"Nano-Mechanics and Phonon Transport in Bio-Inspired and Engineering Functional Materials"

Shangchao Lin
Department of Mechanical Engineering
Materials Science & Engineering Program
FAMU-FSU College of Engineering
Florida State University

Wednesday, March 16, 2016 -- 3:30 P.M. 499 Dirac Science Library

Abstract:

Bio-inspired materials, such as synthetic spider silk fibers, are promising mechanical building blocks for functional materials in biomedical applications. These robust and bio-compatible artificial fibers hold great potentials for wound sutures, artificial nerve guides, and tissue regeneration. I will present meso-scale dissipative particle dynamics (DPD) simulation of shear flow-assisted self-assembly of synthetic polypeptides (a multi-block copolymer) to produce artificial spider silk fibers. We find that intermediate hydrophobic to hydrophilic block ratios observed in natural spider silks and longer chain lengths lead to outstanding silk fiber formation. This design by nature is based on the optimal combination of protein solubility, self-assembled aggregate size, and polymer network topology, all features controlled at the nanoscale. The original homogeneous network structure becomes heterogeneous after spinning, enhancing the anisotropic network connectivity along the shear flow direction. Extending beyond the classical polymer theory, with insights from the percolation network model, we illustrate the direct proportionality between network conductance and fiber stiffness.

On the other hand, solid-state phonon transport in nano- and engineering functional materials are very important in their energy-related applications, such as thermal management and thermoelectric energy conversion. I will present all-atomistic molecular dynamics (MD) simulations for predicting phonon transport properties of one-dimensional carbon-chains (carbyne and cumulene) and hybrid organic-inorganic perovskites. We discovered ultrahigh thermal conductivities for carbyne and cumulene, much higher than that for graphene, attributed to high phonon energies and group velocities, and reduced scattering from non-overlapped acoustic and optical phonon modes. This makes them ideal material candidates in thermal management and heat removal for electronic devices. For hybrid perovskites, we found ultralow and anisotropic thermal conductivity, attributed to small phonon group velocities, short phonon lifetimes from highly-overlapped branches, and preferential orientations of organic cations. This could lead to potential applications in efficient thermoelectric energy conversion, while care must be taken to actively manage heat flow in their devices (solar cells and light-emitting diodes).