Patrick Hopkins1
University of Virginia1
Materials in radiative environments (e.g., nuclear reactors, spacecraft, research instruments, among others) are subject to unique forms of damage. The microstructural damage results in a change in electron and phonon scattering rates in materials, which typically leads to a reduction in thermal conductivity. For example, in a study focused on silicon wafers irradiated with an array of ions (C2+, N2+, Al2+, Si2+, P2+, or Ge2+), we demonstrate that an increase in ion dose results in a significant reduction in thermal conductivity (up to an order of magnitude reduction) of the silicon within damaged region [1]. These reductions in thermal properties are general predicted based on the displacements-per-atom (dpa). While dpa and resulting defects lead to reductions in thermal conductivity in crystalline materials, we observe the opposite trend in thermal conductivity of amorphous materials and thermal boundary conductance across interfaces. For example, we demonstrate the ability to increase the thermal conductivity of amorphous solids through ion irradiation, in turn, altering the bonding network configuration. We report on the thermal conductivity of hydrogenated amorphous carbon implanted with C+ ions, in which the films’ thermal conductivities reveal significant enhancement after ion irradiation, up to a factor of 3, depending upon the preirradiation composition [2]. Films with higher initial hydrogen content provide the greatest increase, which is complemented by an increased stiffening and densification from the irradiation process. This enhancement in vibrational transport is unique when contrasted to crystalline materials, for which ion implantation is known to produce structural degradation and significantly reduced thermal conductivities. At solid/solid interfaces, we also observe this ion irradiation-induced increase in thermal conductance. We experimentally demonstrate this increase in thermal boundary conductance (TBC) by ion irradiating Gallium Nitride to produce near interface point defects. GaN is bombarded with varying doses of C+, N+, and Ga3+ ions, with a maximum target near-interface defect density of 2%. We show an increase in the measured Al/GaN TBC. Our results show an increased level of scattering within the defected material, which assists in the thermalization between interfacial and bulk vibrational modes and decreases TBR. This is in contrast to conventional formalisms, where scattering across the interface is said to dominate thermal transport.<br/><br/>1. E. A. Scott, K. Hattar, E. J. Lang, K. Aryana, J. T. Gaskins, and P. E. Hopkins. Reductions in the thermal conductivity of irradiated silicon governed by displacement damage. Physical Review B, 104:134306, 2021.<br/>2. E. A. Scott, S. W. King, N. N. Jarenwattananon, W. A. Lanford, H. Li, J. Rhodes, and P. E. Hopkins. Thermal conductivity enhancement in ion-irradiated hydrogenated amorphous carbon films. Nano Letters, 21(9):3935–3940, 2021.