New bismuth alkoxide and oxo-alkoxide complexes have been prepared from the salt metathesis reaction of NaOR with BiCl3. When R = CH(CF3)2, the product is [Bi(μ- OR)(OR)2(THF)]2 Similar work with R = C6F5 has not yielded a simple alkoxide, but complexes of formulation NaBi3(μ3-O)(OR)8(THF), NaBi4((μ3-O)2(OR)9(THF)2, Na2Bi4(μ3- O)2(OR)10 and Bi6(μ3 -OR)(μ3 -O)4[μ3-OBi(OR)4]3 have been observed. The reaction of BiPh3 with HOC6F5, however, did produce the desired alkoxide which has been characterized as [Bi(OR)2(μ-OR)(toluene)]2 and [Bi(OR)2(μ-OR)(toluene)[2.2 toluene. The reaction of this alkoxide with NaOC6F5 led to the production of Bi6(ν3--O)2 (μ4-O)(OR)12 and NaBi3(μ3- O)(OR)8(THF)3. Reaction of BiPh3 with HOC6F5 in THF led to the formation of Bi6(μ3-OR)(μ3-O)4[μ3-OBi(OR)4]3 (THiF)2. Surprisingly the reaction of BiEt3 and HOR (R = C6F5 or Ph) displaced only one Et group to give [Et2Bi(μ-OR)]∞ which exist as infinite chain polymers with alternating Bi-O-Bi backbones. These spiral chains form chiral helices in the crystal lattice.

Homogeneous solutions of yttrium, barium and copper 2-(2-methoxy)-ethoxyethoxides in 2-(2-methoxy)ethoxyethanol of appropriate lY:2Ba:3Cu stoichiometry have been prepared and used in sol-gel synthesis of YBa2Cu3O7-δ. To our knowledge, this is the first report of the preparation of a homogeneous solution of yttrium, barium and copper homoleptic alkoxides dissolved only in the parent alcohol. These solutions provide an excellent starting point for sol-gel synthesis of YBa2Cu3O7-° since the metals are mixed on the molecular scale, and because the solutions are stable with no possibility of ligand exchange to give insoluble products. Hydrolysis and careful heat treatment of gels produced from these solutions yield nearly pure tetragonal YBa2Cu3O7-δ at temperatures as low as 725°C. The preparation of these solutions and their characterization by small-angle x-ray scattering measurements and TEM are reported, as well as preliminary data pertaining to their use in the sol-gel synthesis of YBa2Cu3O7-δ.

The starling acetate sols of molar compositions of Bi:Pb:Sr:Ca:Cu=I-2:O:2:l: 2, 11–2:0:2:2:3 and 111–1.6:0.4:2:2:3 were prepared by NH4OH addition at various rates, followed by evaporation. Sols were gelled by further evaporation to shard or to microspheres (diameter below 100 μm) by water extraction from the sol emulsion drops formed in 2-ethyl-l-hexanol. The gels were converted to BSCCO phases by thermal treatment. The processes were characterized by XRD and IR examinations. The influence of the sol preparation step connected with formation of polynuclear cations on the following stages of the process was examined.

The preparation of materials from gas and liquid phase precursor reactions is well documented. However, solid-state precursor routes have remained largely unexplored. This synthetic void led us to develop rapid (< 2 s), solid-state metathesis (exchange) reactions with a very broad range of synthetic applications. An example is the self-propagating reaction:

where the Nal is simply washed away with water. Analogous reactions allow the preparation of carbides, suicides, chalcogenides (O, S, Se, and Te), and pnictides (N, P, As, and Sb), of main-group, transition-, and rare-earth metals. These methods can also be exploited to produce mixed-metal and mixed-nonmetal solid solutions.

To gain insight into some of the mechanistic factors involved in these reactions, such as initiation, nucleation, and propagation, we have employed a variety of physical characterization methods. This paper will review the range of materials accessible by our synthetic approaches, as well as the control of product crystallinity, phase, and homogeneity through the optimization of reaction conditions.

Cordierite powders were prepared using silicon ethoxide, Mg(NO3).2.6H2O and Al (OBu)2etac (etac = ethylacetoacetate) and conditions of hydrolysis have been checked in order pure a cordierite to be obtained. D.T.A. measurements reveal a stepwise crystallization process with intermediate formation of metastable μ cordierite. Infrared studies allow to follow the evolution of the Si-O-Al network with temperature.

Gels form on addition of seven or more moles of water diluted with i-PrOH to one mole of M[Al(O-i-Pr)4]3, M = La, Ce dissolved in i-PrOH. Thermal treatment of xerogels derived from La[AI(O-i-Pr)4]3 gives amorphous powders below 900°C. LaAIO3 crystallizes out at 900°C and residual alumina remains amorphous. CeO2 starts to separate out above 600°C from the xerogels prepared from Ce[M(O-i-Pr)4]3. Thermally induced structural changes in xerogels prepared from the mixtures of La[AI(O-i-Pr)4]3 and Ce[AI(0-i-Pr)4]3 in 1:1 or 1:3 ratio are different from those prepared from individual precursors. These xerogels remain amorphous below 700°C. Crystalline CeO2 forms on heating at 900°C but alumina remains amorphous. Sols are formed on hydrolysis of M[AI(0-/-Pr)4]3 in excess water and acidifying the reaction mixture with acetic acid. Sols can be converted to gels by slow evaporation of volatiles. This method is useful in preparing coatings.

Ion-exchange reactions, particularly with molten salts, are a convenient low temperature route to prepare new materials. Six different complex oxides (KTaO3, K2Ti4O4, NaAlSiO4, Na2Si2O5,ß-NaAlO2 and K2Ge4O9) were reacted with different salts (NaCI, KC1, Na2SO4, NaNO3, LiCl and LiNO3) by the ion exchange route with molten salts. In this work, new phases and substitution reactions are reported.

Sol-Gel processing and Metallo-Organic Decomposition are contrasted from the thermodynamics and chemical reactivity points of view. Differences and similarities on precursor requirements, processing characteristics, processing intermediates, and product formation pathways are outlined. Some specific examples are presented and their reaction thermodynamics are compared. Some conclusions regarding thermo-dynamic versus kinetic control are drawn from these examples.

The sol-gel synthesis of metal oxides can be performed via the hydroxylation and condensation of metal cations in aqueous solutions. The complexation of these ionic species by anions leads to the chemical modification of inorganic precursors at a molecular level. The whole process of hydrolysis and condensation can then be modified allowing a chemical control of the morphology, the structure and even the chemical composition of the resulting powder.

The role of anions during the formation of condensed phases from inorganic precursors in aqueous solutions has to be taken into account. The complexing ability of these anions is described in the frame of the Partial Charge Model as a function of pH and the mean electronegativity of anionie and cationic chemical species. Experimental evidence for the complexation of zirconyl species in aqueous solutions will be given using multinuclear NMR of anions.

Sol-gel processing has emerged as an important new technology for the fabrication of a wide variety of glass, ceramic, and composite materials. In order to better understand and control the gelation process, a theoretical model has been developed which quantitatively links features of fractal growth with measurable properties of a solution during the sol-gel transition. The result of that effort is the fractal growth model of gelation (FGMG) which expresses the solution viscosity, light scattering, and refractive index changes as a funtion of time and the fractal dimension. In this paper, the application of FGMG to silica gels and multi-component system, such as titanium aluminosilicates, is presented.

To understand how solvent quality can be used to tailor structure in sol-gel processing of silicon alkoxides, polymerized tetramethoxysilane polymers were synthesized and fractionated to give relatively stable, narrow molecular weight dispersion samples. These polymers had molecular weights ranging from 8000 to 45,000. The solubility parameter range for these polymers is 8.9–14.5 (cal/cm3)1/2. Light scattering confirmed that this range could be used to predict solvent quality. Bulk gels prepared using good versus poor solvents demonstrated that solvent quality can be used to tailor properties of the gels, presumably by modifying the extent of interpenetration of the growing polymers.

Multichannel detection Raman spectroscopy was used to follow the hydrolysis and condensation reactions of TEOS/ethanol solutions. Effects of alkoxide concentration, catalyst concentration, watenTEOS ratios, and temperature were studied. It was found that the initial oligomeric composition of hydrolyzed solution was dependent upon the amount of water added to the solution: not only did reaction rates differ, but so to did the reaction products. Higher water concentrations resulted in formation of an oligomer which may contain a trisiloxane ring.

The hydrolysis and initial condensation processes of the mixtures of TEOS and MTES have been studied by GC-MS and 29Si-NMR in the systems with TEOS:MTES:ethyl alcohol:water:HCl=1:1:14:28:6×1O-4 and 0.5:1.5:14:28:6×10-4. Thedimer CH3(HO)2SiOSi(OH)3 possessing both TEOS- and MTES-derived units was identified, indicating that the condensation between a hydrolyzed TEOS-derived monomer and a hydrolyzed MTES-derived monomer was one of dominant dimerization.

This work is part of a more general study on the structural characterization of organically modified silica gels that are usually prepared from tetrafunctional, trifunctional and difunctional alkoxides. The systems that were investigated in this paper are gels prepared from co-hydrolysis of dimethyldiethoxysilane (DEDMS) and tetraethoxysilane (TEOS). The condensation process was followed by 29Si and 17O liquid NMR and shows clearly evidence for co-condensation reactions. Gels with various DEDMS/TEOS ratios were studied by 29Si and 1H MAS-NMR. The results are clearly correlated to the change in physical properties of the materials, from flexible polymeric gels at high DEDMS content to crosslinked brittle networks at low DEDMS content.

The Partial Charge Model has been modified to take into account the detailed structure of any molecular sol-gel precursors or inorganic solid networks. Starting from these structure-dependent partial charges, the classical theory of nuclear shielding is applied to compute the electronic cloud compacity <r-3>p, the population unbalance Pu and also the mean excitation energy ΔE. With these three parameters it is possible to explain the chemical shifts variations, spanning from +40 down to -140 ppm, of more than 50 precursors. Depending on the ligands, the well-known upside-down U-curves for series SiXnY4-n (n=0‥4) can be ascribed either to the population unbalance term Pu or to a competition between the two other terms <r-3>p and ΔE.

Organic additives are commonly used in sol-gel chemistry [1]. They can operate as stabilizers of reactive metal alkoxides [2] towards hydrolysis or as drying control chemical additives (such as dimethylformamide). These organic compounds and the by-products of sol-gel reactions can drastically modify the physical properties of final oxide materials, so that it is necessary to optimize the use of these additives. The first steps of hydrolysis of metal alkoxides stabilized by organic additives have been studied by many sophisticated analytical techniques which are difficult to use at technical level and often strongly alter the sol-gel reactions [3]. We chose to use FT-Raman and Infrared spectrocopies, which allow to remain as close as possible of the sol-gel process without altering it. The first steps of hydrolysis of group (IVb) metal alkoxides, more particularly titanium, modified by chlorides, carboxylic acids and alkanolamines have been investigated.

Titanium alkoxides modified by carboxylic acids have been widely studied as the molecular precursors to ceramic materials. These alkoxide complexes have also been very useful in the formation of stable, porous, optically clear films having many novel applications such as chemical sensors, catalytic supports, and ion-exchange media. To improve the processing of these materials, it is essential to better understand the kinetics of the chemical transformations which occur.

The kinetics of the hydrolysis reaction are studied for selected carboxylic acids using Raman spectroscopy to probe the chemistry of the process. The study has a special emphasis on the titanium isopropoxide-valeric acid system due to the superior quality of these films over other carboxylates. Greater knowledge of the hydrolysis kinetics allows increased control over the quality of the film materials and should be of general interest to those working with modified metal alkoxides.

Combination of SAXS and USAXS measurements provide an extended q-range (0.006–3.0 nm-1) to study fractal growth of both aging silica gel as well as precursors of zeolite-A. Mass (silica) and surface (zeolite) fractals are observed. Scanning transmission X-ray microscopy (STXM) proves to be an extremely useful technique to obtain direct images of wet samples in the 0.1–100 micron range, confirming the SAXS/USAXS results on even larger length scales.

The rheology of zirconia-alumina composite slurries for gelcasting was studied in order to maximize the solids loading. The viscosity and yield stress were controlled by adjusting the pH. This approach allows the solids loading to be maximized for the gelcasting of near-net-shape composites. A strong correspondence exists among the rheological behavior, the surface charge on the particles, colloidal stability, and the maximum solids loading. The best pH conditions for gelcasting composites depends on the specific binary composition.

A method of preparation of spherical powders (with particle diameters <100µm) of metal oxidcs, their homegeneous mixtures or compounds, composites and metals has been elaborated. Depending on the nature of cations, the starting solutions were prepared by extraction of anions with Primene JMT from salt solutions, by addition of dopants to the sols or by complexing with acetic acid or/and ammonia. The sols or broths were then emulsified in 2-ethyl-l-hexanol containing SPAN 80. Drops of the emulsion were solidified by extraction of water with the solvent. The process was carried out continuously in a laboratory and/or in a pilot plant.