THERMITE-BASED COMBUSTION SYNTHESIS OF NIOBIUM SILICIDES/Al_2O_3 COMPOSITES

摘要

In situ formation of NbSi_2- and Nb_5Si_3-Al_2O_3 composites with a wide composition range was achieved by self-propagating high-temperature synthesis (SHS) involving thermite reactions. Two thermite mixtures, Al-Nb_2O_5 and Al-Nb_2O_5-SiO_2, were incorporated into the Nb-Si combustion system. As the extent of aluminothermic reduction involved in the SHS process increases for a higher content of Al_2O_3, the combustion temperature and reaction front velocity are enhanced due to the increase of combustion exothermicity. Although the SiO_2-added reaction is less energetic, the Al-Nb_2O_5-SiO_2 thermite mixture contributes to the final product with a higher content of Al_2O_3, especially for the NbSi_2-Al_2O_3 composite. Based upon the X-ray diffraction analysis, the NbSi_2-Al_2O_3 composite was produced with a small amount of Nb_2Al. On the formation of the Nb_5Si_3-Al_2O_3 composites, Nb_5Si_3 of the α form was yielded as the dominant silicide when stoichiometric samples were adopted, while β-phase Nb_5Si_3 was favored for the green samples containing 10% excess Si.