Preparation method and preparation mechanism of alumina hollow spheres

The aluminum oxide hollow sphere material has a cavity structure inside, and the thickness of the shell layer is at the nanometer or micronano level. Compared with other bulk materials, the hollow sphere material with a shell thickness of nanometer to micrometer level has the advantages of high specific area, low density, etc., and the shell of this hollow sphere material can be adjusted accordingly, Get special properties. Due to this special structure, compared with other materials of the same particle size, it has high specific surface area, low density, surface permeability, thermal insulation and its light scattering energy performance, hollow sphere material is widely used as a new type of functional material At the same time, because the refractive index of the shell layer is much higher than that of the core layer, the hollow spherical material is easy to form a reflected electromagnetic field and its "black hole" isolation. Based on this performance, It can be applied to high-performance radar stealth materials.

1. Preparation method of alumina hollow sphere material

So far, the preparation methods of hollow sphere materials have become more and more mature, and a variety of methods have been studied to prepare hollow sphere materials. The methods that are widely used in inorganic hollow sphere materials include hard template method, soft template method and non-template method. Wait for three.

1.1 Hard template method

The hard template method is to adsorb or deposit metal ions on the surface of a template with a rigid structure to form a core-shell structure, and remove the template in subsequent processing to obtain the required hollow sphere material. The samples prepared by this method have the advantages of controllable particle size, uniform spherical shape and good repeatability.

(1) Use inorganic substances as templates

Monodisperse SiO2, Au particles, porous anodized alumina, TiO2 and other inorganic substances are usually used as templates to prepare hollow sphere materials, among which SiO2 templates are mostly used, and SiO2 templates include porous SiO2, SiO2 gel, quartz glass and their arrangements. Neat array of nanometers. When using SiO2 as a template, it is usually necessary to modify the surface of the SiO2 template. The commonly used surface modifiers are silane coupling agent, cetyltrimethylammonium bromide, cetyl sodium sulfate and succinate Sodium sulfonate, etc., the principle of using silane modifier is that the silane modifier connects with SiO2 to form a bond, and the shell material is further adsorbed on the surface of SiO2. At present, monodisperse SiO2 particles are used as templates to carry out surface treatment on monodisperse SiO2 particles, introduce some special groups on the surface of SiO2, and then improve the surface adsorption capacity of SiO2 balls, and deposit the precursor of the target product on the pretreated surface. On the SiO2 surface, the template was removed in the subsequent treatment, and the corresponding hollow sphere material was successfully prepared. The method can control the particle size and shell thickness of the target hollow sphere material by adjusting the particle size of SiO2 microspheres, and is suitable for the preparation of non-metal oxide hollow spheres and metal hollow sphere materials.

(2) Using polymer as a template

Disperse the colloidal template in a solvent, add products or precursor substances to the dispersed system, metal ions are adsorbed on the surface of the template through chemical bonds or electrostatic interactions, and then form a core-shell structure, which can be desorbed by roasting or a suitable organic solvent. After removing the template, the corresponding hollow sphere material can be obtained. The principle of this method is relatively simple, easy to operate, and highly repeatable. It is a method commonly used to prepare hollow sphere materials at present. Various inorganic hollow sphere materials such as CdS, Fe3O4, TiO2, and CuO have been successfully prepared. In this method, common templates mainly include polystyrene balls (PSt), styrene-methacrylic acid copolymer (PSMA), polymethyl methacrylate (PMMA), styrene-acrylic acid copolymer (PSA), etc. . Zhao et al. prepared ZnS hollow spheres using styrene-methyl methacrylate copolymer (PSMA) as a template. Figure 1.9 is a schematic diagram of the preparation of ZnS hollow spheres. It can be seen from the figure that the surface of PSMA is negatively charged, and Zn2+ is adsorbed on the surface of PSMA spheres through electrostatic interaction. In this experiment, Y-ray irradiation can promote the decomposition of thioacetamide (TAA). S2-, the generated S2- will be further adsorbed on Zn2+, and the template can be removed at high temperature to obtain ZnS hollow spheres.

(3) Carbon microspheres as templates

Since the discovery of fullerenes and carbon nanotubes, there have been more and more studies on carbon materials, and the preparation methods of carbon materials with different structures have continuously surfaced. In the 1960s, pitch in the formation of coke During the calcination process, the mesophase will form a spherical mesophase. Therefore, a comprehensive study on the properties of the mesophase has been carried out, and it is found that the mesophase carbon microspheres have properties that many other carbon materials do not have, making them widely used. For lithium battery negative electrode materials, catalyst carriers, hollow sphere material templates, etc., the current preparation methods of carbon microspheres mainly include hydrothermal method, chemical vapor deposition method, reduction method, template method, high temperature pyrolysis method, ultrasonic method, etc. Among them, the hydrothermal method has the advantages of simple operation, good product dispersibility, and high purity, and is widely used. Glucose, fructose, xylose, starch, and cellulose are often used as biomass raw materials.

1.2 Preparation of hollow sphere materials by soft template method

Due to their special structures, surfactants and amphiphilic block copolymers can form ordered aggregates in solution, such as micelles, reverse micelles, vesicles, droplets, etc. These ordered structures can be hollow spherical materials The formation of metal ions provides a good environment, and metal ions can be adsorbed on its surface through precipitation reaction or polymerization reaction to form a shell structure. Micelle method, vesicle method, emulsion droplet template method and other methods are often used to prepare hollow sphere materials.

(1) Emulsion droplets used as templates to prepare hollow sphere materials

The basic process of using emulsion droplets to prepare alumina hollow spheres is as follows: adding reactant precursors to a system composed of water, surfactants and their oil phases, and the precursors undergo chemical reactions at the water-oil interface to obtain target products. The key to preparing hollow spherical materials from emulsion droplets is to obtain stable droplets. It is very important to choose suitable surfactants and solvents, and the solvents cannot dissolve each other. At present, hollow spherical materials such as Cu2O, SiO2, CuS and TiO2 have been successfully synthesized by this method. Jiang et al. synthesized CuS hollow sphere materials by the emulsion droplet method. In this article, copper naphthenate and thioacetamide were used as raw materials, and n-butanol and sodium dodecyl sulfate were added to deionized water to form water /oil phase, first dissolve sodium naphthenate in the oil to form a blue solution, add a certain amount of thioacetamide to the solution, add thioacetamide and copper naphthenate in the droplet The interface reacted to generate CuS, and after a period of reaction, the obtained product was alternately washed with deionized water and absolute ethanol. That is, the CuS hollow sphere material can be obtained.

(2) Preparation of hollow sphere materials by micellar method

Due to the special amphiphilic properties of ionic surfactants, when their concentration exceeds the critical micelle concentration (CMC), they will spontaneously aggregate to form micelles, which can provide templates for the preparation of hollow sphere materials, block copolymers It is composed of blocks of different components, and the difference in solubility of block components can promote self-assembly to form core-shell micelles, which can provide templates for the synthesis of hollow sphere materials. Ma et al. prepared ZnS hollow spheres using micelles formed by nonionic triblock copolymers as templates. Qi et al. used a composite micelle (PEO-block-PMMA-SDS) formed by oxyethylene-methacrylic acid block copolymer (PEO-block-PMAA) and sodium dodecyl sulfate (SDS) as a template to synthesize CaCO3 and Ag hollow spheres. By controlling the concentration of SDS, SDS first forms micelles in the solution, and the hydrophilic end of PEO-block-PMAA, that is, the PEO end, will dissolve in SDS to form a nucleus, and PMAA will stay outside the micelles formed by SDS To form a shell, since PMAA is negatively charged, metal ions can be adsorbed on the surface of PMAA through electrostatic interaction, and then cover the entire micelle.

(3) Preparation of hollow sphere materials using vesicles as templates

At present, there are more and more reports about the preparation of hollow sphere materials using vesicles formed by surfactants as templates. There are two main synthetic routes: one reactant reacts directly in the bilayer membrane of vesicles The shell is formed; the other is to deposit metal ions on the outer wall of the vesicle through chemical reactions such as sol-gel method or precipitation method. Hentze et al. used vesicles formed by cetyltrimethylammonium bromide or sodium dodecylbenzenesulfonate and sodium perfluorooctanoate as a template, and tetramethoxysilane would hydrolyze and coat the vesicles under acidic conditions. Bubble the surface to get SiO2 hollow spheres with a particle size of 60-120nm. Chen et al. synthesized organic-inorganic doped hollow spheres using vesicles formed by polyethylene glycol-polymethyloxysilane amphiphilic block copolymers in a 1:1 mixed solvent of methyl and methanol as templates. Through the action of the catalyst, the reactant forms a sol-gel method on the surface of the vesicle wall to cover the surface of the vesicle.

1.3 Preparation of hollow sphere materials by combining hard and soft templates

The hard template method can adjust the size of the inner cavity of the hollow sphere by controlling the size of the template, and the prepared particle size is relatively uniform. However, the preparation process is cumbersome and requires high temperature heating or organic solvents to remove the template; the soft template prepares the hollow sphere The material does not need a template to come out, and the hollow sphere material can be obtained in one step. The preparation process is relatively simple. However, the uniformity of the shape of the hollow sphere material prepared by the soft template method needs to be improved, and the use of organic solvents will pollute the environment. And, The yield of the soft template method is low, and it is not suitable for industrial production. At present, some people use the combination of hard template and soft template to prepare hollow sphere materials.

1.4 Preparation of hollow sphere materials by self-assembly method

Although the size of the inner cavity of the hollow sphere can be adjusted by controlling the size of the template in the preparation process of the hard template method, the wall thickness is difficult to control. Common preparation methods have the problem of uncontrollable wall thickness. Caruso et al. It is proposed to use the L-b-L self-assembly method to prepare hollow spheres. This method uses latex particles formed by high molecular polymers as a template, and polyelectrolytes and shell material precursors with opposite charges are coated on the surface of the template layer by layer through electrostatic interaction. Layered shell structure. The hollow sphere material can be obtained by removing the polyelectrolyte and the template. The experiment can be divided into three steps, one step: depositing the oppositely charged polyelectrolyte (positively charged) on the surface of latex particles (process 1); the second step: the nanoparticles (negatively charged) are further charged by electrostatic interaction Adsorbed on the above-mentioned polyelectrolyte surface deposited on the latex particle surface; the third step: repeat the above process to obtain a multilayer hollow sphere material, and repeat steps 1 and 2 to obtain the core. Nanoparticle/polymer shell composite material; the fourth step : The hollow sphere material can be obtained by calcination or solvent removal. The key point in the above process is that each adsorption of polyelectrolytes or nanoparticles will replace the surface charge and promote the next adsorption. When each adsorption is completed, it is necessary to remove the polyelectrolyte or nanoparticles that are not adsorbed on the particle surface. This method can not only control the size of the inner cavity of the hollow sphere by changing the size of the template, but also adjust the wall thickness of the hollow sphere by controlling the number of cycles of the above operations. The prepared hollow sphere has the advantages of uniform particle size and good dispersion.

1.5 Preparation of hollow sphere materials by spray drying method

The spray drying method sprays the precursor solution into a fine mist through a spray device and then enters the reactor with a high-temperature atmosphere. At high temperature, the solvent will evaporate quickly, and the metal salt will undergo chemical reactions such as thermal decomposition or combustion, and precipitate to form a hollow. ball. . The advantage of this method is that the morphology of the product can be adjusted by controlling the air flow pattern, atomization conditions, temperature and humidity of the reactor, etc. The method combines many advantages of the liquid phase method and the gas phase method, and is convenient for continuous operation and large-scale production. The schematic diagram of the preparation of hollow sphere materials by spray drying method is shown in Figure 1.12. At present, SiO2, SiO2/γ-Fe2O3, and TiO2 hollow sphere materials have been successfully synthesized by this method.