Distinct impacts of RAF _filler vs. RAF _crystal on thermal transport processes recorded in amorphous vs. semicrystalline polymer CNT-based nanocomposites.

In this work, the influence of semi-conductive SiC nanoparticles on the AC breakdown voltage and partial discharge development in natural ester oil FR3 is examined. Primarily, the dielectric constant and the electrical conductivity of the nanoparticles are measured following the broadband dielectric spectroscopy technique. The nanoparticles are added into the matrix following the ultrasonication process in three weight percentage ratios in order for their effect to be evaluated as a function of their concentration inside the base oil. The processing of the results reveals that the nanofluid containing SiC nanoparticles at 0.004% w/w demonstrates the highest AC dielectric strength improvement and shows the greatest resistance to the appearance of partial discharge activity. The mechanisms behind the aforementioned results are discussed in detail and confirmed by the broadband dielectric spectroscopy technique, which reveals that this particular nanofluid sample is characterized by lower dielectric constant and electrical conductivity than the one with double the weight percentage ratio.

The improvement of dielectric and thermal properties of insulating oils used in High Voltage (HV) equipment has been a key part of research over the last three decades. In this study, a natural ester oil is used as matrix oil and dispersions of two semi-conducting nanoparticles (NPs), titanium oxide (TiO₂) and silicon carbide (SiC), are prepared in 0.004 weight percentage ratio (0.004% w/w). In terms of thermal features, thermal diffusivity and thermal conductivity are measured over a large temperature range between 25 and 90 °C. By means of the dielectric properties, the Lightning Impulse Breakdown Voltage (LI BDV) of nanofluids (NFs) is measured and compared to the corresponding ones of the matrix. Moreover, the dielectric constant and dielectric dissipation factor (DDF) are determined for the NF and matrix samples in the frequency range of 1 to 10⁶ Hz. The obtained results show that the NF containing SiC NPs is characterized from the greatest increase in thermal conductivity, by 58% with respect to natural ester oil’s, as well as the lowest dielectric constant in both 25 and 90 °C, most likely due to their higher permittivity and thermal conductivity. Lastly, the LI BDV of the same NF is the most enhanced, by 10% relatively to the matrix, while a numerical model in COMSOL Multiphysics is developed and is found to successfully verify the LI BDV results.

This study is focused on exploring the change in the dielectric and cooling efficiency of vegetable oil following the integration of semiconducting SiC and TiO ₂ nanoparticles in two different weight percentages by comparing the corresponding behavior of nanofluids at the time of their synthesis and after one month of ageing. The thermal response has been evaluated by means of diffusivity and conductivity, while dielectric properties are measured with non-destructive tests (relative permittivity and dissipation factor) and destructive ones in terms of lightning impulse withstand voltage. The results of the latter property measurements are also compared to numerical model findings. The analysis of results proves that the properties under study are worsened with increase in nanoparticles’ concentration, probably attributing it to quicker agglomeration, as it is also confirmed by the simulation results.

Magnesium oxide (MgO) nanoparticles are used for nanofluid preparation in an effort to investigate their dielectric response and thermal properties. The matrix oil for the prepared samples is natural ester oil. Lightning impulse voltage (1.2/50 μs) is used to investigate the electrical performance of the different concentration samples. In addition, broadband dielectric spectroscopy is used for the analysis of the dielectric constant and electrical conductivity through a temperature range of 30 °C -90 °C and a frequency spectrum of 0.1 Hz - 1MHz. Thermal properties are studied by means of Laser Flash analysis and Differential Scanning Calorimetry. Thermal conductivity is calculated and the understudied results are evaluated. Results reveal an improvement in different properties for the prepared nanofluid samples, including an enhancement by 15.15 % and 12.63 % of the lightning impulse breakdown voltage and the thermal conductivity, respectively.

Nowadays, nanoparticles are used in insulating oils to enhance their dielectric and thermal properties significantly. On the other hand, natural ester oil is considered one of the most environmentally friendly types of insulating oil. In this study, magnesium oxide nanoparticles with their excellent physical and thermal properties, are used to prepare natural ester oil-based nanofluids. Their electrical performance is investigated by subjecting samples of different nanoparticles concentration to lightning impulse voltage (1.2/50 µs) by following the step-up method. For the generation of the pulses, a one-stage impulse voltage generator was used, while measurements from 20 pulses per voltage level were applied to a needle-to-plane gap so as to estimate the relevant breakdown probabilities. Furthermore, relative permittivity and electrical conductivity are measured for a range of frequencies (0.1 Hz - IMHz) and a range of temperatures (30 °C – 90 °C) by means of broadband dielectric spectroscopy. The thermal properties are studied based on thermal diffusivity measurements by LFA (Laser Flash analysis) for different concentration values and a wide range of temperatures (30 °C – 90 °C). Results reveal the improved performance regarding the dielectric and thermal properties for the prepared natural ester oil-based nanofluids. A 15.15 % improvement in lightning impulse breakdown voltage, more specifically of the U50% voltage, is observed. Additionally, results show a 12.63 % improvement for thermal diffusivity.