Properties of Material Flashcards
How are different microstructures achieved
By different cooling rates and the way in which the solid is processed subsequently
How do solids form
When liquids cool and solidify
Or when a gas condenses
As the particles lose energy and their motion becomes slower, the pack closely together - the way in which those particles pack together will determine its microstructure
Describe Amorphous structures
Disordered and resemble the arrangement of particles in a liquid - no long-range order within their molecular structure
Describe Crystalline Structures
Ordered, they have long range order within their molecular structure
What happens to poly-crystalline as they solidify
They form crystals which grow towards each other and they meet at grain boundaries
What is the distance the human eye can distinguish two points between
0.2 mm given sufficient light, without the aid of any additional lenses
What is the limiting condition of resolution known as
Rayleigh’s Criterion
When are images said to be just resolved
When the central maximum of one image falls on the first minimum of another image
Formula for the minimum angular resolution for a circular aperture
Theta = 1.22 (wavelength of the light / diameter of the apature)
What is the resolution of a light microscope
200 nanometers
How do Scanning Electron Microscopes work (SEM)
Specimens are gold sputter coated and then mounted in a vacuum chamber
A fine beam of electrons is scanned across the surface
A detector picks up the backscattered electrons to form an image
How do Transmission Electron Microscopes work (TEM)
Hot filament produces the electrons
Anode accelerates electrons
Magnetic lenses deflect the electrons as they pass through the coil
Further magnetic lenses focus the electrons that pass through the sample onto a
fluorescent screen
Image formed on the screen
How do Scanning Tunelling Microscopes work (STM)
Scanning a very sharp metal wire tip over a surface
Apply an electrical voltage to the tip or sample
Image the surface at an extremely small scale - down to resolving individual atoms
Resolution of SEM
1-20 nm
Resolution of TEM
0.2 nm
Basic model of metallic structure
Positively charged metal ions surrounded by a sea of mobile delocalised electrons
What occupies the lattice points in the crystal lattice
Atoms, molecules or ions
Metal characteristic properties
High Young Modulus
Toughness
Malleability
Ductility
Lustrous
Good Thermal Conductors
Good Electrical Conductors
What is a co-ordination number
The number of atoms a given atom is formed
Packing name for a co-ordination number of 4
Square Close Packing
Packing name for a co-ordination number of 6
Hexagonal Close Packing
What are dislocations
Type of defect in the crystal lattice
Areas where the atoms are out of position in the crystal lattice
Where are dislocations generated
Generated and move when a stress is applied
What does the motion of dislocation allow
Slip-Plastic Deformation to occur
Movement of the edge dislocation
When a force is applied - dislocations move through the lattice structure
Why are metals so malleable and ductile
Ease of movement of dislocations
What do moving dislocations cause
Plastic deformation of the metal
What is toughness
Ability of a material to absorb energy and plastically deform without fracturing
Amount of energy per unit volume that a material can absorb before rupturing
Where can toughness be found
Area under a stress-strain curve
Why are metals malleable
The bond between the atoms can be broken easily and reformed
Force binding the metals are non directional
What does the ductility of a metal describe
Its ability to be drawn out into a wire
What makes a metal ductile
The ability of the layers of atoms to slip past each other
How can atoms slide over one another
Applying a force that makes the atoms in one of the planes slide past the atoms in an adjacent plane
What does a band structure of a solid describe
Ranges of energy that an electron within the solid may have and ranges of energy called band gaps
What is the conduction band
Band of electron orbitals that electrons can jump up into from the valence band when excited
What happens when electrons are in the conduction band
They have enough energy to move freely in the material
This movement of electrons creates an electric current
Band theory in solids
Atoms are packed closely together
For each original level there will be many levels
Spacing between the atoms varies - levels will also be altered by different amounts
Results in a band of extremely closed spaced levels corresponding to one particular level in the isolated atoms
Solids can be considered to have energy bands that are shared by all atoms
Band theory of conductors, insulators and semiconductors
Electrical conductivity of a solid will depend on the spacing of its energy bands and the extent to which they are occupied by electrons
Electrons in the conduction band are able to move freely
If Electrons in the valence band are provided with some additional energy - they can move up into the conduction band
The fermi level is half the bandgap energy
In a conductor - the valence and conduction bands overlap - making it very easy for the valence electrons to move up into the conduction band
Outermost electrons in a metal
Responsible for metallic bonding
Also the conduction electrons involved in the flow of electric current through the metal
Means there is no gap between the valence and conduction bands
There is a single band which is partially filled instead
Band theory for insulators
The valence band is full and the conduction band is empty - therefore there are no available free electrons for the conduction of electrical current
Also a very wide energy gap between the two bands - a large electrical potential is for electrons to cross into the conduction band
Thermal Conduction
Tight packing of atoms in a metal means that kinetic energy can be transferred from one atom to another both rapidly and efficiently
Dleocalised electrons can also collide with ions in the lattice
Thermal insulators
The denser the material the closer its atoms are together
Means the transfer of energy of one atom to the next is more effective
Less dense materials are better insulators
Band theory used to explain optical properties of metals
Metals are opaque
This is because incident radiation has frequencies within the visible range and this excites electrons into unoccupied energy states
Most of the absorbed radiation is remitted from the surface in the form of visible light of the same wavelength - which appears as reflected light
What makes metal appear lustrous
A portion of the energy captured when the metal absorbs light is turned into thermal energy
Rest of the energy is re-radiated by the metal as reflected light
Free electrons are able to absorb energy
What is the Fermi Level
The highest energy level that an electron can occupy at the absolute zero temperature - lies between the valence band and conduction band
