| SiCf/SiC composites are regarded as the best candidates for LWR nuclear fuel cladding due to their excellent high temperature strength,good chemical inertness,and excellent stability in strong neutron irradiation environments.However,at present,brittleness is still a key difficulty limiting the application of SiCf/SiC composites.To solve this problem,in this paper,by introducing Cr and Ni catalysts into the low-density SiCf/SiC cladding tube,the in-situ growth behavior of SiC nanowires in the cladding,and its effect on the densification process and mechanical properties of SiCf/SiC cladding were deeply studied.To avoid the adverse effect of residual Ni catalyst particles on the irradiation performance of the cladding,a purification method of nanowires was obtained by investigating the in-situ and ex-situ oxidation behavior of SiC nanowires.The main conclusions obtained are as follows.（1）The effect of metal catalyst solution type,concentration and CH4on the in-situ growth of SiC nanowires in low-density SiCf/SiC bodies were studied.The results showed that when the catalyst source is Cr（NO3）3·9H2O,short needle-like nanostructures with a diameter of about20-40 nm are mainly obtained;when the catalyst source is Ni（NO3）2·6H2O,SiC nanowires with diameters around 100 nm were mainly obtained.In addition,with the increase of Ni（NO3）2·6H2O solution concentration from0.01 mol/L to 0.05 and 0.1 mol/L,the diameter of SiC nanowires increased from 40-80 nm to 100 and 120 nm;and with the increase of CH4 flow rate the number of SiC nanowires gradually increased and grew denser.（2）By simulating the thermal process in SiC nanowire growth by heat treatment of two catalyst sources,Cr（NO3）3·9H2O and Ni（NO3）2·6H2O it is clear that whether the metal salt can be reduced to metal by CH4 is the key control mechanism for the change of the SiC nanowire growth mechanism.During the nanowire growth process,the Ni salt can generate metallic Ni,which makes the SiC nanowires grow in the V-L-S mechanism,while the Cr salt cannot generate metallic Cr,which makes the nanowire growth follow the V-S mechanism.The different growth mechanisms will directly affect the length,diameter and quantity of SiC nanowires.（3）The CVI densification process and mechanical properties of SiCf/SiC cladding containing SiC nanowires were investigated.When the growth process is:nickel nitrate solution concentration of 0.01 mol/L,CH4flow rate of 6 sccm,silicon source of Si:SiO2 molar ratio of 2:1,reaction temperature of 1300°C,and pressure of 10 kPa（sample 0.01Ni-6）,a loose and cross-linked SiC nanowire network can be constructed in the low-density SiCf/SiC body,which provides more nucleation sites for the densification of the SiC matrix,enabling the density of the SiCf/SiC cladding to reach 2.75 g/cm3,higher than that specimen of 2.42 g/cm3without SiC nanowires,after 150 hours of densification.Due to the strengthening and toughening mechanisms such as pull-out and bridging of SiC nanowires,the specimen 0.01Ni-6 has a higher crush strength（403.11 MPa）and compression displacement（0.79 mm）.Compared with the specimens without SiC nanowires,78.5%（225.94 MPa）and 108.6%（0.39 mm）were improved,respectively.（4）To remove the residual Ni catalyst particles on top of the SiC nanowires,the oxidation behavior of SiC nanowires at 900°C was investigated by using in-situ oxidation and ex-situ oxidation experiments.During the first 1 h of in situ oxidation,Si diffuses to the surface of the residual catalyst particles and is preferentially oxidized to form crystalline/amorphous SiO2;while Ni nucleates in the center of the catalyst particles without oxidation.In the samples after 1-5 h of ex-situ continuous oxidation,Ni diffusion outward and Ni O generation in the peripheral SiO2amorphous layer was observed.In contrast,theβ-SiC phase was completely separated from the catalyst particles after 5 h of oxidation,and the crystalline shape was maintained intact;pureβ-SiC nanowires could be prepared by hydrofluoric acid etching.53 Figures,11 Tables,132 References... |
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