Research

During my PhD, my research was primarily focused on development of dispersible nano-structures for the enhancement of insulating and thermal properties of liquid insulations such as transformer oil and its alternatives. I have also worked in the field of solar energy harvesting by plasmonic nanofluids.

In this dissertation, novel 2D nanomaterials like chemically synthesized Amorphous Graphene (a-Gs), Anatase Titanium Dioxide (a-TiO2), exfoliated Hexagonal Boron Nitride (h-BN), Porous Boron Nitride (PBN) have been proposed and explored for efficient thermal management and enhancement of breakdown voltage (BDV), to combat with ever increasing stresses on Transformer Oil.

This research work covers the easy and cost effective synthesis process of these 2D nanofillers and preparation of the Transformer Oil nanofluid with superior electrical and thermal properties. The major contribution of this dissertation is to design next generation nano-engineered Transformer Oil using surfactant free 2D nanofillers, with improved thermal conductivity (around 20-32% increment), BDV (15-41% enhancement) at a very low filler fraction (0.0025–0.01 Wt.%). Further, the unchanged flash and pour points along with oxidation stability of the nanofluids are really appreciable. Finally lesser van der Waals forces at lower filler fraction lead to a surfactant free stable liquid insulation over a long period (several months) with a promising future of industrial applications.

link to download the dissertation

This invention discloses a nanomaterial including amorphous graphene nanosheets and nanofluids involving the same with a base medium or matrix having significantly enhanced electrical insulation and the thermal conductivity useful for high performing transformer oils and the like utilities. The amorphous graphene nanosheets comprises both sp2 and sp3 bonded regions of carbon and variable sized combination of the sp2 and sp3 bonded clusters to provide for selective high thermal conductivity and high electrical insulation, whereas the nanofluid comprises said quasi 2D nanomaterial of amorphous graphene (a-GS) in nanosheets form dispersed in the base medium or matrix to synergistically enhance electrical insulation and thermal conductivity of said base medium or matrix.

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.

This paper explores the fascinating properties of two-dimensional (2D) nanofillers based transformer oil (TO) nanofluids. Nanofluids of 2D hexagonal boron nitride (h-BN) nanosheets in TO demonstrate stable dispersion with improved dielectric breakdown strength and superior thermo physical properties like thermal conductivity, viscosity and stability. An appreciable augmentation in AC breakdown voltage (BDV) is observed compared to the state-of-the-art boron nitride (BN) particles. This enhancement in BDV is elucidated by the role of the greater surface area of Maxwell-Garnet ‘oil-sheet’ interfacial region of the 2D morphology in charge trapping perspective. The faster rate of heating and cooling along with noteworthy enhancement in thermal conductivity is due to the interfacial heat transfer via 2D nanoadditives prompting good phonon transport which agrees with Maxwell's forecasts. Addition of 2D nanofiller at diluted concentration exhibits better stability and high thermal efficiency compared to its particle counterpart. Hence, 2D nanofillers are better choices for next generation transformer oil nanofluids, due to their high surface area, lower filler fraction and better stability.

Rutile TiO₂ nanoparticle with high dielectric constant has already become an automatic choice as a favorable nanofiller for efficacious transformer oil (TO) nanofluids (NFs) development, thanks to their large surface area and splendid electron trapping capacity. In this work, the role of phase and morphology of TiO₂ on the properties of TO has been deciphered for the first time. For this, solvothermally prepared ultra-thin, square nanosheets of purely anatase phase TiO₂ (a-TiO₂) with 50–70 nm dimensions are used for NF preparation in two types of commercial mineral oil. The dielectric breakdown strength and thermo-physical properties of the resultant TO NFs have been investigated thereafter. A significant enhancement (27–30%) in breakdown voltage (BDV) at very low filler fraction (0.005 wt%) due to accelerated charge scavenging capability by virtue of the inherent surface oxygen vacancies, high electronegativity and slightly higher band-gap than rutile phase is observed. Additionally, the thermal conductivity of TO is seen to rise by 13–23% (±5%) at 50C which is facilitated by long phonon lifetime mediated inter-sheet transfer of heat. Further, the electronegativity induced strong repulsive forces among the dispersed nanofillers at ultra-low concentrations leads to a surfactant-free stable liquid insulation over a prolonged time. Besides, the acidity and viscosity, the increment of which with filler fraction causes a serious drawback for NFs, are below par the alarming level of TO due to low filler fraction of a-TiO₂. All the above beneficial features along with improved flash and pour points makes the synthesized 2D a-TiO₂ a budding candidate for efficient NF development at very low filler fraction without any usage of surfactant.

Stable nanofluids of ethylene glycol (EG) containing RGO-TiO₂ composite structures has been achieved at two weight percentages (wt %) (0.015 & 0.025) without usage of surfactant. The composite material was synthesized via chemical treatment of previously synthesized TiO₂ and GO. Nanofluids are witnessed to possess higher thermal conductivity than pure EG or EG containing same amount of TiO₂; whereas no significant change in viscosity was noticed. Maximum 11 % enhancement in thermal conductivity is obtained with low viscosity and high zeta potential (-21.5 mV). The thermal response and infrared images of nanofluid surfaces support the rising slope in thermal conductivity.

Nanofluids has emerged as a prominent and promising substitution of liquid dielectrics in industrial applications. Nevertheless, their sedimentation has been a consequential stumbling block for extensive and effective exploitation. Present work reports, stable, surfactant free and dilute homogeneous dispersion of a novel 2D dielectric nanomaterial: “Amorphous Graphene Sheets” (a-GS; with high I_D/I_G ratio), in transformer oil (TO) at lower nanofiller (0.0012–0.01 wt %) concentration. Nanofluids smartly address the much needed efficient thermal and electrical management with high resistivity and low loss compared to base oil. Persistent and high degrees of enhancement in the breakdown strength (40%) is observed. The electric double layer (EDL) development and prompted polarization (under electric stress), of the material leads to efficient charge trapping and de-detrapping in non-localized states. This phenomena uplifts the breakdown voltage and other electrical parameters than base TO. Lattice vibration of nanostructure along with EDL and cluster formation explains thermal transport phenomena at the nanoscale. Whereas confirmation of superior heat conduction by a-GS NFs were obtained by surface imaging and systematic study of spatial heat flow distribution. Hence the proposed hybrid nanofluid holds great promise for utilizing in the field of high voltage electrical insulation application.

Well-dispersed exfoliated white graphene (h-BN) nanosheet in transformer oil was prepared at various weight percentages. The nanofluid transformer oil showed excellent stability over long time duration and significant improvement of thermal conductivity (>45% for 0.05 wt.%)due to large surface area and high thermal conductivity of h-BN nanosheets.