Materials T7 - Functional Properties

Conductors

Allow for free flow of electrons (i.e. metals)

Semiconductors

Allow for some flow of electrons (e.g. silicon)

Insulators

Allows little/no flow of electrons (e.g. plastics, ceramics)

Electrical Current

Motion of electrically charged particles from externally applied E field

Solid Metal Energy Band Structures (copper)

Metal Energy Band Structures (magnesium)

Overlap of empty + filled band (3s and 3p bands overlap)

Semiconductor Band Structures

One band filled, separated from empty conduction band w/ small gap

Insulator Band Structures

Same as semiconductor, but w/ large gap

Valence Bands

e- band filled with electrons

Conduction Band

Empty e- bands

Conduction in Metals

Excited e- from filled states promoted to adjacent energy states w/ little energy required

Conduction in Semiconductors and Insulators

Energy gap must be overcome to move e- to conduction band

Inpurities

Reduces mobility and conductivity in metals

Lattice Defects (dislocation and grain boundaries)

Reduces mobility and conductivity in metals

Atomic Vibrations at T > 0K

Reduces mobility and conductivity in

Intrinsic Semiconductor

Pure materials (Silicon, Germanium) - for every e- excited, there is missing e- left behind

Opposite Direction

Movement of e- and holes

Extrinsic Semiconductors

Dictated by impurities, impurity atoms added to increase conductivity

n-type Extrinsic Semiconductor

Impurity atom w/ valence of 5 added as a substitutional impurity, creates filled E level just below conduction band

Donor State

An E level just below the conduction band (no holes are created when conducting)

p-type Extrinsic Semiconductor

Impurity atom with valence of 3 is added, creates E level just above valence band

Acceptor State

An E level just above valence band (only holes are move when conducting)

Material at T = 0K (1)

Conduction band empty

Freeze-Out Region (2)

More e- and holes excited from donor/acceptor states to conduction/valence bands

Extrinsic Region (3)

All e- and holes excited to conduction/valence bands (e- concentration plateaus)

Intrinsic Region (4)

Temperature high enough to excite more e- to conduction band and leave holes in valence band

Mobility of e-

Greater than the mobility of holes (as holes are heavier)

p-n Junction

At depletion zone, e- and holes combine and annihilate, E-field generated which creates a voltage barrier across junction

Forward Bias

Reduces p-n junction’s depletion layer, allows high current flow

Reverse Bias

Increases p-n junction’s depletion layer, blocks current flow

Advantages of Semiconductor Devices

Small size, low power consumption, no warm-up time

Photovotaic Solar Cells

When excited by sunlight, e- attracted toward p-type region and holes attracted toward n-type region

Magnetic Properties

Consequence of magnetic moments from e-

Types of Magnetic Moments

e- orbits around nucleus + e- spin on axis

Relative Permeability

Measure of degree to which material can be magnetised

Diamagnetism

Weak form of magnetism that only persists when external field is applied

Diamagnetic Properties

Induced by change in orbital motion, with extremely small magnitude and direction opposite to applied field

Paramagnetism

Each atom possesses a permanent dipole moment by incomplete cancellation of e- spin and/or orbital magnetic moments

Paramagnetism Properties

No net magnetism without external magnetic field. With external field, dipoles free to rotate indiviudally - dipoles align in external field

Ferromagnetism

Permament magnetic moments result from atomic magnetic moments due to: e- spin, orbital magnetic moments, and coupling interactions

Ferromagnetism Properties

Displayed by transition meetals, magnetic susceptibilities reach up to 10^6

Hysteresis loop

Indicates magnetic energy loss per volume (converts to heat, increases temp)

Large Hysteresis Loop

Hard magnet, energy loss larger

Small Hysteresis Loop

Soft magnetic, energy loss smaller

Hard Magnet

Once magnetised, stay magnetic for a long time and cannot easily have their directions switched

Soft Magnet

Easy to magnetise and demagnetise, and lose little energy when switching directions

Increasing Temperature of Magnet

Increases thermal vibration of atoms, leads to dipole misalignment (decrease in saturation magnetisation)

Max Saturation of Magnet

0 Kelvin

Curie Temperature

Transition temperature where certain materials lose their permanent magnetic properties

Heat

Transfer of thermal energy due to a difference in temperature

Thermal Conductivity

Heat transferred from high- to low-temp regions

Direction of Flux

Perpendicular to heat flow direction

Heat Flux

Thermal conductivity x termperature gradient

Phonons

Lattice vibrations, through which heat is transported

Means of Heat Transport

Phonons and movement of free electrons

Vibrational Heat Capacity

Consists of elastic waves with range of distributions/frequencies

Effect of Alloying

Decreases conductivity (impurities slow electron motion)

Light

Electromagnetic radiation consisting of waves made of photons

Forms of EM Radiation

Light, heat, radar, laser, microwave, radio wave, infrared, x-ray

Radiation (light intensity)

Energy being trasmitted per unit time across unit area perpendicular to direction of propagation

400-800nm

Visible light wavelength

White Light

Mixture of all colours (wavelengths)

Light Transmission

Light completely passes through object (transparent object)

Light Absorption

Atoms absorb photons as energy

Electronic Polarisation

Caused by E-field component interacting with electron cloud (some energy absorbed)

Emission of Photon

Stimulated electron cannot remain excited, decays to ground state

Transparent

Little/no aborption or reflection

Translucent

Light transmitted diffusely, scattered within interior

Opaque

All light absorbed or reflected

Electrically Insulating Materials (Polymers/ceramics)

Can be made transparent or opaque

Can be opaque or transparent

Semiconductors

Intrinsically Transparent

Made transparent/opaque because of interor reflection and refraction

Sources of Internal Scattering

Grains boundaries in polycrystalline materials, two-phases in materials, internal pores

Polymer Translucency

Influenced by the extent of crystallinity, some scattering occurs between boundary of crystalline and amorphous regions

Highly Crystalline Polymers

Translucent or opaque due to high degree of scattering

Highly Amorphous Polymers

Completely transparent due to low degree of scattering

Refraction

Change in direction of a wave due to change in its speed (passing from one medium to another)

Refractive Index, n

The ratio comparing the speed of light in a vacuum to the speed of light in a specific material

Bends towards normal

Light enters medium with higher refractive index

Bends away from normal

Light enters medium with lower refractive index

Snell’s Law

The formula used to find the angle of refraction of light

Velocity of Light in a Medium

A function of electric permittivity and magnetic permeability of the medium

Isotropic Refractive Index

Independent of crystallographic direction (for ceramics w/ cubic crystal structures and glasses)

Anisotropic Refractive Index

Dependent on crystallographic direction (for non-cubic crystals)

Direction with highest density of ions

Highest refractive index for an anisotropic crystal

Reflection

When light radiation passes from one medium to another with different n, some light is reflected at interface of media

Reflectivity (R)

Intensity ratio between reflected and incident beams (how much light gets reflected)

Absorption Coefficient

A materials tendency to absorb light, large coefficient = more absorption

No absorption and transparent

Material with a band gap energy more than 3.1eV - band gap energy too great to absorb any photons

Full absorption and opaque

Material with a band gap energy less than 1.8eV - band gap small enough so all photons are absorbed

Partial absorption and coloured

Material with a band gap energy between 1.8 and 3.1eV - some wavelengths are absorbed

Colourless

Effect of uniform absorption for all visible wavelengths

Colour of Material

The result of the combination of wavelengths that are transmitted AND re-emitted (after absorption)

Optical Fibres

Transmits signals photonically through opitcally transparents fibres

Benefits of Fibre optics

Improved transmission speed, density & information density, reduction in error rate, with no electromagnetic interference

Components of Optical Fibre

Core, cladding, coating

Core

Innermost part of optical fibre where light signal passes through, made of high-purity silica glass

Cladding

Middle layer of optical fibre that constrains light rays within core

Coating

Outer layer of optical fibre that protects internal components from damage

Total Internal Reflection

Effect that occurs when angle of incidence exceeds the critical angle (very shallow angle), 100% of light reflects back into core