Single-Source-Precursor Synthesis and Properties of SiMC(N) Ceramic Nanocomposites (M = Hf, Ta, HfTa)

Industrial and aerospace demands on future technologies have created an urgent need for new material properties that are beyond those of materials known today and that can only be fabricated by designing the respective microstructure at the nanoscale. Taking advantage of the correlation between the molecular structure of preceramic precursors and the microstructure of the derived ceramic materials, the single-source-precursor route offers possibilities to fabricate novel ceramic materials that are inaccessible by conventional synthesis.[1] The motivation of this Ph.D. work is to further develop the concepts for fabrication of novel ceramic nanocomposites with a tailor-made microstructure and versatile properties by molecular design of their precursors. With this motivation, a series of dense monolithic SiMC(N) ceramic nanocomposites (M = Hf, Ta, HfTa) were fabricated using single-source-precursor synthesis plus spark plasma sintering. The chemical synthesis, polymer-to-ceramic transformation as well as high-temperature microstructural evolution was characterized using FT-IR, MAS solid NMR, TG/MS, XRD, Elemental analysis, SEM, TEM and Raman spectroscopy. Moreover, electrical conductivity, microwave absorption capability, electromagnetic interference shielding performance and laser ablation resistance of the as-prepared dense monoliths were investigated as well. In the synthesis part, a series of M-containing single-source precursors were synthesized upon reactions between a commercially available allylhydridopolycarbosilane (SMP10) and metal compounds, including Hf(NMe2)4, Hf(NEt2)4 and Ta(NMe2)5. The polymer-to-ceramic transformation was characterized using FT-IR, 13C and 29Si MAS solid NMR as well as in situ TG/MS. The precursors synthesized using Hf(NMe2)4 and Ta(NMe2)5 lead to higher ceramic yield (≈ 80 wt.%) than that of Hf(NEt2)4 (≈ 71 wt.%), while the ceramic yield of the latter can be improved to ≈ 78 wt.% by introduction of BH3·SMe2. Several thermal stable SiMC(N) ceramic nanocomposites (powders) were prepared upon high-temperature annealing of the amorphous SiMC(N) ceramics, including SiHfC(N), boron-doped SiHfC(N), SiTaC(N), SiHf7Ta3C(N) and SiHf2Ta2C(N). XRD, Raman and TEM results reveal that the ceramic nanocomposites mainly comprise β-SiC and MCxN1-x as well as free carbon (M = Hf, Ta, HfyTa1-y). Rietveld refinement of XRD patterns and the TEM images confirm that the grain size of both β-SiC and MCxN1-x are less than 100 nm even after annealing at 1900 oC for 5 h. The grain growth of β-SiC can be effectively suppressed by introducing M elements into the single-source precursors. Hf0.7Ta0.3CxN1-x and Hf0.2Ta0.8CxN1-x solid solutions with an expected Hf/Ta atomic ratio can be controlled precisely by adjusting the mole ratio of metal compounds during synthesis of the single-source precursors. It is worth emphasizing that a unique MCxN1-x-carbon core shell microstructure is observed within all the SiMC(N) ceramic nanocomposites, and the Hf-rich phase (e.g., HfCxN1-x and Hf0.7Ta0.3CxN1-x) seems to facilitate the formation of the carbon shell more easily. The carbon shell on the MCxN1-x core is able to hinder the coarsening of MCxN1-x grains during high-temperature processing. Thus, dense monolithic SiMC(N) ceramic nanocomposites are fabricated successfully upon spark plasma sintering of the amorphous SiMC(N) ceramics at 2200 oC. The achieved maximum diameter is 35 mm, which is rarely reported in the literature. Laser ablation behavior of the SiHfC(N) ceramics was investigated on dense monolithic SiHfC(N) ceramic nanocomposites and Cf-reinforced SiHfC(N) ceramic matrix composites. With addition of the HfCxN1-x phase, the rim of the ablation pit is covered by Hf-containing materials (e.g., HfO2), which are able to suppress the growth of the ablation pit. The dielectric properties and microwave absorption performance of the SiHfC(N) ceramics were investigated in the X-band (8.2 ~ 12.4 GHz) at room temperature. The minimum reflection loss and the maximum effective absorption bandwidth amount to -47 dB and 3.6 GHz, respectively. Free carbon, including graphitic carbon homogeneously dispersed in the SiC-matrix and less ordered carbon deposited as a shell on HfCxN1-x nanoparticles, accounts for the unique dielectric behavior of the SiHfC(N) ceramics. Electromagnetic interference (EMI) shielding performance of the dense monolithic SiHfC(N) ceramic nanocomposites were investigated in the X-band (8.2 ~ 12.4 GHz) at temperatures up to 600 oC. At room temperature, the SiC/C-35mm and SiC/15HfCxN1-x/C-35mm exhibit an average total shielding effectiveness (SET) of ≈ 21 dB and ≈ 42 dB, respectively, and at 600 oC, ≈ 22.6 dB and ≈ 40.2 dB, respectively. That means that with addition of a small amount of conductive HfCxN1-x, the SET is highly improved at both room and high temperatures. In summary, the synthesis, ceramization, densification as well as microstructural evolution of SiMC(N) ceramic nanocomposites are deeply investigated in this work. With the addition of an M-containing phase, the as-prepared SiMC(N) ceramic nanocomposites exhibit enhanced electrical conductivity, microwave absorption capability, electromagnetic interference shielding performance and laser ablation resistance. Moreover, the correlations regarding to molecular design, microstructure and properties of the SiMC(N) ceramic nanocomposites are carefully discussed.