Topological control of mechanical resilience and tunable thermal transport in 3D boron nitride honeycombs.

The rational assembly of low-dimensional building blocks into three-dimensional (3D) architectures offers an exceptional framework for tailoring macroscopic thermal transport while maintaining structural integrity. In this work, we employ non-equilibrium molecular dynamics simulations to systematically investigate the thermal conductivity and mechanical response of topologically porous 3D boron nitride honeycomb (3D-BNHC) nanostructures. Our results reveal that 3D-BNHCs exhibit an ultralow in-plane thermal conductivity below 5 W mK-1, representing an unprecedented reduction of approximately two orders of magnitude compared to pristine 2D hexagonal boron nitride. Spectral heat current analysis and spatial flux mapping demonstrate that this severe suppression originates from intense phonon scattering at the monatomic junctions, coupled with localized, closed heat flux trajectories that effectively trap thermal carriers. Furthermore, the continuous yet porous network induces a profound spatial thermal anisotropy. Out-of-plane transport remains highly efficient, scaling linearly with structural density, whereas in-plane propagation obeys a distinct, geometric path-dependent power law characteristic of highly porous macroscopic cellular media. Uniaxial deformation reveals that the network preserves robust structural integrity, displaying stretch-dominated load-bearing capacity under tension and a multi-stage, buckling-driven progressive collapse under compression that provides exceptional energy absorption and mechanical damping. Crucially, this structural flexibility enables robust and reversible dynamic tuning of thermal transport, driven by the competing mechanisms of lattice buckling and transport-axis alignment. Providing a deep microscopic understanding of topology-property relationships, our findings position 3D-BNHCs as premier candidates for extreme-environment thermal insulation, protective mechanical damping, and strain-engineered thermal management systems.