TEM observation of electrical tree paths and micro-structures in polyethylene

The starting point of AC electrical trees in polyethylene and the surrounding microstructures were observed by a transmission electron microscope (TEM). To minimize the size of tree paths the authors developed an apparatus for preparing specimens in which the tree growth was suppressed to less than 10 mu m in length. The tree was started from a needle electrode made of carbon-filled polyethylene. The tree was a hollow pipe of about 100 nm in diameter immediately after its generation. There were constrictions inside the tree, resulting in a corrugated surface. The starting point of the tree was a microscopic convex region of several tens to 100 nm across, which results from the cohesion of carbon particles of the electrode. The AC tree path was virtually parallel to lamellae.<>