Tree propagation speeds in epoxy nanocomposites

It was newly found that nano filler particles will decelerate tree growth or accelerate it according to the level of applied voltage. It is extremely interesting as for how they will act on tree propagation. There are three regions recognized for tree propagation speeds. That is, a tree grows starts but stops under a certain lower voltage (Region 1), it grows up to the opposite electrode above a certain higher voltage (Region 3), and it grows at a constant speed between the two voltages (Region 2). Nano filler particles will decelerate tree growth in Regions 1 and 2 and accelerate it in Region 3. “Cross over phenomenon” in tree length vs. applied voltage characteristics obtained in the past can be explained by the new basic finding. Two different mechanisms must work on tree growth. It is demonstrated that a PD erosion process is dominant in Region 1, while a rapid tree dielectric breakdown process is major in Region 3. Especially acceleration by nano particles is interesting to be explored in terms of interaction of tree propagation with the electrostatic force predictively emerged around nano filler particles. Experiments were carried out as follows. Tree propagation speeds were obtained in epoxy and its nanocomposite in successive three steps until a tree grows up to 10, 100, and 3000 μm, respectively. This measurement was enabled by detecting a certain PD magnitude leading to a power source shutdown. Detailed discussion is given in the paper particularly on mechanisms for above new findings.