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 [1
]. 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 [6
]. 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] [11
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 [14
]. 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 summarizes previous studies on the EEWL method.
Previous works on electrical explosion of wires
A schematic illustration of the experimental system is shown in Figure . 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 .
Schematic diagram of the experimental system for the EEWL process.
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:
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].