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Now showing items 1 - 13 of 13

  • Statistical mechanical model of bonding in mixed modifier glasses

    Goyal, Sushmit   Mauro, John C.  

    Oxide glasses often consist of multiple network formers that create the backbone of the glass network and modifiers that serve as either charge compensators or creators of non-bridging oxygens. The variety of bonding preferences results in very rich composition-property relationships. In this work, we present a statistical description of the glass structure governed by the relative enthalpic and entropic contributions to the bonding preferences in a glassy system. Using the proposed model, we derive an analytical expression to represent the bonding in mixed modifier glasses and explain the role of composition and fictive temperature on glass structure. The model provides the criteria for nonlinearity in bonding preference and reveals regions where high fluctuations in local structure are predicted.
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  • Atomic picture of structural relaxation in silicate glasses

    Song, Weiying   Li, Xin   Wang, Bu   Krishnan, N. M. Anoop   Goyal, Sushmit   Smedskjaer, Morten M.   Mauro, John C.   Hoover, Christian G.   Bauchy, Mathieu  

    As nonequilibrium materials, glasses continually relax toward the supercooled liquid state. However, the atomic-scale origin and mechanism of glass relaxation remain unclear. Here, based on molecular dynamics simulations of sodium silicate glasses quenched with varying cooling rates, we show that structural relaxation occurs through the transformation of small silicate rings into larger ones. We demonstrate that this mechanism is driven by the fact that small rings (<6-membered) are topologically overconstrained and experience some internal stress. At the atomic level, such stress manifests itself by a competition between radial and angular constraints, wherein the weaker bond-bending constraints yield to the stronger bond-stretching ones. These results strongly echo von Neumann's N - 6 rule in grain growth theory and suggest that the stability of both atomic rings and two-dimensional crystal grains is fully topological in nature.
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  • Statistical Mechanical Model of Bonding in Mixed Modifier Glasses

    Goyal, Sushmit   Mauro, John C.  

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  • Implicit glass model for simulation of crystal nucleation for glass-ceramics

    McKenzie, Matthew E.   Goyal, Sushmit   Loeffler, Troy   Cai, Ling   Dutta, Indrajit   Baker, David E.   Mauro, John C.  

    Predicting crystal nucleation behavior in glass-ceramic materials is important to create new materials for high-tech applications. Modeling the evolution of crystal microstructures is a challenging problem due to the complex nature of nucleation and growth processes. We introduce an implicit glass model (IGM) which, through the application of a Generalized Born solvation model, effectively replaces the glass with a continuous medium. This permits the computational efforts to focus on nucleating atomic clusters or undissolved impurities that serve as sites for heterogeneous nucleation. We apply IGM to four different systems: binary barium silicate (with two different compositions), binary lithium silicate, and ternary soda lime silicate and validate our precipitated compositions with established phase diagrams. Furthermore, we nucleate lithium metasilicate clusters and probe their structures with SEM. We find that the experimental microstructure matches the modeled growing cluster with IGM for lithium metasilicate.
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  • Adhesion of Organic Molecules on Silica Surfaces:A Density Functional Theory Study

    McKenzie, Mathew E.   Goyal, Sushmit   Lee, Sung Hoon   Park, Hyun-Hang   Savoy, Elizabeth   Rammohan, Aravind R.   Mauro, John C.   Kim, Hyunbin   Min, Kyoungmin   Cho, Eunseog  

    Understanding the interface between organic and inorganic materials presents many challenges due to the complex chemistries involved. Modeling and experimental work have elucidated only a few facets of the physical and chemical nature of the adhesion between such surfaces. In this work, we use density functional theory to, understand the adhesion between five different inorganic crystal surfaces (two-dimensional silica, both sides of kaolinite, hydroxylated quartz, hydroxylated albite) with five different organic molecules (benzene, phenol, phthalimide, N-phenylmaleimide, diphenyl ether). In the analysis, we explore the binding motifs that constitute parts of a polyimide monomer and examine their interactions with increasingly complex crystal surfaces. Comparing these systems, we elucidate the key factors (such as electrostatic interactions, hydrogen bond formation, and cation effects) that affect adhesion of organics on inorganic surfaces. It is found that the presence of cations and the availability of the oxygen species, in either the organic or inorganic layers, allows for increased hydrogen bonding. The most significant contribution to adhesion is from the rearrangement of surface electrostatic interactions. These factors can be used to optimize adhesion by decomposing both the organic and inorganic materials into the constituent interactions and help design improved interfacial properties.
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  • Structure and transport properties of polymer grafted nanoparticles

    Goyal, Sushmit   Escobedo, Fernando A.  

    We perform molecular dynamics simulations on a bead-spring model of pure polymer grafted nanoparticles (PGNs) and of a blend of PGNs with a polymer melt to investigate the correlation between PGN design parameters (such as particle core concentration, polymer grafting density, and polymer length) and properties, such as microstructure, particle mobility, and viscous response. Constant strain-rate simulations were carried out to calculate viscosities and a constant-stress ensemble was used to calculate yield stresses. The PGN systems are found to have less structural order, lower viscosity, and faster diffusivity with increasing length of the grafted chains for a given core concentration or grafting density. Decreasing grafting density causes depletion effects associated with the chains leading to close contacts between some particle cores. All systems were found to shear thin, with the pure PGN systems shear thinning more than the blend; also, the pure systems exhibited a clear yielding behavior that was absent in the blend. Regarding the mechanism of shear thinning at the high shear rates examined, it was found that the shear-induced decrease of Brownian stresses and increase in chain alignment, both correlate with the reduction of viscosity in the system with the latter being more dominant. A coupling between Brownian stresses and chain alignment was also observed wherein the non-equilibrium particle distribution itself promotes chain alignment in the direction of shear. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3657831]
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  • Structure and transport properties of polymer grafted nanoparticles

    Goyal, Sushmit   Escobedo, Fernando A.  

    We perform molecular dynamics simulations on a bead-spring model of pure polymer grafted nanoparticles (PGNs) and of a blend of PGNs with a polymer melt to investigate the correlation between PGN design parameters (such as particle core concentration, polymer grafting density, and polymer length) and properties, such as microstructure, particle mobility, and viscous response. Constant strain-rate simulations were carried out to calculate viscosities and a constant-stress ensemble was used to calculate yield stresses. The PGN systems are found to have less structural order, lower viscosity, and faster diffusivity with increasing length of the grafted chains for a given core concentration or grafting density. Decreasing grafting density causes depletion effects associated with the chains leading to close contacts between some particle cores. All systems were found to shear thin, with the pure PGN systems shear thinning more than the blend; also, the pure systems exhibited a clear yielding behavior that was absent in the blend. Regarding the mechanism of shear thinning at the high shear rates examined, it was found that the shear-induced decrease of Brownian stresses and increase in chain alignment, both correlate with the reduction of viscosity in the system with the latter being more dominant. A coupling between Brownian stresses and chain alignment was also observed wherein the non-equilibrium particle distribution itself promotes chain alignment in the direction of shear. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3657831]
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  • Modeling the thermal poling of glasses using molecular dynamics. Part 2:Effects on elastic properties

    Reveil, Mardochee   Tandia, Adama   Luo, Jian   Vargheese, K. Deenamma   Goyal, Sushmit   Mauro, John C.   Clancy, Paulette  

    In this work, thermal poling of ternary oxide glasses, including borosilicates and aluminosilicates, is studied using molecular dynamics. For the glass compositions simulated, results show that thermal poling lowers the Young's modulus of those ternary oxide glasses. Additionally, comparisons between the structures of as-melted and poled glasses, as well as comparisons with their binary as-melted counterparts of the same composition, reveal that the changes in elastic properties can be partially attributed to changes in void size distributions within those glass networks. Overall, this study shows that thermal poling can effectively be used to alter the elastic properties of oxide glasses.
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  • Effect of Nanoscale Roughness on Adhesion between Glassy Silica and Polyimides:A Molecular Dynamics Study

    Lee, Sung Hoon   Stewart, Ross J.   Park, Hyunhang   Goyal, Sushmit   Botu, Venkatesh   Kim, Hyunbin   Min, Kyoungmin   Cho, Eunseog   Rammohan, Aravind R.   Mauro, John C.  

    The effect of nanoscale roughness on the adhesion between glassy silica and polyimides is examined by molecular dynamics simulation. Different silica surfaces, with varying degrees of roughness, were generated by cleaving bulk structures with a predefined surface and a desired average roughness, with different roughness periods and hydroxylation densities in an effort to study the influence of these surface characteristics on adhesion at the silica-polyimide interface. The calculated results reveal that average roughness R-a is the primary controlling factor within the considered conditions. Further, an energy decomposition analysis of the pulling process suggests that hydrogen bonding contributes to the adhesion on all the rough surfaces, while the Coulombic energy contribution becomes significant at higher R-a. From a structural analysis of the vacant volume and surface area, it is shown that the periodicity of roughness provides a rather interesting trend for the adhesion energy. Adhesion can increase with a reduction in period due to the corresponding surface area expansion; however, if vacant volumes exist at the interface, the level of adhesion can decrease. Competition between two opposing tendencies leads to the maximum adhesion, and hence, both R-a and period are key parameters to control the adhesion in nanoscale roughness.
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  • Grafting Functional Groups in Polymeric Binder toward Enhancing Structural Integrity of LixSiO2 Anode during Electrochemical Cycling

    Min, Kyoungmin   Rammohan, Aravind R.   Lee, Sung Hoon   Goyal, Sushmit   Park, Hyunhang   Stewart, Ross   He, Xiaoxia   Cho, Eunseog  

    Development of a novel polymeric binder material is necessary for improving the electrochemical performance of silicon-based anodes for Li-ion batteries, suffering from irreversible capacity loss due to their huge volume change during the electrochemical cycling. However, relevant mechanisms on how adhesion and mechanical properties of the binder are correlated to the stability of Si anode are still lacking. In this study, we investigate the role of functional groups attached in the polymeric binder on the structural stability of LixSiO2 using molecular dynamics simulations. A pulling test reveals that the binder with a polar group shows better adhesion properties with LixSiO2 than that with a nonpolar group. In addition, cohesive failure dominates the failure mode for the nonpolar group, but an adhesive to cohesive failure transition occurs for the polar group as the amount of lithiation is increased. For mechanical properties, the polar binder exhibits a larger maximum stress, while the nonpolar one can hold a larger strain. Finally, the polar group works more effectively to suppress the volume expansion of LixSiO2 from lithiation. The current study reveals detailed mechanisms on how polar and nonpolar polymeric binders work differently with glasses of varying degrees of lithiation and can guide the design of future generations of Si-based anodes.
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  • Characterizing the Fundamental Adhesion of Polyimide Monomers on Crystalline and Glassy Silica Surfaces:A Molecular Dynamics Study

    Goyal, Sushmit   Park, Hyun-Hang   Lee, Sung Hoon   Savoy, Elizabeth   McKenzie, Mathew E.   Rammohan, Aravind R.   Mauro, John C.   Kim, Hyunbin   Min, Kyoungmin   Cho, Eunseog  

    Understanding the interaction between polyimide and inorganic surfaces is vital in controlling interfacial adhesion behavior. Here, molecular dynamics simulations are employed to study the adhesion of polyimide on both crystalline and glassy silica surfaces, and the effects of hydroxylation, silica structure, and polyimide chemistry on adhesion are investigated. The results reveal that polyimide monomers have stronger adhesion on hydroxylated surfaces compared to nonhydroxylated surfaces. Also, adhesion of polyimide onto silica glass is stronger compared to the corresponding crystalline surfaces. Finally, we explore the molecular origins of adhesion to understand why some polyimide monomers like Kapton have a stronger adhesion per unit area (adhesion density) than others like BPDAAPB. We find this occurs due to a higher density of oxygen's in the Kapton monomer, which we found to have the highest contribution to, adhesion density.
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  • Characterizing the Fundamental Adhesion of Polyimide Monomers on Crystalline and Glassy Silica Surfaces: A Molecular Dynamic Study

    Goyal, Sushmit   Park, Hyunhang   Lee, Sung Hoon   Savoy, Elizabeth   Mckenzie, Matthew E.   Rammohan, Aravind Raghavan   Mauro, John C.   Kim, Hyunbin   Min, Kyoungmin   Cho, Eunseog  

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  • Constructing surface models of silicate glasses using molecular dynamics to understand the effect of pH on the hydration properties

    Stewart, Ross J.   Goyal, Sushmit   Lee, Sung Hoon   Rammohan, Aravind   Park, Hyun Hang   Min, Kyoungmin   Cho, Eunseog   Heinz, Hendrik  

    In this work, we use realistic silicate glass surface models, with molecular dynamics simulations, and present an algorithm for proper atomic partial charge assignment, consistent with measurable internal dipoles. The immersion energy is calculated for different silicate glass compositions in solutions of varying pH. We use molecular dynamics to elucidate the differences in the structure of water between mono- and divalent cations. The immersion energy of the glass surface is found to increase with an increase in ionic surface density and pH. This can be attributed to the stronger interaction between water and cations, as opposed to the interactions between water and silanol groups. The developed models and methods provide new insights into the structure of glass-solution interfaces and the effect of cation surface density in common nanoscale environments. Published under license by AIP Publishing.
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