Dielectric properties dependent on crystalline morphology of PP film for HVDC capacitors application

Abstract

In this paper, the effects and mechanism of crystalline morphology on dielectric loss and breakdown strength of PP film were studied. The crystalline morphology was modified by adding the β-nucleating agent (β-NA). The crystalline types and crystalline morphologies of samples were analysed by the X-Ray diffraction (XRD) and the polarized optical microscopy (POM), respectively. The current integration, representing the dielectric loss, was investigated using the Q(t) method. The DC conductivity was measured at temperature from 25 to 85 °C. The effects of crystalline morphology on the trap energy level distribution were analysed by the isothermal discharge current (IDC). The DC breakdown strength was tested using a pair of ball-plate electrodes at 85 °C. The results of XRD and POM indicate that the addition of β-NA introduced β-spherulites in PP film and the spherulite size of samples with β-NA are smaller the spherulites size of PP without β-NA. The results of IDC indicate that the special structure of β-spherulites introduced deep trap energy levels. The results of Q(t) and DC conductivity indicate that β-spherulites can effectively suppress the current though samples. Besides, the addition of β-nucleating agent increases the DC breakdown strength at 85 °C.