**5. Conclusions**

In summary, two NF samples, containing Al2O3 (iNF) and SiC (sNF) NPs in the same concentration (0.004% *w*/*w*), were synthesized and characterized. Electron microscopy showed that primary particles have a narrow particle size distribution. DLS has revealed rather good stability of the dispersion towards agglomeration over a period of five days. Both dispersions exhibit similar agglomeration behavior. AC, positive and negative LI BDV, and PD activity of the NF samples were measured and compared to the corresponding dielectric properties of a pure base oil sample. AC BDV results indicated that sNF demonstrates the highest improvement in terms of dielectric strength by 16.3%. The statistical analysis also showed that the same sample demonstrates increased AC BDV by 15.6 and 13.7% at 10 and 1% probability levels, providing an indication of enhanced reliability for the uninterrupted operation of the power transformer. These findings have been attributed to the greater ability of SiC NPs to trap charge carriers in shallow traps, because of their higher conductivity.

On the other hand, the increase in positive LI BDV of iNF can be attributed to the positive charge induced by alumina NPs, which results in delaying the streamer propagation from the positive point. Finally, sNF impregnated paper also demonstrates the greatest resistance to the appearance of PD, delaying its initiation by 92% with respect to the paper impregnated to matrix. This finding could be attributed to the role of charge carriers' trapping from the augmented interfacial zone between SiC NPs and matrix. Our future research work includes an attempt to find an appropriate surface modification technique in order to achieve long-term stability of the Al2O3 and SiC dispersions, which will be followed by the conduction of measurements of dielectric and thermal properties to address the effect of surface modification on them.

**Author Contributions:** Conceptualization, K.N.K., I.A.N., E.C.P. and V.P.C.; methodology, K.N.K., I.A.N. and A.E.A.; validation, E.C.P., I.F.G. and S.N.Y.; formal analysis, K.N.K. and I.A.N.; investigation, K.N.K., I.A.N., E.C.P. and V.P.C.; resources, E.C.P.; data curation, I.F.G., A.E.A. and S.N.Y.; writing—original draft preparation, K.N.K. and V.P.C.; writing—review and editing, I.F.G. and S.N.Y.; supervision, E.C.P. and S.N.Y. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** Authors want to express their gratitude to Christina Polyzou of the Laboratory of Physical, Inorganic and Nuclear Chemistry, Department of Chemistry, University of Patras for the assistance in the synthesis of the NF samples.

**Conflicts of Interest:** The authors declare no conflict of interest.
