Structure & Crystalline Solids
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what are the two types of main structures of solid materials?
amorphous and crystalline
what is the structure of a crystalline material?
they have atoms situated in a periodic array over large atomic distances
give some examples of materials that have crystalline structures?
metals and most ceramics
what are amorphous materials?
materials that have no long range order, they are a non-repeating pattern (non crystalline) - these include glasses like silica, plastics and rapidly cooled metals
how do crystalline structures form?
if we cool something down or if we precipitate something out of something
how do ice crystals form on lakes? (can be applied to any formation of crystal structure)
• ‘nuclei’ form during solidification, each of which grows into crystals, nucleation - when the molecules/atoms are initially coming together • crystals grow and meet • eventually they cannot grow anymore and we form out solid crystal • these grains are separated by an amorphous grain boundary because the orientation of the crystals are all mismatched
how can we reveal the grain structure of a crystal?
use polishing and etching (using acids)
what type of atomic packing does a crystalline structure have?
dense, regular packing (tend to have lower bond energy than amorphous structures)
what type of atomic packing does an amorphous structure have?
non-dense, random packing (varying bonds lengths) - they tend to have larger bond energy and bond lengths compared to crystalline structures) - more reactive, less energy needs to be put in
how do we describe the arrangement of crystalline solids?
using the unit cell - the smallest repeating arrangement (basic building block)
what is the difference between the hard sphere model and the reduced sphere representation?
hard sphere model - atoms donated by hard, touching spheres reduced sphere representation - reduced spheres
what are the 4 main unit cells we find in metallic crystal structures?
SC - simple cubic FCC - face centred cubic BCC - body centred cubic HCP - hexagonal close packed
why do metallic crystalline structures tend to have dense packing?
due to non-directional bonding in metals
what is the simple cubic structure? (SC)
• cubic unit cell - very rare, only Po has this structure (polonium)
what does the coordination number refer to?
the no. of nearest neighbours or touching atoms
what is the coordination number of simple cubic structures?
6
how do we describe the packing?
• using the atomic packing factor • we can expect values less than 1 because it is a ratio • APF = volume of atoms in unit cell / volume of unit cell (assuming hard sphere model)
what is the equation for the atomic packing factor?
(no. of atoms per unit cell ) x (4/3π(r)^3) / a^3 where r is written in terms of a (lattice parameter of a cubic cell)
what is the atomic packing factor (APF) for a simple cubic structure?
0.52 (has 8 x 1/8 atoms = 1 atom per unit cell) r = 0.5a
what is the face centred cubic structure (FCC)?
• atoms located at each corner and the centre of each cube faces • some examples include copper, aluminium, silver, gold (ductile metals) • coordination number = 12 • APF = 0.74 (has 8 x 1/8 atoms + 6 x 1/2 = 4 atoms per unit cell) • r = 2^0.5a / 4 (because then face diagonal of the cube is equal to 4r which is equal to root2xa)
what does ductility relate to in terms of crystal structure?
ductility (ease of plastic deformation) is linked to crystal structure and close packed planes
how do slip systems work?
slip occurs on specific atomic planes and in specific crystallographic slip directions i.e slip systems
what are the slip planes?
they are the most close packed planes (note: only count different ones, ignore parallel)
for the unit cell what is the order of directions?
(x, y, z) - helps us define a plane or a direction