We have applied state-of-the-art semi-empirical molecular orbital methods to a study of the anionic polymerization of silanols to form silica. In particular, we have considered nucleophilic attack on silanols and subsequent reactions of the products. Hydroxide addition proceeds without activation to form five-coordinate silicate anions. Five-coordinate structures can also be formed by oligomerization following the attack of hydroxide on neutral silanols to abstract a proton. These five-coordinate structures are predicted to play a key role as intermediates in the polymerization process. Water can be eliminated from these anions, but with a substantial activation barrier. The activation barrier appears to be lower for the larger, more complex systems. These predictions are consistent with a rapid pre-equilibrium to form dimer anions followed by the slower reaction to form higher oligomers.

Ceramic microstructure-property relationships dominate any and all attempts at“better ceramics through chemistry”. Required is some universal calculus to allow analytic, universal, reversible scalable computation of one from the other. Static Synergetics universality-principle provides such a flexible versatile tool, heretofore not available. It is a reexpression of the very basic three laws of thermodynamics into r-,k-,and w-domainsthe equivalence of(symmetry-breaking/defect) Pattern-recognition(the “structure”) to signal-processing(the frequency-dependent FM) properties/ Functions. More basically, it is a manifestation of Noether's theoremthe basis of mechanics and orgin of the energy continuity equation that is the thermodynamic first law. Such an algorithm, the”software”of yield optimization(quality and quantity) real-time Q.A. and(in parallel with a specificity dominated process- model)interactive Q.C.requires as “hardware”produced input the small-angle-scattering(SAS)dominated diffraction-pattern/static structure factor SSAS(k) or Fourier transform r-domain Pattern/ photomicrograph-recognition imaging(of processing-introduced property-detrimental defects(heterogeneity heirarchy)). Output are universal FM:I/f flicker(voltage and/or current)noise power spectrum, signal-to-noise ratio over dynamic rangemulti-level system dominated anomalous low temperature/frequency thermalracoustic. properties, and 1/f relaxation response susceptibility polarization catastrophe derived dielectric, electrical, optical, noise, viscoelastic/mechanicalmagnetic,...property Functions. Input can be the ubiquitous,...universal Mandelbrot fractals,dominating ceramics;output(and internal)Functions are Berry-Nye-Jakeman wdomain diffractals,dominating all properties universally.How and Why it works are detailed exactly;universality, reversibility and scalability are analytically insured for self-similar(or selfaffine) fractal scaling-relation Pattern-recognition input;approximate. deviations from universal Functions output obtains from less than perfect mathematically ideal fractal scaling-relation Pattern-recognition input.Static Synergetics provides a new practical use for external radiation small-angle-scattering(SAS) diffraction-pattern/static structure factor measurements in ceramic material microstructure-property relationships during processing

The Advanced Ceramics Based on Polymer Processing Program is sponsored by the Defense Advanced Research Projects Agency (Materials Science Division). The objectives of the program include development of a family of Si-C, Si-N, and Si-C-N ceramic fibers which may be used for reinforcement in ceramic, metal and plastic matrices, and development of economical process technology for fabricating ceramic matrix composites with high fracture toughness, high temperature performance and no inherent limitations to forming complex shapes. Attainment of these objectives would circumvent two limitations of existing ceramic material technology - lack of design reliability due largely to catastrophic failure related to brittleness, and inability to fabricate complex shapes [1].

The preparation of new, hybrid pre-ceramic organosilicon polymers are described. These hybrid polymers were prepared by reactions of polysilazanes and polysiloxanes. Ceramic materials containing Si-O-N were obtained with high yields (∼ 80%) by pyrolyzing these new polymers at low temperatures (< 800°C). A wide range of chemical compositions and properties can be obtained by using different ratios of polysilazanes and polysiloxanes. Microstructural characterization and densification studies of these ceramic materials are presented.

Al2O3 has been synthesized from aluminum propionate [A1(CO2CH2CH3)3] bySol-Gel techniques. Methods of characterization include: x-ray powder diffraction, scanning electron microscopy (SEM) and nuclear magnetic resonance(NMR). Solution and solid state 13C and 27Al NMR data is coupled with bothx-ray diffraction powder data and SEM in order to understand the transitionfrom sol + gel + amorphous powder + crystalline powder In the sonicated andunsonicated aluminum propionate precursor, and A12O3 powders. Resultsindicate that the sonicated dried powder is crystalline, having the γA12O3structure while the unsonicated dried powder is amorphous and crystallizesat 800°C with the a A12O3 structure. These differences in crystallinity arefurther substantiated by solid state 27AL NMR chemical shifts and linewidths at half height.

For the past twenty years we have successfully synthesized a wide range of ceramic materials by using organic precursors. The organic precursors are formulated into a glass before pyrolysis. Synthesized compounds include, for example, titanates, zirconates, silicates, chromites, niobates, tantalates, ferrites and molybdates.

This preparation technique is based upon having individual cations complexed in separate weak organic acid solutions. The individual solutions are gravimetrically analyzed for the respective cation concentration to a precision of 10–100 ppm. In this way it is possible to precisely control all of the cation concentrations, and to mix the ions on an atomic scale in the liquid state. There is no precipitation in the mixed solution as it is evaporated to the rigid polymeric statein the form of a uniformly colored transparent glass. The glass retains homogeneity on the atomic scale, and may be calcined at a relatively low temperature of only a few hundred degrees Celsius to the homogeneous single phase of precise cationic stoichiometry and particle size of a few hundred Å.

The advantage of having such well characterized, thermodynamically defined compounds is illustrated by the resulting understanding of the solid state chemistry of multicomponent compounds that has occurred.

Good dispersion of oxide ceramics in organic solvents can be achieved using many different dispersants. Several types of dispersants, including fatty acids, coupling agents, polar aromatic compounds and polymers, are discussed to illustrate the important phenomena. Many new problems arise in actual slips during ceramics processing; these are briefly discussed.

The hydrolysis and initial condensation reactions of Si(OC2H5)4 and low order polyethoxysiloxanes have been studied with high resolution 29Si NMR and size exclusion chromatography/FTIR spectroscopy. The effects of various parameters such as H2O/TEOS mole ratio, catalyst, pH, solvent, temperature and aging have been observed in the different nature of the reaction products. The products include partially hydrolyzed and unhydrolyzed Si(OC2H5)4, Si2O(CH2H5)6, and Si3O2(OC2H5)8. The rate of formation and concentration of each product obtained under different conditions provides information about the reactivity of various silanol-containing species and the factors that affect the overall reaction scheme.

A sequential inductivly coupled plasma-atomnic emission spectrrnetric (ICP-AES) method has been dewvloped for the cnmplete quantitative analyses of silicate glasses. B, P, Ge, and Si have been determined with 2%precisicn and accuracy in bulk glass and glass films on Si wafer substrates. Two chemical sample processing steps, dissdution in hydrofluoric acid at roam temperature and fusion in a sodium carbonate flux, were equally effective for deconposing samples without loss of volatile components. Rapid analysis for controlling glass film composition during IC process devlopment wer provided by the ICP-AES maethod. Where accurate film w~ghts were not obtainable, coatpositions wee established by complete analyses of all glass components except cxygen. The sum of the weights of components converted to their cides served as the sample weight for calculation purposes.

Perturbed Angular Correlation spectroscopy of 181Ta nuclei occupying cation sites has been used to investigate microscopic structural properties of several pure and stabilized zirconia materials. The PAC spectra are used to derive the electric field gradients at the tracer nucleus. Thse are compared with calculations based on a simple point-ion model. Agreement between experiment and theory is good for stabilized material and qualitatively reasonable for pure zirconia.

SiO2 glass has been prepared by the hydrolysis of TEOS in alcohol using HCl and NH4OH to control the pH. Copper ions were incorporated from the acetate to act as structural probe ions during the process of gellation and calcination. The local structure of copper ions was investigated using EPR determined at 300 K and 10 K.

It was found that the pH of the solution had a profound effect on the EPR spectra of the copper in the gels and glasses, indicating differences in the local structure.

Alumina sols derived from aluminum sec-butoxide (Yoldas) were characterized. The distribution of the polymer sizes within the sol, determined by gel filtration chromatography (GFC), was found to be dramatically affected by small changes in the chemical processing or preparative procedure. Aging the sol at room temperature for two weeks produced no significant change in the GFC elution curves of the alumina sol. Sols with a “milky” appearance were found to exhibit a wider distribution of polymers by GFC than transparent sols. Rotary evaporation of the sol followed by redissolution of the residue was found to change the polymer size distribution described by the gel filtration elution curves. These observations coupled with 27Al NMR spectroscopy and viscometry measurements were used to elucidate the effects of process conditions and aging on the molecular structure of the sol.

The kinetics of hydrolysis of hexamethylcyclotrisiloxane and di-t-butyldimesitylcyclodisiloxane in tetrahydrofuran solution have been determined and compared to hydrolysis rates of silica defects. In the presence of sufficient excess witer, the first-order rate constant of the cyclotrisiloxine, k= 3.8 × 10−3 min is similar to the rate constant, k = 5.2 × 10−1 min, of the disappearance of the D2 Raman silica defect band it has been proposed to model. Limited hydrolysis rate data for the cyclodisiloxane suggests that it hydrolyzes at least four times faster than does the cyclotrisiloxane. These data are consistent with rate data available for silica crack growth and support the assignment of highly strained siloxane bonds at the crack tip to cyclodisiloxanes. Infrared spectra determined for the cyclodisiloxanes lend further support to this model.

The use of Raman spectroscopy can be greatly hindered by the presence of fluorescing impurities. Even at low concentrations, fluorescence can completely obscure the Raman signal. In the current study, Raman spectra were recorded for various titanium alkoxides (ethoxide, isopropoxide, isobutoxide) as a function of concentration and laser excitation wavelength. It has been shown that fluorescence can be avoided by using uv-excitation (363.8 nm). In addition, titanium alkoxides exhibit a preresonance Raman enhancement as the excitation wavelength appreaches the UV. This result is confirmed by a uv-visible absorption spectrum of the isopropoxide.

The precipitation product of the titanium isopropoxide (TiPT) hydrolysis reaction was followed using photon correlation spectroscopy. This technique followed the mean size of the precipitate as the reaction progressed, as well as giving an indication of the number of particles being produced. From this analysis, the induction time for the onset of nucleation, nucleation rate (or the rate of particle production) and mean growth rate were correlated with reactant concentrations. In the concentration ranges studied, it was found that the induction time was a very strong inverse function of the reactant concentrations. Both nucleation rate and average growth rate were found to increase very rapidly as reactant concentration increased, with the TiPT concentration being more important. This reaction was also followed using absorbance spectrophotometry in the 500 to 320 nm range. After mixing of the reactants, an absorbance peak was seen between 330 and 335 nm, depending on the conditions. This peak increased slowly during the induction period and then increased rapidly after the onset of nucleation due to the increasing turbidity.

Gel drying is a critical step in the sol-gel synthesis of Al2O3 and Al2O3-SiC composites. Problems exist during the drying stage that affect the monolithic properties of the sintered products. Classical drying theory was applied to the drying behavior of Al2O3 and Al2O3-SiC composites in an effort to optimize the drying process and understand the controlling mechanisms.

The need for structural materials that can withstand severe environments up to 4000°F has promulgated the investigation of sol-gel derived ceramic and composite coatings on carbon/carbon composite materials. Alumina and zirconia sols have been deposited via thermophoresis on carbon/carbon substrates.

Cr3+ and Al3+ doped TiO2 can be easily made via the sol-gel process. Mixing the metal-organic solutions give rise to a random dispersion of the doping ions into the TiO2 network. Amorphous and crystalline phases have been characterized all the way from the gel to the crystalline products by thermal analysis, X-ray diffraction, vibrational spectroscopy and E.S.R. Organic groups appear to be involved in the formation of the gel network and could lead to a better control of the morphology of the xerogel. A thermal stabilization of the anatase phase, up to 900°C, is observed when Cr3+ is introduced as a dopant. This would lead to the preparation of anatase TiO2 photoanodes.

In separative process, the use of mineral membranes instead of organic ones is more interesting for it offers numerous advantages as follows :

- High temperature and pressure resistant (no compression of the membrane).

- Corrosion and abrasion resistant.

- Not sensitive to bacterial action.

- Steam sterilisable.

- Longer life time.

The effects of hydrolysis on the degree of polymerization during metallo-organic solution deposition of lead zirconate titanate (PZT) films have been investigated. The reaction of lead 2-ethylhexanoate, zirconium n-tetrapropoxide, and titanium tetrabutoxide in isopropanol with water were studied using thermogravimetry, specular reflectance Fourier transform infrared spectroscopy (FTIR) and optical and electron microscopy.

Films prepared from coating solutions having varying amounts of water exhibited dramatic differences in morphology. The films were spin-coated on platinum coated fused silica substrates and annealed at 525°C for 30 minutes. Unhydrolyzed coating solutions and solutions with a mole ratio of water to total metal of 0.5 yielded perovskite films with 0.5–5μm grains. A mole ratio of 1.5 (the amount of water required to completely hydrolyze the metallo–organics in the solution) formed amorphous, porous films. The stability of the prepolymerized films inhibits crystallization and densification at moderate temperatures.