3D lattices

Question 1

what are porous solids

Answer 1

materials having a large amount of porosity, 70+ vol.%, made of an interconnected porous network of solid struts or plates which form the edges of the cells or pores.

they have 2 phases:
- continuous solid phase
- continuous or not gas phase (pores)

they can be natural (cork, wood, human bones, marine sponges, flower stems) or artificial ( honeycomb panels, expanded polymers, 3d structures)

Question 2

different cell structures of pores

Answer 2

• open-cell structure:
  cells are interconnected by their cell edges are permeable and have large internal surface area per unit volume
  ✓ filters, gas-flow control devices
  ✓ electrodes, batteries, catalysts
  ✓ heat exchangers, bearings
  ✓ biomedical implants
• closed-cell structure:
  cells are isolated from each other by solid faces
  ✓ vibration damping, sound attenuation, energy absorption
  ✓ thermal insulation
• cell architecture can be periodic (= have a precise single unit cell) or stochastic

Question 3

introduction to foams

Answer 3

Polymeric (much more) and metallic (less diffused) foams are considered as commercial semi-products for a wide range of fields, for functional and structural applications
- Closed-cell foams are mainly addressed to lightweight parts
- Open-cells are mainly targeted toward functional applications

examples:
- Metallic foams are semi products, used for dampers, or as structural part, they are really light
- polymeric foams for Packaging applications to protect from impacts the object inside

Question 4

foams applications

Answer 4

• heat exchangers
• filters
• bearings
• sound absorbers
• silencers
• catalyst supports
• bio-medical implants
• energy absorbers

Question 5

outstanding properties of metallic foams

Answer 5

3 stages os stress-strain curve

• almost linear elastic behaviour
• long plateau of platicity ( corresponds to the situation where all the cells collapse)
  –> high mechanical work needed to collapse the material, high impact energy
• densification of the cells and deformation close to behaviour of fully dense material

Question 6

Manufacturing methods of metallic foams

Answer 6

difficult to assure homogeneous
distribution and size of pores.

there are several manufacturing methods:
- deposition (vapour or electrochemical deposition)
- frequently with metals (but also powders) we use them in liquid stage or powder

we have seen 3 example of manufacturing routes:

• liquid state processing ( we add calcium to increase viscosity, add foaming agent, isothermal foaming to activate the foaming agent, cooling)
• mixing of powder with blowing agent (mix powder and foaming agent, consolidation and extrusion, put extruded bar in a furnace to activate foaming agent)
• casting around space holder and then dissolving them

Question 7

nano-porous metals

Answer 7

• we scale down porosity to the size of micrometers
• npms artificially designed to take the form of monolithic small size bodies
• by definition, NPMs have porosity fraction greater than 0.4 and pore diameters of 1-100 nm
• they can be used for many applications such as catalysis, separation and sensin
• Size of pores comparable to wavelength of light We can use them as filters for light (they catch some wavelength and avoid some others)
  -ultrahigh specific surface area due to
  nanosized porosity, good electrical conductivity, high structural stability, and tunable qoptical properties.
• surface area becomes increasingly high now surface tension effect becomes predominant