1 Gas phase method
(1) Chemical vapor deposition technology
The high purity ultrafine alumina particles are synthesized by gas phase pyrolysis or chemical reaction, mainly using metal elemental, halide, hydride or organic compound as raw material. Among them, chemical vapor deposition technology is widely used. For example, Italian researchers used alkyl aluminum and N2O with high vapor pressure at room temperature as reactants, added ethylene as a reaction accelerator, and successfully synthesized spherical α-Al2O3 particles with a particle size of 15-20nm by CO2 laser heating.
(2) Laser induced vapor deposition technology
Laser-induced vapor deposition uses a laser filled with neon, xenon, and HCl to provide energy, producing a laser of a specific frequency that is focused on a rotating aluminum target, melting it and producing alumina vapor, which is cooled to produce ultra-fine alumina powders. The method has high heating and cooling speed, uniform particle size distribution and small reaction pollution.
(3) Plasma vapor synthesis technology
Plasma vapor synthesis techniques include high frequency plasma method, DC arc plasma method and composite plasma method. The high frequency plasma method has low energy efficiency and poor product stability. The DC arc plasma method uses the high temperature produced by the arc to vaporize or melt the electrode in the ionization process of the reaction gas. Combined with the first two methods, the composite plasma method does not require electrodes, the product purity is high, and the production efficiency and system stability are improved.
(4) Inert gas condensation and in-situ pressure technology
The technology is usually filled with low pressure inert gas in the vacuum evaporation chamber, and the raw material is vaporized or formed by heating, and the energy is lost by colliding with the inert gas atoms, and then the rapid cooling condenses into ultra-fine powder. However, this method has high cost and is not suitable for large-scale industrial production. The gas phase method has the advantages of controllable reaction conditions and easy refining of products. By controlling the reaction gas and the degree of rarefaction, ultrafine powders with little or no agglomeration can be obtained, with good particle dispersion, small particle size and narrow distribution. However, this method requires the raw material to be completely vaporized before the reaction, consumes a lot of energy for the products with high melting point, and requires a lot of inert gases in the reaction, resulting in low production efficiency, difficult process control, difficult powder collection, and high equipment requirements, which is not conducive to large-scale industrial production.
2 Solid phase method
Solid phase method is a common method to prepare α-Al2O3 powder, simple process, large yield, low cost, easy to achieve industrial production. However, its energy consumption is high, the efficiency is low, the prepared powder particles are not uniform, the shape and function are limited by the process, and it is difficult to obtain fine and high-purity α-Al2O3 powder. At present, the solid phase method is mainly divided into mechanical crushing method, amorphous crystallization method and pyrolysis method.
(1) mechanical crushing method
Mechanical crushing method uses ball mill, planetary mill, air mill and other equipment to directly crush raw materials into ultrafine powder. Ball mill is used more, through vibration and rotation to provide energy for raw materials, so that it is crushed into fine particles under the impact of hard balls. For example, the researchers prepared α-Al2O3 powders with particle size of 18~40nm by ball milling submicron α-Al2O3 powders. The method is simple in principle and easy to operate, but it is easy to introduce impurities, which affect the purity of the product, and the prepared alumina powder has shortcomings in particle size distribution and morphology.
(2) Amorphous crystallization method
Amorphous aluminum compounds are first prepared by amorphous crystallization process and then converted to crystalline state by annealing treatment. The amorphous state is thermodynamically unstable and tends to be transformed into a crystalline state. Crystallization occurs under the conditions of heat or radiation. Crystalline alumina can be obtained by controlling the conditions.
(3) Pyrolysis method
The α-Al2O3 powder was prepared by thermal decomposition reaction of aluminum salt. The common pyrolysis methods are ammonium aluminum sulfate pyrolysis and ammonium aluminum carbonate pyrolysis. The pyrolysis of ammonium aluminum sulfate will produce waste gas, pollute the environment, and the process is complicated. Ammonium aluminum carbonate pyrolysis pollution is small, more suitable for industrial production, but the calcination temperature is high, energy consumption is large, equipment requirements are high. In addition, ultrafine α-Al2O3 powder can also be directly obtained by rapidly burning aluminum powder at high temperatures, such as the mixture of aluminum nitrate, urea and a small amount of dextrin is ignited in a Muffle furnace to obtain a foamlike white alumina powder. However, the equipment of this method is complicated, the risk is high, the powder collection is difficult, and the application prospect is limited.
3 Liquid phase method
Liquid phase method is one of the most widely used methods, the basic principle is to select a suitable soluble aluminum salt to prepare a solution, and then through the precipitation agent or evaporation, sublimation, hydrolysis and other means to make the metal particles uniformly precipitate, dry dehydration to obtain ultrafine powder. Compared with solid phase method, its advantages include: precise control of chemical composition; The shape and size of nanoparticles are easy to control, and the dispersion is good. Product particle shape, particle size is easy to control; Easy to add trace active ingredients for regulation; The obtained products have good surface activity. The preparation of homogeneous and high purity oxide ultrafine powder is especially suitable for liquid phase method, including precipitation method, sol-gel method, microemulsion reaction method, hydrothermal method, etc.
(1) precipitation method
In the precipitation method, a precipitating agent is added to the metal salt solution to obtain the precipitation, and then the ultrafine powder is obtained by filtration, washing, drying and calcining. It is divided into direct precipitation method, co-precipitation method and uniform precipitation method. The operation is simple, the process is short, the cost is low, but the particle size is not easy to control. The properties of the prepared ultrafine powder are related to the mixing method, adding speed, order, concentration and pH value of the reactants. In recent years, the main precipitation methods for preparing ultrafine alumina powders are aluminum nitrate + ammonium carbonate system, aluminum ammonium sulfate + ammonium bicarbonate system and inorganic salt + urea homogeneous precipitation system.
(2) Sol-gel method
Sol-gel method is a common method for preparing nanomaterials and ultrafine alumina. The aluminum alkoxide is dissolved in organic solvent, the alkoxide brine is decomposed and polymerized to form sol after distillation, and then the gel is formed by adding water. After drying, the ultrafine alumina powder is calcined at high temperature. The synthesis temperature is low, the process is simple and easy to control, the equipment is simple, and the prepared alumina powder has high purity, good chemical uniformity and good dispersion. However, if aluminum salt is used as raw material, corrosive gas may be produced in the process of reaction and calcination, polluting the environment; If organic alkoxide is used as raw material, the preparation cost is high, and the activity is strong and not easy to store.
(3) Microemulsion reaction method
Microemulsion reaction method makes Al3+ dissolve in water to form a tiny water nucleus, surrounded by surfactant and oil phase, so that the nucleation, growth, agglomeration, agglomeration and other processes of alumina are limited to small spherical droplets to avoid agglomeration between particles. The key lies in the formation of stable W/O microemulsion and the selection of appropriate reaction conditions. Surfactant selection and reactant concentration are important factors to control alumina particle size. The alumina prepared by this method has good dispersion and uniform structure, but it needs to add surfactants and requires high reagent.
(4) hydrothermal synthesis method
Hydrothermal method In a sealed reaction container, with water or organic solvents as the medium, heating to create a high temperature and high pressure environment, so that insoluble or insoluble substances dissolve or recrystallize, or the mixture of chemical reactions to prepare materials. This method is an excellent method for preparing ceramic powders. The prepared ultrafine powders have good grain development, uniform particle size distribution, small size, no agglomeration, no calcination and good dispersion. However, the temperature of water heat treatment needs to be higher than 450℃, which requires high performance of autoclave and is difficult to achieve. Ultrafine alumina powder can be prepared by adding crystal seeds, using organic solvents or other substances, and hydrothermal salt solution pressure relief method. The nodular liquid phase method is the best choice for preparing α-Al2O3 powder, and is also the most widely used method in industry. The product prepared by liquid phase method is easy to be accurately controlled, and has significant advantages over the other two methods, and is suitable for preparing high-purity and ultra-fine α-Al2O3 powders.