Ch 1 Crystals-4

Calculate the coordination number ar ratio and apf

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Unformatted text preview: for SC, BCC, and FCC with one atom per lattice site. Determine the basis, a/r ratio, and APF for structures with more than one atom per lattice site (Si, GaAs, CsCl, and NaCl). 2.0 Resources [1]. Callister, Materials Science and Engineering: An Introduction, 8th edition (John Wiley and Sons, New York, 2009), pg. Ch 1 & Ch 2 for background, Ch 3.1-3.7, 3.11-3.12. 3.0 Materials Applications Understanding crystal structure is critically important to understanding why materials behave the way they do and to design processes for specific applications. For example, knowledge of the crystal structure, and the amount of dislocations, allows the engineer to predict whether a material could fail in a brittle catastrophic mode, or would bend and yield before fracture. This knowledge also helps the engineer design new manufacturing processes so that performance of materials can be improved. This all plays important roles in energy generation, transportation, medical devices, space exploration, and construction. Another example of the importance of crystal structure is in the semiconductor industry. Integrated circuit chips are fabricated on single crystal silicon wafers. Performance and reliability of chips relies on the entire wafer having the same unit cell repeated throughout. To ensure this, growth of the wafers is carefully controlled. Properties such as electron mobility and density of charge carriers depends critically crystal structure and purity. Knowledge of the crystal structure and control of purity and dislocations are tasks materials engineers deal with constantly. 4.0 Theory of Atomic Arrangements 4.1 Bonding There are three primary types of bonds in crystalline solids: ionic, covalent, and metallic. Mechanical and electronic properties of solids vary significantly depending on which type of bonding the solid has. Ceramic materials have ionic bonds, which are the strongest type of bonds, producing very hard materials. Semiconductors have covalent and sometimes ionic bonds that are spherically symmetrical. Rev 4.1 1-5 Crystals 1 MatE 25 San Jose State University Lab Notes 4.2 Metallic Bonding In metals, the bonds are isotropic or spherical. Metallic bonding can only occur among a large aggregate of atoms, such as in a crystal. For example in face-centered cubic and hexagonal closepacked metals, each atom has 12 nearest neighbors and thus is bonded in all directions. In bodycentered cubic metals there are 8 nearest neighbors. The valence electrons from each atom are shared throughout the crystal. The valence electrons are loosely attracted to the nucleus of the atom, and they are spread out so far from the nucleus that they may be closer to another nucleus in the solid. Thus all the metal atoms in the solid are bonded together by these free valence electrons. These electrons are hence free to travel throughout the crystal, resulting in the large electrical conductivity of metals. The atoms in metal crystals can slide easily by each other, because the bonds are not restricted to one direction or a strict angle, making it easy to deform most metals. This is why we can make so many structural parts from metal...
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