Small Angle X-Ray Scattering Study On The Structure Of Crystalline Particles In SiC Fibers

SiC fiber is a kind of ceramic fiber.Owing to its high strength,good corrosion and oxidation resistance and compatibility with ceramic matrix,SiC fibers have important applications in fields such as aerospace industry and nuclear reactors.Its microstructure especially the SiC crystals inside determines the macro properties of the fiber.Hence it is vital to investigate the structure of the crystalline particles changes under different fiber synthesis conditions in order to obtain the optimal production condition.In this thesis research,as a preliminary investigation,we firstly study the microstructure of commercial SiC crystals.The structure and size of SiC nanoparticles were studied by different characterization methods including Small Angle X-ray Scattering(SAXS),Transmission Electron Microscope(TEM),and X-ray diffraction(XRD).The results showed that particle size distributions determined respectively from SAXS and TEM are comparable and follow the lognormal function.The size distribution of the particles is between 10 to 100 nm with most of them being in the range of 20-50 nm.The average particle size is around 42 nm.XRD identifies the phase of the SiC nanoparticles and suggests the average size of the single crystalline domain to be around 23 nm.The combined results from XRD and TEM suggests the existence of many polycrystals,which is confirmed by the HRTEM observation of particles with twins and stacking faults.All the methods together provide an overall structure image of the SiC nanocrystals.Secondly,we apply the same characterization techniques to study the SiC fibers processed with different pyrolysis temperature and time.We obtain the average sizes of crystalline domain and particles at each processing condition.Combined with mechanical test results,we find that as temperature and time increase,tensile strength and modulus of the fiber also increase.In the meantime,-SiC particles and the crystalline domain grow larger and larger,while the growth rate of the particles is faster than the average crystalline domain.Hence the domain number increases as the fiber strength goes up.The increased number of grain boundaries might contribute to the improvement of the strength of the material.