US-German Materials Collaboration: Unusual Stability of Amorphous

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This research program is focused on the relationship between synthesis, structure and properties of a new class of ceramic materials that are made directly from polymers. They are becoming known as PDC's, or polymer-derived-ceramics. Currently two families of PDC's, silicon carbonitride (SiCN) and silicon oxycarbide (SiCO), have been identified. The PDC's are neither a crystal nor a glass; instead they contain nanodomains, about 1-5 nm in size. These nanodomains persist to very high temperatures, up to 1500oC; that is, their nanodomain structure resists large-scale crystallization at temperatures where the stoichiometric constituents (Si3N4, SiC and SiO2) would readily crystallize. The excess carbon in the composition is known to influence this behavior but the materials science of PDCs remains largely un-understood. In this project Professor Navrotsky and her research group at the University of California Davis will collaborate with the faculty and students at the University of Colorado and with scientists and students from three institutions in Germany (Universities of Stuttgart, Darmstadt and Aachen) to elucidate (i) the evolution of the nanodomain structure from the structure of the organic polymeric state during controlled pyrolysis, and (ii) the relationship between the nanostructure of SiCN and SiCO, and their alloys, and their properties, e.g. crystallization, resistance to creep, and thermodynamic and kinetic behavior. One of the key features of the program is to highlight the use of non-conventional techniques for characterizing the nanostructure and thermodynamics and the modeling of properties of these non-traditional ceramics. NMR and FTIR spectroscopies will be used to identify the first (and to some extent the second) nearest neighbor configuration of molecules, Raman spectroscopy to quantify the short range and long-range bonding of carbon atoms, and small-angle-X-ray-scattering will be employed to characterize the size and the distribution of nanodomains. The thermodynamic predictions of the structure derived from the above spectroscopic information and molecular modeling will be checked against measurement of the enthalpies of formation and crystallization by high temperature calorimetry. The interfacial energies of the nanodomains predicted by molecular modeling will be measured directly in this way. These calorimetric studies are the major and unique contribution of the group at Davis. This information will be built into models that can explain the unusual resistance of PDC's to crystallization, for example. This project will educate young people in interdisciplinary and international research. Students from both countries will visit sister institutions across the Atlantic for extended periods to foster international scientific collaboration.Nanotechnologies for high temperatures (space and energy are two examples) require materials which must have a special property: their structure must remain stable at temperatures where atoms can move and change the structure. This program is focused on a new class of ceramics, called polymer-derived-ceramics or PDC's that resist this time and temperature dependent change. The PDC's are robust and multifunctional materials which will usher in breakthrough technologies such as sensors, electronics and optoelectronics, and coatings that can be used in extreme environments. The objective of this project is to develop a scientific base of fundamental understanding of PDC's, which is not present, and which is essential for these new technologies to move forward. The program derives great leverage through international collaboration with German scientists, and involves two major universities, one in Colorado and the other in California in this important endeavor. This project will educate young people in interdisciplinary and international research. Five to seven doctoral graduate students and/or post-doctoral fellows from the US and Germany will participate in this collaborative effort. Undergraduate students from Colorado and UC Davis will serve as research interns in this program for long terms. Students from both countries will visit sister institutions across the Atlantic for extended periods to foster international scientific collaboration.