T.N.M. Bernards

Hydrolysis-condensation processes of the tetra-alkoxysilanes TPOS, TEOS and TMOS in some alcoholic solvents

Abstract The effects of methanol, ethanol, 1-propanol and 2-propanol on the kinetics and the mechanisms of the hydrolysis-condensation reactions of the silanes TPOS, TEOS and TMOS were studied. Also, the influence of the amount of water on the hydrolysis-condensation processes of TMOS and TPOS was investigated. Accordingly, hydrolysis time versus condensation time curves were recorded and 29 Si-NMR investigations were performed. The hydrolyzability of the silanes in an acidic environment was found to decrease in the sequence TMOS > TEOS > TPOS. The effect of the alcohols, in the sequence methanol > ethanol, 1-propanol > 2-propanol, upon the hydrolysis rate of each of the silanes is explained by differences in degree of dissociation of HCl in the different alcohols. Also, the exchange of alkoxy groups, observed when an alcoholic solvent was used with an alkoxy group different from the alkoxy group of the silane, may influence the hydrolysis rate of the silane. Differences in dimerization are ascribed to differences in base strength of the activated silanol complexes with regard to those of the protonated alcohols.

The influence of the addition of alkyl-substituted ethoxysilane on the hydrolysis—condensation process of TEOS

Abstract The effect of methyltriethoxysilane, dimethyldiethoxysilane or phenyltriethoxysilane on the hydrolysis-condensation process of TEOS was investigated using hydrolysis time-condensation time curves. Viscosity measurements and 29 Si-NMR investigations were also performed. It is found that the gelation process of such mixtures is slower than that of TEOS itself although, compared with TEOS, both the hydrolysis rate and the condensation rate of each of these compounds are higher.

The electrical and optical properties of thin layers of nano-sized antimony doped tinoxide particles

For the use in anti-static films on glass or polymeric substrates, transparent conductive layers can be prepared by spinning an aqueous suspension of nano-sized antimony-doped tinoxide (ATO) particles. These layers have a resistivity which is substantially higher than that of homogeneous ATO layers which are deposited by physical vapour deposition techniques. By curing the films to temperatures up to 700 °C, the resistivity of the particle layer can be decreased by two or three decades. Because the nano-sized particles are prepared by a low-temperature process a different mechanism can contribute to this decrease in resistivity. Possible effects which may influence the conductivity are sintering of the particles, change of the bulk material and the presence of an insulating layer at the outside of the particles. This decrease can be explained by the presence of an insulating antimony-rich layer on the outside of the particles, the thickness of which is reduced when the layer is cured. At temperatures above 350 °C, sintering of the particles also highly influences the decrease in resistivity. At temperatures above 700 °C, the resistivity is increased due to segregation of the antimony to the surface of the particle.

The effect of the solvent on the cross-link density of SiO2 coatings

With increasing screen size of television and computer monitor tubes the spin-coating of tetraethylorthosilicate (TEOS) based sol-gel coatings becomes an increasingly difficult task. To retain sufficient uniformity and scratch resistance of the coatings, changes in the composition of the coating liquids and the coating procedures are needed. Changing solvents and adding catalysts can lead to increased cross-link density which has been measured by 29 Si-NMR. The cross-link density increases with increasing average number of hydroxy groups on the Si atoms in the drying phase, which can be tailored by adjusting the water concentration, by using water vapour or by using solvents which do not cause the re-esterification of alkoxy groups on the Si atoms during drying.

17O-NMR of Sol-Gel Processes of TEOS and TMOS

Water consumption and formation in the acid catalyzed sol-gel processing of TEOS and TMOS can be followed using 17O-NMR. By using 17O-enriched water, insight into the hydrolysis and condensation in the acid step of the sol-gel process can be obtained. It is found that, after initially strong consumption of water due to hydrolysis, a steady state water concentration results. This amount of water increases upon dilution of the reaction mixture with alcohol. For a hydrolyzed TMOS-methanol-water system lower water contents are found than in a comparable TEOS-ethanol system. Addition of ethanol to a hydrolyzed TMOS methanol system enhances the condensation and a higher water concentration is found.

THE EFFECT OF HF IN A TWO-STEP SOL-GEL PROCESS OF TEOS

The addition of HF to a TEOS-ethanol-water mixture results in faster gelation compared to a system without HF. When HF is added in the basic step the effect is most pronounced. The addition of HF in the acid step only results in an enhancement of the condensation reactions. The effect of HF in the acid step is largely determined by the H+ concentration due to the low acidity of HF. At low HF concentrations a maximum in gelation time is found as a function of the H+ concentration. At low H+ concentrations the F− enhances gelation; at high H+ concentrations the influence of F− is negligible but gelation is enhanced by the protons.

Spin coating of titanium ethoxide solutions

Titanium oxide films are used in anti-reflection interference filters. These layers can be deposited by spinning titanium ethoxide (TEOTi) solutions. By thorough hydrolysis of the TEOTi precursor, optically acceptable coatings can be made even when cured at temperatures below 150°C.The use of acetylacetone to control the reactivity of a TEOTi solution is not suitable when curing the films at 150°C, because at this curing temperature the acetylacetone remains in the coating. A more suitable method to control the reactivity is by adjusting the H+ concentration of the solution. Besides the curing temperature the hydrolysis time of the TEOTi solution also has a major influence on the morphology of the final TiO2 layer.

The effect of TEOG on the hydrolysis-condensation mechanism of a two-step sol-gel process of TEOS

The effect of the addition of tetraethoxygermane (TEOG) in the basic step of a two-step silica sol-gel process on the hydrolysis-condensation mechanism of tetraethoxysilane (TEOS) was investigated. Gelation time versus hydrolysis time curves were recorded for mixtures of TEOS, ethanol and water to which TEOG was added. It is found that an ethoxy group of TEOG reacts with silanols under ethanol formation. The rate of this reaction is almost independent of the base concentration. Because unhydrolysed TEOG has four reactive groups, it is efficient in cross-linking chains, which implies that in general its addition results in shorter gelation times. This effect becomes more apparent for mixtures with low water content. At high TEOG: silanol ratios, however, an increase in the gelation time is found to be due to the capturing of silanols by TEOG. The cross-linking effect of TEOG becomes weaker if it is partly hydrolysed before addition to the TEOS mixture.

Hydrolysis-condensation mechanism of a two-step sol-gel process of mixtures of TEOS and TEOG

Abstract The effect of the addition of tetraethoxygermanate (TEOG) on the hydrolysis-condensation mechanism of a two-step silica sol-gel process was investigated. Gelation time versus hydrolysis time curves were recorded for mixtures of TEOS, ethanol and water to which TEOG was added. It was found that TEOG reacts with silanols under ethnaol formation. Because TEOG has four reactive groups, it is efficient in cross-linking chains, which implies that, in general, its addition results in shorter gelation times. At high TEOG concentrations, an unexpected dissolution of the previously obtained gel was observed.

Characterisation of Sol-Gel TiO2 Films by Etching

TiO2 films that have been prepared from a hydrolysed tetra-ethyl-ortho-titanate (TEOTi) solution are characterised by etching the films in a 0.25% HF solution. The etch rate is influenced by a number of process parameters that are applied for the preparation of the coating solutions. The etch rate of the TiO2 film increases when the hydrolysis is performed in a more diluted system. However, after a hydrolysis time of 30 min, further dilution of the hydrolysis mixture has only a minor effect on the etch rate of the final coating. An increase of the water versus TEOTi molar ratio and an increase of the HCl concentration in the hydrolysis mixture result in a decrease of the etch rate of the TiO2 film. The etch rate of the film is highly influenced by the morphology of the film. TEM measurements of the films have shown that a low etch rate is found for films consisting of small particles. In particular, the hydrolysis stage of the processing of the coating solution has a large effect on the morphology of the final film.