GOALI: Fluctuation Electron Microscopy Studies of Ultra-Low Expansion Glasses and Ceramics GOALI: Fluctuation Electron Microscopy Studies of Ultra-Low Expansion Glasses and Ceramics Copyright ULE is an ultra-low expansion amorphous material, developed by Corning Research and Development Corporation, that has near-zero coefficient of thermal expansion (CTE) over a wide range of temperatures. It is used in precision measurement technology and is critical for suppressing thermal drift in precision lithography. The temperature at which the CTE is zero depends on the composition (TiO2-SiO2), which yields various structures ranging from glasses to glass ceramics (collectively referred to as ULE glass). Ideally, the material should have a low rate of change of CTE with temperature. Knowledge of the detailed structure of ULE glass is important if we are to optimize its temperature response. Determining the structure of diffraction-amorphous materials is much harder than it is for crystals. The structural tools that work well for crystals are insensitive to the details of disordered structures. Fluctuation Electron Microscopy (FEM), which will be used and further developed in this study, examines the variations in diffraction between small volumes of the material. The statistics of the scattered intensity (mean and variance) depend sensitively on the details of any medium-range order (MRO) present at length scales less than about 2 nm. With modeling, FEM data allow us to infer the basic MRO granular structures of the material. Because amorphous materials are damaged easily by the electron beam, data is strongly affected by displacement decoherence, and static models do not reproduce the data accurately. This study will apply FEM, and other electron microscopy methods, to study ULE materials and will advance FEM as a quantitative experimental technique, by exploring and modeling the various decoherence mechanisms, which are strong in beam-sensitive materials such as ULE materials. Experimental FEM work on ULE glass opens up the possibility of gaining a clearer view of the framework structure and, in particular, how the titania component blends with the silica. It is already established that titanium is tetrahedrally coordinated (like silicon) at low Ti concentrations, but is octahedrally coordinated at higher concentrations, indicating phase segregation with associated medium- to long-range order. The structural transition from tetrahedral to octahedral coordination of Ti will be explored by FEM, which is exquisitely sensitive to medium-range order. This project offers an opportunity to develop FEM experimental protocols for optimizing the signal-to-noise for structural information, while minimizing the structural disruption by the beam-an important advance for beam-sensitive materials, such as the ULE glass/ceramic phases. In parallel, new methods to simulate diffraction in the presence of displacement decoherence (beam damage) will be developed. Together, these advances will allow us to more accurately invert FEM data to give a clearer view of the structure details underlying medium-range order in ULE materials. The development of an advanced ULE glass, with low rate of change of CTE as a function of temperature, will benefit applications where temperature-stable precision measurement and manufacturing are of great importance. Developing FEM so that it becomes a more quantitative technique will benefit all areas of research into the structural details of amorphous materials. The graduate student will get to interact with researchers at the Corning Research and Development Corporation and will get to see how the glass industry functions first-hand. As part of the Education and Outreach in this project, High School students will work with the PI and the graduate student on basic structural models. Students will build physical models to learn about crystalline and disordered structures and will engage in research on flexibility and segregation in network materials. Students will present their work at a local Science Fair each spring.
|Effective start/end date||7/1/19 → 6/30/23|
- National Science Foundation (NSF): $536,611.00
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