Physics based models for metal hydride particle morphology, distribution, and effective thermal conductivity
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Physics based models for metal hydride particle morphology, distribution, and effective thermal conductivity Kyle C. Smith and Timothy S. Fisher School of Mechanical Engineering and Birck Nanotechnology Center, Purdue University, West Lafayette, IN 47906, U.S.A. ABSTRACT This paper describes a modeling approach to target aspects of heat conduction in metal hydride powders that are essential to metal hydrides as viable H2 storage media, including particle morphology distribution, size distribution, particle packing properties at specified solid fraction, and effective thermal conductivity. An isotropic fracture model is presented that replicates features of particle size and shape distributions observed experimentally. The discrete element method is used to simulate evolution of metal hydride particle contact networks during quasi-static consolidation of decrepitated metal hydride powders. Finally, the effective thermal conductivity of such a powder is modeled assuming that contact conductance is the same for each interparticle contact. INTRODUCTION Metal hydrides offer high volumetric hydrogen storage density for on-board fuel cell vehicles [1], but the particulate nature of these materials inhibits heat flow, and therefore often limits hydrogenation reaction rates. Composites have been developed to increase the effective thermal conductivity of these materials, but the mechanisms by which enhanced conduction occurs have only been explained with empirical models lacking essential physics. For loose metal hydride powders, the dependence of effective thermal conductivity on the degree of cyclic hydriding is not well understood. The evolution of packing structure due to applied pressure via particle rearrangement and plastic deformation strongly influences thermal transport. Systematic reductions in average particle size have been observed as the number of hydriding cycles increases [2, 3]. Also, the powder can reach a state at which particles become mechanically stabilized and the size distribution becomes invariant with further cycling [2].
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Figure 1 – a) Ti1.1CrMn particles with crack fissures produced from cyclic hydriding and dehydring. The particles produced after further cycling possess irregular faceted shapes. b) The modeled morphology of particles generated via the modeled described herein with Vc = V0/100. Blue, green, and red particles have Vp < Vp,max/3, Vp,max/3 < Vp < 2Vp,max/3, and Vp < 2Vp,max/3, respectively.
Particle morphology resulting from hydrogen-induced fracture can result in faceted particles having irregular shapes (Fig. 1), in contrast to faceted particles produced from growth of single crystals. The particular metal hydride of interest here, Ti1.1CrMn, is prepared by water-cooled arc melting [3] and typically possesses an initial polycrystalline microstructure. Therefore, we
expect crystal defects and grain boundaries at which cracks initiate to have random spatial and directional distribution. Stacked plate-like nanostructures resulting from phonon confineme
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