Preparation and properties of copper-oil-based nanofluids
© Li et al; licensee Springer. 2011
Received: 29 January 2011
Accepted: 5 May 2011
Published: 5 May 2011
In this study, the lipophilic Cu nanoparticles were synthesized by surface modification method to improve their dispersion stability in hydrophobic organic media. The oil-based nanofluids were prepared with the lipophilic Cu nanoparticles. The transport properties, viscosity, and thermal conductivity of the nanofluids have been measured. The viscosities and thermal conductivities of the nanofluids with the surface-modified nanoparticles have higher values than the base fluids do. The composition has more significant effects on the thermal conductivity than on the viscosity. It is valuable to prepare an appropriate oil-based nanofluid for enhancing the heat-transfer capacity of a hydrophobic system. The effects of adding Cu nanoparticles on the thermal oxidation stability of the fluids were investigated by measuring the hydroperoxide concentration in the Cu/kerosene nanofluids. The hydroperoxide concentrations are observed to be clearly lower in the Cu nanofluids than in their base fluids. Appropriate amounts of metal nanoparticles added in a hydrocarbon fuel can enhance the thermal oxidation stability.
Nanofluid is a novel heat-transfer fluid prepared by dispersing nanometer-sized solid particles in traditional heat-transfer fluid to increase thermal conductivity and heat-transfer performance. Nanofluid was coined by Choi and colleagues [1–3] in 1995 at Argonne National Laboratory of the USA. Nanofluids with water, ethylene glycol, or oil as the base fluid were of great significance primarily because of their enhanced thermal properties. There are compelling needs in many industrial fields to develop oil-based heat transfer fluids with significantly higher thermal conductivity for energy-efficient heat exchangers. Many efforts have been focused on the oil-based nanofluids. Transformer oil, mineral oil, silicon oil, hydrocarbon fuels, and some organic solutions are used as the base fluids for studying nanofluids. The dispersion and thermal conductivities of the oil-based nanofluids containing Cu, CuO, Ag, or Al2O3 particles have been recently reported [4–6].
When nanoparticles are introduced into oil, the particles are usually sedimented within several minutes because of the poor compatibility between the nanoparticles and the base oil. The agglomerated particles are gradually settled over time, which leads to the poor stability and low heat-transfer capability of the suspensions. Thus, an appropriate lipophilic modification process is needed for the formation of a stable oil-based nanofluid. Surface modification on metallic particles with hydrophobic ligands and addition of dispersant can be employed to improve the compatibility between the nanoparticles and the oil-based fluid. The organic ligands with long hydrocarbon chains coordinated to the nanoparticles prevent the particles from clustering, and the surface-modified nanoparticles possess good dispersion behavior in oils [4, 7–9].
Kerosene, a typical hydrocarbon fuel, circulated in aircraft for cooling can serve as the primary thermal sink by dissipating waste heat from aircraft subsystems. However, it has relatively low thermal conductivity. As is well known, a kerosene-based nanofluid can improve the heat transfer property and cooling capacity. In this study, we attempted to synthesize lipophilic Cu nanoparticles and to prepare oil-based nanofluids. The hydrophobic layers formed on the surface of copper nanoparticles can protect the particles against oxidation and improve dispersion stability of oil-based nanofluids [10–12], which are important for exploiting the potential benefits and applications of the enhanced thermal properties of the nanofluids. In the meanwhile, the effects of the lipophilic Cu nanoparticles on the viscosity, thermal conductivity, and thermal oxidation stability of the nanofluids are also investigated.
Materials and preparation of ligand
All the materials and solvents used in this study, P2S5, cetyl alcohol, anhydrous ammonia, benzene, cupric acetate, ethanol, sodium hypophosphite, hydrazine hydrate solution (85%), toluene, decahydronaphthalene, and dichloromethane were analytic grade agents.
The Cu nanoparticles were prepared and modified by O, O-di-n-cetyldithiophosphoric acid. The O, O-di-n-cetyldithiophosphate  was synthesized by heating P2S5 (0.02 mol) and cetyl alcohol (0.07 mol) at 80°C for 3 h. The suspension was cooled to room temperature followed by the addition of 50 mL dichloromethane. The mixture was filtered, followed by evaporation of the solvent. Anhydrous ammonia was subsequently bubbled through the solution under stirring. The ligand, ammonium (O, O)-dialkyldithiophosphate, was then precipitated and recrystallized in benzene, washed with absolute ethyl ether, and dried in vacuum.
Preparation and characterization of Cu nanoparticles
Cupric acetate (0.002 mol) was dissolved in 20 mL deionized water used as the precursor of Cu nanoparticles. A mixture of the ligand (O, O-di-n-cetyldithiophosphate) and sodium hypophosphite (NaH2PO2, 0.0015 mol) in 100 mL solvent of ethanol/water was stirred uniformly at 60°C. The solution of cupric acetate was introduced dropwise into the mixture, and the reaction system turned from colorless solution to yellow suspension. Then, the hydrazine solution (10 mL) was added to the mixture, and a dark colloid was observed. The mixture was stirred at 60°C for 0.5 h and then cooled to room temperature. The precipitate was separated by centrifugation and was washed subsequently with water and ethanol. After separation, the nanoparticles were dried in a vacuum oven at 45°C for 2 h.
The surface-modified Cu nanoparticles with various molar ratios of P to Cu (1:2, 1:5, and 1:10) were prepared by fixing the concentrations of copper salt and reductant, and varying the concentration of O, O-di-n-cetyldithiophosphate. Because the ligands act as particle protectors through coordinating the S-containing end groups on the copper particle surfaces and the hydrophobic carbon tails are pointed outward from the particles, the resulting copper nanoparticles with the modification layers should be hydrophobic and be dispersed in nonpolar solvents.
The phase properties of the surface-modified Cu nanoparticles were characterized by X-ray powder diffraction (XRD) using a Thermo X-ray diffractometer (Bruker, Germany) with monochromatized Cu Kα radiation (λ = 1.5405 Å). The differential scanning calorimeter (DSC/TG, NETZSCH STA 409 PC/PG) was used to analyze the thermal decomposition process of the particles with a heating rate of 10 K/min in N2 with a flow rate of 20 mL/min. Transmission electron microscopy (TEM) images were taken with JEM-200CX (JEOL, Japan) instrument using an operating voltage of 160 kV. Scanning electron microscopy (SEM) images were taken with field-emission scanning electron microscope, and the energy dispersive X-ray analysis (EDX) was carried out on the SEM equipped with energy-dispersive spectrometer (FEI SIRION-100, GENENIS-4000, Netherlands). A Nexus 470 Fourier transform infrared spectrometer (NICOLET, USA) was employed to observe the changes of organic functional groups.
Preparation of nanofluids
Three types of nanofluids were prepared by dispersing different mass fractions of the surface-modified Cu nanoparticles in kerosene, toluene, and decahydronaphthalene as the base liquids without a dispersant. The samples were homogenized for about 5 min using an ultrasonic disrupter to ensure proper dispersion of the nanoparticles. The color of the suspension was observed to be puce.
Measurements on viscosity and thermal conductivity
where ρ and ν are the density and kinematic viscosity of the nanofluid, respectively, at the same temperature.
Measurements of the thermal conductivities of Cu nanofluids were performed by means of a computer-controlled transient calorimeter . The schematic diagram of the apparatus has been described previously in detail . The nanofluid samples were added into the thermal conductivity cell, and a series of voltage differences (ΔV) of the unbalanced bridge were recorded with the time at each temperature. These data were utilized to calculate the slope of the voltage against time (dV/dt) of the unbalanced bridge. The thermal conductivities of the base fluids and nanofluids were calculated from the established equation between λ and dV/dt, and the enhanced ratios of thermal conductivity were then obtained. All the measurements were performed at atmospheric pressure.
The Cu/kerosene-based nanofluids (0.1% Cu nanoparticles) were thermally oxidized in an isothermal apparatus. Each test tube containing 100-mL sample of Cu nanofluid was placed in the heated test well. The investigated samples were subjected to thermal oxidation at 120 or 140°C. The temperature remained steady within ± 1°C. The flow meters were employed to regulate the oxygen flow with the rate of 30 mL/min into each sample by means of a gas dispersion tube. A small number of aliquots (<0.5 mL) of the samples were removed from the test tubes at fixed time intervals for the hydroperoxide measurements. The hydroperoxides formed in the samples during the thermal oxidization process were determined through measuring the absorption spectra of the iodine-starch solutions using ultraviolet-visible spectrometry [16, 17].
Results and discussion
Characterization of surface-modified Cu nanoparticles
The Cu nanoparticles are surface-modified by the organic ligands containing hydrocarbon tail. The coating layers should not easily separate from the surface of the Cu nanoparticles when the Cu nanoparticles are dispersed in the oil-based fluids. The lipophilic surface-modified Cu nanoparticles should be dispersed in hydrophobic solvents, such as toluene, chloroform, and liquid paraffin. It should not be dispersed in water and should not stay at the aqueous-organic interface. Therefore, the dispersion capability of Cu nanoparticles in hydrophobic solvents is improved by the surface modification, which enables the surface-modified Cu nanoparticles to be used as additives in oils.
Viscosities and thermal conductivities of nanofluids
Hydroperoxides in the Cu/kerosene-based nanofluids
The Cu oil-based nanofluids have been prepared by dispersing Cu nanoparticles modified with O, O-di-n-cetyldithiophosphate in kerosene, toluene, or decahydronaphthalene. The modified ligand is effective in improving the lipophilic property of Cu nanoparticles. The modified layers can be effectively coated on the surfaces of the Cu nanoparticles even when they are dispersed in the oil-based fluids. The thermal conductivity of nanofluids increases with the mass fraction of nanoparticles to some extent. The hydroperoxide concentrations are observed to be lower in the Cu nanofluids than in their base fluids. Appropriate amounts of metal nanoparticles added into a hydrocarbon fuel can enhance its thermal oxidation stability.
energy dispersive X-ray analysis
selected area electron diffraction
scanning electron microscopy
transmission electron microscopy
X-ray powder diffraction.
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