Cu2S nanofluids: Synthesis and thermal conductivity

International Journal of Heat and Mass Transfer 53 (2010) 1841–1843

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International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt

CuS/Cu2S nanofluids: Synthesis and thermal conductivity Xiaohao Wei a, Tiantian Kong a, Haitao Zhu b, Liqiu Wang a,* a b

Department of Mechanical Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong Department of Materials Science, Qingdao University of Science and Technology, Qingdao 266042, PR China

a r t i c l e

i n f o

Article history: Received 8 April 2009 Received in revised form 11 September 2009 Accepted 31 December 2009 Available online 28 January 2010

a b s t r a c t We apply the chemical solution method to synthesize CuS/Cu2S nanofluids and experimentally measure their thermal conductivity. The measured thermal conductivity shows that the presence of nanoparticles can either upgrade or downgrade fluid conductivity, a phenomenon predicted by the recent thermalwave theory of nanofluids. Ó 2010 Elsevier Ltd. All rights reserved.

Keywords: Thermal-waves Nanofluids CuS/Cu2S nanoparticles Chemical solution method Thermal conductivity Conductivity enhancement

1. Introduction

2. Synthesis

Efforts to synthesize nanofluids have often employed either a single-step physical method that simultaneously makes and disperses the nanoparticles into base fluids [1,2] or a two-step approach that first generates nanoparticles and subsequently disperses them into base fluids [3,4]. In addition to the challenge of how to effectively prevent nanoparticles from agglomerating or aggregating, the key issue in either of these two approaches is the lack of effective means for synthesizing nanofluids with various microstructures and properties due to either the limitation of available nanoparticle powers in the two-step method or the limitation of the system used in the single-step physical method. For creating nanofluids by design, we have recently developed a one-step chemical solution method (CSM), which is capable of synthesizing nanofluids of various microstructures [5,6] and has been successfully applied to produce the CePO4 and Cu2O nanofluids [7,8]. Here we apply this method to synthesize CuS/Cu2S nanofluids of either core-shell or hollow structure, and present experimental evidence that fluid conductivity can be either increased or decreased by the presence of nanoparticles, a phenomenon predicted by the recent thermal-wave theory of nanofluids [5,6,9–11].

Synthesizing CuS/Cu2S nanofluids by a chemical solution method (CSM) is based on following chemical reactions in solution [5,6]:

* Corresponding author. E-mail address: [email protected] (L. Wang). 0017-9310/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijheatmasstransfer.2010.01.006

CuSO4 þ 2NaOH ¼ CuðOHÞ2 þ Na2 SO4 ; D

CuðOHÞ2 ¼ CuO þ H2 O;

ð1Þ ð2Þ

4CuO þ N2 H4 ¼ 2Cu2 O þ N2 þ 2H2 O;

ð3Þ

CuO þ C2 H5 NS þ H2 O ¼ CuS þ CH3 COONH4 ; Cu2 O þ C2 H5 NS þ H2 O ¼ Cu2 S þ CH3 COONH4 :

ð4Þ ð5Þ

The reaction between cupric-sulfate (CuSO4) and sodium-hydrate (NaOH) yields cupric-hydroxide (Cu(OH)2) and sodium sulfate (Na2SO4). Under the heating by a constant-temperature water bath with the magnetic stirring (80 °C; JB-2, Shanghai Leici Equipment Ltd., China), Cu(OH)2 is decomposed into cupric-oxide (CuO) and water (H2O). The hydrazine-hydrate (N2H4) is then mixed as a reducer to reduce the cupric-oxide (CuO) into cuprous-oxide (Cu2O). Nitrogen (N2) and water (H2O) are also produced at the same time. Subsequently, thioacetamide (C2H5NS) is added as a sulfur source to generate covellite/cuprous-sulfide (CuS/Cu2S) particles with ammonium acetate (CH3COONH4) solution as the by-product. To enhance the nanofluid stability and prevent the particle aggregation, some polyvinyl pyrrolidone (PVP; chemical surfactant) is added into CuSO4 solution. In the process, the sodium-hydrate (NaOH) serves not only as a reagent, but also as a mean of adjusting the PH value for changing particle shape. All the chemicals used in

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Fig. 1. Core-shell CuS/Cu2S nanofluids. (a) 24 h after their preparation (CuSO4 molar concentration: S1 – 0.005 mol/L; S2 – 0.01 mol/L; S3 – 0.015 mol/L; S4 – 0.02 mol/L; S5 – 0.025 mol/L). (b) TEM image and SAED pattern of some core-shell CuS/Cu2S nanoparticles (CuSO4 molar concentration is 0.025 mol/L; at ambient temperature).

our experiments are with a nominal purity higher than 99% and are purchased from Taikangda Ltd., China. The water is prepared in our laboratory by double distillation. In our synthesis of nanofluids by the CSM, the used solution amount is 5 ml, 20 ml, 25 ml and 4 ml for CuSO4, PVP, NaOH and N2H4, respectively. The PVP and NaOH mass fractions in the solution are fixed at 25 g/L and 0.004 g/L, respectively. The PH value of NaOH solution and the molar concentration of N2H4 solution are 10 and 0.1 mol/L, respectively. The added amount of C2H5NS is determined by keeping its molar mass as 5 times as that of CuSO4 which is varied from 0.005 mol/L to 0.025 mol/L. The chemical reaction after adding C2H5NS lasts for 30 min. Fig. 1(a) shows the photos of the synthesized nanofluids with different values of CuSO4 molar concentration 24 h after their preparation. The fluid is very stable, and no bulk phase separation has been observed. Fig. 1(b) typifies the TEM images of nanoparticles (Transmission Electron Microscope; Philips Tecnai G2 20 S-TWIN, Oxford Instruments, UK), showing that nanoparticles have basically a core-shell spherical structure. The selected area electron diffraction (SAED) patterns show that the solid particles mainly consist of CuS with small amount of Cu2S [Fig. 1(b)].

follow [5,7,8] in using a THW system for the thermal conductivity measurement. The THW system used in KD2 system (Decagon Devices, USA) infers thermal conductivity from the temperature response of a thermocouple a short distance away from an electrically heated wire. The relationship between the thermal conductivity and the temperature change rate for the THW system is available in [9,12,13]. The variation of conductivity ratio k=kr with the CuSO4 molar concentration and nanofluid temperature is shown in Fig. 2. Here k and kr are the thermal conductivity of the nanofluid and the residual fluid (the left fluid after removing nanoparticles by strongly centrifuging the nanofluid for 60 min), respectively, measured by the standard KD2 system (Decagon Devices, USA). The probe radius and length in the KD2 system are 0.64 mm and 60 mm, respectively. Its controller waits for 30 s to ensure temperature stability, and then heats the probe for 30 s. It then monitors the cooling rate for 30 s. The accuracy of the KD2 system (5% for thermal conductivity) has been verified by a careful calibration before experiments through measuring thermal conductivities of water and various oils and comparing with those well-documented in the literature [5,13]. For every sample and temperature, we repeat our measurement three times with a time gap of 5 min in between and an average values over the three readings are used in Fig. 2. The measured conductivity shows a high nonlinearity to both the CuSO4 molar concentration and the temperature, which is consistent with the theory of thermal-waves and resonance [9,14,15]. The nanofluid thermal conductivity also shows a strong sensitivity to the temperature. The most striking feature is the variation of k=kr from 0.82 (smaller than 1) to 1.21 (larger than 1). Therefore, the presence of nanoparticles can either enhance or weaken fluid heat conduction, a phenomenon predicted by the thermal-wave theory [5,6,9–11]. Nanofluids, fluid suspensions of nanometersized particles, involve molecular, nano- and macro-scales. The particle activities at the molecular scale and the interactions between nanoparticles and base fluids at the nanoscale manifest themselves as heat diffusion and thermal-wave at the macroscale, respectively [5,6,9–11,16]. The overall macroscopic manifestation of molecular and nanoscale activities shifts the Fourier heat conduction in the base fluids into the dual-phase-lagging heat conduction in nanofluids [5,6,9–11,14]. Depending on factors like material properties of nanoparticles and base fluids, nanoparticles’ geometrical structure and their distribution in the base fluids, and interfacial properties and dynamic processes on particle–fluid

3. Thermal conductivity The transient hot-wire method (THW, also called transient line heat source method in the literature) is well established as the most accurate, reliable and robust measurement technique for the thermal conductivity of nanofluids [3,4,12,13]. Therefore, we

Fig. 2. Variation of k=kr with the CuSO4 molar concentration and the core-shell CuS/ Cu2S nanofluid temperature (k: nanofluid thermal conductivity; kr : residual fluid conductivity).

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ductivity particles at the same volume fraction of particles [17,18]. It has been confirmed experimentally as well by the oilin-water emulsion experiments which show extraordinary water conductivity enhancement by adding some oil that has a much lower conductivity value [5,19]. If we reduce the volume of N2H4 solution from 4 ml to 2 ml, increase added amount of C2H5NS to a level with a molar mass equal to 10 times of CuSO4 and allow the chemical reaction after adding C2H5NS lasting for 10 h. The synthesized nanoparticles become hollow spherical (Fig. 3). The conductivity ratio k=kr is now larger than 1 for most cases (Fig. 4). Therefore, the thermal-waves and their interaction with the heat diffusion can be varied by nanoparticle structure through adjusting synthesis parameters. 4. Concluding remarks The Cus/Cu2S nanofluids can be synthesized by using the chemical solution method. The nanoparticles can be varied from a hollow spherical shape to a core-shell one by adjusting synthesis parameters. The nanofluid thermal conductivity can also be controlled by either synthesis parameters or its temperature. The presence of Cus/Cu2S nanoparticles can either enhance or weaken fluid heat conduction so that the conductivity ratio varies from 0.82 to 1.21. Acknowledgement The financial support from the Research Grants Council of Hong Kong (GRF718009 and GRF717508) is gratefully acknowledged. References Fig. 3. Hollow spherical CuS/Cu2S nanofluids. (a) 24 h after their preparation (CuSO4 molar concentration: S1 – 0.01 mol/L; S2 – 0.015 mol/L; S3 – 0.02 mol/L; S4 – 0.025 mol/L; S5 – 0.03 mol/L). (b) TEM image of some hollow spherical CuS/Cu2S nanoparticles (CuSO4 molar concentration is 0.01 mol/L; at ambient temperature)

interfaces, the heat diffusion and thermal-wave may either enhance or counteract each other. Consequently, the heat conduction may be enhanced or weakened by the presence of nanoparticles. This thermal-wave theory can also explain the experimental finding that the fluid conductivity enhancement with high-conductivity particles is not necessarily higher than that with low-con-

Fig. 4. Variation of k=kr with the CuSO4 molar concentration and the hollow spherical CuS/Cu2S nanofluid temperature (k: nanofluid thermal conductivity; kr : residual fluid conductivity).

[1] J.A. Eastman, S.U.S. Choi, S. Li, W. Yu, L.J. Thompson, Anomalously increased effective thermal conductivities of ethylene glycol-based nanofluids containing copper nanoparticles, Appl. Phys. Lett. 78 (2001) 718–720. [2] C.H. Lo, T.T. Tsung, L.C. Chen, Shaped-controlled synthesis of Cu-based nanofluid using submerged arc nanoparticle synthesis system (SANSS), J. Crystal Growth 277 (2005) 636–642. [3] G.P. Peterson, C.H. Li, Heat and mass transfer in fluids with nanoparticle suspensions, Adv. Heat Transfer 39 (2006) 257–376. [4] S.K. Das, S.U.S. Choi, W.H. Yu, T. Pradeep, Nanofluids: Science and Technology, John Wiley & Sons, New Jersey, 2008. [5] L.Q. Wang, X.H. Wei, Nanofluids: synthesis, heat conduction, and extension, ASME J. Heat Transfer 131 (2009) 033102. [6] L.Q. Wang, M. Quintard, Nanofluids of the future, in: L.Q. Wang (Ed.), Advances in Transport Phenomena, Springer, Heidelberg, 2009, pp. 179–243. [7] X.H. Wei, H.T. Zhu, L.Q. Wang, CePO4 nanofluids: synthesis and thermal conductivity, J. Thermophys. Heat Transfer 23 (2009) 219–222. [8] X.H. Wei, H.T. Zhu, T.T. Kong, L.Q. Wang, Synthesis and thermal conductivity of Cu2O nanofluids, Int. J. Heat Mass Transfer 52 (2009) 4371–4374. [9] L.Q. Wang, X.S. Zhou, X.H. Wei, Heat Conduction: Mathematical Models and Analytical Solutions, Springer-Verlag, Heidelberg, 2008. [10] L.Q. Wang, X.H. Wei, Equivalence between dual-phase-lagging and two-phasesystem heat conduction processes, Int. J. Heat Mass Transfer 51 (2008) 1751– 1756. [11] L.Q. Wang, M.T. Xu, X.H. Wei, Multiscale theorems, Adv. Chem. Eng. 34 (2008) 175–468. [12] P. Vadasz, Heat conduction in nanofluid suspensions, ASME J. Heat Transfer 128 (2006) 465–477. [13] S. Tavman, I.H. Tavman, Measurement of effective thermal conductivity of wheat as a function of moisture content, Int. Commun. Heat Mass Transfer 25 (1998) 733–741. [14] D.Y. Tzou, Macro- to Microscale Heat Transfer: The Lagging Behavior, Taylor & Francis, Washington, 1997. [15] M.T. Xu, L.Q. Wang, Thermal oscillation and resonance in dual-phase-lagging heat conduction, Int. J. Heat Mass Transfer 45 (2002) 1055–1061. [16] L.Q. Wang, Generalized Fourier law, Int. J. Heat Mass Transfer 37 (1994) 2627– 2634. [17] J.A. Eastman, S.R. Phillpot, S.U.S. Choi, P. Keblinski, Thermal transport in nanofluids, Annu. Rev. Mater. Res. 34 (2004) 219–246. [18] S. Lee, S.U.S. Choi, S. Li, J.A. Eastman, Measuring thermal conductivity of fluids containing oxide nanoparticles, ASME J. Heat Transfer 121 (1999) 280–289. [19] X.H. Wei, L.Q. Wang, 1 + 1 > 2: extraordinary fluid conductivity enhancement, Current Nanosci. 5 (2009) 527–529.