G. H. Frischat

Atomic force microscopy as a tool to correlate nanostructure to properties of glasses

Atomic force microscopy (AFM) provides high resolution images of surfaces even if they are non-conducting. Thus, glass can be investigated without conductive preparation or other complicated preparation techniques. So far, the great advantage of this microscopy in ambient atmosphere impeded atomic resolution on vitreous surfaces. However, AFM proved to be an appropriate tool for imaging the structure on a nanometre scale, of glasses, glass ceramics and coatings on glass. AFM serves to detect surface defects and changes in the overall surface topology after different treatments, such as polishing, cleaning, aging and corroding. With the help of suitable preparation, volume properties can also be investigated with a spatial resolution in the nanometre range. AFM contributions to glass research fields like fracture mechanics, crystallization, interfaces and gel consolidation are reviewed in this paper.

Direct view of the structure of a silica glass fracture surface

Abstract The fracture minor surface of a silica glass, prepared at 1×10−11 mbar in the chamber of an ultra-high-vaccum-atomic-force microscope (UHV–AFM), was imaged at 1×10−8 mbar with high spatial resolution. The features directly seen were interatomic distances, e.g., O–O and Si–O, groupings of atoms, e.g., SiO4 tetrahedra and rings of tetrahedra, and network holes. A comparison with a structure calculated by a molecular dynamics simulation gave good coincidence. This direct view of the silica glass surface supports Zachariasens network structure model of glass.

High-resolution solid-state NMR studies of mixed-alkali silicate glasses

Abstract Magic angle spinning (MAS) 29Si and 23Na NMR of 74SiO2·(26×x)Na2O·xRb2O glasses has been used to study the distribution of Qn units and the effect of alkali substitution. Clear deviations from a “binary” Qn/Qn−1 distribution have been found even for glasses containing only one alkali modifier oxide. Substitution of one alkali oxide by another does not affect the distribution substantially, nor does it cause an additional distortion of the glass network.

Direct view of the structure of different glass fracture surfaces by atomic force microscopy

Abstract Silica, silicate, borosilicate and heavy metal fluoride glasses were fractured under UHV conditions. The fracture mirror surfaces then were imaged by atomic force microscopy. The atomically resolved images display a fully interconnected random network arrangement for the silica glass, modified random network geometries with percolation paths for the silicate glasses, a structure with higher connectivity with some short percolation paths only for the borosilicate glass and a strongly interconnected structure for the heavy metal fluoride glass, probably with some chain- and ring-like geometric elements. Sodium diffusion, which is supposed to follow mainly these percolation paths, is qualitatively in line with the number, length and width of these paths.

The formation and infrared optical properties of some chalcogenide and chalcohalide glasses

Abstract Chalcohalide glasses based on heavy metal halides of the systems MnXnAg2SAs2S3 (M=Hg, Pb, Ag, X=Cl, Br,I) and MnXmAs2Se3As2Te3 as well as chalcogenide glasses of the system As(Ge)SeTe have been prepared. Large glass-forming regions have been found for the chalcohalide glasses based on heavy metal iodides. The transparent regions of these glasses were shifted towards longer wavelengths after the addition of HgI2, PbI2 and Te. The absorption peaks caused by oxides and hydrides were weakened or eliminated. The interesting optical properties of these glasses based on heavy metal halides make them candidates for potential applications in the mid-infrared region and for CO2 laser power delivery at 10.6 μm.

On the origin of the tin hump in several float glasses

Abstract It has long been known that some commercial float glasses display an anomalous tin hump several micrometers below the glass surface at the tin bath side. The literature offers different explanations. After precise measurement of the in-depth profiles of all relevant species and also the in-depth profile of the Sn 2+ /Sn 4+ ratio, a diffusion-reaction model was developed. Its main assumption is that when the oxidized glass melt enters the strongly reducing float chamber, a drastic and rapid change in the oxidation state occurs in the glass melt near its surface. The anomalous tin hump follows then as an interaction between the ongoing diffusion processes and redox reactions, at a position identical with that of the maximum in the Sn 4+ profile.

Thin SiO2TiO2ZrO2 films from alkoxide solutions

Abstract Different solutions containing alkoxides of silicon, titanium, and zirconium have been prepared. Some of their properties like the time dependent viscosity and the gelling time have been measured and are reported here for different H 2 O, HCl, ethanol and/or formamide contents. Microscope slides have been dip coated in these solutions. After baking, film thickness and chemical durability have been determined. In order to get good SiO 2 TiO 2 ZrO 2 glass coatings, the withdrawal speed should not exceed 5 cm/min unless the viscosities of the solutions are reduced by the addition of ethanol. By such a dilution, the film thickness could also be reduced, while the addition of formamide caused a delayed increase of the viscosity and increased gelling times. For this reason, solutions containing formamide can be used for longer periods. The chemical durability of the substrates against boiling NaOH solution is enhanced by the SiO 2 TiO 2 ZrO 2 glass coatings.

Characterization of nitrogen‐containing glasses by x‐ray photoelectron spectroscopy

Nitrogen incorporated into the matrix of oxide glasses substantially changes the physical properties of the glasses even with very low N content (1–3 wt. %). These novel oxynitride materials are known to show improved electrical resistance, dielectric constant, and mechanical properties in comparison to N‐free glasses. This behavior has been attributed to the substitution of bridging O by trivalent N, causing a higher degree of linkage in the matrix. We employed XPS to address the question of the chemical environment and coordination of N in Na2O ⋅ CaO ⋅ SiO2 and Na2O ⋅ B2O3 glasses. A splitting of the N(1s) core level by ∼1.5 eV, observed for both groups of materials, demonstrates the existence of two different structural states. The core levels of N(1s) found in the N‐containing glasses differ from the positions of N(1s) in reference materials such as Si3N4, BN, SiXOYNZ, and B2O3 ⋅ N suggesting a modified configuration.

On the origin of the aging process of porous SiO2 antireflection coatings

Broad-band and low-cost antireflection coatings are necessary for solar collector covers. Porous SiO2 coatings of ≈100 nm thickness, prepared by a dip-coating process with silica sol as a precursor, can increase the transmittance of float glass up to 99%. However, exposure of the coated glass to ambient atmosphere or even storage in a desiccator leads to an aging with a successive loss in transmittance. This aging process is reversible and may be removed either by a repeated heat treatment at 500°C or by a cleaning with a polar solvent such as water or acetone. The aging process was investigated and it could be shown that there are two overlapping mechanisms. In a faster process with a reaction time of weeks, residual Na+ ions from the silica sol react with H2O and/or CO2 from the air. In a slower process with a reaction time of months, Ca2+ ions diffuse from the float glass substrate into and even through the coating and form products such as CaCO3. In both cases, the pores are filled and the refractive index of the SiO2 coating is increased.

Radiation resistant optical glasses

Abstract Hostile environments created by short wavelength elcctromagnetic radiation, for example UV. X-ray and γ-radiation, or from particle fluxes, for example α-particles. s-particles. protons and neutrons, can produce defects within optical glasses. This loss of transmission is detrimental to the performance of the optical system and must eliminated or reduced to a manageable level. For applications within these hostile environments, radiation-stabilized optical glasses have been developed. Radiation test results will be presented, discolouration of stabilized and not stabilized glasses will be compared and colour-centers will be discussed.