In recent years, near-nano (submicron) and nanostructured materials have attracted increasingly more attention from the materials community. Nanocrystalline materials are characterized by a microstructural length or grain size of up to about 100 nm. Materials having grain size of ∼0.1 to 0.3 μm are classified as submicron materials. Nanocrystalline materials exhibit various shapes or forms, and possess unique chemical, physical or mechanical properties. When the grain size is below a critical value (∼10–20 nm), more than 50 vol.% of atoms is associated with grain boundaries or interfacial boundaries. In this respect, dislocation pile-ups cannot form, and the Hall–Petch relationship for conventional coarse-grained materials is no longer valid. Therefore, grain boundaries play a major role in the deformation of nanocrystalline materials. Nanocrystalline materials exhibit creep and super plasticity at lower temperatures than conventional micro-grained counterparts. Similarly, plastic deformation of nanocrystalline coatings is considered to be associated with grain boundary sliding assisted by grain boundary diffusion or rotation. In this review paper, current developments in fabrication, microstructure, physical and mechanical properties of nanocrystalline materials and coatings will be addressed. Particular attention is paid to the properties of transition metal nitride nanocrystalline films formed by ion beam assisted deposition process.

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Nanocrystalline materials and coatings

Nouveau Traité de Chimie Minérale.

The chemical and structural properties of ferrite-based nanoparticles, precursors for magnetic drug targeting, have been studied by Raman confocal multispectral imaging. The nanoparticles were synthesised as aqueous magnetic fluids by co-precipitation of ferrous and ferric salts. Dehydrated particles corresponding to co-precipitation (CP) and oxidation (OX) steps of the magnetic fluid preparation have been compared in order to establish oxidation-related Raman features. These are discussed in correlation with the spectra of bulk iron oxides (magnetite, maghemite and hematite) recorded under the same experimental conditions. Considering a risk of laser-induced conversion of magnetite into hematite, this reaction was studied as a function of laser power and exposure to oxygen. Under hematite-free conditions, the Raman data indicated that nanoparticles consisted of magnetite and maghemite, and no oxyhydroxide species were detected. The relative maghemite/magnetite spectral contributions were quantified via fitting of their characteristic bands with Lorentzian profiles. Another quality parameter, contamination of the samples with carbon-related species, was assessed via a broad Raman band at 1580 cm−1. The optimised Raman parameters permitted assessment of the homogeneity and stability of the solid phase of prepared magnetic fluids using chemical imaging by Raman multispectral mapping. These data were statistically averaged over each image and over six independently prepared lots of each of the CP and OX nanoparticles. The reproducibility of oxidation rates of the particles was satisfactory: the maghemite spectral fraction varied from 27.8 ± 3.6% for the CP to 43.5 ± 5.6% for the OX samples. These values were used to speculate about the layered structure of isolated particles. Our data were in agreement with a model with maghemite core and magnetite nucleus. The overall oxidation state of the particles remained nearly unchanged for at least one month.

Molecular composition of iron oxide nanoparticles, precursors for magnetic drug targeting, as characterized by confocal Raman microspectroscopy

Two palettes, characteristic of the production of the Sevres Factory (Manufacture Nationale de Sevres), and representative of ancient glazing techniques, were studied. One was made with coloured glazes/enamels on Bisque (1050 °C) and the other one was composed of Moufle (880 °C) painting colours. Spectra specific to the white (opacifiers), yellow (including Naples yellow), green, pink, ‘brown–red’ and black coatings/paintings were considered. Emphasis was placed on the different Raman peaks arising from coloured coatings specific to ancient artefacts: greens obtained with dissolved Cu ions or from chromium pigments (Victoria green garnets), Naples yellows (Pb2Sb2O7 pyrochlore), green and black spinels and pink sphene. Attention was paid to the effect of pigment grain size on the Raman spectrum. To illustrate the capability of the method, we analysed some old artefacts from the eighteenth and nineteenth centuries. Copyright © 2001 John Wiley & Sons, Ltd.

Differentiation of antique ceramics from the Raman spectra of their coloured glazes and paintings

The use of metal nanoparticles dispersed in an optically clear matrix by potters and glassmakers from the Bronze Age up to the present time is reviewed from the solid state chemistry and material science point of view. The nature of metal (gold, silver or copper), the importance of some other elements (Fe, Sn, Sb, Bi) added to control metal reduction in the glass in relation to the firing atmosphere (combined reducing oxidizing sequences, role of hydrogen and water) are considered in the light of ancient Treatises and recent analyses using advanced techniques (FIB- TEM, EXAFS,…) and classical methods (optical microscopy, UV-visible absorption). The different types of colour production, by absorption/reflection (red, yellow) or diffraction (iridescence) and the relationship between nanostructure (metal particle dispersion, layer stacking) and lustre colour are discussed. The very specific interaction between light and the metal nanoparticle makes Raman scattering a very useful "bottom up" technique to study the local glass structure around the metal particles as well as to detect incomplete metal reduction or residues tracing the preparation route, hence making it possible to differentiate between genuine artefacts and fakes.

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The Use of Metal Nanoparticles to Produce Yellow, Red and Iridescent Colour, from Bronze Age to Present Times in Lustre Pottery and Glass: Solid State Chemistry, Spectroscopy and Nanostructure

Optically clear monolithic gel was prepared by slow alkoxide hydrolysis. Subsequent heating induces transformation first into optically clear amorphous porous monoliths and then into dense translucent or optically clear ceramics, the densification occurring at low temperature (∼ 0.6 Tmelt). The densification of aluminosilicates and alumina gels, prepared under various conditions of hydrolysis (solvent, water/alkoxide ratio, time, …), have been studied by dilatometry, thermogravimetry and N2(Kr) adsorption/desorption analysis. Results are discussed in the light of previous electron microscopic and Raman scattering investigations. The specific area and the sample volume decrease regularly on heating to the nucleation temperature, whereas the mean pore size remains approximately constant. Three types of pore distribution are observed: (i) mesopores (4–6 nm) for alumina and most mullites ‘glasses’; (ii) micropores (< 2 nm) for, for example, mullite prepared in acetone; (iii) fractal pores (< 2–20 nm) for aluminosilicates prepared by hydrolysis of an Si-O-Al ester. The sintering is directly related to the densification of the polymeric network. The departure of protonic species stabilizing the oxide surface launches the sintering and the nucleation of the crystalline phase.

Sintering of alumina and mullites prepared by slow hydrolysis of alkoxides: the role of the protonic species and pore topology

The thermal evolution of gels, glasses and ceramics of various more or less refractory compositions (Al2O3, 3Al2O32SiO2, 7Al2O33SiO2, Al2O32SiO2, Al2O32SiO20.7B2O3, Al2O32SiO22B2O3, Al2O32SiO26B2 O3) have been studied by dilatometry, DTA, and helium density measurements. Comparison is made for materials prepared by rapid (powder) or by very slow gelation (optically clear monoliths). The influence of atmosphere sintering (air, H2, vacuum) is reported. Densification and kinetic laws are discussed.

Densification mechanisms of alumina, aluminosilicates and aluminoborosilicates gels, glasses and ceramics: Code: DP18

Optically clear monolithic (OCM) gels of alumina and aluminosilicates can be prepared by hydrolysis-polycondensation of alkoxides. Subsequent heating induces transformation firstly into OCM mesoporous glasses and then into dense translucent or optically clear ceramics, the densification occurring at low temperature. Immersion of alumina or aluminosilicate gels and glasses in aqueous solutions containing transition metal (cobalt, iron, nickel) ions leads to diffusion of these ions into the water within the pores of the monoliths. After heat treatment under hydrogen, metallic nanoprecipitates are obtained. The size of the precipitates, as studied by transmission electron microscopy, ranges from 5 to 70 nm depending on the host matrix. The thermal stability in air of these materials was studied by differential thermal and analysis X-ray diffraction. The oxidation of porous composites occurs above 600 °C in air. Magnetic parameters such as coercitive field, magnetization, etc. were also measured.

Elaboration and thermal stability of (alumina, aluminosilicate/iron, cobalt, nickel) magnetic nanocomposites prepared through a sol-gel route

The hydroxyl group and water contents of alumina, mullite and alumino(boro)silicate gels and porous glasses (xerogels) are discussed on the basis of thermogravimetric, gas adsorption and density measurements. Comparison is made for powder prepared by instant hydrolysis and optically clear monoliths by slow hydrolysis, heated in various atmospheres (air, vacuum, hydrogen). The hydroxyl content appears to depend on the Al/Si ratio: 6 OH−/nm2 for alumina mesoporous glasses, 2 OH−/nm2 for aluminosilicate mesoporous glasses and 3 OH−/nm2 for microporous ones, typically.

V. Vendange

Papers

Sintering of alumina and mullites prepared by slow hydrolysis of alkoxides: the role of the protonic species and pore topology

Densification mechanisms of alumina, aluminosilicates and aluminoborosilicates gels, glasses and ceramics: Code: DP18

Elaboration and thermal stability of (alumina, aluminosilicate/iron, cobalt, nickel) magnetic nanocomposites prepared through a sol-gel route

Determination of the hydroxyl content in gels and porous “glasses” from alkoxide hydrolysis by combined TGA and BET analysis

Chemical reactions at the liquid-solid interface in meso/microporous inorganic gels and glasses