A novel one-step chemical method for preparation of copper nanofluids

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Abstract

This paper presents a novel one-step method for preparing of copper nanofluids by reducing CuSO4·5H2O with NaH2PO2·H2O in ethylene glycol under microwave irradiation. Nonagglomerated and stably suspended Cu nanofluids are obtained. The influences of CuSO4 concentration, addition of NaH2PO2, and microwave irradiation on the reaction rate and the properties of Cu nanofluids were investigated by transmission electron microscopy, infrared analysis, and sedimentation measurements. It is found to be a fast, efficient one-step chemical method to prepare Cu nanofluids.

Introduction

There is a strong need in many industrial fields to develop heat transfer fluids with significantly high thermal conductivity. It is well known that crystalline solids have higher thermal conductivities than traditional fluids (such as water, ethylene glycol, and oil) by 1–3 orders of magnitude. Therefore, fluids containing suspended solid particles can be reasonably expected to have higher thermal conductivities than pure fluids. Nanofluids [1] consisting of metallic or nonmetallic nanoparticles have attracted research attention. Several groups have obtained nanofluids containing a small amount of Cu [2], Al2O3 [3], CuO [3], or SiC [4] nanoparticles or nanotubes (CNTs) [5]. It has been found that the thermal conductivity of a nanofluid consisting of ethylene glycol and only 0.3 vol% Cu nanoparticles increased by up to 40% over that of pure ethylene glycol [6].

At present, Cu nanofluids are prepared by dispersing Cu nanoparticles into base liquids [2]. This is a step-by-step method that isolates the preparation of the nanofluid from the preparation of Cu nanoparticles. As a result, agglomeration of nanoparticles may take place in both steps, especially in the process of drying, storage, and transportation of nanoparticles. The agglomeration will not only result in the settlement and clogging of microchannels, but also decrease the thermal conductivity. So it is necessary to develop one-step methods that combine the preparation of nanoparticles with the preparation of nanofluids, so that the processes of drying, storage, transportation, and redispersion of Cu nanoparticles are avoided. One-step methods can also reduce production costs. Choi has developed a one-step physical method [6], [7], in which Cu vapor is directly condensed into nanoparticles by contact with a flowing low-vapor-pressure liquid (ethylene glycol). To our knowledge, one-step chemical methods have not been applied to the preparation of nanofluids.

Different methods have been used for the preparation of copper nanoparticles (not nanofluids), such as microemulsions [8], reverse micelles [9], reduction of aqueous copper salts [10], γ-irradiation [11], UV light irradiation [12], and the polyol process [13], [14], [15]. Among these, the polyol process is a successful method for preparing monodispersed, nonagglomerated copper nanoparticles, in which heating a suitable inorganic/organic metallic salt in polyol gives rise to metal particles, and polyol act as a solvent and reducing agent [13]. But the solution of the copper salt should be heated to its boiling point and kept under refluxing conditions for a long time.

The present investigation was performed to prepare Cu nanofluids of metallic Cu nanoparticles dispersed in ethylene glycol by one-step chemical methods, in which NaH2PO2·H2O was added as a reducing agent and microwave irradiation was used for heating.

Section snippets

Preparation of Cu nanofluids

All the reagents used in our experiments were of analytical purity and were used without further purification. In a typical procedure, 25 ml ethylene glycol solution (0.1 M) of copper sulfate pentahydrate (CuSO4·5H2O) was mixed with 5 ml of ethylene glycol solution (0.01 M) of polyvinylpyrrolidone (PVP-K30) in a 100-ml beaker, followed by magnetic stirring for 30 min. Then 25 ml of ethylene glycol solution (0.25 M) of sodium hypophosphite (NaH2PO2·H2O) was added and stirred for another 15 min.

Results and discussion

The XRD pattern of a typical sample is shown in Fig. 1. The diffraction peaks can be indexed to those of pure face-centered cubic (FCC) Cu (JCPDS, File No. 04-0838), corresponding to the (111), (200), (220), and (311) planes, respectively.

Figs. 2a and 2b depict a TEM image and the corresponding selected area electron diffraction (SAED) pattern of the as-prepared product, respectively. The micrograph reveals that the product consists of spherical particles. The diffraction pattern further proves

Conclusions

In summary, a novel one-step method for preparing copper nanofluids by reducing CuSO4·5H2O with NaH2PO2·H2O in ethylene glycol under microwave irradiation has been developed. Nonagglomerated and stably suspended Cu nanofluids are obtained. The results show that the addition of NaH2PO2·H2O and the adoption of microwave irradiation greatly affect the reaction rate and the properties of Cu nanofluids. Further research on the purification and improvement of the effective thermal conductivity of Cu

Acknowledgements

We are grateful for the kind assistance from Dr. De-Bao Wang and Professor Xun Fu, Institute of Chemistry and Molecular Engineering, Qingdao University of Science and Technology.

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