Combustion Synthesis And Characterization Of TaSi₂ Powder

In this paper,high purity Tasi₂powder were prepared by combustion synthesis process in magnesium reduction system（Mg-Si-Ta₂O₅system and Mg-Si O₂-Ta₂O₅system）and elemental synthesis system（Ta-Si system）.Combustion reaction mechanism,combustion process craft,microstructures and powder characteristic were studied by means of various experimental and theoretical methods.Combustion synthesis in magnesium reduction system have the advantages of high reaction heat release,low price of raw materials and low product grain size,but it is difficult to remove impurities and thereby the product purity is affected.Based on thermodynamic theory,the adiabatic temperature and free energy of Mg-Si O₂-Ta₂O₅system were analyzed.The adiabatic temperature calculation shows that the adiabatic temperature of Mg-Si O₂-Ta₂O₅system is 2956K when the system does not contain any dilution components.The calculation of the reaction free energy shows that,In the part of Ta-Si system in the magnesium reduction system,the compounds Ta Si₂,Ta₅Si₃and Ta₂Si between Ta and Si are all possible to form,and the reaction free energy of Ta Si₂is higher and the occurrence possibility is lower.In the magnesium reduction system in addition to the reaction between Ta and Si,the free energy of the reaction between Mg and Ta₂O₅,silicon and Ta₂O₅,Mg and Si O₂,Si O₂and Mg O are all less than zero,there is a tendency of occurrence,and the tendency of occurrence is different.The study on the process craft shows that in the Mg-Si O₂-Ta₂O₅system,with the addition of magnesium and silicon oxide,the content of Mg₂Si O₄and Ta₅Si₃impurities in the product decreases to a certain extent,but the absolute content of impurities in the product is still very high,and the feasibility of the Mg-Si O₂-Ta₂O₅system is not enough.The adiabatic temperature of Mg-Si-Ta₂O₅system is 2857K.Combined with quenching experiment and thermodynamic analysis,the formation process mechanism of combustion synthesis reaction of Mg-Si-Ta₂O₅system was studied.The quenching experiment of Mg-Si-Ta₂O₅system showed that the reaction process of Mg-Si-Ta₂O₅system could be divided into three stages:In the initial reaction stage,the morphology of Mg and silicon changes when heated.Mg reacts with tantalum oxide first to generate tantalum,tantalum and silicon to generate Ta Si₂.The reaction between magnesium and tantalum oxide also continues.In the intense reaction stage,a large number of Ta₅Si₃phase were formed due to the insufficient diffusion degree or the insufficient total amount of silicon.At the reaction completion stage,the remaining silicon gradually reacts with Ta₅Si₃phase to convert Ta₅Si₃to Ta Si₂,and the remaining raw materials continue to be consumed.The remaining magnesium oxide and silicon oxide generate magnesium silicate until all raw materials are consumed up.In the Mg-Si-Ta₂O₅system,with the addition of magnesium and silicon,the content of Mg₂Si O₄and Ta₅Si₃impurities in the product decreased significantly.When the additional amount of silicon was more than 20%,the Ta₅Si₃impurities in the product disappeared,but free silicon phase appeared in the product.When the additional amount of magnesium was more than 50%,the Mg₂Si O₄impurities in the product disappeared.In the Mg-Si-Ta₂O₅system,there is no obvious difference in product composition between 2MPa and 4MPa.Ta Si₂powders with high purity were obtained by pickling and alkaline washing of Mg-Si-Ta₂O₅system with 50%Mg excess and 15%Si excess.Ta-Si system is characterized by high product purity,complete reaction,mature process and other advantages,but the original product is a block,the need for additional grinding and other shortcomings.Based on thermodynamic theory,the adiabatic temperature and free energy of Ta-Si system were analyzed,The adiabatic temperature of the Ta-Si system is 1794K,at which the combustion synthesis of the Ta-Si system cannot establish a self-propagating reaction front.After theoretical calculation and experimental screening,the combustion front can be maintained when the adiabatic temperature is above about 2000K.If the energy of the system is improved by preheating,it is possible to complete the combustion when the preheating temperature is above 300℃.The free energy analysis of Ta-Si system has been carried out in magnesium reduction system,and various compounds between tantalum and silicon are possible to form.Based on thermodynamic calculation and differential thermal analysis,the reaction mechanism of Ta-Si system was studied.The DSC-TG experiment showed that the reaction of Ta-Si system occurred after700℃,and the silicon melting occurred in the reaction system near 1410℃,but there was no exothermic peak in the whole process,that is,there was no violent reaction.According to the mechanism of Mg-Si-Ta₂O₅system,It can be inferred that a small amount of Ta Si₂is generated in the early stage of reaction,and a large amount of Ta Si₂and Ta₅Si₃are generated in the middle stage of reaction.In the late stage of reaction,Ta₅Si₃is gradually transformed into Ta Si₂through the diffusion of silicon.The study of combustion process craft of Ta-Si system shows that the content of Ta₅Si₃phase decreases with the increase of the percentage of silicon in the ratio of Ta-Si system.When silicon is more than 4%excess,there is no tantalum Ta₅Si₃phase in the reaction product.In this case,the purity of Ta Si₂powder product is relatively high,and the particle size is about 3.6 microns.In the three different systems studied in this paper,the product impurities in Mg-Si O₂-Ta₂O₅system are high,the potential of industrial production and research in Mg-Si O₂-Ta₂O₅system is low.The product purity of Mg-Si-Ta₂O₅system is high,and the Mg-Si-Ta₂O₅system has the advantages of no preheating and low energy consumption.Therefore,the significance of Mg-Si-Ta₂O₅system in industrial production is general,but it has great potential in industrial production and research.The products of Ta-Si system have the highest purity among the three systems,so it has the highest significance in industrial production at the present stage.However,Ta-Si system has the disadvantages of requiring preheating of raw materials and high raw material prices.