Chunlei Yan

Zirconium carbide, hafnium carbide and their ternary carbide nanoparticles by an in situ polymerization route

An in situ polymerizable complex method to produce zirconium carbide, hafnium carbide and their ternary carbides at a relatively low temperature (1300 °C) using simple and mainly nontoxic starting reagents is presented. In this aqueous process, citric acid (CA) was used to chelate the metal ion and ethylene glycol (EG) to form a polymerized complex resin. We suggest that, based on the results of FT-IR and 13C NMR spectroscopies, a very stable metal–CA chelate complex formed in the starting solution, which was thermally stable upon gelation even up to 350 °C. Immobilization of the metal ion in a rigid polymer can largely guarantee the in situ charring, resulting in carbon adjacent to the metal oxide in the pyrolysed product. The contiguous carbon and metal oxide led to in situ reaction (1100 °C) with a minimum of diffusion, which involved the formation of large numbers of metastable phases. Afterwards, well-defined binary and ternary carbide nanoparticles (∼100 nm) were formed through localized particle coarsening by Ostwald ripening.

Synthesis and formation mechanism of submicrometer ZrB2 powders via the Pechini-type polymerizable complex route

ZrB2 powders were synthesized by a polymerizable complex method based on the Pechini-type reaction route, wherein a precursor solution of citric acid, glycerol, boric acid, and zirconium ions was prepared, and polymerized to form a semitransparent resin without any precipitation at 150 °C. The precursor solutions and the resulting resins were characterized by FT-IR and 13C NMR spectroscopy. The results show the formation of a hybrid polymer with zirconium and boron arrested within the polymeric chain by complexation. The submicrometer ZrB2 powders (200–600 nm) are formed after pyrolysis of the polymeric precursor with 4 B/metal molar ratios at 1400 °C. Investigation of the formation mechanism of ZrB2 powders indicates that ZrC is the intermediary phase and two reduction reactions determine the specific pathway leading to ZrB2 formation: (1) ZrC formation, (2) the formed ZrC directly reacts with B2O3 to form ZrO2 and ZrB2. In the whole conversion process, ZrC formation by carbothermal reduction is a fast reaction, while the direct reaction of ZrC with B2O3 to form ZrO2 and ZrB2 is the rate-limiting step.

Synthesis of zirconium carbide nanoparticles by polymerised complex route

A polymerised complex route, based on polyesterification between tartaric acid–zirconium complex and ethylene glycol (EG), has been demonstrated to synthesise ZrC powders at a relatively low temperature (1300°C). The Fourier transform infrared results confirmed the coordination of tartaric acid to the zirconium ion. Furthermore, this complex reacted with EG through polyesterification to immobilise the metal ion in the rigid polymer. The intimate mixing of the reaction components has thus been achieved in ZrO2/carbon ‘coke” due to the suppression of segregation of the reaction components during pyrolysis. Finally, well crystallised ZrC nanoparticles (50–150 nm) were obtained through the carbothermal reduction of ZrO2/carbon ‘coke’, concerning the formation of the intermediate oxycarbide phase.

Mechanical behaviour and microstructure of Cf/ZrC, Cf/SiC and Cf/ZrC – SiC composites

Cf/ZrC, Cf/SiC and Cf/ZrC–SiC composites were successfully prepared by polymer infiltration and pyrolysis (PIP) using polycarbosilane and a liquid ZrC precursor. The densification process, mechanical properties and microstructures were studied in a view of comparison. After the same total 20 PIP cycles, the Cf/ZrC, Cf/SiC and Cf/ZrC–SiC composites had flexural strengths of 50.1±5.3, 285.7±22.6, 141.5±13.1 MPa respectively; elastic moduli of 7.8±0.9, 57.1±3.2 and 45.1±2.6 GPa respectively; and fracture toughness of 2.5±0.2, 10.4±0.9 and 10.9±1.1 MPa m1/2 respectively. With the introduction of high modulus SiC phase into the ZrC matrix, the densification and modulus of the matrix were improved; as a result, the Cf/ZrC–SiC composite showed higher mechanical properties compared to Cf/ZrC.

Effect of 3D-Braided Structure on Thermal Expansion of PIP–Cf/SiC Composites

3D3d, 3D4d, 3D5d-braided Cf/SiC composites were fabricated by precursor infiltration and pyrolysis (PIP) with polycarbosilane as the matrix precursor. The coefficient of thermal expansion (CTE) of Cf/SiC composites was measured in longitudinal and transversal directions in the temperature range from –150°C to 25°C. The longitudinal CTE varies in the range (0.09–0.68) × 10–6/°C, and the transversal CTE varies in the range (0.21–1.95) × 10–6/°C. Various CTEs of 3D-braided Cf/SiC composites were mainly determined by different braided structures of carbon fibers. The longitudinal CTE is lower than transversal CTE for the negative axial expansion of carbon fibers at cryogenic temperature. Microcracks were examined to understand the effect of structure on the thermal expansion of composites.

Preparation and characterization of diamond-silicon carbide-silicon composites by gaseous silicon vacuum infiltration process

Diamond-SiC-Si composites have been prepared using gaseous silicon vacuum infiltration. The evolution of the phases and microstructures of the composites have been analyzed using X-ray diffraction technique and scanning electron microscopy. It has been found that the diamond-SiC-Si composite is composed of β-SiC, diamond, and residual Si. The diamond particles were distributed homogeneously in the dense matrix of the composites. Besides, the effects of particle size and content of diamond on the properties of diamond-SiC-Si composites have been analyzed. The thermal conductivity of the composites increases with particle size and content of diamond. When the particle size and content of diamond are 300 μm and 80 wt %, respectively, the thermal conductivity of the composites approaches the value of 280 W·m−1·K−1.