The Research On Preparation Of Metallic Ultra-fine Powders By Uncontacted Wire Electrical Explosion Method

When a pre-stored electrical energy loaded upon a thin conductor (including wires and foils)fleetly in some ambience, a pulsed current generated through the conductor and the energy will be transferred to heat energy in high speed, supervene the fast electrothermal effect. The conductor will be melted, evaporated and ionized, resulting in a plasma that expands into the medium. This process is commonly called Conductor Electrical Explosion, and it can be used to produce ultra-fine powders of pure metal, alloy, metal nitrides and metal oxides. This application is provided with many advantages, such as high efficiency of energy, governable process variables, applicability of different metals, environmental friendliness, and so on. And this technics have more significant points in preparing powders, the characteristics of powders as follows: high purity, reactive and well-dispersed with narrow distribution.The device of wire electrical explosion(WEE)is generally designed as: the wire conductor is fixed by the clamps collocated in each electrode, or one end of the conductor is nipped, and another is non-restraint. The wire length always restricted by the interspace between two electrodes, and the wire diameter cannot be too large or too small, additionally, there are several defective aspects of device such as energy unable released completely, the two electrodes would be possibly broken in the forceful blast, and some tinpot presents in the powders prepared by this method.A novel method of preparing ultra-fine powder by wire explosion was proposed in respect of above devices, the metal wires could be exploded electrically in the high voltage electric field without contacting with electrodes. The limitations citied above are smoothly resolved.The un-contacted equipment of WEE is advisedly developed in this paper, which can be called "UCE-WEE". This equipment consists up "arranging-feeding" parts of wire, the vessel of wire electrical exploding, the powders collecting system, high voltage circuit and its controlling system. The makeup becomes more simple, and the operation is relatively flexible. The primary feature is that wire is completely non-restraint, wire no longer be fixed by clamps, and therefore this device can adjust wires with different length and diameter. In addition, the clamps, the gap switch and its operating system are removed, so the configuration of the equipment become more compact, and the managing reliability is well improved. We can explode wire continually using this new method, with presently frequency reaches 10 times/min.We exploded sets of Cu wires with different diameters and different lengths, in order to research the electrical explosion of un-contacted wires, and attest the possibility of application in preparing ultra-fine powders.The crucial factors which markedly influence the whole explosion are concluded generally as follows:1) Arc discharge and preliminary formation of plasma in the initial heating stage. Discharge and plasma in the initial heating stage determine the starting state of electrical explosion, because the transformation above observably exert an influence on the whole heating action which from wire solid state, melting, melted to liquate state till gasification. Many research reveals the transformation is controllable, so we can shift this energy-cumulating process by changing some parameters.2) Energy density. Energy density is named as the energy deposited the volume unit of metal. The explosion of materials inclines to start in the high-density sections if energy density is increased obviously, in addition, the explosion behave as fastexplosion when energy density excesses over the integral action of specific current.We discover that powders show some susceptivity to the process parameters when applying this new method to preparing powders. The energy deposited in wire during explosion of wire in some conditions is controllable to reach a high-point, and the explosion up to the utmost. If the energy is high enough, the process seemed as fast explosion, most of the material vaporizes into the medium and forms superfine droplets, the droplets impelled by the blast and turns into small particles. But if the energy is quite low, there is a small part of material vaporizes into the medium, the rest forms some large droplets and fragments, the last products contains a lot of large particles which always have irregular shapes.People can explode the wires under well-matched process parameters to obtain conceivable powdersAnd next, exploding copper wire experiments with "UCE-WEE" were carried out with a variety of process parameters to prepare the ultra-fine powder which includes Cu and oxides. Particle size distributions measured by laser particle sizer, the results indicate that large number of quadratic particles present in powders. In SEM and TEM images of the powders, particles display as round and polygon shaped, with size reach nanosize. We measured a finite number of particles in these images, and analyzed the data with the "log-normal" function , the results show that the median diameters of some powders are lower than 60nm, and granularity have the range from 0.02 μm～0.3μm.