5 Werkstofftechnik
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Organisationseinheit der BAM
Glasses in the systems Me2O-ZnO-B2O3 with Me = Li, Na, K, Rb (MeZB), Na2O-ZnO-CuO-B2O3 (NZCuB), CaO-ZnO-B2O3 (CaZB), and Li2O-PbO-B2O3 (LPbB) as a reference, were studied by differential thermal analysis, dilatometry, rotational viscometry, and heating microscopy. A decrease of viscosity and sintering range was found with decreasing number of fourfold coordinated boron. The viscosity of the alkali zinc borate glasses varies only slightly. LPbB and CaZB stand out by their reduced and increased viscosities, respectively. Sodium, potassium, and calcium zinc borate glasses possess a fragility above 76. All glasses were sintered to full density before crystallization. Mostly binary zinc borate phases govern crystallization. A ternary crystalline phase was detected only in the potassium containing sample. The Weinberg glass stability parameter ranges between 0.07 and 0.12. This is caused by the presence of several crystalline phases and varying melting points of even the same crystalline phase in different glass matrices.
In this review article, the impact of dissolved water on the viscous properties of soda lime silicate melts is addressed against the background of the upcoming switch from natural gas to hydrogen combustion. This change will lead to an increase in the total water content of the glasses by up to 0.4 mol%. In order to better define possible influences of water speciation, water-rich glasses were synthesised under increasing pressure up to the kbar range. It is shown that a distinction must be made between the influence of dissolved OH-groups and H2Omolecules in order to accurately reflect the dependence of isokom temperatures on water content. In addition, an increase of one order of magnitude in the tolerance to higher deformation rates was observed for the range of expected increased water contents during isothermal deformation processes, which is based on the timetemperature superposition principle, i.e. congruent flow curves were determined under isokomal conditions.
To explore the impact of ambient and structural water on static fatigue, the initiation and growth of 3279 Vickers induced median radial cracks were automatically recorded and analyzed. We find that humidity is more efficient in initiating cracks and promoting their growth than water, which is dissolved in the glass structure. In particular for slow crack growth (< 3x10-6 m s-1), tests in dry nitrogen showed a considerable decrease in the crack growth exponent with increasing water content of the glasses. On the other hand, if tests were performed in humid air, the crack growth exponent was independent of the water content of the hydrous glasses, while stress intensity decreased slightly. These observations indicate that water promotes the processes at the crack-tip regardless of its origin. However, ambient water is more efficient.
Glasses with compositions of 21Gd2O3-63MoO3-(16-x)B2O3-xTeO2 (mol%) (x= 0, 2, 4, 8) were prepared using a conventional melt quenching technique, and the crystallization behavior of ferroelastic β′-Gd2 MoO4)3 Crystals was examined to clarify the mechanism of self-powdering phenomenon and to design bulk crystallized glasses. It was found that the self-powdering phenomenon appeared significantly during the crystallization at temperatures near the crystallization peak temperature, but the phenomenon is suppressed in the crystallization at temperatures much higher than the glass transition temperature. It was also found that the substitution of TeO2 for B2O3 in the base glasses suppresses the self-powdering phenomenon and consequently bulk crystallized glasses were obtained in the glass with x=8 mol%. The densities at room temperature of the base glasses are d =4.755–4.906 g/cm3, being much higher than the value of d=4.555 g/cm3 for β′-Gd2(MoO4)3 crystal. It is proposed that the stresses in the inside of crystals induced by large density differences (i.e., large molar volume differences) between the glassy phase and crystals might be relaxed effectively in the glasses containing TeO2 with weak TeeO bonds and fragile character.
A double cantilever beam technique in air equipped with ultrasound modulation was used to measure the crack velocity v in borate and silicate glasses. In all glasses v and the stress intensity KI followed the empirical correlation v ~ KIn. Indicated by its smallest KI at v = 1 μm s − 1, KI* = 0.27 MPa m0.5, the silicoborate glass containing 70 mol% B2O3 was found most susceptible to stress-corrosion enhanced crack growth. Contrarily, the sodium calcium magnesium silicate glass appeared least susceptible with KI* = 0.57 MPa m0.5. No clear correlation is evident between KI*, reflecting the stress-corrosion susceptibility, and the hydrolytic resistance for all glasses under study, but values of n obtained from the present study and taken from previous literature for 35 glasses tend to decrease with increasing network modifier ion fraction. Energy dissipation during stress-corrosion enhanced crack propagation is assumed to cause this trend.
Stable crack growth was measured for nominal dry and water-bearing (6 wt%) soda-lime silicate glasses in double cantilever beam geometry and combined with DMA studies on the effects of dissolved water on internal friction and glass transition, respectively. In vacuum, a decreased slope of logarithmic crack growth velocity versus stress intensity factor is evident for the hydrous glass in line with an increase of b-relaxation intensity indicating more energy Dissipation during fracture. Further, inert crack growth in hydrous glass is found to be divided into sections of different slope, which indicates different water related crack propagation mechanism. In ambient air, a largely extended region II is observed for the hydrous glass, which indicates that crack growth is more sensitive to ambient water.
The effect of structural water on density, elastic constants and microhardness of water-bearing soda-lime-silica glasses of up to 21.5 mol% total water is studied. It is found that the Poisson ratio and the water content are positively correlated, while density and the elastic moduli decrease with increasing water content. Vickers hardness decreases by approximately 27% from the dry to the most hydrous glass. For water fractions <3 mol%, the dependencies are non-linear reflecting the non-linear change in the concentrations of OH and H2O molecules dissolved, whereas for water fractions >3 mol% linear dependencies are found. To distinguish the effect of structural water and environmental water, indentations were performed in toluene, nitrogen gas and air. Timedependent softening was evident for testing dry glasses in humid atmospheres as well as for tests of hydrous glasses in dry atmospheres. This indicates that the response times of dissolved water species are effectively equal in both scenarios.
Barium silicate glass powders 4 h milled in CO2 and Ar and sintered in air are studied with microscopy, total carbon analysis, differential thermal Analysis (DTA), vacuum hot extraction mass spectroscopy (VHE-MS), Fourier-transformed infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and time-of-flight secondary-ion mass spectrometry (TOF-SIMS). Intensive foaming of powder compacts is evident, and VHE studies prove that foaming is predominantly caused by carbonaceous species for both milling gases. DTA Shows that the decomposition of BaCO3 particles mix-milled with glass powders occurs at similar temperatures as foaming of compacts. However, no carbonate at the glass surface could be detected by FTIR spectroscopy, XPS, and TOF-SIMS after heating to the temperature of sintering. Instead, CO2 molecules unable to rotate identified by FTIR spectroscopy after milling, probably trapped by mechanical dissolution into the glass bulk. Such a mechanism or microencapsulation in cracks and particle aggregates can explain the contribution of Ar to foaming after intense milling in Ar atmosphere. The amount of CO2 molecules and Ar, however, cannot fully explain the extent of foaming. Carbonates mechanically dissolved beneath the surface or encapsulated in cracks and micropores of particle aggregates are therefore probably the major foaming source.
Hydrogen gas diffusivity of fourteen glasses of the Na2O-Al2O3-SiO2 system are studied along the joins quartzalbite-jadeite-nepheline (Qz-Ab-Jd-Np, fully polymerized) and albite-sodium disilicate (Ab-Ds, depolymerized).
Density measurements show that ionic porosity decreases from 54.4% (Qz) to 51.5% (Np) and from 52.4% (Ab) to 50.2% (Ds). Hydrogen diffusivity D follows similar trends but at another scale. D at 523 K decreases from 4×10−12 to 3×10−14m2 s−1 (Qz-Np) and from 4×10−13 to 3×10−15m2 s−1 (Ab-Ds). Charge compensating Na+ acting as a filling agent in fully polymerized network structures leads to up to one order of Magnitude higher diffusivities as depolymerized glass structures of the same SiO2 content where Na+ takes the role of a
modifier ion. Temperature dependence of the diffusivity indicates that both the activation energy involved with the moving H2 molecule as well as the accessible volume in the structure contribute to this compositional trend.
The permeation of hydrogen gas was studied in meta-aluminous (tectosilicate) glass powders of Li2O×Al2O3×SiO2 (LAS), Na2O×Al2O3×SiO2 (NAS) and MgO×Al2O3×SiO2 (MAS) systems by pressure loading and vacuum extraction in the temperatures range 210–310 °C. With this method, both the solubility S and the diffusivity D were determined, while the permeability was given by the product SD. For all glasses, S was found to decrease with temperature, while D increased. Since the activation energy of diffusion of H2 molecules exceeded that of dissolution, permeation increased slightly with temperature. When extrapolated to standard conditions (25 °C), the permeability of tectosilicate glasses was found to be only 10-22–10-24 mol H2 (m s Pa)-1, which is 8–10 magnitudes lower than most polymers. Thin glass liners of these compositions are expected to be the most effective barrier for tanks of pressurised hydrogen.