Comparative dielectric and physicochemical assessment of surfactant-free TiO2 and SiO2 nanofluids for high voltage transformer mineral oil

This study compares semiconductive TiO 2 and insulating SiO 2 nanoparticles as separate, surfactant-free additives to the same transformer mineral oil under a matched preparation and testing framework. The novelty is not the first use of nanofluids, but the controlled head-to-head evaluation of two electronically distinct oxide classes while minimizing the protocol differences that commonly prevent direct comparison across published studies. Five low loadings (0.01–0.10 wt%) were screened for kinematic viscosity, moisture content, and room-temperature AC breakdown voltage; relative permittivity and DC conductivity were additionally characterized from 30 °C to 90 °C. TiO 2 produced the largest measured AC breakdown improvement, 22.16% at 0.10 wt%, whereas SiO 2 reached its maximum improvement, 15.00%, at only 0.01 wt%. At 0.10 wt%, TiO 2 also produced a larger viscosity and DC-conductivity penalty than SiO 2 , revealing a clear performance trade-off between dielectric strengthening and charge transport. The results therefore provide a comparative concentration-selection map: semiconductive TiO 2 favors higher-loading breakdown enhancement, while insulating SiO 2 achieves its optimum response at trace loading with a smaller conductivity increase. These conclusions are restricted to the investigated concentration and test windows; quantitative long-term stability, temperature-dependent breakdown, thermal/oxidation performance, and oil-paper ageing remain necessary before transformer implementation.