Materials Diffusion Lecture 3

Question 1

What are the types of Diffusion?

Answer 1

Vacancy diffusion. Atoms are not prefectly arranged in reality. In our models we sometimes assume they are prefectly arranged but this is far from true. There are gaps between them, atoms leaving spaces etc. Vacancy diffusion is when there is a vacancy in the ordered arrangement. Either an atom which makes up the lattice can move to fill this vacancy (creating another vacancy where it just was because it moved) which is called Self-diffusion or a smaller solute atom can move to fill this vacancy. Both processes use vacant sites which are called Substitutional sites so this diffusion can be known as substitutional diffusion. The rate of Vacancy diffusion depends on the number of Vacant Sites.

Interstitial Diffusion. “Interstices”- an intervening space, especially a very small one. Small Solute atoms move through the lattice or ordered arrangement and because they are small they can move around many interstices. There are always many interstices so it is always faster than Vacancy Diffusion (/Substitutional diffusion).

Atom Exchange. Atoms swap locations. This requires a lot more energy than both processes above so does not occur often.

Question 2

What happens between solids?

Answer 2

You get diffusion between two elements in a solid. If a solid contains more than one element Impurity or Inter Diffusion takes place and the atoms of both elements mix within the solid.

Question 3

What is Flux?
Denoted by?
What must be true in equilibrium?

Answer 3

Flux is the amount of substance passing through a unit area per unit time. mass/surface area x time. Can have units mol/ cm^2 x seconds or Kg/m^2 x seconds.

It is Denoted by the letter J and the flux can measure the number of atoms (or mass of atoms) that are passing through a region per unit area per unit time or the number of vacancies which are being filled/ becoming available per unit area per unit time.

In equilibrium the number of vacancies being formed must equal the number of vacancies being destroyed.

Question 4

What must there be for Diffusion to occur?

Answer 4

A concentration gradient must exist.
A concentration gradient is the difference in the number of the diffusion species at different positions. dc/dx where C is the concentration of the diffusing species and x is the position. Check slides. The greater the difference in the concentration of the diffusing species between points Xa and Xb then the greater the Concentration Gradient.
Flux is proportional to the Concentration Gradient.

Question 5

Fick’s First Law?

Answer 5

The flux (the rate of mass transfer per area) is proportional to the Concentration Gradient.

J= -D dc/dx

D is the Diffusion Coefficient with units m^2/s

dc/dx is the concentration gradient with units atoms/m^4

J is flux with units mol or Kg or atoms/ unit area x time.

There is a negative sign because the atoms flow down the concentration gradient.

Question 6

How can we purify Hydrogen?

Answer 6

We use the process of diffusion to purify hydrogen. Hydrogen atoms are mixed with atoms of another element. The atoms of the other element are too large to pass through the Palladium membrane so remain on one side whilst the Hydrogen atoms are able to diffuse through the Palladium membrane to the region of low concentration.

Question 7

How can diffusion occur in Polycrystalline materials?

Answer 7

The Polycrystalline materials are made up of crystal grains. Adjacent crystal grains meet at grain boundaries. At these grain boundaries the arrangement of the atoms is less dense and diffusion can occur more easily along the grain boundary.

Question 8

What does Diffusion require?

Answer 8

Diffusion requires an activation energy. Some forms of diffusion require a higher activation energy than others. Vacancy diffusion has a much higher activation energy than interstitial diffusion and Atom Exchange diffusion has an even greater activation energy than vacancy diffusion. This is why interstitial diffusion occurs more often than vacancy diffusion on average.

Question 9

What factors affect the rate of diffusion/ promote diffusion?

Answer 9

The lower the melting point (so the weaker the bonds between atoms in the lattice) the more easily atoms can move around as they are not being held together as strongly.
The less dense that the crystal structure the more readily diffusion occurs.
The smaller the diffusing atoms the more readily diffusion occurs.
The less tightly packed the Unit Cells are (so therefore the overall crystal structure) the more easily diffusion can occur.

Question 10

Revise the Arrhenius Equation and the graphs in book. Look over Lecture 3 notes.

Question 11

What would interstitial diffusion look like in a BCC Unit Cell Lattice structure?

Answer 11

The smaller atoms will fit into the small interstices that are left between the main atoms (the middle of each face of the BCC Unit Cell.) Whereas vacancy diffusion would have to be actually swapping locations with one of the main atoms in the BCC Unit Cell.