Philippe Colomban

RAMAN SCATTERING AND LATTICE DEFECTS IN NANOCRYSTALLINE CEO2 THIN FILMS

Abstract The results of Raman scattering studies of nanocrystalline CeO 2 thin films are presented. The spectra have been described using the spatial correlation model from which the correlation length has been determined as a function of grain size. A direct comparison between the concentration of defects related to correlation length and CeO 2 non-stoichiometry has been achieved. The relationship between the lattice disorder and the form of the Raman spectra in nanocrystalline CeO 2 is discussed.

Nanostructure of Nafion® membranes at different states of hydration: An IR and Raman study

Abstract 117 and 112 grades of H+ Nafion® membranes were studied using near-IR FT Raman scattering, IR absorption and ATR techniques. Tested membranes were as received, n water molecules per –HSO3 groups (n(117I)∼10, n(112I)∼2), equilibrated in water at RT (n(117II)∼20) or at 100°C (n(112II)∼16), air dried at 80°C (117III grade, n∼1) and 110°C (117IV grade, n∼3×10−1) and completely dried over P2O5 (117V). Infrared (ATR) and Raman spectra show that whatever the water uptake, Nafion® (117 and 112) skeleton retains the helical zigzag conformation of the polytetrafluoroethylene (PTFE) and that the (ue5f8CF2ue5f8CF2ue5f8) links prevail on the fluorocarbon part of the side-pendant chains. Comparison with PTFE allows for the calculation of the crystallinity degree of Nafion® membranes. Two ν2 bending OH vibrations are observed at 1650 (water, acid water) and 1720xa0cm−1 (weakly-hydrated H3O+ ion). After drying, H3O+⋯SO3− interactions become visible and explain why the OH stretching massif shifts towards smaller wave numbers. At 80°C (117III), sub-maximum at 2980xa0cm−1 is assigned to H3O+ hydrogen bonded to SO3− groups. The acid water ν2 band disappears while the H3O+ band persists. In Nafion® 117IV, almost all the water content is eliminated and many sulfonic groups are undissociated. Formation of the hydrated cluster follows from the helical conformation of the (CF2)n backbone and from the relative position of adjacent chains.

Polymerization degree and Raman identification of ancient glasses used for jewelry, ceramic enamels and mosaics

Abstract We demonstrate the utility of Raman spectroscopy as a technique for the identification of ancient glasses and enamel coatings of ceramics. As for any silicate glasses, the addition of network modifiers breaks the Si–O linkages and modifies the degree of polymerization and hence the relative intensity of the Si–O bending and stretching modes. We demonstrate empirically that the ratio of these envelopes is well correlated to the glass structure and to the firing technology used. Spectral Qn components assigned to isolated and connected SiO4 vibrational units allow more precise analysis. Selected porcelains, faiences, potteries and glasses representative of the different Asian, Islamic and European production technologies were studied. Modern porcelain enamels are used as compositional references.

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

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.

NON-DESTRUCTIVE MECHANICAL CHARACTERIZATION OF SiC FIBERS BY RAMAN SPECTROSCOPY.

The paper provides a comprehensive study on Raman spectroscopy versatility as a fast and non destructive tool for the prediction of the mechanical properties of SiC fibers derived from a polymeric precursor (NLM™, Hi™, Hi-S™, SA™ and Sylramic™ grades) or produced by CVD (SCS-6™ fiber), including in situ analysis in CMCs or MMCs. We show how the results of very simple spectra fitting are correlated with Youngs modulus, tensile strength and microhardness. The reason why such a correlation exists, the common dependency of Raman signal and mechanical behavior to the micro/nanostructure of ceramics, is discussed.

Probing the Nanodomain Origin and Phase Transition Mechanisms in (Un)Poled PMN-PT Single Crystals and Textured Ceramics

Outstanding electrical properties of solids are often due to the composition heterogeneity and/or the competition between two or more sublattices. This is true for superionic and superprotonic conductors and supraconductors, as well as for many ferroelectric materials. As in PLZT ferroelectric materials, the exceptional ferro- and piezoelectric properties of the PMN-PT ((1−x)PbMg1/3Nb2/3O3−xPbTiO3) solid solutions arise from the coexistence of different symmetries with long and short scales in the morphotropic phase boundary (MPB) region. This complex physical behavior requires the use of experimental techniques able to probe the local structure at the nanoregion scale. Since both Raman signature and thermal expansion behavior depend on the chemical bond anharmonicity, these techniques are very efficient to detect and then to analyze the subtitle structural modifications with an efficiency comparable to neutron scattering. Using the example of poled (field cooling or room temperature) and unpoled PMN-PT single crystal and textured ceramic, we show how the competition between the different sublattices with competing degrees of freedom, namely the Pb-Pb dominated by the Coulombian interactions and those built of covalent bonded entities (NbO6 and TiO6), determine the short range arrangement and the outstanding ferro- and piezoelectric properties.

Ancient Portuguese Ceramic Wall Tiles (“Azulejos”): Characterization of the Glaze and Ceramic Pigments

Ancient ceramic wall tiles, called “azulejo”, firstly used on Portuguese churches, monasteries and palaces (15-18th century) have progressively been used in particular houses till the last century. These tiles and its use in huge decorative panels can be considered as a precious but fragile cultural heritage from Brazil to India, in several countries influenced by Portuguese culture. Morphologically, these tiles are composed by a porous clay-based ceramic body, the terracotta, covered by a protective glassy phase, the glaze. As artistic paintings, these murals incorporated various kinds of pigments in the glaze layer to create a pictorial impact on the walls of rich palaces or churches, real and durable monumental works-of-art. In the 21st century, degradation marks are visible on these ceramic tiles because of their use under corrosive conditions (moisture, atmospheric cycles…) along centuries. In order to promote their conservation and enhance their restoration, the physical-chemical characterization of the azulejos is performed in the present work, using mainly non-destructive processes like micro-Raman spectroscopy or X-Ray diffraction. In particular, Raman spectroscopy allows the detection of some nano/microcrystals present in the amorphous glaze due to pigments or opacifying agents or related to the elaboration process of the azulejo. Based on the observation of various selected fragments, one states that very few pigments have been used as colouring agents in this ceramic art during 17-18th centuries. Thus, the relationship between the different colours, the introduced pigments and the structural aspects of the glass will be focused. Some features related with the ancient ceramic technology will also be discussed.

Proton content and nature in perovskite ceramic membranes for medium temperature fuel cells and electrolysers.

Recent interest in environmentally friendly technology has promoted research on green house gas-free devices such as water steam electrolyzers, fuel cells and CO2/syngas converters. In such applications, proton conducting perovskite ceramics appear especially promising as electrolyte membranes. Prior to a successful industrial application, it is necessary to determine/understand their complex physical and chemical behavior, especially that related to proton incorporation mechanism, content and nature of bulk protonic species. Based on the results of quasi-elastic neutron scattering (QNS), thermogravimetric analysis (TGA), Raman and IR measurements we will show the complexity of the protonation process and the importance of differentiation between the protonic species adsorbed on a membrane surface and the bulk protons. The bulk proton content is very low, with a doping limit (~1–5 × 10−3 mole/mole), but sufficient to guarantee proton conduction below 600 °C. The bulk protons posses an ionic, covalent bond free nature and may occupy an interstitial site in the host perovskite structure.

Rocks as blue, green and black pigments/dyes of glazed pottery and enamelled glass artefacts – A review

Among the four different ways (and their combinations) to colour a solid, the dispersion of a phase, already coloured and stable in the fired/molten matrix – a pigment – is the technique offering the largest palette. The pigment preparation involves sophisticated high-temperature routes and/or the use of selected, sometimes rare minerals. Consequently, ancient painters, potters, craftsmen and glassmakers selected natural rocks or minerals to obtain blue, green, red and black colours. For instance, the blue coloration requires the use of a rare element: cobalt. Pure cobalt ores spots were exceptional and the difficult purification of cobalt derivatives could be obtained during the 19th century only. Thus, potters and glassmakers solved the problem by using recycling or specific minerals, for instance lapis lazuli, and developed convenient techniques with the use of mixed ores to obtain the blue, green, red or black colour. We discuss here enamelling/glazing technologies of ceramic and glass masterpieces with emphasis on the lapis lazuli use as blue or green pigment. We will reveal how the recent Raman microscopy studies deeply change our knowledge on the history of colouring techniques.

Testing the Chemical/Structural Stability of Proton Conducting Perovskite Ceramic Membranes by in Situ/ex Situ Autoclave Raman Microscopy

Ceramics, which exhibit high proton conductivity at moderate temperatures, are studied as electrolyte membranes or electrode components of fuel cells, electrolysers or CO2 converters. In severe operating conditions (high gas pressure/high temperature), the chemical activity towards potentially reactive atmospheres (water, CO2, etc.) is enhanced. This can lead to mechanical, chemical, and structural instability of the membranes and premature efficiency loss. Since the lifetime duration of a device determines its economical interest, stability/aging tests are essential. Consequently, we have developed autoclaves equipped with a sapphire window, allowing in situ Raman study in the 25–620 °C temperature region under 1–50 bar of water vapor/gas pressure, both with and without the application of an electric field. Taking examples of four widely investigated perovskites (BaZr0.9Yb0.1O3−δ, SrZr0.9Yb0.1O3−δ, BaZr0.25In0.75O3−δ, BaCe0.5Zr0.3Y0.16Zn0.04O3−δ), we demonstrate the high potential of our unique set-up to discriminate between good/stable and instable electrolytes as well as the ability to detect and monitor in situ: (i) the sample surface reaction with surrounding atmospheres and the formation of crystalline or amorphous secondary phases (carbonates, hydroxides, hydrates, etc.); and (ii) the structural modifications as a function of operating conditions. The results of these studies allow us to compare quantitatively the chemical stability versus water (corrosion rate from ~150 µm/day to less than 0.25 µm/day under 200–500 °C/15–80 bar PH2O) and to go further in comprehension of the aging mechanism of the membrane.