J.M. Fernández Navarro

Structural considerations about SiO2 glasses prepared by sol-gel

Abstract Silica gels were prepared from alcoholic solutions of tetraethylorthosilicate (TEOS) with different H2O/TEOS molar ratios. In the present work the transformation with temperature of these gels has been studied using IR spectroscopy together with measurements of density and specific surface area, thermal analysis and SEM. The IR spectra of these gels, measured as a function of temperature and H2O/TEOS ratio, confirm that the relative concentration of OH groups is temperature dependent. The total OH content, also depends on the H2O/TEOS ratio in the gel. The combined results of IR, density and specific surface area measurements allow some structural considerations to be advanced about the final SiO2 glasses obtained.

Optical properties of glass coatings containing Fe and Co

Abstract Glass films of SiO 2 M x O y (M = Fe, Co, FeCo) have been prepared on soda-lime-silica flat glass by sol-gel employing the dip-coating technique. Different parameters affecting solution preparation and application and coating processing (H 2 O/TEOS and R-OH/TEOS ratios, type of catalyst, time and temperature of thermal treatment) were studied, determining the limits in the R-OH, H 2 O, TEOS diagram within which good coatings can be produced. Film thickness varies from 0.2 to 0.9 μm and its dependence on viscosity and oxide concentration in the solution and on the withdrawal rate has been studied. Furthermore, it has been determined the relation between film thickness and microcracks appearance using ROM. Optical properties of coatings have been investigated by spectrophotometric techniques, optical parameters obtained being compared with the corresponding parameters of soda-lime-silica glasses containing the same oxides. From these measurements, the coloration mechanism in each case has been determined.

Characterization of B2O3−SiO2 glasses prepared via sol−gel

B2O3-SiO2 glasses were prepared by the sol-gel method from boron and silicon alkoxides. The gels were densified by several heat-treatments at temperatures above 800° C. The gel-glass transition was studied with the data obtained from differential thermal analysis, dilatometry and thermal evolution of density. The structural features of the glassy materials prepared were analysed by means of infrared (IR) and near-infrared (NIR) spectroscopy. The IR spectra indicated the existence of mixed Si-O-B bonds, preferential located at the outside of the material particles. The NIR spectra, recorded from gels heat-treated at low temperatures, demonstrated the existence of a great number of OH− groups varying in nature, as well as their evolution and their positional changes on the silica particle surface as a function of the heat-treatment.

UV highly absorbent coatings with CeO2 and TiO2

Abstract Coatings of SiO2CeO2 have been prepared on microscope slides using the dip coating technique to obtain films with a high UV absorption. Different compositions have been prepared by varying the ratios of SiO2/TiO2 and TiO2/CeO2. The optical behaviour of these films have been followed by spectrophotometric measurements. Absorption spectra of the coatings show that a TiCe equimolar complex is responsible for an absorption band centered at 290 nm, due to Ce3+ ions. Thick films (0.5–1 μm) of intense yellow colour with high reflectance and high UV absorption have been obtained which may find interesting applications.

ZrO2-SiO2 materials prepared by sol-gel

Abstract Gels with composition xZrO2−(100−x)SiO2, x = 10−55, were prepared in different conditions using zirconium acetylacetonate and TEOS as precursors. Gels treated at different temperatures up to 1100°C were characterized by X-ray diffraction, IR spectroscopy and TEM. Preparation conditions determined the subsequent development of crystalline phases following thermal treatment. Monoclinic zirconia segregation dispersed in a silica matrix occurred when the gels were prepared in a strong hydrocloric acid medium. Preparation with a lower acid content favours instead the formation of very small crystals of tetragonal zirconia.

Optical Spectroscopy of Hybrid Sol-Gel Coatings Doped with Noble Metals

Sol-gel coatings in the xM⋅ (100-x) SiO2 system, (M = Cu, Ag and Au) x =0.1–10 mol%), are deposited on soda lime glass slides by using silicon tetramethoxide Si(OCH3)4) and methyltriethoxysilane (SiCH3[OCH2CH3]3) as silica precursors. Anhydrous CuCl, CuCl2 ⋅ 2H2O, Cu(NO3)2 ⋅ 3H2O, CuSO4 ⋅ 5H2O, AgNO3 and HAuCl4 ⋅ 3H2O are used as copper, silver and gold sources. Coatings with thicknesses ranging from 100 to 900 nm are deposited on the subs trates by dip-coating and subsequently densified at 500°C for 1 h in air. Spectroscopic studies of the coatings as a function of the thicknesses and the metal concentration are carried out by photoluminescence (PL) and optical absorption (OA). In addition, direct observations of some gold coatings were performed by transmission electron microscopy (TEM). Results indicate that for silver and copper containing coatings the excitation and emission spectra arise from electronic transitions in Ag+ and Cu+ ions and no significant absorption bands due to colloidal precipitation are observed. Gold containing coatings show purple coloration due to an absorption peaking in the 520–560 nm range, which is characteristic of gold colloids. The presence of these colloids is confirmed by TEM observations.

Characterization of titanium induced optical absorption bands in phosphate glasses

Abstract A detailed study of the optical absorption bands induced by titanium in phosphate glasses has been carried out. Optical absorption, diffuse reflectance and EPR techniques have been used to characterize the centers responsible for these bands, which occur in the near ultraviolet and visible region of the spectrum. Three kinds of centers have been identified: Ti3+ ions in a distorted octahedral environment, which show the 10 Dq value characteristic of these type of centers (17 700 cm−1); Ti4+ ions in octahedral coordination presenting a charge transfer transition in the near UV region; and Ti3+ −Ti4+ mixed complexes, which originate an optical absorption band at 450 nm.

Colour of silicate sol-gel glasses containing CuO

Abstract Glasses with compositions x CuO·(100 − x )SiO 2 and 5R n O· x CuO. (95−x)SiO 2 , where R = Li, Na, Ca and x = 0.25–10, were prepared by the sol-gel method. Samples were thermally treated between 60 °C and 1000 °C in oxidizing and reducing atmospheres. Copper incorporation was studied by spectrophotometry, X-ray diffraction and TEM. All the samples are transparent and present a bluegreen colour at 600 °C in oxidizing atmosphere. The obtained results prove that copper is mainly incorporated as Cu 2+ . At higher temperatures all the samples present α-cristobalite, the samples containing Li 2 O or CaO also show α-quartz as crystalline phases. In reducing atmosphere particle segregation takes place, producing in some cases opaque materials. Under specific conditions, transparent ruby glasses were obtained.

TiO2SiO2 glasses prepared by the alkoxide route

Materials in the composition range xTiO2 · (100 − x)SiO2 (x = 10 and 30 mol%) were prepared with different ratios H2O/alkoxide EtOH/alkoxide and with different thicknesses. The precipitation of anatase was observed in all samples heat treated at 550 and 850°C. In samples heat treated at 1300°C, anatase and α-cristobalite were observed. Rutile precipitation was observed only in the samples prepared with high water content heat treated at 1300°C. Infra-red studies indicate that TiO2 enters the structure in tetrahedral and octahedral coordination in all samples. Density and Vickers hardness showed a strong dependence on preparation conditions.

Melting and solidification of Bi nanoparticles in a germanate glass

A very large melting?solidification hysteresis of Bi nanoparticles embedded in a bulk alkali germanate glass is reported. Heating and cooling cycles are reproducible and show reversible transitions. High resolution transmission electron microscopy reveals that the glass contains nanocrystals of elementary Bi which are a few tenths of a nanometre in size. Upon heating above the Bi melting temperature the glass transmission increases up to 10% with respect to the initial value, which is most likely related to Bi melting. Upon cooling this high transmission state remains up to temperatures as low as 436?K. This behaviour is confirmed by Raman spectroscopy measurements. This nanostructured glass with a high refractive index can be used in nonlinear optical applications as well as an optical thermo-sensor.