The Effect of Long-Term Electron Irradiation on Trap Distribution Characteristics of Polyimide

Polyimide is one of the key insulation materials for electrical and electronic components, including solar arrays and their drive assembly, which is subjected to long-term exposure to low-energy electron irradiation of electronvolt to kiloelectronvolt in low-earth orbit. The charge accumulation in dielectric materials due to electron radiation is the significant cause of discharges and anomalies in aerospace systems, and trap distribution characteristic plays an important role in revealing the charge accumulation mechanism. Therefore, in this article, the thermally stimulated depolarization current (TSDC) experiment is carried out to investigate the trap distribution characteristics of polyimide after long-term exposure to 20-keV, 8-nA/cm
$^{\mathbf{2}}$
electron irradiation. The results indicate that with increasing electron irradiation time, the trap levels gradually increase. The
$\alpha$
trap density increases with time and reaches a maximum increase of 1032.1% at 432 h compared to the original sample. The growth rate of the
$\alpha$
trap density is small during 0–48 h, increases during 48–192 h, and decreases during 192–432 h. The
$\gamma$
and
$\beta$
traps decrease during 0–48 h and then increase by 210.7% and 174.4% during 48–432 h. The effects of structural changes on trap distribution characteristics are investigated through electron paramagnetic resonance (EPR), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD) experiments. It is found that the presence of free radicals during electron irradiation increases the
$\alpha$
trap density. The growth rate of free radical relaxation time is positively correlated with the growth rate of
$\alpha$
trap density. During 0–48 h, a small amount of free radicals in the sample mainly induce imidization reactions and C–C cross-linking. This leads to a decrease in end groups and the formation of cross-linking networks, resulting in a decrease in
$\gamma$
and
$\beta$
trap density separately. From 48 to 432 h, a large number of free radicals induce C–C bond breakage, C–O, and C
$=$
O bond recombination, resulting in molecular chain fragmentation and an increase in
$\gamma$
and
$\beta$
trap density. The findings of this study can provide a reference for the failure mechanisms of polyimide applied on spacecraft and the design of radiation-resistant materials.