Shaoming Dong

Characterization of nearly stoichiometric SiC ceramic fibres

A comparative study of the chemical composition and microstructure of Hi-Nicalon, Hi-Nicalon type S, Tyranno SA, Sylramic and Carborundum fibres has been conducted. This analysis has confirmed results already published but has also evidenced some original features. The Hi-Nicalon type S fibre has a near stoichiometric composition but it still contains some oxygen (≈1 at. %) and free carbon (≈2 at. %). The expected near stoichiometric composition of both the Tyranno SA and the Sylramic fibres is only effective near the edge region, while the core of the fibres contains some amount of free carbon (e.g., up to ≈14 at. % and ≈6 at. % respectively in large diameter fibres) as well as some residual oxygen (≈0.5 at. %). The composition of the Carborundum fibre is very close to stoichiometric SiC except rare and localised free carbon or B4C inclusions. The properties of the different fibres, some of them still beeing at a development stage, are discussed from a chemical and a phase composition point of view, on the basis of what is known about their respective preparation process.

Preparation and Property of SiC/(ZrB 2 -SiC/SiC) 4 Anti-oxidation Coatings for C f /SiC Composites: Preparation and Property of SiC/(ZrB 2 -SiC/SiC) 4 Anti-oxidation Coatings for C f /SiC Composites

以Cf/SiC复合材料为基体,采用浆料浸涂法和脉冲CVD法制备了SiC/（ZrB2-SiC/SiC）4涂层,借助XRD、扫描电镜及能谱对涂层的结构及组成进行了分析研究,并初步考查了其高温抗氧化性能.结果表明,涂层总厚度约100μm,主要由ZrB2-SiC涂层与脉冲CVD SiC涂层交替覆盖而成.在1500℃空气中氧化25h,未涂层试样失重明显; 脉冲CVD SiC涂层试样氧化失重率为5.1%; 而SiC/（ZrB2-SiC/SiC）4涂层试样出现增重现象,增重率达2.5%,表现出优异的抗氧化性能.

Correlation of PyC/SiC interphase to the mechanical properties of 3D HTA C/SiC composites fabricated by polymer infiltration and pyrolysis

3D braided carbon fiber preforms were used to fabricate C/SiC composites using polymer infiltration and pyrolysis (PIP). Before the PIP, the preforms were coated by isothermal chemical vapor infiltration with methane to produce pyrocarbon (PyC) and then with hexamethyldisilazane to form SiC and produce the PyC/SiC interphase. The correlation of the PyC/SiC interphase to the microstructure and the mechanical properties of the fabricated composites was investigated. The flexural properties of the composites were measured using the three-point-bend test, and the fracture surfaces observed by SEM. The bend strengths were 247 MPa and 46 MPa, with and without the PyC/SiC interphase respectively. Long fiber pullout dominated the fracture surface for the composite with the PyC/SiC interphase, while for the one without the interphase, almost no fiber pullout was observed.

Synthesis of nanocrystallized zirconium carbide based on an aqueous solution-derived precursor

Nanocrystallized zirconium carbide (ZrC) powder was synthesized by an aqueous solution-based process using zirconium acetate and sucrose as starting reagents. Polyvinyl pyrrolidone (PVP) was used to combine the reactants to form a suitable precursor for ZrC. Through such a novel aqueous solution-based process, fine-scale mixing of the reactants is achieved in an environmentally friendly manner. The formed precursor can be converted to ZrC by carbothermal reduction reaction at 1600–1650 °C. Thanks to a limited amount of residual free carbon, the as-synthesized ZrC powder had an ultra-fine particle size (50–100 nm) and a low oxygen content less than 1.0 wt%. The conversion mechanisms from as-synthesized pre-ceramic precursor to ZrC powder were investigated systematically and revealed by means of FTIR, TG-DSC, XRD, Raman, SEM and TEM.

In situ growth behavior of boron nitride nanotubes on the surface of silicon carbide fibers as hierarchical reinforcements

BNNTs grown in situ on the surface of silicon carbide fibers via a simplified ball milling, impregnation and annealing method using boron powder as the raw material were synthesized. The morphology, microstructure and chemical composition of as-grown BNNTs were well characterized by field-emission scanning electron microscopy, field emission transmission electron microscopy, electron energy loss spectroscopy and Raman spectroscopy. The analysis results indicate that the nanostructures are BNNTs with the diameter of 50–130 nm, exhibiting a multi-walled and bamboo-like structure and possessing two distinct morphologies of the tube walls: flat walls and bubble-chain walls, respectively. Based on the experimental characterization of BNNTs and as-milled powders, the possible growth process of BNNTs has been described and the formation mechanisms of two types of bamboo-like BNNTs have been proposed, including the tip growth mode and the base growth mode.

Characterization and Alignment of Carbon Nanofibers under Shear Force in Microchannel

This work presents a novel method to align CNFs by using shear forces in microchannels. Effect of two different microchannel sizes 1 mm × 0.1 mm and 1 mm × 0.2 mm on CNFs alignment is investigated. SEM images of CNFs preform display significant alignment by both microchannels, which can be interpreted using a second-order alignment tensor and a manual angle meter. In the second-order alignment tensor description, an elongated ellipse can signify high degree of alignment in the direction of the major axis. When the microchannel size is 1 mm × 0.2 mm, the lengths of major and minor axes of the ellipse are 0.982 to 0.018. An angle meter manually shows that 85% of the CNFs are aligned in the direction between 60° and 90° when the microchannel is 1 mm × 0.2 mm. Both methods can demonstrate that better alignment of CNFs can be obtained using the 1 mm × 0.2 mm microchannel.

Ablation Properties of ZrB 2 -SiC Ultra-high Temperature Ceramic Coatings: Ablation Properties of ZrB 2 -SiC Ultra-high Temperature Ceramic Coatings

为了提高C/C复合材料的抗氧化烧蚀性能, 采用浆料浸涂与原位反应复合工艺在材料表面制备了ZrB 2 -SiC超高温陶瓷涂层, 利用氧-丙烷火焰测试了涂层的抗烧蚀性能。结果表明: 采用复合工艺所制备的ZrB 2 -SiC超高温陶瓷涂层与基材具有较高的结合强度; 在氧-丙烷火焰冲刷条件下, 涂层具有良好的抗烧蚀性能, 涂层经1500℃下烧蚀600 s, ZrB 2 -SiC涂层无明显烧蚀, C/C复合材料保持完好。微观结构观察表明: 烧蚀测试后, 涂层中存在ZrO 2 和大量超高温陶瓷相, 涂层抗烧蚀形式主要表现为热化学烧蚀和机械剥蚀。为了提高C/C复合材料的抗氧化烧蚀性能, 采用浆料浸涂与原位反应复合工艺在材料表面制备了ZrB 2 -SiC超高温陶瓷涂层, 利用氧-丙烷火焰测试了涂层的抗烧蚀性能。结果表明: 采用复合工艺所制备的ZrB 2 -SiC超高温陶瓷涂层与基材具有较高的结合强度; 在氧-丙烷火焰冲刷条件下, 涂层具有良好的抗烧蚀性能, 涂层经1500℃下烧蚀600 s, ZrB 2 -SiC涂层无明显烧蚀, C/C复合材料保持完好。微观结构观察表明: 烧蚀测试后, 涂层中存在ZrO 2 和大量超高温陶瓷相, 涂层抗烧蚀形式主要表现为热化学烧蚀和机械剥蚀。

Microstructure, mechanical properties and oxidation resistance of SiCf/SiC composites incorporated with boron nitride nanotubes

SiCf/BNNTs–SiC hierarchical composites were fabricated via firstly in situ growth of boron nitride nanotubes (BNNTs) on the surface of silicon carbide (SiC) fibers using boron powder as a raw material and subsequently matrix densification by chemical vapor infiltration (CVI) and polymer impregnation/pyrolysis (PIP) methods. With the incorporation of BNNTs, energy dissipation mechanisms at the nanoscale triggered by BNNTs such as pullout, debonding, crack deflection and crack bridging are observed. But the positive effect of BNNTs on mechanical properties have not been raised due to the offset from the negative effect of pores in composites. Additionally, the residual boron powder results in an improved oxidation resistance and parabolic oxidation kinetics of SiCf/SiC composites at 900 °C, thanks to the protective effect of the B2O3 glassy phase formed by the oxidation of boron. Consequently, a better strength retention after oxidation is obtained. Moreover, it is believed that the remaining strengthening and toughening mechanisms aroused by BNNTs surviving after oxidation probably also make a contribution to the better strength retention.

Fabrication and Properties of Boron Carbide/Graphite Ceramic Pellets

Boron carbide (B4C)/graphite (C) ceramic pellets containing 25 wt% B4C as an ideal structural component in Chinese 10 MW high temperature gas-cooled test reactor (HTR-10) were successfully prepared by a two-stage operation method at low temperature. Initially, the samples were pressurelessly sintered at 1373 K and subsequently infiltrated with phenolic resin, pyrolysed at 1473 K, then coated with carbon by chemical vapour deposition at 1273 K. Morphology and structure of the samples were analysed by SEM and X-ray diffraction. The mechanical properties of the sintered bodies were also investigated. Superior properties were obtained due to the formation of pyrolytic carbon during the two-stage operation process. Density and compression strength of the samples were 1.94 g.cm–3 and 620 N, respectively. In addition, the rate of abrasion and moisture resistance were near zero for the samples.

Ablation behavior and mechanism of 3D Cf/ZrC-SiC composites in a plasma wind tunnel environment

Abstract Three-dimensional needle-like Cf/ZrC-SiC composites were successfully fabricated by polymer infiltration and pyrolysis combined with ZrC precursor impregnation. The ablation properties of the composites were tested in a plasma wind tunnel environment at different temperatures and different times. The microstructure and morphology of the composites were examined after ablation by scanning electron microscopy, and their composition was confirmed by energy dispersive spectroscopy. The composites exhibited good configurational stability with a surface temperature of greater than 2273 K over a 300–1000 s period. The formation of ZrSiO4 and SiO2 melts on the surface of the 3D Cf/ZrC-SiC composites contributed significantly to improvement in their ablation properties. However, these composites exhibited serious ablation when the temperature was increased to 2800 K. The 3D Cf/ZrC-SiC composites obtained after ablation showed three different layers attributed to the temperature and pressure gradients: the ablation central region, the ablation transition region, and the unablation region.