Practical Notes on the Measurement and Interpretation of Thermally Stimulated Discharge Currents in Biaxially Oriented Polypropylene

Abstract

The measurement and interpretation of high-field thermally stimulated depolarization current (TSDC) of capacitor-grade biaxially oriented polypropylene (BOPP) films are investigated in order to facilitate the derivation of trap parameters. Two main relaxations associable with the glass transition and detrapping of space charge from deep traps are observed, and their dependence on the polarization/depolarization conditions are studied. The complex bipolar nature of the space charge relaxation makes the derivation of trap parameters challenging. Lastly, a brief attempt is made to correlate the TSDC features with morphology and other short-term dielectric properties.

Charge trap spectroscopy in polymer dielectrics: a critical review

“…Rytöluoto et al [131] reported recently on TSDC from Polypropylene capacitors. Trap energies in the range from 0.3 to 1.3 eV were obtained by multi-peaks fitting, with positive or negative contributions to the glow peaks.…”

Biaxially oriented silica–polypropylene nanocomposites for HVDC film capacitors: morphology-dielectric property relationships, and critical evaluation of the current progress and limitations

“…For neat BOPP films, the typically observed TSDC features include 18,86 : (i) a small heteropolar (α-) relaxation peak at −6 °C associable with de-trapping of charge from shallow traps as the polymer passes the glass-transition (cf. Table S1) and (ii) a strong (bipolar) space charge (ρ-) relaxation in high temperature region (~100 °C) which is due to the thermal release of captured charges from deep traps.…”

“…For the silica compounds however, the TSDC spectra above 𝑇 𝑔 became more pronounced and complex, indicating higher presence of shallow (or rather, "inter-level" 87 ) traps with trap depths in the 0.7-0.9 eV range in comparison to neat BOPP. In the high temperature region (~90 °C and above), the unfilled BOPP films exhibited strong TSDC peaks corresponding to charges released from deep traps (in the range of 1.1-1.3 eV), often being anomalous in the sense that the current flow was in the same direction as the polarization current (polarity reversal) which is a result from drifting of thermally de-trapped charge under the internal field caused by the space charge 86 . For the silica-BOPP films however, the (bipolar) TSDC intensity in the deep trap region was greatly reduced, being indicative of modification of the apparent (occupied) deep trap density with nanosilica.…”

“…The fact that polymers exhibit complex TSDC spectra comprising of several overlapping of (quasi-) continuous trap bands-along with the TSDC polarity reversal effect observed in the deep trap region in Figure 9a-complicate the analysis and evaluation of trapping parameters (trap depth and density of occupied states) as discussed in detail elsewhere 86,88 . Due to these reasons we do not attempt e.g.…”

“…Due to these reasons we do not attempt e.g. peak-fitting 86 , deconvolution 89,90 or numerical methods 91,92 to derive trap parameters here, as the results would be largely inconclusive 88 . Nevertheless, the TSDC results presented herein clearly indicate a modification of trap density of states (DOS) and a more homogeneous distribution of charge in the silica-containing samples, and the results are consistent with the PEA space charge depolarization behavior measured from the nonoriented cast films (Figure 4), thereby confirming the modification of charge trapping properties also in silica-BOPP thin films.…”