109 / 2022-03-13 16:22:17

Role of free volume on breakdown of epoxy nanocomposites: measurement and manipulation

free volume,surface modification,electrical breakdown,epoxy nanocomposites

Purpose/Aim

Internal free volume, which is strongly associated with the chain mobility and transport behaviors of charge carriers in polymer dielectrics, has been proved capable of affecting electrical properties. In this paper, an experimental method was applied to quantitively measure the internal free volume (V_f) of epoxy nanocomposites. V_f was effectively reduced by incorporating organically modified nano-Al₂O₃ fillers. As a result, breakdown filed of nanocomposites was enhanced. The underlying mechanism of the resulted filler-matrix interaction which was responsible for the adjustment of V_f was also unveiled at molecular level.

Experimental/Modeling methods

Four types of EP/Al₂O₃ nanocomposites were prepared via casting method with filler contents at 0, 1, 5, and 10 wt%. Nano-Al₂O₃ was surface-modified by 3-methacryloxypropyltrimethoxylsilane (KH570) at a mass ratio of 1:1 through mixing, reacting, and drying. Fourier transform infrared spectroscopy (FTIR) and scanning electron microscope (SEM) were employed to investigate the filler-matrix interaction. V_f was obtained via linear thermal expansion experiment adopted with an empirical expression based on the classic Free Volume Theory. E_B was measured using spherical electrodes with ramping DC voltage.

Results/discussion

FTIR results unveiled the chemical mechanism of the filler modification by using KH570, which was through consecutive reactions of dehydration and condensation, leading to the formation of hydrogen bond and covalent bond between the nano-Al₂O₃ and the epoxy molecular chains. SEM images ascertained the satisfying fillers dispersion and filler-matrix bonding at low content (1 wt% and 5 wt%). Consequently, V_f was considerably reduced as the filler content increased to 5 wt%, at which the highest E_B (159.43 kV/mm) was obtained. With further increase of nano-Al₂O₃ to 10 wt%, V_f rose and E_B decreased owing to filler agglomerations as well as the weakened filler-matrix interaction.

Conclusions

By introducing organically modified nano-Al₂O₃ which contributed an effective filler-matrix interaction via hydrogen bonding as well as covalent bonding, the free volume fraction of EP/Al₂O₃ nanocomposites was successfully altered, and a maximum DC breakdown strength was obtained when the filler content was 5 wt%.