IEEE Transactions on Dielectrics and Electrical Insulation Vol. 22, No. 4; August 2015 2185

DOI 10.1109/TDEI.2015.004678

Thermal Conductivity Enhancement of Transformer Oil using Functionalized Nanodiamonds

Gaurav Shukla and Hemantkumar Aiyer

Crompton Greaves Global R & D Center Crompton Greaves Limited

Kanjur Marg (East) Mumbai 400042, India

ABSTRACT To efficiently manage the increasing heat generation within the new generation power transformers, it is essential now to develop advanced dielectric nanofluids having high thermal conductivity. We report here the synthesis and characterization of stable high thermal conductivity Newtonian nanofluids using functionalized nanodiamonds in naphthenic transformer oil without compromising its viscosity and electrical insulating property. Functionalized nanodiamonds are dispersed in naphthenic oil, used as standard transformer oil, forming stable suspensions with high shelf life. The electrical resistivity, dissipation factor and viscosity of the base oil is maintained for the nano-oil containing small weight fraction of the filler (0.12 wt%), whereas the thermal conductivity was enhanced by 14.5%.

Index Terms - Nanodielectrics, nanofluid, dielectric liquids, dielectric materials, insulation, thermal conductivity, power transformer cooling, power transformer insulation.

1 INTRODUCTION

EFFECTIVE and efficient cooling is one of the critical requirements for many industrial applications in order to achieve optimum performance and avoid product degradation/failure. Despite the considerable amount of research and development focusing on industrial heat transfer requirements, major improvements in cooling capabilities have been lacking because conventional transformer oils -which function both as an electrical insulation and a heat transfer fluid- have rather poor heat transfer properties [1]. Thermal conductivity and heat-transfer performance of conventional fluids can be enhanced by dispersing nanometer-sized solid particles in it [2]. Such novel dispersions are called nanofluids [3]. Reports of experimental data have shown great potential; adding less than one volume percent of nanoparticles has led to double-digit percentage enhancements in thermal conductivity [4-7]. However in order to achieve such results, the nanomaterial must be well dispersed and stable in the base fluid. Most of the metallic nano-particles like Cu, Ag, Au, Sn, Ti, Zn, Al, In as well as carbon nanotubes and graphene can be utilized to obtain nanofluids with higher thermal conductivity compared to the traditional heat-transfer fluids [4-7]. However, for applications where good electrically insulating properties of the heat transfer fluid is an essential requirement, for example in power transformer cooling, such nanofluids are not desirable [8-10]. For power transformer cooling the heat transfer fluid has to be mixed with novel nanomaterials which can exhibit

very high thermal conductivity combined with exceptional dielectric properties [8].

Apart from hexagonal boron nitride (h-BN) modified systems [11], nanodiamonds are emerging as a new class of carbon based systems for enhancement in thermal conductivity of nanofluids [12-14]. Nanodiamond is an attractive material for applications in heat-transfer fluids because of diamond’s very high thermal conductivity, high hardness, relatively low density, very low electrical conductivity and the fact that nanodiamond’s surface is well-suited for chemical modification. Nanodiamonds are also unique due to their excellent bio-compatibility unlike many other nanoparticle additives for nanofluids [15]. Development of nanodiamond as an additive to heat-transfer fluid systems, however, has been hindered by the tendency of the primary nanodiamond particles to form the tightly bound aggregates that cannot be easily separated [16]. De-aggregation of nanodiamond in suspensions by milling with ceramic microbeads (ZrO2 or SiO2) or by microbead-assisted ultrasonic disintegration was developed by Osawa and co-workers, yielding colloidal solutions of individual nanodiamonds (4–5 nm in diameter) [17]. The disadvantages of microbead milling are contamination with bead material and generation of graphitic layers on the nanodiamond surface [17]. De-aggregation from the micrometer-sized aggregates down to stable nanodiamond particles of diameters 5–20 nm has recently been achieved by dry milling [18], using low-cost, abundant and cheap milling media (such as water-soluble salts and sugars) that do not introduce contaminants.

Manuscript received on 19 April 2014, in final form 31 January 2015, accepted 24 February 2015.

2186 G. Shukla and H. Aiyer: Thermal Conductivity Enhancement of Transformer Oil using Functionalized Nanodiamonds

In addition to specialized processing techniques, often surface functionalization or the use of surfactants is required to achieve high-quality colloidal nanodiamond dispersion. Thermal conductivity enhancements have been observed to various degrees for nanodiamond (ND) dispersions in base fluids containing OH groups, such as water, ethylene glycol (EG), EG/water, or Midel oil (synthetic as well as natural ester) [19-23]. Stable dispersions are most commonly achieved using surface-oxidized ND with pH adjustment. At 30 °C, thermal conductivity enhancements of 7.2% and 17.2% have been reported, respectively, for ND:water (3 vol% ND loading) [19] and ND:EG (1 vol% ND loading) [20] static dispersions. However achieving stable ND:transformer oil dispersions remains a critical challenge relevant to cooling of high power transformers [24].

This paper reports on characterization of functionalized nanodiamonds and homogeneous dispersion in naphthenic transformer oil while avoiding the aggregation and precipitation for optimum thermal and electrical performance.

2 EXPERIMENTAL DETAILS Ultra dispersed diamonds (UDD) synthesized through

detonation (uDiamond® Molto Nuevo) were obtained from Carbodeon Ltd Oy (Finland). The product was a high quality nanodiamond powder with low polarity surface chemistry and low moisture content. UDDs were heated for two hours in a muffle furnace at 450°C to increase the carboxylic acid functionality. Next, an acid treatment is applied to increase the number of –COOH groups on NDs surface and remove impurities. After acid treatment the UDD was water washed several times and air dried at 50o C. The dry UDD was then mixed with sucrose in a 1:10 weight ratio and milled for 10 hrs. Finally it was water washed again and air dried at room temperature. A 200 mL round bottom flask was charged with 1.0 gram of processed UDD, 2.0 gram oleic acid, and 50 gram of octane. The light-gray solution was placed in an ultrasonic bath for one hour. The solution was then subjected to high energy ultrasonication for 30 mins. This solution of de-aggregated nanodiamond was combined with an appropriate amount of naphthenic transformer oil and sonicated for an additional hour. The solution was then placed in an evaporator under reduced pressure and elevated temperature in order to remove octane.

All the chemicals used in this experiment were of GR grade and purchased from Spectrochem (Mumbai). The transformer mineral oil was DIVYOL TRANSFORMER OIL IEC 60296 I supplied by Gandhar Oil, Mumbai (India) and used as received. The basic properties of above mentioned transformer oil are presented in Table 1.

Particle size analysis was performed using a Malvern Instruments Zetasizer. Surface functionalization of the nanodiamond was determined by confocal Raman Spectroscopy and Fourier Transform Infrared Spectroscopy, performed using Horiba Jobin-Yvon Lab Ram IR system.

Thermal conductivity measurements were performed with a Decagon Devices KD2 Pro, which was calibrated against a sample of glycerol with known thermal

conductivity. The single hot-wire probe was immersed in 100 mL of sample with at least 2.5 cm of separation between the probe and sample container. For each concentration and temperature data point, multiple measurements were recorded to ensure that the sample was at thermal equilibrium.

Table 1. Properties of uninhibited transformer oil used for making stable nanodiamond dispersions.

Property Experimental Values Appearance Clear and Transparent, No

sedimentation Density@20o C 0.86 Kg/m3

Kinematic Viscosity 15.74 mm2/s Flash point > 140o C

Electrical Breakdown Strength

58 kV

Dissipation factor @ 50 Hz 1.35 x 10-4

Electrical Resistivity @ 90o C

1.42 TΩ-m

Water content 20 mg/Kg Interfacial tension @ 25o C 0.04 N/m

Structural, surface morphological and agglomeration

analysis was performed using PHILIPS CM 200 transmission electron microscope (TEM) (Specification: Operating voltages: 20-200 kV Resolution: 2.4 Ǻ), Jeol JSM-7600F Field emission scanning electron microscope (FE-SEM) (Specification: Resolution: 1.0nm (15 kV), Accelerating Voltage: 0.1 to 30 kV) and X-ray diffraction (XRD), Philips X’Pert System.

The electrical breakdown strength of the transformer oil samples was measured using BAUR DTA 100C Oil Breakdown Voltage Tester with maximum 100 kVrms AC output voltage. The dimension and design of test vessels used for measurement were according to IEC 60156 Figure II i.e. spherical electrodes with 25 mm radius. The electrode gap was fixed at 2.5 mm and 30 kV was the initial applied voltage. The voltage was ramped up at a rate of 2 kV/s. For each sample, 300 measurements were performed. In this system, breakdown detection and current disconnection took place in less than 1 ms, which together with a low energy in the test circuit prevented carbonation of the oil samples.

3 RESULT AND DISCUSSION

The nanodiamonds have diameters of 4–8 nm, but they tend to aggregate and typical suspensions of nanodiamonds in non-polar solvents like naphthenic oil may contain larger aggregates (which can withstand ultrasonic treatment). Larger aggregates tend to settle at a faster rate while dissipation of thermal energy is also drastically reduced. Hence the expected enhancement in thermal properties of transformer oil is heavily affected by such agglomeration. Figure 1a shows the TEM image of as-received nano-diamonds and Figure 1b TEM image of nanodiamonds after de-aggregation treatment. It can be observed that as-received nanodiamonds were highly agglomerated with an average agglomerate size ~ 1-2 micrometers while after de-aggregation process it reduced to the size of ~ 10 nm.

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Figure 1a. TEM image of as-received nanodiamond powder showing strong agglomeration [scale – 20 nm].

Figure 1b. TEM image of de-aggregated functionalized nanodiamond powder + Oleic Acid + Toluene solution showing nano particle dispersion (0.12% loading) [scale – 100 nm].

Prolonged particle-dispersion stability, biggest challenge in commercializing nanodiamond nanofluids [25], requires the use of appropriate surface functional groups for the target fluid. In addition to counteracting particle agglomeration, effective surface functionality is important to achieve good interaction between the nanoparticle surface and the dispersing medium. In the case of nanodiamond, which has a high surface energy and poor dispersion characteristics, some form of surface functionalization is needed to achieve the desired dispersion properties.

Figure 2 shows the X-ray diffraction (XRD) pattern of as-received nanodiamond powder and functionalized nanodiamond powder confirming the purity and phase. No crystalline or turbostratic graphitic carbon features could be determined in XRD measurements. The mean primary particle size of the functionalized nanodiamond powder determined from the XRD data was about 4 nm while that of the as-received powder was 8 nm.

Raman spectroscopy provides information on the structure, composition and homogeneity of the material, and also information on the surface groups. But its interpretation for nanodiamond is not straightforward [14].

Figure 2. XRD image of (A) functionalized nanodiamond powder and (B) as- received nanodiamond powder.

For example, the peak at ~1640 cm–1 in the Raman spectrum of detonation nanodiamond was only recently associated with O–H vibrations [26]. Raman spectra depend on structure, purity, sp3/sp2 ratio, crystal size and surface chemistry. Because of the small Raman scattering cross-section of diamond and the shielding effect of graphitic and amorphous carbon around the diamond core, ultraviolet lasers with excitation energy close to the bandgap of diamond (5.5 eV) are needed to amplify the Raman signal of nanodiamond and suppress the D band of graphitic carbon that may overlap with a weak diamond peak.

Raman spectrum recorded with a 325 nm laser excitation wavelength for as- received ND powder and purified NDs after air oxidation is shown in Figure 3, where Raman peak at 1329 cm-1 corresponds to diamond (sp3-bonded carbon), and a group of Raman peaks around 1600 cm-1 come from amorphous sp2-bonded (graphitic) carbon (~1550 cm-1), –OH (~1600 cm-1), and C=O (~1650 cm-1) groups [26]. The diamond cores in as-received nanodiamond powder seem to be completely covered by graphitic shells, and this is confirmed by the Raman spectrum [black line (A)], which is dominated by the G-band of graphitic carbon at 1,590 cm–1 and has no diamond peak. After purification the graphitic layer is completely removed and a diamond peak can be seen at 1,328 cm–1 in the Raman spectrum [blue line (B)]. The broad feature at 1600–1800 cm–1 in the spectrum of air oxidized nanodiamonds originates from surface functional groups and adsorbed molecules, with some contribution from sp2 carbon atoms.

Plots of thermal conductivity enhancement in ND:Transformer Oil nanofluids are shown in Figure 4 as a function of ND additive loading by weight percent. The temperature dependence of thermal conductivity enhancement in ND:Transformer Oil nanofluids for the optimum ND additive loading (0.12 %) is shown in Figure 5. In transformer oil, an enhancement of 14.5% at 40o C is achieved at ND additive loading of 0.12 weight%. The thermal conductivity enhancement in ND:Transformer Oil

2188 G. Shukla and H. Aiyer: Thermal Conductivity Enhancement of Transformer Oil using Functionalized Nanodiamonds nanofluid is temperature dependent and increases at higher temperature (upto 120o C). However the temperature dependence of thermal conductivity enhancement in ND:Transformer Oil nanofluid is non-linear and depends upon the ND loading as can be seen from Figure 5.

Figure 3. Micro-Raman spectrum of as-received (A) and functionalized nanodiamond powder after purification (B).

The losses in a power transformer are mainly due to the electric resistance of windings and stray losses. The magnitude of winding loss increases by the square of the load current [27]. The 14.5% enhancement in thermal conductivity of transformer oil should allow for ~ 3.5-4% overloading of the transformer.

Figure 4. Enhancement in thermal conductivity of transformer oil with ND concentration (inset) thermal conductivity variation up to 5weight % ND loading.

The non-linear temperature dependence of thermal conductivity enhancement can be attributed to Brownian motion of nanodiamonds in base fluid [28]. Brownian motion acts as an important factor when the viscosity of a base fluid changes significantly with temperature [29], which is certainly the case for transformer oil. It is often argued that when Brownian motion effects are important, nanofluid

thermal conductivity enhancement increases with increasing temperature [14, 30]. The observed temperature dependence of thermal conductivity enhancement for ND nanofluid correlated with ND loading suggests that Brownian motion of nanodiamond nanoparticles actually have a significant effect on the thermal conductivity of nanofluids.

Figure 5. Temperature dependence of thermal conductivity enhancement

of transformer oil with ND concentration of 0.01%, 0.12% and 1.00%.

Figures 6 and 7 show the effect of nanodiamond addition on the dissipation factor and electrical resistivity of base transformer oil, respectively. Above 0.2% of ND loading, decrease in dissipation factor as well as electrical resistivity can be observed with increasing ND loading. However below 0.2% of ND concentration no significant effect of ND addition on the electrical resistivity and dissipation factor has been observed.

Figure 6. Effect of nanodiamond concentration on dissipation factor of transformer oil.

Viscosity of heat transfer fluids is an important parameter in determining their thermal conductivity properties and in nanofluids the filler loading is often limited by

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corresponding increase in viscosity of the fluid. Hence it is important to find nano fillers which can significantly improve the thermal conductivity of base fluid at very low loading. NDs are one such candidate and as can be seen from Figure 8 that there is negligible change in viscosity of base fluid upto 1% of ND loading.

Figure 7. Variation in electrical resistivity of transformer oil with nanodiamond concentration.

Figure 8. Change in kinematic viscosity of transformer oil with nanodiamond concentration at 40o C.

The electrical breakdown strength (BDV) of uninhibited transformer oil and nanodiamond dispersed transformer oil samples were measured and results are shown in figure 9. Initially BDV was found to be increasing with increase in ND concentration and there was peak enhancement of ~ 10% in BDV at 0.5% ND loading compared to base transformer oil. The BDV of transformer oil was not found to be enhanced further by addition of nanodiamonds up to 1% concentration rather it decreased slightly. This can be attributed to ND agglomeration at higher concentration which can reduce the initial positive effect of very high dielectric strength of NDs (~ 2 MV/cm) dispersed in transformer oil.

Stability test on ND:transformer oil samples were performed using UV- visible spectrometry technique in the range of 300 to 800 nm. Absorbance values of homogeneously dispersed NDs (up to 0.12% concentration) in transformer oil were measured over a period of 3 months and no change in absorbance values of NDs was observed till 960 hours. These measurements confirmed the stability of ND:transformer oil suspensions (up to 0.12% ND loading) for 960 hours in steady state conditions.

Figure 9. Variation in breakdown voltage strength of transformer oil with nanodiamond concentration at room temperature.

4 CONCLUSION Sucrose based media milling and non-covalent surface modification of de-aggregated ND particles by oleic acid forms stable, static ND nanofluids in naphthenic transformer base fluid with additive loadings up to 0.12 vol%. The thermal conductivity of base fluid is enhanced by 14.5% with merely 0.12% nanodiamond loading at 40o C which increased further with the increase in temperature. The electrical resistivity and dielectric dissipation factor measurements confirmed that electrical insulating properties of base oil remain intact after nanodiamond loading up to 5% by weight. Change in the viscosity of base fluid up to 1% of ND loading was very small and within acceptable limits. By designing ND covalent surface modifications that optimize ND/base fluid solvation, even greater enhancements of base fluid thermal conductivity are expected.

ACKNOWLEDGMENT The authors gratefully acknowledge the financial support

from Crompton Greaves Global R & D for performing this work. Support from SAIF IIT Mumbai is acknowledged for TEM, FE-SEM and micro Raman measurements.

REFERENCES [1] T.O. Rouse, "Mineral insulating oil in transformers", IEEE Electr.

Insul. Mag., Vol. 14, No. 3, pp. 6-16 1998. [2] J.C. Maxwell, A Treatise on Electricity and Magnetism, 2nd ed.,

Clarendon Press: Oxford, 1881.

2190 G. Shukla and H. Aiyer: Thermal Conductivity Enhancement of Transformer Oil using Functionalized Nanodiamonds [3] S. U. S. Choi, “Nanofluids: From Vision to Reality through

Research”, J. Heat Transfer, Vol. 131, pp. 033106/1-033106/9, 2009.

[4] C. Zhi, Y. Xu, Y. Bando and D. Golberg, “Highly Thermoconductive Fluid with Boron Nitride Nanofillers”, Amer. Chem. Soc. (ACS) Nano Vol. 5, pp. 6571–6577, 2011.

[5] S. S. Botha, P. Ndungu and B.J. Bladergroen, “Physicochemical Properties of Oil-Based Nanofluids Containing Hybrid Structures of Silver Nanoparticles Supported on Silica”, Ind. Eng. Chem. Res., Vol. 50, pp. 3071–3077, 2011.

[6] T.T. Baby and R. Sundara, “Synthesis and Transport Properties of Metal Oxide Decorated Graphene Dispersed Nanofluids”, J. Phys. Chem. C, Vol. 115, pp. 8527–8533, 2011.

[7] J.A. Eastman, S.U.S. Choi, S. Li, W. Yu and L.J. Thompson, “Anomalously Increased Effective Thermal Conductivities of Ethylene Glycol-Based Nanofluids Containing Copper Nanoparticles”, Appl. Phys. Lett., Vol. 78, pp. 718–720, 2001.

[8] C. Choi, H.S. Yoo and J.M. Oh, “Preparation and Heat Transfer Properties of Nanoparticle-in-Transformer Oil Dispersions as Advanced Energy-Efficient Coolants”, Curr. Appl. Phys., Vol. 8, pp. 710–712, 2008.

[9] J.G. Hwang, M. Zahn, F.M. O’Sullivan, L.A. A. Pettersson, O. Hjortstam and R. Liu, “Effects of nanoparticle charging on streamer development in transformer oil-based nanofluids”, J. Appl. Phys., Vol. 107, pp. 014310, 2010.

[10] Y. Du, Y. Lv, C. Li, M. Chen, Y. Zhong, J. Zhou, X. Li and Y. Zhou, "Effect of semiconductive nanoparticles on insulating performances of transformer oil", IEEE Trans. Dielectr. Electr. Insul., Vol. 19, No.3, pp.770-776, 2012.

[11] J. Taha-Tijerina, T. N. Narayanan, G. Gao, M. Rohde, D. A. Tsentalovich, M. Pasquali and P. M. Ajayan, “Electrically Insulating Thermal Nano-Oils Using 2D Fillers”, Amer. Chem. Soc. (ACS) Nano, Vol. 6, pp. 1214-1220, 2012.

[12] V. N. Mochalin, O. Shenderova, D. Ho and Y. Gogotsi, “The properties and applications of nanodiamonds”, Nature Nanotechnology, Vol. 7, pp. 11-23, 2012.

[13] B.T. Branson, P.S. Beauchamp, J.C. Beam, C.M. Lukehart and J.L. Davidson, “Nanodiamond Nanofluids for Enhanced Thermal Conductivity”, Amer. Chem. Soc. (ACS) Nano, Vol. 7, pp. 3183-3189, 2013.

[14] J. J. Taha-Tijerina, T. N. Narayanan, C. S. Tiwary, K. Lozano, M. Chipara, and P.l M. Ajayan, “Nanodiamond-Based Thermal Fluids”, Amer. Chem. Soc. (ACS) Appl. Materials Interfaces, Vol. 6, pp. 4778-4785, 2014.

[15] A. M. Schrand, H. Huang, C. Carlson, J. J. Schlager, E. O. Sawa, S. M. Hussain and L. Dai, “Are Diamond Nanoparticles Cytotoxic?”, J. Phys. Chem. B Letters, Vol. 111, pp. 2-7, 2007.

[16] M. Ozawa, M. Inaguma, M. Takahashi, F. Kataoka, A. Krüger and E. Ōsawa, “Preparation and behavior of brownish, clear nanodiamond colloids”, Adv. Mater. Vol. 19, pp. 1201–1206, 2007.

[17] E. Osawa, “Recent progress and perspectives in single-digit nanodiamond”, Diamond Relat. Mater., Vol. 16, pp. 2018–2022, 2007.

[18] A. Pentecost, S. Gour, V. Mochalin, I. Knoke and Y. Gogotsi, “Deaggregation of nanodiamond powders using salt- and sugar-assisted milling”, Amer. Chem. Soc. (ACS) Appl. Mater. Interfaces, Vol. 2, pp. 3289–3294, 2010.

[19] H. Xie, W. Yu and Y. Li, “Thermal Performance Enhancement in Nanofluids Containing Diamond Nanoparticles”, J. Phys. D: Appl. Phys., Vol. 42, 095413/1-095413/5, 2009.

[20] W. Yu, H. Xie, Y. Li, L. Chen and Q. Wang, “Experimental Investigation on the Thermal Transport Properties of Ethylene Glycol BasedNanofluids Containing Low Volume Concentration Diamond Nanoparticles”, Colloids Surf., A: Physicochem. Engr. Aspects, Vol. 380, pp. 1-5, 2011.

[21] M. Yeganeh, N. Shahtahmasebi, A. Kompany, E.K. Goharshadi, A. Youssefi and L. Siller, “Volume Fraction and Temperature Variations of the Effective Thermal Conductivity of Nanodiamond Fluids in Deionized Water”, Int’l. J. Heat Mass Transf., Vol. 53, pp. 3186-3192, 2010.

[22] S. Torii, W.-J. Yang, “Heat Transfer Augmentation of Aqueous Suspensions of Nanodiamonds in Turbulent Pipe Flow”, J. Heat Transf., Vol. 131, 043203/1-043203/5, 2009.

[23] T. Tyler, O. Shenderova, G. Cunningham, J. Walsh, J. Drobnik and G. McGuire, “Thermal Transport Properties of Diamond-Based Nanofluids and Nanocomposites”, Diamond Relat. Mater., Vol. 15, pp. 2078-2081, 2006.

[24] T. Tyler, O. Shenderova, G. Cunningham, J. Walsh, J. Drobnik and G. McGuire, “Thermal transport properties of diamond-based nanofluids and nanocomposites”, Diamond and Related Materials, Vol. 15, pp. 2078-2081, 2006.

[25] A.C. Ferrari and J. Robertson, “Raman spectroscopy of amorphous, nanostructured, diamond-like carbon, and nanodiamond”, Phil. Trans. Royal Soc. A, Vol. 362, pp. 2477–2512, 2004.

[26] V. Mochalin, S. Osswald and Y. Gogotsi, “Contribution of functional groups to the Raman spectrum of nanodiamond powders”, Chem. Mater., Vol. 21, pp. 273–279, 2009.

[27] J. Perez, "Fundamental principles of transformer thermal loading and protection," 63rd Protective Relay Engineers Conf., pp.1-14, 2010

[28] S.P. Jang and S.U.S. Choi, “Role of Brownian Motion in the Enhanced Thermal Conductivity of Nanofluids”, Appl. Phy. Lett., Vol. 84, pp. 4316-4318, 2004.

[29] J.S. Park, C.K. Choi and K.D. Kihm, “Temperature Measurement for a Nanoparticle Suspension by Detecting the Brownian Motion Using Optical Serial Sectioning Microscopy”, Measurement Sci. Technol., Vol. 16, pp. 1418-1429, 2005.

[30] S. Sundar, M. K. Singh, E. V. Ramana, B. Singh, J. Grácio and A. C. M. Sousa, “Enhanced Thermal Conductivity and Viscosity of Nanodiamond-Nickel Nanocomposite Nanofluids”, Scientific Reports, Vol. 4, pp. 4039-4052, 2014.

Gaurav Shukla was born in Gorakhpur, India in 1983. He received M.Sc. degree in physics from the DDU Gorakhpur University, Gorakhpur India in 2003 and the Ph.D. degree in physics from the Indian Institute of Technology Guwahati, India in 2010. Subsequently he joined the Crompton Greaves Global R & D Center at Mumbai, India as a senior executive (technology) in 2010 where he has

been working towards development of energy efficient materials, technologies and devices for various applications. His current research interests lies in the areas of semiconductors for optical and electrical applications, dielectrics, thin films, opto-electronics, magnetic materials and energy storage materials.

Hemantkumar Aiyer was born in Itarsi, India in 1968. He received the Ph.D. degree in materials science from the Indian Institute of Science, Bangalore India in 1997. Next he worked at Tokyo Institute of Technology and Japan Science and Technology Corporation in Japan. Subsequently he worked for Laird Technologies and General Electric R and D Centre. Currently he is working at Crompton Greaves Global R and D Centre as

Senior Manager for Technology Development. His research interests include product development and novel solutions for the power industry, solar, sensing, lighting, healthcare and wireless industries. He has also worked extensively in designing and processing of ceramics, solar, electronic materials, oxide sensors, display materials, magnetic materials, nanophotonics and microwave materials.