Dielectrics- Capacitance Flashcards

1
Q

What are capacitors?

A

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

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2
Q

Capacitance equation

A

C=q/V

Charge of over volume

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3
Q

Why are ferroelectics ideal to sit between plates of capacitors?

A

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

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3
Q

Why are ferroelectics ideal to sit between plates of capacitors?

A

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

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4
Q

How to increase capacitance

A

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

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5
Q

Relative permittivity of BaTiO3

A

About 10,000 at Tc

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6
Q

What is volume efficiency?

A

Capacitance over volume (Fmm^-3)

C/V=ε0εr/d^2

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7
Q

How to improve volume efficiency

A

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)

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8
Q

Design of multi-layer ceramic capacitors

A

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

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9
Q

Why can capacitance vary with temperature?

A

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

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10
Q

Ideal behaviour of capacitors

A

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

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11
Q

How thick is a MLCC?

A

0.8-2 microns with 100-800 layers

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12
Q

What is charge stored efficiency?

A

Volume efficiency times working voltage

Units Cmm^-3

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13
Q

Electrolytic capacitors

A

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)

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14
Q

Polymer capacitors

A

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

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15
Q

Ceramic capacitors

A

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

16
Q

Considerations when choosing capacitors

A

Voltage, frequency and temperature range of operation

17
Q

Key things to achieve when making MLCCs

A

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

18
Q

How to make MLCCs

A

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