Ultra-stable Transformer Oil nanofluids with significant AC Breakdown Voltage enhancement at ultra-low filler fraction of functionalized boron nitride nanosheets

This paper reports the purposeful fluorination of hexagonal boron nitride (h-BN) nanofillers and its impact on reinforcement of AC breakdown strength and stability of transformer oil (TO) nanofluid. Fluorine functionalized boron nitride nanosheets (f-BNNs) of ˜5 nm thickness was synthesized in house via wet synthetic exfoliation route of pristine h-BN utilizing Ammonium Fluoride (NH4F) as the shedding agent. This promotes attachment of some highly electro- negative fluorine atoms to boron. This tailored f-BNNs exhibit a diminished band gap and induced electrical conductivity which helps in elevating the AC breakdown Voltage to 26 %- 20% and a surge in resistivity at appreciably low nanofiller fraction of 0.005-0.01wt. %. These noteworthy improvement of electrical insulation properties compared to the state-of-the-art Boron nitride nanoparticles or nanosheets is explained by the parallel role played by fluorine in charge trapping as well as the role played 2D morphology of the nanofillers. Here, fluorine facilitated extrinsic energy bands in the oil-nanofiller interface acted as efficient charge trapping sites and helped to accumulate large quantities of streamer charges, more than h-BN nanosheets or BN nanoparticle for a longer time and improved the electric insulation properties to a large extent. The ultra-high steadiness of the nanofluid is also observed at these lower filler concentrations. 2D morphology, lipophilicity and electro-negativity induced electrostatic repulsion between the f-BNNs nanosheets are attributed to achieve this alluring property of the nanofluid. Such significant improvements at very low filler fractions justifies the fluorination of hexagonal boron nitride as a novel idea and a better alternative among all the reported BN brothers for high voltage applications of nano engineered liquid insulation