Analysis of Selected Dielectric Properties of Epoxy-Alumina Nanocomposites Cured at Stepwise Increasing Temperatures

The paper presents the effects of gradual temperature curing on the dielectric properties of epoxy nanocomposite samples. Samples were prepared based on Class H epoxy resin filled with nano-alumina (Al₂O₃) for different wt% loadings (0.5 wt% to 5.0 wt%) and two different filler sizes (13 nm and <50 nm), i.e., two different specific surface area values. During the research, specimen sets were cured gradually at increasingly higher temperatures (from 60 °C to 180 °C). Broadband dielectric spectroscopy (BDS) was used to determine the characteristics of the dielectric constant and the dielectric loss factor in the frequency range from 10⁻³ Hz to 10⁵ Hz. As a result, it was possible to analyze the impact of the progressing polymer structure thermosetting processes on the observed dielectric parameters of the samples. The nano-Al₂O₃ addition with 0.5 wt%, 1.0 wt%, and 3.0 wt% resulted in a decrease in dielectric constant values compared to neat epoxy resin samples. The most significant reductions were recorded for samples filled with 0.5 wt% of 13 nm and <50 nm powders, by about 15% and 11%, respectively. For all tested samples, the curing process at a gradually higher temperature caused a slight decrease in the dielectric constant (approx. 2% to 9%) in the whole frequency range. Depending on the nanofiller content and the curing stage, the dielectric loss factor of the nanocomposite may be lower or higher than that of the neat resin. For all tested samples cured at 130 °C (and post-cured at 180 °C), the differences in the dielectric loss factor characteristics for frequencies greater than 100 Hz are low. For frequencies < 100 Hz, there are prominent differences in the characteristics related to the size of the nanoparticle and the individual wt% value. At a small nanofiller amount (0.5 wt%), a decrease in the dielectric constant and dielectric loss factor was observed for frequencies < 100 Hz for samples with nanofillers of both sizes.