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Porosity in Materials

What is porosity?

Pores are spaces in materials.

Pores are present in many materials, resulting in a material that is not fully dense. An example of pores are in sponges, where the gaps can hold water and help you wash dirty plates and dishes! However, pores are also found in other materials such as ceramics, metals and polymers.

Understanding porosity:

Pores can change the behaviour of a material. Therefore, as a materials scientist we need to understand the following points:

  1. How do the pores affect material properties – is the material a better insulator of heat or is it easier to break with more pores?
  2. Can we control the number of pores and their size?
  3. Can we image the pores and understand where pores are located?

The following resources and activities were displayed at the Big Bang Fair, National Exhibition Centre (NEC), Birmingham in Summer 2023.

1) Materials with pores: Sponges and Rocks

Examples of materials include foams, aerated chocolate bars, sponges and rocks (e.g. pumice stones).

2) Four boxes demonstrating packing

The boxes are a model for particle packing, and contain different sizes and geometries of particles. This is suitable for thinking of crystal structures or for powder processing in manufacturing ceramics.

Box 1: Big spheres, all the same size.

  • How many balls can I fit in a box?
  • How could I calculate how many balls are in the box?

Box 2: Small spheres, all the same size.

  • If I use smaller balls, can I fit more in?
  • What about the total volume of the balls and pores, does this change? (No, assuming a completely full box)

Box 3: A combination of the spheres from Box 1 and Box 2.

  • I can achieve better packing if I use bigger and smaller particles.
  • Why is calculating the number of balls now more difficult? (Fill the interstitial sites between the balls)

Box 4: A combination of different geometries.

  • What limits me using spheres? Particles can be different shapes.
  • How could we fill all the gaps? (Apply understanding from Box 3 and Box 4 – Use different sizes and use different shapes).

Further details on calculating packing fractions for particular crystal structures can be found here: PDF_Packing_Fraction_Calculations_20230617_DS.

3) Squash test (mechanical properties of porous materials)

Foam balls placed between Perspex discs show how pores affect the mechanical properties of ceramics. When there are less balls between the perspex discs, it is easier to squash the discs. This relates to the stiffness of materials as a function of porosity. Denser materials are stiffer.

  1. Can you think of examples of stiff materials? Bone, Concrete, Ceramics.
  2. Can you think of examples of materials which are easy to stretch? Elastic, Polymers and Rubber.

Further examples of porous materials:

  • Polymers – Sponges and foams

Why do we want pores in these materials? How do we make the materials to get pores? Introduce gases in manufacture.

  • Ceramics – Thermal Barrier Coatings for aeroplane jet-engines

Pores in these materials make them better insulators. Why do pores make better insulators? (Hint: Think about conduction,       convection and radiation mechanisms for heat transfer).

  • Metals – Additively Manufactured Components (3D printing)

Metals containing pores can make them easier to break. Taking images of pores can be done using X-ray computed tomography. You may know about these if someone has broken a bone and has had an X-ray scan.

Intended Learning Outcomes:

  • Describe materials where pores are present.
  • Describe the relationship between porosity and stiffness.
  • Propose a method of calculating the number of balls in a box.
  • Explain how pores can influence the properties of materials.
  • Evaluate whether pores are good or bad in these materials.

Curriculum links:

  • Maths – Geometry and Shapes – Volume and Surface Area of Spheres. Pythagoras’ Theorem in 2D and 3D. 3D Shapes.
  • Physics – Particle model and differences in arrangements, shape and density.
  • Chemistry – Materials properties and applications.

Extension: Common crystallography of metals

Metals have different crystal structures. The different crystal structures of metals affect the packing of metal ions. Some examples of different crystal structures include: Face-centered cubic (FCC) – Aluminium; Body-Centered Cubic (BCC) – Iron, and Hexagonal Close Packed (HCP) –  Zinc. Further examples of materials with particular crystal structures and materials containing porosity can be found here: PDF_Crystal_Structure_Examples_20230617_DS.

Raw Materials

The resources used at the Big Bang Fair 2023 stand are listed here: DM_Porosity_In_Materials_Resource_List_20230617_DS.

For more information, or questions regarding this activity, please contact use by using the ‘Contact Us’ form.

 

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