Structure and Dynamics in Boron-Containing Oxide Glasses andLiquids: High-Resolution and High-Temperature Nuclear Magnetic Resonance Studies

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9802072 Stebbins The objective of this project is to use nuclear magnetic resonance (NMR) spectroscopy to acquire quantitative structural and dynamical information on borate, borosilicate, and boraluminate glasses and liquids at ambient and high temperature, and to interpret and model existing macroscopic thermodynamic and transport property data in light of these new findings. The fundamental difference between a glass and the corresponding liquid is that the structure of the latter can change with temperature rapidly enough to remain in equilibrium. This process results in often large effects on properties that must be assessed to accurately describe melt behavior during glass melting and processing. Structural change with temperature also controls the mechanical relaxation during annealing of glasses, which in turn is an essential step in making technologically useful glassy materials. Extensive data sets exist on these macroscopic properties, and sophisticated phenomenological models have been formulated to describe and quantify relaxation. However, these models in general lack empirical data on temperature effects on either molecular-scale structure or dynamics and thus are limited in their predictive value: when extrapolated hundreds (or even 1000) degrees up to temperatures of glass melting furnaces, they may even be grossly in error. The studies proposed here will use NMR on a variety of nuclides at ambient temperature to determine the effects of varying fictive temperature on glass structure, which will constrain T effects on liquid structure in the vicinity of the glass transition. In situ, high resolution, high T NMR on these nuclides will also be used to determine T effects on average coordination environments to temperatures as high as 1500 C. The effects of observed structural changes on entropy, enthalpy, activities, molar volumes, and other properties will be assessed using simple thermodynamic models. At lower temperatures but sti ll in the liquid range, static and MAS NMR will be used to observe exchange among structural species and measure their exchange rates. Spin-lattice relaxation times will be measured to gain dynamical information over wider ranges in temperature. These results for molecular-scale dynamics will be compared to shear relaxation times derived from existing data on viscosity, and to data on electrical conductivity and diffusivity. Models linking the microscopic and macroscopic will be formulated and tested. %%% Boron-containing oxide glasses, and their corresponding high temperature liquids, are of wide-ranging importance in industry and society, finding use in corrosion- and temperature resistant containers, tanks and pipes, fibers in structural composites, optical components, computer display screens, etc. Borosilicate glasses are also likely to be of major importance in the solidification and sequestering of radioactive wastes. This research project will have a major impact on the understanding of glass formation and relaxation needed to improve the properties of commercial products. ***