MEMS1054_Week3_Presentation2_3

MEMS1054_Week3_Presentation2_3 - MEMS 1054 Materials...

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Unformatted text preview: MEMS 1054 Materials Science 1 or Structure of Crystals and Diffraction Week 3 #2 - Crystalline State 1 Professor Jrg Wiezorek, PhD, Materials Science and Metallurgy, University of Cambridge, UK, 1994. Meets in room G37 BEH, Mon/Wed/Fri, 1-1:50PM Crystal Structures and Hard-Sphere Packing ; Mostly after - Chapter 3.4, Allen & Thomas text (see upload) Electronic Structure and Bonding strongly affect the SYMMETRY of the crystal structure formed from combinations of atoms of the same kind (elements) and for compounds (alloys, intermetallic, ionic, molecular). Some important Structures of crystalline materials can be understood and described quantitatively using the very simply approach and Descriptors based on the CONCEPT of Hard-Sphere Packing. These include the diamond cubic structures of diamond, silicon, germanium and related compounds, e.g. ZnS, GaAs (sphalerite structures), as well as many metals (Cu, Au, Zn, Zr, Ti, Be, Ni, Al, -Fe) and ionic compounds, e.g. NaCl, MgO etc Close-Packed Structures and Derivative Structures 3 More than 4500 Close- Packed cubic, hexagonal structures related rhombohedral and also tetragonal and simple cubic structures For example, Cu, FCC archetype, a cubic F-lattice ( cF), with space group close-packing is a useful descriptor, particularly for capturing geometric aspects and for discussions related to crystal growth and crystal defects in cp-structures. Fm 3 m (After Guy and Hren) Geometric Descriptor of a large number of crystalline materials (metals, alloys, inorganic compounds) => Close Packed (CP) Structures: here one kind of atoms are located approximately at the locations of incompressible spheres of equal size that are close packed, with possible other atoms distributed in the voids among the first kind of atoms. The resulting structure may not be ideally close-packed (cp) but nevertheless are described effectively as cp-structures. Planar cp-arrangement is a hexagonal array or layer of touching spheres, (A); each sphere, A, has 6 nearest neighbors (NN). It contains two types of triangular voids marked B and C. Using the unit cell shown, the locations, mnp, of the centers of mass of the locations marked A, B, C are 000, 1/3 2/3 0, 2/3 1/3 0....
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MEMS1054_Week3_Presentation2_3 - MEMS 1054 Materials...

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