Design,Fabrication And Gas Sensing Properties Of SiC Nanofibers By The Polymer Derived Ceramics Route

The gas sensors,which can be applied in harsh environments with fast response and high reproducibility,are urgently demanded in aerospace and nuclear industries.One-dimensional silicon carbide?SiC?nanomaterials are promising sensing materials for harsh environment applications due to their high-temperature and corrosion resistance,large surface area and high thermal conductivity.In this work,flexible SiC nanofibrous membranes and hollowed SiC nanofibers constructed by SiC nanorods?HSiC?were designed and fabricated based on the polymer derived ceramics route.Platinum?Pd?nanoparticles were loaded on the surface of HSiC through chemical reduction method.The compositions and microstructure of different SiC nanofibers were analyzed.The mechanical properties,hydrophobicity and high-temperature sensing performance of the obtained SiC nanofibrous samples were studied and the corresponding mechanisms were also proposed.A novel polycarbosilane?PCS?-based precursor solution for electrospinning was prepared by introducing co-polymer and surfactant.SiC nanofibers with the average diameter of 500 nm were fabricated by electrospinning,pre-oxidation and high-temperature treatments.The morphology and compositions of SiC nanofibers were influenced by the co-polymer,surfactant,ambient humidity and pyrolysis procedures.The obtained SiC nanofibers are uniform and stable under high-temperature and corrosive conditions.The sensing response of SiC nanofibers to 500 ppm H₂ is 22.6%at500oC.The flexible Pd-doped SiC nanofibrous membranes were prepared by adding Pd?acac?₂ into the spinning solution,followed by pre-oxidation and high-temperature treatments.The in-situ formed Pd and PdO nanoparticles inside SiC nanofibers can decrease the average pore size and volume of SiC nanofibers.The Pd-doped SiC nanofibrous membranes exhibited outstanding flexibility and high tensile strength of33.2 MPa,which is 66 times higher than the pure SiC nanofibrous membrane.We concluded two mechanisms for the improvement of mechanical properties from nanoparticles:one is that the in-situ formed nanoparticles can decrease the pore size and volume of SiC nanofibers;the other one is that nanoparticles can retard the development or change the extension direction of cracks through the pinning effect.The flexible SiC nanofibrous membranes possess excellent hydrophobic properties under bending,high-temperature and corrosive environments.The sensing response of free-standing SiC nanofibers to 500 ppm H₂ is 1.5%.The response and recovery time was 3 s and 1 s,respectively.The variable range hopping conduction was supposed to the sensing mechanism for SiC nanofibers at high temperature.The core-shell PS@PCS nanofibers can be prepared by regulating the dispersion of polystyrene?PS?and PCS in the spinning solution followed by electrospinning.After high-temperature treatments,PS decomposed completely and PCS transformed to SiC.Finally,the hollowed SiC nanofibers can be obtained.HSiC was constructed by large numbers of SiC nanorods with the diameter of10²0 nm.The decomposed products of SiO and CO from amorphous SiOC were catalyzed into SiC nanorods by NiO nanoparticles.Platinum?Pt?nanoparticles were uniformly loaded on the surface of HSiC through chemical reduction method.The high-temperature NH₃ sensing properties of Pt/HSiC were determined by the synergetic effects of Pt nanoparticles,hierarchically porous structure of HSiC and high crystallinity of SiC nanorods.The sensing response of Pt/HSiC to 100 ppm NH₃ was 6.5%.The response and recovery time was 2 s and 5 s,respectively.Furthermore,Pt/HSiC exhibited high sensing reproducibility to NH₃ at high temperature.The spill-over effect from Pt/HSiC was the main sensing mechanism to NH₃.Besides the above work,other two tasks were also completed in this dissertation.One is electrospun interconnected Fe-N/C nanofiber networks as efficient electrocatalysts for oxygen reduction in acid media;the other one is facile synthesis of FeCo@NC core-shell nanospheres supported on graphene as an efficient bifunctional oxygen electrocatalyst.These two tasks were added as appendix because of the difference of materials and applications.