Traditionally, small angle neutron scattering (SANS) has been used to study dilute concentrations of defects 1 -100 nm in size. Recent extensions of the scattering theory have allowed the expansion of the technique to include larger sizes through the use of multiple scattering. With multiple small angle neutron scattering, defects (pores, microcracks, precipitates) up to 10 μm in size can be studied. SANS is inherently a non-destructive, bulk probe of microstructure, with wide applications in the characterization of materials.

A number of studies of ceramic materials using multiple and traditional (single particle diffraction) small angle neutron scattering will be discussed. The emphasis will be on the strength of the technique in the characterization of materials. Particular examples will include: the assessment of pore size distributions in spinel compacts as a function of sintering and agglomeration, the characterization of primary and secondary particle sizes in precipitated aggregates, and the determination of microporosity in MDF cements.

27Al NMRsp ectra were obtained on a series of alumina sols prepared by the hydrolysis of aluminum sec-butoxide. Subtle but distinct differences were observed in the solution 27Al NMR spectra of sols which varied in appearance from being very milky to completely transparent. No changes were observed in the 27Al spectra of sols which had been aged. The adddition of sufficient quantities of acid or base to gel the sol precipitated dramatic changes in the Al spectra. Aluminum-27 NMR was found to be a highly useful tool for probing the sol-gel transformation of this system.

Transmission electron microscopy, spectroscopy, light scattering and surface area measurements have been used to characterize the microstructural details of transparent silica aerogels. Light scattering intensity measurements have also been used to differentiate acid and base catalyzed hydrolysis and condensation reaction kinetics of Si(OC2H5)4. A plot of logarithm of the light scattering intensity vs. logarithm of time diring gelation gives distinctly different slopes for acid and base catalyzed systems (6 for acid and 2 for base). These slopes for the two systems can be explained on the basis of the point of zero charge of silica and the resulting interactions of the uncharged and charged particles in the systems.

Sol-gel top and cross-fractured surfaces have been Pt-C replicated (9.7–11.3 Å thick) and carbon film backed (100–145 Å thick) and separated from the gel with an HF solution. Samples made from both base catalyzed LUDOX/Kasil mixtures and acid catalyzed alkoxides were studied with tilt series of 104 to 105 magnification TEM mlcrographs of the surface replicas. The LUDOX/Kasil gels contained spherical particles with a raspberry-like morphology and the acid catalyzed alkoxIdes are composed of densely packed filaments and no apparent particles. The smallest structural entity in both gels are Pt/C coated filaments (12–14Å) estimated to be 1.5–2.7 Å in width. The pore sizes were characterized in both gel types, though TEM pore measurements were only made on the LUDOX/Kasil gels.

A technique is described for directly observing the movement of a drying front through a thin layer of porous material. The observations show that the front moves in abrupt steps and develops an irregular morphology as it advances. Comparison of the experimental images with computer simulations indicates that several features of the drying front can be accounted for by a model based on invasion percolation.

A model is presented for the stresses and strains that develop in a gel during drying. The driving force for shrinkage is assumed to be the interfacial energy, and the gel is considered to be viscoelastic. The liquid seeks to flow into the dry region of a gel in order to replace the solid-vapor interface with a solid-liquid interface having lower specific energy. This creates a “redistribution pressure” that causes the wet region to contract. The free contraction rate can be calculated by equating the decrease in surface energy with the energy dissipated in viscous flow as the gel contracts. The permeability of the gel to the liquid in the pores is especially important in the early stages of drying, and may control the contraction rate. The model allows quantitative predictions of contraction rate and stress during drying. In this paper, the model is applied to a plate drying from both sides.

The use of the high frequency elastic modulus, G∞, to probe particle interactions in weakly agglomerated, concentrated suspensions is discussed. We show that a model based on pair interaction potentials and a statistical description of pair spatial distribution yields accurate prediction of the volume fraction dependence of G∞. The use of statistical treatments of particle distributions as predicted from particle interaction potentials is found to provide insight on how surface chemistry affects the uniformity of powder compacts.

High purity silica was synthesized by hydrolytic decomposition of tetramethylorthosilicate (TMOS), followed by polycondensation, supercritical drying and sintering. The key factors which govern the hydrolysis and polycondensation were systematically studied. Gas chromatography/mass spectrometry was used to monitor the hydrolysis rate. The hydrolysis of TMOS is the only reaction at ambient temperature in an acidic environment. The rate of polycondensation was also controlled by temperature as well as pH. The structural transformation from alkoxide to gel network and to glass was illustrated by FTIR. The structure of the TMOS derived supercritically dried gel sintered at 1000°C is identical to that of silica. The shrinkage kinetics and microstructural development of the gel heated at different temperatures was examined by dilatometry, B.E.T., TGA, and STEM. This combination of techniques showed that structural relaxation is not an important factor in the densification of these gels. Below 1000°C most shrinkage is due to condensation and water evolution. Above 1000°C, viscous sintering is the primary mechanism for shrinkage.

The densification of sol-gel processed silica was studied by comparing calculated and measured viscosities between 700 and 850°C. Linear shrinkage was measured in a dilatometer. From shrinkage data, relative bulk densities were determined. The densities were used in a model for viscous sintering to calculate viscosities. Beam-bending viscosimetry was used to determine viscosities directly. The deflection rate of a centrally loaded beam was measured. Both the calculated and measured viscosities had the same time dependence. The temperature dependence was also determined.

Fluorine has a remarkable and diverse action in silica sols, gels and gel-derived glasses. Introduced in ionic form, it causes more rapid gelation, it reduces surface area and water adsorption of dry gels, and it helps to eliminate bubble formation during sintering and reheating of gel-derived glasses. The mechanisms for these actions of the fluoride ion are reviewed, with a more detailed discussion of the rate of gelation. Several previously held views of the way fluoride accelerates gel formation are compared with a new suggestion that it is caused by an electrostatic attraction between the proton of the OH of silicic acid and F− present as Si-F groups.

Raman spectroscopic studies on gel-derived silicates have confirmed that narrow bands near 607 cm-1 and 492 cm-1, first observed in the Raman spectrum of fused silica, are associated with three- and four-fold siloxane rings. Using these results, we have identified three- and four-fold siloxane rings in other high-surface-area silica materials, including leached glasses and Cab-OSil. This Raman spectroscopic evidence not only shows that small siloxane rings are a common characteristic of a number of silica materials but also suggests that they form preferentially at silica surfaces. This paper reviews the Raman spectroscopic evidence that led to the identification of the vibrational frequencies of the small siloxane rings and presents the results of Raman experiments on high-surface-area silica materials in which the concentration of small siloxane rings is enhanced compared to fused silica.

Chemically ultra-homogeneous gels have been prepared, in the SiO2-Al2O3 system, using new metal-organic precursors with Al-O-Si linkages. By thermal treatment at 900°C, transparent monolithic gels lead to aluminosilicate glasses and above 1000°C, to optically transparent glass-ceramics (for instance by homogeneous crystallization of mullite in an amorphous silica matrix).

The concept of fractal geometry is used to describe the structure of silica polymers, colloidal aggregates, and critical systems. We illustrate the interpretation of scattering curves (X-ray, neutron and light) for fractal systems, and review simple growth models which generate fractal structures. We describe the polymerization of silica under various conditions and demonstrate that, depending on chemical conditions, polymerization maps onto simple fractal growth processes. The key factors which control growth are monomer-cluster vs. cluster-cluster growth, and reaction-limited vs. diffusion-limited growth.

Small angle X-ray scattering was used to characterize the structure of sols and gels in the TMOS-MeOH-H2O-CHONH2 system. The scattering curves were analyzed in both the Porod and Guinier regions. A fractal analysis shows that the structure evolves with time and temperature from a linear type polymer to a more highly branched polymer and then towards a “particle”. However, these structural units disappear when the temperature increases above a critical value.

Small-angle x-ray scattering measurements on partially hydrolyzed silicon tetraethoxide solutions indicate the formation of colloidal particles which have fractal structures or fractally rough surfaces. The structures and growth kinetics are consistent with chemically limited nucleation and growth of the particles from slowly generated reactive silanol species. Two dimensional computer simulations of nucleation and random growth of clusters from partially hydrolyzed monomers generate the same range of non-fractal, fractally rough and fractal clusters observed in the experiment.

Using N2 adsorption (B.E.T.), small angle X-ray scattering and molecular adsorption (tiling) we attempt to describe the internal pore morphology and surface structure of three porous silica gels. The results are analyzed from both a classical euclidian and a fractal scaling viewpoint. The applicability of both viewpoints are shown to be limited but important in describing the structure of porous silicas.

The activity in our laboratory and in collaboration with other laboratories in applying fractal geometry for the characterization of irregular surfaces and materials and for the analysis of molecular interactions with such objects are summarized in this report. Most of the studies were performed on the xerogel of silica. Various theories and experimental techniques have been employed for this purpose. They include adsorption studies, computerized image analyses in one (boundary lines), two (textures) and three (proteins) dimensions; non-radiative, Förster-type, one step electronic energy transfer (EET); small angle X-ray scattering; and analysis of chemical reactivity of fractal objects. The relation between porosity and fractal dimension has been studied by EET, by the photophysics of adsorbed pyrene and by monolayer adsorption studies. A direct method for the determination of adsorption conformations has been developed. Limitations of the fractal approach have been identified and outlined.

Silica gels were prepared by two methods; from the alkoxides under conditions where moderate degrees of crosslinking are expected and from LUDOX - alkaline silicate sols from which the alkali was subsequently removed. By selective thermal treatment from 300 to 850°C, pore diameters (excluding micropores) in the range 35Å to 400Å are seen. The pore size increased with temperature while the surface area and total porosity remained essentially constant. Extensive high resolution TEM analyses showed both colloid and polymeric structures with particle sizes between 60Å and 250Å which decreased on heating. An intra particle structure, presumably micropores of 10Å to 30Å is also seen. By using a shadow cast Pt-C replica technique, stereoimages show the particles to have a filamentry type structure.

Packings of uniform disks with bond orientational order (BOO) are described. These are distinct from both ordered structures, that have translational correlations as well as BOO, and fully disordered structures that lack both features.

A technique for inducing ordered, close-packed arrangements of various symmetries among colloidal particles is discussed. An external alternating electric field applied to the colloid induces dipole interactions of variable strength by polarizing either the dielectric material of the particles or their electrostatic double layers. Ordering in various symmetries can be obtained by switching the field rapidly between pairs of electrodes, thereby changing the orientation of the induced dipoles. A small dc bias serves to deposit and compact the aligned particles.