Characteristics and Properties of Silicon

Silicon has the crystal structure of diamond and its properties are influenced by the crystal orientation. ] CVD silicon can be 

Crystalline form cubic (similar to diamond) Lattice parameters a, = 0.54307 nm 

Semiconductor properties are imparted by doping its structure with boron, phosphorus, or arsenic atoms. Silicon is relatively inert chemically but is attacked by halogens and dilute alkalies. It has good optical transmission especially in the infra-red. 

Several precursors and CVD reactions are available to deposit silicon. The deposit can be either single crystal (epitaxial), polycrystalline, or amorphous. 

Silicon Epitaxy. Silicon epitaxial films have superior properties. The applications are, however, limited by the high temperature of deposition, which is generally above 1000°C. These reactions use chlorinated compounds of silicon (tetrachloride, trichlorosilane, or dichlorosilane) as precursors as follows  

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Characteristics and Properties of Silicon Carbide and Boron Carbide... 

The characteristics and properties of silicon carbide are summarized in Table 8. If Wioi and reviewed in more detail in Secs. 4-8. Values quoted are for hot-pressed material and are an average of the values reported in the literature. 

The results reported confirm that tin or silicon composite configuration is the correct approach in bringing the related lithium alloy electrodes to the commercial stage. In fact, their use is now seriously considered by battery manufacturers. For example, the Sn-C-Co ternary composite is presently used by a Japanese manufacturer to produce a battery under the commercial name of Nexelion. The characteristics and properties of metal alloy anodes are summarised in Table 3.1. 

A chemical property of silicones is the possibility of building reactivity on the polymer [1,32,33]. This allows the building of cured silicone networks of controlled molecular architectures with specific adhesion properties while maintaining the inherent physical properties of the PDMS chains. The combination of the unique bulk characteristics of the silicone networks, the surface properties of the PDMS segments, and the specificity and controllability of the reactive groups, produces unique materials useful as adhesives, protective encapsulants, coatings and sealants. 

Germanium was the semiconductor material used in the development of the transistor in the early 1950s. However, it exhibits high junction leakage current due to its narrow bandgap and is now largely replaced by silicon. It is a brittle metalloid element with semiconductor characteristics. The properties of germanium are summarized in Table 8.3.1 lP l... 

Silicon dioxide (Si02), also known as silica, is a major industrial material with many applications particularly in the semiconductor industry in the form of coatings, which are produced mostly by CVD. It is an excellent electrical insulator with very low thermal expansion and good resistance to thermal shock. Its characteristics and properties are summarized in Table 11.4. 

Silicone rubbers have one more very significant advantage in comparison with rubbers based on organic elastomers, and that is high dielectric characteristics. E.g., rubbers based on silicone elastomers do not conduct electric current even at 250-300 °C, whereas rubbers based on organic elastomers become conductive already at 120-150 °C. Insulating properties of silicone rubbers are preserved even at contact with water. 

Renewed interest in the optical properties of silicon films prepared by various new methods, like chemical vapor deposition (CVD), glow-discharge deposition, and sputter deposition, also stimulated investigations of the characteristics of these films for optical recording. 

The hydrophobicity of the surface prevents the wetting by tear and tends to expose dry surface of a contact lens. Therefore, rapid dehydration of the corneal tissues could occur, which could cause the damage of corneal epithelium. However, this explanation seems to be oversimplified in light of the adsorption of protein, which makes a hydrophobic surface wettable by tear fluid, as described in Chapter 26. Moreover, the highly hydrophobic surface characteristic of silicone rubber tends to encourage the deposition of protein and mucus of the tear on the surface of the lens. Lipids and lipid-soluble materials follow the same track and eventually penetrate into the bulk phase of the contact lens. Because of these undesirable factors, the use of silicone contact lenses of various chemical compositions and with surface treatments has not been successful but rather disastrous because of the interfacial characteristics of silicone contact lens on the cornea, which cannot be oflfset by these efforts. It indicates that more profound surface modification to cope with the problems rather than mere surface treatment is needed in capitalizing on the advantageous bulk properties of silicone polymers. 

Silicone foam thus formed has an open cell structure and is a relatively poor insulating material. Cell size can he controlled by the selection of fillers, which serve as bubble nucleating sites. The addition of quartz as a filler gready improves the flame retardancy of the foam chat yields of >65% can he achieved. Because of its excellent flammability characteristics, Silicone foam is used in building and construction fire-stop systems and as pipe insulation in power plants. Typical physical properties of silicone foam are listed in Table 10. 

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