PACKING OF PARTICULATE MATERIALS

Mathematically, compression of particulate materials is related to the problems of close packing or "jamming" (Torquato et al., 2000). It has been proved that the volume fraction of the densest possible packing for identical spheres in three dimensions is 74.05% in the crystalline packing corresponding to the close-packed face-centered cubic (FCC) lattice. For random close packing of identical spheres (i.e., amorphous monodisperse sphere packing), however, the volume fraction is lower than that for the crystalline packing. The number could vary depending on the packing protocol used. For ellipsoidal geometries, higher values of packing fractions were reported in literature for both crystalline and random packing (Donev et al., 2004).

Real materials systems, however, exhibit important features not captured by these idealized models. In the present work, the effects of friction and deformability of particles on the compression of particulate systems were investigated using finite element analysis. The established relationship between interfacial friction and jamming fraction in spherical to ellipsoidal systems, was used to correlate maximum stresses and different frictional coefficients, with morphology (obtained by image analysis) of graphite particles in Li-ion anodes. The simulated results were compared with the experiments, showing good agreements. It was also concluded that use of maximum jamming fractions derived from idealized mathematical models to assess likely configuration of mixtures is unrealistic in real manufacturing processes. Thus, consideration of material properties is clearly merited in composites having volume fractions of particles near percolation onset. The picture below shows the stress distribution at the end of packing simulation using ABAQUS/EXPLICIT.

Reference

Yi, Y. B., Wang, C. W. and Sastry, A. M., 2006, Compression of Packed Particulate Systems: Simulations and Experiments in Graphitic Li-ion Anodes, ASME Journal of Engineering Materials and Technology, 128, pp.73-80.

Torquato, S., Truskett, T. M., and Debenedetti, P. G., 2000, "Is random close packing of spheres well defined?" Physical Review Letters, 84, pp. 2064-2067.

Donev, A, Cisse, I., Sachs, D., Variano, E., Stillinger, F. H., Connelly, R., Torquato, S., and Chaikin, P. M., 2004, "Improving the density of jammed disordered packings using ellipsoids," Science, 303, pp. 990-993.

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