An Electron and Hole Trap Energy Distribution Estimation Model and Its Application in Polypropylene Nanocomposites

To differentiate the electron and hole traps inside polymers, a model based on the pulsed electro-acoustic (PEA) method is proposed. It is premised that under short-term DC electric field polarization at relatively low temperature, the homocharge is injected into the sample to a shallow depth and dominates the space charge distribution. Thus, the homocharge near the ground electrode and the high voltage electrode can be separated. During depolarization, only the homocharge near the ground electrode is analyzed to reduce the influence of ultrasonic wave attenuation, which also results in unipolar space charge decay behavior being obtained. The isothermal relaxation current (IRC) theory is then employed to obtain the unipolar quasi-continuous trap energy distribution based on the space charge decay. Thereby, the electron and hole traps can be differentiated by applying positive and negative DC electric field polarization, respectively. The experimental results of low density polyethylene (LDPE) verify the validity of the proposed model. Based on the model, polypropylene (PP) and PP/MgO nanocomposites are studied to explore the effect of MgO doping on the trap characteristics. The results show that numerous electron deep traps are introduced into the nanocomposites due to the intrinsic properties of nanoparticles. Besides, the heterogeneous nucleation of the nanoparticles increases the crystal/amorphous and crystal/vacuum interfaces, which may be the main reason for the increase of hole trap density of nanocomposites.