The effect of surface treatment of silica nanoparticles on the breakdown strength of mineral oil

In previous work, the results of AC breakdown tests showed that unmodified silica nanoparticles improve the breakdown strength of mineral oil based nanofluids, especially at a relatively high humidity level of around 25 ppm. It was proposed that, since the hydrophilic surface of unmodified silica nanoparticles can absorb water, this would lead to a reduction of free moisture in the bulk of the oil, which has a strong influence on the breakdown strength. In the present study this proposition is verified, by comparing the breakdown strength of two mineral oil based nanofluids: a reference with unmodified silica nanofluid and a nanofluid with Z-6011 modified silica. The silane coupling agent Z-6011 turns the surface of silica nanoparticles hydrophobic, thus preventing water adsorption.