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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
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
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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- Spring '08