Ceramics and Glasses Simulations and Machine Learning: Atomistic Modeling
Sponsored by: ACerS Glass & Optical Materials Division
Program Organizers: Mathieu Bauchy, University of California, Los Angeles; Peter Kroll, University of Texas at Arlington; N. M. Anoop Krishnan, Indian Institute of Technology Delhi
Monday 2:00 PM
November 2, 2020
Room: Virtual Meeting Room 18
Location: MS&T Virtual
Session Chair: Peter Kroll, UT Arlington; Mathieu Bauchy, UCLA; Anoop Krishnan, IIT Delhi
2:00 PM Invited
Ab-initio and Reactive MD Simulations of Polymer Pyrolysis and Formation of Silicon-based Ceramics
: Peter Kroll1; 1University of Texas at Arlington
We perform ab-initio molecular dynamic (aiMD) simulations of polymer pyrolysis of different polysiloxanes and polysilazanes. Models comprise 400 to 1000 atoms and exhibit different polymer side groups. Simulations are performed for 20 to 100 ps at high temperatures. We detect developing gaseous species and follow trajectories of fundamental processes in detail. We observe the Kumada-type rearrangement, hence, insertion of carbon from aliphatic side groups into the polymer back-bone. This process changes the local environment of Si and facilitates formation of mixed SiCnO4-n-tetrahedra. Time and length scales of the aiMD simulations are augmented by orders of magnitude using a complex reactive force field (ReaxFF) that we continuously develop. We show that formation of carbon segregations in amorphous SiCO is linked to early stages of polymer degradation, when organic and inorganic portions of the polymers partition and segregate. Further reactions within the organic portion then yields sheet-like or tubular carbonaceous segregations.
2:30 PM
Theoretical Calculation of Formation Energies and Site Preference of Substitutional Divalent Cations in Carbonated Apatite: Tatasushi Saito1; Tatsuya Yokoi1; Atsutomo Nakamura1; Katsuyuki Matsunaga1; 1Nagoya University
Carbonated apatite (CAp) is hydroxyapatite (HAp) containing carbonate ions (CO32-) and is used in bone grafts. It was reported that substitutional CO32- increases solubility of foreign cation impurities into CAp, which affects bone tissue formation ability. However, underlying mechanisms are still unclear. In this study, first principles calculations were performed to investigate defect formation energies (ΔEf) and site preference of substitutional divalent cations (M2+) in CAp. For all M2+ studied, it was found that ΔEf for the most stable substitutional sites are lower in CAp than in HAp. This indicates that M2+ are preferentially substituted into CAp over HAp. Detailed analyses of atomic environments indicated that the presence of CO32- vary the bond lengths and coordination number of Ca sites. As a result, M2+ is favorably substituted for particular Ca sites at which mismatches in the ionic-size and coordination number are minimized between Ca2+ and M2+, decreasing ΔEf.
2:50 PM
The Role of Pore Pattern on The Ductility Enhancement of Crystalline Silicon Nitride Nanoporous Membranes: Ali Shargh1; James McGrath1; Niaz Abdolrahim1; 1University of Rochester
Silicon nitride nanoporous membranes are extremely permeable silicon based ceramics that were first developed at University of Rochester in 2014. Recent studies show that those nanostructures possess sudden failure with negligible ductility which limits their biomedical applications. Here, we use molecular dynamics simulations to investigate the role of pore pattern on failure behavior of crystalline nanostructures. With change of pore pattern, nanostructures show three different mechanical behaviors with distinct fracture surfaces upon loading. A key outcome is observation of pronounced enhancement in ductility upon arranging diagonal neighbor pores along the preferred slip direction. In this case, a network of embryonic shear bands is formed in the nanostructure which leads to ductility enhancement. The origin of this enhancement is found to be associated with a large area with compressive stress in front of the propagating crack that opposes the crack opening and distort it toward a zigzag path which delayed fracture.
3:10 PM Invited
Beyond the Average: Fluctuations in Glass-forming Systems: Katelyn Kirchner1; John Mauro1; 1Pennsylvania State University
The macroscopic properties of any material system are dictated by atomic structure. Within disordered structures, such as glass, long-range atomic arrangement is impossible to precisely predict; however, statistical mechanical modeling can be used to quantify the presence of topological fluctuations within these disordered structures to predict the performance of glass-forming systems. This work presents a general modeling approach to describe structural and topological fluctuations by linking statistical mechanics and topological constraint theory. The model is then used to explore how fluctuations within glass-forming systems impact the distribution of glass structural units, the ability of atoms to self-organize in adaptable network topologies, the thermodynamic properties of the system, heat capacity, and a glass-forming system’s ability to nucleate crystals.
3:40 PM
The Energy Landscape Governs Brittle-to-Ductile Transitions in Glasses: Longwen Tang1; Mathieu Bauchy1; 1University of California, Los Angeles
Based on their structure, non-crystalline phases can fail in a brittle or ductile fashion. However, the nature of the linkages between structure and propensity for ductility in disordered materials has remained elusive. Here, based on molecular dynamics simulations, we investigate the fracture of a Lennard-Jones system with varying degrees of disorder. We find that that structural disorder results in an increase in ductility. By applying the activation-relaxation technique (an accelerated sampling method to identify transition states), we show that the propensity for ductility is controlled by the topography of the energy landscape.