L14 - SUMMARY FROM LAST CLASS Crystalline solids For all...

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Dr. P. Lucas U of A MSE 110 Crystalline solids SUMMARY FROM LAST CLASS For all types of solids, every atomic bonds have maximum energy at a specific distance r o , hence ordered crystalline arrangements are energetically favored. When atoms or spherical molecule do not favor any particular bonding directions, they behave as hard sphere and the bonding is maximized by adopting high packing density. These close packed structures are formed by stacking dense atomic layers. Faced Center Cubic : (FCC) • ABCABC. .. sequence. • 4 atoms per cubic cell. Hexagonal Close Packed :(HCP) • ABAB… sequence. • 6 atoms per hexagonal cell. •a / c= 1 . 6 3 3 Body Centered Cubic : (BCC) • 2 atoms per cubic cell. 2 2 R
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Dr. P. Lucas U of A MSE 110 Packing density: CRYSTALLINE STRUCTURE The packing density can be calculated for each structure by taking the ratio of the volume of the sphere included in the cell versus the cell volume itself. % 68 ) ( 2 3 3 4 3 3 4 2 = × = R R cell of Volume atom of Volume π % 52 ) 2 ( 3 3 3 4 1 = = R R cell of Volume atom of Volume SIMPLE CUBIC: 1 atom per cell Packing density= BODY CENTERED CUBIC: 2 atoms per cell Packing density= % 74 ) 2 2 ( 4 3 3 3 4 4 = × = R R cell of Volume atom of Volume FACE CENTERED CUBIC: 4 atoms per cell Packig density= FCC has the same packing density as HCP and is the densest of all structures.
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Dr. P. Lucas
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L14 - SUMMARY FROM LAST CLASS Crystalline solids For all...

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