Preparation And Properties Of Nano-alumina By Process Strengthening Method

Nanoα-alumina with excellent mechanical and chemical performance are widely used in catalysts,catalyst carriers,composite reinforcement materials,ceramic materials,biomedical materials,optical materials,semiconductor materials,etc.In order to synthesize nanoα-alumina with good performance,several techniques have been developed.Because of its simple operation and low energy consumption compared with other methods,liquid phase co-precipitation is widely used to prepare nanoα-alumina powders in industry.However,the particles prepared by the traditional liquid-phase precipitation method have large size and uneven distribution.In this paper,a process-enhanced impinging stream microreactor with high-through is proposed.The micromixing performance of impinging stream microreactor is investigated by using Villermaux-Dushman fast parallel competition reaction system and simple reaction of BaCl₂ and Na₂SO₄.Nano-alumina is prepared by traditional co-current precipitation method and micro-channel reaction method.Nano-alumina is prepared by traditional co-current precipitation method and microchannel reaction method.The effects of pH,Al³⁺concentration,flow rate,and calcination conditions on the phase transition,morphology,and sintering behavior of the synthesized alumina powder are investigated.And the morphology and dispersion of the two alumina powders were compared.This study also enables efficient integration of multiple reaction processes.Nano-flaky aluminum is prepared by microreaction method combined with aging and hydrothermal technology.The effects of the coupling of different processes on the morphology,specific surface area and mesoporous structure of alumina are investigated.The optimal dispersion conditions of flake alumina aqueous suspension are analyzed by Zeta potential.The specific research work is as follows.（1）The micromixing performance of the microreactor is evaluated by the Villermaux-Dushman method.When the flow velocity is extremely high,the segregation index is close to 0.001.At high flow rates,an increase in flow rate will enhance the reactor’s micromixing performance,but instead will increase the pressure drop.Subsequent experiments should consider the use of a relatively high flow rate for the liquid-liquid precipitation reaction under the condition that the pressure drop is less affected.The effect of reactant concentration and flow rate on the product was studied using a simple reaction of BaCl₂ and Na₂SO₄.At the flow rate of 120 L·h^-1 and the concentration of 0.1 mol·L^-1,BaSO₄ nanoparticles with good dispersion is prepared by the microreactor.The particle size of the prepared BaSO₄ particles is about 50 nm and the distribution is narrow.（2）Nano-alumina powder was prepared by traditional parallel flow precipitation and microreaction method.The effects of different feeding methods,pH,Al³⁺concentration,flow rate,calcination conditions on the morphology,phase transition and sintering behavior of synthetic alumina powder are studied.Precursor was synthesized with the aid of a high throughput impinging stream microreactor.Theα-Al₂O₃ powder with higher crystallinity,dispersion and sintering activity is obtained by calcining the precursor at 1200℃ for 2 h in air.The average particle size ofα-Al₂O₃ powder prepared by traditional parallel flow precipitation method is about 180 nm,and the dispersion is poor.The specific surface area ofα-Al₂O₃ powders prepared by microreaction method is above 20m²·g^-1,the average particle size is about 110 nm and the sintering performance is high.From room temperature to 1520℃,the green compact exhibited shrinkage of 19.34%.The aggregation coefficient(D_SEM/D_XRD)by microreaction method and conventional method was 1.36 and 1.75 under the calcination temperature of 1200℃ for 2 h,respectively.It is indicated that the powders synthesized by the microreactor had a good dispersion compared with the conventional precipitation method.It was demonstrated that the as-prepared powders exhibited better dispersity and more homogeneous distribution of particle size than that prepared by conventional parallel flow precipitation method due to the drastic collisions and homogeneous explosive nucleation in microchannel during the precipitation process.This work opens a door for developing ultrafineα-Al₂O₃ powders with uniform size distribution,high crystallinity,and excellent thermal expansion property.（3）Microreaction,aging and hydrothermal method are used to prepareγ-Al₂O₃ flakes.The phase changes,morphology,specific surface area and mesoporous structure of alumina powders prepared by traditional hydrothermal,microreaction with hydrothermal,microreaction and aging with hydrothermal,microreaction method,microreaction and aging method are systematically studied.The particle morphology prepared by traditional hydrothermal method is an uneven morphology composed of rods and irregular flakes.The microreaction-hydrothermal method can produce flakyγ-AlOOH with a purity of 99.7%,a particle size of 30～100 nm and a thickness of 2～5 nm.The same shape ofγ-Al₂O₃ can be obtained after calcination.Combined with the aging process,the rhomboid lamellar structure can be regulated to develop into a hexagonal lamellar structure.The morphology,specific surface area,and mesoporous structure of the lamella powder are controlled by different technological processes.Dispersity performace ofγ-AlOOH aqueous suspension prepared by microreaction-hydrothermal method with different ultrasonic time,pH and temperature conditions are analyzed by zeta potential.The dispersion Is better when the ultrasonic time Is 10 min,the pH Is 4,and the temperature Is 40℃.It provides a good scientific research support for the industrial preparation of self-dispersing nano-flake aluminum oxide.