Optimal synthesis and characterization of Ag nanofluids by electrical explosion of wires in liquids

  • Eun Ju Park1,

    Affiliated with

    • Seung Won Lee2,

      Affiliated with

      • In Cheol Bang2 and

        Affiliated with

        • Hyung Wook Park1Email author

          Affiliated with

          Nanoscale Research Letters20116:223

          DOI: 10.1186/1556-276X-6-223

          Received: 30 October 2010

          Accepted: 15 March 2011

          Published: 15 March 2011

          Abstract

          Silver nanoparticles were produced by electrical explosion of wires in liquids with no additive. In this study, we optimized the fabrication method and examined the effects of manufacturing process parameters. Morphology and size of the Ag nanoparticles were determined using transmission electron microscopy and field-emission scanning electron microscopy. Size and zeta potential were analyzed using dynamic light scattering. A response optimization technique showed that optimal conditions were achieved when capacitance was 30 μF, wire length was 38 mm, liquid volume was 500 mL, and the liquid type was deionized water. The average Ag nanoparticle size in water was 118.9 nm and the zeta potential was -42.5 mV. The critical heat flux of the 0.001-vol.% Ag nanofluid was higher than pure water.

          Introduction

          As noble metal materials, silver nanoparticles exhibit significantly distinct physical, chemical, and biological properties. Silver nanoparticles have attracted attention in a wide range of application fields [14]. Their unique properties result from particles on the nanoscale that are monodispersed and unagglomerated.

          Nanofluids, dispersed nanoscale particles suspended in a base fluid [5], have drawn tremendous interest from scientific and industrial communities because of their unique properties. They have been used in many industrial applications such as heat transfer, automotive, electronic, biomedical device manufacturing, and others [610]. In particular, nanofluids have gained interest as heat transfer fluids. Due to the high thermal conductivity of nanoscale metal particles, metal-nanofluids may significantly enhance thermal transport capabilities. Nanofluids have shown the most promise as coolants because they enhance critical heat flux [CHF] [1113].

          Nanofluids are produced by two methods: a one-step method and a two-step method. The two-step method forms nanoparticles using physical or chemical synthesis techniques and disperses them in basic fluids. The one-step method forms nanoparticles directly in basic fluids [1416]. A promising one-step method, a physical synthesis technique, is the electrical explosion of wires in liquids [EEWL]. EEWL has advantages, such as high-purity nanoparticle production without surfactants (non-toxic), in contrast to chemical techniques, oxidation prevention using dense media, and spherical nanoparticle production. The greatest advantages are simple evaporation and condensation, short production times, and possibility of mass production. The EEWL method has been studied by many researchers. Most previous studies were conducted with various materials under gaseous conditions. Table 1 summarizes previous studies on the EEWL method.
          Table 1

          Previous works on electrical explosion of wires

          Reference

          Material

          Condition

          Control parameter

          Average size

          Karioris and Fish [20]

          Au, Ag, Al, Cu, Fe, W, Mo, Ni, Th, U, Pt, Mg, Pb, Sn, Ta

          Air

          Capacitance

          30-50 nm

             

          Applied voltage

           
             

          Wire mass

           
             

          Ambient gas

           

          Couchman[21]

          Al, Ni, Au, Pt

          Air

           

          10 nm

          Phalen [22]

          Ag

          Air

          Voltage: 0-30 kV

          0.3 μm

          Cho [23]

          Ag

          Water

           

          <100 nm

          Park [24]

          Ag

          Water

          One-step

          88.8 nm

             

          Two-step

           
          A schematic illustration of the experimental system is shown in Figure 1. The system consists of a container for liquid media for the explosion process and a simple discharge circuit, which includes a high-voltage power supply, a capacitor, and a spark-gap switch. A metal wire (conductor) was placed in the container filled with the liquids. The capacitor was charged using an applied voltage. The amount of stored energy in the capacitor was W = 0.5CV2, where C is the capacitance and V is the charged voltage. By closing the switch, the current was allowed to flow through the wire. The current deposited the electrical energy in the wire due to its finite resistance. Thus, the wire located between the two electrodes melted, vaporized, and turned into plasma. Finally, nanoparticles were formed by interaction with the liquid. The vaporized particles were condensed more efficiently in the liquid than in ambient air. The basic principle of the method is illustrated in Figure 2.
          http://static-content.springer.com/image/art%3A10.1186%2F1556-276X-6-223/MediaObjects/11671_2010_Article_172_Fig1_HTML.jpg
          Figure 1

          Schematic diagram of the experimental system for the EEWL process.

          http://static-content.springer.com/image/art%3A10.1186%2F1556-276X-6-223/MediaObjects/11671_2010_Article_172_Fig2_HTML.jpg
          Figure 2

          Production principles for nanoparticles produced by EEWL.

          Although the formation of nanoparticles by EEWL is complex, including melting, vaporization, and condensation, it can be explained based on the energy deposited in the wire. In the EEWL process, the energy deposited in the wire can be calculated by time integration using the measured voltage and current waveform of the power dissipated in the wire according to the following equation:
          http://static-content.springer.com/image/art%3A10.1186%2F1556-276X-6-223/MediaObjects/11671_2010_Article_172_Equa_HTML.gif

          where W is the energy deposited in the wire, v is the voltage, and i(τ) is the time integration.

          Parameters that can influence the properties of particles synthesized by EEWL include electrical circuit parameters (voltage, capacitance, inductance); the amount of energy deposited in the wire; the properties of the exploding wire (diameter, length, defects); sublimation of the metal; and properties of the liquid (viscosity, thermal conductivity, breakdown strength).

          In this study, we produced and characterized pure Ag nanofluids by EEWL. We examined the energy deposition in the wire under various conditions and focused on controlled particle size and stability. To identify the effects of key parameters in EEWL, we designed the experiments using Minitab and observed the Ag particle size, morphology, and dispersibility in nanofluids. For applications such as cooling system for electronics and nuclear reactors, it is important to increase the CHF. Thus, to determine potential for increased CHF, we used the pool boiling test of the Ag nanofluids. Finally, to decrease the particle size and improve the dispersibility of Ag nanofluids, we optimized the processing parameters for EEWL using a response optimization technique [ROT].

          Experimental setup

          The Ag wire (0.1 mm in diameter) was installed in the cylinder filled with the liquids. The capacitor was charged to 3 kV and the current flowed through the wire when the spark-gap switch was closed. High-temperature plasma was generated by the electrical energy deposited in the wire and was condensed by the basic fluids. A self-integrated Rogowsky coil and a high-voltage probe were used to measure the current and voltage waveforms, respectively.

          Many parameters can influence the particles produced by EEWL. In this work, we examined the effects of capacitance, wire length, and liquid type and volume. The experimental parameters are summarized in Table 2.
          Table 2

          Summary of experimental parameters

          Capacitance

          7.5 μF, 30 μF

          Charging voltage

          3 kV

          Material

          Silver

          Wire diameter

          0.1 mm

          Wire length

          2.8 mm, 3.8 mm

          Liquid

          Deionized water, Ethanol

          Liquid volume

          500 mL, 1,000 mL

          Ag concentration

          0.001 vol.%

          We analyzed the effects of the energy deposited in the exploding wire on the size and shape of the Ag nanoparticles. The morphology was observed by high-resolution transmission electron microscopy [TEM]. The size and zeta potential of the nanoparticles were measured using a submicron size and zeta potential measuring system (Nano ZS, Malvern, Worcestershire WR14 1XZUK, UK). Adsorption spectra were analyzed using UV/Vis spectroscopy. Design of experiments [DOE] was performed to optimize the control factors for EEWL.

          Results

          Manufacturing of Ag nanofluids

          Figure 3 shows the current and voltage waveform measurements. When the capacitor was discharged, the current decreased, and after a certain time, a sudden current drop and sharp rise in voltage occurred. The current drop and voltage rise resulted from an increase in the resistivity of the wire due to vaporization. The resistance increased several orders of magnitude, and the current in the circuit was interrupted. The inductive energy stored in the stray inductance of the circuit generated a high voltage, which was higher than the initial voltage of the capacitor. Once vaporization occurred, an arc discharge formed between the electrodes, and the current flowed through the low-resistance arc plasma. Thus, the wire was heated and vaporized by the energy deposited in the wire until the arc discharged.
          http://static-content.springer.com/image/art%3A10.1186%2F1556-276X-6-223/MediaObjects/11671_2010_Article_172_Fig3_HTML.jpg
          Figure 3

          Voltage, current, and energy deposited in the wire during the explosion process.

          Figure 4 shows the voltage and current waveforms under various capacitances. Under the same discharge voltage, the pulsed voltage was faster and higher with 30 μF than with 7.5 μF. The increase in capacitance resulted in a slight increase in current and energy deposition.
          http://static-content.springer.com/image/art%3A10.1186%2F1556-276X-6-223/MediaObjects/11671_2010_Article_172_Fig4_HTML.jpg
          Figure 4

          Voltage and current waveforms under various capacitances: Voltage (a), current (b), and deposited energy (c) as a function of time (black line, 30 μF; red line, 7.5 μF).

          The energy deposited in the wire affected the size and shape of the particles (Table 3). As the energy deposited in the wire increased, the particle size decreased. Figure 5 shows the X-ray diffraction [XRD] patterns of silver nanoparticles generated through the EEWL. Only a silver peak was observed. The average size of the synthesized Ag nanoparticles calculated from the XRD patterns by Scherrer's equation is around 23.9 nm. Figure 6 shows the size distribution from TEM images and the morphology of the Ag nanoparticles at different capacitances. The charging time and duration of plasma can influence the formation of particles. The Ag particles were spherical in shape and the particle distribution approached a bimodal distribution, which may have resulted from liquid droplet formation-induced aggregation of particles [17].
          Table 3

          Average particle size and zeta potential of Ag nanofluids produced by EEWL in deionized water

          Capacitance (μF)

          30

          7.5

          Average particle size (nm)

          123.5

          1,283

          Zeta potential (mV)

          -37

          -11.80

          http://static-content.springer.com/image/art%3A10.1186%2F1556-276X-6-223/MediaObjects/11671_2010_Article_172_Fig5_HTML.jpg
          Figure 5

          XRD pattern of Ag nanoparticles produced by EEWL.

          http://static-content.springer.com/image/art%3A10.1186%2F1556-276X-6-223/MediaObjects/11671_2010_Article_172_Fig6_HTML.jpg
          Figure 6

          Size distribution and morphology of Ag nanoparticles under different capacitances: 7.5 μF (a, c), 30 μF (b, d).

          Figure 6b shows the coagulation of Ag nanoparticles, which was likely a result of three potential processes. First, agglomeration may have occurred during handling or processing due to the high specific surface area of the nanoparticles. Second, it may have occurred during the drying process of sample preparation. Finally, it may have occurred during condensation.

          Figure 7 shows the effect of liquid type on the current and energy. Deionized water resulted in smaller particle sizes and good stability (Table 4). The plasma expanded in the medium due to the temperature difference between the plasma and the medium. A low viscosity led to a larger expansion in the plasma volume; thus, the particle size was smaller.
          http://static-content.springer.com/image/art%3A10.1186%2F1556-276X-6-223/MediaObjects/11671_2010_Article_172_Fig7_HTML.jpg
          Figure 7

          Effect of liquid type on current and deposited energy in the wire. Ethanol (black line), water (red line)

          Table 4

          Average particle size and zeta potential of Ag nanofluids

           

          Deionized water

          Ethanol

          Average particle size (nm)

          123.5

          1,873

          Zeta potential (mV)

          -37

          1.43

          Defects were observed in the particle structure. Under non-equilibrium process conditions, the synthesized particles were imperfect and had defects, such as twins and dislocations. The UV/Vis absorption spectra of Ag nanofluids are shown in Figure 8. Maximum absorbance occurred at ~398 nm, similar to previous reports [18].
          http://static-content.springer.com/image/art%3A10.1186%2F1556-276X-6-223/MediaObjects/11671_2010_Article_172_Fig8_HTML.jpg
          Figure 8

          UV/Vis absorption spectra of Ag nanofluids.

          Typical boiling curves for pure water and Ag nanofluids are shown in Figure 9. The boiling curves of Ag nanofluids showed no significant change due to the very low concentration. The increase in the CHF can be explained by the changes in surface properties [19]. Analysis of the wire surface showed that a silver layer built up during nanofluids boiling, as shown in Figure 10.
          http://static-content.springer.com/image/art%3A10.1186%2F1556-276X-6-223/MediaObjects/11671_2010_Article_172_Fig9_HTML.jpg
          Figure 9

          Boiling curves for pure water and 0.001 vol.% Ag nanofluids.

          http://static-content.springer.com/image/art%3A10.1186%2F1556-276X-6-223/MediaObjects/11671_2010_Article_172_Fig10_HTML.jpg
          Figure 10

          Ag particles coating the surface of the wire after boiling. Magnification: ×400 (a), ×4,998 (b).

          Process optimization

          DOE was used to compare the discharge features and to optimize process parameters. EEWL is a function of many parameters, including wire diameter, length and material, electrical circuit properties, and ambient conditions. In this experiment, there were four parameters of interest that affected the size and dispersibility of particles: capacitance, wire length, liquid volume, and liquid type. Each parameter had two levels or values. The experimental parameters are summarized in Table 5. The experiments were set up using Minitab. The DOE used a 1/4 fractional factorial design of the full factorial design with two repetitions. This resulted in eight total experiments, which was less than the full factorial design of 16, as shown in Table 5. Designs with factors that are usually set at two levels assume that the effects of the factors are linear. Generally, a factor is only evaluated at the low and high points. Ideally, there should be one or more center points to improve the reliability. Further analysis can compare the measurements of the dependent variable at the center point with the average values of the design.
          Table 5

          Experimental conditions

           

          Capacitance (μF)

          Length (mm)

          Liquid volume (mL)

          Liquid

          Run 1

          30

          38

          1,000

          Ethanol

          Run 2

          7.5

          38

          1,000

          Deionized water

          Run 3

          7.5

          38

          500

          Ethanol

          Run 4

          30

          28

          500

          Ethanol

          Run 5

          7.5

          28

          500

          Deionized water

          Run 6

          30

          28

          1,000

          Deionized water

          Run 7

          7.5

          28

          1,000

          Ethanol

          Run 8

          30

          38

          500

          Deionized water

          Run 9

          15

          33

          500

          Deionized water

          Run 10

          15

          33

          1,000

          Deionized water

          Run 11

          15

          33

          500

          Ethanol

          Run 12

          15

          33

          1,000

          Ethanol

          Figures 11 and 12 show the main effects plot, interaction plot, and cube plot for size and zeta potential to quantitatively assess the effects of the parameters of EEWL. In the cube plot, the mean responses of the factors are displayed on the corners of the cube. Low levels of the factors are to the left, front, or bottom and high levels to the right, back, or front of the cube. Figure 13 represents the residual plot for size and zeta potential. The normal probability plot did not show any unusual features.
          http://static-content.springer.com/image/art%3A10.1186%2F1556-276X-6-223/MediaObjects/11671_2010_Article_172_Fig11_HTML.jpg
          Figure 11

          Quantitative assessment of the effects of the parameters of EEWL for particle size. Main effects (a) and interaction plots (b).

          http://static-content.springer.com/image/art%3A10.1186%2F1556-276X-6-223/MediaObjects/11671_2010_Article_172_Fig12_HTML.jpg
          Figure 12

          Quantitative assessment of the effects of the parameters of EEWL for particle zeta potential. Main effects (a) and Interaction plots (b)

          http://static-content.springer.com/image/art%3A10.1186%2F1556-276X-6-223/MediaObjects/11671_2010_Article_172_Fig13_HTML.jpg
          Figure 13

          Residual plots for Ag nanofluids. Particle size (a) and zeta potential (b).

          The process was optimized when capacitance was 30 μF, wire length was 38 mm, liquid volume was 500 mL, and liquid type was deionized water. These conditions corresponded to a particle size of 49 nm and zeta potential of -39.15 mV, as shown in Figure 14. These predicted optimized parameters were confirmed by repeating the experiment, and the results for particle size and zeta potential were 118.9 nm and -42.5 mV, respectively. There were differences between the prediction and the experimental results. However, the experimental results showed the smallest and most stable Ag nanoparticles.
          http://static-content.springer.com/image/art%3A10.1186%2F1556-276X-6-223/MediaObjects/11671_2010_Article_172_Fig14_HTML.jpg
          Figure 14

          Response optimization conditions.

          Conclusions

          In this study, we produced Ag nanofluids by electrical explosion of wires in liquids. By optimizing control parameters, we decreased the particle size under fast explosion conditions and long plasma duration with low-viscosity media. Low viscosity decreased the particle size and dispersion stability due to greater expansion in the plasma volume. The process was optimized by ROT when capacitance was 30 μF, wire length was 38 mm, liquid volume was 500 mL, and the liquid type was deionized water. For the repeated experiment, the average particle size of the Ag nanoparticles in water was 118.9 nm and the zeta potential was -42.5 mV. The CHF of the 0.001-vol.% Ag nanofluid was higher than that of pure water.

          Declarations

          Acknowledgements

          This research was partly supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010-0023511) and the Fostering of Regional Strategic Industry Program though the Ministry of Knowledge Economy (70007094).

          Authors’ Affiliations

          (1)
          School of Mechanical and Advanced Materials Engineering, UNIST 100 Banyeon-ri
          (2)
          Interdisciplinary School of Green Energy, UNIST

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          Copyright

          © Ju Park et al; licensee Springer. 2011

          This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://​creativecommons.​org/​licenses/​by/​2.​0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.