Unit 1_crystal structures.pptx
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This presentation gives brief introduction to crystal structures. The difference between crystalline and non crystalline materials, classification of crystalline materials and different types of lattices are also covered.
Unit 1_crystal structures.pptx
- 1. crystalline and non-crystalline materials Solids exist in nature in two principal forms: crystalline and non-crystalline (amorphous) Crystalline material: Ordered arrangement (long range periodicity) of their ions, atoms or molecules Repeating periodic array over large atomic distances Crystals exhibit sharp melting point Any single crystal- a single grain no grain boundaries Most crystalline solids more grains polycrystalline Whiskers single crystals - dia/thickness to length ratio- very high Crystalline Single crystal Whiskers
- 2. Crystalline materials Anisotropy Material properties - directional dependent in single crystals Isotropic nature Material properties - directional independent of polycrystalline materials During solidification Atomic arrangement at grain boundary
- 3. Noncrystalline (amorphous) materials No ordered arrangement (long range periodicity) Also called as supercooled liquids Example: ordinary glass, glycerine and most of the polymers Can be produced by preventing crystallization by high cooling rate (106 K/s) Gradually softens (gel like) on heating Atoms in crystals- closely packed high density than amorphous Non-crystalline
- 4. Space lattice Lattice points and space lattice Infinite array of points arranged in 3D Unit cell-the smallest unit forms space lattice
- 5. The Bravais lattices A 3D space lattice can be generated by three vectors a, b and c 14 ways of arranging points in 3D - therefore 14 Bravais lattices They belong to 7 crystal systems.
- 7. 14 ways of arranging points in 3D - therefore 14 Bravais lattices
- 8. BCC 8 atoms at the corners, I atom at the centre Centre and corner atoms touch one another along cube diagonal a=unit cell length, R = radius of the atom a=4R/3 Ex: Cr, W, iron..etc 1 atom from center +1/8 atom at each corners Total, 2 atoms are associated with one BCC unit cell The coordination number-nearest neighbours-8 a a
- 9. Atomic packing factor (APF): = Volume of atoms in a unit cell/total unit cell volume = Vs/Vc BCC unit cell - APF=0.68 Vs = average number of atoms (n) X volume of one atom (Va) Simple Cubic structure APC= 0.52 Va = (4/3) * r3 APF = (nVa)/Vc
- 12. FCC 8 atoms at each corner, 6 atoms at the centers of all the cube faces a and R related with a=22R Corner atoms touches the face atoms Each corner atom shares among 8 unit cells Face centered atoms belong to 2 unit cells 1/8th of corner atoms and 遜 of the face centered atoms total 4 atoms associated with FCC The coordination number -12 Ex: copper, aluminium, silver, gold etc Metals show high APF electron cloud
- 14. We can calculate theoretical density, if the crystal structure is known Density = n A/Vc NA n = number of atoms associated with each unit cell A = atomic weight Vc =Volume of the unit cell NA= Avagadros number (6.023 X 1023 atoms/mole)
- 16. HCP Another common crystal structure The top and bottom faces-6 at the regular hexagons and surround a single atom at the centre Midplane 3 atoms 1/6th of top and bottom 12 corner atoms 遜 of each of the two centre face atoms 3 midplane interior atoms Total number of atoms associated : 6 The coordinate number :12 Atomic packing factor (APF): 0.74 Ex: Mg, Ca, Ti..etc
- 17. APF = 0.74
- 18. Summary Stability-metastability Atomic bonds Crystalline and non-crystalline materials Space lattice, unitcell Different crystal structures BCC, FCC and hcp structures