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Basic properties of silicon dioxide and synthetic method
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Huayuan Wollastonite, Xinyu Wollastonite, Jiangxi Wollastonite
Silicon dioxide (chemical formula: SiO2) is an acidic oxide, and the corresponding hydrate is silicic acid (H2SiO3). Silicon dioxide is one of the most important compounds of silicon. The natural silica existing on the earth accounts for about 12% of the crustal mass. Its existence forms are crystalline and amorphous, which are collectively called silica.
 
Basic properties
 
Physical properties
 
Hafnium dioxide is also called silica, with the chemical formula SiO₂. There are two kinds of crystalline silica and amorphous silica in nature.
 
Crystalline silica is divided into quartz, scaly quartz and cristobalite due to different crystal structures. Pure quartz is a colorless crystal. Large, transparent prismatic quartz is called crystal. If the crystals containing trace impurities have different colors, there are amethyst, tea crystal, ink crystal and so on. Ordinary sand is small quartz crystals, including yellow sand (more iron impurities) and white sand (less impurities, more pure). In silica crystals, the four valence electrons of the silicon atom and the four oxygen atoms form four covalent bonds. The silicon atom is located at the center of the tetrahedron, and the four oxygen atoms are located at the four top corners of the tetrahedron. Many Each of these tetrahedrons is connected by oxygen atoms at the top corners. Each oxygen atom is shared by two tetrahedrons, that is, each oxygen atom is combined with two silicon atoms. SiO₂ is the simplest form of the composition, and only the ratio of the number of silicon and oxygen atoms in the silicon dioxide crystal. Silica is an atomic crystal.
 
The bond energy of Si—O bond in SiO₂ is very high, and the melting point and boiling point are high (melting point 1723 ° C, boiling point 2230 ° C). The refractive index is approximately 1.6.
 
The refractive index of various silica products is: 1.547 for quartz sand; 1.544 for quartz powder; 1.542 for vein quartz; 1.42 to 1.48 for diatomaceous earth; 1.46 for fumed silica; and 1.46 for precipitated silica.
 
The diatomaceous earth existing in nature is amorphous silicon dioxide, which is the remains of diatoms of lower aquatic plants. It is a white solid or powder, porous, light and soft solid with strong adsorption.
 
Chemical properties
 
Chemical properties are relatively stable. Insoluble in water and does not react with water. It is an acid oxide and does not react with ordinary acids. Gaseous hydrogen fluoride reacts with silicon dioxide to form gaseous silicon tetrafluoride. Reacts with hot, strong alkali solution or molten alkali to form silicates and water. It reacts with a variety of metal oxides at high temperatures to form silicates. Used in the manufacture of quartz glass, optical instruments, chemical utensils, ordinary glass, refractory materials, optical fibers, ceramics, etc. Silica is inert in nature. It does not interact with halogens other than fluorine and hydrogen fluoride, hydrogen halides, sulfuric acid, nitric acid, and perchloric acid (except hot concentrated phosphoric acid). Common concentrated phosphoric acid (or pyrophosphate) can corrode silica at high temperatures to form heteropolyacids [2]. Molten borate or boric anhydride can also corrode silica at high temperatures. In view of this property, boric acid Salt can be used as a flux in the firing of ceramics. In addition, hydrogen fluoride can also be an acid that can dissolve silicon dioxide to generate water-soluble fluorosilicic acid: SiO₂ + 4HF = SiF4 ↑ + 2H₂O
 
  resolve resolution
 
[5] 1. Ion exchange method: The diluted water glass is filtered to remove impurities and then cation exchanged, anion exchanged, pH adjusted, and concentrated by evaporation or ultrafiltration to obtain a silica sol.
 
2. Silicon powder method: Distilled water and sodium hydroxide (reagent grade) are added to the reaction tank, the temperature is raised to 65 ° C, a certain amount of ammonia water is added to adjust the alkalinity, and the silicon powder is added in batches under stirring, and the temperature is controlled below 8.3 ° C After the silicon powder is added, stirring is continued for 2 to 3 hours. When the pH value drops to 9 to 10, sampling and analysis are performed. After the reaction is completed, the mixture is cooled to 50 ° C under stirring and naturally filtered to obtain a silica sol product. Unreacted silicon powder can be recycled.
 
3. Sulfuric acid method: The diluted water glass is mixed with dilute sulfuric acid at 20-30 ° C to prepare a silicone gel, which is aged for 4 hours, soaked with 2% -2.5% dilute sulfuric acid for 1 hour, and washed with water at 40-50 ° C , Dehydrated in a drum dryer to a moisture content of ≤80%, and dried to a moisture content of ≤10%, activated in an activation furnace at 500-550 ° C, and obtained coarse-pored microsphere silica gel after screening.
 
4. When smelting pig iron, silicon dioxide is contained in the waste residue.
 
In microelectronics process, SiO 2 film is widely used because of its superior electrical insulation and process feasibility. In semiconductor devices, it can be used as a thin film light absorption layer of amorphous silicon solar cells to improve the light absorption efficiency by using the variable band gap width of SiO 2. It can also be used as a metal 2 nitride 2 oxide 2 semiconductor (MN SO Charge storage layers in memory devices, CMOS devices and SiGeMOS devices in integrated circuits, and gate dielectric layers in thin film transistors (TFTs). In addition, with the increase in the integration of large-scale integrated circuit devices, multi-layer wiring technology has become increasingly important. For example, the intermediate dielectric layer of logic devices will increase to 4-5 layers, which requires reducing the parasitic capacitance brought by the dielectric layer. . In view of this, many researchers have conducted in-depth research on the types, preparation methods and properties of low dielectric constant dielectric films. The requirements for new low dielectric constant dielectric materials are: low loss and low power consumption in terms of electrical properties; high adhesion and high hardness in terms of mechanical properties; corrosion resistance and low water absorption in terms of chemical properties; In terms of performance, it has high stability and low shrinkage. SiO 2, which is commonly used for preparing dielectric layers, has a dielectric constant of about 4.0, and has good mechanical properties. For example, it is used to passivate the die plane and mesa of a silicon high-power bipolar transistor, improve or maintain the breakdown voltage of the die, and improve the stability of the transistor. This technology completely achieves the purpose of protecting the passivation device, making the device's performance stable and reliable, reducing external pollution and interference to the chip, and improving the reliable performance of the device.
 
In the late 1980s, the research on Si-based SiO₂ optical waveguide passive and active devices has made great progress, making these devices not only have excellent conduction characteristics, but also have basic functions such as light amplification, light emission and electro-optic modulation. It has great application prospects in optical integration and optoelectronic integration devices, and can be used as waveguide film, anti-reflection film and anti-reflection film. With the rapid development of optical communication and integrated optics research, glass film optical waveguides are widely used in optical passive devices and integrated optical circuits. It is important to prepare films with good properties for use as optical waveguides. General requirements for optical waveguides in integrated optical circuits: single-mode transmission, low transmission loss, and high coupling efficiency with optical fibers. Waveguide loss sources are mainly divided into three parts: material absorption, substrate loss, and scattering loss. By using a high-roughness, flat optical glass sheet or an ordinary glass substrate sputtered with a sufficiently thick SiO₂ film in advance, the instantaneous field distribution of the waveguide mode is kept away from the rough surface to reduce the substrate loss. Research on antireflection films for lasers has also made great progress. The sol-gel method of the China Academy of Engineering Physics and the Institute of Chemistry successfully developed an ultraviolet laser SiO2 anti-reflection film. The results show that the porous SiO2 film prepared by the immersion coating method has better anti-reflection effect than the SiO₂ film prepared by the earlier vacuum evaporation and spin coating methods. The transmittance at the wavelength of 350nm reaches more than 98%, and the highest transmittance in the ultraviolet region reaches more than 99%. This SiO₂ film is expected to be used as an antireflection film for light-transmitting elements for inertial confinement fusion (ICF) and X-ray laser research. At present, some progress has also been made in the preparation of protective films and anti-reflection films by the sol-gel process. The SiO₂ optical film prepared by this method has become an important means in laser devices for inertial confinement fusion, and is widely used in antireflection optical elements, such as spatial filters, windows, target chamber windows, or target lenses. Protective and anti-reflection coatings are deposited on the KDP crystal of the harmonic conversion element by a sol process, which can improve the working conditions of the KDP crystal, improve the quality of the harmonic beam and focusable power.