Solids electrical classification

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. 

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. 

When we study a solid that does not have the characteristic lustrous appearance of a metal, we find that the conductivity is extremely low. This includes the solids we have called ionic solids sodium chloride, sodium nitrate, silver nitrate, and silver chloride. It includes, as well, the molecular crystals, such as ice. This solid, shown in Figure 5-3, is made up of molecules (such as exist in the gas phase) regularly packed in an orderly array. These poor conductors differ widely from the metals in almost every property. Thus electrical conductivity furnishes the key to one of the most fundamental classification schemes for substances. 

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... 

Interpreting Data Nonmetals can be solids, liquids, or gases. They do not conduct electricity and do not react with acids. If a nonmetal is a solid, it is likely to be brittle and have color (other than white or silver). Write the word nonmetal beneath the Classification heading in the data table for those element samples that display the general characteristics of nonmetals. 

Classifying the elements There are three main classifications for the elements—metals, nonmetals, and metalloids. Metals are elements that are generally shiny when smooth and clean, solid at room temperature, and good conductors of heat and electricity. Most metals also are malleable and ductile, meaning that they can be pounded into thin sheets and drawn into wires, respectively. Figure 6-6 shows several applications that make use of the physical properties of metals. 

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. 

---