Point Defects Enhance Cross-Plane Thermal Conductivity In Graphite

Point defects typically reduce the thermal conductivity (κ) of a crystal due to increased scattering of heat-carrying phonons, a mechanism that is well understood and widely used to enhance or impede heat transfer in the material for different applications. Here an opposite effect is reported where the introduction of point defects in graphite with energetic particle irradiation increases its cross-plane κ by nearly a factor of two, from 10.8 to 18.9 W m K^-1 at room temperature. Integrated differential phase contrast imaging with scanning transmission electron microscopy revealed the creation of spiro interstitials in graphite by the irradiation. The enhancement in κ is attributed to a remarkable mechanism that works to the benefit of phonon propagation in both the harmonic and anharmonic terms: these spiro interstitial defects covalently bridge neighboring basal planes, simultaneously enhancing acoustic phonon group velocity and reducing phonon-phonon scattering in the graphite structure. The enhancement of κ reveals an unconventional role of lattice defects in heat conduction, i.e., easing the propagation of heat-carrying phonons rather than impeding them in layered materials, inspiring their applications for thermal management in heavily radiative environments.