Günter Völksch

The degradation of EBSD-patterns as a tool to investigate surface crystallized glasses and to identify glassy surface layers.

Surface crystallized samples of glass-ceramics containing cordierite, rhombohedral BaAl₂B₂O₇ and fresnoite were analyzed using electron backscatter diffraction (EBSD). The first two materials were chosen because surface crystallized samples of these materials have previously been shown to contain crystals covered by a very thin layer of glass. In all materials, EBSD pattern degradation occurs if the step size of a scan is chosen to be small. It is shown that the minimum step size enabling an evaluable EBSD-scan increases notably, if the crystals are covered by a thin layer of glass. It is also shown that pattern degradation may be utilized to prove the existence of such a thin glass or otherwise thermally sensitive layer. This provides significant information concerning the nucleation process of glasses also with respect to nucleation theory of glass-ceramics. It is also possible to describe the quantity of crystalline surface covered by the thermally sensitive layer.

Electron backscatter diffraction of BaAl2B2O7 crystals grown from the surface of a BaO·Al2O3·B2O3 glass

A glass with the composition BaO·Al2O2·B2O3 was annealed at 780 °C for 8 h. The samples were investigated by electron backscatter diffraction (EBSD). At the surface of the samples a first layer composed of BaAl2B2O7 crystals oriented with their crystallographic c-axes perpendicular to the surface was formed. This layer was highly oriented and up to 20 µm thick. In a subsequent layer, the crystals were oriented with their crystallographic c-axes parallel to the surface. While the first layer was caused by oriented nucleation at the surface, the orientation of the secondary layer was caused by kinetic selection of spherulitically grown crystals.

Structure, chemical durability and microhardness of glass-ceramics containing apatite and leucite crystals

Abstract Glass-ceramics based on SiO 2 –Al 2 O 3 –K 2 O–Na 2 O–CaO–P 2 O 5 prepared by controlled crystallization of glass are studied. Glass melted from a batch of high purity raw materials was quenched and milled. The powder was heat treated at 850°C, 900°C and 1150°C for 1 h, quenched in water, milled, formed and sintered at 1000°C for 1 min. The study focused on the phases of crystallization and microstructure of both heat treated and sintered samples. Microhardness and chemical durability were measured of glass-ceramics produced by sintering. Leucite and needle-shaped fluorapatite crystals were developed. The microstructure changed depending on heat-treatment and sintering temperatures. The chemical durability and microhardness of glass-ceramics prepared for restorative dental applications are influenced by the properties of the bulk glass.

Electron-microscopical studies of glass

Abstract During the past 25 years, the use of the electron microscope in glass research has substantially expanded our knowledge of the microstructure of glasses, for instance of the phase separation processes in glasses. The use of electron microscope in glass research calls for special knowledge the lack of which would certainly lead to misinterpretations and wrong conclusions. Direct electron transmission of the specimen: low voltage beams create images of high contrast. On the other hand, there is always the risk of altering the specimen. If very high beam voltages are employed this is at the expense of image contrast. The optimum beam voltage range in the direct electron transmission of glass is 50 to 120 kV, depending very much on the kind of glass to be examined. The replica technique: this technique in electron microscopical investigations of glasses has more advantages than disadvantages but a highly developed technique is necessary. The most important factors will be described. Surface treatment of glass samples prior to making the replica: suitable etching processes of a freshly fractured glass surface sometimes help to enhance the visibility of structural peculiarities but faulty preparations have to be avoided. Use of the scanning electron microscope and the electron-beam microprobe: enormous advantages and problems will be described. Finally selected examples of electron-optical examinations of glass will be given.

Dissolved gases and minor component effects on glass crystallization

Abstract Surface crystallization is always observed when a cordierite (2MgO2Al2O35SiO2) glass is heat treated. Nucleation was studied by mass spectrometric evolved gas analysis, optical microscopy, scanning and transmission electron microscopy, and atomic force microscopy. The formation of metastable (μ-cordierite, X-phase) and stable (indialite) crystals are correlated with sample mass loss and gas release. H2O is the main component of the evolved gas. The influence of the surrounding atmosphere and of the polished state of the surface on the formation of μ-cordierite and X-phase were characterized by microscopy. Polished and fractured surfaces are strongly deformed during crystallization. AFM shows crystals on the surface and in the material to about 20 nm underneath the surface.

Crystallisation of iron containing glass–ceramics and the transformation of hematite to magnetite

Crystallisation of a glass with the composition 16Na2O·10CaO·49SiO2·25Fe2O3 led to the formation of hematite and magnetite crystals with various morphologies. Crystals containing both the hematite and the magnetite phases were detected and a strict orientation relationship between the phases is described. It is argued that hematite (Fe2O3) is the phase primarily crystallised and magnetite (Fe3O4) is the result of a phase transformation after the primary crystal growth. The main method of characterization was scanning electron microscopy (SEM) combined with electron backscatter diffraction (EBSD).

Composition and texture of barium silicate crystals in fresnoite glass-ceramics by various scanning electron microscopic techniques

Highly oriented fresnoite glass-ceramics were prepared from a non-stoichiometric melt (Ba2TiSi2O8 + 0.75 SiO2) by electrochemically induced nucleation. By using electron microscopy, cathodoluminescence (CL), electron backscatter diffraction (EBSD) and energy dispersive X-ray (EDX) spectroscopy as well as wavelength dispersive X-ray (WDX) spectroscopy, two additional barium silicate phases free of titanium were identified besides the fresnoite phase predominantly occurring throughout the sample. A barium disilicate phase (BaSi2O5) was found within a radius of approximately 300 µm from the Pt-wire where the nucleation was induced. At distances >300 µm a phase with the composition BaSiO3 was detected which occurred in between the large dendritic fresnoite crystals next to an additional glassy phase which mainly consists of SiO2. Both barium silicate phases show more intense cathodoluminescence than the fresnoite phase and the maxima in the attributed spectra were shifted to smaller wavelengths in comparison to fresnoite.

On the interdiffusion of Ag+ and Na+ ions in the glass Na2O·2SiO2 around the glass transition temperature

Abstract The Ag+/Na+ ion-exchange is studied for the glass Na2O·2SiO2 below and above the glass transition temperature using a eutectic melt (AgCl)0.42(AgI)0.58. The interdiffusion coefficients D Ag/Na are calculated from the Ag concentration profiles in the glass measured by X-ray micro-analysis. The evaluation gives concentration-independent values D Ag/Na corresponding to a low degree of exchange (1%). The Arrhenius diagram 1g D = f(1/T) shows a distinct bending at the glass transition temperature. The observed change of the activation parameters D 0 and E A is discussed in relation to the generation of free volume for T > Tg and to chemical bonding of Ag and Na in the glass network. The results are compared with ion-exchange experiments using an AgCl(T > Tg) and AgNO3 melt (T

On the ion exchange behaviour of the glass Na2O · 2SiO2 against a melt (NaNO3)0.9(TINO3)0.1

The Na + /Tl + exchange was studied on the glass Na 2 O·2SiO 2 ( T g = 465 °C) by contact with a melt (NaNO 3 ) 0.9 (TlNO 3 ) 0.1 in the temperature range 400–550 °C (19 h). Between 400 and 500 °C Fickian behaviour has been stated. At temperatures higher than 500 °C the Na + /Tl + exchange is disturbed by degradation effects of the salt melt and by partial dissolution of the glass (etching). The interdiffusion coefficients D Na/Tl (400–500 °C) increase with ascending Tl 2 O content (0–10 mol%) in the ion-exchanged region of the glass. The activation energy E A falls off in this direction. The results of the Na + /Tl + interdiffusion are discussed in connection with the distinctly decreased glass transition temperature ( T g ) of the ion-exchanged region of the glass.

Electron Microscopic Investigations of Electrochemically Induced Mullite Crystallization in a Glassy Matrix

The electrochemically induced nucleation and crystallization in a glass with the basic composition (mol%) 52.6 SiO2 · 18.7 Al2O3 · 14.3 MgO · 7.8 TiO2 · 4.7 B2O3 · 2.0 CaO leads to the formation of oriented mullite crystals. Electron microscopy techniques (energydispersive X-ray microanalysis and electron backscatter diffraction) were used to analyze the composition and the orientation of the crystals in the glassy matrix. The mean composition of the mullite crystals matches the formula Al4,65Ti0,11Si1,24O9,675. The electron backscatter diffraction pattern is consistent with data from the literature that needle-like mullite crystals are fast-growing parallel to the crystallographic c-axis [001]. The composition of the melt grown mullite crystals does not depend on the melting temperature in the range between 1230 °C and 1350 °C.