14. EXTRA PART 1 Flashcards Preview

4C3 Advanced Materials > 14. EXTRA PART 1 > Flashcards

Flashcards in 14. EXTRA PART 1 Deck (29)
Loading flashcards...
1
Q

What 2 things determine the field at the surface of a hard magnet?

A

Its shape and its remnant magnetisation

2
Q

What are the key features of the M-H curves for type 1 and type 2 superconductors?

A

The slope of the downwards section is equal to the susceptibility therefore = -1, and the intercepts with the x axis are the critical fields

3
Q

If the geometry of the question is ‘large’ what does this allow you to do?

A

Neglect geometric effects

4
Q

What is dD/dt inside a superconductor?

A

0, as there cannot be any electric field inside a superconductor

5
Q

What are the origins of the thermal time constant for a pyroelectric element?

A

The thermal time constant (tau t) essentially depends on how quickly absorbed heat in the element is rejected to the environment. It depends, therefore, on the heat capacity of the element, H, and on the thermal conductivity of the element to its surroundings Gt.
tau t=H/Gt

6
Q

What are the origins of the electrical time constant?

A

The electrical time constant (tau e) depends on the capacitance of the element and the amplifier. It is a measure of how quickly charge is transferred from one to the other.
tau e=Rg(Ce+Ca)

7
Q

What is the feature of pyroelectric materials that allows them to produce a charge with the change in temperature?

A

Pyroelectric crystals have a rare asymmetry due to their single polar axis. This causes their polarization to change with temperature.

8
Q

Why is a modulated source necessary for use with pyroelectric detectors?

A

As a pyroelectric element responds only to changes in infrared radiation.

9
Q

In current mode operation of a pyroelectric detector, what are we concerned about and under what conditions does this occur?

A

In current mode we are concerned about current generated by unit of input power.
This is the case at low frequency of modulation of the input flux – say in a PIR detector.

10
Q

In voltage mode operation of a pyroelectric detector, what are we concerned about and under what conditions does this occur?

A

In voltage mode we are concerned about voltage generated by unit of input power.
This is the case at high modulation frequencies as might be expected in an imaging system where the input scene is deliberately chopped.

11
Q

Is pure iron a soft or hard magnetic material?

A

soft

12
Q

Name an example of a hard magnetic material

A

SmCo

13
Q

Explain why the maximum magnetisation, Ms, obtained in a conventional ferromagnet is a materials property, whereas in a bulk superconductor it depends on
sample size.

A

The magnetisation from a conventional magnetic material is due to the alignment of the spin (both electron and orbital) associated with the atoms within the material. The number of Bohr magnetons available per volume is fundamentally limited and thus maximum Magnetisation is sample size independent. It depends on the material itself.
In a superconductor however, the Magnetisation depends on a current loop and thus depends on the size of the sample as well as the maximum critical current available.

14
Q

What is the origin of the demagnetising field Hm?

A

The Magnetisation leads to a magnetic field in the magnetic material that acts back through the material itself – hence the term demagnetisation.

15
Q

Equation for demagnetising field

A

Hm = -NM

N- demagnetising factor

16
Q

Explain how the charge displacement arises and consequently gives rise to the observed behaviour for the case of each of piezo- and pyro- electric materials.

A

Both pyroelectric and piezoelectric materials are sub-classes of dielectric materials. Their electrical properties arise from asymmetry in their crystal structure.

17
Q

Explain why, when pyroelectric or piezoelectric behaviour is required, the material of choice is, in fact, often a ferroelectric.

A

Ferroelectrics are a subset of pyroelectrics, but where their polar axis can be changed by the application of an electric field. This means they exhibit both pyro and piezo electricity, and polycrystalline materials can be used which allow for the axis for the polar properties to be defined by applying a high enough electric field in that direction.

Ferroelectrics form a subset of the set of pyroelectrics because they are polar materials in which the direction of the polar axis can be changed by the application of an electric field. As a consequence they are both pyroelectric and piezoelectric. As many of the largest pyroelectric and piezoelectric effects occur in ferroelectric materials, they have become very important technologically. Furthermore, the fact that the polar axis can be reoriented by the application of a field means that polycrystalline ferroelectrics in which the crystallographic axes of the component crystallites are randomly oriented can be made to show polar properties by applying a sufficiently large electric field (the process of “poling”).

18
Q

Why do tensor equations arise?

A

Tensor equations arise when a change in one variable in one direction causes a change in a second variable in a different direction.

19
Q

Explain why, although there are 18 components to the reduced piezoelectric coefficient, only three piezoelectric coefficients are normally of technological
interest.

A

Piezoelectric materials are usually formed in thin plates which means that large field or strain is only developed across the thickness, therefore 2 of the stress and field values fall to 0, resulting in only 3 piezoelectric coefficients being of interest

20
Q

What is a benefit of working a piezoelectric material in shear mode?

A

Charge is only generated when the it is under shear, therefore the device is insensitive to the pyroelectric effect and will not generate charge with changes in temperature

21
Q

Describe the changes in gate oxide material in MOSFETS and why it was made

A

Changed from SiO2 to a thicker layer of a high k (dielectric constant) oxide such as HfO2
Done to stop tunnelling conduction through the gate oxide

22
Q

Describe the changes in material for interconnects in MOSFETS and why it was made

A

Al changed to Cu

To increase conductivity

23
Q

Describe the changes in material for interconnect dielectric in MOSFETS and why it was made

A

Changed from SiO2 to low k (dielectric constant) oxides such as SiOF
Replacing the silicon dioxide with a low-κ dielectric of the same thickness reduces parasitic capacitance, enabling faster switching speeds and lower heat dissipation.

24
Q

Describe the changes in material for the channel in MOSFETS and why it was made

A

Changed from Si to high mobility SiGe

To increase electron mobility (how fast an electron can move in a material)

25
Q

What is passivation, and what are 2 typical passivants?

A

passivation is the removal of gap states from the gap, often by a chemical reaction with a
passivant. Typical passivants are SiO2 or hydrogen.

26
Q

Describe the nature of states in amorphous silicon (a-Si) in the different energy ranges, in terms of their origin and conduction properties, and explain how they are related to those of crystalline silicon.

A

Extended states conduct electricity at 00K, like the band states in a crystal.
Localised states of band tail are created by the disorder. They do not conduct electricity at 00K.
Conduction for these occurs by hopping from 1 state to the next (tunnelling) or by thermal
activation to the extended states.

27
Q

Name to hard magnetic materials with a high Hc

A

NdBCO and SmCo

28
Q

What are the 2 types of gap states in a-Si

A

localised (tail) states and (deep) defect states.

29
Q

What are the cause of the tail/localised states in a-Si

A

Tail state due to angular and lattice disorder