Silicon, Silica, and Silicates

If we were to list the materials of technology, sihcon would certainly be prominent on that list. As we mentioned in the previous section, silicon is the basic material in the semiconductor devices that make up CD players, computers, and other electronics gear. (Silicon Valley, an area in Cahfomia that is headquarters for many computer companies, takes its name from the element silicon.) But oxygen compounds of silicon also are important to technology. Quartz crystals, a form of silica (also called silicon dioxide, Si02), for example, are used to control frequencies in radio transmitters and watches. And many practical materials such as cement and bricks are silicate materials. Silicates are compounds of sihcon and oxygen with one or more metallic elements. In this section, we will look at silicon, sUica, and silicates. 

Silicon wafers form the base material for integrated circuit chips used in solid-state electronic devices. 

Sihcon occurs in the earth s crust, the outermost solid layer of the planet, as compounds with oxygen. About 95% of the earth s crust is silica and silicate rocks and minerals. Elemental silicon is obtained by reducing quartz sand (Si02) with coke (C) in an electric furnace at 3000°C. 

Sihcon has a diamond-hke structure (in which silicon atoms bond tetrahedraUy to four other silicon atoms). It is a hard, lustrous gray solid and is used to make alloys and solid-state electronic devices. 

For the manufacture of solid-state devices, it is necessary to start with extremely pure silicon (no more than 10 % impurities). You first convert the impure element to silicon tetrachloride. Si, which is a low-boiling liquid (b.p. 58°C) that can be purified by distillation. 

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In the following sections, we look at several nonmetallic materials with applications in modem technology. We begin with a discussion of the different allotropes or forms of carbon—diamond, graphite, and the fullerenes—where research has produced some exciting discoveries (Section 13.4). The fullerenes are recently discovraed molecular forms of the element carbon, in which the carbon atoms form hollow balls and tubes that may make them important as catalysts or possibly as drug-delivay mataials. Diamond shows promise as a material that might supersede silicon in its role as a matmal for sohd-state electronics. Silicon and diamond can act as semiconductors, which we discuss in Section 13.5. We end the chapter with sections on silicon, silica, and silicates (Section 13.6), ceramics (Section 13.7), and finally composites (Section 13.8). 

Amorphous silica, 21 780t 22 380-406. See also Amorphous silica and silicates Amorphous silicon (a-Si) Vitreous silica... 

Silicon [7440-21-3], Si, from the Latin silex, silicis for flint, is the fourteenth element of the Periodic Table, has atomic wt 28.083, and a room temperature density of 2.3 gm/cm3. Silicon is brittle, has a gray, metallic luster, and melts at 1412°C. In 1787 Lavoisier suggested that silica (qv), of which flint is one form, was the oxide of an unknown element. Gay-Lussac and Thenard apparently produced elemental silicon in 1811 by reducing silicon tetrafluoride with potassium but did not recognize it as an element. In 1817 Berzelius reported evidence of silicon occurring as a precipitate in cast iron. Elemental silicon does not occur in nature. As a constituent of various minerals, eg, silica and silicates such as the feldspars and kaolins, however, silicon comprises about 28% of the earth s crust. There are three stable isotopes that occur naturally and several that can be prepared artificially and are radioactive (Table 1) (1). 

Silicon is the most important constituent of igneous and many sedimentary rocks, occurring in combination with oxygen in feldspars, micas, quartz, sands and shales. The element is used in electronic devices, while silicon in combination with oxygen as silica and silicates finds application in concrete, bricks, pottery, enamels, glasses, optical fibers for telecommunications, and refractory (high-temperature resistant) materials. 

Polymer structures that hold silanols at the interface. Good examples of hydrolytically stable crosslinked structures are silica and silicate rocks. Although every oxane bond in these structures is hydrolyzable, a silicate rock is quite resistant to water. Each silicon is bonded to four oxygens under equilibrium conditions with a favorable equilibrium constant for bond retention. The probability that all four bonds to silicon can hydrolyze simultaneously to release soluble silicic acid is extremely remote. With sensitive enough analytical techniques it is possible to identify soluble silica as it -leaches from rocks, but an individual rock will survive in water for thousands of years. 

Silicon is next to oxygen the most abundant element in the lithosphere the average content amounts about 30% by weight. Inorganic silicon compounds such as silica and silicates form the basis of most of the rocks forming the earth s crust. In the atmosphere, there is no silicon present, except as dust of cosmic and terrestrial origin. The silicon content in the hydrosphere, mainly in form of dissolved silica (silicic acid), is also very small. 

White carbon black is the global title for synthetic hydrated silica and silicate, scientifically termed hydrated silicon dioxide (Si02nH20), and is an important additive of rubber etc. [183]. Because of its white color and properties comparable with carbon black, it is termed White Carbon Black , for which the Chinese quality standard is given in Table 13.1. 

FIGURE 2. Glassy silica needles (2 mm x 30 gm) produced in copious amount by a marine sponge. Each needle contains an occluded axial filament comprised of silicateins, enzyme-like proteins that catalyze and spatially direct the polycondensation of silicon alkoxides and silicic acid at neutral pjj41,42 Reprinted from Reference 3, copyright (1999), with permission from Elsevier Science... 

Materials based on silicon are found everywhere in nature in form of silica and silicates which count for more than ninety percent of the earth s crust... 

Silicon, the second most abundant element in the earth s crust, is a semimetal found widely distributed in silica and silicates (see Section 16.5). About 85% of the earth s crust is composed of these substances. Although silicon is found in some steel and aluminum alloys, its major use is in semiconductors for electronic devices (see Chapter 16). 

I ILICON-CONTAINING CERAMICS include the oxide materials, silica and silicates the binary compounds of silicon with nonmetals, principally silicon carbide and silicon nitride silicon oxynitride and the sialons main group and transition metal silicides and, finally, elemental silicon itself. Throughout the world, research activity on the preparation of all of these classes of solid silicon compounds by newer preparative techniques is vigorous. 

A survey of the literature [28, 58-65, 71, 72, 74-80] indicates that the most frequent exposure to silicon involves exposure to silica and silicates mainly in their crystalline forms. Health risks associated with the exposure to other sihcon containing compounds were reported in the mortality study of 16.661 manmade mineral fiber workers employed during 1945 to 1963 at one of 17 U.S. manufacturing plants [75]. Fiber exposure in the plants producing fibrous glass or mineral wool, or both, was associated with increased... 

Use Smoke screens manufacture of ethyl silicate and similar compounds production of silicones manufacture of high-purity silica and fused silica glass source of silicon, silica, and hydrogen chloride lab reagent. 

Many of the transformations shown in Table 9 were known early in the history of silicon chemistry. Alcoholysis of SiCl4 provided one of the first volatile, covalent derivatives with all silicon-oxygen bonds104. The properties of this silicon-oxygen derivative were certainly quite different from silica and silicates, which also contain all silicon-oxygen bonds. Successive substitution by alcohols was one of the earliest reaction... 

In the study of silica and silicates, MD was primarily used to simulate molten and vitreous states using ionic and pairwise interatomic potentials [1, 2, 3, 4], where the interatomic potential was obtained empirically by trial and error. Although the fourfold oxygen coordination of silicon atoms and non-linear Si-O-Si angle widely observed in silica and silicates seem to suggest covalent character of the Si-0 bonds, it has been shown that these structural properties are roughly reproduced by pairwise interatomic potentials. 

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