Basic Design Of The Microstructure And Composition Of SiC/SiC Composites With High Thermal Conductivity For Nuclear Reactor

Continuous Silicon Carbon Fiber Reinforced SiC Composites?SiC/SiC?owe many excellent performances such as low chemical activity,low specific density,low thermal expansion coefficient,high energy transformation ratio,and excellent high temperature strength and corrosion resistance.Which make SiC/SiC becomes a promising cladding material for General IV reactor.Meanwhile,SiC/SiC composites have low induced radioactivity,high dimensional stability,low thermal decay and tritium permeability under 14MeV neutron irradiation,also make SiC/SiC very noticeable in the concept design of first wall structure materials in many fusion reactors recent years.SiC/SiC composites are facing a series of challenges,such as how to improve the thermal conductivity and gas tightness and how to solve the joint problem of composites.Chemical vapor infiltration?CVI?is a mature technique to fabric SiC/SiC composites;the SiC matrix produced by CVI is pure,crystallized?-SiC with few defects.?-SiC has good irradiation resistance,so CVI is the most suitable technique to fabric SiC/SiC composites for the structural materials of nuclear reactors.However,low thermal conductivity is the key problem of CVI SiC matrix.In the harsh environment of nuclear reactors,it is necessary to decrease the heat load of structure materials.Thermalconductivityisthekeyfactorwhichcandirectlyaffectthe efficiency of energy conversion,the service life and the safety of nuclear reactors.Generally,the thermal conductivity of CVI 2D SiC/SiC composites is only around 10¹5W·m^-1·K^-1 in room temperature,so how to improve the thermal conductivity of SiC/SiC composites needs to solve urgently in the application of nuclear reactors.This paper is focus on the theme of optimizing the thermal conductivity and ensuring the mechanical properties of CVI SiC/SiC composites to meet the requirement of nuclear reactors.The microstructures of fiber,interphase and matrix are designed and controlled;the effects of each component on the thermal conductivity and mechanical properties are studied deeply combined with heat transport model and calculation.The main contents and results are shown as follow:?1?The effects of three different oxygen content,C/Si and crystallinity on the microstructure,heat stability and thermal conductivity of SiC fiber are studied.Results show that,the 3#SiC fiber with stoichiometric,low oxygen content and high crystallization has the best stability in microstructure and phase composition in high temperature,and it has the highest thermal conductivity.Which make it very suitable as the reinforced phase of SiC/SiC composites for nuclear reactors.?2?The effects of PyC,SiC_p-SiC,CNTs and CNTs-PyC four different interphases on the mechanical properties and thermal conductivity of SiC/SiC composites are studied and compared.Results show that,it can effectively improve the thermal conductivity of SiC/SiC composites by introducing a thin layer of CNTs into interphase by EPD to form a CNTs-PyC compound interphase,and the thermal conductivity,bending strength and fracture toughness of SiC/SiC with CNTs-PyC interphase are improved 115%,17%and 26%than SiC/SiC with PyC interphases,respectively.?3?The effects of four different structure design of layered?SiC_f/SiC+SiC_w/SiC?,SiC_f/?SiC_w+SiC?,multi layered SiC_f/?SiC-CNTs?and layered?SiC_f/SiC+CNTs/SiC?on the microstructure,mechanical properties and thermal conductivity of SiC/SiC composites are studied and compared.Results show that,SiC_f/?SiC_w+SiC?composites prepared by CVI combined with brushing have the best performance in fracture toughness and thermal conductivity which are 151%and 179%of SiC/SiC composites,respectively.From the fracture surface morphnology of SiC_f/?SiC_w+SiC?,it can be seen that,the introduced SiC_w has good bonding status with SiC matrix.The pull-out of fiber and?SiC_w+SiC?layer take place alternatively and pull-out of SiC_w is observed in?SiC_w+SiC?layer,which can effectively toughing and strengthing the composites.The?SiC_f+SiC_w?/SiC composites have satisfied bending strength,higher fracture strength and thermal conductivity.?4?Based on the resuls of all optimal designs;3#SiC fiber.CNTs-PyC interface and SiC+SiC_w matrix fabricated by brushing combined with CVI are selected to prepare SiC/SiC composites for nuclear reactor.The density of optimal designed SiC/SiC composites is2.51±0.05 g·cm^-3,the porosity is 12.2±0.3%,bending strength is 530±15 MPa,the fracture toughness is 35.7±1.3 MPa·m^1/2,and the thermal conductivity increases to 28.5 W·m^-1·K^-1,which increase 22%,150%and 94%compared with tranditional SiC/SiC composites,respectively.