AIR FORCE RESEARCH LAB KIRTLAND AFB NM KIRTLAND AFB
The transport of electrons through a highly disordered insulating material HDIM is complex and drives our understanding of how spacecraft develop and dissipate potentially damaging electrostatic charge when exposed to the energetic charged particle environment of space. The electrical conduction of spacecraft materials is the prime driver of spacecraft charging. Theories developed to describe doped semiconductors predict that the conduction mechanisms are determined by the distribution and occupancy of electrons in localized electron trap states above the dark current Fermi level. It follows that the density and energy distribution of these trap states plays a key role in the conduction process. However, the density and energy distribution of these traps are not static on orbit and are constantly changing as a result of radiation damage. As a result the electrical conductivity and the optical properties reflection, absorption, and transmission are dynamic and cannot be assumed to be the same throughout the mission life of a spacecraft. To elucidate this problem we have undertaken this study to identify the chemically specific damage pathways that result as a consequence of electron bombardment. Here we have shown that radicals formed during electron bombardment play a key role in enhancing the conductivity of polyimide. These radicals are shown to be the likely result of phenol ring rupture in the polyimide monomer. This radiation induced structural change is also observed in the infrared absorption spectrum and the fundamental band gap as shown by UVVIS transmission spectroscopy. We also show that exposure to air causes these radiation induced morphological changes to rapidly revert to their pre-damaged state. This calls into question the material handling procedures of the last 40 years of similar studies. Polyimide is the prime material under investigation in the study however, Mylar and low density polyethylene will also be discussed.