Study On The Regulation Mechanism Of Reversible Phase Transition Between Nanodiamonds And Onion-like Carbon And Its Application

In recent years,the research on reversible phase transition between nanodiamonds(NDs)and onion-like carbon(OLCs)has provided a new model for exploring the controllable path between graphite and diamond.For example,OLCs have been synthesized by thermal annealing of NDs,and superhard diamonds have also been prepared from OLCs.However,the high temperatures,high pressure,and long time-consuming involved in the above conditions are not the path pursued by industrialization.As a DC pulse electric field assisted sintering technology,spark plasma sintering(SPS)is expected to provide new technical advantages for the above research.In addition,OLCs,as an outstanding representative of non-planar carbon chemistry in recent years,are destined to show its application value in many fields.Therefore,it is of great engineering significance to further explore the application of OLCs in new fields.Herein,the regulation mechanism of reversible phase transition between NDs and OLCs in SPS and its application in titanium matrix composites are mainly studied.The thermal stability of NDs and its phase transition to OLCs under DC pulse electric field were studied.Experimental results showed that NDs could almost remain stable until 950?under 60 MPa pressure.As the temperature was increased,amorphous carbon appeared on the surface of NDs,and then,graphitization began.At1300?,lamellar graphite structures were formed and kept stable with increasing holding time(to 30 min),but no OLCs was found.The optimum parameters for the synthesis of OLCs from NDs via SPS are the temperature of 1400?and holding time of 15 min under a pressureless condition.The main transformation mechanism is that the pressureless condition during the SPS processing creates a more favourable environment for the NDs graphitization and curling into spherical OLCs.The existence of pressure during the SPS processing can improve the thermal stability of NDs,delay the initial temperature for graphitization transition of NDs and inhibit the graphite layer curling to form OLCs.The structural stability of OLCs and its transformation to diamond under DC pulse electric field were studied.As a result,under the normal temperature(1200-1400?)and pressure(0-60 MPa)in SPS,the stable carbon layer structure of OLCs could not be changed even if the holding time was extended.When the temperature reached 1500?and the pressure was 80 MPa,the carbon bonds of OLCs began to break and the free carbon layer was distorted.Fe Ni₃₀catalyst promoted the conversion of OLCs to diamond under relatively mild conditions of 1200?and 80 MPa pressure in SPS.Without the addition of external catalyst,OLCs with diamond core give external carbon layer the ability to transform to diamond under mild conditions(1200?,60MPa)in SPS.The main mechanism is that the introduction of Fe Ni₃₀ significantly reduces the high activation energy required to overcome the energy barrier during the phase transition from OLCs to NDs.On the other hand,the OLCs with diamond core can eliminate the higher activation energy required by the spontaneous nucleation of OLCs and give OLC particles natural diamond growth sites.Especially,using OLCs with diamond core as the precursor and Fe Ni₃₀ as the catalyst can significantly reduce the conversion conditions of OLCs to diamond.The application of OLCs as a new reinforcement in titanium matrix composites was studied.The results showed that OLCs were partially in-situ reacted with the Ti matrix forming nano-to submicron Ti C particles and some OLCs were retained after the SPS processing.The unreacted OLCs and fine dispersed Ti C particles give the titanium matrix composites excellent mechanical properties.The Ti-0.35 wt.%OLCs composites possessed excellent compressive and tensile properties with acceptable ductility,which are 54%and 40%higher than those of pure Ti,respectively.The strengthening of the composites reinforced with the hybrid reinforcements of Ti C/OLCs can be attributed to a group of mechanisms:solution strengthening,grain refinement,load transferring effect,and dispersion strengthening.