John Kieffer

Dispersions of Aramid Nanofibers: A New Nanoscale Building Block

Stable dispersions of nanofibers are virtually unknown for synthetic polymers. They can complement analogous dispersions of inorganic components, such as nanoparticles, nanowires, nanosheets, etc. as a fundamental component of a toolset for design of nanostructures and metamaterials via numerous solvent-based processing methods. As such, strong flexible polymeric nanofibers are very desirable for the effective utilization within composites of nanoscale inorganic components such as nanowires, carbon nanotubes, graphene, and others. Here stable dispersions of uniform high-aspect-ratio aramid nanofibers (ANFs) with diameters between 3 and 30 nm and up to 10 μm in length were successfully obtained. Unlike the traditional approaches based on polymerization of monomers, they are made by controlled dissolution of standard macroscale form of the aramid polymer, that is, well-known Kevlar threads, and revealed distinct morphological features similar to carbon nanotubes. ANFs are successfully processed into films using layer-by-layer (LBL) assembly as one of the potential methods of preparation of composites from ANFs. The resultant films are transparent and highly temperature resilient. They also display enhanced mechanical characteristics making ANF films highly desirable as protective coatings, ultrastrong membranes, as well as building blocks of other high performance materials in place of or in combination with carbon nanotubes.

Molecular dynamics study of cristobalite silica using a charge transfer three-body potential: Phase transformation and structural disorder

Structural and dynamic properties of cristobalite silica have been studied using molecular dynamics simulations based on a charge transfer three-body potential model. In this potential model, the directional covalent bonding of SiO 2 is characterized by a charge transfer function of the interatomic distance between Si and O atoms, and in the form of Si‐O‐Si and O‐Si‐Othree-body interactions. The dynamic properties such as infrared spectra and density of states at room and elevated temperatures are in excellent agreement with experiments, and are also consistent with the recently proposed rigid unit modes model. The a- and b-cristobalite crystallographic structures are well reproduced in this model, and the transition between these modifications occurs reversibly and reproducibly in simulations, both as a result of changes in pressure and temperature. The thermally induced transition results in a significantly more disordered b-cristobalite than the pressure-induced b-cristobalite at room temperature. While simulated a-cristobalite exhibits a positive thermal expansion coefficient, it is almost zero forb-cristobalite up to 2000 K and slightly negative at higher temperatures, confirming results from recent x-ray diffraction experiments and other simulations with potential models based on ab initio calculations.

Continuum and Molecular-Level Modeling of Fatigue Crack Retardation in Self-Healing Polymers

A numerical model to study the fatigue crack retardation in a self-healing material (White et al., 2001, Nature, 409, pp. 794-797) is presented. The approach relies on a combination of cohesive modeling for fatigue crack propagation and a contact algorithm to enforce crack closure due to an artificial wedge in the wake of the crack. The healing kinetics of the self-healing material is captured by introducing along the fracture plane a state variable representing the evolving degree of cure of the healing agent. The atomic-scale processes during the cure of the healing agent are modeled using a coarse-grain molecular dynamics model specifically developed for this purpose. This approach yields the cure kinetics and the mechanical properties as a function of the degree of cure, information that is transmitted to the continuum-scale models. The incorporation of healing kinetics in the model enables us to study the competition between fatigue crack growth and crack retardation mechanisms in this new class of materials. A systematic study of the effect of different loading and healing parameters shows a good qualitative agreement between experimental observations and simulation results.

Fractal dimensions of silica gels generated using reactive molecular dynamics simulations

We have used molecular dynamics simulations based on a three-body potential with charge transfer to generate nanoporous silica aerogels. Care was taken to reproduce the sol-gel condensation reaction that forms the gel backbone as realistically as possible and to thereby produce credible gel structures. The self-similarity of aerogel structures was investigated by evaluating their fractal dimension from geometric correlations. For comparison, we have also generated porous silica glasses by rupturing dense silica and computed their fractal dimension. The fractal dimension of the porous silica structures was found to be process dependent. Finally, we have determined that the effect of supercritical drying on the fractal nature of condensed silica gels is not appreciable.

The influence of the representative volume element (RVE) size on the homogenized response of cured fiber composites

The influence of the representative volume element (RVE) size (in terms of fiber packing and number of fibers for a given fiber-volume fraction) on the residual stresses created during the curing process of a continuous fiber-reinforced polymer matrix tow is investigated with the ultimate goal of finding a minimum unit cell size that can be used later for a homogenization procedure to calculate the response of woven fiber textile composites and in particular, fiber tows. A novel network curing model for the solidification of epoxy is used to model the curing process. The model takes into account heat conduction, cure kinetics and the creation of networks in a continuously shape changing body. The model is applied to the curing of a fiber/matrix RVE. The results for the minimum size of the RVE, obtained on the basis of the curing problem, are compared with a similar RVE, modeled as an elastic-plastic solid subjected to external loads, in order to compare the minimum RVE sizes obtained on the basis of different boundary value problem solutions. (Some figures may appear in colour only in the online journal)

Flame made nanoparticles permit processing of dense, flexible, Li+ conducting ceramic electrolyte thin films of cubic-Li7La3Zr2O12 (c-LLZO)

Ceramic electrolytes are proposed as key components in resolving challenges extant in developing next generation, high energy density Li batteries by replacing liquid electrolytes to improve safety and performance. Among numerous candidates, c-LLZO offers multiple desirable properties: high ionic conductivities (0.1–1 mS cm−1), Li stability, a wide electrochemical operating window (∼6 V) and pH stability (7–11.5). However, incorporation into prototype cells has yet to be demonstrated as c-LLZO membranes at thicknesses <50 μm have not been achieved. Processing dense, thin films matching bulk counterpart properties remains a very difficult target arising from energy and/or equipment intensive sintering, Li volatilization, and contamination from substrates. We show that using metalloorganic derived flame made nanoparticles can overcome these processing challenges resulting in a significantly reduced energy input required for densification, 10–40 fold shorter dwell times at sintering temperatures, compared to common solid state reaction derived c-LLZO. Furthermore, surface/volume ratios of the films are determined to be a critical factor affecting final microstructures and phase compositions of the sintered films. Through careful control of the processing variables, 10–15 grains thick, dense (94 ± 1%) c-LLZO thin (<30 μm), flexible films with high ambient ionic conductivities (0.2 ± 0.03 mS cm−1) are achieved using conventional casting–sintering of flame made nanoparticles. These c-LLZO membranes greatly increase the selection of complementary cell components and simplify battery configurations broadening opportunities for cell designs.

Polyamorphic transitions in vitreous B2O3 under pressure

We have studied the nature of structural transitions in B2O3 glass under pressure using molecular dynamics simulations, based on a newly developed coordination-dependent charge transfer potential, to complement the results from our earlier Brillouin and Raman scattering experiments and to interpret these findings. This interaction model allows for charges to re-distribute between atoms upon the formation and rupture of chemical bonds, and accommodates multiple coordination states for a given species in the course of the simulation. The macroscopic observables of the simulated vitreous B2O3, such as the variation of density and elastic modulus with pressure, agree well with those seen in experiments. The compaction of simulated structures is based on a polyamorphic transition that involves transitory four-coordinated boron atoms at high pressures. While the coordination of boron completely reverts to trigonal upon pressure release, without this transitory coordination increase permanent densification would not be manifest in the recovered glass. The response of vitreous B2O3 to pressure is virtually independent of the concentration of boroxol rings in the structure. In simulated glass, boroxol rings dissolve when subject to pressure, which explains the disappearance of the breathing mode in the Raman spectrum of compressed B2O3 glass. (Some figures in this article are in colour only in the electronic version)

Vibrational spectra in fluoride crystals and glasses at normal and high pressures by computer simulation

Molecular dynamics computer simulation methods to obtain the infrared and Raman spectra of ionic liquids and glasses are described. These methods are applied to two problems: (1) the simulation and interpretation of the vibrational spectra of heavy metal fluoride glasses, and (2) the problem of reversible collapse of network glasses and the dramatic spectroscopic consequences of this collapse observed in recent experiments on SiO2-like glasses. As a contribution to this latter problem, the case of BeF2, for which our rigid ion potentials should be good approximations to the actual interactions, is studied. In the spectra, an initial network stiffening with increase of pressure, followed by rather sudden collapse to a spectrum characteristic of a higher coordinated state, is observed. This is correlated with a sudden increase in average coordination numbers. The changes are mostly reversible on decompression, so the behavior is reminiscent of the transition between ‘vitreous polymorphs’ observed in the case of vitreous water and vitreous silica.

Origins of thermal boundary conductance of interfaces involving organic semiconductors

We measure the room temperature thermal conductance of interfaces between an archetypal organic semiconductor copper phthalocyanine (CuPc) and several metals (aluminum, gold, magnesium, and silver) using the 3−ω method. The measured thermal boundary conductance (TBC) scales with bonding strength at the CuPc-metal interface, a correlation that is supported by molecular dynamics (MD) simulation, allowing the extrapolation of the effective interface Youngs modulus. The trend in modeled interface modulus is in agreement with that deduced from adhesion tests, e.g., approximately 2 GPa for CuPc-gold and CuPc-silver interfaces, comparable to the van der Waals interaction strength of the materials. Using MD simulations in which the effects on thermal transport can be studied as a function of interfacial bond strength only, we isolate the relative contribution of acoustic mismatch and interface bond strength to TBC. Furthermore, measurements and modeling of organic/organic (e.g., CuPc/C60) interfaces reveal that ...

Molecular dynamic simulations of the infrared dielectric response of silica structures

The molecular dynamic simulation technique was used to model the vibrational behavior of crystalline (α and β cristobalite) and amorphous silica structures. To this end a refined potential function was developed, which allows one to reproduce the correct structural geometries, the corresponding infrared spectra, and to observe a reversible phase transformation between α and β cristobalite. The complex dielectric constants in the infrared frequency range were calculated from the dipole moment time correlation functions. While idealized cristobalite exhibits the simplest spectrum with only two narrow bands, the increase of structural complexity and reduction of symmetry characteristic for the real cristobalites and amorphous silica, creates additional features in the infrared spectra. These structural changes predominantly affect the coordination of oxygen, and generate a broader spread in the normal modes characterizing the vibrations of this species. A unique method for the identification of atomic trajec...