Growth Mechanism And Influence Factors Study Of Nanodiamond Synthesized By Liquid Pulsed Laser Irradiation

Compared with conventional diamond,nanodiamond has found broad application value in the fields of machinery,electronics,chemical industry,medical etc,attributing to its unique mechanical,optical,thermal,magnetic properties.Compared with traditional methods,liquid-pulsed-laser irradiation is a green pollution free method,the operation is flexible and the parameters are controllable,the effect is remarkable in the preparation of new nano-materials.In this paper,the traditional carbon of graphite was taken as the research object,we implemented the high energy pulsed laser irradiation experiments of graphite suspension to study the growth mechanism of nanodiamond and graphene nanoribbons and factors influencing its yield.The main research contents and conclusions are as follows:(1)It was found that the graphite was transformed into the different nano-structure materials after high power density(109~1011W/cm2),short pulse(10ns)laser irradiation of the circulating graphite suspension: nanodiamond,grapheme nanoribbons,spherical amorphous carbon cluster,carbon nanotube-like material,disorderly layer of carbon cluster.With different laser energy,the number of layers of graphene ranged from 1~14.A few layers of short and ordered graphene fragments were spun off from graphite surface alone under the combined action of laser thermal stress and shock wave.With the increase of laser energy,the number of layers of graphene increased and the degree of disorder was getting higher.(2)The nanodiamond was synthesized continuously by the method of the liquid pulsed laser irradiation.The components,morphology,microstructure,micro-stress and various defects were analyzed.It was found that with the increase of laser energy,the yield of diamond didn't change obviously,and the surface morphology was spherical or elliptical.The particle size of nanodiamond ranges from 2~10nm,the average size is about 5nm.Compared with conventional diamond,the Raman peaks of nanodiamonds are broadened and shifted to lower frequencies because of the ultrafine size,crystal lattice distortion and large residual stress inside the obtained diamond.(3)The growth mechanism of nanodiamond was proposed based on physics,thermodynamics and kinetic theories.The diamond growth temperature could be calculated according to carbon phase diagram,during the endothermal process of the graphite particles,the gaseous plasma evaporated from the graphite surface and collapsed into liquid carbon droplets.At this time,the liquid carbon droplets entered into the graphite metastable and diamond stable region under thermodynamic conditions,carbon clusters may recrystallize in to diamond structure and grow rapidly.During the process of cooling,the carbon droplets might pass through the point(4300K,12.5GPa),a lot of twisted graphene nanoribbons were found composed of enclosed nano-spaces which the diamond would nucleate and grow inside.Phase transition mechanism could be demonstrated that high energy pulsed laser irradiation of graphite suspension is a solid-vapor-liquid-solid or solid-vapor-plasma-liquid-coexistence state-solid process.It was concluded that why nanodiamond grew only nanometers: considering from the growth resistance mechanism of nanodiamond,high degree of supercooling helped prove the growth velocity.In addition,crystal nucleus growth time was very short,limiting the further growth of nano-diamond.(4)Graphite with smaller particle size(2?m,5?m)was found easier and more favorable for the formation of nanodiamond under the condition of the fixed laser energy and different particle sizes.The formation heat of graphite with smaller particle size is higher than graphite with larger particle size and is more unstable.Therefore,it could transform into nanodiamond more simply.After the experimental setup has been improved,the interaction area between the laser and the graphite is increased and the synthesis efficiency of the nanodiamond is greatly improved under the combined action of the dual-beams-pulsed laser.The phase transition route from graphite to nanodiamond could go along as solid-vapor-plasma-liquid-coexistence state-solid or solid-vapor-liquid-solid route.Moreover,the existence of nanodiamond inside of amorphous carbon clusters and at the edge of disorderly-layer carbon clusters was the evidence of nanodiamond nucleating and growing in a liquid phase.Nanometer size effect induced the decrease of the melting point was another mechanism for the synthesis of ultrafine nanodiamond as well as the important reason of restructuring spontaneously to form Bucky-diamond like structure that the diamond particles were surrounded by carbon atoms.