**2. Materials and Methods**

This section includes the steps followed for the preparation of NF dispersions and the experiments undertaken for the study the stability and the conduction of measurements of dielectric properties.

EnvirotempTM FR3TM, purchased by Cargill, was used as matrix. It is a renewable, bio-based natural ester dielectric coolant, which is formulated from seed oils and performance enhancing additives with a density of 0.92 g·cm−<sup>3</sup> at 20 ◦C, relative permittivity 3.2, conductivity 5 <sup>×</sup> <sup>10</sup>−<sup>14</sup> <sup>S</sup>·m<sup>−</sup>1, and flash and fire points greater than 250 and 300 ◦C, respectively. The commercially obtained dielectric alumina and semi-conducting SiC NPs were used as received without any further treatment. These NPs were selected considering their similar nominal size and permittivities. Some of the most important dielectric and thermal properties of the NPs are presented in Table 1.


**Table 1.** Dielectric and thermal properties of the NPs.

## *2.1. Preparation of Nanofluid Samples*

A two-step method was implemented for the production of NF samples. Primarily, a volume of 500 mL of FR3TM was picked as base oil for the formation of NFs. Each matrix sample was filtered and dried in hot air oven at 120 ◦C overnight in order to achieve moisture reduction within the prescribed limits. The level of moisture within each sample after drying was confirmed with Karl Fischer titration method, following the procedure reported elsewhere [31]. In brief, Mitsubishi Chemicals Co. CA-100 moisture meter and Metrohm KF Coulometer 831 measured and confirmed, respectively, that the remaining moisture was below the recommended values.

Subsequently, 18.4 mg of the two types of NPs under study were immersed in appropriate base oil volumes to achieve the desired *w*/*w* concentration. As NPs have high surface areas [1,32,33] they tend to agglomerate shortly after their addition in the matrix forming clusters, where primary particles are held together by van der Waals interaction forces [32,33].

In order to avoid the agglomeration of NPs, which diminishes their beneficial effects on dielectric strength [1,7,32,33], each mixture was subjected to ultrasonication and vigorous magnetic stirring to achieve good dispersion of the NPs and homogenization of the final NF samples. In fact, each sample was sonicated for 30 min with the assistance of a Raypa UCI-50 ultrasonic bath, followed by a 15 min stirring with the use of a Biosan MSH-300 magnetic stirrer. The maximum input power of the sonicator is about 300 W with a frequency of 35 kHz. Each NF sample was subjected to ultrasonication under full power and the amount of ultrasonication energy was in the range of 540 kJ. This cycle was repeated three times for both NF samples, ensuring uniform dispersion of the colloidal suspensions, as indicated in [7]. Table 2 depicts the labeling of the NF samples, as well as the base oil, which will be submitted in the same experiments as well, while images of the two NF samples are shown in Figure 1 immediately after their preparation.

**Table 2.** Details of nanofluids studied in this work.


**Figure 1.** The NF samples shortly after their synthesis: sNF left; iNF right.
