Graduate Student Seminar Series - 2015

December 17, 2015

Structure-Properties relationships in Ti containing silicate glasses

Garth Scannell, Department of Materials Science and Engineering, Rensselaer Polytechnic Institute

TiO2 is known to produce unusual properties in silica glass, such as zero thermal expansion glass used in high thermal shock applications.  However, the role of Ti4+ in the silica structure to produce these anomalous properties is still poorly understood, largely due to ability of Ti4+ to adopt four-, five- and six-fold coordinations with oxygen.  My research focuses on the TiO2-SiO2 and Na2O-TiO2-SiO2 glass systems.  The structure and properties of the glass are studied from both elastic and plastic perspectives.  Elastically, structural information and elastic moduli are examined in in-situ high temperature and high pressure conditions using Raman light scattering and Brillouin light scattering, respectively.  Dilatometry experiments measure thermal expansion.  Plastically, in-situ high temperature indentation experiments are used to examine deformation and cracking behavior, while viscosity measurements provide a link between elastic moduli and deformation behavior.  The mode of plastic deformation can be measured through atomic force microscopy.  By examining how these various properties change with composition and in response to external stimuli, such as temperature or pressure, and comparing them to the observed changed in structure, it is possible to suggest structural roles for the glass components.

Predicting the evolution of misfit dislocation arrays in heteroepitaxial semiconductor thin films through experimentally refined simulations

Dustin Andersen, Department of Materials Science and Engineering, Rensselaer Polytechnic Institute

Dislocations can negatively impact the optoelectronic and electronic properties of semiconductor thin films and thus semiconductor devices. In particular, while III-Nitrides have very useful (opto)electronic properties, they also have significant densities of dislocations due to III-nitride (hetero)epitaxial layers generally being grown on non-native substrates. Misfit between substrate and epilayer produce a driving stress that generates new dislocations and extends pre-existing dislocations in the interface between substrate and epilayer where they relieve the misfit strain. A simulator of misfit dislocation nucleation and extension during heteroepitaxial film growth and annealing has been previously developed by Dr. Hull for the SiGe/Si(100) system, and which has recently been extended to the SiGe/Si(110) system. Experimental data is collected and used to refine the parameters of the simulator through comparison between the predicted and actual data sets. This experimental data is collected using TEM from as grown layers, and using in-situ annealing to observe dislocation motion and nucleation rates. This simulator is being further extended to the III-Nitride system, where it is being implemented for AlGaN grown on non-polar GaN substrates due the misfit stress being resolved onto the most operative slip planes.

November 19, 2015

Glass Toughness & Crack Arrest

J.H. Seaman, Department of Mechanical, Aerospace and Nuclear Engineering, Rensselaer Polytechnic Institute

Glass products are an integral part of modern society.  We appreciate the benefits and unique properties of glass when we look through a clear window, when we communicate via our smart phones and iPads, and more indirectly when we enjoy the internet and intercontinental communication.  Ordinary soda-lime float window glass, chemically tempered alkali-containing glass, and ultra-pure silica communication optical fibers are but a few advancements from over a century of glass science research.

More recent glass research has focused on leveraging the unique surface properties of glass, specifically the interaction of surface stresses and strain with diffused molecular water.  From this research has come a novel method of strengthening optical fibers, but also a tool to help explain many interesting phenomena found in the field of glass science, specifically in crack growth behavior.

This presentation will compare and contrast the strength and toughness of glass to other engineering materials, then discuss experimental evidence and quantitative modeling for special cases where glass exhibits higher toughness than expected.

In-Situ Initiation of Pitting Corrosion on Aluminum

Pinkowitz, Department of Materials Science and Engineering, Rensselaer Polytechnic Institute

While the growth of pits in passive metals exposed to chloride solutions is reasonably well understood and has been attributed to changes in chemistry in occluded cavities in the metal surfaces, the initiation of pits is still a matter of some conjecture. In particular, the processes associated with the propagation of stable pits, versus pits that form and apparently re-passivate, are not well understood. 

The goal of this work is to investigate the mechanisms of pit initiation by observation in situ to the TEM. A major challenge in observing the microstructural effects in pit initiation by electron microscopy in the past has been that there are structural and chemical changes between the hydrated passive film on aluminum, in situ to the corrosive medium, and the film after exposure to air during transfer to the electron microscope. The use of a microfluidic TEM holder allows us to observe the microstructure of an oxide film in its hydrated state and the microstructural detail of the metal from which the film is formed, with the intent of directly correlating local microstructure to stable pit initiation.

This presentation will focus on the advances made towards observing pit initiation, and the comparison of the thin film corrosion cell behavior to that observed in a bulk analogous system. Comparison of the Tafel behavior of the two systems, including variances in open circuit potential and stable/metastable pitting potential. In addition, ex-situ post-mortem analysis of several pits formed on the thin film cell was done, including Auger Electron Spectroscopy and Atomic Force Microscopy, in order to understand the chemistry changes that had occurred as a result of the corrosive environment.

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