Nano-Polycrystal Diamond Synthesizing At High Pressure And High Temperature

Diamond is the hardest material in the world,and has unique properties make it widely used in many fields,such as industrial applications,national defence and so on.However,due to the anisotropy and brightness of diamond single crystal,it is easy to cleavage along(111)when subjected to external force.Compared to diamond single crystal,nano-polycrystalline diamond(NPD)exhibit isotropy,ductility,wear resistance and thermal stability.These properties are even better than the natural diamond,and the hardness of nano-polycrystalline diamond is comparable to that of natural diamond,even more harder than natural diamond.So nano-polycrystalline diamond is a promising material in industrial application.Hence it is considered to be the best way to synthesize nano-crystalline diamond without additives.A lot of methods were performed,but the synthetic conditions are harsh,graphite can only partially converted to diamond.The precursor for the preparation of nano-crystalline diamond can be in various forms,such as glass carbon,onion carbon,carbon black and so on.The synthesis conditions with different precursors is not the same,using graphite as precursor for preparing nano-polycrystalline diamond to the most harsh conditions,but by the direct conversion of graphite to diamond synthesis of nano-polycrystalline diamond is the highest hardness,mechanical properties are the best.It is of great significance to reduce the pressure and temperature required for the synthesis of nano-diamond under the condition of no additives.In order to study on the synthesis of nano-diamond,we calibrate the pressure of both multi-anvil cubic high pressure apparatus and 6/8 Walker type high pressure apparatus.A new high temperature assembly for cubic high pressure apparatus is designed,which can reach 2000 ? and improve the stability and safety.At the same time,the relationship between the oil pressure and the pressure of the synthetic sample chamber of cubic press and Walker type 6/8 press is obtained,and the curve of the relationship between the two is fitted.The temperature of the sample chamber of the 6/8 press is calibrated.These provide reference basis for pressure and temperature.By using high energy ball milling nano-graphite powder,nano-polycrystalline precursor was synthesized,and then use the high pressure synthesis method in 18 GPa,2100 ? and 2300 ? to achieve the transition from graphite to diamond.Micro XRD of synthesized nano-polycrystalline diamond indicates that after sintered the precursor can easily transiform to diamond at 2100 ? and 2300 ? under high pressure of 20 GPa.But for the precursor which has not sintered,the graphite transformation is not completed under high temperature and high pressure.The Raman spectra show that the Raman peak at 1330cm-1 is the first order Raman spectra.The results of SEM showed that the synthesized nano-diamond particles around 15 nm,measurement results of Vickers hardness tester show that precursor sintered samples of Vickers hardness can reach 140 Gpa.This result is larger than that of sigle crystal diamond(120 GPa)by nearly 17%,confirming that NPD is a super hard material with excellent properties.