This item is under embargo and not available online per the author's request. For access information, please visit http://libanswers.wustl.edu/faq/5640.
Date of Award
Doctor of Philosophy (PhD)
Controlling nucleation is key for the manufacture of glasses and glass-ceramics. It has been observed by different researchers in many silicate glasses that at low temperatures the critical work of cluster formation (i.e. the nucleation barrier) slowly rises rather than decreasing with decreasing temperature. However, this experimental observation is in contradiction with nucleation theories. In this dissertation, crystal nucleation was studied in BaO·2SiO2, 5BaO·8SiO2, and Na2O·2CaO·3SiO2 glasses. The main research topics include measurements of the nucleation rate, the structural evolution of the glass, and the low-temperature nucleation behavior. A special focus is given to the low-temperature nucleation anomaly. In total, there are four projects in this dissertation. In the first project, the nucleation rates are measured in BaO·2SiO2 and 5BaO·8SiO2 glasses following the time scale commonly used for silicate glasses by others in the literature. Similar to the results reported in many silicate glasses by different researchers, it is found that at low temperatures the observed nucleation barrier stabilizes or slowly increases instead of decreasing with decreasing temperature. In the second project, the structural evolution of BaO·2SiO2 glass is studied using synchrotron X-ray scattering and neutron scattering techniques with in-situ heating at different nucleation temperatures to determine whether this can provide insights about the nucleation process. The results show no systematic trend in the structural evolution as a function of either temperature or time. In particular, there is no structural phenomenon observed that can be the reason for the nucleation barrier anomaly at low temperature. In the third project, to test if the widely reported low-temperature anomaly is an artifact because of inadequate heating time at low temperatures, long-time nucleation measurement were made at low temperature for a 5BaO·8SiO2 glass. The heating time at the low temperature is much longer than any used in the nucleation measurements of silicate glasses reported in the literature. It is found that the widely reported low-temperature nucleation anomaly is actually an experimental artifact, at least in the 5BaO·8SiO2 glass composition. In the fourth project, a similar long-time low-temperature nucleation test was made in a Na2O·2CaO·3SiO2 glass. However, due to the significant crystallization that happens from concurrent nucleation and growth processes at low temperatures, no conclusive answer can be given about the low-temperature nucleation behavior in this glass. In summary, this dissertation gives the first direct experimental work conclusively showing that the widely reported nucleation anomaly at low temperatures is an artifact, at least in the 5BaO·8SiO2 glass composition, and serves as a motivation for further study of the low-temperature nucleation behavior in other silicate glasses.
Kenneth F. Kelton
Kenneth . Kelton, Katharine Flores, Erik Henriksen, Michael J. Krawczynski,
Available for download on Wednesday, August 17, 2022