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Organisationseinheit der BAM
The effect of elastic stress and relaxation on crystal nucleation in lithium disilicate glass
(2004)
In the framework of a general theoretical approach recently developed, the effect of elastic stresses on the rate of nucleation of crystallites of critical sizes in the process of crystallization of lithium disilicate glasses is studied. Hereby both the evolution of elastic stresses due to changes in the specific volume of the crystallizing system (i.e., differences in the densities of the melt and the crystal) and their relaxation connected with viscous flow are taken into consideration. If the classical theory of nucleation is employed for the determination of the nucleation rates (modified by taking into account elastic stresses), elastic stresses may lead, for the system under consideration, to a decrease of the nucleation rate by up to two orders of magnitude. As shown, the theory allows an at least qualitatively correct incorporation of the effect of elastic stresses and stress relaxation into the description of nucleation phenomena in glass-forming melts.
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 (550560 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.
Theory of nucleation in viscoelastic media: application to phase formation in glassforming melts
(2003)
Glassforming melts behave, in the vicinity of the temperature of vitrification Tg, as viscoelastic bodies. A general theory of nucleation in a viscoelastic body developed elsewhere is applicable to the description of phase formation processes in such systems. The present contribution is directed to the demonstration of the relevance of this proposed general theory to describing phase transformation processes in glassforming melts. The application of the theory is shown to explain a number of experimental results on crystallization of glassforming melts, which have not found a satisfactory interpretation so far.
Lithium disilicate, leucite and apatite glass-ceramics have become state-of-the-art framework materials in the fabrication of all-ceramic dental restorative materials. The goal of this study was to examine the crack propagation behaviour of these three known glass-ceramic materials after they have been subjected to Vickers indentation and to characterize their crack opening profiles (δmeas vs. (a-r)). For this purpose, various methods of optical examination were employed. Optical microscopy investigations were performed to examine the crack phenomena at a macroscopic level, while high-resolution techniques, such as scanning electron microscopy (SEM) and atomic force microscopy (AFM), were employed to investigate the crack phenomena at a microscopic level. The crack patterns of the three glass-ceramics vary from fairly straightforward to more complex, depending on the amount of residual glass matrix present in the material. The high-strength lithium disilicate crystals feature a high degree of crosslinking, thereby preventing crack propagation. In this material, the crack propagates only through the residual glass phase, which constitutes 30%40% by volume. Having a high glass content of more than 65% by volume, the leucite and apatite glass-ceramics show far more complex crack patterns. Cracks in the leucite glass-ceramic propagate through both the glass and crystal phase. The apatite glass-ceramic shows a similar crack behaviour as an inorganicorganic composite material containing nanoscale fillers, which are pulled out in the surroundings of the crack tip. The observed crack behaviour and the calculated View the MathML source values of the three types of glass-ceramics were compared to the Kkic values determined according to the SEVNB method.
Green compacts of ceramics, glass ceramic composites and sinter glass ceramics contain
different amounts of organic materials added as pressing aids or binders. Before sintering, these
organics have to burn out completely. In oxidising atmospheres, the debindering process is mostly
exothermic and therefore difficult to control. This uncontrolled heat production due to locally enhanced
debindering and respective gas release may cause damages in the green compact microstructure.
Therefore, debindering is usually operated with very low heating rates (< 3 K/min) which requires
long processing times of many hours. In this paper, we will show that it is possible to reduce
the processing time for debindering dramatically by using the decomposition rate of the organic
binder, detected by the weight loss of the sample, as a control factor of the furnace.
The viscosity of silica glasses with different water contents was measured by penetration of sapphire microspheres in the range from 1012-1014·7 Pas at ambient pressure. Commercial silica glasses were used together with hydrous silica glasses, which were prepared by melting glass powder plus water in an internally heated pressure vessel at 2-3 kbar and 1350-1450°C. The temperature dependence of the viscosity of silica glasses with total water contents Cw of 5·4×10-4, 7·0×10-4, 1·00×10-2, and 1·55×10-2(Cw in mass fractions) can be described by Arrhenius equations. From the results, the linear dependence of the T12 isokom (K) with the logarithm of the water content was determined as T12=192-409 logCw. The analysis of the compositional dependence of the T12 isokom in the H2O-Na2O-SiO2 system reveals a temperature depression of up to 200 K for mixed water- and sodium-bearing glasses compared to H2O-SiO2 and Na2O-SiO2 glasses with same molar silica content. From these findings we conclude that protons may contribute significantly to the mixed alkali effect in glasses.
We measured and collected literature data for the crystal growth rate, u(T), of µ-cordierite (2MgO · 2Al2O3 · 5SiO2) and diopside (CaO · MgO · 2SiO2) in their isochemical glass forming melts. The data cover exceptionally wide temperature ranges, i.e. 8001350 °C for cordierite and 7501378 °C for diopside. The maximum of u(T) occurs at about 1250 °C for both systems. A smooth shoulder is observed around 970 °C for µ-cordierite. Based on measured and collected viscosity data, we fitted u(T) using standard crystal growth models. For diopside, the experimental u(T) fits well to the 2D surface nucleation model and also to the screw dislocation growth mechanism. However, the screw dislocation model yields parameters of more significant physical meaning. For cordierite, these two models also describe the experimental growth rates. However, the best fittings of u(T) including the observed shoulder, were attained for a combined mechanism, assuming that the melt/crystal interface growing from screw dislocations is additionally roughened by superimposed 2D surface nucleation at large undercoolings, starting at a temperature around the shoulder. The good fittings indicate that viscosity can be used to assess the transport mechanism that determines crystal growth in these two systems, from the melting point Tm down to about Tg, with no sign of a breakdown of the StokesEinstein/Eyring equation.
The effect of hydration on the kinetic fragility of soda-lime-silica glasses was investigated by viscometry in the glass transition range. Water-bearing glasses were prepared from industrial float glass (FG) and a ternary model glass (NCS = 16Na2O 10CaO 74SiO2 in mol%) by bubbling steam through the melt at 1480 °C and up to 7 bar. Additionally, a sodium borosilicate glass (NBS = 16Na2O 10B2O3 74SiO2 in mol%) was hydrated under equal conditions. As detected by infrared spectroscopy water dissolves in the glasses exclusively as OH-groups. The hydration resulted in a total water content CW up to ~ 0.2 wt% for FG, NCS and NBS glasses. Kinetic fragility, expressed by the steepness index m, was determined from the temperature dependence of η at the glass transition. Viscosity data from previous studies on hydrous float glasses (CW > 1 wt%) were surveyed together with literature data on the (H2O)Na2OCaOSiO2, (H2O)Na2OSiO2 and (H2O)SiO2 systems to expand the range of water concentration and bulk composition. We could demonstrate that m decreases for all glasses although water is dissolved as OH and should depolymerize the network. An empirical equation of the general type m = a - b logCW where a, b are fitting parameters, enables m to be predicted, for each glass series as function of the water content CW. The enlarged data base shows that the parameter B of the Arrhenius viscosity-temperature relation decreases much stronger than the isokom temperature at the glass transition.
The short- and long-term structural evolution and aging of solgel 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 donoracceptor-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 solgel 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.
Glass bricks are important transparent building materials. They are produced by joining two halves of glass pressings at 600700 °C. During this production process alkali oxides evaporate and are redeposited at the cooler inner front surfaces of the bricks. This surface layer reacts with H2O and CO2 from the residual brick atmosphere, leading to the formation of an alkali-rich silicate-hydrate layer of ≥50 nm thickness, which could be evidenced leading to a reduced nano-hardness of similar thickness, and from which NaHCO3 crystals can finally grow. Climate chamber experiments (repeated cooling between at -8 and -14 °C and reheating to 0 to 15 °C) resulted in reversible NaHCO3 crystallization and redissolution, presumably influenced by water evaporation or condensation and driven by the NaHCO3 supersaturation of the silicate-hydrate layer. Depending on the timetemperature schedule, different crystal morphologies became visible in this closed system, e.g. isolated spherical crystals, crystals arranged in chains and in double-chains, respectively, which can limit already the transmittance of the glass bricks. When a crack occurs or the brick is opened, the hygroscopic NaHCO3 crystals take up more H2O from the ambient, react irreversibly with the glass surface, finally leading to a total loss of transmittance.
We propose a model to describe the sintering kinetics of polydispersed glass particles, having no adjustable parameter. The model is based on three sintering stages: a pure Frenkel' (F) first step, a mixed Frenkel/MackenzieShuttleworth' stage, and a third, pure MackenzieShuttleworth' (MS) step. The model considers sample shrinkage as the sum of the partial shrinkage of several clusters, each consisting of equally sized particles and each showing independent F or MS behavior. The overall set of clusters mimics the specimen's real particle size distribution. We then introduce the concept of neck forming ability, which allows the formation of necks among particles of different sizes, relaxing the clustering condition. Using experimental physical parameters: particle size distribution, viscosity, surface energy, and the theoretical neck forming ability, the model describes well the sintering kinetics of an alumino-borosilicate glass powder having polydispersed, irregular shaped particles in a variety of temperatures. The sintering kinetics of the real powder is slower, but not far from the calculated kinetics of a monodispersed distribution containing only particles of average size. Thus the model provides a tool for estimating the sintering kinetics of real glass powders, for any size distribution and temperature, thus minimizing the number of laboratory experiments.
Literature data on the effect of water on the glass transition in silicate melts are gathered for a broad range of total water content cw from 3 × 10-4 to 27 wt%. In terms of a reduced glass transition temperature Tg*=Tg/TgGN, where TgGN is Tg of the melt containing cw?0.02 wt% total water, a uniform dependence of Tg* on total water content (cw) is evident for silicate melts. Tg* decreases steadily with increasing water content, most strongly at the lowest water content where H2O is dominantly dissolved as OH. For water-rich melts, the variation of Tg* is less pronounced, but it does not vanish even at the largest water contents reported (?27 wt%). Tg* vs. cw is fitted by a three-component model. This approach accounts for different transition temperatures of the dry glass, hydroxyl and molecular water predicting Tg* as a weighted linear combination of these temperatures. The required but mostly unknown water speciation in the glasses was estimated using IR-spectroscopy data for hydrous sodium trisilicate and rhyolite.
For the first time evidence is provided that a nanocrystalline and stacking-disordered, chemically stabilized β-cristobalite form of AlPO4 occurs in a sewage sludge ash (SSA). This proof is based on a combined X-ray powder diffraction and X-ray fluorescence investigation of an SSA produced at a large-scale fluidized bed incineration facility serving a catching area with a population of 2 million. The structural and chemical characterization was carried out on 'as received' SSA samples as well as on solid residues remaining after leaching this SSA in sodium hydroxide solution. Thus, it was ascertained that the observed nanocrystalline and stacking-disordered cristobalite-like component belongs to the aluminum phosphate component of this SSA, rather than to its silicon dioxide component. In addition, a direct proof is presented that the chemically stabilized β-cristobalite form of AlPO4 does crystallize from X-ray amorphous precursors under conditions that mimic the huge heating rate and short retention time (just seconds at T ≈ 850°C), typical for fluidized bed incinerators.
A glass of the composition 37BaO·16CaO·47SiO2 wt% produced on an industrial scale is crystallized at 970 °C for times ranging from 15 min to 2 h. The crystallization at the immediate surface as well as the crystal growth into the bulk are analyzed using scanning electron microscopy (SEM) including energy dispersive X-ray spectroscopy (EDXS) and electron backscatter diffraction (EBSD) as well as X-ray diffraction in the Θ–2Θ setup (XRD). The immediate surface shows the oriented nucleation of walstromite as well as the formation of wollastonite and an unknown phase of the composition BaCaSi3O8. All three phases also grow into the bulk where walstromite ultimately dominates the kinetic selection and grows throughout the bulk due to a lack of bulk nucleation. Walstromite shows systematic orientation changes as well as twinning during growth. A critical analysis of the XRD-patterns acquired from various crystallized samples indicates that their evaluation is problematic and that phases detected by XRD in this system should be verified by another method such as EDXS.