| In the 60s of last century,the Polycryatalline Diamond(PCD),which is made up of many diamond particles,gradually replaced natural single crystal diamond as the main consumables in the field of polishing and abrasive.The prospect of the application of nano polycrystalline diamond in the ultra fine processing industry,such as optical devices and semiconductors,has tens of billions of dollars in huge economic benefits and hundreds of millions of carats of diamond demand.The ideal pure polycrystalline diamond not only has the excellent physical properties similar to the single-crystal diamond,but also has the advantage of the isotropy that the single-crystal diamond does not have.Therefore,many scientists are concerned about the preparation and synthesis mechanism of PCD.The method of explosion is unique in the field of diamond synthesis because of its advantages of fast speed,high yield,low energy consumption and simple operation.In this paper,a new method for detonation sintering of 50?200nm nano polycrystalline diamond by DND as raw material and oxide as binder phase is studied.Two aspects of this method must be taken into consideration:the oxidation and graphitization of diamond at high temperature and pressure.Combining with the detonation characteristics of explosives and the properties of DND,two effective ways to avoid oxidation ablation and reverse graphitization of diamond are put forward in this paper:(1)the appropriate detonation parameters should be chosen.Detonation should occur in diamond stable region or near stable region in the carbon phase diagram,which essentially reduces the driving force of diamond reverse graphitization.Reduce the action time of unloading high temperature and prevent the reverse graphitization of nano diamond in the stable region;(2)pretreatment of nano diamond particles.The nano diamond particles are pretreated and the coating oxide protective layer can isolate the oxidation of nano diamond during the detonation process.The details are as follows:First,in order to improve the oxidation resistance of nano diamond synthesized by detonation,surface modification of nano diamond powder by molten salt method was carried out.The nano diamond particles were surface modified by molten salt method of boron oxide,sodium silicate,chromium chloride.And the modified nano diamond powder was characterized by thermogravimetry(TGA),X-ray diffraction(XRD),Fourier transformation infrared spectrometer(FTIR)and transmission electron microscopy(TEM).The results show that:(1)the initial oxidation temperature of the nano diamond powder has been obviously improved after the surface modification.The rate of oxidation has been reduced and the amount of the remaining diamond powder has been greatly increased after annealing.(2)the functional groups of C-B-O,C-Si-O,C-Cr-O has been grafted on the surface of nano diamond.The barrier effect of these functional groups on the surface of nano diamond particles can increase the antioxidant capacity of diamond.(3)In addition,these kinds of functional group have a good hydrophilicity and can reduce the wetting angle of the diamond.Therefore,the modified method also provides a new way to solve the water dispersibility of nano diamond.Then,experiments on detonation sintering were carried out.The surface modified nano diamond was mixed with the explosive containing aluminum,silicon and chromium,and the experiment of the nano polycrystalline diamond by detonation was carried out.XRD,TEM,scanning electron microscopy(SEM)and energy spectrum(EDS)was used to characterize and analyze the phase,morphology and composition of nano crystalline polycrystalline materials.The following results are obtained:(1)the shock waves produced by the detonation of explosives accompanied by high temperature and pressure made the nano diamond with the size of 4?10nm be closely bonded with the oxide(alumina,silicon oxide and chromium oxide),and aggregate into the polycrystalline diamond with the size of 50?200nm.(2)when discussing the XRD diagram of the detonation product,it could be found that the alumina in the detonation product contained a variety of crystalline forms.And a small amount of alumina hydrate was found.(3)the wettability between boron oxide,silicon oxide,chromium oxide and diamond should be better.Their existence is helpful to improve the wettability between diamond and alumina,and to strengthen the interface bonding between diamond and alumina.(4)the surface modified diamond can withstand high temperature and pressure without oxidation and graphitization.And no obvious graphite was found in the HETEM diagram of polycrystalline diamond particles.Finally,the mechanism of detonation sintering was studied.The pressure and temperature time-histories relation of the product was calculated by the detonation theory.Then,a theoretical model of the reverse graphitization probability of diamond was established,and the reverse graphitization probability of nano polycrystalline diamond by detonation sintering is discussed.The detonation parameters of polycrystalline diamond were calculated based on the BKW and JWL state equation of the detonation products.A detailed description of the dynamic scattering problem of explosives by Autodyn software was to calculate the relationship of chemical components-pressure-temperature-time.The following conclusions are obtained:(1)when analyzing the state of detonation parameters in the carbon phase diagram,we can know that the oxide protective layer on the surface of DND could protect it from oxidation in a rapid explosion reaction.At the same time,it was found that there was no obvious reverse graphitization in the process of detonation sintering polycrystalline diamond in the unstable region of the diamond.(2)through the numerical calculation results of Autodyn,it was known that the process of detonaion sintering polycrystalline diamond was the reaction of high temperature,high pressure and short duration of 2500?3500K,10?30GPa and ten to dozens of microseconds.(3)the reverse graphitization model was established by using the probability of atomic motion.The calculation results showed that the size,temperature,pressure,cooling speed and reaction time would influence the reverse graphitization probability of diamond.(4)through the calculation of the reverse graphitization probability of the process of detonation sintering polycrystalline diamond,it was found that the probability of reverse graphitization was only about dozen parts per million.Therefore,the reverse graphitization probability of the process of detonation sintering polycrystalline diamond is almost negligible,which was in agreement with the experimental results of HRTEM. |
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