The Effect Of Raw Materials On Self-propagating Hightemperature Synthesis Of ZrB₂

The optimum raw material ratio and mechanism of Self-propagating Hightemperature Synthesis?SHS?of ZrB₂ powders in Mg-ZrO₂-B₂O₃-MgCl₂ and Mg-ZrO₂-B₂O₃-Na₂B₄O₇ systems,while choosing Mg,ZrO₂,B₂O₃,MgCl₂ and Na₂B₄O₇ as raw materials.In the present paper,the thermodynamic calculation of the SHS was studied by FactSage7.0.The phase composition and microstructure of the final product were characterized with X-ray diffraction?XRD?and field-emission scanning electron microscopy?FESEM?with energy dispersive spectrometer?EDS?.The mechanism of ZrB₂ prepared by SHS was characterized by Differential Scanning Calorimetry?DSC?and thermogravimetric?TG?.The results showed that,in terms of thermodynamics,the adiabatic temperature?Tad?of Mg-ZrO₂-B₂O₃ systems is 3097.91 K,though the Tad decreased while adding MgCl₂ the Tad matched the Merzhanov empirical criteria before the extra MgCl₂ is less than 41 wt.%;while chossing Na₂B₄O₇ to institute B₂O₃ as recombination B source with B₂O₃,Tad decreased as the institution increased,when the institution is more than 72 wt.%,Tad was less than 1800 K,which didn't match sustaining action criteria.The Gibbs free energy and reaction enthalpy of the reactions in Mg-ZrO₂-B₂O₃-MgCl₂ and Mg-ZrO₂-B₂O₃-Na₂B₄O₇ systems are all less than zero in the setting temperature.The ZrB₂ purity of the final product could be improved when the raw materials were mixed with extra amount of Mg and B₂O₃ in Mg-ZrO₂-B₂O₃ system,it was found that the optimum extra amounts of Mg and B₂O₃ were 40 wt.% and 30 wt.%,respectively.Based on the optimum ratio,adding extra MgCl₂ could decrease the grain size of ZrB₂ effectively,while there is few significant improvment in ZrB₂ purity.Based on the optimum ratio of Mg-ZrO₂-B₂O₃ system,choosing Na₂B₄O₇ instead of B₂O₃ gradually,the purity of ZrB₂ in the final product is the highest,when the institution ratio of Na₂B₄O₇ is 15 wt.%.Na₂B₄O₇ not only work as molten salt decreasing the ZrB₂ grain size,but also work as B source,Tad and grain size of ZrB₂ decreased while Na₂B₄O₇ increasing.The result of DSC-TG showed that the Mg-ZrO₂-B₂O₃-Na₂B₄O₇ reaction took place from 625?,there is a wide exothermic peak during the temperature range of 625? to 700?,Mg and Na₂B₄O₇ melted by heat released by strong exothermic reaction.It's beneficial for ZrO₂ converting to ZrB₂ when there was more liquid,the synthesized ZrB₂ grain precitated from liquid,the grain size of the ZrB₂ was limited by Na₂B₄O₇ coating on the ZrB₂.