Filtern
Erscheinungsjahr
- 2016 (6) (entfernen)
Dokumenttyp
- Zeitschriftenartikel (2)
- Vortrag (2)
- Posterpräsentation (2)
Schlagworte
- Sintering (3)
- Glass (2)
- SOFC (2)
- 3D High-temperature shape screening (1)
- Borate glasses (1)
- Boratgläser (1)
- Crystallization (1)
- Foaming (1)
- Glas (1)
- Glass powder (1)
Eingeladener Vortrag
- nein (2)
Chemical variability is a main strength of glass. Glass powders are therefore promising candidates for manufacturing a broad diversity of sintered materials like sintered ¬glass-ceramics, glass matrix composites or glass bonded ceramics with tailored mechanical, thermal, electrical and optical properties and complex shape. Its wide and precise adjustability makes this class of materials, even if it may not be obvious at first sight, a key component of advanced technologies. Manufacture and processing of initial glass powders often allow even more flexibility in materials design. At the same time, however, they can cause additional problems. The lecture illustrates possible consequences of glass powder processing upon glass crystallization and sintering as well as chances for targeted utilization. Simple kinetic models describing the effect of particle size distribution, surface crystallization and rigid inclusions on sintering as well effects of different milling and seeding on sinter crystallization are presented.
The manufacture of sintered glasses and glass-ceramics, glass matrix composites, and glass-bounded ceramics or pastes is often affected by gas bubble formation. Against this background, we studied sintering and foaming of barium silicate glass powders used as SOFC sealants using different powder milling procedures. Sintering was measured by means of heating microscopy backed up by XPD, differential thermal analysis, vacuum hot extraction (VHE), and optical and electron microscopy. Foaming increased significantly as milling progressed. For moderately milled glass powders, subsequent storage in air could also promote foaming. Although the powder compacts were uniaxially pressed and sintered in air, the milling atmosphere significantly affected foaming. The strength of this effect increased in the order Ar ≈ N2 < air < CO2. Conformingly, VHE studies revealed that the pores of foamed samples predominantly encapsulated CO2, even for powders milled in Ar and N2. Results of this study thus indicate that foaming is caused by carbonaceous species trapped on the glass powder surface. Foaming could be substantially reduced by milling in water and 10 wt% HCl.
Glasses are can serve as exceptionally tight hydrogen barriers e.g. used for hydrogen storage in micro glass containers or cover glasses in micro electronic systems. Respective glass development, however, requires precise measurements of minimal hydrogen permeability, PH2. Recent studies showed that PH2 can be measured down to 2 10-21 mol s-1 Pa-1m-1 by means of Vacuum Hot Extraction (VHE) powder methods [1]. In this respect the isothermal gas release from glass powder particles is fitted in terms of classical diffusion models assuming spherical particles of uniform size thus obtaining the hydrogen diffusion coefficient, DH2. PH2 is then given by D H2 × S H2, where hydrogen solubility, SH2, is obtained from VHE studies of glass powders exposed to hydrogen atmosphere for different exposure time and hydrogen pressure. Measurements of minimal values of hydrogen permeability, however, require a careful evaluation and error discussion of this method. Against that background, we modeled hydrogen degassing during heating and subsequent isothermal annealing of glass powder particles of different shape and particle size distribution by means of COMSOL Multiphysics® [2] and verified related effects on DH2 obtained by the VHE powder method.
Advanced methods for 3D green density characterization like computed tomography and 3D FE sinter modeling can be utilized for increasing the reliability of sintered components. The experimental in situ observation of sintering, however, is currently restricted to silhouette methods, i.e. heating microscopy. For complex shaped samples, in situ shape screening during shrinkage would allow much better validation of 3D sinter simulation models. Further, by revealing temporary sinter warpage, 3D high-temperature shape screening allows to locate potential defects of complex sintered components. Against this background, BAM developed a testing device for in situ 3D high-temperature shape screening for ceramic and glass-ceramic tapes up to 1000°C [1-3]. Current work is focused on dropping this restriction in sample shape and temperature. The poster illustrates the current state of this work and possible applications of the method e.g. in detecting sinter warpage of metallized glass-ceramic LTCC tapes.
In the present study we have investigated whether the effect of water on properties of borate glasses resembles that of alkali oxide. Soda-lime-borate glasses with nominal compositions of x Na2O, 10 CaO, (90-x) B2O3 (x = 5, 15 and 25 mol%) were doped with up to 8 wt.% H2O by processing glass powder + distilled water in platinum capsules in an internally heated gas pressure vessel at 1523 K and 500 MPa. The water content of hydrous glasses was determined by Karl-Fischer titration and near-infrared spectroscopy. The glass transition temperature T-g. was derived from DTA and micropenetration experiments for which the effect of water loss at the surface of the hydrous glasses was studied. Heating glass samples at 10 K min(-1) in the DTA resulted in T-g values which are close to T-12 isokom temperatures confirming the equivalence of enthalpy relaxation and viscous relaxation for borate glasses. For all three glass series it is shown that T-g strongly decreases whereas the liquid fragility strongly increases upon the addition of water. These findings reveal that H2O primarily causes breaking of B-O-B bonds rather than supporting 4-fold coordinated boron as it is well-known for alkali oxides in this concentration range. (C) 2015 Elsevier B.V. All rights reserved.