Preparation,Properties And Application Of Alumina-based Hollow Microspheres

Micro/nano hollow microspheres have attracted intense interest because of their unique morphology and structure.Their unique hollow structure can accommodate other materials,and the size effect imparts them special optical,electrical and magnetic properties,so it has been applied in many fields.At present,the high efficiency,low cost and environment-friendly preparation of micro/nanometer hollow microspheres with controllable morphology has become hot spots for researchers.The methods of the preparation of hollow microspheres are mainly composed of template method and template-free method,each of which has its specific application and limitations.For the template method,the shape and size of the hollow microspheres can be well controlled,but the process is complicated and low yield.Template-free method is simple,but the controllability is poor and the type of hollow product is limited.Based on this situation,hollow microspheres were prepared by the template method in this paper,and then the template-free process was explored,and the“rapid sol-gel combined with induced self-transformation”process was put forward to prepare a series of alumina-based micro-nanometer hollow microspheres.The main research contents and achievements are summarized as follows:?1?Using colloidal carbon spheres as the template,urea as the precipitant,ethanol and deionized water as the mixed solvents,monodisperse alumina hollow microsphere with homogeneous particle size and a certain shell thickness were prepared.Urea in 60 ^oC solution can be slowly decomposed,which makes the solution alkaline.Under the action of the adsorption of carbon spheres onto the aluminum ion and the alkaline solution,aluminum ions precipitated on the surface of the carbon spheres to form a thick shell.The effect of solvent environment on the morphology and pore structure of hollow microspheres were studied.The results showed that the presence of ethanol can reduce the polarity of the solution,making it easier for aluminum ions to precipitate on the surface of the carbon spheres.However,excess ethanol prevents the decomposition of urea,so that the urea metathesis reaction cannot proceed and aluminum ions are difficult to precipitate.The optimum dosage of ethanol and water is 120 ml and 40 ml,respectively.Under this condition,the specific surface area of the alumina hollow microspheres was 328.52 m²/g at600°C.As the calcining temperature is increased from 600°C to 1000°C and1100°C,the crystal of alumina transforms from?-Al₂O₃ to?-Al₂O₃ and then to?-Al₂O₃,and the surface morphology of alumina hollow microspheres appeared to change from a relatively rough surface formed by nanoparticles to a smooth surface formed by lamellar grains,which lead to the closure of pore channels and the reduction of specifc area reduced from 328.52 m²/g at 600 ^oC to 88.81 m²/g at 1100oC.?2??-AlOOH hollow microspheres were successfully synthesized for the first time by using the rapid sol-gel combined with induced self-transformation process.This process is simple,low-cost and large-scale preparation.The prepared hollow microspheres have a size of 2-3?m and urchin-like shell structures with a specific surface area of 316 m²/g.Through the study of the effect of solvent environment on the appearance of hollow microspheres,it was found that excessive water in the solvent cannot proceed with sol-gel reaction,while overdose of ethanol will cause the reaction to complete instantaneously,resulting in precipitation and failure to obtain hollow microspheres during the subsequent aging process.The optimal experimental solvent conditions studied were 6 ml of ethanol and 4 ml of deionized water.Through the study of the influence of the amount of the propylene oxide on the morphology of hollow microspheres,it was found that when the addition of propylene oxide is too small,the time of sol-gel reaction will take longer,and the gel formed is transparent;when the propylene oxide is added too much,it causes the sol-gel reaction to be completed in a moment,and the granulated precipitate is generated,so the hollow structure microsphere cannot be obtained in the aging process.Only if a moderate amount of propylene oxide was added can the hollow structure microspheres be obtained in the subsequent aging process.The optimum amount of epoxy propane in the study was 7 ml.Through the study of the effect of different precursors on the appearance of hollow microspheres,it was found that the anion not only plays a certain role in the sol-gel process,but also plays a crucial role in the formation of hollow microspheres.Cl^-ions not only participate in the ring-opening reaction but also have strong nucleophilic properties relative to NO₃^-.In the reaction process,the Cl^-ions can be coordinated with the metal cations to affect the hydrolysis and polymerization of the metal cations in the solvent.?-AlOOH hollow microspheres have low thermal conductivity and good dispersion.When used as fillers in thermal insulation coatings,they can increase the thermal insulation effect of thermal insulation coatings.The adding quantity of?-AlOOH hollow microspheres on the thermal insulation effect was studied,and the optimal amount of?-AlOOH hollow microspheres was 1.25 g.?3?After research,it has been found that there are two deficiencies of the alumina hollow microspheres using in thermal insulation materials.The first one is that the strength in the thermal insulation materials is not high,and they can only appear in the form of powder,which cannot be used as supporting materials.The second one is that in high temperature,the surface morphology changes due to the transformation of the aluminum crystal,thereby affecting its thermal insulation effect.Al2O3-SiO2 and Al₂O₃-ZrO₂ composite hollow microspheres were successfully prepared by the rapid sol-gel combined with induced self-transformation process.The results show that the prepared composite hollow microspheres have good high temperature stability.Al₂O₃-SiO₂ composite hollow microspheres can be presented as a bulk material with certain strength.In order to solve the problems of low strength and poor toughness of bulk hollow microspheres,composites of foamed ceramic-based hollow microspheres and polyester fiber-filtered hollow microspheres were successfully prepared which play a supportive and resilient role in the thermal insulation materials respectively.The thermal conductivity of the materials were 0.12 W/m·K and 0.04 W/m·K,respectively.The composite hollow microspheres used as water-based fire retardant coating can improve the fire resistance greatly,the best dosage is 1.5 g,and the prepared fireproof coating can protect a 0.6 mm steel plate from concaving after 2hours test with a continuous back surface temperature of about 125°C under the action of the alcohol torch.The hollow microspheres modified study found that Al₂O₃-SiO₂ composite hollow microspheres can be modified to hydrophobic hollow microspheres after using of dimethyl triethyl silane.The contact angle is 120°.?4?Hollow microsphere luminescent materials were successfully prepared using the rapid sol-gel combined with induced self-transformation process.It was found that hollow microspheres can maintain hollow structure and has a certain afterglow performance at the condition of 1050°C.Influence of adding different amounts of H₃BO₃ on the pore structure and the afterglow property of hollow microspheres were studied.It was found that the optimum amount of was 0.02 g,and the optimal calcination temperature was 950°C,the addition of H3BO3 decreased the optimum calcination temperature by 100°C.When the experimental test sample was 0.2 g,the time of afterglow and the initial brightness were 32min and 0.31cd/m²,respectively.The rapid sol-gel combined self-induced transformation process was extended.The photoluminescence material Al₂O₃:Eu³⁺was successfully prepared and emits intense red light under UV irradiation.The results show that after calcination at 600?,Al₂O₃:Eu³⁺has not only a complete hollow structure,but also a larger specific surface area with stronger luminescence intensity.?5?A mechanism of“rapid sol-gel combined with induced self-transformation”was proposed to prepare alumina and multi-component inorganic micronano hollow microspheres based on alumina.Al₂O₃,Al₂O₃-SiO₂,Al₂O₃-ZrO₂,SrAl₂O₄:Eu²⁺,Dy³⁺and Al₂O₃:Eu³⁺micronano hollow microspheres were prepared.The formation of hollow microspheres is mainly divided into two steps.The first step is the rapid sol-gel process actuated by the epoxy propane.Under a certain external environment,the sol-gel process is completed within a few seconds.The product is a solid spherical particle;the second step is to complete the hollowization transformation process by aging at a certain temperature and in an organic solvent environment.Preparation of alumina-basedhollowmicrospheresbytherapidsol-gelcombined self-transformation process provides new ideas and approaches for the preparation of other inorganic hollow microspheres.