| Summary: | 碩士 === 逢甲大學 === 航太與系統工程所 === 99 === An experimental study on the formation of Al2O3-added niobium silicides, tantalum silicides, and tantalum borides was conducted by self-propagating high-temperature synthesis (SHS) involving thermite reactions. Four types of thermite mixtures, Al-Nb2O5, Al-Ta2O5, Al-B2O3, and Al-SiO2, were utilized in the Nb-Si, Ta-Si and Ta-B combustion systems. This study is to examine the dependence of product composition and morphology, combustion wave velocity, and reaction temperature on the sample stoichiometry.
Experimental observations showed that the combustion process is characterized by a self-sustaining reaction front. For the formation of NbSi2 and Nb5Si3, the reaction temperature and propagation velocity of the combustion front were increased by increasing the extent of aluminothermic reduction. Although the inclusion of SiO2 lowers the exothermicity of the thermite reaction, the Al-Nb2O5-SiO2 mixture broadens the composition range of the final product, especially for the formation of NbSi2 and Al2O3. Based upon the XRD analysis, the NbSi2-Al2O3 composite was produced with a small amount of Nb2Al. It was found that ??Nb5Si3 was the dominant silicide in the composite synthesized from the sample under an exact stoichiometry. However, the sample containing 10%-excessive Si favored the formation of ??Nb5Si3.
For the preparation of Ta-Si compounds, the combustion front temperature and propagation velocity increased with the extent of thermite reactions for the systems adopting the thermite mixture of Al-Ta2O5, while both of them decreased for those using Al, Ta2O5, and SiO2 as the thermite reagents. Among four silicide compounds, a better degree of phase evolution was observed for TaSi2 and Ta5Si3 when compared to that of Ta2Si and Ta3Si. The XRD analysis indicated the presence of a small amount of Ta5Si3 in the TaSi2-Al2O3 composite. On the formation of Ta5Si3 with Al2O3, the minor phase was Ta2Si for the Al-Ta2O5-containing system.
On the synthesis of tantalum borides from the samples adopting the Al-Ta2O5 thermite, the increase of elemental B content in the sample led to an increase in the combustion temperature and velocity for the formation of TaB, but a decrease for producing TaB2. When the samples with the thermite of Al-Ta2O5-B2O3 were used, the increase of B2O3 decreased the combustion temperature and velocity on the formation of both TaB and TaB2. The samples with Al-Ta2O5 achieved better product conversion. The intermediate borides and unreacted Ta were substantially reduced in the final products by an excessive amount of boron.
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