Preparation Of TiC And SiC Nanomaterials And Their Microwave Absorption Properties

With the raipd development of electronic equipments and wirelesscommunications, electromagnetic (EM) interference and radiation pollution problemshave been become serious in people’s life and national security, in which radar stealthtechnology has become one of the commanding heights of the current militarystrategy. At present, the research for EM wave absorbing materials is still ontraditional absorbing materials, and the developments for new absorbing materials areobviously inadequate, especially for high-temperature and high-frequency absorbingmaterials. Therefore, investigating the new absorbing materials not only has thepractical significance for enriching their types and enlarging the applications, but alsohas high theoretical value.New nanostuctured materials show highly efficient microwave absorptionproperties due to their multi-microwave loss mechanisms. As the typical no-oxygenicceramics, TiC and SiC have high melting point, low mass density, good electricalconductivity and thermal conductivity, excellent environmental stability and so on.Combin the very best of their good properties and nanostructure, TiC and SiCnanomaterials become the potential microwave absorbing materials. In this thesis, asthe preparation and application of nanomaterials as basic, the TiC nanowires, TiCnanoparticles/Ti and ordered inter-filled structural SiC/SiO2composites wererespecvtively prepared by chloride-assisted carbothermal reaction, thermo-chemicalreaction and nanocasting methods. And then the optimal as-prepared materials’microstructures, the room and high temperature microwave absorbing performancesin X-band (8.2~12.4GHz) and the corresponding absorbing attenuation mechanismswere investigated. The main contents and results are listed as foolows:(1) TiC nanowires prepared by a chloride-assisted carbothermal reaction methodwere investigated. The results showed that the single crystal TiC nanowires withface-centered cubic structure were successfully generated using Ni(NO3)2·6H2O ascatalyst, NaCl as additive, sucrose and TiO2nanopowders as C and Ti source. Thesample had the typical aspect ratio of80~100and the specific surface area of186.7m2/g. The growth mechanism for TiC nanowires was VLS at beginning growthprocess and the speculative chemical reaction equations were also provides. (2) TiC nanoparticles uniformly in-situ grown on surface of metal Ti particleswere investigated, using acetone as carbon source. The results were listed as follows.When the flowing Ar of50ml/min carried the0oC acetone into the chamber bybubbling, the TiC nanoclusters with the size of around300nm uniformly and denselygrew on surface of metal Ti paiticles at the typical parameters of800oC and2.0h.The polycrystal TiC nanoclusters had a face-centred cubic structure and the meangrain size was about8.0nm. According to thermodynamic analysis, the smallmolecule CHxgas from acetone pyrolysis prioritized reaction with Ti to generate theTiC grains.(3) The ordered inter-filled structural SiC/SiO2monolithic composites wereprepared by nanocasting and cold-pressing, using polycarbosilane (PCS) as precursorand ordered mesoporous silica SBA-15as a hard-template. The results showed thatthe SiC grains derived from PCS polymer almost full-filled the nanochannels of themesoporous SBA-15. The crystallinity of the composites increased with increase ofthe sintering temperatures and they were β-SiC. Moreover, when the sinteringtemperature was not higher than1400oC, the SiC/SiO2composites not onlymaintained the ordered inter-filled structure, and showed certain mechanicalproperties.(4) Microwave absorption performances were evaluated in X-band for thecomposites of the TiC nanowires, TiC nanoparticles/Ti or ordered inter-filledstructural SiC as EM wave absorber and paraffin or SiO2as EM wave transparentmatrix. The results were listed as follows. At room-temperature, the TiC nanowiresdispersed in paraffin with30wt%hybrid exhibited the biggest absorption strength: aminimum reflectin loss (RL, dB) was-51.0dB at the thickness of1.7mm. And theSiC/SiO2composite prepared at1400oC showed the widest absorbing band: aneffective absorption bandwidth covered the whole X-band. When the environmentaltemperature was300oC, the TiC nanowires/SiO2with7.5wt%composite exhibited astrong absorption of the minimum RL of-61.0dB at thickness of3.0mm. And theSiC/SiO2composite prepared at1300oC showed a wide absorbing band at25~500oC,in which there was always a matching thickness and the corresponding effectiveabsorption bandwidth (RL<-10dB) covered the whole X-band.(5) The microwave loss mechanisms were investigated for above composites.And the microwave absorbing properties of the single-layer absorber were analyzed using quarter-wavelength principle and impedance match degree. The result showedthat the excellent absorbing performances of the above materials were attributed to themutli-attenuation mechanisms: the electric conductance path can be easily formed byTiC nanowires with large length/diameter ratio, and the electric conductance loss canbe caused by complex network-like TiC nanowires in microwave transparent matrix;the TiC nanoparticles/Ti composites had the heterogeneous microstructure, whichmainly caused the interface and dielectric relaxation losses; the attenuation of theordered inter-filled structural SiC/SiO2composite was the multi-reflection and electricconductance loss, which caused by the ordered microstructure and network-likestructure. The single absorption materials exhibited excellent microwave absorbingproperties, in which the thickness obeyed the destructive interference principle andthe electromagnetic parameters satisfied the impedance match. This provided the basisfor designing the single-layer absorber.