Chemical analysis of tree growth in epoxy resin using AFM-IR spectroscopy

Abstract

Recent advances have allowed submicron features of electrical trees to be imaged. Until recently it has not been possible to analyze chemical aging on the same scale. Here, the use of atomic force microscopy — infrared spectroscopy (AFM-IR) to characterize chemical degradation around a needle tip, and in the vicinity of electrical tree branches grown in epoxy resin is explored. The spectral signatures associated with the degradation of epoxies during electrical treeing are successfully identified with 50 nm resolution. This allows identification of chemical degradation in unprecedented detail. The method of polishing and grinding to prepare the samples was found to remove degraded material from exposed channels. This issue was overcome by not fully exposing the tree, but bringing it close enough to the surface that the AFM-IR was able to see within the channel. A short tree examined immediately after initiation showed no chemical degradation, indicating that its growth was not a chemically active process, suggesting electromechanical fracturing is more likely. Mature channels showed no singular chemical signature. Carbonyl groups were identified in all mature channels and were particularly concentrated at the branching points studied, suggesting higher levels of degradation during branching or accumulation at points of bifurcation. It is believed that these reactions are precipitated by hot electron impacts during discharges. AFM-IR has provided unprecedented detail concerning chemical damage incurred during the treeing process and can provide vital information to inform models of aging.