) In cases dealt with in this paper, chemical routes are used to generate oxides in a porous preform of their final geometry The geometries of interest are fibers (5), thin films (6) and shapes called monoliths (7,8)

Extruded ceramic fibres of modified lead titanate including sol-gel precursors

Abstract: An original procedure to obtain extruded ceramic fibres of calcium modified lead titanate has been developed. Slurry preparation was made by homogeneous mixture of crystalline ceramic powder with a gel of analogous composition acting as plasticizer. In addition, the gel phase provides a binder elect to the green fibres after extrusion and during the first stage of the firing process. On the other hand, the high reactivity of the gel accelerates the sintering mechanisms. Fibres of ∼ 1 mm final diameter and 97% of the theoretical density were obtained for moderate sintering conditions (1000° C, 3 to 7 h). The microstructure of the fibres was studied in comparison with those of ceramics prepared from each of the two components separately. Since electromechanical properties of these types of ceramics strongly depend on the microstructure, the procedure here developed is considered very appropriate for their processing.

Thin films-preparation, structure and properties. Photoelectrochemical properties of Sb-doped TiO2 film semiconductor electrode prepared by the sol-gel method.

Abstract: ゾル.ゲル法によりアンチモンをドーピングしたTiO2薄膜を調製し,その光電気化学的性質を調べた。ESCAにより調ぺた結果,600℃で20分間熱処理したTiO2(アナタース)中のアンチモンはSb3+として存在することがわかった。Sb3+はTiO2中に侵入型で固溶してTio2の電気伝導度を増加させるが,その固溶度限は約1mol%である。光電流は薄膜の電気伝導度の増加の結果として0.5mol%Sb2O3,で約26mA.cm-2となり,若干増加するが,それ以上ドーピングするとかえっでいちじるしく低下し,とくに5mol%Sb2O3では約3mA.cm-2と約1/10に減少した。これはSb3+がTiO2のバンドギャップ中に不純物準位を形成し,それが再結合中心として働くためであると考えられる。TiO2中のSb2O3は光照射下で溶解し,ドーピング種としては好ましくないという結果を得た。