The concept of ultrafine Alumina was put forward in the 1960s for distinguish "ordinary alumina powder" produced by the traditional Bayer process.The difference between the two lies in: the former by artificial synthesis, its purity is betweem  99.9%~99.999% (3-5N) , the average particle size is  below tens of microns, specially used in a variety of artificial crystals (YAG), artificial gems, precision electronic components, LCD substrate and other high-tech fields;The latter is a natural mineral bauxite, using powder metallurgy method production, purity is generally less than 99.9% (3N), it is mainly used in metallurgy, refractory materials, chemical industry, traditional ceramics and other industrial fields.The known preparation methods of ultrafine alumina powder are as follows:

1. Gas phase method

The process of condensing a gaseous material into a nano powder by means of a physical or chemical reaction in the gaseous state, followed by rapid cooling.

1.1 Evaporation and condensation

The alumina is heated and vaporized in an inert gas, and then cooled and condensed in an inert gas to form ultrafine particles.According to the different heating sources can be divided into plasma heating, electron beam heating, arc heating and laser heating.

1.2 Gas phase hydrolysis method

It is also known as flame hydrolysis method and pyrogenesis method.In this method, aluminum salt is hydrolyzed at high temperature in hydrogen and oxygen flame, and ultrafine particles are precipitated in the gas phase.

Gas phase method can be used to produce powders with no agglomeration or less agglomeration by controlling the gas flow rate, but its disadvantages are large energy consumption and complex equipment.

2, solid phase method

2.1 Mechanical crushing method

It is a method of using the mechanical force exerted on the material by crushing equipment to make the material undergo structural change, chemical change and physical and chemical change.The principle is to use the interaction between the grinding medium and the raw material to eventually make the raw material broken.The most widely used ultrafine grinding equipment is ball mill, high energy ball mill, planetary mill, tower mill and airflow mill.

Its advantages are simple equipment requirements, low production cost and high output;The disadvantage is that the treated alumina powder contains a certain amount of impurities, which reduces its performance. At the same time, the particle size range is relatively wide. The sharp decrease of the surface area will lead to the agglomeration of the powder.

2.2 Solid state reaction method

It is a method of fully mixing aluminum salt or aluminum ammonium with a certain molar ratio, then grinding and activating it, and finally calcining it, and directly obtaining nano powder or re-crushing nano powder through solid-phase interface reaction.

Its advantages are high product and high reaction purity. Its disadvantages are: the temperature required for the mutual reaction of solid substances is high, and the required gas pressure is large, which requires high requirements on the equipment.Large particle size range, etc.

2.3 the combustion method

The combustion method of aluminum powder is to prepare ultrafine alumina powder by burning aluminum powder with particle size less than 40 microns in the flame of oxygen and propane.

2.4 Amorphous crystallization method

Amorphous crystallization method is to prepare amorphous combined state aluminum, and then after annealing treatment to make amorphous crystallization.With this method, nanomaterials with accurate composition can be produced, and nanometer alumina can be prepared directly from amorphous state without molding.

3, liquid phase method

3.1 precipitation

It is a method of adding appropriate precipitant to aluminum salt solution to get precursor precipitates, and then preparing corresponding ultrafine powders by filtration, drying and calcination.

The alumina powder produced by precipitation method has not only excellent performance indexes, but also simple preparation technology, low equipment and raw material cost. It is one of the most commonly used methods for preparing alumina powder in modern industrial production.However, the existing problem is that the products in each link of the preparation are prone to agglomeration phenomenon, which will seriously affect the product performance indicators.

3.2 Sol-gel method

The precursors used are inorganic salts or metal alkyl salts. The main reaction steps are that the precursors are dissolved in solvents (water or organic solvents) to form a uniform solution. The solute and the solvent produce hydrolysis or alcoholysis reaction to produce sol, which is converted into gel after drying.

Al2O3 prepared by sol-gel method has high chemical homogeneity, easy to uniformly doping trace elements, easy to carry out chemical reaction, and narrow particle size range of synthesized powder.Disadvantages: high price of raw materials and some of them are harmful to health;Long process engineering time.

3.3 hydrothermal method

A method for preparing powders by creating a high temperature and high pressure hydrothermal condition in a sealed system with water as a solvent.The powders prepared by this method have good properties, small grain size, narrow particle size distribution and low degree of agglomeration.

This method creates a special physical and chemical environment, the obtained powder morphology and particle size can be controlled, but in the closed condition of chemical reaction, to mass production needs to have high temperature and high pressure resistant equipment, virtually will bring certain risks to the production.

3.4 Microemulsion method

The required powder is obtained by chemical reactions in microemulsion droplets to form solids.In order to obtain monodisperse nanoparticles it is necessary to adjust the water volume and reaction concentration in microemulsion to control the nucleation and growth of particles.

Microemulsion method features: 1) easy to control the size of the core of microemulsion to control the particle size of the product;2) can add surfactant to modify the particle surface to get functional nano powder;3) The particles can be coated by surfactant, which promotes the repulsion between particles and is not easy to produce agglomeration;4) The surface layer of particles can be replaced by specific organic groups to synthesize functional nanomaterials;5) By coating the particles, the interface properties of the material are improved, and finally the optical and electrorheological properties of the material are changed.Its main disadvantage: its low production rate makes it difficult to carry out industrial production.

3.5 Solution evaporation method

The solution was made into small droplets and evaporated quickly to minimize the segregation of components, and then heated and decomposed to prepare nano powders.Spray method is a typical solution evaporation method, the method refers to the use of high pressure spray gun will precipitate into the high temperature kettle and aluminum ions quickly react to produce nano alumina method.

The advantage of this method is that the drying time is short and the whole process is usually completed within a few seconds to tens of seconds. Therefore, each multi-component fine droplet has no time to segregate in the reaction process, so as to obtain the ultra-fine powder material with uniform composition.