Brief Introduction of Nanometer Silica Materials

1. physical method

physical method refers to the mechanical crushing method we often say. It is generally pulverized aggregates of SiO2 by a high-energy ball mill or a super jet mill, thereby obtaining an ultrafine product, and the particle size of the obtained product is generally 1 to 5 μm. Although the physical method has a simple process, it is easy to mix impurities, it is difficult to control its powder characteristics, it has a low pulverizing efficiency, and the particle size distribution prepared by it is wide.

2. Gas phase method

Silica prepared by gas phase method is an amorphous, loose, pollution-free and non-toxic white inorganic nonmetallic oxide. SiO2 prepared by this method not only has small particle size and average particle size distribution between 0.30-10μm, but also has very high purity and good dispersion, and its specific surface area is between 50-380m2/g. Because of its nano effect, gas phase SiO2 has good anti-flow, thixotropy, extinction, thickening, reinforcement, insulation and other properties in the material, and has a wide range of applications in the field of sealant, adhesive, coating, plastic, rubber and other polymer.

In the production process, gas-phase SiO2 is not only costly, but also consumes a lot of energy. Its main raw materials are generally organic silicon or inorganic silicon halides, which are produced by high-temperature hydrolysis in water generated by hydrogen-oxygen flame. At present, the world mainly uses or the mixture of the two as raw materials, and the chemical reaction formula for its preparation is as follows:

3. Precipitation method

SiO2 is mainly prepared by reacting sodium silicate (water glass) in sulfuric acid or hydrochloric acid:

SiO2 is prepared by precipitation method. Although it has cheap and extensive raw materials, it is difficult to control its pore size and shape, and the prepared nano-SiO2 has a wide pore size distribution, so its products are mainly used as rubber reinforcement and applied in industrial production. The precipitation method of preparing SiO2 not only has a large water absorption, compared with the gas phase method, its heat resistance and electrical properties are also weak, and when the extrusion molding is easily accompanied by foaming phenomenon, making hot air difficult to vulcanize, compared with the gas phase method, the reinforcement effect is poor. The specific surface area of the SiO2 prepared by this method is generally 200 m2/g, and the average particle size is generally distributed between 2.0 and 25 μm. The precipitation method mainly includes the following technologies: ① carbonating water glass to prepare SiO2;② mixing alkali metal silicate and inorganic acid to prepare SiO2 hydrosol, and subjecting the gel particles to a series of treatments (drying, washing, drying and calcination) to obtain SiO2;③ using organic solvent SiO2, which has high dispersibility; ④ using acidifier to react with silicate aqueous solution to obtain its sediment, which is separated and dried, thus, SiO2 was prepared. ⑤ Non-spherical SiO2 was prepared using a spray granulation technique, which had smooth edges.

4. Sol-gel method

Sol-gel method is the most important method for preparing SiO2 materials and has unique characteristics. The sol-gel method generally uses metal alkoxides or inorganic salts as raw materials, adds a certain catalyst to hydrolyze and polycondensate, thus forming a gel, and then the gel is aged, dried and other post-treatment to produce materials with different needs. Preparation of nano-SiO2 by sol-gel method, different catalyst types (acid or alkali), different types of silicate, different types of alcohol, different reactant concentrations and reaction temperature have different effects on the particle size, by regulating these factors can be prepared with different structures of nano-materials.

5. elemental silicon method

The method of making elemental silicon react under alkaline conditions, adjusting the pH of the product, and preparing its aqueous dispersion after suction filtration is called elemental silicon method. The reaction formula is as follows:

Si + OH-+ H2O → SiO32-+ H2 ↑(1-4)

The prepared product w(SiO2)= 25%-30%, and in its aqueous dispersion (pH = 9), the particle size of nano SiO2 particles is 8-15nm. After a series of characterization analysis, it can be seen that the preparation of nano SiO2 is a stable amorphous material, nano SiO2 will be agglomerated, so in order to use it, it must be modified.