Ingenieurwissenschaften und zugeordnete Tätigkeiten
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An understanding on crystallization behavior and kinetics in glasses is one of the important topics in the glass science and technology. Tsukada et al.1) found an extremely unique and curious phenomenon in the crystallization of multiferroic (ferroelectric and ferroelastic properties) ’-Gd2(MoO4)3 crystals in Gd2O3-MoO3-B2O3 glasses. That is, crystals formed in the crystallization break into small pieces with a triangle prism or pyramidal shape having a length of 50-150 um spontaneously during the crystallization in the inside of an electric furnace, not during the cooling in air after the crystallization. They proposed to call this phenomenon “self-powdering phenomenon”. To the best of our knowledge, such a self-powdering phenomenon has not been observed in the crystallization of any other glasses. To investigate this phenomenon is very important for the glass science and for the preparation of bulk crystallized glasses with ferroelastic crystals. In this work, the preparation of crystallized glasses keeping the original shape is challenged, and the mechanism of self-powdering phenomenon is proposed. Glasses with the compositions of 21Gd2O3-63MoO3-(16-x)B2O3-xTeO2 (mol%) (x=0, 2, 4, 8) such as 21Gd2O3-63MoO3-16B2O3 (GM16B) and 21Gd2O3-63MoO3-8B2O3-8TeO2 (GM8T8B) were prepared using a conventional melt quenching technique. The quenched glasses were heat-treated at some temperatures, and the crystalline phase present in the crystallized samples was identified by X-ray diffraction (XRD) analyses, and the morphology and birefringence of crystals formed were observed using a polarized optical microscope (POM). The densities of GM16B and GM8T8B glasses are 4.76 g/cm3 and 4.91 g/cam3, respectively, and these values are higher than the density of ’-Gd2(MoO4)3 crystal, 4.555 g/cm3. These differences in the density might induce the large stresses in the inside of crystals during the crystal growth. The crystallized glasses of GM16B with ’-Gd2(MoO4)3 crystals do not keep the original glass shape, showing self-powdering phenomenon (breaking into small pieces). However, the crystallized glasses of GM8T8B keep the original glass shape without indicating any self-powdering. It is proposed that the stress at the interface between the glassy phase and ’-Gd2(MoO4)3 crystalline phase might be reduced in GM8T8B glasses containing TeO2 with weak Te-O bonds.
185 cracks were initiated in a soda-lime-silica glass by Vickers indentation and grown under controlled dry nitrogen atmosphere. Direct observation of subcritical crack growth (SCCG) upon indentation was performed using a video camera with an image acquisition rate of 3 ms. The results show that the validity of K(c) relations used to determine the indentation fracture toughness from a single imprint is questionable. It seems that correlation of crack length to stress intensity necessitate the use of statistical significant data of multiple crack events.
Comparison of fluorescence measurements performed on different fluorescence instruments, analyte quantification from fluorescence intensities as well as the determination of fluorescence quantum yields require instrument calibration and consideration of the wavelength-dependent instrument-specific quantities spectral photon flux reaching the sample and spectral responsivity. Here, we present guidelines and recommendations for the qualification of fluorescence instruments and introduce suitable chromophore-based reference materials. Moreover, the design concepts of the different BAM fluorescence standards are discussed.
Solar glass in arid and semi-arid regions is exposed to sand storms which can affect the durability of PV modules. Related erosion processes have been extensively studied but the results given are difficult to compare due to being obtained by different variables like particle speed and sand mass. This study correlates the damage of solar panels to the cumulative impact energy as a global parameter.
Compared to its intrinsic strength, the practical strength of glasses is substantially
lowered by microscopic flaws. Their sub critical growth from the surface is strongly
affected by ambient humidity.
A new device was designed to enable the observation of sub critical crack growth
(SCCG) in vacuum and controlled atmosphere. It allows measurements in transparent and
semi-transparent materials in double cantilever beam (DCB) geometry. The aim of the
present work is to study the effect of crystals on crack growth.
Partially crystalline glasses are predominantly used as solid oxide fuel cell (SOFC) sealants due to their superior long term durability. However, cracks caused by thermal cycling still remain a substantial bottleneck in developing durable SOFC sealants inasmuch as, in contrast to crystal free glasses, large crystal volume fractions can retard healing. Hence, the basic understanding of crack healing in glassy crystalline materials and the effects of micro structure are important for finding optimum micro structures for both, durability and crack healing.
For studying these effects, several model glass matrix composites (GMC), for which simultaneous crystal growth and crack healing can be excluded, have been synthesized. Sodium calcium silicate glass – zirconia GMC turned out to provide sufficiently homogeneous, dense and durable model GMC for our studies. The microstructure of this GMC shows large crystal free glassy regions embedded in network of finely dispersed ZrO2 nanoscale crystals. Whereas the glassy regions allow easy local crack healing, the network of dispersed crystals increases the effective viscosity on a global scale. This effect substantially retards crack broadening during later healing stages, which often ends up in large pores. Therefore, this type of microstructure seems to be an interesting candidate for crack healing optimized sealants.
Crack healing in glasses
(2017)
Fundamental understanding of crack healing in glassy crystalline materials is very important for many applications, especially for solid oxide fuel cells (SOFC) sealants since cracks caused by mechanical stress or thermal cycling still remain a substantial bottleneck in developing durable SOFC. Previous studies on soda lime silicate glass published by Singh showed that crack healing is driven by viscous flow. There he postulated that the healing progress is proportional to time, t, and the inverse viscosity. This finding would allow to present for a given glass data of crack healing measured at different temperatures in a master curve, if the healing progress is plotted versus t/η. Such master curves would be a helpful tool in understanding crack healing kinetics. To verify the applicability of such master curves, crack healing in non-crystallizing soda-lime-silicate (NCS) and sodium-borosilicate glasses (NBS) was studied. Cracks were generated by Vickers indention and healed isothermally at different temperatures. Crack healing progress was monitored by optical and electron microscopy. The results show that the above mentioned proportionalities applies to the two glasses. In both cases the afore developed master curve could be obtained.
The practical strength of glasses under ambient conditions is substantially lower compared to its intrinsic strength because of sub-critical crack growth (SCCG) from microscopic flaws. While SCCG is related to the humidity of the ambient atmosphere, leading to stress corrosion phenomena, the detailed kinetics are still not fully understood. To get better insight to the contribution of water on the crack-tip, highly water bearing glasses will be investigated by a new device for inert SCCG-measurements using double cantilever beam (DCB) geometry specimens. This device was designed to investigate the stress intensity factor in modus I and crack velocity in vacuum, but different atmospheres can also be introduced.
For validation of the new device, first experiments were performed on microscope slides as well as on a soda-lime silicate and a borosilicate crown glass. The results achieved will be presented in comparison to the published results of Wiederhorn.
Development of durable transparent enamel colors for the manufacture of decorate glass panels
(2017)
Today glass is broadly used in modern architecture. For applications indoor it is possible to produce decor glass by using enamel colors and glass painting techniques without any problems. However, this is more limited for applications outdoor. Humidity and environmental pollution attack the surface of the coating and damage it strongly. There are only few colors on the market which are resistant towards acids and bases until now. Additionally, most of those colors are opaque. In order to extend the color palette, chemically resistant colored glasses are being developed which are transparent, relatively low melting and intensively toned even in thin coat thickness. To achieve such ambitious aim, many parameters have to match which act in complex manner. Metal oxides were used to color the glasses. A lead-free glass composition was developed to avoid an exposition of heavy metals to the environment. The glasses were characterized, in particular in terms of their thermal properties, their crystallization behavior and their chemical durability. Different practical applications will be shown.
The development of glassy hydrogen barriers requires the determination of low H2 permeabilities. Previous studies and numerical simulations have shown that the VHE powder method (vacuum hot extraction with mass spectrometric gas detection) is suitable for this purpose. The measured isothermal gas emission is fitted to a classical diffusion model for spherical particles from which the diffusion coefficient of the glass is calculated. The H2 permeability is determined by means of the solubility determined from the same measurement data. This presentation is referring to the optimization of the method regarding sample preparation and measurement data evaluation using suitable experimentally determined particle size distributions is reported. For quartz glass, it is shown that the accuracy of classical measuring methods is achieved with the VHE powder method. Furthermore, other examples of substantially gas-tighter glasses are given.