Zhean Su

Influence of Preparation Technology on the Structure and Phase Composition of MoSi2-Mo5Si3/SiC Multi-coating for Carbon/Carbon Composites

Oxidation protective MoSi2-Mo5Si3/SiC multi-coatings for carbon/carbon composites were prepared by chemical vapor reaction and slurry-sintering method. The influence of preparation technology on the structure and phase composition of the coating was investigated by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) analyses, and then their relationship was discussed. The results indicate that the Si/Mo ratio of the slurry and sintering processing were two main factors that significantly affected the structure and phase composition of the multi-coating. Appropriate sintering process and relatively high Si/Mo ratio were essential for preparing the multi-coating with dense structure and favorable phase composition. After being sintered at 1723 K for 2 h and with the Si/Mo ratio of the slurry being 4.5 (weight ratio), a dense structure accompanied by favorable phase composition of the coating can be obtained. When heat treated at 2373 K for 1 h, this coating became more compact and continuous. Oxidation tests (performed at 1623 and 1823 K) demonstrated that both of these two obtained multi-coatings exhibited better anti-oxidation property than single layer SiC coating.

Effects of mannitol on the synthesis of ultra-fine ZrB2 powders

ZrB2 particles were synthesized by a sol–gel method using zirconium oxychloride, boric acid, and mannitol as starting materials. Mannitol helps to form the Zr–O–B in the sol–gel system to improve the stability of the precursor. Mannitol acts both as a modifier, as well as carbon source. High purity of ZrB2 powder can be obtained with a molar ratio of B/Zr = 2.3, C/Zr = 6.6 after carbothermal reduction reaction at a low temperature of 1350 °C for 1 h. The yield rate of the modified ZrB2 precursor can be increased to 53.4%. Additionally, the ultra fine ZrB2 powder exhibits rod-like and well-crystallized particles morphology with a particle size of ~200–300 nm. For comparison purpose, sucrose or phenolic resin substituted for mannitol, acting as carbon source were carried out as contrast experiments. It revealed that mannitol helps to form chelate complex with H3BO3, which promotes the stability of the precursor, increases the yield of ZrB2, and reduce the preparation temperature of the ZrB2 powders.Graphical abstract(a) SEM image of sample 3 heated at 1350 °C for 1 h. (b) XRD patterns of sample 3 heated at different temperatures for 1 h from 1000 to 1450 °C.