2,366,516 l/1945 Gefficken et al. ................... 17/124 2,466, 19 4/1949 Moulton et al. .......................... 88/1 3,094,436 6/1963 Schroder ........... 7/25 4,059,658 11/1977 Shoup et al. ... ... 264/43 4, 65,224 8/1979 Irven et al. ............ ... 65/3.2 4,323,381 4/1982 Matsuyama et al. .. ... 65/90 4,605,428 8/986 Johnson et al. .... ... 65/18. 4,680,046 7/1987 Matsuo et al. ..... ... 6.5/90 4,680,048 7/1987 Motoki et al. ..... ... 65/90 4,840,653 6/1989 Rabinovich ........ ... 65/90 4,872,895 10/1989 Fleming et al. ...................... 65/18.1 4,92.73 5/1990 Clark et al. ........ ... 427/376.4 4.941,905 7/1990 Narasimham ......................... 65/90 4,944,895 7/1990 Buckley et al. ... ... 65/18. 4,952,225 8/1990 Le Sergent et al.................. 65/3.2

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.

We have investigated interface reactions between expitaxially-grown magnetite (Fe3O4) and maghemite (γ-Fe2O3) films with MgO substrates using Rutherford backscattering (RBS), channeling, and X-ray diffraction (XRD). Annealing these films in 2.0×10−6 Torr of O at temperatures up to 970 K enhances Mg outdiffusion into the films and increases the film thickness depending on temperature. The Fe3O4 film thickness reach a limiting value at 870 K anneal while the γ-Fe2O3 film thickness did not maximize after annealing at 970 K. After the annealing at 970 K, both films produced a compound with composition close to magnesioferrite (MgFe2O4). XRD results reveal the formation of MgFe2O4 films after annealing both films at 970 K in O.

The objective of this study was to understand the kinetics of hydrolysis and condensation reactions and structures of zirconium hydrous polymers formed by these reactions. Small angle X-ray scattering (SAXS) measurements were made on solutions for an aging period ranging from 109 ms to few days. Results prove that condensation reactions start before completion of hydrolysis and as a result polymers formed at the earlier stages (t≈100 ms) are weakly cross-linked (linear). On further aging, due to the internal condensation process these weakly cross-linked polymers convert to highly branched polymers with a structure identical to an ideal Gaussian randomly branched polymer with a persistent length (building block) of 8.3±0.5 Å.

Standardized experimental and calculated X-ray powder diffraction data for a new synthetic zeolite, cesium titanium silicate, CsTiSi2O6.5, are reported. In addition, a structure model in space groupIa3d(230), which is isomorphous with the mineral pollucite (CsAlSi2O6·xH2O), is proposed for CsTiSi2O6.5. This structure model is the basis of the reported calculated X-ray powder diffraction data for CsTiSi2O6.5. The experimental pattern for CsTiSi2O6.5shows this compound crystallizes in a body-centered cubic (BCC) unit cell witha=13.8423 (1) Å. The measured value of the reference intensity ratio (I/Ic) of CsTiSi2O6.5is 2.37, while the contrasting calculated value ofI/Icis 4.45. The experimental density (Dm) of CsTiSi2O6.5is 3.48±0.09 g/cm3, in agreement with the calculated density (Dx) of 3.42 g/cm3. Chemical analysis of CsTiSi2O6.5by atomic absorption spectroscopy gives its composition as 54±2 wt% Cs; 23±2 wt% Ti; 23±2 wt% Si, which compares favorably with the theoretical composition of 56 wt% Cs; 20 wt% Ti; 24 wt% Si.

Knowledge of the equilibrium conditions under which the superconducting phases in the Tl‐Ca‐Ba‐Cu‐O system are stable would enable the controlled preparation of these materials. Isothermal equilibrium between samples in this quaternary system and Tl2O vapor from a condensed thallium oxide source has been studied. None of the superconductor phases coexisted in equilibrium with thallium oxide under the conditions employed. Instead, two previously unknown ternary thallium‐calcium‐barium oxides and one previously unknown thallium‐calcium oxide were observed. The equilibrium phase assemblage at temperatures between 825° and 925°C consisted of the most thallium‐rich ternary oxide plus CuO, although small amounts of the other thallium‐rich phases may be required to balance stoichiometry. The vapor pressure of Tl2O established by pure, condensed thallium oxide clearly exceeds the stability limit for the superconductor phases. If the thallium‐containing superconductors are stable phases at high temperature, they will exist over some range of lower Tl2O partial pressure.

Inorganic silicate polymers are prepared by the hydrolysis of silicon alkoxides and subsequent condensation of the resulting silanols. The structures can be described only in a statistical fashion by means of fractal geometry. The structures are controlled by the relative rates of the hydrolysis reactions that generate reactive silanol groups and the condensation reactions of those groups. Under alkaline conditions, hydrolysis tends to be slow with respect to condensation, and nonfractal colloidal structures form by a nucleation- and reaction-limited growth process. Under acidic conditions, hydrolysis tends to be fast with respect to condensation, and "polymeric" fractal structures with Df (mass fractal dimension) = 2.1 are formed by reaction-limited cluster aggregation. The exact structure in real systems may only approach these idealized cases.

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.

The recent discovery of superconducting oxide ceramics with critical temperatures (Tc) near 100°K has stimulated research at an unprecedented pace. Single phase YBa2 Cu30x (x = 6.9), the subject of most of the interest, is an oxygen deficient 1:1:3 stacked perovskite derivative, (The structure of this materials has been described by a number of authors, many of whom are referenced by Jorgensen et al.) At room temperture it has an orthorhombic structure with lattice parameters near: a = 3.82Å, b = 3.88Å, and c - 11.68 Å. At lower oxygen values (x≤6.5) the structure is tetragonal with lattice parameters near: a = 3.86 and c = 11.80. It has been suggested that tetragonal YBa2 Cu30x is not superconducting. YBa2 Cu30x, regardless of oxygen content, is referred to here as 123.

A process for forming thin films of superconducting material on a metal substrate by heating the metal substrate in an oxidizing atmosphere to form a metal oxide on the substrate surface and then coating the surface of the substrate with a solution containing dissolved salts of the metals which make up the superconducting film. The solution on the substrate is heated to evaporate the solvent and form a film of salts on the substrate surface. The dried film is further heated at an elevated temperature, either in the presence or absence of O{sub 2}, to complete reaction between constituents of the film. The resulting ceramic coated substrate is finally heated in an oxidizing atmosphere to form a superconducting film on the substrate.

Several porous silicas have been studied in order to determine the origin of porosity in random porous media. The silica system offers a unique opportunity to study the origin of porous structures because a variety of different structures can be synthesized depending on precursor chemistry and physics. In solution‐grown materials, for example, it is possible to grow particles with randomly rough surfaces, smooth surfaces, as well as polymerlike structures. Porous materials can be made from all these synthetic precursors and the structure of the final product reflects the geometry of the precursors. In addition, porous materials can be made by a phase‐separation/leaching process which gives rise to a completely different type of porosity network. Structures from these various classes have been studied by small‐angle x‐ray and neutron scattering and in some cases it is possible to directly trace the structure of the porous material to the geometry of the precursor macromolecules.

The transformation of tetragonal zirconia to monoclinic in a glass‐ceramic as a function of depth from a fracture surface was measured by X‐ray diffraction employing synchrotron radiation. The large number of photon energies which can be used in such an experiment permt rigorous testing of models of the transformation zone. A crack plane intersection model has been developed to interpret the observations for two phase‐transformation‐toughened glass‐ceramics of grossly different inicrostructures.

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.

Aerogels are highly porous, synthetic materials composed of continuous, interpenetrating solid and gaseous phases. Normally these materials are prepared by extracting the liquid phase from a gel using supercritical drying (SCD). In this process, the portion of the gel originally occupied by liquid (normally alcohol) is replaced by a non-condensed gas with little or no associated shrinkage. Aerogel density therefore generally scales with the volume fraction (concentration) of condensed species in the original gel.

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.

In situ FTIR, NMR and SAXS were used to investigate the synthesis and molecular structure of xLi2O·(1−x)B2O3 gels derived from tri-n-butyl borate (TBB) and lithium methoxide. In solution the fraction of tetrahedrally coordinated borons (N4) increases linearly with x, and a critical value of N4 must be exceeded in order to form gels. The primary criterion affecting gel formation is the kinetic stability of borate bonds toward molecules which are able to undergo dissociative chemisorption.

Using small-angle x-ray scattering, we show that porous silica aerogel has a fractal backbone structure. The observed structure is traced to the underlying chemical (polymerization) and physical (colloid aggregation) growth processes. Comparison of scattering curves for aerogel with silica aggregates confirms this interpretation.

We report the growth and structure of fractally rough silicate particles in solution. Using small-angle x-ray scattering, we find fractal surfaces both in solution and in a porous solid made from the solution precursor. Finally, we develop a simple model which both is consistent with silicate chemistry and generates fractally rough structures.

Solutions of tetramethoxysilicon are reacted until very near the sol-gel transition. At this point the sol is diluted and elastic and quasielastic light scattering measurements are made at intermediate momentum transfer. The static scattering exponent for the diluted sol is found to be in good agreement with the percolation prediction of 1.6, and the quasielastic scattering measurements demonstrate the flexibility of the sol clusters.

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.