Nano-powder Production By Electrical Explosion Of Metallic Wires

The electrical explosion of metallic wires for the nano-powder production is performed by rapidly heating the wires to the vaporization temperature with a high-density current pulse. The metallic vapor produced by electrical explosion of metallic wires is cooled by the collisions with the molecules of the embedding gas, forming nano-powders from the condensed vapor. Being composed of a pulsed current generator and a discharge chamber, a facility for the nano-powder production by electrical explosion of metallic wires was designed and built. Eight metallic wires housed in the discharge chamber are exploded one by one before opening the chamber for the collection of the produced nano-powders.With the focus on increasing the rate of the energy deposition into a metallic wire for exploding the wire in the overheat mode, the electrical behavior of the exploding wire was studied by numerically solving nonlinear differential equation describing the discharge circuit. For the metal wires of high conductivity and low sublimation heat, such as copper, the circuit simulation was well conducted based on the resistivity model developed by Tucker in which the resistivity of an exploding wire was expressed with the explicit functions of specific action. For the metals such as titanium and zinc of their resistivity anomalously changing, i.e., decreasing rather than increasing, with the liquid heating, the circuit simulation was performed using the implicit relationship betweenρand g that was obtained by data acquisition, point by point, from the experimentally measured curve. The results from the circuit simulation showed that the best method for increasing the energy deposition rate is to reduce the capacitance and increase the charging voltage of the energy-storage capacitor, while keeps the storage energy constant.For the purpose of more thoroughly understanding the physical processes of the nano-powder formation by the metallic vapor, a Mach-Zehnder interferometer was used to record the time evolution of the metallic vapor as well as the plasma. It was found that the exploding wire is characterized by a central dense core as well as the surrounding plasma and a rapidly expanding neutral gas shell outside. Based on the fringe shifts on the interferograms of the titanium wires exploding in air at a pressure of 10 kPa, the averaged electron density of the plasma and the averaged density in the neutral gas shell were calculated to be about 1×1018 cm-3 and 6×10-4 g/cm-3, respectively. The expanding speed of the neutral gas shell was determined to be in the range of 1.4 km/s ~ 1.7 km/s, which is much higher than the sound speed in air at room temperature, leading to a shock wave. A thermal expansion lag of the dense vapor core was observed for the first time and this thermal lag was shortened by an increased overheat factor. More than one times of the vapor burst from a titanium wire exploding in the mode of underheat were usually observed, which suggested there exist two heating mechanisms, i.e., Joule heating followed by the plasma heating. The evolution of an exploding copper wire was found to be much different from that of a titanium wire. Even in the mode of underheat, there appears only one burst of the metallic vapor from the exploding copper wire.Nano-powders of titanium nitrides, titanium dioxides, copper oxides and zinc oxide were produced by electrical explosion of metallic wires. The smallest averaged diameters for these powders are 16.05 nm, 13.86 nm, 40.70 nm, and 50.18 nm, respectively. The influences of the experimental conditions on the dimension of the nano-powders were investigated. Although many parameters, such as the energy-storage capacitor as well as its charging voltage, the embedding gases as well as their pressures, the wire materials and diameters, were found to have influences on the dimension of the nano-powders, only the overheat factor, the gas pressure and the gas temperature, were considered as the intrinsic parameters that are directly related to the dimension of the nano-powders. All the experimental results of the nano-powder production could be well explained with these three intrinsic parameters. It was important to find that the dimension of the nano-powders is not only determined by the overheat factor, but also by the gas pressure and the deposited energy into the plasma that determines the gas temperature. While the dimension of the nano-powders was usually decreased by increasing the overheat factor in the mode of overheat, the overheat factor was not dominant in determining the dimension of the nano-powders in the mode of underheat. It was also found that the dimension of the nano-powders is usually increased by raising the gas pressure and that the overheat factor is indeed enhanced by increasing the rate of energy deposition into the wire.