For most atoms, the electrons fill up the lowest possible energy states in the electron shells and subshells, two electrons having opposite spins per state. The energy structure for a sodium atom is represented schematically in Figure 2. When all the electrons occupy the lowest possible energies in accord with the foregoing restrictions, an atom is said to be in its ground state. However, electron transitions to higher energy states are possible, as discussed in Chapters 18 and The electron configuration or structure of an atom represents the manner in which these states are occupied. For example, the electron configurations for Principal quantum number, n E ne rg y 1 s s p s p s p s p df s p s p df d d d f 2 3 4 5 6 7 Figure 2.
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Twin plane boundary Figure 4. One type of catalyst exists as a solid; reactant molecules in a gas or liquid phase are adsorbed5 onto the cat- alytic surface, at which point some type of inter- action occurs that promotes an increase in their chemical reactivity rate.
Several types of surface defects, repre- sented schematically in Figure 4. One important use of catalysts is in catalytic converters on automobiles, which reduce the emis- sion of exhaust gas pollutants such as carbon monoxide CO , nitrogen oxides NOx, where x is variable , and unburned hydrocarbons.
See the chapter-opening diagrams and photograph for this chapter. Air is introduced into the exhaust emissions from the automobile engine; this mix- ture of gases then passes over the catalyst, which adsorbs on its surface molecules of CO, NOx, and O2. Pairs of nitrogen atoms combine to form N2 mol- ecules, and carbon monoxide is oxidized to form carbon dioxide CO2. Furthermore, any unburned hydrocarbons are also oxidized to CO2 and H2O.
One of the materials used as a catalyst in this application is Ce0. Figure 4. Indi- vidual atoms are resolved in this micrograph as well as some of the defects presented in Figure 4. These surface defects act as adsorption sites for the atomic and molecular species noted in the previous paragraph.
Consequently, dissociation, combina- tion,and oxidation reactions involving these species are facilitated, such that the content of pollutant species CO, NOx, and unburned hydrocarbons in the exhaust gas stream is reduced significantly. Individual atom sites are represented as cubes.
It should not be confused with absorption, which is the assimilation of molecules into a solid or liquid. Surface defects represented schematically in Figure 4. Stark, L. Maciejewski, S. Pratsinis, A. Reproduced by permission of The Royal Society of Chemistry. Twins result from atomic displacements that are produced from applied mechanical shear forces me- chanical twins , and also during annealing heat treatments following deformation annealing twins.
Twinning occurs on a definite crystallographic plane and in a spe- cific direction, both of which depend on the crystal structure. The role of mechanical twins in the deformation process is discussed in Section 7. Annealing twins may be observed in the photomicrograph of the polycrystalline brass specimen shown in Figure 4.
The twins correspond to those regions having relatively straight and parallel sides and a different visual contrast than the untwinned regions of the grains within which they reside.
An explanation for the variety of textural contrasts in this photomi- crograph is provided in Section 4. Miscellaneous Interfacial Defects Other possible interfacial defects include stacking faults and ferromagnetic domain walls. For fer- romagnetic and ferrimagnetic materials, the boundary that separates regions hav- ing different directions of magnetization is termed a domain wall, which is discussed in Section Associated with each of the defects discussed in this section is an interfacial en- ergy, the magnitude of which depends on boundary type, and which will vary from material to material.
Normally, the interfacial energy will be greatest for external surfaces and least for domain walls. Concept Check 4. Does this surface energy increase or decrease with an increase in planar density? These include pores, cracks, foreign inclusions, and other phases. They are normally introduced during processing and fabrication steps. Some of these defects and their effects on the properties of materials are discussed in subsequent chapters. In a sense, these atomic vibrations may be thought of as imper- fections or defects.
At any instant of time not all atoms vibrate at the same fre- quency and amplitude, nor with the same energy. At a given temperature there will exist a distribution of energies for the constituent atoms about an average energy. Over time the vibrational energy of any specific atom will also vary in a random manner.
With rising temperature, this average energy increases, and, in fact, the tem- perature of a solid is really just a measure of the average vibrational activity of atoms and molecules. Many properties and processes in solids are manifestations of this vibrational atomic motion.
For example, melting occurs when the vibrations are vigorous enough to rupture large numbers of atomic bonds.
A more detailed discussion of atomic vibrations and their influence on the properties of materials is presented in Chapter Microscopic Examinat ion 4.
Some structural elements are of macroscopic dimensions; that is, they are large enough to be observed with the un- aided eye. For example, the shape and average size or diameter of the grains for a polycrystalline specimen are important structural characteristics.
Macroscopic grains are often evident on aluminum streetlight posts and also on highway guardrails. Relatively large grains having different textures are
Callister Uma Introdução A Ciencias Dos Materiais 8a Edição