Diffusion

Question 1

What is diffusion?

Answer 1

Diffusion is mass transport by atomic motion.

It is the consequence of the constant vibrational motion of atoms, molecules and particles that results in material moving from areas of high to low concentration.

Question 2

What are the two key diffusion mechanisms for metals?

Answer 2

Vacancy or interstitial diffusion

Question 3

What two conditions must be met for an atom to be able to move?

Answer 3

1. There must be an empty adjacent site
2. The atom must have sufficient vibrations energy to break bonds with neighbouring atoms and then cause lattice distortion during the displacement.

The number of atoms capable of motion by diffusin increases with increasing temperature.

Question 4

What is vacancy diffusion?

Answer 4

Atoms exchange with vacancies, (applies to self and inter-diffusion).

The rate of vacancy diffusion depends on the number of vacancies and the activation energy to exchange.

Question 5

What is interstitial diffusion?

Answer 5

Smaller atoms (e.g. H,C,O and N) can diffuse between larger atoms.

This is more rapid than vacancy diffusion due to more mobile small atoms and more (empty) interstitial sites.

Question 6

Name three processes which employ diffusion.

Answer 6

1. Case hardening
2. Hot-dip galvanising
3. Doping

Question 7

What is case hardening?

Answer 7

An example of interstitial diffusion can be seen in a case hardened gear. Carbon atoms are diffused into the host iron atoms on the surface. This produces an alloy which is difficult to deform or crack.

The carbon atoms lock the planes from shearing and put the surface in compression.

Question 8

What is hot-dip galvanising?

Answer 8

Process of coating iron, steel or aluminium with a thin zinc layer, by passing the metal through a molten bath of zinc at temp of approx 460C.

When exposed to atompshere the zinc layer reacts with oxygen (forms zinc oxide) which further reacts with CO2 (forms zinc carbonate). This a dull, strong material.

Question 9

What is doping by diffusion?

Answer 9

Controlled concentrations of impurities are diffused into specific regions of a device (e.g. integrated circuit) to change its properties (e.g. improve electrical conductivity).

Question 10

What 4 characteristics of a material allow for fast diffusion?

Answer 10

1. Open crystal strcuture
2. Lower melting temperature
3. Materials with secondary bonding
4. Smaller diffusing atoms

Question 11

What 6 characteristics of a material do not allow for fast diffusion?

Answer 11

1. Close-packed structures
2. High melting temperature
3. Materials with covalent bonding
4. Larger diffusing atoms
5. Anions
6. Higher density materials