John Kieffer

Professor

kieffer@umich.edu

2018 HH Dow

T: (734) 763-2595

Bio

Projects

Publications

Facilities

Group

Openings

CV


Characterization of Amorphous Systems via Inelastic Light Scattering

Sponsor: National Aeronautic Space Agency (NASA)
Specific glass classes, like the chalcogenides, are interesting because of their practical application to applications that can take advantage of nonlinear optics, such as optical waveguides. To better understand and manipulate these properties it is necessary to investigate their origins, which is related to the atoms and molecules but also to the very structure of these materials. With the myriad of glasses that can be formed, one of the primary challenges in working with amorphous materials is simply picking the right glass for the right application. While the properties of these glasses are readily observed, it is less obvious how the structure of these glasses is related to these properties. Continuum random network (CRN) theories offer much insight into the formation and behavior of glasses, but the abundance of modified CRN theories, seems to suggest that there is more to the properties of these materials than a random network.
We are exploring the evolution of structures in glassy materials by exploring the connectivity between the molecules using Raman and Brillouin inelastic light scattering. Brillouin light scattering allows for high frequency phonons to be probed, which can in turn yield information about the complex modulus and viscoelastic properties of the sample. We will be producing amorphous materials via sol gel synthesis in order to fully control the stoichiometry and formation of the glasses we study. By using an array of techniques to more completely characterize these glasses, fundamental questions can be investigated. Does polyamorphism play a role in determining the properties of glassy materials? What are the exact mechanisms of glass formation, and how do they change in different environments? The possibilities for novel materials in the optoelectronics field make these very interesting science questions into valuable engineering problems.
Highlights (Click an image for more information)
  • Irreversible Transformations in Simulated Silica Glass

    We observed reversible and irreversible transformation in simulated silica glass under pressure. Reversible transformation explains the anomalous thermo-mechanical properties of silica glass, while irreversible transformation leads to the permanent densification in recovered silica glass. Note: arrows indicate the compression and decompression cycle.

  • Negative Thermal Expansion in Cristobalite and Quartz Silica

    Using molecular dynamics simulations, we successfully reproduced the first-order and second-order alpha to beta transformation in cristobalite and quartz silica, respectively. Our studies reveal the structural origin of the negative thermal expansion in the high-temperature beta phases. Note: arrows indicate the transformation temperatures.

  • High Pressure Silica Phase

    Based on concurrent molecular dynamics simulations and ab initio calculations, we uncovered a ubiquitous mechanism for the densification under pressure of minerals, such as silica, that have open structures due to their rather rigid polyhedral building blocks. Our findings finally solve the mystery with regard to the nature of the so-called X-I phase (high-pressure cristobalite silica), concluding an elusive quest that originated over a decade ago. Note: red spheres represent oxygen atoms, the blue ones are silicon atoms.