Capillary gas chromatography has been used to monitor the stoichi-ometries, structures, and abundances of low molecular weight polysilicate esters formed by partial hydrolysis of methanolic tetramethylorthosili-cate and redistribution of methanolie hexamethyoxydisiloxane under acidic (HC1) and basic (KOH) conditions. The polysilicate molecular size distributions, expressed in terms of mole percent of total silicon present as a function of degree of polymerization, show maxima near the number average degree of polymerization under acidic conditions. Under basic conditions, the distribution has its maximum at the monomer percent and extends to very high molecular weights, yielding a distribution far broader than the one observed under acidic conditions. These features are in agreement with covalent network theories of silica gelation, as opposed to particle aggregation theories. Weight fraction distributions of the type observed under acidic conditions are characteristic of linear polymers with a low degree of crosslinking and weight fraction distributions of the type observed under basic conditions are characteristic of branched polymers with high degree of crosslinking. The low degree of crosslinking obtained under acidic conditions is observed to arise from steric crowding that discourages branching in polysilicate structures.

Identification of polysilicate intermediates formed during sol-gel processing of silica gels and glasses is an important step toward understanding the nature of this process on a molecular level. Although capillary gas chromatography using mass spectrometric detection gives the molecular formulas for low molecular weight intermediates [1], this approach is only semi-quantitative and provides no structural information. An improved protocol has been developed that allows structural assignment and quantitative determination of the principal mono- through hexasilicate structural isamers formed by hydrolysis of methanolic tetramethylorthosilicate in the presence of HC1. This protocol combines quenching by diazomethane, fractionation using spinning band column distillation, identification by capillary gas chromatography, and structural characterization using 29Si{1H} NMR techniques (l-pulse, ID-INADEQUATE, and 2D-INADEQUATE) to provide structural assignments and response factors for the components separated by gas chromatography.

The early time behavior of an acid catalyzed Si(OCH3),4 (TMOS) sol-gel was studied by high resolution 29Si nuclear magnetic resonance (NMR). Both the water producing and the alcohol producing condensation reactions were found to contribute significantly to the overall condensation rate. A general theoretical kinetic formalism which specifically treats the temporal evolution of the various chemical function groups about a specific silicon atom was developed. The experimentally observed functional group distribution was in agreement with the distribution predicted by a simplified statistical reaction model. The mathematical framework for the study of chemical speciation at the next-to-nearest functional group level was developed. This framework was used to assign several fine structure resonances, and to show that the formation of various dimeric species is also largely statistical in nature.

TEOS has been hydrolysed under acidic condition with stoichiome-tric or excess amount of water. Evolution of the silicon species is followed by 29Si NMR. The data are analyzed at different levels of detail; first, analysis of the by products of polymerization reactions, second determination of the extents and overall rate constants of hydrolysis and condensation reactions and finally kinetics simulations of the evolution taking into account all the present silicon species. We have shown that the hydrolysis rate increases with the number of hydroxyl groups, and the reesterification reactions have a significant contribution. We also found that condensation reactions preferentially occur with loss of water between the more hydrolyzed monomers; their rates rapidly decrease with the degree of condensation. We compare the two compositions as a function of their water content and pH.

The complex sequence of the hydrolysis/polymerization reaction of certain types of alkoxysilanes is modelled using a novel approach based on the branching theory. Network formation is described, resulting from the reaction of multifunctional oligomers. Expressions are derived for the critical silane conversion at the gel point, the molecular weight, and changes in the sol and gel fractions as a function of time.

High resolution 29Si NMR spectroscopy has been used to study the hydrolysis and condensation reactions of TEOS and related silicon alkoxides under a range of conditions selected to model initial steps in the formation of ceramic precursors. Kinetic results have led to a detailed description of the role of various reaction parameters and to differences in reactivity of various polyethoxysiloxanes. Reactivity studies have provided insight into the processes that form multicomponent alkoxide/oxide materials. Titanium alkoxides in particular can act as catalysts for silanol condensation in addition to forming Si-O-Ti linkages, and the former role may predominate in certain systems.

We report here an experimental technqiue based on changes in reaction medium volume which we have used to observe the kinetics of hydolysis and polymerization of TEOS in aqueous ethanol solutions containing NH3. Our results show that there are three distinct reaction regions. By combining these results with determinations of change in reaction medium conductivity and average particle size, we are able to link the short and long time regions to TEOS hydrolysis and particle growth respectively. The intermediate reaction time region is not associated with hydrolysis, loss of ionic species due to condensation or to particle growth.

The condensation kinetics and the role of Ion pairing are investigated for silicate anions in alkaline solutions. Selective, and non-selective inversion recovery experiments are performed using the Si NMR spectrum of alkali metal silicate solutions. In this way the rates of condensation of silicate monomers is studied as a function of the nature of the base. These rates exhibit maxima with increasing cation size. The maxima are interpreted in terms of the concentration and activity of ion pair intermediates of the Si exchange reaction.

Interactions between alkali metal cations and larger silicate anions in silicate solutions are investigated using NMR spectroscopy of the cations. The chemical shift and the resonance llnewidths are used to estimate the concentration of cation-silicate ion pairs. The concentration of pairs involving large anions increases with increasing cation size. Thus it is expected that condensation kinetics are influenced by the size of the silicate fragment.

Dynamic oscillatory measurements at low strains are used to monitor the gelation kinetics of particulate aqueous sols composed of alkoxide-derived silica powders. The dynamic storage modulus is sensitive to the sizes of the growing clusters below the gel point. A transition in the modulus is observed and large normal forces are measured at the gel point. These tests do not disrupt gel structure (in contrast to shear viscosity measurements) and therefore the measurement process does not alter the gelation kinetics. To facilitate gelation of sols composed of low-surface area silica particulates, silica powders doped with F ions were added to the undoped powder in various concentrations (10, 15 and 20 wt.%). These additions allowed a change in gelation time from 80 min. (for 10% of the F containing powder) to 17 min. (for 20%).

Beginning with the work of Dislich, metal alkoxides have been increasingly used for the preparation of oxide-ceramics by the ‘sol-gel’ process. In spite of higher initial cost of alkoxides, their main advantages are (i)homoge-neous mixing at molecular level in the initial solution; (ii)ease of hydrolysis; and (iii) lower sintering temperatures.

Although the formation of new chemical bonds between different alkoxides employed in the starting mixed solution has been envisaged, the actual isolation of a large variety of polymetallic alkoxides in our laboratories has given an entirely new turn to the expectations (e.g., preparation of spinel from magnesium bis-tetraalkoxyaluminates) from this process. Some such possibilities are being presented for synthesis of novel ceramic materials.

Reactions of polymetallic alkoxides with water, alcohols andβ-diketones would be discussed to throw light on their hydrolysis process in the presence of other alcohols and chelating ligands.

Vanadium oxide gels are synthesized through vanadium oxo-alkoxide hydrolysis condensation processes. Different precursors and hydrolysis conditions lead to different sorts of gels. V0(0Amt)3 hydrolyzed with a large excess of water results in red jammy gels with a layered structure. They exhibit electronic and ionic behavior comparable to vanadium pentoxide gels from inorganic precursors. Hydrolysis of VO(OPrn)3 in an alcoholic medium, leads to orange transparent monolithic gels. They have a highly branched polymeric structure. Controlled hydrolysis of vanadium oxo-alkoxide precursors has the further advantage of giving good adherent thin films.

Raman and solid state 17O NMR spectroscopies have been used to determine the molecular structure of amorphous hydrous sodium titanate catalyst support materials prepared by hydrolysis of titanium isopropoxide. Three types of oxygens have been identified: oxygens bridging three titanium atoms, oxygens bridging two titaniums, and non-bridging oxygens. Changes in the relative populations of the different oxygen types have been used to help understand the ion-exchange and polymerization properties of hydrous titanates.

Zirconia powder was produced in aqueous solution from zirconyl nitrate using electrogenerated base. Both divided and undivided electrochemical cells were used. In the divided cell, hydroxide ion was discharged in the cathode compartment, and hydrated zirconia was produced. The as-produced material was weakly agglomerated, amorphous, and had a surface area of up to 316 m2/g. The surface area of the powder did not vary systematically with the electrosynthesis current density, but did depend on subsequent processing. Crystalline zirconia was produced by calcining in air at temperatures above 400°C. Tetragonal zirconia was the only phase observed until about 800°C. After calcining at 800°C the crystallite size increased to about 20nm and about 34% monoclinic zirconia was produced. When an undivided cell was used, the pH remained constant (1–1.5) throughout the electrosynthesis, and amorphous zirconia deposited on the cathode.

A new versatile method to prepare monolithic metal-organic derived gels is presented in which the sol-to-gel transition is achieved through hydrolysis and condensation reactions in a reversed micelles microemulsion. C7 to C10 alkanes were used as solvents for the reaction and water molecules were solubilized using a non ionic surfactant. The general rules governing the sol-to-gel transition for several alkoxides such as Ti (OPri)4, Ti(OBut)4 and Fe(OPri)3 have been laid down and the exchange of water between reversed micelles related to hydrolysis and condensation rates has been investigated. Hydrolysis and condensation kinetics were monitored for different water/surfactant ratio u-sing gas chromatography. At the sol state growing of colloidal particles is mainly affected by the structure of the water inside the reversed micelles. It has been demonstrated using light scattering measurements that gelation times depend on the particle size evolution into the sol.

Gels have been prepared in the H2O, HOiPr, (BuO)2Al-O-Si(OEt)3 ternary phase diagram in a large range of oxide concentrations. The gelation time is governed by the initial water concentration, showing that the hydrolysis of Si-OR groups is the rate-determining process for the gelation.

The structure of clusters and the kinetics of aggregation have been studied by SANS. We observe mass-fractal particles characterized by a fractal dimension of 1.9, in agreement with the existence of a cluster-cluster aggregation mechanism during the gelation. The scattering curves exhibit a maximum intensity corresponding to interparticle correlations due to purely excluded-volume effects.

Het erosiloxanes containing the Group IV metals, Ti, Zr, and Hf, are conveniently prepared by the facile silanol-induced cleavage of organometallic compounds. The isoleptic metallasiloxanes, M(OSiR3)4, vary from mobile liquids to insoluble non-melting powders depending upon the constituents bonded to the silicon. They can be fired into oxides at 650°C. The resulting materials have the formula MSi4o10. FT-IR and ESCA data suggest that the M-o-Si linkage remains intact during this process. Thus the MSi4O10 material can be considered a mixed oxide alloy rather than a simple mixture of the oxides MO2 and SiO2. Lower temperature treatments cures the liquid heterosiloxane, Hf(OSiE3),4 into a thin film which still contains some degree of organometallic character.

A similar silanol cleavage reaction involving Zr(CH2SiMe3)4 and (Ph)2(OH)SiOSi(OH)Ph2 affords (Me3SiCH2)2Zr[OSi(Ph)2OSi(Ph)2O], a cyclic metallasiloxane with a Zr:Si ratio of 1:2. This compound can be passivated and fired into an oxide material. It also reacts via the remaining organo groups with substrates containing reactive surface hydroxyls, e.g. silica, to form an organometallic coating which can In turn be fired into a refractory coating.

The role of dimethylformamide (DMF) in the sol-gel processing of alumina at high acid/Al ratios was studied at different hydrolysis and aging temperatures by 27A1 NMR and rheology measurements. The composition of sols in which aluminum sec-butoxide was hydrolyzed in 50/50 mixtures of DMF and H2O was similar to those hydrolyzed without DMF, although the DMF sols contained a slightly lower concentration of mobile alumina species. The Al13O4 (OH)24(H2O)12 7+ cation predominated in both DMF and aqueous sols hydrolyzed at room temperatures at acid/Al ratios between 0.4 and 0.6. However, aging the room temperature DMF sols at 90°C for 24 hrs was found to cause rapid gelation whereas a similar treatment of the aqueous sols resulted in a decrease in viscosity. This gelation process was correlated with a more rapid decay of the Al13 and A1(H2O)6 3+ cations in the DMF sols. In contrast, DMF sols hydrolyzed at 90°C were found to be less viscous than their aqueous counterparts at high acid ratios.

Synthesis of pure Y2O3 Powders have been performed through the sol-gel process. Stable colloids in the range of 500 Å to 2000 Å have been obtained by hydroxylation of yttrium-aquo ions performed through an anion exchange resin.

The chemical nature of these sols and gels investigated by GTA, DTA and EXAFS is in close agreement with Y(OH)3nH2O. Scattering experiments (S.A.N.S, S.A.X.S, light scattering) show that these colloids are made of anisotropie particles, that can be described as platelets 30 Å thick, 500 Å large and a few thousand Angstroms long.

This synthesis can be extended to other rare earth colloids.

Aqueous processing of 1- to 20-% v/v binary suspensions containing 0.50-μm α-Al2O3 and 0.71-μm m-ZrO2 was investigated in the presence and absence of AlCl3 from pH 2 to 12 using sedimentation, rheological, electrokinetic, and potentiometric techniques. Differential sedimentation of the oxide particulates leads to a nonuniform distribution of ZrO2 when colloidally stable slurries are used. However, processing under conditions that promote weak association of the oxides improves uniformity, with optimal conditions in the absence of AlCl3 being in the pH range 5 to 6. Thus, prefired microstructure depends on the pH at which processing occurs, behavior derived from the different, pH-dependent surface properties of the oxides. Pre-exposure of m-ZrO2 to AlCl3 in the pH range of 4 to 8 alters the final composite microstructure via adsorption of Al-species and renders the two solids more similar electrostatically, thereby reducing the forces responsible for the weak association and uniform distribution.

Models of the oxide and sulfide structure-types of quartz and cristobalite have been made using potential energy surfaces derived from MO calculations on small molecules. The bond length and angle and the volume compressibility data calculated for quartz match those observed for pressures up to 40 kbars. An analysis of the force constants that define the potential energy surface indicates that the bulk modulus of the mineral is governed primarily by the bending force constant of the bridging angle. Similar calculations were completed for the GeO2 form of quartz, but the agreement with the observed data is somewhat poorer. A modeling of the CO2 form of quartz predicts that it would be significantly harder, more incompressible and show less expansibility than the SiO2 form.