Synthesis Of Nanocrystalline ?-Al₂O₃ For Seeding And Its Application In Preparation Of ?-Al₂O₃ Fiber

There are two main problems in the preparation of ?-Al₂O₃ fibers by sol-gel method.On the one hand,when the transition phase of alumina is transformed into a phase,vermicular structure is formed and thus affect the density of alumina fibers.On the other hand,?-Al₂O₃ is prone to grain coarsening during sintering,which affects mechanical properties of the fibers at high temperature.In this paper,nano-scale ?-Al₂O₃ particles were prepared by homogeneous precipitation method and then introduced into the aluminum sol as seeding material,which aims to increase the nucleation density of ?-Al₂O₃,to reduce the temperature of nucleation,and to inhibit the vermicular structure;Meanwhile,silica sol is also introduced into the aluminum sol to suppress the growth of ?-Al₂O₃ grains at high temperatures.The following main conclusions were obtained through research:1.Aluminum sulfate?Al₂?SO₄?₃·18H₂O?was used as aluminum source,ferric nitrate?Fe?NO3?3·9H₂O?was used as iron source,and formamide?NH₂CHO?was used as precipitant.The nano-scale ?-Al₂O₃ particles were prepared by homogeneous precipitation method.By adjusting the process parameters such as calcination temperature and Fe/Al molar ratio,nano-scale spherical ?-Al₂O₃ particles with good dispersibility can be obtained.It was found that the main factor affecting the grain size is calcination temperature.When calcination temperature is 800? and the Fe/Al molar ratio is 5/1,the average grain size of final product ?-Al₂O₃ is 26.2 nm.2.The above ?-Al₂O₃ suspension is introduced into the aluminum sol as seeding material,which can significantly increase the nucleation density of ?-Al₂O₃ and induce amorphous alumina to transform into ?-phase directly.In addition,self-seeding can also lower the phase transition temperature of ?-Al₂O₃ to ?-Al₂O₃.When the content of ?-Al₂O₃ seeds is 1.5wt.%,the temperature at which alumina fibers started to transform into the a-phase is 700?,and the phase transition temperature of ?-Al₂O₃ to ?-Al₂O₃ is reduced by 200?.Since self-seeding increases the nucleation site of ?-Al₂O₃,vermicular structure accompanied by phase transformation is suppressed,and the density of alumina fibers is improved obviously.3.The silica sol significantly reduces the grain size of ?-Al₂O₃.When the content of ?-Al₂O₃ seeds is 1.5wt.%and the content of silica sol is 1.0 wt.%,the grain size of alumina fiber sintering at 1200? is reduced from 63.5 nm to 50.5 nm.At the same time,since the silica sol inhibits the formation of ?-Al₂O₃ critical nucleus,the temperature at which alumina fiber completely transforms into the ? phase is increased from 900? to 1000?;In addition,the silica sol hinders boundary diffusion of grains,and thus inhibits the sintering of ?-Al₂O₃.4.A two-step sintering process is used to help fibers obtain refined grains and dense structure.The first step?rapid heating up to high temperature?will promote fibers to completely transform into a-phase and reduce the porosity of alumina fiber;The second step?heat preservation at a temperature without the rapid grain growth?will promote fibers to be densified witihout significant changes in grain size.The dense ?-Al₂O₃ fiber with an average grain size of less than 200 nm can be obtained by rapidly raising the temperature to 1400? and then holding at 1000? for 2 hours as a condition of two-step sintering.