Dielectrics- Capacitance

Question 1

What are capacitors?

Answer 1

Insulating barriers in electric circuits. They block the flow of current. Current only flows when the charge on one side becomes large enough to breakdown the barrier and cross it. Used for charge strorage, block dc current, ac-dc conversion

Question 2

Capacitance equation

Answer 2

C=q/V

Charge of over volume

Question 3

Why are ferroelectics ideal to sit between plates of capacitors?

Answer 3

They are insulators so prevent charge flowing. They polarise in the field created by the plates, further stabilising the charge and allowing a greater charge to build up on the plates

Question 3

Why are ferroelectics ideal to sit between plates of capacitors?

Answer 3

They are insulators so prevent charge flowing. They polarise in the field created by the plates, further stabilising the charge and allowing a greater charge to build up on the plates

Question 4

How to increase capacitance

Answer 4

Shrink the gap between plates. Increase the area of plates. Increase the permittivity.
C=ε0A/d
C’=ε0εrA/d
εr=C’/C

Question 5

Relative permittivity of BaTiO3

Answer 5

About 10,000 at Tc

Question 6

What is volume efficiency?

Answer 6

Capacitance over volume (Fmm^-3)

C/V=ε0εr/d^2

Question 7

How to improve volume efficiency

Answer 7

Use thin layers (e.g polymer dielectrics 1-10μm thick but small εr).
Oxidise surface of a metal foil (electrolytic capacitors) to get 0.1-1μm thick insulating surface.
Tape cast ferroelectric BaTiO3-based ceramics (0.8-1μm thick layers) and stack layers together to form multi-layer capacitor (normally 100-400 layers)

Question 8

Design of multi-layer ceramic capacitors

Answer 8

Two main external electrodes parallel to each other. Internal electrodes perpendicular to them coming out into gap (alternate from which external electrode). Between internal electrodes they are separated by ceramic. Each layer contributes to overall A while separation is only very small gap between each electrode

Question 9

Why can capacitance vary with temperature?

Answer 9

The polarisation of materials varies with temperature. Phase changes in BaTiO3 cause shifts in permittivity

Question 10

Ideal behaviour of capacitors

Answer 10

Electrical engineers want capacitors with same behaviour (capacitance) across a temperature range. Permittivity stabilised. Capacitor design often focussed around optimising temperature coefficient of capacitance (TCC) for certain temperature ranges

Question 11

How thick is a MLCC?

Answer 11

0.8-2 microns with 100-800 layers

Question 12

What is charge stored efficiency?

Answer 12

Volume efficiency times working voltage

Units Cmm^-3

Question 13

Electrolytic capacitors

Answer 13

Could be Al (low cost) or Ta (high cost). Both good CSE. Both high capacitance. Both can only operate at low frequencies (less than 10kHz)

Question 14

Polymer capacitors

Answer 14

Cheap. Poor CSE. Wide frequency range. But only work at low voltage (less than 1V)

Question 15

Ceramic capacitors

Answer 15

Cheap. Good CSE. High capacitance and voltage. Wide frequency range. But high tanδ (losses) and high TCC

Question 16

Considerations when choosing capacitors

Answer 16

Voltage, frequency and temperature range of operation

Question 17

Key things to achieve when making MLCCs

Answer 17

Control over each layer thickness. Alternating ceramic and electrode layers. Identical ceramic layers (so same properties)

Question 18

How to make MLCCs

Answer 18

Start with barium titanate powder and dissolve in organic solvent. Add any binder or additives to make a slurry and homogenise. Pump slurry into slurry container. Doctor blade allows certain thickness of slurry onto flexible tape on a support table. This then goes through the drying zone where it is heated. At the end of the table the tape is peeled off and the layer is fed onto a take-up reel. Work in layers pasting and drying until have desired number of layers. Sinter in an oven. Process called tapecasting