Low Temperature Synthesis And Properties Of Si-C-N Series Nanomaterials

As outstanding structural and functional materials, Si3N4, SiC and CNTs/SiC nanomaterials have potential and promising prospects in military, commercial and civil applications. Hollow carbon nanospheres (HCNSs) possess potential applications in electricity, electrochemistry and biomedicine field due to the excellent properties including low density, high specific surface area, high damping performance, good biocompatibility, and so on. Nitrogen-doped carbon materials as cathode for Li-ion batteries show improvement in specific capacity to pure carbon. Therefore, it is of great theoretical and practical significance to explore low-temperature synthesis routes with high efficiency and to investigate the related performances for the above-mentioned nanomaterials. In this paper, we put emphasis on the large-scale synthesis of Si3N4, SiC, CNTs/SiC, HCNSs and carbon doped with N and S at relatively low temperature and study their related properties.(1) Si3N4nanorods and nanoparticles with several nanometers in diameter were prepared at about250℃via a simple organic-inorganic reaction between CH3SiCl3and NaN3. The resulting product is a mixture of a-and β-Si3N4with high crystallinity. The relative amount of a-Si3N4increases with raised temperature in the range of250-550℃. The formation of nanocrystalline Si3N4by the organic-inorganic reaction goes through an intermediate product of NaSi2N3during the initial reaction of and NaN3, followed by the subsequent reaction of NaSi2N3with and NaN3to yield Si3N4. It is found that pure Si3N4can hardly be obtained by substituting (CH3)2SiCl2and (CH3)3SiCl for CH3SiCl3to react with NaN3even at550℃.(2) SiC nanomaterial was synthesized at as low as150℃or so via the reaction of Si and graphite induced by Na and S (or I2). Curly β-SiC nanowires with smooth surface were obtained by the induction of Na and S, which were several millimeters in length and50-70nm in diameter. A prominent peak at387nm is observed in the visible photoluminescence emission for the curved nanowires. A mixture of straight and rough β-SiC nanowires with30-50nm in diameter and nanoparticles with several nanometers were synthesized via Si and graphite induced by Na and I2. The molar ratio of S (or I2) to Si (or graphite) has significant influence on the synthesis of SiC at relatively low temperature. Increasing the molar ratio of S (or I2) to Si (or graphite) can prepare SiC nanomaterial at lower temperature or improve the crystallinity of the resulting product. Because the reaction can be achieved in a short time, holding time affects very slightly on both the yield and crystallinity of the final product.(3) CNTs/SiC composites were synthesized via the reaction between Si and CNTs induced by that of Na and S at200℃or so. The resulting product contains one-dimensional structures comprised of porous SiC spheres and CNTs, and SiC nanoparticles. Raising the reaction temperature could promote the yield of SiC spheres, and therefore is helpful for improving microwave absorbing performance. The CNTs/SiC composites obtained at600℃exhibit outstanding microwave absorbing properties, which could reach a minimum reflection loss of-38.7dB, and the absorption bandwidth lower than-10dB (90%absorption) is about2GHz. The interfacial polarization aroused from the interfaces between CNTs and SiC, grain boundaries between SiC nanoparticles and high density of stacking faults in SiC crystallites, as well as the porous SiC spheres are responsible for the excellent microwave absorbing performances.(4) One-dimensional CNTs/SiC porous nanocomposites were prepared via an in situ reaction between Si and MWCNTs induced by the reaction between Na and I2at as low as200℃. The resulting nanocomposites mainly comprised of severely etched CNTs and SiC nanoparticles about50nm in diameter grown on them. The as-prepared porous nanocomposites exhibit excellent microwave absorbing properties, and both the absorbing intensity and bandwidth improve with raising the reaction temperature. For the nanocomposites obtained at400℃, the minimum reflection loss can reach-44.2dB with an absorber thickness of2.0mm, and can achieve99%absorption in the range of10.6-18GHz. The synergistic effect of dangling bonds, interfaces, small size effect and wide range of size and the dielectric polarizations aroused from them play crucial roles in the intense and wide microwave absorption.(5) HCNSs about30nm in diameter were prepared on large scale by the reaction of AlCl3·6H2O and CaC2at around250℃. The crystallinity of the HCNSs increases by raising temperature. Confirmed by a series of comparative experiments, both the water of crystallization and AlCl3play crucial roles in the formation of HCNSs. Particularly, it is found that AlCl3exhibits a strong graphitization behavior on carbon nanospheres even at low temperature, which contributes to their evolution to HCNSs. HCNSs can also be prepared via the reaction between CaC2and CaCl2·6H2O (or MgCl2·6H2O) at250℃or so. Based on the comparative experiments, the mixture of a chloride of CaCl2or MgCl2and a hydroxide Ca(OH)2or Mg(OH)2is essential for obtaining HCNSs with high degree of graphitization using C2H2as carbon source. The electrochemical performance of the as-prepared HCNSs was investigated. By comparison, the HCNSs obtained by the reaction of CaCl2·6H2O and CaC2exhibit higher discharge/charge performance than those by AlCl3·6H2O and CaC2.(6) Nitrogen-doped carbon microspheres were prepared by the pyrolysis of C4H5N at about550℃. The electrochemical performances using the as-prepared nitrogen-doped carbon microspheres as anode material for rechargeable lithium-ion battery were evaluated. The product obtained exhibits a reversible capacity of482mAh g-1after80cycles. The carbon materials co-doped with N and S elements were synthesized by the reaction between C4H5N and S powder at550℃. The product exhibits a reversible capacity of415.3mAh g-'after40cycles, and shows excellent rate performance even at IOC.