Ultralow thermal conductivity of a packed bed of crystalline nanoparticles: A theoretical study

Theoretical thermal conductivity of a packed bed of crystalline spherical nanoparticles is reported. Thermal conductivity is dominated by surface and constriction thermal resistances and surface energy of the nanoparticles. Depending on the surface energy and size of the nanoparticles, thermal conductivity of the solid phase can be smaller than the minimum thermal conductivity given by the Einstein limit. It is also shown that depending on the surface energy and size of the nanoparticles, thermal conductivity of the nanoparticle bed can be smaller than the thermal conductivity of air. The range of surface energies under which these conditions are achievable for silicon-based nanoparticle beds is reported. Finally, it is shown that nanoconstrictions are more efficient in reducing thermal conductivity than superlattices and nanowires.