Date: Tuesday, September 7, 2021
Time: 4:30 p.m.
Location: Whitaker Lab 303
Lehigh University's Materials Science and Engineering department would like to remind you about its upcoming speakers in it Fall 2021 Seminar Series. Seminars are open to all visitors. Attendance is required for all full-time MSE graduate students.
On Tuesday, September 7 at 4:30 p.m. EDT, Dr. Shen J. Dillon, Materials Science and Engineering, University of California, Irvine, will present, "Is Densification During Sintering Diffusion Rate Limited?," as part of Materials Science and Engineering's Fall 2021 Seminar Series. The event will be held in person at Whitaker Lab Room 303.
Abstract:
Sintering is an industrially important powder consolidation process that has gained renewed interest as a convenient process for densifying 3D printed powder compacts and materials of novel chemistry. Despite ≈70 years of scientific investigation, practical aspects of sintering continue to present challenges to our fundamental understanding of the process. This is highlighted by ongoing contentious debates around the roles of electric fields, high heating rates, and chemical additives in affecting sintering kinetics. Such experiments are primarily interpreted in the context of diffusion rate limited sintering models, where geometry, surface and grain boundary diffusivities, and surface and grain boundary energies are the primary variables. Coble first demonstrated that sintering kinetics were consistent with his celebrated model for a grain boundary diffusion rate limited process. Shortly thereafter Johnson and Cutler, however, demonstrated that bulk sintering experiments cannot effectively distinguish kinetic models due to the magnitude of the inherent noise in such data. The generally assumed kinetic model, as a result, has not been sufficiently experimentally validated to justify its broad application.
Revisiting this problem requires knowledge of the aforementioned diffusive and thermodynamic materials properties as well as geometric observations of model structures that can be quantitatively analyzed with errors sufficiently small to distinguish kinetic models. To address these issues, we have developed ultrahigh temperature small-scale mechanical testing methods based on laser heating and in situ transmission electron microscopy imaging. The methodology enables investigation of individual interfaces under conditions of creep under applied load and pressure-less sintering. Performing a series of bicrystal Coble creep, zero-creep, and unconstrained sintering experiments provide a basis for measuring interfacial energies and diffusivities, grain boundary point defect formation volumes, and activation volume for the rate limiting process. These data can identify the diffusion mediating defect and rate limiting mechanisms in different experimental regimes. They are applied to interpreting the sintering behavior of model 2-particle sintering configurations in fluorite oxides, and Al2O3-GdAlO3 composites observed in situ. The associated kinetic analyses indicate that, for the particles characterized, densification kinetics are interface reaction rate limited. Densification occurs during discrete steps, whose kinetics follow measured diffusivities, with longer intermittent incubation periods. It is hypothesized that these events are preceded by nucleation of climb mediating grain boundary dislocations. The associated temperature dependence provides valuable insights into the effect of temperature on sintering kinetics and residual stress evolution during sintering that will be discussed.
About The Speaker:
Shen J. Dillon is a Professor in the Department of Materials Science and Engineering at the University of Califonia Irvine. He received his B.S. and Ph.D in Materials Science and Engineering from Lehigh University in 2007. He worked as a Research Associate at Carnegie Mellon University and a Visiting Research Scientist at the Massachusetts Institute of Technology. He was on the faculty at the University of Illinois at Urbana-Champaign between 2009 and 2021. His scientific interests relate to understanding the key role played by inorganic interfacial structure-property relationships in affecting the performance of systems in extreme environments. Much of his recent work relates to developing and applying novel in situ characterization techniques that can be applied to understanding the dynamic properties of materials and their interfaces. He is the author of over 100 articles, and was a recipient of the 2011 Department of Energy Early Career Award, the 2013 National Science Foundation CAREER Award, and the 2015 American Ceramic Society’s Robert L. Coble Award for Young Scholars.