1) Structural Basis for Tougher and Less Brittle Glasses
Using a combination of Brillouin and Raman light scattering techniques, and large-scale computer simulation and modeling methods, the main objective of this project is to develop a fundamental atomistic understanding of the response of glasses to external thermal, mechanical and radiation impacts, and an ability to tune these properties by changing the structure of glass through processing in a controllable way. In this research, one of our major efforts is to develop a scientific basis for utilizing light scattering techniques as structural characterization tools for glass, by studying the pressure and temperature dependent vibrational and elastic properties of glass using Raman and Brillouin light scattering (Figure 1).
Figure 1. Schematic diagram of the concurrent Brillouin and Raman light scattering setup. Abbreviations are: M, mirror, BS/FM, beam splitter or flipping mirror. Sample holder can be a goniometer for measurements under ambient conditions, a heating stage for high temperature measurements, or a diamond anvil cell (DAC) for high pressure measurements.
In this project, we will develop a pressure-quenching technique and synthesize glasses that have a high thermo-mechanical resistance when subjected to large temperature and pressure fluctuations, which will find a wide range of uses in pressure sensors, optoelectronics and space applications. By quenching silica glass under pressure, its propensity to undergo irreversible densification under UV radiation can be eradicated. Such pressure-quenched silica glass can potentially increase the lifetime of many optical components in the semiconductor industry. The pressure-quenching technique established through this program can be generalized to synthesize glasses with novel properties.
Supported by NSF (DMR -907076)
2) Thermometry from Nanoparticle Assembly Microstructure Fingerprinting
Nanoparticles are intrinsically characterized by a large ratio of surface to volume atoms. This dramatically modifies many electronic, vibrational as well as thermodynamic properties that are well defined for the bulk. A notable example is the size-dependent melting point depression, which has been observed in many nanoparticles (e.g. Au, Al, Sn, CdS, Ge, etc) as shown the left pane of Figure 2. A closely related behavior is the sintering temperature depression in nanoparticles that strongly impacts nanoparticle coalescence. Another phenomenon associated with the particle size reduction is the phonon confinement effect, which comes from the fact that a description of atomic vibrations in nanoparticles in terms of phonons with a well-defined wave vector is no longer valid because of the crystal surface. The lattice vibration waves get reflected and/or decay at the boundary, but still remain confined within the particles. This spatial confinement inside nanoparticles gives rise to the peak shift in Raman spectra compared to bulk crystals (see right pane of Figure 2).
Figure 2. (Left) Size dependence of the melting points of Sn nanoparticles (Ref: P. Buffat et al., 1976); (Right) Raman frequency shift in anatase TiO2 nanoparticles as a function of grain size (Ref: V. Swamy et al., 2005).
In this project, we will investigate the combined use of phonon confinement effect together with size-dependent melting/sintering temperature depression in core-shell nanostructures as thermosensors. Raman spectra of the TiO2 core can be measured using a hand-held spectrometer on site without much sample preparation. This will determine sizes of the initial nanoparticles used after a thermal event, no matter what the final microstructures look like. At the same time, the assembly microstructures of nanoparticles induced by size-dependent coalescence/melting in the shell will record the temperature range they experienced. Such thermosensors have the potential to forensically retain the complete thermal history (spatial and temporal variation) of a thermal event under extreme conditions ranging from tens to hundreds of kilopascals of pressure (100 KPa), hundreds of degrees of temperature (up to 700°C), and microsecond (1 µs) changes of these conditions.
Supported by DTRA (HDTRA1-09-1-0046)
3) Multiscale simulation and modeling of chemical reactions with diffusion for energy applications:
This project is to develop and test a suite of methods spanning the electronic, atomistic and continuum scales to treat simultaneous reaction, diffusion and adsorption. These methods will be applied to the study of chemical reactions of energy interest, including: (a) decomposition reactions of small molecule (e.g., CH4 and H2O) on bare and transition metal decorated carbons (Figure 3) to produce hydrogen in the absence of carbon oxides, (b) applications of metal catalyst/titania nanocomposites (e.g. Pt-TiO2, and Au-TiO2) for splitting of water to produce hydrogen, and CO oxidation at low temperatures to purify hydrogen feedstock for fuel cells, and (c) electrochemical reactions and transport processes in fuel cell and supercapacitor electrodes.
Figure 3. (a) SEM image for Ordered Hierarchical Nanostructured Carbon (OHNC) and (b) HRSEM image for the Pt (60 wt %)/OHNC. (Ref: B.Z. Fang et al., 2009)
Supported by NSF (CHE-1012719), in collaboration with Prof. Keith E. Gubbins in the Department of Chemical and Biomolecular Engineering Department at the North Carolina State University.
4) Quantitative understanding of atomic wear using accelerated molecular simulations:
The goal of this proposal is to develop a molecular-level understanding and a quantitative description of atomic wear during single-asperity sliding. We will employ a novel accelerated molecular dynamics algorithm to approach the experimental time scales. By simulating tip-sliding at various speeds, loads, contact areas and temperatures that are close to experimental values, we will establish a quantitative relation between the wear rate and loading conditions, and verify the applicability of the linear wear law and the nonlinear bond rupture model in the atomic wear regime.
Supported by NSF (CMMI-1031408), in collaboration with Prof. Yunfeng Shi in the Department of Materials Science and Engineering Department at Rensselaer.
5) GOALI/Collaborative: Impact of Mixed Network Formers on the Structure and Properties of Oxide Glasses:
In this project, we will establish a general methodology for developing a new set of interatomic potentials for systems consisting of mixed network formers with a focus on aluminosilicate, borosilicate, and boroaluminosilicate glasses. These potentials are based a common functional form to capture the coordination variation and charge transfer in such complex systems (see figure below). A general procedure will be developed to fit potential parameters to the structure and properties of glasses obtained from our integrated experimental work on well-designed glass compositions of these systems. Subsequent simulations will be performed to obtain detailed structures of these glasses and to understand the normal and anomalous thermal, dynamic and mechanical properties of these glasses. The information obtained will guide the rational design of glass compositions for various technological applications utilizing these glasses.
(a) Graphical representation of the three-body potential term as a function of the angle between adjacent bonds and the effective coordination number. Negative energies are plotted upwards for better visualization; (b) sketch of the neighbor-cutoff spheres and the function to determine the effective coordination number Zi of the central particle i.
Supported by NSF (DMR-1105238), in collaboration with Prof. Jincheng Du in the Department of Materials Science and Engineering at the North Texas University and Dr. John Mauro in Corning, Inc.
Funding: Our research is currently supported by NSF (DMR-907076; CHE-1012719; CMMI-1031408), DTRA (HDTRA1-09-1-0046) and RPI start-up.