The Division of Materials Research and the Office of Cyberinfrastrcture contribute funds to this award. It supports computational research and education to further develop advanced materials simulation methods and to characterize the mechanisms and dynamics of the high-temperature reactive processes associated with the oxidation and hydroxylation in one class of ionic-covalent materials: metallic nitrides. Nitrides are chosen as the focus of this work both as archetypes of high-temperature reactive materials and because of the large amount of available experimental information, largely driven by their wide range of applications.
The dynamics of reactive processes at surfaces are often very different at high temperatures than at low temperatures. In some cases, entirely new processes are activated as the thermal energy allows ever higher energy barriers to be overcome. In addition, even well-known low-temperature processes, such as those associated with surface oxidation and hydroxylation, manifest new personas at high temperatures. This is most evident in systems with mixed ionic and covalent bonding, which are largely non-reactive at low temperatures, but become highly reactive at high temperatures.
Using a judicious combination of electronic-structure density functional theory calculations, density functional theory-molecular dynamics simulations, and classical, atomistic molecular dynamics and temperature-accelerated dynamics simulations that will be compared to, and validated by, data from experimental collaborators, the PIs will focus on answering three key questions: (1) How does composition and structure control surface reaction mechanisms? (2) How do the surface reactions modify the surface? and (3) How does surface microstructure and stress affect surface reactivity?
Atomistic simulations will be facilitated by, and build upon, the PIs previous work by extending the many-body, variable-charge, reactive Charge Optimized Many Body potentials, called COMB potentials, they have previously developed. The PIs will model the chemical processes associated with the reaction of molecular oxygen and water vapor on the nitride surfaces. In particular, the key issues to be examined include adsorption and dissociation of oxygen and/or water molecules on the surfaces, the incorporation of individual oxygen atoms into the surface structure, oxidation of the first atomic layer of the nitride, and diffusion of oxygen through the material.
The PIs aim to obtain a systematic atomic-level understanding of the complex processes involved in the development of passivating oxide layers, catastrophic oxidation, and hydroxylation processes. In addition, the research will provide the foundational understanding needed for the development of systematic approaches to the mitigation of these degradation processes, key to the further exploitation of this class of materials at high temperatures. The continued development of the COMB methodology will firmly establish it as a powerful method for the simulation of systems in which metals, ionics, semiconductors and gaseous phases coexist and interact both physically and chemically. This will open up a large number of physio-chemical problems involving the chemical degradation of a wide range of materials to investigation with atomistic methods. Among these problems are: oxidation of nitride nuclear fuels, complex chemistries involving oxygen and nitrogen for fossil fuel application, catalysis, corrosion, and protection against hot corrosion and thermal oxidation of alloys.
The PIs will visit colleges and universities in the Southeast, especially Historically Black Colleges and Universities and Hispanic-serving institutions, to discuss the graduate program in Materials Science and Engineering at the University of Florida in general, and their computational program in particular.
They will disseminate implementations of the COMB framework in well-known computer codes for lattice statics and molecular dynamics simulations, respectively. The PIs will also bring together the community working on reactive potentials in an MRS Symposium and a symposium at the Fall MS&T meeting. The PIs intend for the MRS activity to be accompanied by a one-day tutorial program aimed primarily at graduate students and postdocs.
NONTECHNICAL SUMMARY The Division of Materials Research and the Office of Cyberinfrastrcture contribute funds to this award. It supports computational research and education to develop advanced computer simulation tools and to apply them to study materials at high temperatures. The computational tools will be able to simulate materials at the atomic level and include not only physical processes but chemical processes as well. This is a challenge in computational materials research. Materials can behave quite differently at temperatures well above room temperature. For example, some materials are strong and seemingly inert in their environment at room temperature, but at the elevated temperatures typical inside, say, aircraft engines they may oxidize and degrade rapidly. The PIs will use their advanced simulation tools to understand reactions that occur on the surfaces of materials and how they affect the surfaces of materials. They will focus on classes of materials that are well studied in the laboratory and have technological applications. These include some metallic nitrides that are used to make hard coatings on cutting tools and machine parts and for hard coatings on prosthetic hips.
The PIs will disseminate computational tools developed under NSF support through computer code packages for materials simulation that are popular in the materials research and other scientific communities. The PIs will also disseminate information about their simulation tools by bringing the materials simulation community together through professional society meetings that will include tutorial programs for graduate students and postdocs.
The PIs will visit colleges and universities in the Southeast, especially Historically Black Colleges and Universities and Hispanic-serving institutions, to discuss the graduate program in Materials Science and Engineering at the University of Florida in general, and their computational program in particular.
The dynamics of chemical reactions at surfaces are often very different at high temperatures than at low temperatures. In some cases entirely new processes take place. In other cases, even well-known low-temperature processes, such as those associated with surface oxidation (attack by oxygen) and hydroxylation (attack by water), are very different at high temperatures, and are often much more aggressive. This is most evident in systems with mixed ionic and covalent bonding, such as nitrides, which are very resistant to oxidation and hydroxylation at low temperatures, but become highly vulnerable at high temperatures. The intellectual merit of this project was two-fold. First, we characterized the mechanisms and time evolution of the high-temperature reactive processes associated with the oxidation and hydroxylation of metallic nitrides. Second, we developed new mathematical descriptions of the interatomic interactions for materials systems of interest, most particularly for TiO2 – TiN – O2. The broader impacts of this work include the development of powerful computer simulation methods to address oxidation and hydroxylation problems and their dissemination to other researchers through a widely used, and freely available, computer code distributed by Sandia National Laboratories. The technical results of this work have been disseminated in multiple technical publications and in multiple invited and contributed presentations at scientific conferences. The vibrant and dynamic collaborative environment that PI and Co-PI engendered for this project directly supported the education of 5 undergraduate students (including 3 women) and 7 graduate students (including 3 women), 5 of whom have graduated (4 PhDs, 1 terminal MS). The PI and Co-PI also mentored the summer research experiences of 4 high-school students, 2 of whom were from under-represented groups.
