Silicon dioxide (silica), SiO2, is a naturally occurring substance comprising 12% of the earth's crust. Known since antiquity, it occurs most frequently in nature as the crystal quartz; however, other natural and artificial forms are known. The varied properties of silica allow it to be applied to a diverse array of fields.
SiO2 is vital to microelectronics fabrication. It is used in IC manufacture as a mask to control the diffusion of dopants (impurities that alter electrical conductivity) throughout the IC's silicon wafer. Its ability to inhibit the diffusion of select impurities makes it ideal for this purpose. SiO2 also serves as a protective insulator for IC devices: it prevents chemical contamination of the wafer, and separates different electrical paths on the wafer. Furthermore, colloidal silicas are used in the polishing of silicon wafers.
The piezoelectric property of quartz (i.e., its ability to convert mechanical energy into electrical energy, and vice versa), combined with its chemical stability (it is completely oxidized, and almost insoluble in water) and abundance, makes it indispensable to the electronics industry. The quartz crystal resonator (QCR) is a widely used piezoelectric device. Radio and TV broadcasting, and mobile radio communications (from air/ground vehicles) use QCRs for stabilizing radio transmitter frequencies. The telecommunications industry uses QCRs for electromagnetic wave filters that allow for carrier frequency separation in telephone systems. QCRs are used in radio receivers for selecting desired frequencies. Quartz-based sonic and ultrasonic transducers are used when the generation or detection of high-frequency vibrations is required (e.g., underwater ranging equipment--sonar). Applied to clocks, quartz resonators have allowed for inexpensive and accurate time-keeping over small periods, and find themselves in a variety of electronics equipment.
In the laboratory, silica has many important applications. Silica glass, a non-crystalline, supercooled liquid, is used for making scientific apparatus where resistance to thermal shock/expansion, low electrical conductivity, and transparency to UV light are necessary. Fibers of vitreous (supercooled) silica are essential for precision equipment such as balances, galvanometers, and gravimeters, The enantiomorphism of quartz crystals (i.e., the ability to rotate the plane of polarization of light) is used in instruments such as monochrometers. Also, quartz crystals are used as prisms in spectroscopes.
Quartz is used in refractory (heat-resistant) materials, such as insulation, firebrick, foundry molds, and electrical insulators because of its high melting point, low thermal expansion, and chemical inertness. Quartz is very hard (7 on the Mohs scale), and resists fracture as it lacks easy cleavage. Hence, it is used as an abrasive, in sandblasting, in polishing and cutting materials, and to provide traction on streets. Sand--an essential part of concrete and mortar--is largely composed of quartz, as are sandstone, and quartzite (which are themselves used as construction materials). The strong and durable cement used 2000 years ago by the Romans, was composed chiefly of amorphous (non-crystalline) silica. Filaments of partially amorphous quartz can be stronger than steel and are used in high-stress applications.
As well as its direct applications, silica is also a raw material for other products, such as water glass (a cementing agent), various sols (used in water-resistant coatings), organic silicates and silicones, silicon carbide (semiconductor, ceramic, abrasive), elemental silicon, ordinary glass, ceramics, fiber optic cables, and alloys. Extraction and processing industries find silica invaluable: steelmaking and copper industries use silica in smelting fluxes and acidic slags, while silica gel is used in the petroleum industry as a catalyst for cracking large hydrocarbon molecules (in crude oil) into gasoline and fuel oils.
Biologically, silica is essential. Diatoms, the earliest form of life to absorb sunlight and release oxygen, formed their skeletons from silica. Many plants use silica to stiffen their stems, as well as to form protective needles. Animals also require silica, which is present in collagen and arterial walls, and without which bones could not be formed. There are also small amounts of silica in the normal human brain; some researchers have found that aluminum (implicated in Alzheimer's disease) wreaks its damage by chemically combining with the brain's natural silica.
Along with its practical applications, silica has also been used for its aesthetic qualities. Various forms of quartz crystals have been used since ancient times as gemstones. A hydrous form of sifica--opal--has been used as a gemstone since Roman times.
Thus, silicon dioxide is a compound which finds application to a plethora of fields. Indeed, many of the technological pillars of the modern, industrial world, as well as those of the biological world rely heavily on this unique oxide of silicon for its many properties.
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