Filtern
Erscheinungsjahr
Dokumenttyp
- Zeitschriftenartikel (35) (entfernen)
Schlagworte
- Glass (8)
- Sintering (6)
- LTCC (4)
- Viscosity (4)
- Crystallization (3)
- EBSD (3)
- Glass powder (3)
- Internal friction (3)
- Water content (3)
- Aluminosilicate glasses (2)
- Diopsid (2)
- Foaming (2)
- Glass ceramic composites (2)
- Glass transition (2)
- Inclusions (2)
- Infrared spectroscopy (2)
- Ionic porosity (2)
- NMR spectroscopy (2)
- SOFC (2)
- Silicates (2)
- Soda-lime-silica (2)
- Water speciation (2)
- 3D etching (1)
- Acid-leaching (1)
- Al2O3-inclusions (1)
- Alkali aluminosilicate glasses (1)
- Alumina dissolution (1)
- Analytical scanning electron microscopy (1)
- Automated analysis (1)
- Bond energy (1)
- Borate glasses (1)
- Boratgläser (1)
- Borosilicate glass (1)
- Brittle fracture (1)
- Bubble formation (1)
- Calculated intrinsic fracture toughness (1)
- Crack growth (1)
- Crystal growth (1)
- Crystal lattice (1)
- DCB geometry (1)
- DTA (1)
- Debindering (1)
- Densification (1)
- Diffusivity (1)
- Effective Viscosity (1)
- Elastic constants (1)
- Fracture surface energy (1)
- Fragility (1)
- Glas Matrix Composites (1)
- Glass Ceramic (1)
- Glass Matrix Composites (1)
- Glass ceramic (1)
- Glass ceramics (1)
- Glass liner (1)
- Glass matrix composites (1)
- Glass-ceramic (1)
- High pressure (1)
- Hydrogen diffusivity (1)
- Hydrogen permeation (1)
- Hydrogen storage tank (1)
- Hydrous glass (1)
- Indentation fracture toughness (1)
- Ion beam erosion Sectioning (1)
- Kinetic Modelling (1)
- Kinetik (1)
- Kristallisation (1)
- Kristallwachstumsgeschwindigkeit (1)
- Lithiumionen-Batterie (1)
- Microhardness (1)
- Milling (1)
- Mixing (1)
- Modeling (1)
- Modelling (1)
- Nucleation tendency (1)
- Orientation (1)
- Oxide glasses (1)
- Permeability (1)
- Phasenentwicklung (1)
- Phosphate (1)
- Powder preparation (1)
- Powders (1)
- Relaxation (1)
- Rheology (1)
- Rietveld analysis (1)
- Rigid Inclusion (1)
- SEM (1)
- Sample preparation (1)
- Silicate glass (1)
- Silicoborate glasses (1)
- Silikatgläser (1)
- Sinter retardation (1)
- Sintering Kinetics (1)
- Sintering glass composit bio material (1)
- Sintering kinetics (1)
- Slow crack growth (1)
- Soda-lime silicate glass (1)
- Sodium ion batteries (1)
- Solid-state reaction (1)
- Solubility (1)
- Stable crack growth (1)
- Stress intensity factor (1)
- Structural relaxation (1)
- Surface energy (1)
- Surface nucleation (1)
- Thermal analysis (1)
- Thermische Analyse (1)
- Thermo gravimetry (1)
- Ultrasonic relaxation (1)
- Unterschiedliche Atmosphären (1)
- Vickers indentation (1)
- Viscous sintering (1)
- Viskosität (1)
- Wasserhaltige Gläser (1)
- Water (1)
- Water in glass (1)
- Water-bearing glasses (1)
- hydrogen storage (1)
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.
Crystallization behavior in different atmospheres (air, Ar, and 7% H2/Ar) of lithium iron phosphate glass (33Li2O33Fe2O31Nb2O5–33P2O5 (LFNP)) powders with different sizes of 2–1000 µm was examined. The crystallization peak temperature (Tp) in Ar and 7% H2/Ar decreased with decreasing the particle size of glass powders, but the value of Tp in air was independent of particle size. The crystallized glass obtained by heating to Tp showed the formation of the α-Li3Fe2(PO4)3 and LiFePO4 crystalline phases. Fe2O3 crystals were formed only in the inside of crystallized glass plates in the heating in air. The crystallization of LiFePO4 was largely enhanced in the particle size of <2 µm during the heating in Ar and 7% H2/Ar atmospheres. The main crystallization mechanism in LFNP glass was found to be surface crystallization. Significant shrinkages were observed for glass compacts (pellets) in the heating in 7% H2/Ar. These results suggest that controlling particle sizes and atmospheres enables the design the morphology of LiFePO4 crystals in LFNP glass.
We studied sintering of LTCC-type glass matrix composites (GMCs) consisting of small glass and alumina particles of equal size. Primarily, crystals act as rigid inclusions, decelerating the densification rate. In later stages, they also dissolve, partially increasing the viscosity. Release of alumina finally induces crystallization of alumosilicates, which enables post-firing stability. To study both effects, two model GMCs were prepared: an α-Al2O3 + barium alumoborosilicate glass (BABS)GMC, which shows neither significant dissolution nor crystallization, and an α-Al2O3 + calcium alumoborosilicate glass (CABS)GMC, which dissolves readily and promotes crystallization. The kinetics of shrinkage for both GMC were modeled by utilizing Frenkel theory for the early stage and MackenzieShuttleworth theory for the late stage, assuming that sintering is superimposed by the weighted contributions of triparticle glasscrystal clusters, their random occurrence (ideal mixing), and a shrinkage rate controlled by the GMC effective viscosity. In agreement with modeling, the experimental results showed that the shrinkage rate of BABSGMC decreases progressively for crystal volume fractions Φ > 0.15. The attainable shrinkage is reduced by up to 8% for Φ = 0.45. For the CABSGMC with Φ = 0.25, a reduction of Φ to 0.20 was evident due to partial α-Al2O3 dissolution. This effect was found able to increase the sintering temperature by ~5060 K.
The sintering of Low Temperature Co-fired Ceramics prepared from alumoborosilicate
glass- and Al2O3 powders of similar small particle size was studied by dilatometry, heating microscopy,
microstructure analysis, glass- and effective viscosity measurements. The steric effect of
Al3O3 inclusions was studied using a gnon-reactiveh model composite. With increasing Al3O3 volume
fraction (Φ ≤ 0.45), sintering decelerates and its final stage shifts to higher temperature. The
attainable shrinkage is reduced as Al2O3 particle clusters bearing residual pores become more frequent.
The kinetics of sintering could be described formally superposing the weighed contributions
of differentially sized and randomly composed glass-crystal particle clusters and assuming a sintering
rate controlled by the effective matrix viscosity, which increases with Φ and with progressive
wetting of Al2O3 particles during densification. The "reactive" model composite shows significant
dissolution of Al2O3 into the glass, which has two opposed effects on sintering: reducing Φ and increasing
glass viscosity. For the present case (Φ = 0.25), the latter effect dominates and sintering is
retarded by Al2O3 dissolution. Crystallization of wollastonite starts after full densification. Dissolution
of Al2O3 was found to promote the subsequent growth of anorthite.
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.
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 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.