Brita Unger

The early stages of the sol-gel processing of TEOS

The reactivity of tetraethoxysilane (TEOS) used in many sol-gel syntheses can be increased by prehydrolysis. In contrast to one-step prehydrolysis acidic two-step prehydrolysis causes an increase of the reactivity of the low molecular species provided the total water/TEOS ratios are the same in both cases. By choice of the processing conditions of the hydrolysis—shown on variation of water content, hydrolysis steps and heat treatment—the degree of condensation, the ratio of hydrolyzable (-OC2H5) and hydrolyzed groups (-OH) and the structure of the low molecular species (short chains, 3-membered and 4-membered rings) can be influenced systematically. The development of the building units formed in the early reaction stages is ascertained by molybdate method and 29Si NMR measurements.

Microscopic vs. macroscopic structural evolution of SiO2 sols and gels employing a tailor-made fluorescent reporter dye

The short- and long-term structural evolution and aging of sol–gel materials prepared under mild conditions are studied by fluorometry, dynamic light scattering (DLS), 29Si NMR and gravimetry. In combination with the first, a newly developed donor–acceptor-substituted fluorescent probe with two acidophilic sites of different strength and topology is introduced. Its performance in detecting changes in parameters such as polarity, proticity, and pore volume on the microscopic scale is compared to two other dyes and to the macroscopic changes revealed by DLS, NMR and gravimetry. At early stages of the sol–gel process, fluorometry, DLS and NMR reveal similar changes which are typical of the structural evolution of SiO2 sols having different synthesis parameters. During later times of sol aging, DLS and NMR reflect the growth of the network and indicate that all sols approach similar structural equilibria. Fluorometry supports these findings by signalling a gradual decrease in mobile species with high proton activity. The formation of the spanning cluster upon gelation is strongly evident only from DLS results while the local environment of the fluorophore remains largely unchanged. However, the fluorescent dye is able to report microscopic changes even during long-term aging. Drying of the gels with a pronounced contraction of solvated pores is readily observable by fluorometry and gravimetry. During the late reaction stages, the fluorescence properties of the tailor-made dye are still sensitive toward the environment.

The function of paramagnetic iron species in amorphous materials formed by sol-gel method and conventional melting techniques

Abstract A 9 and 35 GHz electron paramagnetic resonance investigation of conventional Na 2 OAl 2 O 3 SiO 2 and CaOAl 2 O 3 SiO 2 glasses has shown that: (a) a fraction of Fe 3+ acts as a network former, (b) precursors of non-stoichiometric FeO x phases are formed, (c) the ratio of the concentration of Fe 2+ ions to the concentration of Fe 3+ ions controls the formation and stabilization of radiation-induced paramagnetic hole centres. 9 GHz EPR of SiO 2 and x Na 2 O·0.1Fe 2 O 3 ·100SiO 2 ( x = 0, 0.1, 1.0) sols and gels showed that (i) Fe 3+ sites in the sols are less rhombicly distorted than in glasses (in the presence of a chelating agent such as acac an effect of changing Fe 3+ /Na + ratio is not detectable) and (ii) in alcogels, ferric ions are observed in axially distorted sites; (iii) UV irradiation of frozen sols and gels leads to the formation of ·C 2 H 5 and CH 3 ĊHOH radicals, which provide information about the degree of hydrolyses of the samples; (iv) if sodium is added as NaNO 3 , the UV irradiation produces ·NO 2 radicals; (v) the addition of 1 mol% nitrate is sufficient to remove residues of carbon compounds on tempering the gels.

On Aging of Acid-Catalyzed Silica Sols—A Dynamic Light Scattering Study

Dynamic light scattering experiments in acid-catalyzed silica sols are discussed. It is shown that in spite of limited parameter accuracy and limited absolute knowledge of the particle sizes, the scattering data from various experiments are comparable with one another and give information about structural differences. Measurements at various angles indicate changes between non-spherical and nearly spherical particles and allow the estimation of gelling times before gelation.

Reaction kinetics of SiO2 and ZnOSiO2 sol-gel solutions

Abstract Some silica sols prepared from tetraethoxysilane (TEOS) and various amounts of water (molar ratios H2O:TEOS=rw 2−9.6) were tested with regard to their suitability as starting materials for zinc dopant sols. The influence of composition and of preparation conditions on structure were determined by viscosimetric measurements and by the molybdate method (the colorimetric observation of the acidic depolymerization of the silicate network of the colloidal particles). It is shown that the structure of the sols depends not only on the chemical composition but also on applied heat treatments.

Effects of the Sol-Gel Processing on the Fluorescence Properties of Laser Dyes in Tetraethoxysilane Derived Matrices

Two fluorescent dyes were incorporated into sol-gel derived SiO2 matrices. The dye was added to SiO2 precursors of different degrees of pre-condensation and the spectroscopic properties of the immobilized dye were measured at various aging and drying stages of the resulting gels. The significant influence of the processing parameters on the spectroscopic properties is manifested in the relative intensity of a second red shifted emission band (550–560 nm), which was observed besides the typical coumarin emission band (495 nm). The appearance of this long wavelength emission might be attributed to dye aggregation or to other reactions with the ambient matrix forced by micro porosity phenomena.

Glassy Dopant Deposits On Semiconductor Surfaces Prepared By Sol Gel Technique

A suitable (concerning long time stability) homogeneous starting silica sol was investigated using the so-called molybdate method and viscosity measurements. For the intendet doping process of AIIIBV-compounds different solutions of zinc salts were added to the silica sols. The long time aging process (polycondensation, gelling) was controlled by the mentioned methods. The semiconductor surfaces were coated by spinning on. Zinc diffusion profiles after different heating regimes were discussed by SIMS and C/V measurements.