Surface Modification-Dominated Space-Charge Behaviors of LDPE Films: A Role of Charge Injection Barriers

Abstract

Gradually increasing power transmission voltage requires an improved high-voltage capability of polymeric insulating materials. Surface modification emerges as an easily accessible approach in enhancing breakdown and flashover performances due to the widely acknowledged modification of space-charge behaviors. However, as oxidation and fluorination essentially react within a limited depth of 2 μm underneath polymer surfaces, the nature of such bulk space-charge modulation remains a controversial issue, and further investigation is needed to realize enhancement of insulating performance. In this work, the surface oxidation-dependent space-charge accumulation in LDPE film was found to be dominated by an electrode/polymer interfacial barrier, but not by the generation of bulk charge traps. Through quantitative investigation of space-charge distributions along with induced electric field distortion, the functions of surface oxidation on the interfacial barrier of a typical dielectric polymer, LDPE, is discussed and linked to space-charge behaviors. As the mechanism of surface modification on space-charge behaviors is herein proposed, space-charge accumulation can be effectively modified by selecting an appropriate surface modification method, which consequentially benefits breakdown and flashover performances of polymeric insulating films for high-voltage applications.

Keywords: LDPE; charge injection; high voltage; insulating polymer; space charge; surface oxidation.

Figure 1

XPS spectra of as-prepared LDPE film and LDPE films with surface oxidation durations of 1–6 h. (a) XPS spectrum in binding energy range of 200–1200 eV; (b) The C 1s XPS spectrum; (c) The O 1s XPS spectrum; (d) The O Auger electron XPS spectrum.

Figure 2

TSC spectra of as-prepared LDPE film and LDPE films with surface oxidation durations of 1–6 h. (a) TSC spectra in temperature range of 170–365 K; (b) The variations in trap level and trap amount with changes in surface oxidation duration, obtained from TSC results.

Figure 3

PEA results of as-prepared LDPE film and LDPE films with surface oxidation durations of 1–6 h. (a) PEA result of as-prepared LDPE film; (b) PEA result of LDPE film after 1 h surface oxidation; (c) PEA result of LDPE film after 6 h surface oxidation; (d) Comparison of space-charge accumulation of LDPE after surface oxidation, tested by ISPD, PEA and TSC methods.

Figure 3

PEA results of as-prepared LDPE film and LDPE films with surface oxidation durations of 1–6 h. (a) PEA result of as-prepared LDPE film; (b) PEA result of LDPE film after 1 h surface oxidation; (c) PEA result of LDPE film after 6 h surface oxidation; (d) Comparison of space-charge accumulation of LDPE after surface oxidation, tested by ISPD, PEA and TSC methods.

Figure 4

The calculated energy band structures of LDPE with a polymerization index of 10 (C₂₀H₄₂) and surface oxidation treated LDPE (C₂₀H₄₀O).

Figure 5

Variations in energy band structures of LDPE with surface oxidation treatment. (a) Molecular structures of surface oxidized LDPE with gradually increased grafted O atoms; (b) The calculated energy band structures of surface oxidized LDPE with gradually increased grafted O atoms.

Figure 6

Correlations between space-charge quantity (obtained from PEA experiments) and charge injection barrier (obtained from molecular simulations).

Figure 7

Electrode/polymer interfacial barrier-dependent space-charge accumulation characteristics of LDPE. (a) Total quantity of space charges in LDPE under 1 kV DC voltage (1 × 10⁷ V/m) with 1.1–1.5 eV charge injection barriers; (b) Total quantity of space charges in LDPE under 5 kV DC voltage (5 × 10⁷ V/m) with 1.1–1.5 eV charge injection barriers; (c) Total quantity of space charges in LDPE under 9 kV DC voltage (9 × 10⁷ V/m) with 1.1–1.5 eV charge injection barriers; (d) Changes in total quantity of space charges under variations in applied voltage (1–9 kV) with 1.1–1.5 eV charge injection barriers.

Figure 7

Electrode/polymer interfacial barrier-dependent space-charge accumulation characteristics of LDPE. (a) Total quantity of space charges in LDPE under 1 kV DC voltage (1 × 10⁷ V/m) with 1.1–1.5 eV charge injection barriers; (b) Total quantity of space charges in LDPE under 5 kV DC voltage (5 × 10⁷ V/m) with 1.1–1.5 eV charge injection barriers; (c) Total quantity of space charges in LDPE under 9 kV DC voltage (9 × 10⁷ V/m) with 1.1–1.5 eV charge injection barriers; (d) Changes in total quantity of space charges under variations in applied voltage (1–9 kV) with 1.1–1.5 eV charge injection barriers.

Figure 8

Space-charge distributions and induced electric field distortion inside LDPE film, under 5 kV (5 × 10⁷ V/m) applied voltage. (a) Distribution of space charges along film depth direction in LDPE with 1.1 eV charge injection barrier; (b) Distribution of space charges along film depth direction in LDPE with 1.5 eV charge injection barrier; (c) Distribution of electric field along film depth direction in LDPE with 1.1 eV charge injection barrier; (d) Distribution of electric field along film depth direction in LDPE with 1.5 eV charge injection barrier.

Figure 8

Space-charge distributions and induced electric field distortion inside LDPE film, under 5 kV (5 × 10⁷ V/m) applied voltage. (a) Distribution of space charges along film depth direction in LDPE with 1.1 eV charge injection barrier; (b) Distribution of space charges along film depth direction in LDPE with 1.5 eV charge injection barrier; (c) Distribution of electric field along film depth direction in LDPE with 1.1 eV charge injection barrier; (d) Distribution of electric field along film depth direction in LDPE with 1.5 eV charge injection barrier.