Papers by Author: Nathalie Herlin-Boime

Abstract: This document aims at presenting and explaining the mechanism of a simple green process, called Graftfast^©, recently developed in order to graft polymer films onto any type of materials. This process is of great interest as it works in a short one step reaction at room temperature, atmospheric pressure in water. Particularly since this method is a redoxinduced process consisting in the reduction of diazonium salts into aryl radicals in presence of vinylic monomer, the involvement of such radicals was investigated. Moreover, this work demonstrates the efficiency of such process for the preparation of functionalized TiO₂ nanoparticles. The composition and the grafted polymer quantities were investigated showing the successful grafting of the polymer onto the nanoparticles while conserving their morphology.

Abstract: Rare earth doped materials have many familiar applications (TV screens, solid lasers, scintillators…) thanks to their efficient and robust luminescence properties. In recent years, growing interest has been focused on the changes in their optical properties with the size of the host particles. In this work, nanomaterials were produced for the first time by using laser pyrolysis. Y, Gd, and Eu nitrates were dissolved in water and used as precursors. Cubic phases of Y2O3:Eu3+ and Gd2O3:Eu3+ were obtained with sizes ranging from 3 to 40 nm. The spectroscopic properties revealed a new and nanostructure-specific broad band in the excitation spectrum. The emission spectrum was found to be characteristic of nanostructured sesquioxides only when excited in this new band, which was finally assumed to be a new charge transfer band for the smallest nanoparticles in the sample.

Abstract: Two-nanophase SiC-Zn composites were synthesized under pressure up to 8 GPa at up to 1000oC using an high-pressure infiltration method. The advantage of this technique is that in a single, continuous process the ceramic nanopowder is compressed to form the matrix with nanopores; the nanopores are filled with a liquid secondary phase, (here Zn), which crystallizes as nano-scale grains. The key limitation is that the pores in the infiltrated preform have to stay open during the entire process. For this reason only powders of very hard ceramic materials can be used as a matrix. Two types of SiC nanopowders with average crystallite size of 10 nm and 60 nm and average particle size of 30 nm and 100 nm, respectively were used. The measurements of porosity of the green compacts prepared from these powders, pressed at 2.5 GPa and 8 GPa at room temperature, indicated that open porosity was maintained. The nanocomposites obtained show a “nano-nano” type microstructure with a uniform mixture of SiC and Zn phases. The volume fraction of Zn is 20 % independent of the process conditions and initial powder morphology. The process parameters and powder granularity influenced the crystal size of the secondary phase. The average grain size of Zn varied from 20 to 85 nm and was smaller in the composites obtained with the finer matrix, under higher pressure and at lower temperature. The microhardness HV02 of SiCZn nanocomposites varied from 6 to 22 GPa and increased with an increase of pressure and temperature of the infiltration process, and was significantly larger for the finer grained composites.

Abstract: SiC-Zn nanocomposites with about 20% volume fraction of metal were fabricated by infiltration process under the pressure of 2-8 GPa and at the temperature of 400_1000oC. SiC nanopowders used in the experiments formed loosely agglomerated chains of single crystal nanoparticles. The dimension of the agglomerates was in the micrometer range, the mean grain size was up to tens of nanometers. Microstructural investigations of the nanocomposites were performed at a different resolution levels using scanning, transmission electron microscopy and atomic force microscopy techniques (SEM, TEM, AFM, respectively). SEM observations indicate a presence of nano-dispersed, uniform (on the micrometer scale) mixture of two phases. TEM observations show that distribution of SiC and Zn nanocrystallites is uniform on the nanometer scale. High-resolution TEM images demonstrate an existence of thin (on the order of tens of Angstroms) Zn layers separating SiC grains. AFM images of the mechanically polished samples show a smooth surface with the roughness on the order of the SiC grain size (10-30 nm). After ion etching of some samples the AFM topographs show surface irregularities: periodically spaced hillocks 50-100 nm in height. The size of the SiC grains remains equal to that of the initial powder crystallites. The size of the Zn grains varies in the range of 20-100 nm depending on the initial SiC grain size and the composite fabrication conditions.