Structural and preparative studies of doped silica glasses

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Aubonnet, S., 1999. Structural and preparative studies of doped silica glasses. PhD, Nottingham Trent University.

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Abstract

The sol-gel process has been used for the preparation of high optical quality materials. Doped sol-gel glasses were made using the pre-doped method which consists of doping the sol-gel glass during its preparation, as soon as the precursors are mixed together. In this work, solid standards for the UV region were prepared using the sol-gel process.

Initially, Nd3+ and Ce3+ metal ions were used as dopants. The effect of the two different dopants on the silica matrix was studied. During the drying process of doped and undoped sol-gel glasses, the dopant affects the rate at which the glass dries. High concentration (9.48wt%) Nd3+ doped sol-gels showed a slower drying rate than normal sol-gels. At lower concentrations (0.948wt% to 9.48.10-4wt%), the drying rate was quicker than normal sol-gels. At 9.48.10-5wt%, the drying behaviour of the doped sol-gel was very similar to that of the normal sol-gel. However, the drying rate was slowed down in the presence of cerium regardless of its concentration in the glass (8.99% to 0.00128wt%). FTIR(mid and NIR) showed that the dopant, according to its concentration in the sol-gel glass, affects the structure of the silica matrix. The number of hydroxyl functionalities varied between doped and undoped sol-gels. Consequently the dopant interacts with the matrix. To understand the nature of this interaction, the first hour of hydrolysis and condensation reactions was studied. The gelation time was monitored according to the hydrolysis time for doped and undoped samples. In the presence of high concentrations of dopant, the gelation time increased in comparison with undoped sol-gels. At a high concentration of Ce3+ metal ion (0.0128M), the gelation time of the sol-gel solution was longer than in the presence of a high concentration of Nd3+ (0.0128M). At lower concentration ranges, the gelation time of the sol-gel solution was lower than in the presence of Nd3+. A qualitative study of the silica species during hydrolysis and condensation reactions was performed by 29SiNMR. Variation of the silica species according to the type of sol-gel shows that the dopant affects the rates of hydrolysis and condensation reactions. The dopant has a catalytic effect. pH studies demonstrated that the catalytic effect of the dopant was due to the metal ion itself and not to the ligands around it, more specifically to the acidity of the metal ion. Not only has the metal ion an effect on the silica matrix, but the metal ion is affected by its environment. Studies were carried out using Nd3+ which has hypersensitive electronic transitions. Nd3+ ions in crystals, in solution and m sol-gels were studied. For these studies, single crystal X-ray analysis, UV-VIS spectroscopy and EXAFS analysis were performed. As a crystal, the metal ion was sensitive to the type of ligand present. A coordination number of 10 was found with NO3 ligands and a coordination number of 8 with C1 ligands. In solution, the metal ion was influenced by the ligands and the solvents. Only the solvent had an effect on the coordination number. A coordination number of 9 was found in aqueous solution and 7 coordination was present in ethanol solutions. In the glass, the coordination number was not dependent on the ligands and the solvents in which the metal ion had been prepared. The electronic transitions were sensitive to the structure of the glass. A coordination number of 7 was found in the glass suggesting that the metal ion surrounded by its ligands was electrostatically bonded to the matrix. The bond distance between Nd-O was 2.55A. After dehydration, the metal ion loses one water ligand, now having a coordination number of 6. A bond distance between Nd-O of 2.549A was found. Finally when the sample has been thermally treated, a coordination number of only 4 was observed. It was covalently bonded to the silica matrix via four oxygens of the siloxane groups.A bond distance between Nd-0 of 2.495 A was found. Knowing that the metal ions (Nd3+ and Ce3+ interact with the silica matrix, the preparation of standards was carried out. A series of UV solid standards was prepared using Ce3+ metal ion as the dopant. The concentration of the metal ion in the sol-gel glasses was varied in order to obtain a series of standards with bands having intensities between 0.5A to 2A for a pathlength of 1mm±O.1mm. The effect of the environment on the electronic transitions of Ce3+ was also studied by UV-VIS spectroscopy. After dehydration, a shift of the bands was observed. After heat treatment (800°C), the bands characteristic of the metal ion were no longer observable. A change in the oxidation state from Ce3+ to Ce4+ possibly occurred during heat treatment. The Ce4+ ion does not possess any bands in the UV region. Unsuccessful results were obtained when a combined standard for the UV and VIS regions was prepared by using Ce'* and Nd3+. The two metal ions interacted with each other and the bands of Ce3+ ion in the UV region were no longer observable. As a standard, the sol-gel glass needed to be stable with time and no changes in the transmittance should be observed. Preliminary studies were carried out on the aging of the samples. A decrease of the transmittance with time was observed due to the moisture ft om the air which gets into the pores of the glass. Finally, some doped sol-gels for use as visible standards were prepared. The transition elements of the first row were studied. The effect of the environment on their electronic transitions was monitored by UV-VIS spectroscopy. Chromium and vanadium were two potential candidates for the preparation of such standards as they exhibit broad bands in the visible region.

Item Type:

Thesis

Creators:

Aubonnet, S.

Date:

1999

ISBN:

9781369313253

Identifiers: