The Science & Technology
of Glass
Cambridge - Monday 4th to
Wednesday 6th September 2017



Katrin Thieme
<katrin.thieme@uni-jena.de>

article posted 19 June 2017

• Date of birth: January 14, 1989

• 10/2007 - 10/2012 Studies of Materials Science at the Friedrich-Schiller-University Jena

• 12/2012 - 04/2016 PhD student at the Otto Schott-Institute of Materials Research (Friedrich-Schiller-University Jena) in the working group of Prof. Rüssel Thesis title: “Nucleation inhibitors in lithium disilicate glasses”

• Since 10/2016 postdoc in the working group of Prof. Rüssel


Effect of different additives on the crystallization behaviour of BaO-SrO-ZnO-SiO2 glasses
Katrin Thieme1*, Christian Thieme2, Christian Rüssel1

Recently, in the system BaO-SrO-ZnO-SiO2 the phase with the formula Ba1-xSrxZn2Si2O7 was found which possesses a very low or even negative thermal expansion. For example in case of Ba0.6Sr0.4Zn2Si2O7 solid solution crystals, the mean thermal expansion coefficient is negative as determined by high-temperature X-ray diffraction between room temperature and 300 °C. However, since this phase shows a pronounced contraction in the direction of the crystallographic b-axis while it expands in a- and c-direction, the thermal expansion is highly anisotropic (Figure 1).

It is possible to prepare glasses in which Ba1-xSrxZn2Si2O7 crystals can be precipitated in high concentrations during thermal treatment. However, these glasses show solely surface crystallization and often micro cracking occurs which hinders the preparation of large size samples. Hence, the precipitation of volume crystals should be triggered by the introduction of appropriate nucleating agents. Therefore, the effect of different additives such as noble metals, ZrO2, and P2O5 is analysed and their suitability as nucleating agents is studied. The crystallization behaviour is studied by a combination of various methods such as differential scanning calorimetry, X-ray diffraction, scanning electron microscopy, electron backscattered diffraction and high resolution scanning transmission electron microscopy.

For example P2O5 was added to the glass composition BaO·SrO·2ZnO·2SiO2
in different concentrations. While the stoichiometric glass melt crystallizes spontaneously during cooling to room temperature, the addition of P2O5 has a stabilizing effect. This is a much better glass formation than expected. Hence, a glass without any crystals can be obtained by conventional melt quench technique. As seen in the DSC profiles of the P2O5-containing glasses (see Figure 2), the onset of the crystallization peak is shifted to higher temperatures with increasing P2O5 concentration. Depending on the P2O5 concentration, heat treatments of these glasses result in the precipitation of different phases, but with the desired phase as the major one which was proved via X-ray diffraction as well as elementalysis in a scanning electron microscope. In certain concentration ranges, it is possible to achieve volume crystallization in this strongly surface crystallizing glass system (see the SEM micrograph in Figure 3). The effect of P2O5 on the crystallization behaviour as well as on the microstructure of crystallized glasses depends on the P2O5 concentration. The appearance of stresses and micro cracks can be attributed to certain crystallographic orientations of certain crystals, which is studied and discussed in terms of electron backscatter diffraction.









Institutions:

1Otto-Schott-Institut für Materialforschung, Jena University, Germany Fraunhoferstraße 6, 07743 Jena
2Fraunhofer-Institut für Mikrostruktur von Werkstoffen und Systemen IMWS, Walter-Hülse-Straße 1, 06120 Halle, Germany