Study On The Preparation Of Nano-powders By Electrical Explosion Of Wire And Its Mechanism

Nano-powder refers to the particle with the size of 1nm?100nm,which is a typical mesoscopic system range between a single atom,molecule and macroscopic object.Nano-powder shows potential application prospects in national defense,biomedicine,electronics industry,new energy and other fields base on its unique mechanical,magnetic,electrical,thermal and optical properties.Such as the use of nano-metal powder on the special absorption of electromagnetic waves can greatly enhance the stealth performance of military materials;nano-ceramic coating shows high toughness,high strength,high thermal barrier and strong wear-resisting property and is widely used in aerospace,Aviation and navigation.Therefore,the study on the preparation technology of nano-powders and related related mechanism,performance has become a hot research topic.Electrical explosion of wire(EEW)is a new technology for the preparation of nano materials.It is the use of a strong pulse current to heat the wire to vaporization and even plasma state,and then the explosion products condense into nano-particles in a certain medium(such as inert gas,water,etc),some nano-powders also involve the chemical reaction with the media,the extract of elements from the media and other phenomena.The systematic study of the basic principles of electrical explosion and the formation mechanism of nano-powders under high temperature and high pressure are of important engineering application significance for broadening the application of electrical explosion method in nanomaterial preparation and improving the product quality.Meanwhile,the research of electrical explosion method is also of important academic significance,for which is conducive to promote the interdisciplinary integration of electromagnetism,thermodynamics,materials science,etc.The main research work and conclusions are described as follows:An electrical explosion experiment and test system was developed for the preparation of nano-powders and parameter detection of explosion process.The system mainly consists of pulse discharge device,powder collection device and detection device,which can realize the control of medium environment,circuit parameters,and material parameters of blast wire.It can also effectively observe the pulse current,voltage and shock wave in the process of electrical explosion,and provide sufficient experimental dates for the establishment of theoretical model.The electrical explosion process and its formation mechanism are analyzed.Base on the features of current and voltage waveforms,we divided the electric explosion process into three stages:stable heating and phase change stage,vaporization and explosion formation stage,diffusion and condensation stage.The electric,magnetic,thermal and force fields in the process of current heating are analyzed by the skin effect,thermal effect and electromagnetic pinch effect.Based on the analysis of the characteristics of current distribution,pressure distribution and instability,the formation mechanism of shock wave is summarized.It shows that the sudden change of pressure is due to the interruption of the current in the liquid,which lead to the magnetic pressure disappeared and the thermal pressure released.Pure metal,metal-oxide,carbon-coated nanopowders and carbon nanomaterials were prepared by electrical explosion method.The resistance variation and deposition energy characteristics of these materials were calculated by the measured current and voltage.The morphologies of the products were observed by transmission electron microscope(TEM).The composition and average particle size of the products were investigated by X-ray diffraction(XRD),and the formation mechanism of nanopowders was summarized.The results show that the process of preparing nanopowders by electrical explosion is a combination of"top-down" and "bottom-up".The basic steps include gasification of exploding wire,generation of growth substances and crystal nucleus,growth substances were adsorbed to the nucleation surface,growth of solid phase and recrystallization.Taking copper,zirconia and carbon-coated metal nanopowders as examples,the process of grain growth is summarized as three types:free crystal growth,recrystallization of solid state and molecular remodeling.Three influencing factors(circuit parameters,environment medium and material properties)of preparing nano-powders by electrical explosion are analyzed combined with the data of the comparative experiments,the electrical explosion process and the nucleation and growth mechanism of the nanopowders.The results show that the circuit parameters and the material size played an important role in the deposited energy density during the process of"top-down"(the evaporation of wire).The environment medium and material properties wire played a key role in the process of "bottom-up"(the condensation and growth of nanopowders).Therefore,it is necessary to consider the impact of three factors synthetically when electrical explosion of wire method was applied to prepare nano-powders and select the appropriate experimental parameters according to the specific product type and experimental purpose.