Breakdown characteristics of polyethylene/silicon nitride nanocomposites

Silicon nitride (Si3N4) has been utilized as a nanofiller in polymeric insulation due to its good characteristics in both electrical insulation and thermal conduction properties. In this work, a comparative study was performed between unfilled polyethylene and polyethylene containing different amounts of Si3N4 nanofiller. The study showed that the low density polyethylene (LDPE) added with 15 wt% of Si3N4nanofiller could have higher breakdown strength compared to equivalent LDPE with 10 wt% of Si3N4nanofiller. Morphological characterizations of the nanocomposite samples were performed using field emission electron microscopy (FESEM) and the results showed that the breakdown performance of the investigated materials were affected by the agglomeration of Si3N4 nanoparticles.


Methodology
This section presents the sample preparation and the methods to perform breakdown strength test and morphological characterization.

Sample Preparation
Low density polyethylene (LDPE) was chosen as the polymeric matrix since PE has been generally used in manufacturing high voltage underground cables. Si3N4, with a manufacturer-quoted size of 15-30 nm, was chosen as the nanofiller due to its non-oxide surface properties. The nanocomposites were prepared using a mechanical blending method. In order to prepare the nanocomposites, the LDPE and silicon nitride nanofiller were first mixed together in a plastic bag. Then, they were melted at 140 o C for ~25 min in the two-roll mill machine to obtain nanocomposite lumps. Test specimens for breakdown testing were subsequently prepared by melt pressing the nanocomposite lumps using a hydraulic laboratory press at 160 o C temperature and 3 ton load. Each test specimen had a thickness of ~100 µm. Table 1 shows the composition of LDPE with Si3N4 nanofiller at loadings of 0 wt%, 5 wt%, 10 wt% and 15 wt%.

Breakdown Testing
The breakdown strength test was conducted by injecting the step voltage with 2 kV voltage for every 20 s for DC breakdown, and 1 kV voltage for every 20 s for AC breakdown. A digital multimeter was used to display the breakdown voltage after the specimen experienced breakdown. Then, the breakdown voltage readings were recorded and analyzed by using the two-parameter Weibull distribution analysis. Fifteen breakdown values were collected for each specimen type. The breakdown strength value was determined based on (1). The probability of dielectric breakdown strength is determined by using Weibull distribution analysis Bernard's approximation as in (2).
In (2), i represents the rank of dielectric breakdown strength, Ebd of specimen from low to high, while N is the assigned number of measured values from the breakdown tests. The probability of breakdown strength was estimated by using the cumulative distribution function (CDF) shown in equation (3). The probability of dielectric breakdown strength of the specimen is represented as F(Ebd; α, β) while α is the scale parameter and β is the shape parameter of the Weibull graph.

Morphological Characterization
The LDPE, LDPE-5 and LDPE-15 samples were characterized using field emission scanning electron microscope (FESEM). This allowed the dispersion state of Si3N4 nanoparticles in the polymeric matrix to be determined. Prior to FESEM characterization, each sample was coated with platinum using an automated platinum sputter coater. The FESEM machine was set at high magnifications with a spectrum voltage of 10 kV.

Results and Analysis
This section discusses on breakdown and morphological characterization results.

Breakdown Results
The breakdown results are divided into two sections. Section 1 will discuss on the DC breakdown results and the comparison with the previous studies. Section 2 will discuss on the AC breakdown strength test results and the comparison of results obtained with the previous studies.

DC breakdowns results
The Weibull plots of DC breakdown strength are shown in Figure 1 while the Weibull parameters are shown in Table 2. The scale parameter α represents the breakdown strength while the shape parameter β represents the width of the Weibull distribution. Based on Figure 1, the DC breakdown strength for polyethylene nanocomposites of LDPE-5 sample was the closest to pure polyethylene. The lowest breakdown strength result was LDPE-10 sample with a scale parameter of 117 kV/mm. The breakdown result of LDPE-15 sample showed an improvement of 12% with a scale parameter of 131 kV/mm compared to the LDPE-10 sample.

AC Breakdowns Strength Test
The Weibull distribution analysis was carried out and the Weibull plots for AC breakdown strength are shown in Figure 2. The Weibull parameters are shown in Table 3. From Figure 2 and Table 3, the highest AC breakdown results was possessed by LDPE with the shape parameter β=10.04. The breakdown results for the LDPE-5 sample is slightly lower, which was 4.2% lowered than LDPE, with scale parameter 136 kV/mm, followed by the LDPE-10 sample and the LDPE-15 sample, with 106 kV/mm and 90 kV/mm respectively.   Figure 3 shows the difference between AC and DC breakdown results. Clearly, these nanocomposites samples behaved differently under AC and DC applied fields. It can be noticed that the detrimental effect of Si3N4 on LDPE is less significant in AC than in DC. These DC and AC breakdown trends are not unusual and have also been reported elsewhere for SiO2-filled LDPE [24,25]. With increasing amounts of Si3N4, the AC breakdown strength of the nanocomposites reduced accordingly. Meanwhile, the DC breakdown strength for nanocomposites with 15 wt% of Si3N4 increased slightly compared to those with 10 wt% of Si3N4. In this case, the addition of a high amount of Si3N4 resulted in better breakdown performance, albeit that the value was not comparable to that of the unfilled LDPE sample.

Morphological Characterization of LDPE Containing Nanofiller
Morphological characterization of the LDPE/Si3N4 nanocomposite samples was carried out to investigate the dispersion of Si3N4 nanoparticles in the LDPE matrix. The FESEM micrographs for the unfilled LDPE, LDPE-5 and LDPE-15 samples are shown in Figure 4, Figure 5 and Figure 6 respectively. Figure 4 is the micrograph taken by FESEM for the reference, unfilled LDPE sample. There were no nanoparticles present in the LDPE matrix. Meanwhile, the images for the LDPE-5 and LDPE-15 samples shown in Figure 5 and Figure 6, respectively, showed the presence of nanoparticles in the LDPE matrix; these images display the agglomerated state of nanoparticles. Of note, more agglomerations were found in the LDPE-15 sample. While it was expected that increasing nanofiller amounts would lead to reduced breakdown strength, this did not happen for the case of LDPE-15 sample in comparison with the LDPE-10 sample. Nevertheless, an alternative method to fabricate the samples could be explored to improve the dispersion the nanoparticles across the samples in an attempt to improve the breakdown performance.

Conclusion
This study was conducted to investigate the breakdown strength of unfilled polyethylene and polyethylene nanocomposites containing high amounts of silicon nitride nanofillers. It can be concluded that the breakdown strength of polyethylene nanocomposites under AC and DC were lower compared to unfilled polyethylene. However, in comparing the breakdown performance of LDPE-15 and LDPE-10 under the DC field, LDPE-15 sample showed 12% improvement in breakdown strength compared to the LDPE-10 sample. Therefore, the results from this study showed that the presence of a high amount of silicon nitride nanofillers in LDPE did not necessarily result in reduced breakdown strength, but better breakdown performance could be achieved.