Study On The Technology Of Wire Electrical Explosion By Gas Discharge For Nanopowders Preparation

Metal nanopowders are materials with manifold excellent properties and havebroad application prospects in the field of national economy and national defence.However, the applications of metal nanopowders are limited at present because oftheir high price and the lack of preparation technology. Wire Electrical Explosion(WEE) method has many advantages in nanopowders preparation, and is considered asa preparation method for large-scale industrial nanopowders. In present WEE method,a metal wire was first linked with two electrodes, then the current was injected intothe metal wire from the electrodes through the channels which were provided by the"contact" of the electrodes and the wire ends. This method has lots of unavoidablydisadvantages in practice application, such as the high fault rate, the electrodesbeing ablated easily, some micron-size particles remaining in the powders, and so on.Previous studies shows that all of those problems are relevant to the "contact"channels through which the current is injected into the metal wire from the electrodes.Hence, a new method of WEE with gas discharge is proposed in this paper, in whichthe current flows into the wire by gas discharge. Then this WEE by gas discharge isalso investigated systematically.For the current channelling into the wire by gas discharge, two methods of WEEby gas discharge devices are designed, which are hole-to-plane electrode WEE deviceand conical electrode WEE device respectively. And the breakdown properties of theabove two methods are also researched. In the process of hole-to-plane electrode WEE,there are two forms of gas discharge, including the single air-gap discharge and thedouble air-gap discharge. The breakdown voltage of the single air-gap discharge issmaller than that of the double air-gap discharge. The optimum length of the metalwire for electrical explosion is the minimum length when the double air-gap dischargecan occur. However, there is only single air-gap discharge exist in the process ofconical electrode WEE. Hence, the conical electrode WEE is beneficial forengineering application. The gap discharge in this method is similar to that in therod-plane air gap: With the increasing of initial charging voltage, the length of theair-gap increases and the average breakdown electric field strength decreases; Thebreakdown voltage of air-gap is depedent on the distance between two electrodes, andgap increases with the increasing of electrodes distance.In the process of WEE, the current can be injected into the metal wire through the channels which are provided by the "contact" of the electrodes and the wire ends,as well as the "gas discharge" between them. Mechanism of current injection isdeduced according to the experimental results. In these two channels, the photocurrentdetected by photodiodes at the wire ends almost occurs simultaneously with thecircuit discharge current, but that at the center section lags behind the circuitdischarge current obviously; The initial explosion products of the wire ends is liquid,and that of the other parts of wire is gas; When the current is injected by the way ofthe "breakdown", the electrodes ablated is slight obviously, and there is no remainderon the electrodes. These results indicates that gas discharge also occurs at the wireends when the current is injected by the way of "contact". Because the plasmaprovides an alternated current path, the energy density of the wire ends decreases.Hence, the wire ends couldn’t explode and formed remainder on the electrodes.In the process of nanopowders preparation by gas discharge WEE, the powdersfrom wire various sections are collected by a quartz probe, then the position ofmicron-size particles are observed. Moreover, the characters of particle formation arealso analyzed. The results shows that, the micron-size particles proportion of the wireends powders is higher than that of wire center section powders. The main reason forthis phenomenon is that the ways of energy deposition are different. At the center ofthe wire, the energy would deposit into the wire mainly by the Joule Heating.However, at the ends of wire, the charged particles which came from the plasmawould strike on the wire surface at a high speed. Thus, some energies also could beinjected into the wire end by this effect besides of the Joule Heating. When thecharging voltage is too high, the breakdown which is the major reason of theincreasing of micron-size particles proportion will occur surrounding the wire surface,and the plasma which is known as the shunting arc will form by the gas discharge.The optimum charging voltage for nanopowders preparation is the maximum voltageat which the surface discharge could not occur. The above analysis indicates that theaverage breakdown electric field strength for surface discharge is about2.37kV/cm,and the energy density deposition in the wire increases with the increasing of initiallinear energy, However, it will decrease again when the surface discharge occurred.Based on the experimental results, a gaseous layer model which considers the gasdischarge along the wire surface is proposed to understanding the characters of energydeposition and the influence of technological parameters. The electrical behavior ofthe discharge gap WEE is studied by numerically solving nonlinear differentialequation describing the discharge circuit based on this model. The numerical simulation results also shows that, the discharged gap has a little influence on theprocess of WEE. The energy deposition rate increases with the dcreasing of the circuitinductance or the capacitance of the energy-storage capacitor when the storage energyremains constant. Enough energy could deposit into the wire before the explosionwith the rational matching of the wire length and wire diameter.Based on the above research results, a industrialized equipment of gas dischargeWEE in laboratory scale is developed. In this equipment, the range of the wirediameter and length are0.05~0.8mm and25~100mm, respectively. Moreover, theexplosion frequency is higher than2.1Hz and the efficiency of wire feed is more than105mm/s. This gas discharge WEE equipment is better than the other equipments onthe respect of the wire diameter range the and production efficiency.