Properties of Material

Question 1

How are different microstructures achieved

Answer 1

By different cooling rates and the way in which the solid is processed subsequently

Question 2

How do solids form

Answer 2

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

Question 3

Describe Amorphous structures

Answer 3

Disordered and resemble the arrangement of particles in a liquid - no long-range order within their molecular structure

Question 4

Describe Crystalline Structures

Answer 4

Ordered, they have long range order within their molecular structure

Question 5

What happens to poly-crystalline as they solidify

Answer 5

They form crystals which grow towards each other and they meet at grain boundaries

Question 6

What is the distance the human eye can distinguish two points between

Answer 6

0.2 mm given sufficient light, without the aid of any additional lenses

Question 7

What is the limiting condition of resolution known as

Answer 7

Rayleigh’s Criterion

Question 8

When are images said to be just resolved

Answer 8

When the central maximum of one image falls on the first minimum of another image

Question 9

Formula for the minimum angular resolution for a circular aperture

Answer 9

Theta = 1.22 (wavelength of the light / diameter of the apature)

Question 10

What is the resolution of a light microscope

Answer 10

200 nanometers

Question 11

How do Scanning Electron Microscopes work (SEM)

Answer 11

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

Question 12

How do Transmission Electron Microscopes work (TEM)

Answer 12

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

Question 13

How do Scanning Tunelling Microscopes work (STM)

Answer 13

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

Question 14

Resolution of SEM

Answer 14

1-20 nm

Question 15

Resolution of TEM

Answer 15

0.2 nm

Question 16

Basic model of metallic structure

Answer 16

Positively charged metal ions surrounded by a sea of mobile delocalised electrons

Question 17

What occupies the lattice points in the crystal lattice

Answer 17

Atoms, molecules or ions

Question 18

Metal characteristic properties

Answer 18

High Young Modulus
Toughness
Malleability
Ductility
Lustrous
Good Thermal Conductors
Good Electrical Conductors

Question 19

What is a co-ordination number

Answer 19

The number of atoms a given atom is formed

Question 20

Packing name for a co-ordination number of 4

Answer 20

Square Close Packing

Question 21

Packing name for a co-ordination number of 6

Answer 21

Hexagonal Close Packing

Question 22

What are dislocations

Answer 22

Type of defect in the crystal lattice
Areas where the atoms are out of position in the crystal lattice

Question 23

Where are dislocations generated

Answer 23

Generated and move when a stress is applied

Question 24

What does the motion of dislocation allow

Answer 24

Slip-Plastic Deformation to occur

Question 25

Movement of the edge dislocation

Answer 25

When a force is applied - dislocations move through the lattice structure

Question 26

Why are metals so malleable and ductile

Answer 26

Ease of movement of dislocations

Question 27

What do moving dislocations cause

Answer 27

Plastic deformation of the metal

Question 28

What is toughness

Answer 28

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

Question 29

Where can toughness be found

Answer 29

Area under a stress-strain curve

Question 30

Why are metals malleable

Answer 30

The bond between the atoms can be broken easily and reformed Force binding the metals are non directional

Question 31

What does the ductility of a metal describe

Answer 31

Its ability to be drawn out into a wire

Question 32

What makes a metal ductile

Answer 32

The ability of the layers of atoms to slip past each other

Question 33

How can atoms slide over one another

Answer 33

Applying a force that makes the atoms in one of the planes slide past the atoms in an adjacent plane

Question 34

What does a band structure of a solid describe

Answer 34

Ranges of energy that an electron within the solid may have and ranges of energy called band gaps

Question 35

What is the conduction band

Answer 35

Band of electron orbitals that electrons can jump up into from the valence band when excited

Question 36

What happens when electrons are in the conduction band

Answer 36

They have enough energy to move freely in the material This movement of electrons creates an electric current

Question 37

Band theory in solids

Answer 37

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

Question 38

Band theory of conductors, insulators and semiconductors

Answer 38

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

Question 39

Outermost electrons in a metal

Answer 39

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

Question 40

Band theory for insulators

Answer 40

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

Question 41

Thermal Conduction

Answer 41

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

Question 42

Thermal insulators

Answer 42

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

Question 43

Band theory used to explain optical properties of metals

Answer 43

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

Question 44

What makes metal appear lustrous

Answer 44

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

Question 45

What is the Fermi Level

Answer 45

The highest energy level that an electron can occupy at the absolute zero temperature - lies between the valence band and conduction band