Lecture 5 - Atomic Packing Flashcards
What does dense ordered packed structures tend to have
Lower energies
3 types of solids
Crystalline
Amorphous
Semicrystalline
2 properties of crytsalline structures
Atoms packed in an organised structure
Typically metals and ceramics
2 properties of amorphous
Atoms pack together randomly
Typically polymers and glasses
2 properties of semicrystalline
Regions which are crystalline and region which are amorphous
Typically high modulus polymers
What are typical of crystalline materials
Metals
Many ceramics
Some polymers
Non-crystalline materials occurs for …
Non - crystalline is amorphous
Complex structures
Rapid cooling
Carbons
Diamond
Graphite
Fullerenes - bucky balls c60
Nanotubes - half buckyballs at end and tubes between
Graphene - 2 dimensional sheets of graphite
Carbon black
5 things to consider in a crystal structure
How atoms pack
Depends on interatomic bonds
Depends on relative sizes of the atoms
May be perfect of distrorted
Can move between different structures depending on temperature
4 assumptions in a crystal structure
Atoms are spherical
Not deformable
Bonds between atoms are deformable but require energy to deform
Atoms pack to minimise energy requirements
What are the 3 possible methods of packing
Body centred cubic - BCC
Face centred cubic - FCC
Hexagonal close packed - HCP
2 types of close packed structures
FCC
HCP
3 properties FCC
FCC make up a cube
Further atoms are in the centre of the faces
14 atoms involved a unit cell
Unit cell - small repeat entities
FCC total number = 4
3 properties of HCP
Hexagonal close packed
Have basic repeat unit of 7 atoms in each of two layers with three between
17 atoms involved in a unit cell
Number of atoms to a unit cell total= 6
3 properties of BCC
One atom in the centre of the cube
9 atoms involved in the cell
Total number of atoms in a unit cell = 2
