Abstract
The steady-state electrical conduction current for single and multilayer polyimide (PI) nanocomposite films was observed at the low and high electric field for different temperatures. Experimental data were fitted to conduction models to investigate the dominant conduction mechanism in these films. In most films, space charge limited current (SCLC) and Poole–Frenkel current displayed dominant conduction. At a high electric field, the ohmic conduction was replaced by current–voltage dependency. Higher conduction current was observed for nanocomposite films at a lower temperature, but it declined at a higher temperature. PI nanocomposite multilayer films showed a huge reduction in the conduction current at higher electric fields and temperatures. The conclusions derived in this study would provide the empirical basis and early breakdown phenomenon explanation when performing dielectric strength and partial discharge measurements of PI-based nanocomposite insulation systems of electric motors.
Highlights
Polyimide (PI) films are a widely used insulating material for engineering industries. These thin films are mainly used in electronic devices, multilayer surface coatings on metals, coatings on intermetallic compounds, temperature protection blankets in space crafts and magnetic wire enamelling for electric motor insulation [1,2]
The second slope for PI, FPI, PI/SiO2 and PI-PI/SiO2 films obtained at the high electric field has a higher value, which seems to correspond either to the trap-filled region or other conduction mechanisms
space charge limited current (SCLC) relates to the mobility of holes and electrons, while other conductions relate to ion donors and acceptor sites present in bulk, which need thermal or electrical energy to participate in the conduction by giving their space to neighboring electrons or holes, depending on their trap energy level
Summary
Polyimide (PI) films are a widely used insulating material for engineering industries. Fluorine-coated PI films have shown promising improvements in results by applying a thin layer of conductive surface coating on the top and bottom of these films [30,31] These huge improvements in dielectric properties are conditioned on a better dispersion and interface region of nanoparticles, which is not easy to achieve, because these tiny particles can agglomerate. Mott and Gurney have proved that the maximum current density J that corresponds to the space charge saturation (filled traps) inside the material, in a perfect dielectric without intrinsic carriers and without electron holes, will be given by Equation (4) [38]
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