Relation of Structure to Electrical and Optical Properties

Most plastics materials may be considered as electrical insulators, i.e., they are able to withstand a potential difference between different points of a given piece of material with the passage of only a small electric current and a low dissipation energy. When assessing a potential insulating material, information on the following properties will be required  

Typical properties for the selection of well-known plastics materials are tabulated in Table 6.1. 

Some brief notes on the testing of electrical properties are given in the appendix at the end of this chapter. 

The materials in Table 6.1 may be divided roughly into two groups  

Dielectric Constant, Power Factor and Structure 111 Table 6.1 Typical electrical properties of some selected plastics materials at 20°C 

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Relation of Structure to Electrical and Optical Properties It may be shown that for electron polarisation... 

Chapters 22 and 23 in the fifth edition have been combined, reorganized, and revised to provide a systematic introduction to ceramics, electronic materials, and optical materials. Structural, electrical, and optical properties are related to the nature of the chemical bonds in each class of materials. 

The structural study of materials was always of a high priority, because the physical properties of materials depend very much on their structme. There are several levels of structural study, which start with the investigation of the morphology of the material surfaces, closely related to their in-plane ordering. Many nano-structured materials prepared with the help of layer-by-layer deposition techniques, such as LB or electrostatic self-assembly, have a distinctive periodicity in the direction normal to the surface, which determines their main electrical and optical properties. This is why the study of the layer-by-layer structure of such materials is of crucial importance. 

While certain preliminary ideas about the origin of ionic conduction in crystalline solids were derived from the crystal structure investigations of a-AgI by Stiock, and relate to a molten sublattice, i.e., no distinction can be made between a regular Ag lattice site and an Ag on an interstitial site, our knowledge of point defects is based maiidy on the woik of Frenkel, Wagner and Schotlky, Schotlky, and lost, and was developed primarily by studying electrical and optical properties. 

To understand and describe the electrical and optical properties of a semiconductor, it is essential to have knowledge of its electronic band structure, which exhibits the relation between energy and momentum E k) of electrons and holes in the different possible states of the conduction and valence bands at the various symmetry points of the first Brillouin zone of the reciprocal lattice. In particular, the band gap between the valence and conduction bands is important, because it determines, e.g., the optical transition energy and the temperature dependence of the intrinsic conductivity. In the case of the complex boron-rich solids with large numbers of atoms per unit cell, the agreement between theoretical calculations of the band gaps and the experimental results has not yet been satisfactory. 

Solid solutions are very common among structurally related compounds. Just as metallic elements of similar structure and atomic properties form alloys, certain chemical compounds can be combined to produce derivative solid solutions, which may permit realization of properties not found in either of the precursors. The combinations of binary compounds with common anion or common cation element, such as the isovalent alloys of IV-VI, III-V, II-VI, or I-VII members, are of considerable scientific and technological interest as their solid-state properties (e.g., electric and optical such as type of conductivity, current carrier density, band gap) modulate regularly over a wide range through variations in composition. A general descriptive scheme for such alloys is as follows [41]. 

Among the many classes of chiral molecules, helical systems are particularly fascinating. Their structure is relevant to proposed mechanisms of handedness induction in relation to chiral amplification [76], Helicenes ([A]-H) are helical molecules formed from A-ortho-fused benzene rings (Fig. 8) which display considerable rotatory power [77]. Helicenes are presently the subject of intense synthesis efforts that try to functionalize these molecules in order to attain enhanced electric, magnetic, and optical properties [78, 79]. Phenylenes ([A]-P), or heliphenes, constitute another class of helical aromatic compounds for which syntheses have recently been reported [80, 81]. They are made up of N benzene rings fused together with N - 1 cyclobutadiene rings (Fig. 8). 

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