Abnormal Increase in Thermal Diffusivity of Lignin-Based Carbon Fibers during Vacuum Exposure

Vacuum generally suppresses gaseous heat conduction without altering intrinsic transport of solid materials. Here we report an unexpected, time-dependent increase in thermal diffusivity (α) of lignin-based carbon fibers (CFs) during vacuum exposure. At <0.08 Pa, α increases by up to ∼95% and approaches saturation, while its electrical resistance (R) shows smaller, condition-dependent changes. After ∼16 h of air re-exposure, α partially or nearly fully recovers, while R responds less consistently, demonstrating strong dependence on exposure history. Across the vacuum systems examined, the α increase diminishes as vacuum pressure rises from <0.08 to 2.13 Pa, whereas a moderate temperature rise (∼50 K) accelerates the evolution of the α increase under ∼0.21 Pa vacuum. Laser-power-normalized Raman intensities increase during air re-exposure without significant D- or G-band shifts, providing indirect evidence of reversible near-surface-state evolution rather than restructuring of the carbon framework. Atomistic simulations show that adsorbed water shortens acoustic phonon lifetimes and suppresses in-plane thermal transport. These results support vacuum-induced desorption of near-surface adsorbates as reducing phonon scattering and increasing α, as there is negligible change in volumetric heat capacity. Removal of humidity-assisted charge-transport pathways may also contribute to the increase in R observed in some sequences. It is concluded that the exposure history is an important factor in thermal and electrical measurements of carbon fibers under vacuum.