Effects of Different Thermite Reagents on Combustion Synthesis of Mo-Si and W-Si Compounds

• Main Authors: Je-An Peng, 彭哲安
• Other Authors: Chun-Liang Yeh
• Format: Others
• Language: zh-TW
• Published: 2016
• Online Access: http://ndltd.ncl.edu.tw/handle/32602658552193429295

碩士 === 逢甲大學 === 航太與系統工程學系 === 104 === Formation of Al2O3-containing molybdenum (Mo) and tungsten (W) silicides was investigated by self-propagating high-temperature synthesis (SHS) involving thermite reduction. Metal oxides, MoO3 and WO3, were adopted as the primary oxides and SiO2 was used as the secondary oxide. The use of single and two oxides in the reactant mixtures was to broaden the composition range of the final composites. Effects of the sample stoichiometry on the propagation mode of combustion wave, flame-front velocity and temperature, and product composition and microstructure were investigated. In the synthesis of the Mo-Si compounds with Al2O3, the spinning combustion wave accompanying the melting of the sample was observed. This caused a phase separation of Mo5Si3 and Al2O3. The combination of single- and dual-oxide reaction systems produced two composites with the molar ratio of MoSi2/Al2O3 from 0.8 to 4.5 and Mo5Si3/Al2O3 from 0.4 to 1.6. However, the W-Si reaction system showed planar and steady combustion fronts and less melting of the sample. The phase composition achieved for WSi2/Al2O3 varied from 0.8 to 4.0 and for W5Si3/Al2O3 from 0.4 to 2.0. The combustion temperature and flame-front velocity decreased with increasing content of the silicide phase. The addition of SiO2 also caused a reduction in combustion exothermicity. For the Mo-Si reaction systems, the combustion temperature decreased from about 1700 to 1000 ℃, and flame velocity from 6.5 to 2.0 mm/s. For the W-Si reaction systems, the temperature and velocity were ranged from 1700 to 1100 oC and 9.5 to 4.0 mm/s, respectively. In the formation of Mo-Si phases and Al2O3, the yield of Mullite was often observed due to the dissolution of Si. Moreover, -MoSi2 was dominant in the case of the high combustion temperature above 1500 ℃. At the combustion temperature below 1400 ℃, -MoSi2 was the major phase. A phase separation between Mo5Si3 and Mullite occurred because of significant melting. The production of the WSi2/Al2O3 composites was well achieved, except for a minor amount of W5Si3. In the synthesis of the W5Si3/Al2O3 composites, there were WSi2 and remaining W present in the final products.