Study On Crystalline Structure Evolution Of SiO₂/Al₂O₃ Precursor And Its Application In Preparation Of Alumina Fiber

As a kind of high performance new inorganic ceramic fiber,alumina fiber has excellent electrical,thermal and high temperature mechanical properties,and be with not only high strength,low density and low thermal conductivity,but also high temperature stability up to 1000?,corrosion resistance and electrical insulation.With good surface activity,alumina fiber can be used as reinforcement for resin,metal,ceramic matrix,forming a variety of composite materials with excellent properties,which are widely applied in chemical,metallurgy and other fields,In recent years,alumina fiber has attached a lot of attention especially in the aerospace field.With more than ten crystalline states,Al₂O₃ is highly susceptible to change crystal phase at high temperature and eventually transforms into the only thermodynamically stable phase?-Al₂O₃.Once the?phase particles grow up and exceed the critical size d_p^???50nm?,they will be sharply grow to form a typical worm-like porous structure,causing loss of mechanical properties and the increase of brittleness of alumina fibers.As a phase transition inhibitor,SiO₂ have a great influence on the evolution of Al₂O₃ crystal structure at high temperature,and improve the performance of alumina fiber.Therefore,it is of great significance to study the crystal structure evolution of SiO₂/Al₂O₃ precursors for the preparation of high performance alumina fibers.In this paper,a series of alumina sol precursors with different SiO₂ content were prepared by sol-gel method.The effects of SiO₂ on the thermal weight loss and crystallization heat change,chemical structure,crystal structure,microstructure,compaction and the phase transition temperature of?-Al₂O₃ on alumina precursors were analyzed with FT-IR,TG-DTA,XRD,SEM and thermal expansion,and the evolution of the crystal structure of SiO₂/Al₂O₃ precursors were studied.The alumina sol precursors were fiber-formed by dry spinning,Then the mass loss,diameter distribution,mechanical properties,microstructure compactness and macroscopic toughness of two alumina fibers with 10 wt%TEOS-SiO₂ and NS30-SiO₂ were analyzed by means of SEM,fiber fineness analyzer and fiber strength tester;the evolution of the crystal structure of SiO₂/Al₂O₃ precursors in the preparation of high performance alumina fibers was discussed.The results show that the crystal structure evolution of Al₂O₃ follows the law of amorphous state??-Al₂O₃??-Al₂O₃??-Al₂O₃??-Al₂O₃ during heat treatment.With the addition of SiO₂,Al₂O₃ and SiO₂ undergo hydrolytic polycondensation reaction to form Al-O-Si bond.When SiO₂is enough more,it combines with Al₂O₃ to form a small amount of mullite phase at high temperature,the phase transformation process of alumina is gradually delayed,and the growth of Al₂O₃ grains is inhibited.When the SiO₂ content is increased to 10 wt%,it has the most obvious inhibition effect on the phase transformation of?-Al₂O₃.Compared with pure alumina,the phase transition temperature of?-Al₂O₃ can be increased from 1000? to above 1300?,and?-Al₂O₃grain is controlled within the critical size range,significantly reducing the thermal shrinkage of the alumina precursor and improving the compactness of the precursor.For the two alumina fibers with 10 wt%TEOS-SiO₂ and 10 wt%NS30-SiO₂,the fiber mass loss rate does not change much during the Al₂O₃ phase transition from 900? to 1200?.When the temperature reaches 1200?,the microscopically structural compactness of the fiber is remarkably improved,and the toughness of the fiber is improved.The mechanical strength of the fiber with 10 wt%TEOS-SiO₂ is rapidly increased to 1.022GPa,which is 0.01GPa higher slightly than that of the fiber with 10wt%NS30-SiO₂.In the case of similar mechanical strength,the usage temperature is 200? higher than that of the commercial fiber Saffil.Although TEOS and NS30have little difference in fiber properties,the preparation of the fiber with NS30 is easier and more convenient than TEOS,which is beneficial to industrial production,and the cost of raw materials is much lower than that of TEOS.In summary,SiO₂ is an excellent phase transition inhibitor,which can delay the process of the crystal structure evolution of Al₂O₃ by chemical or physical interaction with Al₂O₃,increase the crystal phase transition temperature of?-Al₂O₃,and inhibit the rapid growth of?-Al₂O₃ grains.Therefore the mechanical properties of the alumina fiber is improved,and the usage temperature of the alumina fiber is increased to 1200?.