Study On The Process Of Combustion Syntesis Of Al-Zr（NO₃）₄Highly Exothermic System

Eutectic ceramics is a self-situ composites with good microstructure, excellentmechanical properties and solid phase interface. Besides, it shows excellentroom-temperature, high temperature mechanical properties and high temperature （oreven close to the melting point） structural stability in the preferential growthdirection. In the article, we chose Zr（NO₃）₄and Al as reagent, tested the basicparameters （temperature, pressure） of Al-Zr（NO₃）₄combustion synthesis and studiedthe preparation process of the Al₂O₃/ZrO₂eutectic ceramic under the centrifugalforce. On this basis, we made full use of the heat generated by Al-Zr（NO₃）₄reactionto melt ZrB₂and SiC and explored the preparation process of ZrB₂-SiC eutecticceramics.Based on the thermodynamic theory, the adiabatic temperature and reactantratio of the Al-Zr（NO₃）₄system were calculated. The results show that with theincrease in the percentage of reagent quality system adiabatic temperature rises, andit becomes more easily to achieve the adiabatic temperature of the spontaneousspread1800K. By testing the combustion wave rate of the reactants of differentratios in the open environment and closed environment, we found that the measuredtemperature under both the environments increases with the ratio of reactants.Compared with the theoretical temperature, the open environment measuredtemperature is400K low on average and that of the closed environment is280K low.Open environment, for Zr （NO₃）₄decomposition，O,N escape, resulting in heatloss, and reactant content of Zr（NO₃）₄preheat decomposition ratio increases, theheat loss increases, so the infrared temperature is lower than the theoreticalcalculation results, the combustion wave velocity increase with the reactant contentfirst increased and then decreased trend.The microstructure of products of the Al₂O₃/ZrO₂system were analyzed. Theresults show that eutectic mixed with the lamellar eutectic and rod eutectic wasformed in the products. low reaction temperature, no formation of the completelyliquid, the organization coarse, fully miscible, the formation of fine eutecticstructure, temperaturefurther increased, grain growth, organization coarsening;centrifugal conditions.The Vickers hardness of the products of different reagents was tested and itshowed that the hardness value is between12GPa¹3GPa and with little change.The crack first passes through the Al₂O₃and expands, then bypass the Al₂O₃itextends along the Al₂O₃grains and eutectic interface. The adiabatic temperature and pressure of the ZrB₂/SiC system were calculatedwith the changes of the reagent contents. The results show that the pressure and theadiabatic temperature rises with the increase of the reagent content. The eutectictemperature can be achieved by reactant content of35%. As for the reactant contentof42%, the adiabatic temperature can reach3400K, beyond the ZrB₂melting point（3323K）. In order to reduce the system pressure, we added an excess of Al toreactwith nitrogen. As a result, the reaction pressure of the system was reduced greatly.In theory, the pressure can be reduced to atmospheric pressure. But the measuredpressure is higher than the theoretical pressure, indicating that the N2was produced.By XRD and stress test, the combustion reaction mechanism was analyzed. Theresults show that with the rise of the reactant ratio, the instantaneous pressure of thesystem also increases. And the pressure after cooling is zero, then N combined withAl、O formation Al₅O₆N. The ZrC phase in the product was formed by the reactionof Zr and SiC; After adding ZrO₂, SiC completely decomposed to form ZrSi2andZrC; After adding B₄C, the decomposition of SiC was inhibited while Z and Bcombined to form ZrB₂, and ZrC phase disappeared as a result.The microstructure of the product prepared by different reagent contents wasinvestigated. When the reagent content is less, ZrB₂and SiC can not be fully meltingdue to the low system temperature. As for that of higher, ZrB₂and SiC fully meltedto a mutual solution phase. But with the rise of the temperature, the decompositionof SiC increases and the ratio of SiC in liquid decreases. Finally it affected theformation of the eutectic structure. Besides, the impact of adding the component ofZrO₂and B₄C was investigated. The results show that SiC is completelydecomposed without formation of the eutectic structure in the adding ZrO₂system.As for the B₄C system, the decomposition of SiC was inhibited and formedZrB₂-SiC irregular eutectic in the product.Analyzed the material of the solidification of the law, when the temperature ishigh, due to the SiC decomposition, in ZrB₂/SiC mutual solution phase, a highproportion of ZrB₂, so ZrB₂as a precipitated phase, priority nucleation ofprecipitates formed flake crystal; As the solidification, when of ZrB₂/SiC to reachthe eutectic ratio, SiC priority precipitation, and ZrB₂attached to shaped SiCnucleation and growth, the formation of the irregular organization of ZrB₂-SiCeutectic; when the temperature is lower, thinner melt is not sufficient, to granularevenly distributed in the material systems.