Synthesis and characterization of copper nanofluid by a novel one-step method PDF

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Materials Chemistry and Physics 113 (2009) 57–62 Contents lists available at ScienceDirect Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys Synthesis and characterization of copper nanofluid by a novel one-step method S. Ananda Kumar a,∗ , K. Shree Meenakshi a , B...

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Synthesis and characterization of copper nanofluid by a novel one-step method Ananda Kumar (Scopus Author ID: 36065286300) Srinivasan

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Materials Chemistry and Physics 113 (2009) 57–62

Contents lists available at ScienceDirect

Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys

Synthesis and characterization of copper nanofluid by a novel one-step method S. Ananda Kumar a,∗ , K. Shree Meenakshi a , B.R.V. Narashimhan b , S. Srikanth b , G. Arthanareeswaran c a b c

Department of Chemistry, Anna University, Chennai 600025, Tamilnadu, India National Metallurgical Laboratory, Taramani, Chennai 600112, India Department of Chemical Engineering, National Institute of Technology, Trichy 620015, India

a r t i c l e

i n f o

Article history: Received 24 February 2008 Received in revised form 30 May 2008 Accepted 1 July 2008 Keywords: X-ray diffraction topography Visible and ultraviolet spectrometers Fourier transform infrared spectroscopy Thermal conductivity

a b s t r a c t This paper presents a novel one-step method for the preparation of stable, non-agglomerated copper nanofluids by reducing copper sulphate pentahydrate with sodium hypophosphite as reducing agent in ethylene glycol as base fluid by means of conventional heating. This is an in situ, one-step method which gives high yield of product with less time consumption. The characterization of the nanofluid is done by particle size analyzer, X-ray diffraction topography, UV–vis analysis and Fourier transform infrared spectroscopy (FT-IR) followed by the study of thermal conductivity of nanofluid by the transient hot wire method. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Cooling has become one of the top technical challenges faced by hi-tech industries such as microelectronics, transportation, manufacturing and metrology. There is a strong need in these industrial fields to develop heat transfer fluid [1,2] with significantly higher thermal conductivity than pure fluids. It is a well-known fact that crystalline solids have a higher thermal conductivity by one to three orders of magnitude than traditional fluids like water, ethylene glycol, oil, etc. Therefore fluids containing suspended solid particles are reasonably expected to have a higher thermal conductivity than pure fluids. Nanofluids [3], containing metallic or non-metallic particles have attracted a great deal of research attention due to their higher heat transfer efficiency. Nanofluids having suspensions [4,5] of nanometer sized particles have been proposed as a route for surpassing the performance of heat transfer liquids that are currently available [6]. Recent experiments on nanofluids have indicated that a significant increase in thermal conductivity could be achieved when compared with liquids without nanoparticles or larger particles [7–10]. For example 0.3 vol.% copper nanoparticles dispersed in ethylene glycol is

∗ Corresponding author. Tel.: +91 22203158; fax: +91 22203543. E-mail address: sri anand [email protected] (S.A. Kumar). 0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2008.07.027

reported to increase its inherent poor thermal conductivity by 40% [7]. At present copper nanofluids are prepared by dispersing copper nanoparticles in the base fluid [8]. This is a step-by-step method, which involves agglomeration that takes place during the process of drying, storage and transportation of nanoparticles. Agglomeration will result in settlement and clogging of the microchannels and hence the thermal conductivity of the nanofluids will be decreased. There are several other methods that are similar to one-step physical method, in which copper vapour is directly condensed into nanoparticles by contact with a flowing low vapour pressure liquid [7] but this method appears to be cost ineffective. By polyol process [11], monodispersed, non-agglomerated copper nanoparticles are obtained since polyol acts as solvent and reducing agent. However, the major drawback of this method is that solution of the copper salt should be heated to its boiling point and kept under refluxing conditions for a long time [12]. In the aqueous chemical reduction method, though the rate of the reaction is high, the agglomeration problem exists, as a consequence, a decrease in the thermal conductivity of the nanofluid is observed in most cases [13]. Hence the development of a new and novel method for the preparation of a copper nanofluid is inevitable. With all these ideas in mind, an attempt has been made in the present investigation to synthesize copper nanofluid by a novel one-step method using copper sulphate as a source for copper nanoparticles, ethylene glycol as base fluid and sodium

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Table 1 Effect of concentration on the size of copper nanoparticles Concentration (M)

Diameter (␮)

0.1 0.3 0.5

0.74 2.46 5.35

Scheme 1. Table 2 Effect of dilution on the size of copper nanoparticles synthesized using 0.1 M ‘Cu’ nanofluid Dilution (ml)

Particle size (␮)

25 50 75 100

0.66 0.52 0.21 0.16

hypophosphite as reducing agent by means of conventional heating. In the one-step synthesis method, copper nanofluids of metallic copper dispersed in ethylene glycol are prepared using sodium hypophosphite as the reducing agent and conventional heating is carried out. The method is a unique one, where preparation of nanoparticles is combined with the preparation of nanofluids and hence the process of drying, storage, transportation and redispersion of copper nanoparticles is avoided and ultimately it reduces the production cost as well. These aspects of this work are novel.

Fig. 1. Particle size analyzer results for copper nanofluid (I) of 2 M concentration.

2. Experimental 2.1. Preparation of copper nanofluids All the reagents used in our experiments were of analytical purity and were used without further purification. The beakers used in this procedure were cleaned by an ultrasonic cleaner in an ultrasonic bath. In this procedure, 25 ml of ethylene glycol solution was taken in a 500 ml beaker. To this 15 ml of (0.1 M) copper sulphate pentahydrate, 50 ml of sodium lauryl sulphate (SLS) surfactant and 100 ml water was added. Further few drops of kerosene were added to prevent oxidation of the copper nanofluid being synthesized. The reaction mixture was subjected to magnetic stirring for 15 min in a magnetic stirrer/heater. Then 30 ml of sodium hypophosphite was added and the magnetic stirring was continued for another 30 min. The colour of the mixture turned from blue to dark red after the reaction. Copper nanofluid was obtained after cooling the reaction mixture to room temperature. To hasten the reaction, few drops of dilute sulphuric acid was added and this can be neutralised by the addition of an equal amount of dilute ammonia. Chemical reaction involved is given in Scheme 1. 2.2. Characterization Characterization of the copper nanofluid was done by particle size analyzer, X-ray diffraction topography, UV–vis analysis, and Fourier transform infrared spectroscopy (FT-IR). 2.2.1. Particle size analysis Before the analysis of the particle size, the nanofluids are subjected to ultrasonication, to keep the particles in suspension and to prevent them from agglomeration. The Mie theory [14] principle is followed for the analysis of particles by means of particle size analyzer.

Table 3 Effect of pH on the time of synthesis of copper nanofluid pH

Time (min)

8 7 5 2

90 60 40 20

Fig. 2. Particle size analyzer results for copper nanofluid (II) of 1 M concentration.

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Fig. 3. Particle size analyzer results for copper nanofluid (III) of 0.1 M concentration after 75 ml dilution.

Fig. 4. Particle size analyzer results for copper nanofluid (IV) 0.1 M concentration after 100 ml dilution.

2.2.2. X-ray diffraction topography Copper nanofluid was diluted with absolute ethanol followed by centrifugation at 4000 rpm for 60 min. It was then washed with absolute ethanol and acetone. Further it was vacuum dried at 80 ◦ C for 2 h. X-ray diffraction topography of the obtained powder was performed on a D/max-ra diffractometer using Nickel-filtered Cu K␣ radiation.

at higher concentrations, of 0.3 M and 0.5 M, the nucleation and growth take place simultaneously. Moreover some large particles grow continually and some nascent particles do appear simultaneously, hence the distribution of copper nanoparticles is broadened. The results obtained were optimized and the particle diameter was further reduced by dilution, shown in Table 2.

2.2.3. FT-IR and UV analysis A 510P FT-IR spectrometer was used to identify the ingredients of the reaction solution. The nanofluids were centrifuged at 16,000 rpm for 60 min and supernatant used for FT-IR analysis. Copper is an inorganic species, which shows characteristic absorption that can be identified qualitatively by UV–vis spectroscopy. 2.2.4. Transient hot wire method The thermal conductivity (K) is the intrinsic property of a material, which relates its ability to conduct heat. Determination of thermal conductivity is done by transient hot wire method [8,15–21]. This method involves a wire (typically platinum) suspended symmetrically in a liquid in a vertical cylindrical container. Briefly the method works by measuring the temperature/time response of the wire to an abrupt electrical impulse. The thermal conductivity K is calculated from a derivation of Fourier’s law: K=

q 4(T2 − T1 )

ln

3.1. Effect of dilution Dilution plays an important role in the synthesis of copper nanofluids as the particle size can further be minimized by dilution. From the study, it is found that when the concentration of copper sulphate pentahydrate is high, the particle diameter of

  t2 t1

where q is the applied electrical power and T1 and T2 are the temperatures at time t1 and t2 , respectively.

3. Results and discussion The effect of copper sulphate concentration on the formation copper nanoparticles is shown in Table 1. From the table it is evident that copper nanofluid of 0.1 M concentration seems to be quite effective for obtaining copper nanoparticle of desired particle size. This is mainly due to the fact that the growth and nucleation process take place independently. That takes place separately, and as a result, the distribution range of copper nanoparticle is narrow. But

Fig. 5. XRD Pattern for copper nanofluid.

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Fig. 6. FT-IR spectrum of ethylene glycol.

the copper nanoparticle is also found to be high. This can be clearly seen from Figs. 1 and 2. For example, when the concentration of the copper nanofluid is 2 M, the mean diameter is 21.70 ␮ as shown in Fig. 1 and when the concentration of the copper nanofluid is 1 M, the mean diameter is 13.79 ␮ as depicted in Fig. 2. Based on this the copper nanofluid having 1 M concentration is alone subjected to different dilutions, viz. 0.1 M, 0.3 M and 0.5 M in order to get particle sizes in the nanorange. The results are presented in Table 1. Among the three different diluted solutions, 0.1 M solution alone is taken for our study since the particle size obtained from it approaches the nanorange (740 nm, Table 1). Furthermore, the 0.1 M solution is subjected to dilution with an objective of reducing the particle size as low as possible (within the nanorange) by adding 25 ml, 50 ml, 75 ml, and 100 ml of water, keeping the other components intact. The particle size analyzer

graphs obtained after dilution by 75 ml water and 100 ml water are shown in Figs. 3 and 4, respectively. It is interesting to observe that the dilution of 0.1 M copper nanofluid by 100 ml water is quite effective in reducing the size of the particle and a particle size of 0.16 ␮ (160 nm) is obtained without agglomeration. The effect of dilution on the size of the nanoparticles is presented in Table 2. 3.2. Effect of pH pH plays a significant role in the synthesis of copper nanofluid. A low pH results in a faster reaction time. As shown in Table 3, at a higher pH of 8, the time taken to obtain the copper nanofluid was 90 min. However, at a lower pH of 2, the reaction time was significantly reduced to 20 min indicating the necessity of maintaining

Fig. 7. FT-IR spectrum of copper nanofluid.

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The crystallite size can be found by applying Sherrer’s equation:

D=

0.9 ˇ cos 

where D is the crystallite size,  is the wavelength of X-ray used,  is the diffraction angle, and ˇ is the full-width half maximum (FWHM). The average crystallite size is found to be 130 nm. 3.5. Results of FT-IR spectra The FT-IR spectra for the analytical pure ethylene glycol and for copper nanofluid solution are shown in Figs. 6 and 7 respectively. It can be seen from the FT-IR analysis, that the spectrum corresponding to the copper nanofluid solution resembles the spectrum of pure ethylene glycol. There is no sign indicating the formation of oxidised products of ethylene glycol. This suggests that it is sodium hypophosphite and not ethylene glycol, which acts as a reducing agent. Hence this method preserves the respective advantages of the polyol process and aqueous chemical reduction process as well. It is also determined to be a fast and novel method for preparation of several nanofluids besides ‘Cu’. Fig. 8. Analysis pattern of copper nanofluid by UV–vis method.

the pH as low as possible. Hence a low pH is maintained by the addition of 5 ml of dilute sulphuric acid. However, the copper nanofluid obtained would be in a highly acidic state and it may even corrode the engines when it is used as a coolant. Thus, the copper nanofluid is neutralized with dilute ammonia, which reverts it back to the basic state and can be used in engines without causing corrosion. The effect of the addition of dilute sulphuric acid is that it makes copper more electron releasing and thus hastens the reaction. 3.3. Effect of SLS in stabilizing the nanoparticle In the preparation of copper nanofluids (0.01 M) SLS was added. The introduction of SLS keeps the copper nanoparticles suspended and thus is helpful in their thermal conductivity. It retards the growth and agglomeration of the metal nanoparticles. Sodium hypophosphite is a reducing agent and it is helpful in the reduction of copper from +2 oxidation state to copper zero valent (0) state. The concentration of sodium hypophosphite used is 0.25 M added in the rate of NaH2 PO2 /CuSO4 = 2.5 and results in stable nanofluid, which is stable up to 2 weeks in stationary state. The stabilization of the nanofluid is very important for industrial applications. It has been shown at room temperature that the obtained nanofluid is stable for more than 3 weeks in the stationary state and more than 8 h under centrifugation at 4000 rpm without sedimentation. It could also be suspended for more than 2 weeks in stationary state at 120 ◦ C. The stabilization of the obtained copper nanofluid was found to be better than that of the one prepared by the step-by-step method in which the nanofluid lasted for only 1 week in the stationary state at room temperature. Two factors contribute to this improvement: one is the small size and hence better dispersity of copper nanoparticles and the other is due to the addition of the surfactant (SLS) which prevents the agglomeration and the growth of the metal nanoparticles. 3.4. Data resulting from X-ray diffraction topography studies The X-ray diffraction topography pattern of the sample is shown in Fig. 5. Diffraction peaks can be indexed to those of pure face centred cubic (FCC) Cu (JCPDS, File No. 04-0836), corresponding to the (1 1 1), (2 0 0) and (2 2 0) planes.

3.6. UV–vis and thermal conductivity analysis The result of UV–vis analysis is shown in Fig. 8. The peak at 422.15 nm in the visible region shows the existence of copper. Furthermore, the shape of the absorption band illustrates uneven distribution above and below the peak maximum. There is a gradual decrease in absorbance in the longer wavelength side above the peak maximum, and below the peak maximum there is a sudden drop in absorbance or straightening, which clearly indicates a very narrow size distribution. This supports the result obtained from the particle size analysis of Fig. 4. Furthermore, the thermal conductivity of the copper nanofluid developed in the present study is found to be significantly higher (0.6 W m−1 K−1 ) than the reported value for Cu nanofluid (0.259 W m−1 K−1 ) [12] and that of pure ethylene glycol (0.256 W m−1 K−1 ), which is used as a conventional coolant. 4. Conclusion A novel one-step method was developed for preparing copper nanofluids by reducing copper sulphate pentahydrate using sodium hypophosphite as reducing agent and ethylene glycol as base fluid by means of conventional heating. This novel one-step method for preparing copper nanofluids is advantageous over other more conventional methods due to the following reasons: • It is an in situ, one-step method. • Non-agglomerated and stably suspended copper nanofluids are obtained. • Copper nanofluids can be synthesized in a short time. • This method is economical. • The synthesized copper nanofluid has superior thermal conductivity properties when compared with conventional engine coolant fluids. Over all, the experimental results reveal that the copper nanofluid developed in the present investigation has a better thermal conductivity (0.6 W m−1 K−1 ) than the reported value for Cu nanofluid (0.259 W m−1 K−1 ) [12] and that of pure ethylene glycol (0.256 W m−1 K−1 ), which is used as a conventional coolant. Hence the Cu nanofluid resulted from the present study may be used as an effective coolant in automobile industry as an alternative to the conventional fluids that are currently in use.

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Acknowledgements One of the authors K. Shree Meenakshi is thankful to NML Madras Centre for having provided the facility to carry out this work. Furthermore, she is grateful to Dr. S. Nanjundan, professor and head, Department of Chemistry, Anna University for granting her permission to carryout the work in NML, Madras Centre. References [1] A.E. Bergles, J. Heat Transfer 110 (1988) 1082. [2] A.E. Bergles, Handbook of Heat Transfer Application, Mc-Graw Hill, New York, 1985 (Section 3-1-80). [3] U.S. Choi, Development and Applications of Non Newtonian Fluids, in: D.A. Siginer, H.P. Wang (Eds.), FED, vol. 231, ASME, New York, 1995, p. 99. [4] A.S. Ahuja, J. Appl. Phys. 46 (1975) 3408. [5] A.S. Ahuja, Int. J. Heat Mass Transfer 25 (1982) 725.

[6] J.A. Eastman, U.S. Choi, S. Li, L.J. Thompson, S. Lee, Mater. Res. Soc. Symp. Proc. 457 (1997) 3. [7] J.A. Eastman, S.U.S. Choi, S. Li, W. Yuand, L.J. Thompson, Appl. Phys. Lett. 78 (2001) 718. [8] Y. Xuan, Q. Li, Int. J. Heat Fluid Flow 21 (2000) 58. [9] X. Xang, X. Xu, U.S. Choi, J. Thermophys. Heat Transfer 13 (1999) 434. [10] S. Lee, U.S. Choi, S. Li, J.A. Eastman, J. H...