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A novel single-source precursor was synthesized by the reaction of an allyl hydrido polycarbosilane (SMP10) and tetrakis(dimethylamido)hafnium(iv) (TDMAH) for the purpose of preparing dense monolithic SiC/HfC(x)N(1-x)-based ultrahigh temperature ceramic nanocomposites. The materials obtained at different stages of the synthesis process were characterized via Fourier transform infrared (FT-IR) as well as nuclear magnetic resonance (NMR) spectroscopy. The polymer-to-ceramic transformation was investigated by means of MAS NMR and FT-IR spectroscopy as well as thermogravimetric analysis (TGA) coupled with in situ mass spectrometry. Moreover, the microstructural evolution of the synthesized SiHfCN-based ceramics annealed at different temperatures ranging from 1300 °C to 1800 °C was characterized by elemental analysis, X-ray diffraction, Raman spectroscopy and transmission electron microscopy (TEM). Based on its high temperature behavior, the amorphous SiHfCN-based ceramic powder was used to prepare monolithic SiC/HfC(x)N(1-x)-based nanocomposites using the spark plasma sintering (SPS) technique. The results showed that dense monolithic SiC/HfC(x)N(1-x)-based nanocomposites with low open porosity (0.74 vol%) can be prepared successfully from single-source precursors. The average grain size of both HfC0.83N0.17 and SiC phases was found to be less than 100 nm after SPS processing owing to a unique microstructure: HfC0.83N0.17 grains were embedded homogeneously in a β-SiC matrix and encapsulated by in situ formed carbon layers which acted as a diffusion barrier to suppress grain growth. The segregated Hf-carbonitride grains significantly influenced the electrical conductivity of the SPS processed monolithic samples. While Hf-free polymer-derived SiC showed an electrical conductivity of ca. 1.8 S cm(-1), the electrical conductivity of the Hf-containing material was analyzed to be ca. 136.2 S cm(-1).

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Calculations have been made for single-walled zigzag (n,0) carbon nanotubes containing substitutional nitrogen impurity atoms using ab initio density functional theory. It is found that the formation energies of these nanotubes depend on the tube diameter, as do the electronic properties, and that they show periodic features which result from their different π bonding structures compared to those of perfect zigzag carbon nanotubes. When two nitrogen atoms are doped in the same hexagon per five tube units, the semiconducting tubes exhibit some special electronic structures, in which the impurity level is occupied fully by two excess electrons from doped nitrogen atoms. The electronic structures for the tubes depend on the sites that two nitrogen atoms occupy in the hexagon, by which the impurity states can be near the bottom of the conduction band or can be far apart from the bottom of the conduction band.

Ultra-high temperature ceramics (UHTCs) are proposed to be the most promising material candidates for structural and thermal protection components in the aerospace field, due to their extremely high melting points, high hardness and strength, good thermal and chemical attack resistance. 1-4 When considering engineering manufacturing and application in very harsh environment, silica formers (silicon-containing materials such as SiC, Si 3 N 4 , SiBCN, MoSi 2 , TaSi 2 etc) are generally introduced into UHTCs to significantly improve their sinterability, mechanical property, as well as oxidation and ablation resistance. 5-8 Among them, SiC as a typical effective additive can not only modify the sintering behavior of ZrB 2 ceramic by pinning grains from overgrowth, but also enhance the oxidation resistance by forming a protective surface layer of borosilicate glass or ZrSiO 4. 2,6,9 Meanwhile, it has been shown that the particle size of SiC has important effects on the microstructure and mechanical property of ZrB 2-SiC composites. 10,11 Thus nano-sized

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