Topic 3- Structure of Crystalline Solids
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Define crystalline.
Crystalline materials have atoms situated in an ordered, periodic array over large atomic distances.
What materials are crystalline?
Metals and most ceramics.
Give two examples of properties that depend on the crystal structure of a material.
Density and ductility.
Define amorphous/non-crystalline.
Amorphous materials have no long range order. They are a random arrangement of atoms.
What materials are amorphous?
Glasses (silica), plastics and rapidly cooled metals.
How fast is rapid cooling for a metal?
1x10^5 degrees C/s.
When do amorphous structures usually occur?
When cooling happens too quickly for nucleation to occur.
What is a region in a crystalline structure called?
Grain.
What are the lines in crystalline regions called?
Twins.
Define polycrystalline.
Polycrystalline materials are made up of lots of crystalline regions and form like a lake freezing over.
What is the name of the line where two crystalline regions meet?
Grain boundary.
Describe the formation of a polycrystalline structure.
‘Nuclei’ form during solidification, each of which grows into a crystal. The crystals grow and meet. These crystals are separated by an amorphous grain boundary which influences properties.
How does packing affect energy?
Dense, regular-packed structures tend to have lower energy.
Are dense, regularly-packed atoms stable or unstable?
Stable - this is the preferred atomic arrangement.
For a dense, regularly-packed atoms will separation distance be slightly greater or slightly less than equilibrium position?
Slightly greater.
Why are non-dense, randomly packed atoms less stable?
A range of bond lengths, means average bond length is greater so there is a higher energy state.
What is one negative impact of non-dense, random packing?
Resistance to corrosion would be worse.
What is a lattice?
A 3D array of regularly spaced points.
What is hard sphere representation?
Atoms denoted by hard, touching spheres.
What is reduced sphere representation?
Atoms denoted by small circles allowing position in 3D to be seen.
What is a unit cell?
Basic building block that repeats in space to create the crystal structure, usually a parallelpiped or prizm.
Roughly how many different types of unit cell are there?
14.
What does FCC stand for?
Face Centered Cubic.
What does BCC stand for?
Body Centered Cubic.