"Metallic silicon" (also called industrial silicon in my country) is a commodity name that appeared in the mid-1960s. Its emergence is related to the rise of the semiconductor industry. The international common practice is to divide commodity silicon into metal silicon and semiconductor silicon. Silicon metal is a product smelted by quartz and coke in an electric heating furnace. The content of the main component silicon is about 98% (the content of Si content of 99.99% is also included in the metal silicon), and the remaining impurities are iron, aluminum, calcium, etc. . Semiconductor silicon High-purity metal silicon used to make semiconductor devices. It is sold in the form of polycrystalline and single crystal, the former is cheap and the latter is expensive. It is divided into various specifications due to different uses. According to statistics, in 1985, the world consumed about 500,000 tons of metal silicon, of which about 60% was used for aluminum alloys, less than 30% was used for organic silicon, about 3% was used for semiconductors, and the rest was used for Steel smelting and precision ceramics, etc.
The classification of metal silicon is usually classified according to the content of three main impurities of iron, aluminum and calcium contained in the metal silicon component. According to the content of iron, aluminum and calcium in metal silicon, metal silicon can be divided into 553, 441, 411, 421, 3303, 3305, 2202, 2502, 1501, 1101 and other different grades.
Industrially, metallic silicon is usually prepared by reducing silica with carbon in an electric furnace. Chemical reaction equation: SiO2 + 2C → Si + 2CO The purity of silicon obtained in this way is 97~98%, which is called metal silicon. It is then melted and recrystallized, and impurities are removed with acid to obtain metallic silicon with a purity of 99.7~99.8%.
The composition of metal silicon is mainly silicon, so it has similar properties to silicon. Silicon has two allotropes, amorphous silicon and crystalline silicon. Amorphous silicon is a gray-black powder that is actually a microcrystal. Crystalline silicon has the crystal structure and semiconductor properties of diamond, the melting point is 1410°C, the boiling point is 2355°C, the Mohs hardness is 7, and it is brittle. Amorphous silicon is chemically active and can burn violently in oxygen. It reacts with non-metals such as halogen, nitrogen and carbon at high temperature, and can also interact with metals such as magnesium, calcium and iron to form silicides. Amorphous silicon is almost insoluble in all inorganic and organic acids including hydrofluoric acid, but is soluble in mixed acids of nitric acid and hydrofluoric acid. Concentrated sodium hydroxide solution can dissolve amorphous silicon and release hydrogen. Crystalline silicon is relatively inactive, it does not combine with oxygen even at high temperature, it is not soluble in any inorganic acid and organic acid, but it is soluble in mixed acids of nitric acid and hydrofluoric acid and concentrated sodium hydroxide solution