In Situ Generation Of Nano-graphite Under Laser Irradiation And Study On Its Technology

In recent years,nano-carbon materials have attracted much attention from the material academia,including carbon nanoneedles,carbon nanofibers,carbon nanotubes and so on have been studied and studied.As an inorganic non-metallic nano-material,nano-graphite retains many excellent properties of graphite compared with common graphite materials,such as conductivity,lubricity,and so on,and nano-graphite has quantum effect,macroscopic quantum tunneling effect,small size effect and surface effect.Therefore,nano-graphite has excellent physical,chemical and interfacial properties.Nano-graphite is able to be used as a functional material,an additive,a guided interfering agent,a hydrogen storage material and other civil and military fields.The main problems faced by the application market today are:the nano-carbon materials have such problems as non-uniform particle size,unstable performance and agglomeration,and the preparation cost is high.To solve the above problem,the traditional carbon materials（50μm flaky graphite）were used as the carbon sources,the IPG500 W fiber laser with vibrating mirror scanning was used as the laser irradiation source,the Ar gas was used as the protective gas,and in order to prepare nano-graphite,the micro-graphite preseted on the substrate（Ni and Si）was prepared by laser single track scanning,and a large number of experiments were carried out to optimize the laser irradiation process parameters and preparation conditions.By means of high resolution transmission electron microscopy（HRTEM）,scanning electron microscopy（SEM）and X ray diffraction（XRD）,the effects of the preparation conditions,such as gas flow rate,laser irradiation power,scanning speed,and the type of prefabricated substrate on the composition,crystal structure and microstructure of graphite nanoparticles,were investigated.The experimental results showed that when the laser energy density was 5.00 kJ cm^-2,the sample changes from micron flake to spherical product with good dispersion;when the laser energy density was 6.25 kJ cm^-2,ellipsoid graphite was found in the samples;when the laser energy density increased to 12.50 kJ cm^-2,the micron flake graphite transforms into a large number of spherical graphite,which exists in crystal form,and the average size was 61.5 nm;when the laser energy density increased to 13.75 kJ cm^-2,the sample existed in crystal state,and the product showed large particles.There are many small particles attached to the particles,and the particle size range is large,the minimum particle size is about 150 nm,and the average particle size is 280 nm.At the same time,it is found that with the increase of laser energy density,the micro-morphology of micron graphite gradually changes from the larger sheet to the smaller spherical granule,thermodynamics indicates that any material or system is most stable at the lowest gibbs free energy state,whereas the deposited solid nanoparticlesare thermodynamically unstable or metastable due to the large surface energy of the transition from micron graphite sheets to nanoparticles.In order to reduce the total surface area of materials at the single nanostructure level,the total surface energy is reduced with growth as a globule.Moreover,fixed other conditions unchanged,with the increase of laser power,the temperature gradient between the laser scanning area and the heat affected zone on both sides is obvious,the reasonable Ar gas protection promotes the cooling of the reaction system,further enlarges its supersaturation,and results in the decrease of the critical size of the stable spherical nucleus.At the same time,the nucleation of graphite vapor was inhibited and the further growth of graphite was inhibited to produce nano-graphite with smaller particle size.In this paper,50μm flaky graphite sheeted on different substrates was irradiated by a galvanometric fiber laser with Ar gas was used as protective gas.By changing the laser processing parameters and technological conditions,Preliminarily conditions for preparation of nano-graphite by laser irradiation micro-graphite and morphology and microstructure of the product were obtained,the formation mechanism of nano-graphite particles in-situ formed of micro-graphite by laser irradiation is obtained.