María Angeles Villegas

Photoluminescence of silver in glassy matrices

This work studies the behavior of Ag+ ions incorporated in different silica-based glassy matrices. To this end, Ag-doped silica coatings, prepared via sol-gel and deposited on pure silica and soda-lime glasses, are investigated by means of structural and optical spectroscopy techniques. Silver tends to segregate towards the interface during the annealing process, but in the case of soda-lime glassy substrates the exchange process favors its diffusion into the substrate. The environment of Ag+ ions during the annealing process determines their final oxidation state. In the pure silica matrix, Ag+ ions are found to be unstable and tend to reduce to Ag0, with the subsequent formation of metallic nanoparticles. However, the presence of network formers and modifiers gives rise to the appearance of nonbridging oxygen, which allows the stabilization of Ag+ ions in the matrix.

Mechanism of crystallization of Co-cordierites from stoichiometric powdered glasses

Abstract The influence of cobalt, as a divalent cation, on the crystallization behaviour of the cordierite based glasses was studied. Powdered glass specimens of stoichiometric composition 2MO 2Al 2 O 3 5SiO 2 (M=Co and/or Mg) were obtained and thermally treated at several temperatures and times, and the sequence of crystallization and their microstructural evolution were analysed by X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM). Results on the crystallization sequence showed that the steps in the crystallization path are independent of the nature of the divalent cation. The first crystalline phase detected displayed the μ-cordierite structure, i.e. a solid solution with β-quartz structure. After further thermal treatment a phase with α-cordierite structure was formed. Cobalt μ- and α-cordierites were further characterized by infrared (IR) and UV–vis diffuse reflectance spectroscopies. The final morphology of cobalt-containing hexagonal cordierites depended on the densification degree of the precursor glass.

Crystallization and microstructural development of nickel-containing aluminosilicate glasses with cordierite stoichiometry

Abstract The crystallization behaviour of two glasses with chemical compositions MAl2O4/SiO2=1/2.5, being MNi and equimolar mixtures of Mg and Ni, was investigated. Glasses were prepared by cooling samples melted at 1650 °C. During thermal treatment over the range of temperature up to 1200 °C both glasses were first crystallized to form a β-quartz solid solution, then transformed to α-cordierite or spinel depending on the amount of nickel. The temperatures of both the crystallization of β-quartz solid solution and its further transformation were also dependent on the nominal amount of nickel. Lower temperatures were found for glasses with a large amount of nickel. UV–Vis diffuse reflectance characterization of the starting and thermal treated glasses indicated that the Ni+2 cations located in tetrahedral sites in the glass moved into octahedral sites after forming the final crystalline phases. The microstructural features of α-cordierite-based material from nickel–magnesium cordierite glasses heated at 1100 °C, examined by field emission scanning electron microscopy (FESEM) revealed the formation of well-defined crystals including regular and elongated hexagonal shaped prisms and square shaped particles.

Surface Characterisation of Cerium-Doped Silica Coatings Obtained by Sol-Gel

The chemical composition of cerium-doped silica coatings prepared via sol-gel were studied using X-ray photoelectron spectroscopy (XPS) in conjunction with Ar+-ion sputtering, Rutherford back-scattering spectroscopy (RBS) and photoluminescence (PL) spectroscopy. XPS results showed that cerium was incorporated in the silica network as Ce(III). The absence of PL emissions from Ce(III) was explained by a clustering of the ions producing a quenching of the luminescence. XPS combined with Ar+ and RBS showed that the distribution of Ce is not uniform across the coating, showing a maximum concentration in an inner layer of the coating.

Archeometry in preventive conservation of metal objects from the Naval Museum of Madrid

La evaluacion de las condiciones ambientales, tanto en sala como en vitrinas, realizada en el Museo Naval de Madrid a partir de un protocolo de seguimiento de la acidez ambiental que utiliza sensores opticos basados en la tecnologia sol-gel, sirvio para constatar que las condiciones de conservacion eran adecuadas en la mayoria de los espacios expositivos. Sin embargo, en algunas vitrinas en las que se habian observado problemas de conservacion de objetos metalicos procedentes de pecios, los valores de pH ambiental detectados fueron acidos (valores promedio inferiores a pH = 6,5). Esta contribucion presenta los resultados de la investigacion llevada a cabo para determinar las causas de los problemas de conservacion observados en una vitrina que expone balas de plomo, con el proposito de establecer pautas de conservacion preventiva. Las balas de plomo desarrollaban una capa blanquecina poco consistente tras su limpieza y posterior proteccion contra la corrosion con una resina acrilica comercial. Se realizaron diferentes ensayos en el laboratorio con los objetos tratados y sin tratar dentro de recipientes cerrados, y con los soportes y contenedores de polimetilmetacrilato del interior de las vitrinas, utilizando en ambos casos los sensores opticos mencionados. Los resultados indicaron que en los objetos de plomo se formaba una capa de formiato de plomo producida por la presencia de especies acidas cuyo origen se debe, muy probablemente, al efecto combinado del empleo de la resina acrilica y la presencia de materiales de polimetilmetacrilato en un ambiente cerrado. La determinacion del pH en el ambiente de dicha vitrina y su seguimento en los ensayos de laboratorio ha permitido alertar sobre el medio acido generado, lo cual ha servido para establecer acciones correctoras de conservacion preventiva en el museo.