Search Results (4)

In this study, SiHfCN ceramics were synthesized from a single-source precursor obtained by reacting Durazane 1800 with tetrakis(dimethylamido)hafnium(IV) (TDMAH). In a separate preparation, Ti₃C₂ MXene was incorporated into this precursor to produce MXene-SiHfCN composite ceramics. The samples were pyrolyzed at 1000 °C and heat-treated at 1600 °C in N₂ to investigate amorphous-to-crystalline transformations. Both SiHfCN and MXene-SiHfCN formed a single-phase amorphous structure after pyrolysis at 1000 °C. At 1600 °C, SiHfCN partially crystallized into α/β-Si₃N₄ and HfC_xN_1−x phases within an amorphous/crystalline Si₃N₄ matrix. In contrast, the MXene–SiHfCN matrix remained largely amorphous, evolving into SiOCN with localized Si₂ON₂ crystallization. Additional phases, including HfC_xN_1−x, Hf oxide/oxycarbide, and a Ti carbonitride-rich phase (TiC_0.63N_1.06O_0.18Si_0.99Hf_0.11), were identified within the amorphous SiOCN. No SiC was detected in either system, indicating suppression of carbothermal reduction of Si₃N₄ up to 1600 °C in N₂. While SiHfCN exhibited pronounced macroscopic cracks, MXene-SiHfCN showed no such large cracks, though local microscopic cracking was observed. These results demonstrate that Ti₃C₂ MXene incorporation stabilizes the amorphous matrix, modifies phase evolution, and mitigates severe cracking, offering new insights into non-oxide PDC nanocomposites for ultra-high-temperature applications. Full article

A computational study of model Y…HCN/HNC (Y = FB, OC, N₂, CO, BF) dimers was undertaken to assess the effect on the Y…H hydrogen bond when the Lewis base Y is systematically varied, while another model study of HCN/HNC…XF (XF = HF, LiF, BeF₂, BF₃, ClF, PH₂F, SF₂, SiH₃F) dimers was undertaken to compare the relative binding strengths of the various types of noncovalent interactions between HCN/HNC and the fluorinated Lewis acid XF. The X atoms represent elements that span Groups 1–2 and 13–17 of the periodic table. The optimized trimers Y…HCN/HNC…XF that result from the combined dimer pairs were then studied in order to assess the relative strengths of the cooperative effects of the noncovalent N…X or C…X interactions on the Y…H hydrogen bond. The properties computed for the dimers and trimers include interaction energies, intermolecular separations, bond length changes, vibrational frequencies and their infrared intensity enhancements. Full article

HfC_xN_1−x nanoparticles were synthesized using the urea-glass route, employing hafnium chloride, urea, and methanol as raw materials. The synthesis process, polymer-to-ceramic conversion, microstructure, and phase evolution of HfC_xN_1−x/C nanoparticles were thoroughly investigated across a wide range of molar ratios between the nitrogen source and the hafnium source. Upon annealing at 1600 °C, all precursors demonstrated remarkable translatability to HfC_xN_1−x ceramics. Under high nitrogen source ratios, the precursor exhibited complete transformation into HfC_xN_1−x nanoparticles at 1200 °C, with no observed presence of oxidation phases. In comparison to HfO₂, the carbothermal reaction of HfN with C significantly reduced the preparation temperature required for HfC. By increasing the urea content in the precursor, the carbon content of the pyrolyzed products increased, leading to a substantial decrease in the electrical conductivity of HfC_xN_1−x/C nanoparticle powders. Notably, as the urea content in the precursor increased, a significant decrease in average electrical conductivity values was observed for the R4-1600, R8-1600, R12-1600, and R16-1600 nanoparticles measured at a pressure of 18 MPa, yielding values of 225.5, 59.1, 44.8, and 46.0 S·cm⁻¹, respectively. Full article

High-temperature oxidation resistant amorphous Hf₆B₁₀Si₃₁C₂N₅₀ and Hf₇B₁₀Si₃₂C₂N₄₄ films were deposited by reactive pulsed dc magnetron sputtering. To investigate the oxidation mechanism, the films were annealed up to 1500 °C in air. The evolved microstructures were studied by X-ray diffraction and transmission electron microscopy. A three-layered microstructure was developed upon exposure to high temperature. An oxidized layer formed at the top surface for both films consisting of monoclinic and/or orthorhombic m-/o-HfO₂ nanoparticles embedded in an amorphous SiO_x-based matrix. The as-deposited bottom layer of the films remained amorphous (Hf₆B₁₀Si₃₁C₂N₅₀) or partially recrystallized (Hf₇B₁₀Si₃₂C₂N₄₄) exhibiting a h-Si₃N₄ and HfC_xN_1−x distribution along with formation of t-HfO₂ at its top section. The two layers were separated by a partially oxidized transition layer composed of nanocrystalline h-Si₃N₄ and tetragonal t-HfO₂. The oxidation process initiates at the bottom/transition layer interface with oxidation of Hf-rich domains either in the amorphous structure or in HfC_xN_1−x nanoparticles resulting in t-HfO₂ separated by Si₃N₄ domains. The second stage occurs at the oxidized/transition layer interface characterized by densely packed HfO₂, Si₃N₄ and quartz SiO₂ nanostructures that can act as a barrier for oxygen diffusion. The small t-HfO₂ nanoparticles merge and transform into large m-/o-HfO₂ while h-Si₃N₄ forms amorphous SiO_x matrix. A similar oxidation mechanism was observed in both films despite the different microstructures developed. Full article