Augusto A. Morrone

Recent developments in the preparation and properties of nanometer-size spherical and platelet-shaped particles and composite particles

Abstract By using self-assembly molecules as a template, nanometer-sized plate-like metal oxide and semiconductor particles can be obtained by confined growth inside the lamellar bilayers of microemulsion systems. It was found that a strong chemical affinity between the metal salt and the polar head group of amphiphilic molecules and the anisotropic structure of microemulsion systems play a premier role in the anisotropic growth. Nanometer-sized composite particles (nano-composites) with a core-shell structure have been prepared by arrested precipitation of metal or semiconductor clusters in reverse micelles, followed by hydrolysis and condensation of organometallic precursors in the microemulsion matrices. Temporally discrete nucleation and growth at elevated temperature (70 °C) give the resulting particles a narrow size distribution and defined crystallinity. Both the size of the core particles and the thickness of the coating layers can be varied by controlling processing parameters such as the ratio of water to surfactant and the ratio of water to organometallic precursors. By controlling the pH conditions and aging temperatures, a transparent gel composing the nanometer-sized inorganic clusters has been obtained. Optical properties of nanometer-sized composite particles are reviewed. For silver metal clusters and nano-composites, the shift of the absorption peak at the surface-plasmon resonance frequency due to the classical limited mean-free path of the conduction electrons or quantum size effects has been observed. The enhanced third-order non-linear susceptibility of the silver nano-composites results from the local-field enhancement and size effects, which has been experimentally demonstrated by the optical phase-conjugation technique.

Formation mechanisms and morphological changes during the hydrothermal synthesis of BaTiO3 particles from a chemically modified, amorphous titanium (hydrous) oxide precursor

Abstract The formation mechanism of BaTiO3 under hydrothermal conditions was investigated. A coprecipitated precursor prepared from chemically modified titanium isopropoxide with acetylacetone and barium acetate was used as a starting material. A solid-state kinetic analysis, supported by microstructural evidence, indicates that the formation mechanism of BaTiO3 in the current material system is dissolution and precipitation. The Ba–Ti complex gel dissolves into the aqueous soluble species, followed by direct precipitation from supersaturated solution. It is proposed that crystallization is controlled by dissolution of the hydrous Ti gel at the initial stage and then possibly by dissociation of the acetylacetonate group from the Ti solution species in which the acetylacetonate group is strongly bound to Ti.

Polymer-derived silicon carbide fibers with low oxygen content and improved thermomechanical stability

Abstract Continuous silicon carbide fibers (UF fibers) with low oxygen content (∼1−2 wt%) were prepared in a range of diameters (∼8−50 μm) by the dry spinning of organosilicon polymer solutions and subsequent pyrolysis of the polymer fibers. Room-temperature mechanical properties were similar to commercially-available Nicalon TM fibers, as average tensile strengths ≈3 GPa were obtained for some batches with fiber diameters in the range of ∼10 μm. Furthermore, UF fibers showed significantly better thermomechanical stability compared to Nicalon, as indicated by lower weight losses, lower specific surface areas, and improved strength retention after heat treatment at temperatures up to 1700°C. The structure and composition of UF fibers were also characterized using scanning electron microscopy, transmission electron microscopy, X-ray diffraction, Auger depth profiling analysis, neutron activation analysis, and atomic absorption.

X-ray photoelectron spectroscopy, attenuated total reflectance Fourier transform IR spectroscopy and transmission electron diffraction studies of oriented cadmium iodide prepared in a Langmuir-Blodgett template

Abstract Oriented CdI 2 has been prepared in a deposited Langmuir-Blodgett (LB) film by the reaction of gaseous HI with a 20-layer LB film of cadmium arachidate. Attenuated total reflectance Fourier transform IR spectroscopy shows that the conversion goes to completion, while X-ray photoelectron spectroscopy analysis of the HI-exposed cadmium arachidate film reveals a Cd:I ratio consistent with the CdI 2 stoichiometry. Electron diffraction shows that the CdI 2 is oriented exclusively with its [001] axis normal to the LB film basal plane and for domains ranging up to several microns has a preferred alignment about the [001] axis. The results suggest that the organic template plays a role in orienting the CdI 2 within the LB plane and that some degree of lattice matching exists between the LB template and the (001) face of CdI 2 . The oriented synthesis described here is in contrast with previous attempts at template-directed syntheses using deposited LB films which have resulted in the mediation of particle size only and not particle orientation. The difference here may be that CdI 2 has a layered structure in the bulk, crystalline phase which complements the LB film structure.

TEM microstructural analysis of ceramic powders derived from the pyrolysis of polyvinylmethylsilazane

Abstract Polymers from a cyclo-trivinylsilazane were prepared by free-radical initiation. The cross-linked polymers were pyrolyzed in nitrogen and argon at temperatures up to 1750°C. The powders obtained were analyzed by TEM. The microstructure and composition of the ceramic powders varied as a function of pyrolysis temperature and atmosphere. Crystalline phases identified were α- and β-SiC, and α- and β-Si 3 N 4 . They were present as equiaxed crystals up to 0.3 μm diameter and/or whiskers of diameters under 0.3 μm and aspect ratios up to 60. Subsequent exposure to oxygen at high temperatures was also studied.