An acetic acid/water based sol-gel PZT process II: Formation of a water based solution

The chemical reactions underlying the formation of a water based alkoxide sol gel process for lead zirconate titanate thin films using solutions of Ti and Zr alkoxides are outlined. Titanium isopropoxide and zirconium propoxide are chemically modified by acetic acid in order to use water as a solvent. The nature of the hydrolysis reactions in the presence of acetic acid is reviewed. It is shown that the formation of ions and charged polymeric species reduces their rates of condensation and aggregation.     

An acetic acid/water based sol-gel PZT process I: Modification of Zr and Ti alkoxides with acetic acid

The chemical reactions underlying the formation of a water based alkoxide sol gel solution for lead zirconate titanate thin films have been evaluated using infra-red spectroscopy and viscosity measurements. Titanium isopropoxide and zirconium propoxide are modified by acetic acid in order to use water as a solvent. Substitution reactions initially take place with the formation of titanium or zirconium alkoxide acetate and alcohol. The titanium or zirconium alkoxide acetate gradually associate through the formation of M-O-M linkages to increase the viscosity of the solution.     

Control Preparation of Sulfated Zirconia by Sol-Gel Process: Impact on Catalytic Performances During n-Hexane Isomerization

Sulfated zirconia catalysts were prepared by sol-gel method using CH₃COOH as in situ water source to control hydrolysis of alkoxide, and following two sulfation procedures. The samples were characterized by N₂ adsorption, XRD, chemical analysis, and the activity for isomerization of n-hexane was assessed. It was found that sulfation procedure and the amount of acetic acid added exert a great influence on the catalysts properties. Mixture of sulfuric acid with zirconium propoxide before addition of acetic acid allows the retention of a larger amount of sulfur after calcination and enhances catalytic performances of sulfated zirconia. The use of CH₃COOH reduces the rate of hydrolysis, and improves considerably acidic and catalytic properties.     

Electrochemically induced sol-gel deposition of zirconia thin films

A novel electrochemical method for deposition of ZrO(2) thin films is described. The films, 50-600 nm thick, were obtained by applying moderate positive or negative potentials (+2.5 V to -1.5 V versus SHE) on conducting surfaces immersed in a 2-propanol solution of zirconium tetra-n-propoxide [Zr(OPr)(4)] in the presence of minute quantities of water (water/monomer molar ratios in the range of 10(-5) to 10(-1)), which was the limiting reagent. Oxidative electrochemical formation of solvated H(+) and reductive formation of OH(-) catalyze the hydrolysis and condensation of the metal alkoxide precursor. The magnitude of the applied potential and its duration provide a convenient way of controlling the film thickness. The films consist of an amorphous phase, as revealed by XRD measurements. The effects of different parameters, such as the applied potential and its duration, the amount of added water and the current-time characteristics, were studied. A mechanism for the electrodeposition of the zirconia films which is in accordance with our findings is proposed.     

Role of solvents in the gelation process: can light scattering studies shed some light?

Sol–gel reactions continue to be of interest for the preparation of nanostructured materials. Two chemical reactions that are important in the sol–gel process are the hydrolysis and condensation reactions. The rate of the these two reactions are affected by a number of factors such as reaction pH, temperature, humidity, amount of water, type of alkoxide, molar ratio of alkoxide to water, and nature of solvent. Moreover, there is a physical process, that of particle aggregation that is also important in the overall gelation process. The role of solvents in these chemical and physical processes is still not very clear. In order to clarify the role of solvents in the gelation process, small angle light scattering studies (SALS) were carried out. A model system chosen was a colloidal silica solution that contained preformed silica particles of 10–15 nm in diameter. SALS studies indicate that gelation times are independent of the nature of solvent.     

Distribution of Methoxyl Groups in the Methylation of Starch Alkoxides Obtained by Metallation with Alkali Metal Naphthalene

Abstract

The distribution of the alkoxide groups obtained in the metallation of starch in dimethyl sulfoxide solution by alkali metal naphthalenes was studied. The starch alkoxide was reacted with methyl iodide, and the methylated starch was hydrolyzed and analyzed for glucose and O-methyl glucose derivatives. The metallation reaction was found to be random, as seen from the fact that at low alkoxide concentration (D.S = 0.6), 2,3,6-tri-O-methyl glucose was formed, while at relatively high alkoxide concentration (D.S. = 1.5) unreacted glucose was still present. At low alkoxide concentration (D.S. ? 0.6) there was, to a certain extent, preferential metallation at the C₂ hydroxyl groups, and to a lesser extent at the C₆ hydroxyl groups, as seen from the relative molar ratios of about 10:4:1 of the 2-, 6-, and 3-O-methyl glucose derivatives obtained, respectively. An increase in the metallation at the C₃ hydroxyl occurred with increasing alkoxide concentration. The distribution of the methoxyl groups with the three alkali metals potassium, sodium, and lithium was similar; there were differences only in the ease of metallation of the starch by the various alkali metal naphthalenes and in the efficiency of the coupling reaction with methyl iodide.     

Modification of Silica by Liquid Polybutadienes Containing Alkoxysilane Groups

Summary : The present work describes a method to modify the surface of silica, reducing its polar character and making it compatible and dispersible into hydrocarbon based elastomers. A liquid low molar mass polybutadiene (PB) was grafted with mercaptopropyltrimethoxysilane (MPTS) via radical addition of the thiol group to the double bonds. The silanized PB was reacted with silica via thermal condensation with its silanol groups. ²⁹Si NMR spectra showed that the condensation reaction of the trifunctional silane involved one or two alkoxy groups, while the third alkoxy group remained unreacted, probably for steric reasons. The characterization of the functionalized silica particles was performed by contact angle measurements and TGA analysis.     

Hydrolysis, Condensation, and Tetragonal Phase Formation in Sol-Gel Zirconia Prepared with Electron-Irradiated Alkoxide Solutions

Sol-gel zirconia precursor, zirconium n-butoxide in ter-butanol, was irradiated with 1.3 MeV electrons to a dose of 330 KGy. Gelling was instantaneously produced when an aqueous solution of sulfuric, hydrochloric or acetic acid was added to the irradiated solution; no hydrolysis catalyst was required. Samples were characterized with X-ray powder diffraction, infrared spectroscopy, and electron paramagnetic resonance. The electron irradiation accelerated hydrolysis and condensation, which avoided the stabilization of the tetragonal phase via carboxyls, and decreased the capability of sulfate ions to stabilize it. These results suggest that the stabilization of the tetragonal phase of sol-gel zirconia via carboxyl and sulfate ions depends on their diffusion in the sol.     

FTIR Spectroscopy,SAXS and Electrical Conductivity Studies of the Hydrolysis and Condensation of Zirconium and Titanium Alkoxides

A continuous flowing-rapid mixing technique was combined with FTIR, SAXS and electrical conductivity to study the early stages of polymer formation and growth during the acid-catalyzed hydrolysis and condensation of titanium and zirconium alkoxides. Reaction times as short as 80 milliseconds were investigated. FTIR spectroscopy was used to monitor the water and M−OR concentrations during the reaction. Hydrolysis of ∼25–50% of the alkoxy groups was facile. The FTIR and SAXS data showed that condensation was also very rapid. The activity and mobility of the ions in the solution were monitored by electrical conductivity measurements. The decrease in the normalized solution conductivity during the reaction correlated with the loss of [M−OR]. Furthermore, the radius of gyration of the growing polymers increased rapidly in regimes where the conductivity and [M−OR] decreased fastest. This finding suggests that the mobility of some of the charge carrying species decreases because of the growth in size of the polymers. Managed by Lockheed-Martin Energy Systems, Inc.     

Straightforward synthesis of Ti-doped YSZ gels by chemical modification of the precursors alkoxides

Translucent, homogeneous, and monolithic gels of the [(ZrO₂)_0.92(Y₂O₃)_0.08]_1?x(TiO₂)_x system, where x = 0, 0.05, 0.08, and x = 0.10 (mol), have been reliably obtained, for the first time, by a sol–gel route from zirconium (IV) n-propoxide (Zr(OPrⁿ)₄), yttrium acetate hydrate and titanium (IV) isopropoxide (Ti(OPrⁱ)₄). Chemical modification of both alkoxides, zirconium (IV) n-propoxide and titanium (IV) isopropoxide, by acetic acid allows us to change the hydrolysis and condensation behavior of them. Their modification implies the formation of chelating and bridging acetates avoiding the formation of precipitates. The line width and some shoulders in the FT-IR spectra of the solution, resulting of the mixture of the precursors and the catalysts, during the hydrolysis reaction suggest that both coordinations, chelating and bridging, should occur. Furthermore, the separation of the steps of hydrolysis and condensation allows to achieve conditions under which hydrolysis of the molecular precursor is slowed, whereas condensation is promoted under chemical reversibility to ensure a crystalline product at low calcination temperatures. In addition, the formation of metalloxane bondings (M–O–M’, M and M’ = Zr, Y, and Ti) has been confirmed by FT-IR throughout the sol–gel process. At about 630 °C, the crystallization of yttria stabilized zirconia (YSZ) for x = 0 or a titania-doped yttria stabilized zirconia solid solution (Ti-doped YSZ) for x = 0.05, 0.08, and 0.10 is detected by DTA-TG. By SEM-EDX and TEM-EDX the presence of Zr, Y, and Ti elements, in the adequate proportions according to the nominal compositions, has been proven in both dried and calcined gels.     

Design of Homogeneous Hybrid Materials through a Careful Control of the Synthetic Procedure

Sols for the synthesis of hybrid organic-inorganic materials have been prepared by mixing zirconium n-propoxide and methacryloxypropyltrimethoxysilane (MPS). The synthesis was done in two steps: a 15 minute hydrolysis of a MPS : H₂O : EtOH 1 : 1 : 2 mixture and then addition of 0.5 molar equivalent of zirconium alkoxide. All the experimental parameters—hydrolysis ratio, pH, dilution, pre-hydrolysis time—have been optimized through a detailed ²⁹ Si and ¹⁷O NMR analysis. Immediately after the addition, 94% of the initial water was consumed for the formation of Si–O–Zr bridges. Cleavage of these bonds, associated with formation of Si–O–Si and Zr–O–Zr bridges are then observed during the aging time.     

The effect of different monocarboxylic acids on the gelation behaviour of Cu ethoxide

Cu ethoxide becomes soluble in methoxyethanol when formic, acetic, or acrylic acid is added. For the complete dissolution, the minimum molar ratio of an acid to Cu ethoxide is two for formic acid (HOFA), and one for acetic (HOAC) or acrylic (HOAA) acid. The solutions of Cu ethoxide with any amounts of formic acid are incapable of forming gels. Those with acetic or acrylic acid/Cu ratio of 1 are able to convert to gels. For the range of the water concentration from 1.5 to 10 M, the gelation time is strongly dependent on [H₂O] for the solution with HOAC/Cu ratio of 1, while it is almost independent for the solution with HOAA. It is suggested that the differences in the behaviour of soluble Cu species derived from the three acids are attributed to different coordination forms and the bonding strength between ligands and copper ions.     

Effect of 3-glycidoxypropyltrimethoxysilane precursor on the properties of ambient pressure dried silica aerogels

The effect of an organically modified precursor, 3-glycidoxypropyltrimethoxysilane in an ambient pressure process involving aging in silane solution for silica aerogels is presented. The effect of increasing trialkoxysilane/tetraalkoxysilane precursor ratio and the influence of water to Si molar ratio on the gelation and adsorption properties were investigated. An optimum water to Si molar ratio (8) gave the fastest gelation for all precursor ratios indicating a balance between the increase in rate of hydrolysis and a decrease in concentration of the monomers. Surface area analysis proved that in the dried gel, the organic groups are largely present on the pore walls and prevent the condensation of the silanol groups during drying. This in turn prevents pore collapse and further increases the total pore volume. The inclusion of the organically functionalised silane in the process further enhances the ambient pressure drying through this effect.     

Structural investigations on the hydrolysis and condensation behavior of pure and chemically modified alkoxides. 2. Germanium alkoxides

Structural investigations on the hydrolysis and condensation behavior of germanium alkoxides were for the first time performed by means of X-ray absorption fine structure and Raman spectroscopy. The studies reveal that germanium alkoxides are monomeric in nature and undergo very fast hydrolysis and condensation reactions upon water addition. However, the chelation of germanium alkoxides by acetylacetone does not take place even 48 h after mixing, and any change in hydrolysis and condensation behavior is not observed after acetylacetone addition. When mixed with prehydrolyzed silicon alkoxide, the structures of germanium alkoxides are not modified. Both Si and Ge precursors are insensitive to the presence of each other in the reaction solution even after 48 h of aging. The addition of water to this mixture catalyzes the hydrolysis and condensation reactions very fast and leads to the formation of Ge-O-Ge (and consequently Si-O-Si) homocondensation products.     

一锅法合成3-溴丙基乙酸酯

以1,3-丙二醇、氢溴酸（48％）和冰醋酸为原料,在带水剂的作用下,一锅法合成了3-溴丙基乙酸酯．考察了带水剂及追加醋酸或醋酸酐量对产率的影响,并通过气质联用仪对粗产品进行分析测定,推断其反应机理．结果表明：较适宜的合成条件为1,3-丙二醇0．21mol,1,3-丙二醇与氢溴酸（48％）、冰醋酸摩尔比为1：1：1．3,在带水剂甲苯（21mL）的作用下,反应2．5h,反应结束前1h追加4．8mL醋酸酐,合成效果最好,产率可达到95．7％．     

Study of the drying zirconia gel-precipitates using thermal analysis

Thermoanalytical techniques (TG, DTG, DTA and EGA) can be helpful in studying gel compositions and relating the chemical history and the role of the precipitant to the chemical properties of gels. The objective of our work was to determine the influence of the initial conditions in the reaction mixture on the product characteristics. Zirconia gels were prepared by the gelprecipitation method by the hydrolysis of zirconium tetrachloride dissolved in methanol. The substitution of water by methanol was chosen in order to obtain a reaction medium in which better control of the hydrolysis and condensation reactions of the zirconium precursor were achieved and thus also better control of the final properties of the dried zirconia gel-precipitates could be obtained. The nature of the hydrated zirconia gels obtained during the gelation process is strongly influenced by the conditions of the reaction mixture, e.g. concentration of the zirconium precursor, the mixing rate and particularly by the amount of water added. To distinguish between various types of water in the gel-precipitates, the furnace atmosphere and the partial water pressure were altered appropriately. It was shown that initial zirconium to water molar ratio has a significant effect on the properties of the final products. It was assumed that, according to the preparation conditions, three different types of water are present in the formed zirconia gel-precipitates. Modification of the dehydration processes of zirconia gel-precipitates enables control of the final microstructural and surface properties of the dried gels which are suitable for further catalysts support as well as mixed oxide preparation.     

Liquid-liquid extraction of zirconium with tri-n-butyl-acetohydroxamic acid and spectrophotometric determination in the organic phase with xylenol orange

Zirconium (6–121 μg) is determined with xylenol orange after liquid-liquid extraction by tri-n-butylacetohydroxamic acid (TBAH) from 6 M hydrochloric acid. Under optimal conditions (primarily, 1 1:1.5 ratio of TBAH to xylenol orange, addition of acetic acid, and measurement at 545 nm), the calibration is linear with a molar absorptivity of 34 650 l mol^?1 cm^?1. Ions which are not extracted by TBAH or donot form coloured complexes do not interfere; these include uranium and rare-earth elements. Hafnium interferes; interference from thorium is avoided by a preliminary extraction of zirconium with tri-n-octylamine.     

Silicon tetraacetate as a sol-gel precursor

Variations in the starting materials, solvent, catalyst, and water to alkoxide ratio affect gel times and gel properties in sol-gel reactions. Silicon tetraacetate-derived sols gel surprisingly quickly. This paper discusses the acid catalyzed hydrolysis and condensation of silicon tetraacetate in an attempt to understand this rapid gelation. ²⁹Si and ¹H NMR spectra show that tetraethoxysilane is an intermediate, and ethyl acetate is produced via a secondary reaction. A correlation between gel times and H⁺ concentration is demonstrated.     

Hierarchically structured meso-macroporous aluminosilicates with high tetrahedral aluminium content in acid catalysed esterification of fatty acids

A simple synthesis pathway has been developed for the design of hierarchically structured spongy or spherical voids assembled meso-macroporous aluminosilicates with high tetrahedral aluminium content on the basis of the aqueous polymerisation of new stabilized alkoxy-bridged single molecular precursors. The intimate mixing of an aluminosilicate ester (sec-BuO)(2)-Al-O-Si(OEt)(3) and a silica co-reactant (tetramethoxysilane, TMOS) with variable ratios and the use of alkaline solutions (pH 13.0 and 13.5) improve significantly the heterocondensation rates between the highly reactive aluminium alkoxide part of the single precursor and added silica co-reactant, leading to aluminosilicate materials with high intra-framework aluminium content and low Si/Al ratios. The spherically-shaped meso-macroporosity was spontaneously generated by the release of high amount of liquid by-products (water/alcohol molecules) produced during the rapid hydrolysis and condensation processes of this double alkoxide and the TMOS co-reactant. It has been observed that both pH value and Al-Si/TMOS molar ratio can strongly affect the macroporous structure formation. Increasing pH value, even slightly from 13 to 13.5, can significantly favour the incorporation of Al atoms in tetrahedral position of the framework. After the total ionic exchange of Na(+) compensating cations, catalytic tests of obtained materials were realised in the esterification reaction of high free fatty acid (FFA) oils, showing their higher catalytic activity compared to commercial Bentonite clay, and their potential applications as catalyst supports in acid catalysed reactions.     

Preparation of Zirconia Fibers via a Simple Aqueous Sol‐Gel Method

Polycrystalline tetragonal zirconia fiber was obtained by pyrolysis of precursor fibers from citrate‐acetate‐zirconium complex solution. The viscous zirconia sol with good spinnability was prepared by aging the starting solution of ZrOCl₂ · 8H₂O (ZOC) in the presence of acetic acid (HA) and citric acid (CA). The effects of molar ratio of zirconium cation to carboxylic acid and the aging time on the formation of spinnable sol were investigated. Thermogravimetric (TG) analysis, x‐ray diffraction (XRD), infrared (IR) spectra, and scanning electron microscope (SEM) techniques were used to characterize the sintered fibers. The results show that the fibers obtained at 1400°C are crack‐free with diameter of ca. 5–10 µm.