The influence of particle shape on structure, mechanics, and transport in granular materials
The development of materials with tailored transport properties is essential to energy conversion and storage applications. Utilization of heterogeneous composite materials composed of discrete particles (i.e., granular materials) represents a promising approach to sustainable, scalable materials production. The so-called jamming point, which represents the transition between fluid-like and solid-like regimes of granular materials, has been the subject of recent fundamental studies. Prior studies have incorporated highly simplified grain shapes that do not reflect the diversity commonly observed in advanced composite materials (e.g., nanomaterials). In the present work, the coupling of heat and charge transport to the level of order in jammed microstructures composed of faceted 3D grains is explored. The systems investigated include lithium ion battery cathodes composed of LiFePO4 nanoparticles, solid state H2 storage in packed beds composed of metal hydride particles, and the Platonic solids. Empirical and theoretical representations of particle shape are determined with single crystal growth models, statistical geometric models, and experimental measurements. An energy-based structural optimization method for the jamming of such arbitrary polyhedral grains is developed to model the mesoscopic structure of heterogeneous materials. Diffusion through the resulting microstructures is simulated with the finite volume method. In LiFePO4 systems a strong dependence of jamming on particle shapes is observed, in which columnar structures aligned with the  direction inhibit diffusion along  in anisotropic LiFePO4. Transport limitations are induced by  columnar order and lead to catastrophic performance degradation in anisotropic LiFePO4 cathodes. Further, judicious mixing of nanoplatelets with additive nanoparticles can frustrate columnar ordering and thereby enhance the rate capability of LiFePO4 electrodes by nearly an order of magnitude. In contrast, metal hydride particles (and all Platonic solids except cubes) jam into highly disordered structures, as a result of anisotropic shape and size distribution. Such systems exhibit fundamentally different pathways of heat transport than that of packed spheres and consequently display close agreement with granular effective medium theory predictions. Also, despite possessing rigidity percolation at the jamming point, conductivity percolation does not occur at the jamming point. From these initial studies it is clear that knowledge of particle shape effects on structure and transport provide a pathway for scalable, bottom-up design of materials.
Fisher, Purdue University.
Mechanical engineering|Physics|Materials science
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