Porous materials general properties
- Are materials with a large amount of porosity (70% of volume).
- A solid structure made by interconnected porous network
- Two phases: solid and gas
- Natural cellular materials as: Cork, bones, marine sponges
- Artificial cellular materials are: Honeycomb panels, expanded polymers.
- Open-cell structure in which the cells are connected by edges and are permeable, large internal area per unit volume.
- Damping capabilities, energy absorption.
Classification
Summarizing the classification:
- Stochastic structures: open cell foams, closed cell foams, syntactic foasm, hybrid foams.
- Periodic structures: Lattice truss
Metallic and polymerc foams
- Polymeric foams are very diffused
- Metallic foams are used for light-weight applications.
The applications vary according to porosity, for example: Heat exchangers, filters, catalyst, silencers, sound absorbers, energy absorbers.
Some outstanding properties of metallic foams
- The typical behaviour of a metallic foams is almost linear behaviour, a collaps of cell that mean high ductility, densification of the cells that reach deformations and UTS near to a fully dense material.
Manufactoring routes of metallic foams
- Producing a metallic foams is very challenging and needs a very accurate and repetable to assure an homogeneous distribution of pores.
- Several ways to produce Metallic foams exist
- Vapour deposition
- Liquid metal
- Powdered metal
- Metal ions and electrochemical deposition
Manufactoring methods: liquid state manufactoring
- Liquid state processing: inflating gasses.
- Mixing of powder with blowing agent
- Filling the mould with space holders
Nano porous metals
NPMs are artificially designed to take the form of monolithic small-size bodies containing approximately 10e15 ligaments per mm^3. This correspond to an extramely high area over mass ratio.
Nano porous metals: properties
Since their extramely high specific surface NPMs shows some distictive properties:
- Since the ligaments and pores are smaller the light wavelenght, special optical properties are created.
- The final properties are a combination of bulk and surface forces (van der walls, tension …).
Manufactoring methods in NPMs
- Template methods can be used to replicate a porous template with good control of pores size and periodicity.
- Chemical or electrochemical dealloying is used to fabricate 3d configurations.
- Dealloying refers to the selective dissolution of one or more components of an alloy, generally the metal is couple with a less noble one and trougth a chemical or electrochemical process the less noble material dissolves leaving a porous structure.
Metallurgical aspects in NPMs
- Only some alloys are suitable for dealloying and only if well defined concentration, for example Au-Ag (Au 20-40%) less gold a suspension of gold is obtained, too much and the separation limit is excedeed.
- For Au-Ag: Usually the NPG retains the same polycristalline structure.
Ag tends to segregate at grain boundaries an this promote cracking.
If the recrystallization is promoted in the parent metal prior
NPMs as micro-actuators/chemical actuators
- By electrochemical polarization the extremely high surface area leads to very large space-charge-density at the metal-electrolyte interface.
- This will significantly affect the bonding between metals atoms at the surface, altering the mechanical equilibrium of the sample, so internal stresses are generated to compensate the modified surface bond forces.
- The surface stress is here changed by surface interaction with changing enviroment at the gas-metal interface.
- In NP gold ozone and monoxide of carbon act as a switch between distinct surface states, leading to reversible surface stresses
- By combination of different NP sample in bimetallic foils strains can be greatly amplified and actuations of millimiters can be achived.
3D lattice structures
Lattices are created laying out materials into precisely deisgned elemets to form structures of micro meta and macro dim, this allows to reduce weight and have control over other characteristics.
Is suitable also for structure that need to be semi-permeable as heat-exchangers and batteries.
Lattice design is according to:
- Lightweight design
- Shock absorption and impact control
- Vibration and noise damping
Mechanical behaviour of lattices: stability criterion
- The stability of strut-type is characterized by the Maxwell number M:
M=Ns -3Nn + 6
if M<0: there are few struts causing bending stresses
if M>=0: external loads are equilibrated, no bending occours, the structure are stretch dominated.
- As a result, stretch dominated structures, are stiff and strong, especially considering thier mass
applications
Mechanical part for light weight design and at the same time structural aim.
