Microstructural changes in carbonized wood-lignin, a potential space material, in response to atomic oxygen irradiation

Abstract

Utilizing wood in space-based applications poses challenges because the lack of electrical conductivity of this material can lead to local charging while off-gassing under high vacuum can potentially also have adverse effects. However, carbonized wood exhibits electrical conductivity and does not produce gases in a vacuum, making it a potential candidate for use in spacecraft. Even so, erosion by atomic oxygen (AO) at low altitude orbits could potentially degrade wood surfaces. The present study therefore investigated the effects of AO irradiation on carbonized and uncarbonized milled wood lignin (MWL) obtained from both softwood and hardwood sources. The lignin source was found to significantly affect resistance to AO. Specifically, softwood-derived MWL underwent structural changes upon AO exposure whereas hardwood-derived MWL showed greater resistance due to the higher concentration of oxygen-containing functional groups in the latter. AO irradiation evidently induced changes in the carbon framework while affecting the micropore sizes and peak distribution ranges. These findings highlight the importance of selecting specific wood types and carbonization conditions when producing lignin-derived carbon materials for AO-exposed environments. Softwood MWL is evidently more vulnerable to AO erosion as a result of the guaiacyl structures in this material whereas hardwood MWL resists AO based on its oxygen-rich syringyl groups. The present work underscores the complex relationships between the wood type, concentration of oxygenated functional groups, and AO resistance. This research also suggests potential applications for wood-derived lignin carbon in low Earth orbit vehicles and highlights the need for additional studies as a means of better understanding the underlying mechanisms.