Observation of electrical treeing in epoxy resin by X-ray phase contrast imaging

Epoxy resins are widely used as insulating material in electrical apparatus. As such epoxy resins are non-transparent due to the effect of fillers, it is hard to optically observe electrical treeing in them. In this paper, we observe electrical trees in unfilled and filled epoxy resins by X-ray phase contrast imaging. When X-ray passes through very small tubular material such as electrical trees, edge-enhancement images are obtained. It makes easier to detect trees, but, on the other hand, wave-optical analysis is necessary to obtain a tree diameter accurately from an apparent tree diameter in an X-ray image. Through the computational wave-optical analysis, it turns out that the apparent tree diameter converges to a certain value with decreasing the diameter of measured tree. In addition, optimal arrangement of the equipment and minimum detective value are investigated based on this calculation. In case of unfilled epoxy, minimum detective value in the condition of optimal arrangement is found to be 3.5μm.