Keith D. Keefer

Sol-gel transition in simple silicates II☆

Abstract Silicate gels were prepared under a range of conditions in which the rate of hydrolysis was varied from fast to slow with respect to the rate of condensation. When hydrolysis was fast, larger, more highly condensed polymers were formed during gelation. Conversely, for slow hydrolysis, smaller, less highly condensed polymers were formed. These gels dried to low density coarse textured and high density fine textured gels, respectively. High temperatures, (>800°C) were required to densify the coarse gels by viscous sintering. Lower temperatures were sufficient to densify fine gels by a process which was postulated to consist of polymer relaxation followed by condensation and pore collapse.

Liquidus relations in Y--Ba--Cu oxides

The liquidus relations in the system YO/sub 1.5/--BaO--CuO/sub x/ in air in the compositional region near the superconducting oxide YBa/sub 2/Cu/sub 3/O/sub x/ were studied by differential thermal analysis, x-ray diffraction, electron microprobe analysis, and visual observation. The temperatures of 11 invariant points and the corresponding reactions were determined. YBa/sub 2/Cu/sub 3/O/sub x/ was found to melt incongruently at 1015 /sup 0/C to Y/sub 2/BaCuO/sub 5/, which in turn melts incongruently to Y/sub 2/O/sub 3/ at 1270 /sup 0/C. These reactions mean that preparing the superconducting phase by melting and rapid cooling will result in the presence of these two phases as well. The peritectic reaction YBa/sub 2/Cu/sub 3/O/sub x/+CuO..-->..Y/sub 2/BaCuO/sub 5/+liquid at 940 /sup 0/C accounts for the observation of partial melting, improved synthesis purity, and grain growth at temperatures of 950 /sup 0/C. The determination of these invariant temperatures and reactions provide insight into optimal processing conditions.

The Effect of Hydrolysis Conditions on the Structure and Growth of Silicate Polymers

Small angle scattering experiments have demonstrated that the structure of the silicate species produced by the hydrolysis of silicon alkoxides in non-aqueous solvents ranges from extended, weakly cross-linked polymers to highly condensed, colloidal particles. In contrast, inorganic, aqueous silicate solutions yield primarily colloidal particles because the silicate species have a number of different silanol sites available and the preferred condensation reaction is that of weakly condensed species with highly cross-linked branch sites, such as those on an amorphous silica surface. It is proposed that in the alkoxide systems, however, the hydrolysis reaction may control the number and type of silanol sites available for condensation. In acid catalyzed reactions, the rate of hydrolysis of a silicate tetrahedron tends to decrease as alkoxide groups are removed. This favors the production of silanol sites on the end of chains, thus generating linear polymers. In base catalyzed reactions, it is argued that each subsequent hydrolysis of a tetrahedron should proceed more rapidly than the previous one, producing numerous branch points which are the preferred sites for condensation.

Structure of Soluble Silicates

Small angle x-ray scattering (SAXS) is the technique of choice for the determination of structure on the 10–1000A scale. We have used this technique to study the growth and topology of the macromolecules which precede gelation in several chemical systems used in sol-gel glass technology. The results show that branched polymers, as opposed to colloids, are formed. The alcoholic silica system is akin to organic systems where gelation occurs through growth and crosslinking of chain molecules. Data are reported from both the Porod and Guinier regions of the SAXS curve and these data are interpreted in terms of geometrical structures predicted by various disorderly growth processes. The results indicate that the degree of crosslinking can be controlled by catalytic conditions. The degree of crosslinking may, in turn, control phase separation and processability to a dense glass.

Structure of random silicates: polymers, colloids, and porous solids

Small angle x-ray scattering and light scattering are used to characterize structures grown by random processes within the silica system. Dense colloids, rough colloids, and branched polymers are grown by polymerization in solution. Supermolecular structures are also studied, including gels, colloidal liquids, and aggregates.

Mechanical properties and adhesion of hydrated glass surface layers

Abstract This paper reviews results for interfacial adhesion and fracture of silicate glasses that demonstrate the effect of hydrated glass surface layers on the mechanical properties of glass. First, it is shown how the generation of hydrated surface layers formed on alkali borosilicate glasses can control crack propagation rates. Crack growth data, solution analysis and surface stress measurements are used to support a fracture model that involves the generation of surface stress on the crack walls behind the crack tip. A fracture mechanics based model is used to show that stressed layers can contribute to the crack tip stress intensity in a way that either increases or decreases the rate of crack propagation. In the case of alkali containing silicate glasses, tensile stresses formed on the crack walls increase the crack tip stress and contribute to the formation of a low velocity plateau in the stress intensity vs. crack velocity curve. Second, fracture mechanics test techniques are used to examine the adhesive bond formed between hydrated surface layers and bulk silicate glass. The adhesive bonds formed by sol-gel precursors composed of colloidal silica, hydrolyzed organosilanes and alkali silicate solutions are compared to determine the mechanism of interfacial bonding to dense silica substrates. The formation of siloxane bonds across the interface depends upon the nature of the silicate polyanions in solution. For the case of soluble alkali silicate derived films, heat treatments at temperatures as low as 200°C can result interferfacial adhesion energies as large as the fracture energy of silica glass. These results have important implications to the aging and repair of surface damage in glass as well as the adhesion of sol-gel derived thin films.

Equilibrium structure and rigidity of alumina polymers

Alumina polymers are studied at various degrees of branching using X-ray and light scattering. A combination of static and dynamic data shows that lattice animal-like structures are formed which break apart and swell on dilution. We observe a threshold for large-scale polymerization when there are 2.5 bridging groups per A1. Below this value only polynuclear species occur whereas above this value the system gels.

Growth and Structure of Fractally Rough Silica Colloids

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.

Structure of Random Materials

Light scattering and small angle x-ray scattering results are reported for a variety of random materials. Random processes such as polymerization and aggregation account for the structure of these materials. Materials studied include linear and branched polymers, colloidal aggregates (prepared in solution, in flames and at an air-water interface), and composites. Although the concept of fractal geometry is essential to interpretation of the scattering curves, not all the materials show fractal character.

Fractal Aspects of Ceramic Synthesis

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. 51 refs., 8 figs.