Electrical characterization of polymeric insulation by electrically stimulated acoustic wave measurements

Widespread use of polymeric insulation has motivated interest in understanding the physics associated with the electrical aging process. Under DC bias, materials such as polyethylene (PE) and poly(methylmethacrylate) (PMMA) have been known to accumulate space charge. As charge builds up within the bulk, the electric field becomes modified with regions of higher and lower field. Detection of the accumulated resident space charge profiles resulting from electrical stress gives insight into breakdown and loss mechanisms of the material. This thesis presents the Electrically Stimulated Acoustic Wave (ESAW) method of determining space charge profiles in polymeric insulation. The method is a non-destructive means of observing one-dimensional space charge accumulation, in-situ, during electrical excitation. ESAW was used to observe the effects of DC electric field in samples of PE and PMMA. Polarization was observed at all temperatures measured between 20 and 900 C with fields applied from 1 to 4K in PMMA. Heterocharge extends deep into the bulk of 1 cm thick samples. Negative charge injection appears to increase with temperature above 40* C. At a temperature as high as 900 C, cathode injection overwhelms the positive heterocharge producing a net negative charge within the sample, and continues until the surface field is substantially reduced, demonstrating a space charge limited condition. PE exhibits cathode injection at every temperature measured between 20 and 600 C. At higher temperatures, the conduction appears to become space charge limited once the injected charge increases sufficiently. Evidence of bulk polarization in PE only appears at a temperature as low as 200 C A virtual cathode condition was imposed on PMMA samples by shallow implantation with an electron beam. The charge was then imaged across the unirradiated side with a DC voltage. The voltage needed to cancel the surface image charge was determined using ESAW. This set of experiments allowed the testing of bulk properties of the plastic without influence from the electrodes. Using the virtual cathode data, in combination with experiments of other researchers, approximations have been made for the density of states, electron mobility, equilibrium trapped charge concentration, and the effective activation energy. The conduction in PMMA is consistent with a polaron transport theory within the material and agrees with conductivity results from previous works of other authors. Contributions of this work include: the development of a method for detecting distributed space charge in polymeric insulation with a better resolution than any other available technique, the construction of a density of states model for PMMA, and a unique way to distinguish spacecharge-limited from injection-limited currents in insulators. Thesis Co-Supervisor: Dr. Chatham M. Cooke Title: Lecturer in Electrical Engineering and Computer Science Thesis Co-Supervisor: Professor Stephen D. Senturia Title: Professor of Electrical Engineering and Computer Science I_ __~_C _^_I~I__II____ ~~1~_~__ _______ ___