Katrin Thieme

The mechanism of deceleration of nucleation and crystal growth by the small addition of transition metals to lithium disilicate glasses

The addition of small amounts of niobium or tantalum oxide to lithium disilicate glass provokes a drastic decrease of the steady-state nucleation rates and the crystal growth velocities. The viscosity of the residual glassy matrix is considered as a function of the crystallization degree in the course of a non-isothermal crystallization. For simplification, a homogeneous distribution of the added oxides in the glass matrix is assumed. While the viscosity initially decreases, it significantly increases again for higher crystallization degrees hindering crystal growth. However, it was shown that the additives are enriched at the crystal interface. Several possible reasons for the inhibition of nucleation and growth kinetics such as viscosity, interfacial energy crystal/glassy phase, thermodynamic driving force or impingement rate are discussed. Since the crystallization front is blocked by the additives the impingement rate is decreased with increasing additive concentration. Since small concentrations of Nb2O5 and Ta2O5 have a drastic effect on the nucleation, these components should be enriched at the interface crystal/glass. This will only take place, if it leads to a decrease in the interfacial energy. Since this effect alone should result in an increase of the nucleation rate, it must be overcompensated by kinetic effects.

Surface crystallization of low thermal expansion Ba0.5Sr0.5Zn2Si2O7 from an 8 BaO·8 SrO·34 ZnO·50 SiO2 glass

Thermal treatment of a glass with the composition 8 BaO·8 SrO·34 ZnO·50 SiO2 has led to the crystallization of a Ba1−xSrxZn2Si2O7 solid solution. This solid solution has a very low or even negative thermal expansion. The glass system possesses a strong tendency towards surface crystallization, while bulk nucleation is negligible. The surface crystallization behavior was characterized using differential scanning calorimetry, X-ray diffraction, optical microscopy, and scanning electron microscopy including electron backscatter diffraction. The observed morphology strongly depends on the type of surface pretreatment. Two different surface qualities were compared: a polished surface and a surface obtained by cutting. The as-cut surface shows a significantly enhanced nucleation rate, which leads to smaller crystals caused by a growth selection near to the surface. The crystal orientation related to the inward growth starting from the surface was successfully investigated by EBSD, for the first time, using a recently reported crystal structure. The crystals show a preferred orientation of the c-axis perpendicular to the surface; this has a significant effect on the crack formation.

Redox effects and formation of gold nanoparticles for the nucleation of low thermal expansion phases from BaO/SrO/ZnO/SiO2 glasses

Glasses in the system BaO/SrO/ZnO/SiO2 containing 0.01 and 0.1 mol% gold were used to study the formation of gold nanoparticles with the aim to use them as nucleation agents. In order to promote gold clustering, the glasses were additionally doped with 0.5 mol% Sb2O3. Depending on the heat treatment schedule, Au particle sizes were in the range from 6 to above 50 nm. In contrast to many other gold ruby glass systems, the clustering is completely prevented by the absence of antimony; then the glasses remain colorless. Surprisingly, at higher temperatures, a re-dissolution of gold clusters was also observed, which now allows the formulation of a more comprehensive model concerning the redox and clustering behavior. This growth model is completed by the fact that a high gold concentration enables the stabilization of much smaller Au clusters. Mie theory with the aid of quantum confined size-dependent dielectric functions was successfully used to describe the optical behavior of the gold nanoparticles also for sizes below 10 nm. These results were confirmed using high resolution scanning transmission electron microscopy, including energy dispersive X-ray spectroscopy. It could also be shown that small gold particles up to a size of 50 nm are not effective as nucleating agents.

WO3 as a nucleating agent for BaO/SrO/ZnO/SiO2 glasses – experiments and simulations

Recently, it has been shown that the Ba1−xSrxZn2Si2O7 crystal phase has a negative coefficient of thermal expansion. However, technological applications of this material as low thermal expansion glass ceramics are limited by the undesired surface crystallization of the corresponding glass. Surface nucleation, however, can be turned into bulk nucleation by adding nucleating agents. Here, glasses in the base system BaO–SrO–ZnO–SiO2 with small additions of ZrO2 and WO3 were synthesized and their crystallization behavior was investigated using thermal analysis, X-ray diffraction, electron microscopy, and density functional theory simulations. The addition of WO3 leads to the formation of volume crystals with a scheelite crystal structure (Ba1−xSrxWO4) in high number density. The limited incorporation of Si4+ ions into these crystals is discussed. Possible crystal lattice sites for Si4+ were located by density functional theory simulations. In a much lower number density, crystals with a crystal structure similar to the high-temperature polymorph of BaZn2Si2O7 and the Ba1−xSrxZn2Si2O7 composition crystallize as well. These crystals can reach a larger size than the scheelite crystals that occur in parallel. The overall microstructure is thus formed by small dendritic crystals with a scheelite structure and huge Ba1−xSrxZn2Si2O7 crystals. Both of them seem to grow independently of each other. In spite of the high anisotropy of both phases, the microstructure is revealed to be free of cracks.