Electrical classification of solids

Solids are divided according to their electrical conductivity into three groups conductors, insulators, and semiconductors. If a piece of solid material is placed in an electric field, whether or not current will flow depends on the type of material. If current flows, the material is a conductor. If current is zero at low temperatures but larger than zero at higher temperatures, the material is a semiconductor. If current is zero at all temperatures, the material is an insulator. 

Conductivity and electric current mean motion of electrons, and according to the results of this simple experiment, 

In conductors, electrons can move freely at any voltage different than zero. 

In insulators, electrons cannot move under any voltage (except, of course, when the voltage is so high that an electrical discharge occurs). 

In semiconductors, electrons cannot move at low temperatures (close to absolute zero) under any voltage. As the temperature of a semiconductor increases, however, electrons can move and electric current will flow at moderate voltages. 

For instance, the most common orders of magnitude for energy-band gaps are 5.3 eV (511 kj/mol) for diamond-type la, showing its excellent electric insulating properties, and 1.04 eV (i.e., 100 kJ/mol) for pure silicon (Si) monocrystal used as semiconductors and 0.69 eV (i.e., 67 kJ/mol) for pure germanium (Ge) crystals. The first two examples are both intrinsic semiconductors. Moreover, electrical conductivity of materials is strongly temperature dependent. In fact, as the temperature increases, the conductivity of metals decreases, while the electrical conductivity of pure semiconductors and insulators increases. 

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The great variations among solids make it desirable to And useful classification schemes. Though this topic is taken up much later in the course (Chapter 17), a beginning is provided by a look at the electrical conductivity of solids. 

A possible classification of solids where ionic conductivity plays an important role is given in Table 5.1. In contrast to the situation in solutions, ionic transport in solids is accompanied by an electronic counterpart. For solid electrolytes the ionic contribution to the total electrical conductivity is predominant. Other cases represent MIECs. 

Commercial dryers differ fundamentally by the methods of heat transfer employed (see classification of diyers, Fig. 12-45). These industrial-diyer operations may utihze heat transfer by convection, conduction, radiation, or a combination of these. In each case, however, heat must flow to the outer surface and then into the interior of the solid. The single exception is dielectric and microwave diying, in which high-frequency electricity generates heat internally and produces a high temperature within the material and on its surface. 

NMAB. 1982. Classification of Gases, Liquids and Volatile Solids Relative to Explosion-Proof Electrical Equipment. Report NMAB 353-5. National Academy Press, Washington, DC (August 1982). 

FPN No. 1) For additional information on the properties and group classification of Class I materials, see Manual for Classification of Gases, Vapors, and Dusts for Electrical Equipment in Hazardous (Classified) Locations, NFPA 497M-1991, and Guide to Eire Hazard Properties ofElammable Liquids, Gases, and Volatile Solids, NFPA 325—1994. 

NFPA-325 Guide to Fire Hazard Properties of Flammable Liquids, Gases and Volatile Solids, (1994 ed.), NFPA-321 Basic Classification of Flammable and Combustible Liquids (1991 ed.), NFPA-497A, Classification of Class 1 Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas (1992 ed.), and NFPA-497B, Classification of Class II Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas (1991 ed.), National Fire Protection Association, Quincy, MA. 

Crystalline solids are built up of regular arrangements of atoms in three dimensions these arrangements can be represented by a repeat unit or motif called a unit cell. A unit cell is defined as the smallest repeating unit that shows the fuU symmetry of the crystal structure. A perfect crystal may be defined as one in which all the atoms are at rest on their correct lattice positions in the crystal structure. Such a perfect crystal can be obtained, hypothetically, only at absolute zero. At all real temperatures, crystalline solids generally depart from perfect order and contain several types of defects, which are responsible for many important solid-state phenomena, such as diffusion, electrical conduction, electrochemical reactions, and so on. Various schemes have been proposed for the classification of defects. Here the size and shape of the defect are used as a basis for classification. 

Recommended Practice for the Classification of Combustible Dusts and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids... 

Above all, we must recognize that any classification of a compound that we might suggest based on electronic properties must be consistent with the physical properties of ionic and covalent substances described at the beginning of the chapter. For instance, HCl has a rather large electronegativity difference (0.9), and its aqueous solutions conduct electricity. But we know that we cannot view it as an ionic compound because it is a gas, and not a solid, at room temperature. Liquid HCl is a nonconductor. 

Most elements combine with phosphorus to give binary phosphides exceptions include Hg, Pb, Sb, Bi and Te. Types of solid state phosphides are very varied, and simple classification is not possible. Phosphides of the d-block metals tend to be inert, metallic-looking compounds with high melting points and electrical conductivities. Their formulae are often deceptive in terms of the oxidation state of the metal and their structures may contain isolated P centres, P2 groups, or rings, chains or layers of P atoms. 

Classification of the Elements.—Berzelius was the first to divide all the elements into two great classes, to which he gave the names metals and metalloids. The metals, being such substances as are opaque, possess what is known as metallic lustre, are good conductors of heat and electricity, and are electro-positive the metalloids, on the other hand, such as are gaseous, or, if solid, do not possess metallic lustre, have a comparatively low power of conducting heat and electricity, and are electro-negative. 

The most fundamental classification of the chemical elements is into metals and nonmetals. Metals typically have the following physical properties a lustrous appearance, the ability to change shape without breaking (they can be pulled into a wire or pounded into a thin sheet), and excellent conductivity of heat and electricity. Nonmetals typically do not have these physical properties, although there are some exceptions. (For example, solid iodine is lustrous the graphite form of carbon is an excellent conductor of electricity and the diamond form of carbon is an excellent conductor of heat.) However, it is the chemical differences between metals and nonmetals that interest us the most metals tend to lose electrons to form positive ions, and non-metals tend to gain electrons to form negative ions. When a metal and a nonmetal react, a transfer of one or more electrons from the metal to the nonmetal often occurs. 

In materials science we often divide materials into distinct classes. The primary classes of solid materials are ceramics, metals, and polymers. This classification is based on the types of atoms involved and the bonding between them. The other widely recognized classes are semiconductors and composites. Composites are combinations of more than one material and often involve ceramics, such as fiberglass. Semiconductors are materials with electrical conductivities that are very sensitive to minute amounts of impurities. As we will see later, most materials that are semiconductors are actually ceramics, for example, gallium nitride, the blue-green laser diode material. 

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