Effective Thermal Conductivity of Metal Hydride-Metal Matrix Composites: A Theoretical Investigation
Kyle C. Smitha and Timothy S. Fisherb
Purdue University, Birck Nanotechnology Center, Mechanical Engineering, West Lafayette, IN 47906, USA.
akcsmith@purdue.edu
btsfisher@purdue.edu
ABSTRACT
Metal hydrides have potential to meet on-board hydrogen storage goals for fuel cell vehicles as set by the US DoE. Cyclic strain caused by addition and depletion of hydrogen in metal hydride beds results in brittle fracture and subsequent formation of micron-sized irregular faceted particles that inhibit heat flow and hydrogenation because of poor interparticle heat conduction that increases the bed's temperature during the exothermic hydriding reaction. Metal hydridemetal matrix composites have been developed to enhance effective thermal conductivity and mechanical stability. In this work the effective thermal conductivity of such matrices in which metal hydride is embedded are studied theoretically and computationally via a statistical model for metal hydride particle morphology, the Discrete Element Method, geometric steric exclusion, thermal network modeling, and a model for ballistic-diffusive heat flow. Metal hydride particles are simulated with faceted shapes and polydisperse sizes, while matrix particles are simulated as monodisperse spheres. The dependence of effective thermal conductivity on the size ratio between metal hydride and metal matrix particles, metal matrix volume fraction, and the intrinsic thermal properties of the matrix material are presented. The matrix volume fraction for which conductivity percolation occurs is found to depend strongly on the size ratio between metal hydride and metal matrix particles. Steric exclusion of the matrix by metal hydride inclusions acts to decrease matrix solid fraction and to reduce effective thermal conductivity with increasing effect for large matrix particles. Ballistic resistance to heat flow through small matrix particles decreases effective thermal conductivity. As a result, an optimal particle size exists at which minimum matrix solid fraction may be utilized to achieve a desired solid fraction. This work is the first to quantify these effects for realistic three-dimensional polydisperse particle beds with thermal additives, and the results are expected to aid the development of optimized hydrogen storage materials and metal matrix composites.
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