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

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Solid-state Nuclear Magnetic Resonance will continue to be used to study the structure and dynamics of two classes of oxide glasses that have major technological applications, namely boron-containing glasses and oxyfluoride glasses. We will acquire quantitative microscopic information on borate, borosilicate, aluminoborate, and oxyfluoride glasses at ambient and high temperature, including systems containing diamagnetic analogs of rare earth elements, and will interpret and model existing thermodynamic and transport property data in light of these new findings. We will continue to emphasize temperature effects, with both in situ, high temperature measurements and studies of glasses prepared with varying thermal histories, which record the liquid structure at a range in temperature. We will use a wide range of NMR methods (on nuclides including 11B, 170, 19F, 23Na, 25Mg, 27AI, and 29Si), supplemented where needed by other methods such as Raman and infrared spectroscopy. Instruments available at Stanford include spectrometers with 9.4, 14.1, and 18.8 Tesla magnets (the latter the highest field strength currently commercially obtainable), with sophisticated high-speed magic-angle spinning, double and triple resonance, and high temperature NMR probes. A newly-developed rapid quench apparatus that will allow thermal history studies of small, valuable, isotopically enriched samples to be done. Applications of new NMR methods, in particular multiple quantum NMR, will continue to play a major role, as will the empirical calibration of NMR observables with structure of known crystalline model compounds. Models linking the microscopic and macroscopic will continue to be developed and tested and a new program of ab initio energy calculations, based on density functional theory, will be used to complement experimental results. Boron-containing oxide glasses are widely used in corrosion- and temperature-resistant containers, pipes and tanks, in "fiberglass" composites, in optical components, computer display screens, etc. Borosilicate glasses are also likely to play a major role in sequestering radioactive wastes. The ease with which the boron cation changes its local structural environment as a function of composition and temperature not only contributes to the useful properties of these materials but makes them a unique and intriguing subject scientifically. In oxyfluoride glasses, some oxygen ion is replaced by fluoride, again giving the resulting glasses and glass-forming liquids unique properties, and again posing a wealth of fundamental scientific issues concerning the structure and dynamics of mixed-anion systems. Fluoride has long been used to lower viscosities and melting temperatures of glass-forming liquids without clear understanding of mechanism; in recent high-tech innovations, oxyfluoride glasses are becoming interesting as hosts for rare earth elements in laser and optical amplifier materials. In all of these materials, the ability to tailor their properties to specific technological applications requires quantitative knowledge of their structure at the atomic scale, and the dynamics with which that structure changes in the precursor high-temperature glass melts. Nuclear magnetic resonance "sees" the local atomic structure and dynamics, often in a highly quantitative way, around selected isotopes of many of the most important constituents of oxide glasses. NMR has thus proven to be a near-ideal tool for studying such non-crystalline materials, and has increased our understanding of them enormously. This project should have direct interest to the Materials Science community, but its results will also have real significance to physicists working on the dynamics of glass- forming and other complex liquids, to physical chemists developing new applications of solid-state NMR, and to geochemists trying to model and predict the behavior of silicate magmas in nature. In the past, and hopefully in the future, the discipline-crossing nature of this type of study has broadened the perspectives of students in our group, with backgrounds in spectroscopy or geochemistry, into the fascinating and technologically important world of glass and ceramic sciences.