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Materials 1A > Materials for Devices > Flashcards

Materials for Devices Flashcards

1
Q

Angle between C-C bonds

A

109.5 degrees

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2
Q

Polymers

A

Built up of repeating units (monomers)

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3
Q

Chem drawing for polymers (with (un)shaded triangles)

A

Shaded triangle = bond comes out of the page

Lined triangle = bond retreating into the page

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4
Q

Ends of carbon chains

A

The last carbon at the end of a carbon chain isn’t bonded to 2 carbons, only 1. So, you have to bond it with 2 extra bonds e.g. 2 extra hydrogens to maintain 4 bonds each

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5
Q

Random walk

A

Used to model the actual length of a polymer since they gets tangled up and don’t lie perfectly straight.

Proof on BH6

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6
Q

Kuhn length

A

Polymer chains are stiffer than they shown by the random-walk model. Bonds aren’t free to move in all directions.
The kuhn length is the length for which polymer chains are effectively straight and rigid.

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7
Q

Kuhn length in the random walk model

A

Length squared = (n/k)(l*k)^2
where n is the number of bonds, k is the kuhn length and l is the length of 1 of the bonds

So the rms length = n^(1/2) * k^(1/2) * l

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8
Q

How side-groups affect the stiffness of polymers

A

Increase the stiffness because they limit foldability

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9
Q

Crystalline materials

A

Have long range order i.e. are anisotropic

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10
Q

Anisotropic

A

Properties may differ depending on the direction of measurement

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11
Q

Liquids

A

No long range order i.e. are isotropic

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12
Q

Isotropic

A

Properties are invariant depending on direction

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13
Q

Liquid crystals

A

Anisotropic liquids

Molecules are free to move relative to each other i.e. no long range positional order

Molecules tend to line up i.e. there is orientational order

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14
Q

Orientational order in a LC

A

Defined by a vector called a director

Degree of orientational order depends on the temperature of the LC:
At low temperatures, the LC may crystallise and have long range positional order
At high temperatures, the thermal agitation overcomes the alignment interaction

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15
Q

Order parameter (Q)

A

Used to describe the degree of orientational order

x = mean of (cosy)^2
where y is the angle between a molecule and the director

Q = (3x - 1)/2

With all molecules aligned, Q = 1 since x = 1
With molecules randomly orientated, x = 1/3 so Q = 0

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16
Q

How light interacts with a polymer as it passes through it and how this is affect by long range order or lack of it

A

EM waves travelling through a sample interact with the electron clouds around a polymer.
Along the long axis of the polymer, there’s a lot of interaction; perpendicular to this axis, there’s a lot less interaction.
This has no effect if the molecules are randomly orientated.
There is a net effect if the molecules are aligned preferentially. The axis along which there’s more interaction is the slow axis. The axis with little interaction is the fast axis. Refractive index is different for the different axes.

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17
Q

PVDs

A

Permitted vibration directions

The axes along which light can travel along a molecules

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18
Q

Birefringence

A

The difference between the refractive indices of PVDs

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19
Q

How polarising filters work

A

Aligned polymers with large birefringence. The light along the slow PVD is absorbed

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20
Q

Optical path difference

A

How far behind the slow beam is when the fast beam reaches the end of a birefringent block.

(birefringence)*(thickness)

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21
Q

When light through a birefringent material will pass through crossed polars

A

The optical path difference is (n + 1/2) wavelengths i.e. the phase difference is pi
Plane of polarisation is perpendicular to before

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22
Q

When light through a birefringent material will not pass through crossed polars

A

The optical path difference is n wavelengths i.e. the phase difference is 0
Plane of polarisation is parallel to before

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23
Q

Complementary colour

A

When white light passes through a birefringent material, 1 or more wavelengths of light will be absorbed by the analyzer and the optical path difference is an integer number of wavelengths.
Light observed is the full optical spectrum minus these wavelengths

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24
Q

Why Michel-Levy chart becomes less vivid in higher order

A

More and more wavelengths begin to be absorbed by the analyzer as there are more combinations of integer values of wavelengths that are possible

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25
Q

Extinction positions

A

Occur when the incident plane polarised light oscillated entirely along 1 PVD. There is no phase change and so the analyser absorbs all the light

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26
Q

Determination of which PVD is fast and which is slow

A

Align sample and compensator so both PVDS are 45 degrees to the polarizer/analizer.

Fast aligns with fast or slow aligns with slow:
Optical path difference increases and observed colour moves up Michel-Lecy chart

Fast aligns with slow:
Optical path difference decreases and observed colour moves down Michel-Levy chart

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27
Q

Compensator

A

An anisotropic crystal with a known birefringence e.g. quartz

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28
Q

What happens if light is polarised along the director in a LC

A

The section is isotropic

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29
Q

PVDs in an LC

A

PVDs are perpendicular and parallel to the director

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30
Q

Domains in LCs

A

LCs don’t have single, uniform directors. They are broken into smaller, differently orientated regions

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31
Q

Disclinatior

A

When domains meet in an LC

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32
Q

Schlieren texture

A

Observed when an LC is under crossed polars

Dark regions are in extinction since the director is parallel to the plane of polarisation of the light
Light regions have a uniform director that isn’t parallel to the light’s plane of polarisation

Maltese crosses occur when all point towards a single centre: the centre of the cross

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33
Q

Smectic LCs

A

Molecules organise themselves into layers –> organisational order with some positional order

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34
Q

Chiral nematic aka cholesteric

A

Molecules align themselves into a helical twist

Director rotates in a plane. Therefore, the plane of polarisation of light rotates too

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35
Q

Pitch

A

The distance along the axis of a chiral nematic LC for a complete 360 degree rotation of the director

Depends on how close the stiff part of the molecules is to the chiral centre. The further it is away, the steepness decreases and the pitch increases.

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36
Q

Chiral molecules

A

Lack inversion symmetry

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37
Q

Chiral nematic LCs under crossed polars

A

Light polarised perpendicularly to the axis of the helix produces a stripy pattern like a finger print

When director is in the plane of polarisation, there’s extinction.
When the director is perpendicular to the plane of polarisation, there’s a bight spot.

The pitch is the distance between 3 subsequent dark spots

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38
Q

How to encourage the director of an LC to be in a certain direction

A

Create grooves on the surface the LC is in contact with

An LC sandwiched between 2 surfaces with grooves perpendicular to each other will twist.

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39
Q

Chiral nematic dopant

A

Helps to encourage the director of an LC to twist

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40
Q

LC cells

A

Analyzer and polarizer are parallel to the grooves on the 2 surfaces that the chiral nematic LC is sandwiched between.
Normally, all the PPL is transmitted through the analyzer as the plane of polarisation twists 90 degrees.

If an electric field is applied across the LC cell, there’s an induced dipole moment and the molecules align with the field. Molecules near the grooves maintain with orientation. Twisting of the plane of polarisation is disrupted and the analyzer doesn’t transmit light.

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41
Q

Freedericksz Transition

A

If an electric field is applied across the LC cell, there’s an induced dipole moment and the molecules align with the field.

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42
Q

Dielectric material

A

Electrical insulator for that it can support electrostatic charge

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43
Q

Polarisation

A

Dipole moment per unit volume

44
Q

Mechanisms of polarisation

A

Distortion of the electric cloud about a nucleus

Change the relative position of positive and negative ions

Rotate pre-existing permanent dipole moments

45
Q

Dipole moment

A

A vector

individual charge * displacement between then

46
Q

Charge density of a material aka displacement field

A

Sum of the polarisation of a material and the free space component.

= permitivity of free space * electric field + polarisation

= Electric field * permitivity of the material

47
Q

Dielectric constant of a material

A

permitivity of a material / permitivity of free space

48
Q

Capacitance

A

The units of charge a material can store per unit of potential difference across it

49
Q

Unique direction

A

A lattice vector which isn’t repeated anywhere else by the symmetry present

50
Q

Centrosymmetric

A

Every component is reflected through the centre of symmetry

51
Q

Polar material

A

demonstrates dielectric polarisation

must contain a unique direction

52
Q

Piezoelectricity

A

e.g. quartz

A stress applied to the material induces a shape change which moves the centres of positive and negative charge relative to each other and creates a dipole moment. Overall, there is a net change in polarisation.
e.g. igniters or flashing lights in trainers

OR

An electric field is applied to the material moves the ions which changes the shape of the material
e.g. displacement transducers

53
Q

Pyroelectricity

A

As the temperature of the material changes, the relative positions of positive and negative charges changes. There is polarisation

e. g. thermal imaging cameras or burglar alarms
e. g. ZnS

54
Q

Ferroelectrics

A

Application of an electric field can leave the crystal permanently polarised and this can only be reversed by applying an electric field in the opposite direction.

Ions in a crystal sit in double well potentials. An electric field pushes the ions to specific energy wells. When it’s removed, the ions remain in an energy well, so they don’t move and the material remains polarised.

e.g. BaTiO[3]

55
Q

Why there’s no polarisation of free space across a piezoelectric

A

There’s no external voltage since the voltage developes within the sample due to an applied stress. So, there’s no polarisation of free space as there’s no voltage across free space

56
Q

Curie Temperature (ferroelectrics)

A

Temperature at which the unit cell in a ferroelectic material spontaneously acquires a dipole moment due to a change from high to low crystallographic symmetry

57
Q

What effects the dielectric constant of ferroelectric materials

A
  1. Dopants or impurities
  2. Temperature:
    Approaching the Curie temperature as the sample cools, the dielectric constant increases rapidly. The small ions in interstitial sites vibrate less as they cool and there’s more space left to displace them and create a dipole.
    After passing the Curie temperature as the sample cools, the crystal system gets a greater packing efficiency at expense of symmetry. So, the interstitial sites are smaller. There’s less space to displace the ion into, so the crystal is harder to polarise. Dielectric constant falls rapidly.
    The dielectric constant increases again after more cooling.
58
Q

Energy balance of domains

A

Stray field energy is balances by domain wall energy.

The smallest total energy is optimal

59
Q

Poled ferroelectrics

A

Dipoles are preferentially aligned along the crystallographic direction that’s most close to parallel to the field.

60
Q

Hysteresis for ferroelectrics

A
  1. No E-Field, no polarisation = randomly orientated domains

E-Field increases:

  1. Reversible domain wall motion
  2. Favourably orientated domain grow and non-favourably orientated domain diminish
  3. Saturation polarisation: 1 single domain lined up along the crystallographic direction that most closely aligns with the field

E-Field decreases:
Polsarisation remains principally aligned.
With no E-Field, there’s the remanent polarisation

Reverse Field increases:

  1. Oppositely orientated domain grow.
  2. Coercive field is the strength of the E-Field required to reach 0 polarisation again
  3. Domains aligned with the reverse field continue to grow.
  4. Saturation polarisation in reverse direction
61
Q

Saturation polarisation

A

One single aligned domain in the ferroelectric

62
Q

What affects the size of the hysteresis loop

A

Domain walls can be pinned by crystal defects.

Area of the loop is proportional to the energy required to switch polarisation

63
Q

How reverse domains nucleate and grow

A
  1. Reverse field is applied
  2. oppositely polarised domains grow at the edges of existing domains
  3. Reverse domains grow along the direction of the field
  4. Reverse domains grow laterally aka perpendicularly to the field
  5. Polarisation is reversed when all domains form 1 domain
64
Q

Why ferroelectric is good

A
  1. Low voltage
  2. Non-volatile
  3. Radiation hard
  4. Small size and cost
65
Q

Why it’s good to keep a ferroelectric at a phase boundary

A

Can deform along more crystallographic directions and better align with an applied field as it can deform in both lattices

66
Q

Magnetic dipole

A

Produced by spinning electrons about a nucleus.

Depends on electrons in incomplete shells as these don’t all have partners with opposite spin.

67
Q

Magnetisation

A

Magnetic moment per unit volume (Am^2 / m^3)

68
Q

Magnetic field

A

Applied across a sample

69
Q

Magnetic moment

A

Current * Area

70
Q

Diamagnetism

A

No unpaired electrons
With no external field, no net magnetic moment

In an applied magnetic field, electron orbits react to oppose the field
Susceptibility is negative

e.g. graphite

71
Q

Paramagnetism

A

Some unpaired electrons –> dipoles exist, but they align randomly

With no external field, the overall moment is 0
With an external field, there is partial alignment and moments in the direction of the field
Susceptibility is small but positive
e.g. Na

72
Q

Ferromagnetism

A

Many unpaired electrons
Strong interaction between moments
Moments tend to align as parallel moments have the lowest energy.

Moments align with an applied field and orientation becomes permanent
Susceptibility is high and positive
e.g. Fe

73
Q

Antiferromagnetism

A

Many unpaired electrons
Strong interaction between moments
Moments tend to align BUT antiparallel moments have the lowest energy.

Net magnetic moment = 0
e.g. Cr

74
Q

Ferrimagnetism

A

Many unpaired electrons
Strong interaction between moments
Moments tend to align BUT antiparallel moments have the lowest energy.
Moments in opposite directions are NOT equal magnetudes

Net magnetism
e.g. magnetite

75
Q

Curie Temperature (ferromagnets)

A

Susceptibility becomes very high around the Curie temperature

As temperature increases, vibration of ions increases and coordination of electron spin states decays. Ferromagnetic materials becomes paramagnetic at the Curie temperature

76
Q

Magnetocrystalline anisotropy

A

Preferred direction of the magnetic moment depends on the crystalline direction.
Energy is lowest when the magnetisation points along a specific crystallographic direction.
There are easy and hard directions along which to magnetise a material

77
Q

Shape anisotropy

A

Easier for magnetisation direction to be along a long, thin sample than perpendicular to the long side of the same sample

78
Q

Magnetostriction coefficient

A

fractional change in length on changing magnetisation from 0 to saturation

79
Q

Magnetostriction

A

A magnetised crystal will deform

An applied stress to a crystal can lead to magnetisation

80
Q

Domain walls in magnetised materials

A

Can either transition between anti-parallel domains abruptly or through gradual twisting.

  1. Exchange interaction energy: Misaligned domains have high energy
  2. Magnetocrystalline/anisotropy energy: Moments misaligned with the easy axis have high energy.
    Depending on the material, the optimal scenario is found from 1 and 2
81
Q

Ferromagnetic hysteresis

A

Increasing applied field:

  1. Reversible domain wall motion (domain walls don’t reach crystal defects)
  2. irreversible wall motion as favourably orientated domain grow along the easy access that fit best to the field’s direction
  3. Moment rotates from the easy axis to the direction of the field. Saturation magnetisation is reached

Removing the field:
Spontaneous magnetisation is achieved. Magnetic dipole rotates back to the easy axis. Still 1 single domain

When there’s 0 field, there’s the remanent magnetisation. Small oppositely orientated domains nucleate to reduce the large stray field energy.

Increasing reverse field:

  1. Reverse domains grow
  2. Coercive field is required to reach 0 magnetisation again
  3. 1 single dipole along easy axis
  4. domain rotates to the direction of the field
82
Q

Area of the magnetic hysteresis loop

A

proportional to energy required for 1 cycle i.e. double the energy needed to switch magnetisation

83
Q

How imperfections in ferromagnetic materials affect the coercive field

A

Pin domain walls and increase the coercive field

84
Q

Shapes of magnetic hysteresis loops that fit different needs

A

Large area: permanent magnets that don’t demagnetise easily
Square loop: well defined switches for memory devices
Tall and thin: High saturation magnetisation and low coercive field for transformer cores

85
Q

Making magnetically soft materials

A

Small hysteresis loop area
Get rid of imperfections
Use long heat treatments to grow perfect crystals or use homogeneous materials with no crystalline structure as these can’t have imperfections

86
Q

Making magnetically hard materials

A

Want to incorporate infections to pin domain walls

Either quench the material (rapid cooling) or compress fine powders as these are independent particles

87
Q

Shottky defect

A

Equal number of anion and cation vacancies. Overall neutrality

88
Q

Frenkel defect

A

Ion moved from where it should be to an interstitial site leaving behind a vacancy

89
Q

Why ion conduction is affected by temperature

A

Ions vibrate more at higher temperatures.

Have more energy and can migrate through a lattice more easily.

90
Q

What ion mobility depends on

A

Number of interstitial sites

Energy barrier between adjacent sites

91
Q

Factors that create an ionic current flow

A
  1. concentration gradient (of ions or vacancies) gives rise to a diffusion current
  2. An external electric field gives rise to a drift current
92
Q

Diffusion current density

A

Charge of individual ion * diffusivity * concentration gradient

ONLY applies to solid-state diffusion in a uniform concentration gradient
Ions move down the concentration gradient (oppositely charged vacancies move in the opposite direction) by random diffusion

93
Q

Activation energy

A

Energy barrier between lattice points (i.e. from an energy well to the maximum energy between it and the subsequent energy well)

EITHER in Joules per mole
OR in eV per atom

94
Q

Drift current

A

Addition of an electric field reduces the activation energy in 1 direction (by rotating the energy profile)

95
Q

When to use the Nernst-Einstein equation

A

When diffusion current density is equal to drift current density.

96
Q

Yttrium Stabilized Zirconia

A

Doping zirconia (ZrO[2]) with yttrium to create oxygen vacancies

Zirconia has cations in an FCC lattice with anions filling all the tetrahedral interstices

Y^(3+) ions replace Zr^(4+) ions
2 replacements creates 1 oxygen vacancy

Undoped zirconia is monoclinic at room temperature

Doping with Y[2]O[3] allows the cubic form to be stabilised over a wider range of temperatures including room temperature

97
Q

delta - Bi[2]O[3]

A

FCC lattice with on average 6 tetrahedral sites filled with oxide ions. i.e. on average there are 2 oxygen vacancies per unit cell

98
Q

Oxygen Concentration cell

A

Porous platinum electrodes to catalyse reactions and conduct electrons

If there are different concentrations of oxygen at each electrode, oxygen is transported from high to low concentration.

On the side with a greater oxygen concentration, oxygen is reduced and forms oxide ions.(cathode) These ions are transported across a solid electrolyte e.g. YSZ. At the other side (anode), oxide ions gain electrons.

In databook, oxygen ions flow from pO[2] to pO[1]. Of flow is in the opposite direction, E will be negative

99
Q

Lamdba sensors

A

Measure oxygen levels in vehicle exhausts to make fuel combustion as efficient as possible

Self-generated voltage shows the difference in oxygen concentration between the exhaust and the atmopshere

Small e.m.f = lean burn, fuel should be added

Large e.m.f. = fuel rich, add oxygen

Air-to-fuel ratio by weight is 14.6
Aiming for (measured ratio)/14.6 = 1
100
Q

Oxygen pump

A

Oxide ions driven across a membrane using an applied potential
Used for removing oxygen from molten metals

101
Q

Fuel cell

A

Produces electricity from direct oxidation of fuel. Doesn’t have as many stages for electricity generation and so can be more efficient.

Cathode:
Oxygen is reduced to form oxide ions
Oxide ions travel across solid electrolyte
Oxide ions oxidise fuel (Hydrogen to water or methane to carbon dioxide and water)

Pros:
If hydrogen is used, there’s no polluting emissions
No noise
High efficiency

Requirements:
For effective conduction, electrolyte must be kept at high temperatures to maintain high ion conductivity and low electron conductivity.
Anode must be porous and electrically conducting –> conducting ceramic (cer-met)
Cathode must be permeable to oxygen, not oxidise at high temperatures and electrically conducting –> doped porous manganite

Cons:
High operating temperatures: need stability, no inter-reaction or inter-diffusion and matched thermal coefficients of expansion of all components

102
Q

Polymer electrolyte membrane fuel cells

A

Conduct protons and operate at lower temperatures

e.g. sulphonated fluoropolymer membrane
Has a hydrophilic end that form clusters where water collects.
Proton on hydrated end moves between molecules

103
Q

How to produce hydrogen

A

electrolyse water: requires energy input

pass fossil fuels and water over a heated catalyst: separates in carbon dioxide and hydrogen

104
Q

Easy –> hard crystallographic axes for common ferromagnetic materials

A

Fe (BCC):
Easy <100> –> <110> –> <111> Hard

Ni (FCC):
Easy: <111> –> <110> –> <100> Hard

Co (hex):
Easy: <001> –> <100> Hard

105
Q

Magnetite - reverse spinel

A

Ferrimagnetic

Fe(3+) ions have magnetic moments that all cancel
Fe(2+) ions have magnetic moments that remain

106
Q

Arrhenius equation

A

Diffusivity = D

D = D[0] * exp(Q/RT) if Q in J/mol

D = D[0] * exp(Q/kT) if Q in J/atom

Materials 1A (3 decks)

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