Preparation and characterization of carbon nanofluid by a plasma arc nanoparticles synthesis system
© Teng et al; licensee Springer. 2011
Received: 29 October 2010
Accepted: 5 April 2011
Published: 5 April 2011
Heat dissipation from electrical appliances is a significant issue with contemporary electrical devices. One factor in the improvement of heat dissipation is the heat transfer performance of the working fluid. In this study, we used plasma arc technology to produce a nanofluid of carbon nanoparticles dispersed in distilled water. In a one-step synthesis, carbon was simultaneously heated and vaporized in the chamber, the carbon vapor and particles were then carried to a collector, where cooling furnished the desired carbon/water nanofluid. The particle size and shape were determined using the light-scattering size analyzer, SEM, and TEM. Crystal morphology was examined by XRD. Finally, the characterization include thermal conductivity, viscosity, density and electric conductivity were evaluated by suitable instruments under different temperatures. The thermal conductivity of carbon/water nanofluid increased by about 25% at 50°C compared to distilled water. The experimental results demonstrated excellent thermal conductivity and feasibility for manufacturing of carbon/water nanofluids.
As industrial and technological products demand higher standards of function and capacity, the problem of heat dissipation from electrical appliances becomes a significant issue. To ameliorate this problem, there are four approaches commonly taken: (1) enlarge the heat exchanger area and structure, (2) fabricate the heat exchanger using materials with higher thermal conductivity, (3) increase the working fluid flow rate to the heat exchanger, and (4) improve the heat transfer performance of the heat exchange working fluid. Of these methods, enlargement of the heat exchanger area has reached a physical limit. Increasing the flow rate of heat exchange would create problems of volume, power consumption, and noise from the fan and pump. The thermal conductivity of copper and aluminum heat exchangers are quite high, and the addition of precious metal to improve thermal conductivity further would incur a tremendous increase in the heat exchanger cost. Therefore, we consider that in order to increase heat dissipation, the most feasible approach is to improve the heat transfer performance of the heat exchange working fluid.
The use of nanofluids to improve the heat-transfer performance of heat exchange working fluids deserves consideration. In 1995, Choi  became the first person to use the term "nanofluid" to describe a fluid containing nanoparticles. Nanofluid manufacture involves dispersing metallic and non-metallic nanomaterials with high thermal conductivity, into a suitable "working fluid" such as engine oil, water, ethylene glycol, etc., to enhance the heat transfer performance of traditional fluids . According to literature reports, the thermal conductivity of a nanofluid is strongly dependent on the volume fraction and properties of the added nanoparticles [3, 4]. In addition, for the addition of a given volume of particles, the solid-liquid surface contact area between nano-scale particles and the suspension fluid is greater than that for micro-scale particles. Hence, the size and shape of the particles added will have a significant effect on thermal conductivity and heat transfer characteristics [1, 5–12].
Nanofluids preparation generally follows one of two methods: a one-step and a two-step synthesis. The so-called "one-step synthesis" produces nanofluids by synthesizing the nanoparticles directly into a suspending fluid, while the two-step process produces the nanoparticles and then disperses them in a bulk liquid to form a stable suspension, as separate processes.
Many variations on the one-step synthesis of nanofluids exist. Akoh et al.  used the VEROS method to prepare nanofluids in a one-step by applying vacuum evaporation to a running oil substrate. Wagener et al.  adopted magnetron sputtering to improve the VEROS technique, and succeeded in developing an effective preparation of Ag, Fe nanofluids. Zhu et al.  employed a new chemical method to prepare Cu-ethylene glycol nanofluids from reaction under microwave irradiation. Eastman et al.  also improved on the VEROS technique, by using low-temperature and low-pressure conditions, and letting Cu vapor directly contact and flow with low-vapor-pressure ethylene glycol fluid, causing the Cu vapor to condense directly in the fluid to form Cu nanofluid. Lo et al.  used a submerged arc nanoparticle-synthesis system to prepare Cu-based nanofluids. Lo et al. let Cu vapor, formed by electric arc discharge, directly condense in low-temperature and low-pressure deionized water, or ethylene glycol, to form CuO and Cu nanofluids. These researchers also used this method to produce Ni nanomagnetic fluids , and achieved good results. Chang et al.  synthesized an Al2O3 nanofluid, with high suspension stability, using a modified plasma arc system. The vaporized metallic gas mixed thoroughly with the pre-condensed, deionized water, to form an Al2O3/water nanofluid. The average particle size was in the range 25-75 nm. Hwang et al.  employed a modified magnetron sputtering system to produce Ag/silicon oil nanofluids. The Ag nanoparticles were relatively uniform with primary size less than 5 nm. Kumar et al.  fabricated copper nanofluids, of metallic copper dispersed in ethylene glycol, using sodium hypophosphite as reducing agent and conventional heating. Wei et al.  applied chemical solution methods to synthesize cuprous-oxide (Cu2O) nanoparticles in water, to form Cu2O nanofluids. Abareshi et al.  produced magnetite Fe3O4 nanoparticles by a co-precipitation method at various pH values. The concentration was around 0.25-3.0 vol.%. Generally, the one-step synthesis has the advantage that nanoparticles form directly in the bulk liquid. Normally, this method contains an intrinsic sorting mechanism, in which excessively large particles settle by static placement, and the supernatant, containing finer nano-sized particles as the dispersion, simply collected. This approach provides nanofluids with good suspension properties. Unless required by the preparation process, there is no need to add any dispersant or surfactant to improve the dispersion, and thus, not interference will arise from the addition of such additives. However, a disadvantage of the one-step method is that preparation conditions influence the size, shape and concentration of nanoparticles, the range of particle size distribution is broad, and an accurate control of the concentration is difficult.
Considering reports of two-step nanofluid formation, there are many accounts of Al2O3 nanofluid preparation using ultrasonic dispersion [16, 24, 25]. Murshed et al.  employed ultrasonic dispersion to prepare TiO2/water nanofluid, and applied the same method to prepare Au, Ag, SiC, and carbon nanotube nanofluid. In general, two-step syntheses are more suitable for the preparation of oxide nanofluids, but are less appropriate for the preparation of metallic nanofluids. Wen and Ding  used a high shear homogenizer to solve an agglomeration problem with TiO2 nanoparticles. Operating the homogenizer at 24,000 rpm, with a shear rate of 40,000 s-1 disrupted nanoparticle agglomeration and provided an adequate dispersion of nanoparticles with narrow size distribution. Nevertheless, although this method improved on the agglomeration problem, it was still unavailable to acquire the particle size as observed by SEM and TEM. Choi et al.  used ZrO2 bead milling in a vertical, super-fine grinding mill, to mix Al2O3 and AlN with transformer oil at volume fractions up to 4%, and added n-hexane to regard as dispersant in order to keep good suspension. Hwang et al.  treated carbon black (CB)/water, and Ag/silicon oil nanofluids, to various two-step procedures, using stirrer, ultrasonic bath, ultrasonic disrupter and high-pressure homogenizer methods in order to achieve small particle size, with good dispersion. The high-pressure homogenizer produced average CB and Ag particle diameters of 45 and 35 nm, respectively. Moosavi et al.  demonstrated a two-step synthesis of ZnO nanoparticles, by mixing ethylene glycol and glycerol with the aid of a magnetic stirrer. Moosavi et al. added ammonium citrate to act as a dispersant, and enhance stability of the suspension. This method produced a mean ZnO particle size of 67.17 nm.
Generally, two-step methods are simpler than one-step methods, because the nanoparticles may either be self-made, or purchased, then added to a bulk liquid to form nanofluids. However, in the process of addition, agglomeration can occur easily, resulting in poor suspension, thus, two-step methods often require dispersion methods such as ultrasonic sonication, mechanical stirring, a homogenizer, or the addition of a surfactant or dispersant, to disrupt agglomeration and provide dispersion and stabilize the suspension. The advantages of two-step syntheses are facile and rapid preparation of large volume nanofluids, greater control over nanoparticle concentration and narrower particle size distribution is than that of single-step syntheses.
In this study, we employed a plasma arc system to produce a carbon/water nanofluid with stable suspension, in a one-step process, without addition of any dispersant or surfactant. We fully characterized the microstructure, particle size distribution, and fundamental properties by suitable instrumentation, in order to demonstrate the feasibility of the process described herein.
Preparation of carbon/water nanofluid by plasma arc
The low temperature of the working liquid (distilled water) instantly condenses the vaporized carbon to form nanoparticles, and the magnetic stirrer and stainless steel mesh thoroughly mix the resulting nanofluid, which will be induced out to form stable carbon/water nanofluid by collection pipe. Carbon nanoparticles suspended in cold distilled water have fewer interactions, so less aggregation occurs, resulting in smaller nanoparticles. Finally, we conducted an examination of the collected nanofluids material properties.
Method and procedure for characteristic experiments
All the completed experimental samples had to be statically placed for 48 h to confirm suspension performance, and to be identified concentration of carbon/water nanofluid changes less than 5% in a fixed depth of the container by using the spectrometer. For the particle size analysis, we used transmission electron microscope (FEI-TEM, Tecnai G2 F20, Philips, Holland, the Netherlands) and a field emission scanning electron microscope (FE-SEM, 1530, LEO, Carl Zeiss Smt Ltd., Cambridge, UK) to identify microstructural properties. The suspended particle size and zeta potential of carbon/water nanofluids were measured using a light-scattering size/zeta potential analyzer (Zetasizer Nano ZS, Malvern Instruments, Worcestershire, UK) so as to determine clustering and suspension performance. Regarding the analysis of materials, the dry nanoparticles were obtained by centrifuge and heating the nanofluid to the appropriate speed and temperature. The crystalline phase was determined by X-ray Diffraction (XRD, APEX II, Kappa CCD, Monrovia, CA, USA). All peaks were measured by XRD and assigned by comparison with those of the joint committee on powder diffraction standards data (PCPDFWIN 2.4, JCPDS-ICDD, Newtown Square, PA, USA) . Density, electric conductivity, viscosity, and thermal conductivities were measured by a density meter (DA-130N, KEM, Tokyo, Japan), rheology meter (DVIII+, BROOKFIELD, Middleboro, MA, USA), electric conductivity meter (CD-4306, Lutron Electronics Co., Inc., Taipei, Taiwan) respectively, and a thermal property analyzer (KD-2 Pro, Decagon Devices, Inc., Pullman, WA, USA) was used for determination of carbon/water nanofluids properties at various temperatures.
Equation 2 can be used to convert the weight fraction to volume fraction in order to compare the experimental results with the relevant literatures. However, it should be noted that the density is affected by temperature, so the volume fraction will be slightly changed by temperature.
The precision of the density meter was ±1%. The precision of the RTD was ±0.5°C. Hence, the uncertainty of the density experiment was less than ±2.7%.
The precision of the rheology meter was ±1%. The precision of the RTD was ±0.5°C. Hence, the uncertainty of the viscosity experiment was less than ±2.7%.
The precision of the electric conductivity meter was ±3%. The precision of the RTD was ±0.5°C. Hence, the uncertainty of the electric conductivity experiment was less than ±3.9%.
The precision of the thermal property analyzer was ±5%. The precision of the RTD was ±0.5°C. Hence, the uncertainty of the thermal conductivity experiment was less than ±5.6%.
Results and discussion
List of fabrication parameters and properties for carbon/water nanofluid
Working currents (A)
Working voltage (V)
Working power (kW)
Pulse frequency (Hz)
Plasma Ar (L/min)
Shield Ar (L/min)
Distilled water volume (ml)
Manufacturing time (s)
Particle size (Z-average, nm)a
Zeta potential (mV)a
Using plasma arc in a one-step synthesis successfully produced a carbon/water nanofluid. The resulting nanofluid displayed good suspension performance, and the addition of dispersants was unnecessary. Characterization included thermal conductivity, viscosity, density, and electric conductivity measurements at various temperatures. The thermal conductivity of the carbon/water nanofluid is increased to about 25% at 50°C compared to distilled water. In addition, the manufacturing machine has the potential to produce the nanofluid with a variety of materials in the future. In the aspect of optimal manufacturing parameters for nanofluid, it is worth having a further in-depth study.
The authors would like to thank National Science Council of the Republic of China, Taiwan for their financial support to this research under contract no. NSC 99-2221-E-003-006- and NSC 99-2221-E-003-008-.
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