Microstructure Control And Properties Study Of The SiC Coating Layer

TRISO(tristructural-isotropic)particles comprise the fuel kernels and four coating layers:porous pyrolytic carbon(porous PyC)layer,inner dense PyC(IPyC)layer,silicon carbide(SiC)layer,and outer dense PyC(OPyC)layer.TRISO particles are used as advanced fuel forms for high-temperature gas-cooled reactors and molten salt reactors,even for fully ceramic microencapsulated(FCM)fuels.Among all the coating layers,SiC layer is considered as the most important layer to provide mechanical strength,and prevent the fission products from being released from the inside of the fuel particles,because the SiC layer has chemical inertness,high thermal conductivity,high hardness and low diffusivities for fission products.So far,the overpressure of fission gas,silver release from the silicon carbide layer have limited the safety of the SiC layer.Therefore,many researchers are working on the enhancing of the SiC layerRecent research shows that the preparation of small grain SiC coating layer is an effective way to enhance the properties of SiC layer.Previous fundamental studies show the silver release process in SiC layer is through the grain boundary diffusion.The flexural grain boundary in fine grained SiC layer can retard the release process of silver.Moreover,SiC layer with small grain size shows better mechanical property than SiC layer with larger grain size.This work studied the effects that the deposition parameters have on the SiC layer.A new preparation method for SiC layer with fine grain size and no impurity was proposed.This work also investigated the growth process and growth mechanism of the CVD SiC layer.The SiC layer with small grain size was used to study the high temperature resistence of the SiC layer.This study summarizes the different change of the SiC layers with different grain size after thermal treatment(1)This work studied the effects that the deposition temperature and the carrier gas have on the SiC layer.The SiC layers deposited at lower temperature has smaller grain size and worse crystallinity than higher temperature.Besides,there will be free silicon in the SiC layers when they were deposited at low temperature.The effect of the volume ratio of H2 to Ar have on SiC layers is researched.The content of the free silicon in the SiC layers get lower as the percent of H2 decrease.And there will be C impurity when the percent of H2 is lower than 15%.The free silicon content can be eliminate by adjusting the percent of H2 and the pure SiC layer with average grain size about 150nm can be prepared.The SiC layer with fined grain size had high density of stacking faults and showed higher mechanical property than SiC layer with large grain size(2)The growth process and growth mechanism of the SiC layer was investigated to help to optimize the parameters of CVD process.This work studied the SiC layer fabricated at 1200? and the SiC layer fabricated at 1500?,which have the most different microstructures.A new method is used to study the growth process of SiC by the microstructure and crystallinity of the SiC powders from the tail gas during the deposition of SiC.This work proposes two different formation processes of SiC layers at different temperature.At lower temperatures,the original products deposited on the substrate are the liquid droplets.These liquid droplets form the SiC after the fusion and crystallization process.On the contrary,the SiC produced at high temperature is mainly comes from the growth of SiC crystals by absorbing the small molecules.These two growth processes come from the different nucleation rate,crystallinity and growth speed of the SiC crystals.SiC layer properties such as grain size and crystallinity will change in different growth mechanism(3)TRISO particles will be in high temperature environment when there is an accident in the teactor..Two kinds of SiC layers were fabricated for this research.The sample had the grain size of 150nm and the sample had the grain size of 5 ? m were annealed at different temperatures for 1h.The crystallinity of the SiC layer with small grain size was a little heightened after lower temperature annealing(1500?,1600?,1700?).The grain size of the SiC layer annealed at this temperature had no evident difference with the origin sample.On the contrary,the grain become lager after the thermal treatment at 1800? and 1900?.The misfit dislocation in SiC layer with small grain size can be eliminated after high temperature annealing.The large grains have better thermal stability and have minor size change at high temperature,while the fine grains between the large grains fuse preferentially.The pores in SiC layer comes from the inhomogeneous grain growth process at high temperature.There is a evaporation process of SiC layer at 1800? and above,the surface of the SiC layer will be polyporous at this temperature.Moreover there will be a crack between the SiC layer and the OPyC layer because of the different thermal expansivityThis work is based on the fine grained SiC layer.First part is about the deposition parameters of the fine grained SiC layer.The second and the third parts studied the mechanism during the growth and annealing process of the fine grained SiC layer and the large grained SiC layer.The results of this article can be used into the application of the SiC layers in TRISO particles.