Decomposition Of Carbon Silicon Nanomaterial Under High Pressure And High Temperature

Silicon carbide(SiC)is a superior semiconductor material with high heat resistance,chemical stability,bulk modulus and excellent electronic properties.It is a good candidate for making fireproofing material,cutter and light-emitting diode.SiC is covalent crystal,the arrangement order of carbon-and silicon-atomic layer is the main factor that determines its structure.Up to now,more than 200 SiC allotropes have been reported.Since the discovery of SiC,the phase transformation,decomposition mechanism and decomposition products under high pressure and high temperature(HPHT)have been attracting great research interest.These related research will help us understand the structure stability and property change of SiC under extreme conditions.Moreover,scientists found SiC in many geological objects,meteorolites and carbon rich planets,so that study on SiC's phase transformation will also be connected to geology phenomena and carbon rich planet searching.Thus the related study in the phase transformation of SiC under HTHP is very important.However,there are still many problems remain unsolved,especially the controversy on the decomposition of SiC under HTHP and what decomposition product could be.The difficulties in these studies are mainly due to the two factors: The first is that SiC has many allotropes,and transformation between these allotropes can be induced by pressure and temperature.When SiC is heated unevenly it is possible to form a mixed phases in the materials.Secondly,carbon atoms and silicon atoms could rebond to form SiC or dissolve to each other if they are not well separated after decomposition.These will make obstacles to our research about the decomposition conditions and decompose product of SiC.To solve these problems,we study the HPHT behaviors of SiC by using nanostructured SiC materials as precursors.Compared with bulk material,IV nanomaterial has much smaller grain size,and thus a more homogeneous heating willbe given to the material so that the temperature gradient can be reduced;moreover,once decomposition,the decomposed components will have a shorter diffusion path to separate in nanomaterial,so that it will be more possible to form a pure phase of silicon/carbon.And we use liquid argon as pressure transmitter,which could also make carbon and silicon atoms separate more easily compared to those previous experiments without using pressure medium.This will reduce the possibility of recombination of carbon and silicon atoms.Using SiC nanomaterial also allows us to compare with the bulk material and figure out what difference between the bulk and nanomaterial under HTHP,and further study the size effect on SiC's decomposition products and the mechanism.Our main purpose is to study whether SiC decompose or not under HTHP,what are the decomposition conditions and products of nanostructured SiC,and finally reveal the possible decomposition mechanism.We use Raman spectrum,XRD diffraction spectrum,selected area electron diffraction,transmission electron microscope high resolution graph and energy dispersive spectrometer,etc,in our study.The main results are summarized as follow:1.We compress SiC nanowires to 10,15 and 20 GPa,respectively,and then heat them by laser heating system to high temperature.Under 10 and 15 GPa SiC decomposes obviously.But when pressure is increased to 20 GPa,heat absorption of SiC is dramatically reduced and temperature of the material does not increase very much even when very high power is applied,so that little SiC decomposes.Through a series of experiments we explain how pressure prevents decomposition of SiC nanomaterial.Under 10 and 15 GPa,after SiC's decomposition carbon atoms will form a coating layer on sample's surface and the inner core are SiC or silicon/carbon rich Si-C phase.The structure of this coating layer is pressure dependent,that is,under 10 GPa carbon atoms form a graphitic layer,while diamond structure show up on the surface at 15 GPa.We obtain successfully a core-shell structure with graphitic carbon as shell and inner core of SiC/C,which has been rarely observed in experiment of using bulk material.The coated graphitic carbon can be further transformed into diamond structure by increasing the pressure.2.SiC nanoparticles have been studied under 10 GPa and high temperature.It shows that SiC nanoparticles with different size behave different under HPHT conditions.When particle size smaller than 100 nm,the decomposed carbon and silicon could easily separate due to short diffusion path,for which carbon atoms could form graphite structure,silicon atoms most likely form a hexagonal structure.Note that hexagonal silicon is hardly synthesized at ambient pressure.When particle size is larger than 200 nm,it is likely that graphitic carbon prefer to form on material's surface.We thus discover that particle size should have significant effects on SiC's decomposition behavior and the different decomposition products in SiC particles could also be related to their different diffusion path/length in SiC materials.