Ultrasonic image formation of electrical breakdown region in epoxy resin blended with silica powder as filler material

The only practical way to visualize the electrical breakdown region in insulating materials has long been to section a specimen with a cutting machine like microtome, since they are usually opaque, or at best, translucent. We already demonstrated that electrical trees in simple materials, e.g., polyethylene, could be inspected by an ultrasonic visualization method. As the next step of our study, we began to try to obtain ultrasonic images of the electrical breakdown region that was formed in compound material blended with filler powder. However, it was difficult by far, as was expected, to make the ultrasonic image because the reflection signal from the target (electrical breakdown region) was screened by heavy scattering noises dispersed by filler particles. So we prepared the specimens in which silica powder with different particle sizes and contents were blended. From the experimental results, it became clear that when the mean diameter of filler particle was smaller than the value determined by the theory of Rayleigh scattering, the influence of scattering was reduced abruptly and clear ultrasonic images could be obtained.