J.R. Martínez

Cobalt oxide/silica xerogels powders: X-ray diffraction, infrared and visible absorption studies

Using x-ray diffraction, infrared and visible absorption spectroscopies we have studied: silica xerogel powders when cobalt nitrate is incorporated and mainly the evolution of the precipitated cobalt species in the SiO2 matrix. Samples containing cobalt nitrate and prepared with a molar ratio of H2O/TEOS of 11.67 were studied as a function of annealing temperature. It was found that the added element has a remarkable influence on the final structure of this kind of samples and at relatively high temperatures the cobalt compounds react with the silica matrix forming a cobalt silicate.

Structure of heat-treated sol-gel SiO 2 glasses containing silver

The crystallization of bulk amorphous SiO 2 samples, prepared by the sol-gel method, was obtained by heat treatments in air at temperatures as low as 500 °C. This occurs when silver is added to the precursor solutions in an amount such that it forms aggregates embedded in the glass. Another requirement to observe the low-temperature glass crystallization is that the bulk samples must be prepared from precursor solutions with specific compositions. These compositions, have a high H 2 O/TEOS ratio, which produces an amorphous SiO 2 structure with some structural similarities to cristobalite, the phase in which the SiO 2 glass crystallizes.

Formation of Nickel-Zinc Ferrite Embedded in a Silica Xerogel Matrix

We report a study using X-ray diffraction, refinement Rietveld, IR absorption spectroscopy and vibrating sample magnetometry of the formation of particles of Ni-Zn ferrites embedded in a xerogel SiO2 matrix. Initial solutions were prepared mixing TEOS, distilled water, ethanol, and three different nitrates: iron, nickel and zinc. A molar ratio H2O:TEOS:Et-OH of 11.66:1.0:4.0 was used in all solutions. Formation of Ni0.5Zn0.5Fe2O4 as well as structural modifications of the SiO2 matrix induced by these particles are discussed. Ni-Zn crystals having dimensions varying from 4 to 16.5 nm have been synthesized by a suitable heat treatment schedule at temperatures varying from 700 to 1100°C. The coercivity values for different heat-treated samples are found in the range 11–193 Oe, which are significantly larger than those presented by bulk Ni-Zn ferrites.

A simple model to analyze vibrationally decoupled modes on SiO2 glasses

By using a simple model, the sharp bands at about 495 and 606 cm 21 in the Raman spectrum of vitreous SiO2 have been identified as due to vibrational modes of regular rings with 4- and 3-members, respectively. For that the restrictions in the Si‐ O‐ Si intertetrahedral angle, which depends on the number of members in a ring, are taken into account. Our model also predict infrared (IR) features, corresponding to ring structures, these vibrational IR modes might be weak and quite broad and probably buried by the LO ‐ TO pairs modes at 1160‐ 1250 cm 21 . q 2002 Elsevier Science B.V. All rights reserved.

Magnetic properties enhancement of M-Ba ferrites embedded in a SiO2 matrix

Abstract M–Ba ferrite powders prepared by the coprecipitation method were embedded, using the sol–gel technique, in an inert silica xerogel matrix. Samples were analysed by X-ray diffraction and vibrating sample magnetometry. We compare the magnetic and structural properties of pure ferrite samples and those embedded in the inorganic matrix. The mixed samples had a proportion of 30% BaFe 12 O 19 :70% SiO 2 in weight. We observed an enhancement of the coercive force in ferrite powders, embedded in the SiO 2 matrix, accompanied by a decrease of the saturation magnetization, with respect to pure samples.

Annealing Behavior of Silica Gel Powders Modified with Silver Crystalline Aggregates

The annealing behaviour of silica powders added with silver, prepared by the sol-gel method, was studied using X-ray diffraction. Partial crystallization of amorphous SiO2 samples as low as 600°C has been observed. For that, silver needed to be added to the precursor solution in such a way that it formed aggregates. Silica xerogel samples were prepared using a molar ratio ethanol/H2O/TEOS of 4:11.6:1 and loaded with silver in three different ways: in the form of silver nitrate, silver chloride, or chemically synthesised silver fine particles. The microstructure of the silica xerogel powders was studied as a function of annealing temperature. Attention was paid to the evolution of the glass matrix as well as the silver aggregates in the SiO2 matrix. Partial crystallization of the glass matrix was achieved at temperatures much lower than those specified by the phase diagram, independently of preparation method of the silver aggregates.

The effect of ball milling dispersion on the optical properties of organic dyes trapped in silica films by the sol–gel method

Abstract Silicon dioxide coatings containing different types of dyes were prepared using the sol–gel method. It is found that the optical density of the coatings depends on both the dye concentration and on the degree of dye agglomeration. Coatings in which the dye forms aggregates show lower optical density than those in which the dye is better dispersed in the glass matrix. Dispersion of the dye in the starting material is achieved by a ball milling process. A simple model is used to explain the dependence of the optical density on the size of the dye aggregates.

Infrared Spectroscopy Analysis of Oxyhydroxides as Intermediate Species in the Formation of Iron Oxides-Silica Xerogels

A detailed infrared spectroscopy analysis in the 2500 to 3800 cm−1 region has been used to study the formation of species in samples of iron oxides embedded in silica xerogel matrix. We report the presence of α, β, and γ forms of iron oxyhydroxides as intermediate species in the formation of α-Fe2O3, γ-Fe2O3 and ε-Fe2O3 starting from three different iron precursors: iron nitrate, iron chloride and nanometric Fe particles prepared by chemical reduction. Our results show that under thermal dehydration α and γ forms of iron oxyhydroxides transform into hematite and maghemite, respectively, whereas the β form transform to the ε-Fe2O3 without going through an intermediate iron oxide phase.