Research On The Synthesis Of High Performance Nanocrystalline Coesite And Nanocrystalline Stishovite By High Pressure Phase Transformation Of Microcrystalline ?-SiO₂

The preparation of nanocrystalline ceramics is mostly based on the nanometer powder as the starting material,the problems of agglomeration,adsorption and difficult dispersion of nanometer powder at present have restricted the mechanical properties of the polycrystalline materials,how to inhibit the growth of nanocrystalline grains during the high temperature sintering process and to keep nanometer properties and high density of sintered body is a technical problem in the preparation of nanocrystalline materials.In this experiment,the technical bottlenecks in the preparation of nanocrystalline ceramics were solved by using high temperature and high pressure,and the nanocrystalline materials with uniform distribution and close to ideal density were obtained.Under high temperature and high pressure,the phase transformation is a "new" process,regardless of the grain size of the initial phase?large single crystal or micro crystal?,the birt h of the new phase to undergo the process of nucleation and growth.the new phase intercepted under high pressure is not restricted by the initial grain size,mostly are the nanocrystalline materials with higher density and hardness.In this experiment,nanocrystalline bulk coesite and nanocrystalline bulk stishovite material were prepared by direct transformation of microcrystalline ?-SiO₂ as the initial material under high temperature and high pressure.The experimental results show:1?At the pressure of 5.0 GPa and temperature of 1000?,the nanocrystalline bulk coesite was prepared by direct conversion using microcrystalline ?-SiO₂ as the raw material.the average size of the grains is less than 100 nm and the length is about 200-300 nm.with uniform microstructure and better density.The microcrystalline ?-SiO₂ began to change to coesite at 3.7 GPa,800?,when the pressure is less than 3.7 GPa cannot be converted to coesite phase,with the increase of pressure,the transition temperature decreases first and then increases,at the pressure of 5 GPa,the transition temperature is reduced to 600?,minimum for all pressure gradients.Raman spectroscopy found that there is a new characteristic peak of suspected polycrystalline crystal at 78 cm^-1,the Raman peaks of nanocrystalline coesite contain all the 10 Raman characteristic peaks of coesite.When the loading force is 1.96 N,the hardness of the nanocrystalline bulk coesite is 1406 MPa.2?Nanocrystalline bulk stishovite was obtained at the pressure of 12 GPa and temperature of 1200-1250?,The material has good compactness and the grain size of less than 100 nm.The microcrystalline ?-SiO₂ has changed to stishovite at 8 GPa,1250?,the product is accompanied by the presence of coesite phase;When the pressure rose to 10 GPa,the Raman peak intensity of coesite has been significantly reduced;When the pressure rose to 12 GPa,the Raman peak of coesite disappeared,the product containing only stishovite single phase,without coesite and quartz.Raman spectroscopy fo und that,nanocrystalline bulk stishovite was obtained at the pressure 12 GPa,there are unknown characteristic peak of stishovite at 1346 cm^-1 and 1607 cm^-1.3?Both pressure and temperature will affect the grain size of the polycrystalline material.Under the same temperature conditions,the density of polycrystalline bulk increased with the increase of pressure,while the grain size is gradually reduced;Under the same pressure conditions,increasing the temperature and prolonging the heating time will increase the grain size of the polycrystalline material.When the two factors exist simultaneously,the temperature effect is more obvious.