  While thermal anisotropicity is a desirable materials property for many
applications, including transverse thermoelectrics and thermal management in
electronic devices, it remains elusive in practical natural compounds. In this
work, we show how nanoporous materials with anisotropic pore lattices can be
used as a platform for inducing strong heat transport directionality in
isotropic materials. Using density functional theory and the phonon Boltzmann
transport equation, we calculate the phonon-size effects and thermal
conductivity of nanoporous silicon with different anisotropicpore lattices. Our
calculations predict a strong directionality in the thermal conductivity,
dictated by the difference in the pore-pore distances along the two Cartesian
axes. As the space between pores along the direction of the applied temperature
gradient represents the phonon bottleneck, an anisotropic pores lattice
distortion induces directionality in heat transport. Using Fourier's law, we
also compute diffusive heat transport for the same geometries obtaining
significantly smaller anisotropicity, revealing the crucial role of phonon-size
effects in tuning thermal transport directionality. Besides enhancing our
understanding of nanoscale heat transport, our results demonstrate the promise
of nanoporous materials for modulating anisotropy in thermal conductivity.
