  The thermal conductivity of nanostructures needs to be as small as possible
so that it will have a greater efficiency for solid-state electricity
generation/refrigeration by thermoelectrics. We studied how the period length
and the mass ratio affect the thermal conductivity of isotopic nanoscale 3D
phononic crystal of Si. Simulation results by equilibrium molecular dynamics
show isotopic nanoscale 3D phononic crystal can make a significance reduction
on the thermal conductivity of bulk Si at high temperature,1000 K. Size and
mass effects are obvious in manipulating thermal conductivity. The thermal
conductivity decreases as the period length and mass ratio increases. The
phonon dispersion curves show the decrease of group velocities in 3D phononic
crystals. The phonon's localization and band gap is clearly shown in spectra of
normalized inverse participation ratio in nanoscale 3D phononic crystal
structure.
