Structure and Crystallization of Iron-Rich Borosilicate Based Nuclear Waste Glasses (09/01/24)
Speaker and Affliation:
Dr. Nikhila Balasubramanya
Development Scientist
Corning Incorporated, NY, USA
When?
9th January, 2024 (Tuesday), 3.00 PM (India Standard Time)
Where
K I Vasu Auditorium, Dept. of Materials Engineering, IISc, Bangalore
Abstract:
The vitrification of nuclear wastes with high Fe, Ni, and Cr fractions often results in the unwanted crystallization of spinels in the Joule-heated ceramic melters, posing a threat to its efficiency and potentially limiting waste loading. While empirical constraints are in place to design compositions and limit spinel crystallization, and empirical models are in place to predict the behavior of the compositions, there are always some compositions whose behavior is beyond the predictive ability of the models. My Ph.D. study was an in-depth investigation of the chemo structural descriptors controlling the crystallization behavior in these glasses. It primarily focused on (i) the impact of non-framework cation mixing and (ii) the impact of mixed-network former effect on the structure and crystallization of glasses in the system Na2O - Al2O3 - B2O3 - SiO2 - Fe2O3 - MnO - NiO - Cr2O3 where Fe2O3 = 9 mol%. (iii) Iron redox kinetics in various compositional regimes of interest. A suite of characterization techniques such as X-ray diffraction (XRD), vibrating sample magnetometry (VSM), differential scanning calorimetry (DSC), Raman spectroscopy, magic angle spinning-nuclear magnetic resonance (MAS NMR) spectroscopy, and Mössbauer spectroscopy were employed. The studies suggest that (i) generally, the tendency to crystallize increases when Na+ is replaced with Li+ or Ca2+ i.e higher field cation, and, the tendency to crystallize also increases with (ii) increasing Al2O3/SiO2 and B2O3/SiO2 ratios, which correlates to the preferential charge compensation of the Na+ amongst the tetrahedral network forming units (Al, B, Fe) in the order Al > B > Fe, and further leads to an increase in Fe2+/ΣFe and, (iii) resultant unmixing of the aluminoborosilicate network leads to the preferential ion enrichment in the solution favorable to crystallization.
Biosketch:
In pursuit of my interests in Materials, Mechanics, and Design, I obtained a bachelor’s degree in mechanical engineering. Later, I pursued M.Tech in Materials Sciences at IIT-Kanpur where I worked on synthesizing and electrically characterizing chalcogenide thin films towards understanding the switching rates eventually developing a high-performance, scalable solid-state device. Moving forward, as a Materials Scientist at the R&D center of the Aditya Birla Group, I worked on the synthesis and development of a novel cement product. I also worked on the material modeling of high-strength concrete and fracture analysis of its pre-cast structures. With an interest in amorphous materials, later, at Rutgers- The State University of New Jersey, my Ph.D. research focused on understanding the impact of melt chemistry, structure, and iron-redox on spinel crystallization in iron-rich nuclear wastes under the guidance of Prof. Ashutosh Goel. A suite of Characterization techniques including Raman spectroscopy, Mössbauer spectroscopy, and high-field high-speed Nuclear Magnetic Resonance (NMR) amongst others was employed. The results of the investigation have made it abundantly clear that the alkali and alkaline earth cations with higher ionic field strength promoted crystallization. While the clustered iron-rich regions provided a lower energy path for spinel crystallization, the degree of heterogeneity in the network was found to control crystallization. The empirical relationships that are believed to represent compositional control of crystallization in the regimes of interest to the nuclear waste community are further refined by the results obtained from the study. Currently, I am working as a Glass melting process engineer/Development scientist at Corning Incorporated where I apply rigorous scientific techniques and methods to (i) improve glass melting process capability and (ii) develop incremental technology to mitigate and/or optimize existing process systems in manufacturing. I love to cook and please my taste buds as well as others’ too. I paint and sketch whenever possible. Badminton and Hindu philosophical texts are some of my other interests.