Designing glasses with tunable structure and properties by computer simulation
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1229-LL08-08
Designing glasses with tunable structure and properties by computer simulation
Liping Huang*, Fenglin Yuan and Qing Zhao Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, NY 12180, USA
ABSTRACT A normal solid becomes stiffer when squeezed and softer when heated. In contrast, silica glass behaves the opposite way: its elastic moduli decrease upon compression and increase upon heating. Silica glass is also known to densify under compression and radiations. These have been long-standing mysteries in materials science. Using molecular dynamics simulation, we uncovered the structural origins of the anomalous thermo-mechanical behaviors and mechanisms of permanent densification in silica glass. Accordingly, these anomalies can be attributed to localized structural transitions, analogous to those that occur in the crystalline counterparts. The irreversible densification in silica glass is achieved through structural transition involving bond breaking and re-formation under a combination of high pressure and temperature. We further revealed that the anomalous thermomechanical behaviors are inherently connected to the ability of the glass to undergo permanent densification. Our computer simulations demonstrate that by processing in ways that gradually eliminates anomalous thermo-mechanical behaviors, degree of the glass to undergo densification can be eventually eradicated. This provides the conceptual foundation for the bottom-up design of new glasses with tunable structure and properties.
INTRODUCTION Discoveries in the past three decades strongly suggest that two or more distinct amorphous states may exist for the same material. This phenomenon, termed ‘polyamorphism,’ is one of the most intriguing and puzzling topics of condensed matter physics. It has been observed in various classes of materials, such as amorphous ice, silica, boron oxide, silicon, and chalcogenide glasses1-15. These materials tend to have open and less dense network structures. As one manifestation of their unique topologies, these network structures exhibit a number of anomalous behaviors, some of which are shared among different systems. For instance, both water and silica shrinks when heated in certain temperature regimes16. Among these materials, silica glass is one of the most widely and well studied, not only as an archetypical amorphous material, but also because of its anomalous thermo-mechanical properties. The elastic moduli of silica glass increase with increasing temperature,17-20 and the bulk modulus passes through a minimum upon compression at ~2-3 GPa21-27. Furthermore, this material can undergo irreversible densification under pressure.2, 3, 14, 28-32 Models proposed to explain these phenomena are in one way or another based on the assumption that two or more energetically distinct amorphous states coexist in proportion that vary with pressure and temperature,25, 33 but for the most part little detail is provided as to the structural entities that constitute these states and the a
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