Optical, Magnetic, Thermal and Electrical Properties of Engineering Materials

In electromagnetic radiation, what are the orientations of the electric field, magnetic field and direction of propagation in relation to each other?

Electric field and magnetic field perpendicular to each other

both components perpendicular to direction of waves propagation

What form of radiation has the largest wavelength?

radio waves

What form of radiation has the highest energy?

Gamma rays

What colour in the visible light spectrum has the longest wavelength?

red

What is the approximate range of wavelengths for visible light?

0.4μm to 0.7μm

What is the equation of visible light, c?

c = 1/ rt(ε0*μ0)

What is the result of interaction between the electric field in a light wave and the electron cloud surrounding the atom in a material?

electronic polarisation

electron cloud temporarily distorted creating a temporary dipole moment

one side of electron cloud slightly positive and the other slightly negative

Effect of overlapping energy in metallic materials and partially filled bands on optical properties

electrons easily absorb photon energy and move to higher energy states

near total absorption of visible light makes materials fundamentally opaque

What causes high reflectivity of light in metallic materials?

Absorbed photons are rapidly re-emitted

Effect of band gap separating fully occupied valence band and empty conduction bands on optical properties of non metallic materials

can exhibit` wide array of optical properties

dependent on size of band gap

Refraction in nonmetal

velocity of light reduced due to electronic polarisation

light bends

When does reflection occur?

at the interface between two materials with different refractive indices

When does transmission occur in non metals? (transparency)

if non metals band gap is larger than the energy of visible light photons

passes through without being absorbed

What happens when band gap of non metals is smaller than energy of the photons?

photons are absorbed

leading to opacity

What are the three causes of internal light scattering in non metals?

porosity

grain boundaries

secondary phases

How does porosity impact light scattering in non metals

pores of gas in ceramic or polymer material

light scatters at boundaries of pores due to significant difference in indices of refraction

How do grain boundaries cause light scattering in polycrystalline materials?

variations in crystallographic orientation between adjacent grains

leads to different refractive indices across boundaries

causes scattering

How do secondary phases impact scattering in materials?

Composite materials or materials that contain secondary phase particles have a mismatch in refractive indices between components

causing dispersion of incoming light

Define luminescence

phenomena where material absorbs energy and subsequently emits visible light

Four major applications of optical phenomena

Luminescence, photoconductivity, lasers and optical fibers

Define Photoconductivity

materials electrical conductivity becomes greater upon exposure to light

Prime application of photoconductivity

photovoltaic solar cell

What makes photovoltaic solar cell?

consists of polycrystalline silicon

fabricated to form a p-n junction

How does a photovoltaic solar cell work?

photons strike the solar cell

interact with semiconductor material

if E of photon is sufficient, electrons excite

excited electrons move from valence band to conduction band

excitation creates pair of charge carriers

puts electron in conduction band on n side and a corresponding hole on the p-side

• hole is a positively charged vacancy

new holes and electrons drawn away from p-n junction in opposite directions

flow out of semiconductor material and become part of external circuit

What does laser stand for?

Light amplification by stimulated emission of radiation

Two key features of lasers:

Output beam is

• monochromatic

• coherent

What is a coherent light source?

all emitted waves are completely in phase with eachother

Distinctive feature of a semiconductor laser:

composed of several distinct layers of semiconducting materials

each with different compositions

What are the distinct layers of a semiconductor laser sandwiched between?

metal conductor and a heat sink

Provide an example of compounds that may make up a typical semiconductor laser (according to the summary notes):

• central layer of heavily doped Gallium Arsenide (GaAs)

  • outer layers of p-doped and n-doped Gallium Aluminium Arsenide (GaAlAs)

What is the purpose of the distinct composite layers in the semiconductor laser?

To restrict excited electrons, generated holes, and the resulting laser beam, within the central gallium arsenide layer

Semiconductor laser needs a constant supply of …

charge carriers

constant applied voltage used to drive current through semiconductor

Result: constant and steady supply of electrons and holes

How are photons of light emitted?

Excited electron in conduction band recombines with hole in valence band

energy released in form of photon

What occurs after the first recombination?

Second recombination occurs

both photons have same wavelength and are perfectly in phase with each other

chain reaction occurs - producing avalanche of additional photons

all join the monochromatic, coherent laser beam

How is laser beam amplified?

Acts like optical cavity

one end of the beam is totally reflecting

allows beam of coherent photons to bounce entirely back into semiconductor material at fully reflective end of the beam

stimulates even more recombinations

opposite end of laser is only partially reflective

Benefits of semiconductor laser beam acting like optical cavity

allows portion of beam to be amplified

some of coherent laser beam escapes device for practical use

enough light maintained inside to maintain stimulated emission cycle

Uses of lasers:

non-contact measuring, welding, cutting, hole machining

What are optical fibers and how do they work?

application optical properties of non-metallic properties

confine light and transmit over far distances

Utilise phenomena of total internal reflection

relates directly to index of refraction of the fiber material

Benefits of optical fibers

minimal loss of light

transmit data exponentially faster than traditional copper wires

How do engineers improve performance of optical fibres

manipulate refractive index profile of the fiber

create gradual variation of the index of refraction

create a graded index near outer surface of the fiber

How do engineers create graded index of optical fibers?

deliberately add specific impurities to the material

control concentration of impurities

refractive index engineered to decrease from centre of fibre to outer edge

acts to smoothly bend straying light rays back towards centre axis of fibre

Effect of continuous variation of refractive index within optical fibres

reduces signal dispersion

allows light pulses to travel long distances without distorting

Equation for Magnetic Field Strength:

H = (N*I)/L

H - magnetic field strength (A/m)

N - # of turns in the coil

I - current

L - length

Equation for magnetic flux density/magnetic induction in a vacuum:

B = (μ0)*H

B - magnetic flux density (Tesla, T or Webers per square meter, Wb/m²)

μ0 - permeability of a vacuum

H - magnetic field strength

B = μ*H in material medium

μ - permeability of the medium

What does vector magnetization, M describe?

the magnetic state of a solid

defined as the magnetic moment per unit volume R

Relationship between B, H and M:

B = μ0*H + μ0*M

B is the sum of the external field contribution and the material’s internal contribution

Two sources of electron generated magnetic moments:

Orbital motion

Electron spinD

Describe process of orbital motion

electron orbits nucleus

behaves like small current loop

generates magnetic field along its axis of rotation

Describe process of electron spin:

electron spins on its axis

generates spin magnetic moment

magnetic moment quantised and oriented either up or down

in atoms, electrons usually spin in pairs and magnetic moments cancel each other out

net magnetic moment of atom, as a result of cancellation, usually 0

Why do some atoms have a net magnetic moment and which ones?

some transition metals and rare earth metals

incomplete electron shells result in unpaired electrons

result: net magnetic moment

What is diamagnetism?

exhibited by all matter

often masked by stronger magnetic fields

field applied which induces dipole moment

dipole moment opposes applied field direction R

Result of dipole moment opposing applied field direction:

negative magnetic susceptibility X_m

usually of order x10^-5

Name some typical diamagnetic materials

Aluminium Oxide

copper

gold

silicon

zinc

What behaves as a perfect diamagnet?

Superconductor

B=0

What are the features of paramagnetic materials?

atoms possess permanent magnetic dipoles

dipoles randomly oriented in the absence of an external field

exhibit small, positive susceptibility

What happens when magnetic field applied across paramagnetic material?

dipoles align with field

enhances the magnetic field

Specific examples of paramagnetic materials

Aluminium, chromium, titanium, molybdenum

Both diamagnetism and paramagnetism are considered … because…

non-magnetic

magnetization only occurs while in an external field

effect is very weak

Ferromagnetism in materials such as….

iron, cobalt, nickel

rare earth elements such as gadolinium

Describe the magnetic behaviour of ferromagnetic materials

strong

What is the behaviour of magnetic moments of individual atoms?

spontaneously align with each other

even in absence of external field - retain magnetization when field removed

due to quantum mechanical exchange interactions

Magnetic susceptibilities of ferromagnetic materials

often high as 10⁶

compared with -10^-5 for para and dia

so very large

What is saturation magnetization in ferromagnetic materials?

As applied field increases

more domains align

all dipoles are parallel

material reaches saturation magnetization

max possible magnetization for that material

Describe the alignment of adjacent atoms in Antiferromagnetic materials

coupling between magnetic moments of adjacent atoms results in antiparallel alignment

dipoles of neighbouring atoms point in exactly opposite directions

cancel each other out

RESULT: zero net magnetic moment for solid as a whole

What is the net magnetic moment of an antiferromagnetic material?

zero

Example of an antiferromagnetic material

Manganese Oxide

Mn²⁺ ions have net moments that align antiparallel to their neighbours

O^2- ions have no net moment

How is ferrimagnetism similar to antiferromagnetism?

magnetic moments align in an antiparallel fashion

What differentiates ferrimagnetism from antiferromagnetism?

opposing moments are unequal in magnitude in ferrimagnets

What is the result of opposing moments being unequal in magnitude in ferrimagnets?

causes incomplete cancellation and net spontaneous magnetization

What are the most common ferrimagnets?

cubic ferrites

formula: MFe₂O₄

_{looks at notes for specifics of ions}

hexagonal ferrites e.g. barium ferrite - used in permanent magnets

garnets like Yttrium Iron Garnet - used in microwave electronics

What does thermal energy do to magnetic properties?

thermal energy opposes the orderly alignment of magnetic moments

What is the Curie temperature?

For Ferromagnetic and Ferrimagnetic materials

T increases - causes atomic thermal vibrations to counteract the coupling forces aligning the moments

At Curie T - the spin alignment is destroyed and material becomes paramagnetic

• EG iron is ferromagnetic below 768C and paramagnetic above it

What is the Neel Temperature?

above Neel T, antiparallel alignments is destroyed

material becomes paramagnetic

Describe the alignment of dipoles (WITHIN DOMAINS) in ferromagnetic materials- even when not magnetised

in each domain

all magnetic dipoles are aligned parallel to one another - reaching saturation magnetization

in ferromagnetic materials how are adjacent domains oriented

vectors of adjacent domains are oriented randomly

summing to net zero magnetization for the bulk material

How are adjacent domains separated?

adjacent domains are separated by domain walls

narrow regions where the direction of magnetization gradually changes from the orientation of one domain to another

List the three things that occur when an external magnetic field is applied to an unmagnetized ferromagnet

1. Domain growth

2. Rotation

3. Saturation

What is domain growth?

domains that are parallel/nearly parallel to applied field

grow at the expense of unfavourably oriented domains

What is rotation in the magnetization process?

as the field strengthens

domains that have grown eventually rotate their magnetization vectors

to align with the external field

What is saturation in the magnetization process?

once all domains are aligned

the material reaches saturation flux density

What is hysteresis?

when applied field is reduced from saturation

curve does not retrace its original path

what is remanence, Br?

when the applied field is reduced to 0

material retains some magnetisation

residual flux density is called remanence

why does remanence exist?

resistance to the movement of domain walls

preventing them from returning to their original configuration

What is coercivity?

the magnitude of the reverse field applied in the opposite direction to bring B back to 0

What does the area enclosed by the B-H hysteresis loop represent?

the energy dissipated as heat during a complete magnetization cycle.

crucial in designing AC magnetic devices like transformers

Describe the characterisation of soft magnetic materials and what the characterisation means

characterised by narrow hysteresis loop

meaning they have low coercivity and low remanence

describe the properties of soft magnetic materials

easily magnetised and demagnetised with little energy loss

high initial permeability

low hysteresis energy loss

applications of soft magnetic materials

essential when magnetic field must be rapidly reversed:

• transformer cores

• motors

• generator

Examples of soft magnetic materials

• silicon iron alloys

• amorphus metals: metallic glasses (eg. FeBSi alloys)

  • metallic glasses produced by rapid solidification

  • have non-crystalline structures

  • exhibit extremely low hysteresis losses

  • used in high efficiency transformers

describe the characterisation of hard magnetic materials and what this means

wide hysteresis loop

high coercivity and high remanencedes

describe the properties of hard magnetic materials

once magnetised, they are difficult to demagnetise - permanent magnets

high energy property BHmax - represents the energy required to demagnetise the magneta

applications of hard magnetic materials

permanent magnets in:

• speakers

• motors

• sensors

• MRI machines

Examples of hard magnetic materials

• alnico

• hard ferrites

what is alnico? BLURT

alloys of aluminium, nickel, cobalt and iron

precipitation hardened to create microstructure that impedes domain wall motion

result - high coercivity

rare earth magnets:

• neodymium-iron-boron

  • currently strongest commercially available permanent magnet

  • essential for miniaturised electronics and electric vehicle motors

• samarium cobalt

  • not as strong at common temperatures

  • performs better at T>150C

What are hard ferrites?

barium and strontium ferrites

ceramic permanent magnets

magnetic prop inferior to rare earth magnets

inexpensive and widely used in low cost applications

What is superconductivity?

observed in some materials

0 electrical resistance and perfect diamagnetism below a certain temperature

used in:

MRI

high speed maglev trains - magnetic repulsion for levitation

efficient power transmission lines

primary limitation:

need for cryogenic cooling (liquid helium or nitrogen) to maintain superconducting state