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A combination of electron diffraction and infrared reflectance measurements shows that synthetic silica transforms partially into stishovite under high-intensity (GW/cm 2 ) laser irradiation, probably by the formation of a dense ionized plasma above the silica surface. During the transformation the silicon coordination changes from four-fold to six-fold and the silicon-oxygen bond changes from mostly covalent to mostly ionic, such that optical properties of the transformed material differ significantly from those of the original glass. This phase transformation offers one suitable mechanism by which laser-induced damage grows catastrophically once initiated, thereby dramatically shortening the service lifetime of optics used for high-power photonics applications such as inertial confinement fusion.3

Porous ceramics are of great interest for filtration, catalysis, and reactive separation processes. Performance in these applications is highly dependent on features such as pore size distribution and connectivity and wall composition. Here, we describe a method allowing the rational design and synthesis of mesoporous silica composites with controlled heterogeneous pore architectures and demonstrate its validity by producing structures with predetermined placement of regions having different pore size and pore organization.

Dicumyl peroxide initiator (2wt% of polysilazane) was first dissolved into PSZ. This PSZ and PB-b-PEO were then dissolved separately in a 1:1 (vol.) mixture of tetrahydrofuran and chloroform to obtain 5wt% solutions. Subsequently the solutions of precursor and block copolymer were mixed according to the designed proportion.These homogenous solution mixtures were then cast into Teflon dishes and allowed to dry at room temperature for 6-12 hours. All mixing and drying was conducted in a glove box under nitrogen.The dried polymer blends were transferred into a vacuum oven pre-set to 70°C to eliminate any residual solvent, then annealed at 100°C for 12 hours to allow for self-assembly of PB-b-PEO and PSZ. Films of about 0.5 mm in thickness were then peeled from the Teflon dishes and transferred into a tube furnace with flowing nitrogen. The samples were heated to 400°C for one hour in order to induce cross-linking. Pyrolysis experiments were conducted by further heating the cross-linked samples at a constant rate of 0.5°-20°C/min to final temperatures of 700° to 1500°C, the dwell time at the final pyrolysis temperature was 2-4 hours.Transmission electron microscopy. Polymeric samples were microtomed either at room temperature (after being embedded in an epoxy matrix) or at -150°C. Fully pyrolyzed samples were crushed into fine particles and dispersed onto a thin holey-carbon support film. All samples

The oxidation of Nicalon™ fibers is a concern, because of its potential as a reinforcement of high-temperature composites, whose service conditions involve high-temperature, oxidizing environments. Two limiting types of oxidation mechanisms are often used to describe the kinetics: chemical-reaction-controlled oxidation, at small oxide thicknesses, and diffusion-controlled oxidation, at large oxide thicknesses. Neither mechanism can satisfactorily describe the intermediate region where the oxidation kinetics are controlled jointly by both the chemical reaction rate at the interface and the diffusion of oxygen through the oxide layer. To describe the entire oxidation process with a general relationship, one must consider all stages of the oxidation process, namely (i) adsorption of oxygen at the outer surface of the oxide, (ii) diffusion of oxygen from the outer surface toward the interface where oxidation occurs, and (iii) reaction at the interface to form a new layer of oxide. Previously, a very useful general relationship was derived for the oxidation kinetics for a flat plate, which could account for all three stages of oxidation. However, that equation is inadequate to describe the oxidation of cylindrical fibers, because the effective area for oxygen diffusion changes along the diffusion path and the oxidation interfacial area decreases as the oxide thickness increases for cylindrical fibers. In this paper, we have derived a general kinetic relationship for the oxidation of cylindrical fibers, which can account for all stages of oxidation. Comparison of the theory with experimental data of Nicalon™ fibers shows good agreement.

We report on the first observation of quantum splitting effect in nanocrystalline powders of SrAl12O19:1% Pr, Mg. The nanocrystalline materials were prepared using a surfactant-templated-assisted route yielding high-quality hexagonal nanocrystallites with thicknesses comprised between 30 and 60 nm. Comprehensive optical studies, including emission, photoexcitation, and time-resolved fluorescence measurements, demonstrate that no physical property degradation such as surface-induced loss mechanisms are found and that the quantum splitting properties of the material are fully retained in the nanocrystalline particles.

We report on the selective-area heteroepitaxy and facet evolution of submicron GaN islands on GaN-sapphire, AlN-sapphire, and bare sapphire substrates. It is shown that strain due to the lattice mismatch between GaN and the underlying substrate has a significant influence on the final morphology and faceting of submicron islands. Under identical metalorganic chemical vapor deposition growth parameters, islands with low or no mismatch strain exhibit pyramidal morphologies, while highly strained islands evolve into prismatic shapes. Furthermore, islands grown with relatively low compressive mismatch strain yield more uniform arrays of pyramids as compared to the nonstrained, homoepitaxially grown crystals. It is proposed that the strain dependency of Ehrlich-Schwoebel barriers across different crystallographic planes could potentially account for the observed morphologies during selective area growth of GaN islands.

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