Synthesis and characterization of cerium oxide based nanofluids: An efficient coolant in heat transport applications

Chemical Engineering Journal 255 (2014) 282–289

Contents lists available at ScienceDirect

Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej

Synthesis and characterization of cerium oxide based nanofluids: An efficient coolant in heat transport applications Thadathil S. Sreeremya a, Asha Krishnan a, A. Peer Mohamed a, U.S. Hareesh a, Swapankumar Ghosh a,b,⇑ a b

National Institute for Interdisciplinary Science & Technology (NIIST), Council of Scientific & Industrial Research (CSIR), Trivandrum 695019, India ACTC Div., Central Glass & Ceramic Research Institute, CSIR, 196 Raja S.C. Mullick Road, Kolkata 700 032, India

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 A facile one pot method allows

A simple one pot method produced surface modified ceria nanoparticles based nanofluids in transformer oil possessing long term stability and appreciable enhancement in thermal conductivity.

preparation of surface modified ceria nanoparticles.  The nanoparticles were dispersible in conventional and highly hydrophobic solvents.  Ceria-oil nanofluids exhibited a long term stability > five months.  A maximum TC enhancement of 14.6% was achieved with 0.7 vol% nanoparticles.

a r t i c l e

i n f o

Article history: Received 14 March 2014 Received in revised form 12 June 2014 Accepted 13 June 2014 Available online 21 June 2014 Keywords: Cerium oxide Surface modification Nanofluid Stability Heat transfer

a b s t r a c t Monodispersed oleic acid capped ceria nanoparticles were prepared by a facile one-step strategy involving thermal decomposition of a cerium oleate complex in an organic solvent with high boiling point. The nanocrystals synthesized by this relatively faster and inexpensive route have been optimized and were characterized by X-ray, infrared spectroscopy, dynamic light scattering and transmission electron microscopy. The surface-capped ceria nanoparticles exhibited excellent dispersity in conventional organic solvents and were utilized for the preparation of transformer oil based nanofluids. The suitability of the nanofluids thus obtained as heat transfer fluid was evaluated by measuring the stability of nanofluids, thermal conductivity, viscosity and particle size. The effect of particle loading and temperature on the thermal conductivity of the oil based nanofluids was studied. Oil-based nanofluids containing ceria nanoparticles showed shear-thinning behavior and produced 14.6% enhancement in thermal conductivity at 50 °C with 0.7 vol% solid loading. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction The idea of overcoming the poor heat transfer properties of conventional coolants such as water, ethylene glycol and transformer ⇑ Corresponding author at: ACTC Div., Central Glass & Ceramic Research Institute, CSIR, 196 Raja S.C. Mullick Road, Kolkata 700 032, India. Tel.: +91 33 23223546; fax: +91 33 24730957. E-mail addresses: [email protected], [email protected] (S. Ghosh). http://dx.doi.org/10.1016/j.cej.2014.06.061 1385-8947/Ó 2014 Elsevier B.V. All rights reserved.

oils by dispersing nanoparticulate solids of inherent higher thermal conductivity in it was developed by Dr. Choi of Argonne National Laboratory, US in 1995 [1]. Because of their unprecedented, intriguing properties nanofluids possess potential applications in areas like transportation [2], electronics [3], medical [4], food, and manufacturing of many types [5]. Enormous enhancements in thermal conductivity (TC), long-term suspension stability, and prevention of clogging in microchannels are some among these properties [6]. Nanofluid based heat transport technology is primarily aimed

T.S. Sreeremya et al. / Chemical Engineering Journal 255 (2014) 282–289

at enhancement in thermal properties of fluids and an increase in TC of a heat transferring fluid would eventually result in downsizing the heat transfer systems, lowering of capital costs, and improving the energy conversion efficiencies [7]. Heat transport driven failures are quite often in high voltage power transformers. Dielectric oils in these transformers provide efficient cooling extending its life [8]. These transformer oils require excellent nanoparticle (NP) dispersion, high heat conduction, with simultaneous electrical insulation. Nanofluids with suspension stability for over months are not yet been reported. Philip et al. investigated the temperature dependence of thermal conductivity in non aqueous magnetic nanofluids [9]. Alumina being a insulating ceramic oxide, has a fairly high thermal conductivity (33 Wm1 k1) and has been tested as a candidate in heat transport applications by several researchers [8,10,11]. Choi et al. prepared transformer oil (TO) based nanofluids of Al2O3 and AIN by a simple ball milling process [8]. A TC enhancement of 40% in pump oil was reported by Xie et al. with dispersion of 60 nm alumina NPs (5 vol%) where as 1 vol% of 20 nm alumina NPs enhanced the TC of TO by 14% [11]. Stearic acid modified spherical MoS2 NPs in the size range of 50–100 nm were utilized by Li et al. for preparing oil based nanofluids and reported a TC enhancement of 18% with 1 wt% NP loading at 40 °C [12]. Motivated by their high inherent TC, metallic NPs such as Cu and Ag were also tested as coolants [13–15]. Xuan et al. observed a 6% enhancement with 100 nm sized Cu NPs in TO and Kole et al. reported 24% increase in thermal conductivity with 2 vol% Cu NPs dispersed in gear oil [13,14]. However, such nanofluids may cause arcing in high voltage transformer applications. Though most of these oil based nanofluids offer a reasonable enhancement in TC, they lack in the long term stability. In spite of the fact that cerium oxide (commonly called as ceria) NPs have been well explored for applications in areas such as catalysis [16], chemical–mechanical polishing [17], in SOFC’s [18,19] etc., limited attempts have been made in fabricating ceria nanofluids [20,21]. A versatile method was developed by Kim et al. for continuous synthesis of surface modified cerium oxide nanoparticles [20]. Unlike metallic counterparts, ceramic (metal oxide) nanoparticles have significantly higher chemical stability over longer periods and are not susceptible to surface oxidation. These promising features together with low electrical conductivity make ceria a suitable candidate in high voltage transformer applications [22]. One of the critical issues which limit the widespread use of nanofluids is its poor stability against sedimentation [23]. In order to reduce their enhanced surface energy due to high surface area to volume ratio, NPs tend to cluster and form extended structures which in turn cause clogging of channels in the heat-transfer systems and a consequent decrease of the thermal conductivity. Steric stabilization with the employment of amphiphilic surfactants on the NP surfaces is the most widely used technique for preparing stable NP dispersions [17,24,25]. Oleic acid (OA) surfactant is well known to effectively stabilize dispersions of various metals, semiconductors, and metal oxide nanocrystals. The presence of a carboxylic group chemisorbed onto the NP surfaces isolates the particles and prevents both grain growth, and later, ripening. The hydrophobic tail of the surfactant extends out of the NP surface into the nonpolar solvents and stabilizes the particles in hydrophobic environments producing stable colloidal suspension [17]. Recent reports suggest that synthesis via an organic route may be preferable to aqueous methods for the preparation of stable nanofluids in apolar solvents, due to the greater control of particle size and polydispersity [26]. Numerous methods such as hydrothermal [27], alcothermal [28,29], thermal decomposition [30], and aqueous precipitation [16,17] have previously been reported for the synthesis of organophilic CeO2 nanoparticles with good control of size. Among these, hydrothermal and alcothermal methods require high pressure and/or temperature equipment which involves additional

283

costs, safety measures and processing time. Although aqueous precipitation methods are simple, greener and inexpensive, they require multiple processing steps. Solvothermal decomposition of an organic precursor on the other hand is a facile, one step method to synthesize monodispersed nanoparticles with narrow size distribution [26]. Though the fabrication methods of surface modified ceria NPs and their dispersions in few solvents have been well documented, to the best of our knowledge, there is no report in the open literature on the evaluation of oil based CeO2 fluids for heat transport applications. We report here the synthesis and characterization of a remarkably stable, highly thermo-conductive nanofluids using ceria NPs in transformer oil. Surface modified ceria NPs were prepared by the thermal decomposition of a cerium oleate precursor in diphenyl ether in the presence of oleic acid as the capping agent. Stable oil-based nanofluids with varying ceria content were prepared by dispersing these capped ceria NP’s in transformer oil. The corresponding thermal conductivity and rheological tests were performed and the dependence of TC on the concentration and temperature has been evaluated. The nanofluids demonstrated higher thermal conductivity than the basefluid (oil). An attempt is also made to propose the mechanism of thermal conductivity enhancements in the prepared nanofluids. 2. Experimental 2.1. Materials and methods Cerium nitrate hexahydrate [Ce(NO3)36H2O, 99.9%] was procured from Indian Rare Earth Ltd., India. Oleic acid (90%) and oleyl amine (70%) were supplied by Alfa Aesaer, India and Sigma– Aldrich respectively. Sodium hydroxide, ethanol and cyclohexane were procured from Merck, India and were of analytical reagent grade. All the chemicals utilized in this study were used as received without further purification. Double distilled water was used for preparation of all aqueous solutions. Sodium oleate was prepared by adding NaOH solution (0.0270 mol) drop-wise to oleic acid (0.0270 mol) in a 100 mL beaker. The resultant viscous solution was mixed well by magnetic stirring and sodium oleate was precipitated by adding excess ethanol. The precipitate was filtered and dried at room temperature. 2.2. Preparation of cerium-oleate Cerium nitrate hexahydrate (24.2 mmol) and Na-oleate (72.6 mmol) were dissolved in a solution consisting of 100 mL ethanol, 100 mL H2O and 200 mL cyclohexane. The mixture was heated at 80 °C for 4 h and the organic phase was collected and washed three times with 30 mL water. The remaining cyclohexane was evaporated slowly at 80 °C leaving a solid yellow residue as shown in Fig. 1. 2.3. Synthesis of CeO2 nanocrystals In a typical synthesis, 24.2 mmol of the freshly synthesized cerium oleate was dissolved in 150 mL diphenyl ether in a round bottom flask. About 72.6 mmol portions each of OA and oleylamine were added to the above RB flask and the reaction mixture was refluxed at 265 °C for 1–4 h. The product obtained after 1, 2 and 4 h reflux time are denoted as CeDPE1, CeDPE2 and CeDPE4. As the reaction proceeded, the solution became slightly turbid and brown in color. The reaction mixture was cooled naturally to room temperature. To this 20 mL acetone was added to precipitate the OA coated CeO2 nanoparticles. The precipitate was washed thoroughly with acetone and was dried. The capped nanoparticles

284

T.S. Sreeremya et al. / Chemical Engineering Journal 255 (2014) 282–289

3. Results and discussion Oleic acid coated CeO2 NPs have been fabricated by refluxing cerium oleate precursors in diphenyl ether solvent for 1–4 h. CeDPE4, synthesized through 4 h reflux treatment, was utilized for the preparation and characterization of all nanofluids discussed in the following sections (details are provided in Section 3.4). 3.1. Rheological properties of nanofluids

Fig. 1. Optical photograph of as synthesized bright yellow cerium oleate complex. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

were dispersed in TO by ultrasonication for 10 min to produce a reddish brown suspension.

The flow behavior of ceria based nanofluid systems as measured by viscosity is shown in Fig. 2. Ceria nanofluids up to 0.4 vol% solid loading showed Newtonian behavior with negligible effect on the viscosity against shear rate as there were no significant interactions among the particles. This may be due to the lower concentration of CeO2, resulting in relatively large separation among the particles. Nanofluid with 0.7 vol% CeO2 showed shear-thinning behavior at low shear rates, after which ideal viscous flow was observed (Fig. 2). Higher concentration of ceria NPs in TO produced gel-like suspensions. Shearing causes the three-dimensional network of gel-like suspensions disintegrate, resulting in a decrease in the interaction among the particles and hence a resultant lowering in the viscosity [32]. The effect of temperature on viscosity of nanofluids is presented in Fig. 3. The viscosity data had a standard deviation within ±2% for all the CeO2 fluids. Both the nanofluids and

2.4. Characterization and measurement The crystalline phase composition in the solid products were determined from the powder X-ray diffraction (XRD) patterns using a Philips X’PERT PRO diffractometer with Ni-filtered Cu Ka1 radiation (k = 1.5406 Å) in the 2h range 20–100° at a scanning rate of 2° min1 with a step size 0.04°. The morphology, average size of the CeO2 crystals and crystal structure were determined by high resolution transmission electron microscopy (HR-TEM) using a FEI Tecnai 30 G2 S-Twin microscope operated at 300 kV and equipped with a Gatan CCD camera. A small amount of capped CeO2 CeDPE4 was dispersed in toluene and was ultrasonicated to get a stable suspension. TEM samples were prepared by dropping a micro-droplet of the suspension onto a 400 mesh copper grid and drying the excess solvent naturally. Size measurements for the colloidal cerium oxide NPs in suspensions were performed at 25 °C by photon correlation spectroscopy (PCS) on a Zetasizer 3000 HSA, Malvern Instruments, Worcestershire, UK using a 60 mW He–Ne laser operating at a wavelength of 633 nm with General Purpose algorithm with Dispersion Technology Software (v. 1.61) at 90° detection angle. Fourier transform infrared (FTIR) spectra of the as prepared products were recorded at room temperature using KBr (Sigma Aldrich, 99%) pellet method on a Nicolet Magna IR-560 spectrometer in the range 400–4000 cm1 with average of 50 scans. The effective thermal conductivity (keff) of the nanofluids was measured using a thermal property analyzer (model KD2pro, Decagon Devices Inc., US) based on the transient hot wire method [1,9,31]. The sample vials with the thermal conductivity probe were immersed in a constant temperature (measurement error ± 0.1 °C) water bath while measuring the thermal conductivity at different temperatures. The entire assembly was thermally insulated for avoiding temperature gradient and vibrations. The apparent viscosity of nanofluids was measured using a Physica rotational-type viscometer with the SC4–18 spindle (RheolabÒ MC1vt2 rheometer) employing the concentric cylinder/bob and cup method. The colloidal stability of the nanofluids was monitored with a nephelometer (CL 52D, ELICO, India) as intensity of transmitted visible light through the fluid against time.

Fig. 2. Flow curves [(A) shear stress and (B) viscosity against shear rates] for (a) base TO, (b) 0.08 vol%, (c) 0.27 vol%, (d) 0.41 vol%, and (e) 0.7 vol% CeDPE4 dispersed in TO.

T.S. Sreeremya et al. / Chemical Engineering Journal 255 (2014) 282–289

285

Fig. 3. The variation of viscosity with temperature for (a) base oil, (b) CeDPE4 fluid in TO with solid loading of 0.41%, and (c) the same with 0.7 vol% CeDPE4 loading.

base TO showed a decrease in viscosity with increase in fluid temperature, the difference being the rate of change. This is because, as the temperature increases, the average kinetic energy of the molecules in a liquid increases [33]. The greater average kinetic energy of the molecules more easily overcomes the attractive forces that tend to hold the molecules together. This could possibly imply that the nanofluids are indeed true colloidal solutions. The viscosity of the 0.7 vol% fluid decreased significantly from initial 0.402 Pa.s at 25 °C to 0.058 Pa.s at 50 °C. The CeO2 nanofluid shows a nonlinear decrease in viscosity with temperature with the rate being faster in the lower temperatures (Fig. 3). A similar behavior was reported by Jamshidi et al. [34]. The effect is more pronounced in fluids with higher loading as the surface modified CeDPE4 might form net-work structure among themselves. The reduction in viscosity was about 59%, 74% and 86% for TO, 0.41 and 0.7 vol% nanofluids respectively when temperature was increased from initial 25° to 50 °C. The reduction in viscosity of 0.7 vol% nanofluid at 50 °C (86%) is comparable to that reported by Kole et al. (90%) for 1 vol% Cu based nanofluid [13]. The change in viscosity with temperature above 40 °C is not significant. Accordingly, transformer oil based nanofluids with higher NP loading may find suitable for use as transformer oil at normal temperature of transformer operations (80 °C). 3.2. Thermal conductivity of nanofluids The thermal conductivity enhancement of ceria nanofluids (keff) is calculated using Eq. (1).

keff ¼

ðknf  kbf Þ  100 kbf

ð1Þ

where knf and kbf are the thermal conductivity of nanofluid and basefluid respectively. Fig. 4 shows the TC enhancements in ceria fluid against its loading at 25 °C as well as measurement temperature (30–50 °C). The thermal conductivity measurements were carried out at least three times on each fluid. The standard error in the TC measurements was within ±3% and was found to decrease from an initial ±3.3% in 0.08% fluid to 1.5% with increase in CeO2 loading in 0.7% fluid (Fig. 4A). It is apparent that the TC of each nanofluid was higher than that of its base fluid and increased almost linearly with increased loading of the nanoparticles. A 6.5% increase in TC was observed with 0.08 vol% ceria and 9.5% increase for 0.4 vol% ceria loaded nanofluids. The suspensions turned slowly into gel-like with increasing concentration of ceria.

Fig. 4. (A) Thermal conductivity of ceria fluids with 0.08–0.7 vol% CeDPE4 loading at 25 °C and (B) the variation in thermal conductivity of 0.41 and 0.7% ceria fluids at 30 °C, 40 °C, and 50 °C.

The highest concentration of ceria used in this study is 0.7 vol% and that resulted in a 10.3% increase in TC (Fig. 4) when compared with the TC of base TO. The maximum enhancement observed with ceria fluids which are viscous gel-like suspensions with higher loading of CeO2 NPs. Fluids with lower extent of loading show small increases in TC. A similar behavior was observed in silica system by Burgi et al. and Botha et al. in which they reported maximum TC enhancements with gel type fluids [31,35]. Many authors pinpointed nanoconvection due to Brownian motion as the main mechanism of heat transfer in nanofluids [15,36]. In this study, the heat transfer may possibly be through particle–particle contact also in addition to Brownian motion which becomes less prominent in viscous gel-like suspensions as described in Scheme 1. Despite ceria being a poor heat conductor, observed increase in thermal conductivity may be due to (i) small particle size and, (ii) compatibility of capped particles with TO. The viscosity of nanoparticle-in-oil suspensions increased with increase in particle concentration in the slurry (Fig. 2). However, the particle loading cannot be increased indefinitely as the slurry forms a gel beyond certain critical concentration. A maximum of 14.6% TC enhancement was observed with 0.7 vol% nanofluid at 50 °C. The increase in TC with temperature is consistent with the previously reported results which were all carried at temperatures below 80 °C [12,37]. An increase in temperature leads to enhanced Brownian motion of the particles, which improves the rate of heat transfer. The Brown-

286

T.S. Sreeremya et al. / Chemical Engineering Journal 255 (2014) 282–289

Scheme 1. Enhanced particle–particle contact on moving from low concentration to high concentration (a to b) of NP’s which adds to TC.

ian motion of the particles has not contributed directly to the mass transport enhancement, but it enhanced the convection currents because of an increase in the nanoscale stirring of the liquid [9].

X-ray diffraction. The XRD patterns of CeDPE and FT-IR spectra for CeDPE4 and OA are presented in Fig. 6. The XRD patterns of capped ceria nanoparticles show finger-print reflections of typical  fcc cubic CeO2 of Fm3m) space group [16,18]. Fig. 6 shows the crystallization and growth of CeO2 crystals by thermal decomposition of Ce-oleate by extending the reflux time. One hour reflux treatment on Ce-oleate initiated the formation process of cubic ceria crystals (CeDPE1) as indicated by the appearance of small humps corresponding to (1 1 1), (2 2 0) and (3 1 1) crystal facets which grew sharper on further extending the reflux time to 4 h in CeDPE4. The average CeO2 crystal size estimated from the line broadening of the (1 1 1) X-ray reflection using Scherrer’s formula for CeDPE4 is 4.1 nm [40,41]. A peak at 20° 2h was observed in all the DPE refluxed samples and was attributed to the presence of OA surfactant [25] and the relative intensity of this peak with respect to that of cubic ceria became less prominent with increasing refluxing time. The broadened peaks are due to the nanocrystalline nature of the ceria product. The FT-IR measurements of CeDPE4 also provided supporting evidence for the formation of capped ceria nanocrystals (Fig. 6). The characteristic bands of the oleyl group are present in both the spectra. The CeO2 crystals with chemisorbed OA showed two strong vibrational bands at 2845 and 2927 cm1 due to the methyl ms(ACH3) and the mas(CH) groups. These peaks are known to be the characteristic modes of the methylene (CH2) chains that are present in OA. The rocking vibration at 715 cm1 is also typical of

3.3. Stability of nanofluids The stability of the CeO2 nanofluids was monitored by performing ageing studies by measuring turbidity (transmittance of visible light) over a time period of five months [38,39] and presented in Fig. 5. The ceria nanoparticles dispersed in transformer oil were visibly stable at room temperature for months without changing their appearance, and no detectable precipitate could be found. The transmittance of visible light decreased slightly by 7% from initial of 42 NTU to 39 NTU after 150 days of ageing. In this regard the prepared nanofluids surpassed the stability reported for oil based CeO2 nanofluids (40 days), Cu and MoS2 suspensions (20 days) [12,13,20]. 3.4. Characterization of surface modified ceria nanoparticles The phase composition of the solid products obtained by thermal decomposition of cerium oleate in DPE was determined by

Fig. 5. Stability of 0.7 vol% CeDPE4 loaded CeO2 nanofluid as a function of time. Photographs of ceria nanofluids (a) fresh and (b) after five months are shown in the background.

Fig. 6. (A) XRD patterns of (a) CeDPE1, (b) CeDPE2, (c) CeDPE4, and (B) FT-IR spectra of CeDPE4 and oleic acid.

T.S. Sreeremya et al. / Chemical Engineering Journal 255 (2014) 282–289

287

(CH2)n chains with n > 3 [17]. The free carbonyl stretching mode of the acid is visible at 1711 cm1. In comparison, the intensity of this band is reduced in the capped sample, and strong absorption bands corresponding to the asymmetric and symmetric stretching bands of the RCOO group appeared at 1536 and 1413 cm1 respectively. Furthermore, the difference of 123 cm1 between both stretching modes is characteristic of a bidentate coordination of the carboxylate group to a metal atom. The broad peak at 3400 cm1 is assigned to absorbed H2O or OH groups. The multiple peaks in the range 1000–1700 cm1 can be assigned to the ACH, COOA and H2O modes or their mixtures. The peak corresponding to the Ce–O stretch was observed at 497 cm1 and the bands at 1070 and 1350 cm1 are due to m(Ce–O–Ce) vibration [17]. The size and morphology of the ceria particles were examined by transmission electron microscopy. TEM images of CeDPE4 showed well separated, monodisperse, nanocrystals of CeO2 in the size range 3–7 nm with an average size of 5.8 ± 1 nm which is some what larger than the same calculated from X-ray peak broadening (4.1 nm). Crystal size determined from XRD profile using the Scherrer equation [40,41] underestimates the grain size as it ignores broadening of the diffraction peaks due to the microstrain in the OA coated crystals [17]. There were no evidence of agglomeration and the surfactant capped nanocrystals are mostly cubic in shape with the exception of few irregular shaped particles. The high intensity (1 1 1) lattice fringes characteristic of cubic ceria was observed in the HR-TEM image as shown in the inset of Fig. 7. The XRD patterns together with TEM analysis clearly indicates that 4 h refluxing at 265 °C in DPE is essential for the formation of well crystallized ceria NPs. The particle size (hydrodynamic diameter) of CeO2 NCs (CeDPE4) in toluene suspension, measured by photon correlation spectroscopy (PCS) is shown in Fig. 8. The hydrodynamic diameter of this material is in good agreement with the same from TEM and XRD measurements. The reproducibility of the synthetic procedure in terms of size of the crystalline product was within acceptable limits (3%). TEM provides the core diameter cerium oxide particles and DLS data reports the hydrodynamic diameter of the NP-OA core–shell spherical structures [42]. The z-average (DPCS) of 7.4 nm have shown a difference of 1.6 nm from DTEM mainly due to the surfactant coating, a value of 0.8 nm is very close to 0.9 nm reported for the length of OA chain [17]. The PDI value of 0.33 and un-clustered, isolated particles in TEM images confirm the monodispersed nature of CeO2 nanoparticles. Fig. 7. (A) Bright-field TEM, and (B) high-resolution TEM images of CeO2 nanoparticles in CeDPE4. Prominent (1 1 1) lattice fringes characteristic of cubic ceria is marked in HR-TEM image shown in the inset of (B).

3.5. Thermolysis mechanism Ceria nanoparticles were obtained by decomposing ceriumoleate complex in high boiling (265 °C) diphenyl ether solvent. The reaction conditions e.g., temperature and duration of reflux were varied and was found that no nanocrystal formed by holding the reaction mixture at temperatures below the reflux point (265 °C) even for extended period of time. Nanosized ceria particles were obtained only at 265 °C when oleate complex decomposed and produced nuclei for cerium oxide crystals. These crystallites grew over next several hours through Ostwald ripening. In general, the mechanism can be summarized as follows: metal carboxylates on heating decompose near or above 300 °C to form metal oxide nanocrystals along with some byproducts [43]. It is thought that the decomposition reaction proceeds via the formation of free radicals from metal oleates as shown in Eqs. (2) and (3).

M-OOCR ! M þ R-COO

ð2Þ

M-OOCR ! MO þ R-CO

ð3Þ 

From the reaction between the simultaneously formed Ce–O and Ce species, Ce–O–Ce links arise. This should result in the formation

Fig. 8. Particle size distribution as measured by photon scattering (number distribution) of suspensions of surface modified ceria (CeDPE4) in toluene.

288

T.S. Sreeremya et al. / Chemical Engineering Journal 255 (2014) 282–289

of ceria ‘‘CeO2’’, as a final solid product [44]. Subsequently, oleylamine assisted ionization of oleic acid leads to the formation of CeO2 nanoparticles (NPs) with negatively charged oleate ions adsorbed on the surface and the core of the metal oxide remaining positively charged [26,45]. However, a detailed and stoichiometric description of this process has not been established yet. Crystal growth appears to be consistent with ‘‘Ostwald ripening,’’ where the higher surface free energy of small crystallites makes them less stable and susceptible to higher rate of dissolution in the solvent than the larger counterparts. The net result of this solubility gradient in a dispersion is slow diffusion of material from the surfaces of small particles to the larger particles. Crystal growth by this kind of mass transport can result in the production of colloidal dispersions from systems that may initially be polydispersed [46]. Capping groups can slow down the growth kinetics by offering a significant steric barrier to the addition of material to the surface of a growing crystallite. The oleate molecules coordinate the surface of ceria nanocrystallites and permits slow and steady growth at temperatures above 265 °C. Steady and controlled growth results in nanoparticles of consistent crystal dimensions [47]. 3.6. Evaluation of efficiency of nanofluids Effectiveness of various liquid coolants depends on the flow mode (laminar/turbulent) and can be estimated based on fluid dynamics relations by the Eqs. (4) given below. In the case of fully developed simple laminar flow, use of nanofluid will be beneficial if the increase in the viscosity is less than four times of the increase in thermal conductivity [48],

g ¼ 1 þ Cg; g0

k ¼ 1 þ Ck; kf

Cg <4 Ck

ð4Þ

where Cg and Ck are viscosity and thermal conductivity enhancement coefficients, determined from experimental viscosity and thermal conductivity ratios. Ceria nanofluids up to 0.4 vol% have viscosity increases less than four times the thermal conductivity increase, indicating that these nanofluids are efficient coolants. Nanofluids with reasonably low viscosity (up to 0.4 vol%) offer better performance than nanofluids with highest thermal conductivity and high viscosity (0.7 vol%) because of the high pumping power required for viscous fluids [9]. 4. Conclusions In summary, for the first time, we adopted surface modified ceria nanoparticles for fabrication of a highly stable and thermoconductive fluid. Surface modified and well dispersed ceria nanocrystals of cubic morphology have been successfully synthesized by a comparatively simple method involving the thermolysis of respective oleate precursor. The surface functionalized nanocrystals are ideal for nanofluid preparation as they offer long term stability. The enhanced thermal conductivity offered by ceria nanofluid can be attributed to small size of particles and the compatibility of surface modified nanoparticles with the base fluid. From the stability test, we confirmed that ceria nanocrystals are well dispersed in transformer oil over a long time period up to five months. Remarkable improvements in thermal conductivity of oil (14.6%) were achieved with 0.7 vol% of ceria NPs. It was also demonstrated that evaluation of nanofluids for a particular application requires a proper understanding of all the characteristics and thermophysical properties of nanoparticle suspensions. The simplicity of the method together with extended stability and reasonable enhancement in thermal conductivity at low vol% of nanofluid makes this strategy very potential for the preparation of highly efficient heat transfer fluids.

Acknowledgements This work was supported by Bharat Heavy Electricals Ltd., Bangalore, India. The authors are grateful to the Director, CSIR-National Institute for Interdisciplinary Science & Technology (NIIST) for providing the necessary facilities for the work and CSIR-Central Glass & Ceramic Research Institute for continuing the same. Authors thank the Department of Science & Technology (DST) and CSIR, India for providing HR-TEM facility to NIIST. Author (TSS) thanks BHEL, India, for the fellowship and (AK) acknowledges CSIR for the CSIR-UGC SRF fellowship.

References [1] B.G. Wang, J.C. Hao, H.G. Li, Remarkable improvements in the stability and thermal conductivity of graphite/ethylene glycol nanofluids caused by a graphene oxide percolation structure, Dalton Trans. 42 (2013) 5866–5873. [2] V. Sridhara, L.N. Satapathy, Al2O3-based nanofluids: a review, Nanoscale Res. Lett. 6 (2011) 456. [3] I. Manna, Synthesis, characterization and application of nanofluid – an overview, J. Indian Inst. Sci. A 89 (1) (2009) 21–33. [4] L. Cheng, Nanofluid heat transfer technologies, Recent Patents on Engineering 3 (2009) 1–7. [5] W. Yu, D.M. France, J.L. Routbort, S.U.S. Choi, Review and comparison of nanofluid thermal conductivity and heat transfer enhancements, Heat Trans. Eng. 29 (2008) 432–460. [6] H.Q. Xie, L.F. Chen, Review on the preparation and thermal performances of carbon nanotube contained nanofluids, J. Chem. Eng. Data 56 (2011) 1030– 1041. [7] C.Y. Zhi, Y.B. Xu, Y. Bando, D. Golberg, Highly thermo-conductive fluid with boron nitride nanofillers, ACS Nano 5 (2011) 6571–6577. [8] C. Choi, H.S. Yoo, J.M. Oh, Preparation and heat transfer properties of nanoparticle-in-transformer oil dispersions as advanced energy-efficient coolants, Curr. Appl. Phys. 8 (2008) 710–712. [9] P.D. Shima, J. Philip, B. Raj, Synthesis of aqueous and nonaqueous iron oxide nanofluids and study of temperature dependence on thermal conductivity and viscosity, J. Phys. Chem. C 114 (2010) 18825–18833. [10] H.Q. Xie, J.C. Wang, T.G. Xi, Y. Liu, F. Ai, Q.R. Wu, Thermal conductivity enhancement of suspensions containing nanosized alumina particles, J. Appl. Phys. 91 (2002) 4568–4572. [11] M. Singh, L. Kundan, Experimental study on thermal conductivity and viscosity of Al2O3-nanotransformer oil, IJTARME 2 (2013) 2319–3182. [12] Y.X. Zeng, X.W. Zhong, Z.Q. Liu, S. Chen, N. Li, Preparation and enhancement of thermal conductivity of heat transfer oil-based MoS2 nanofluids, J. Nanomater. (2013) 1–6. 270490. [13] M. Kole, T.K. Dey, Enhanced thermophysical properties of copper nanoparticles dispersed in gear oil, Appl. Therm. Eng. 56 (2013) 45–53. [14] Y.M. Xuan, Q. Li, Heat transfer enhancement of nanofluids, Int. J. Heat Fluid Flow 21 (2000) 58–64. [15] D. Li, B.Y. Hong, W.J. Fang, Y.S. Guo, R.S. Lin, Preparation of well-dispersed silver nanoparticles for oil-based nanofluids, Ind. Eng. Chem. Res. 49 (2010) 1697–1702. [16] T.S. Sreeremya, A. Krishnan, S.J. Iyengar, S. Ghosh, Ultra-thin cerium oxide nanostructures through a facile aqueous synthetic strategy, Ceram. Int. 38 (2012) 3023–3028. [17] T.S. Sreeremya, K.M. Thulasi, A. Krishnan, S. Ghosh, A novel aqueous route to fabricate ultrasmall monodisperse lipophilic cerium oxide nanoparticles, Ind. Eng. Chem. Res. 51 (2012) 318–326. [18] S. Ghosh, D. Divya, K.C. Remani, T.S. Sreeremya, Growth of monodisperse nanocrystals of cerium oxide during synthesis and annealing, J. Nanopart. Res. 12 (2010) 1905–1911. [19] M. Brigante, P.C. Schulz, Cerium(IV) oxide: synthesis in alkaline and acidic media, characterization and adsorption properties, Chem. Eng. J. 191 (2012) 563–570. [20] J. Kim, Y.S. Park, B. Veriansyah, J.D. Kim, Y.W. Lee, Continuous synthesis of surface-modified metal oxide nanoparticles using supercritical methanol for highly stabilized nanofluids, Chem. Mater. 20 (2008) 6301–6303. [21] M.P. Beck, Y.H. Yuan, P. Warrier, A.S. Teja, The thermal conductivity of aqueous nanofluids containing ceria nanoparticles, J. Appl. Phys. 107 (2010). 066101. [22] D. Singh, E. Timofeeva, W. Yu, J. Routbort, D. France, D. Smith, J.M. LopezCepero, An investigation of silicon carbide-water nanofluid for heat transfer applications, J. Appl. Phys. 105 (2009). 064306. [23] X.Q. Wang, A.S. Mujumdar, Heat transfer characteristics of nanofluids: a review, Int. J. Therm. Sci. 46 (2007) 1–19. [24] E.F. Lopez, V.S. Escribano, M. Panizza, M.M. Carnasciali, G. Busca, Vibrational and electronic spectroscopic properties of zirconia powders, J. Mater. Chem. 11 (2001) 1891–1897. [25] A. Ahniyaz, Y. Sakamoto, L. Bergstrom, Tuning the aspect ratio of ceria nanorods and nanodumbbells by a face-specific growth and dissolution process, Cryst. Growth Des. 8 (2008) 1798–1800.

T.S. Sreeremya et al. / Chemical Engineering Journal 255 (2014) 282–289 [26] A. Krishnan, T.S. Sreeremya, E. Murray, S. Ghosh, One-pot synthesis of ultrasmall cerium oxide nanodots exhibiting multi-colored fluorescence, J. Colloid Interface Sci. 389 (2013) 16–22. [27] F. Dang, K. Kato, H. Imai, S. Wada, H. Haneda, M. Kuwabara, Characteristics of CeO2 nanocubes and related polyhedra prepared by using a liquid–liquid interface, Cryst. Growth Des. 10 (2010) 4537–4541. [28] M. Inoue, M. Kimura, T. Inui, Transparent colloidal solution of 2 nm ceria particles, Chem. Commun. (1999) 957–958. [29] A. Nugroho, B. Veriansyah, S.K. Kim, B.G. Lee, J. Kim, Y.W. Lee, Continuous synthesis of surface-modified nanoparticles in supercritical methanol: a facile approach to control dispersibility, Chem. Eng. J. 193 (2012) 146–153. [30] H.L. Lin, C.Y. Wu, R.K. Chiang, Facile synthesis of CeO2 nanoplates and nanorods by [1 0 0] oriented growth, J. Colloid Interface Sci. 341 (2010) 12–17. [31] S.S. Botha, P. Ndungu, B.J. Bladergroen, Physicochemical properties of oil-based nanofluids containing hybrid structures of silver nanoparticles supported on silica, Ind. Eng. Chem. Res. 50 (2011) 3071–3077. [32] H.I. Chen, H.Y. Chang, Synthesis of nanocrystalline cerium oxide particles by the precipitation method, Ceram. Int. 31 (2005) 795–802. [33] P.G. Wright, The variation of viscosity with temperature, Phys. Educ. 12 (1977) 323–325. [34] N. Jamshidi, M. Farhadi, D.D. Ganji, K. Sedighi, Experimental investigation on the viscosity of nanofluids, IJE Trans. B: Appl. 25 (2012) 201–209. [35] N. Shalkevich, A. Shalkevich, T. Buergi, Thermal conductivity of concentrated colloids in different states, J. Phys. Chem. C 114 (2010) 9568–9572. [36] S.P. Jang, S.U.S. Choi, Role of Brownian motion in the enhanced thermal conductivity of nanofluids, Appl. Phys. Lett. 84 (2004) 4316–4318. [37] L. Chen, H. Xie, Silicon oil based multiwalled carbon nanotubes nanofluid with optimized thermal conductivity enhancement, Colloid Surf. A 352 (2009) 136– 140.

289

[38] D. Kim, Y. Hwang, S.I. Cheong, J.K. Lee, D. Hong, S. Moon, J.E. Lee, S.H. Kim, Production and characterization of carbon nano colloid via one-step electrochemical method, J. Nanopart. Res. 10 (2008) 1121–1128. [39] M. Janus, M. Inagaki, B. Tryba, M. Toyoda, A.W. Morawski, Carbon-modified TiO2 photocatalyst by ethanol carbonisation, Appl. Catal. B 63 (2006) 272–276. [40] S.C. Tjong, H. Chen, Nanocrystalline materials and coatings, Mat. Sci. Eng. R 45 (2004) 1–88. [41] P. Dutta, S. Pal, M.S. Seehra, Y. Shi, E.M. Eyring, R.D. Ernst, Concentration of Ce3+ and oxygen vacancies in cerium oxide nanoparticles, Chem. Mater. 18 (2006) 5144–5146. [42] A. Prakash, H.G. Zhu, C.J. Jones, D.N. Benoit, A.Z. Ellsworth, E.L. Bryant, V.L. Colvin, Bilayers as phase transfer agents for nanocrystals prepared in nonpolar solvents, ACS Nano 3 (2009) 2139–2146. [43] T.D. Schladt, T. Graf, W. Tremel, Synthesis and characterization of monodisperse manganese oxide nanoparticles-evaluation of the nucleation and growth mechanism, Chem. Mater. 21 (2009) 3183–3190. [44] F. Kenfack, H. Langbein, Synthesis and thermal decomposition of freeze-dried copper–iron formates, Thermochim. Acta 426 (2005) 61–72. [45] M. Klokkenburg, J. Hilhorst, B.H. Erne, Surface analysis of magnetite nanoparticles in cyclohexane solutions of oleic acid and oleylamine, Vib. Spectrosc. 43 (2007) 243–248. [46] H. Reiss, The Growth of Uniform Colloidal Dispersions, J. Chem. Phys. 19 (1951) 482–489. [47] D.K. Lee, Y.S. Kang, Synthesis of silver nanocrystallites by a new thermal decomposition method and their characterization, ETRI J. 26 (2004) 252–256. [48] R. Prasher, D. Song, J.L. Wang, P. Phelan, Measurements of nanofluid viscosity and its implications for thermal applications, Appl. Phys. Lett. 89 (2006). 133108-133101.