Dielectrics- Polarisation

Question 1

4 polarisation mechanisms in an applied electric field

Answer 1

Atomic/electronic αe
Ionic αi
Dipolar αdp
Space charge or diffusional άsc

Question 2

Atomic/electronic polarisation

Answer 2

The movement of the electron cloud around the nucleus. Present in all solids. When an electric field is applied to the solid, the electron distribution of each atom redistributes so there is a higher electron density towards the positive end of the field. Weak effect as the charges and distances moved are small

Question 3

Ionic polarisation

Answer 3

The small movement of charged particles (ions) in the crystal. Present in ionic but not covalent materials. The ions in the lattice move slightly from their original rest positions in accordance with the direction of the field. The structure and order of ions remains intact

Question 4

Dipolar polarisation (orientational)

Answer 4

The alignment of pre-existing dipoles. The dipoles that were oriented randomly in the material rotate to become more aligned with an applied electric field.

Question 5

Space charge or diffusional polarisation

Answer 5

The movement of charge carriers (e-, h+, ions) across the material. There is no full conduction. The charge carriers move through a grain until they meet a grain boundary where there is a larger energy barrier for the electric field to overcome. Leaves one side of material more negative than the other. Requires atoms/ions that can, under field, change their oxidation state (for e- and h+) and allow partial movement of charge

Question 6

How do the effects of mechanical of polarisation combine?

Answer 6

They sum linearly

αtotal= αe+αi+αdp+αsc

Question 7

What is permittivity?

Answer 7

The macroscopic property that measures polarisation. It is the ability of a material to respond to an electric field. No units for relative. Symbol ε or εr for relative to vacuum permittivity

Question 8

Relative permittivity values and dominant polarisation mechanisms for vacuum, hydrocarbons, metal oxides, inorganic glasses, ferroelectric materials

Answer 8

Vacuum: no mechanisms. 1
Hydrocarbons: electronic. 2
Metal oxides: electronic, ionic (small). 5-15
Inorganic glasses: electronic, ionic (medium). 10-20
Ferroelectric: electronic, ionic (large). 1000

Question 9

How does polarisation change when the electric field is flipped?

Answer 9

The polarisation will flip to align with the field. The speed at which it does depends on the mass of the particles involved and the distance they have to move. The larger the inertia of the polarisation the slower its response

Question 10

What sort of polarisation responds to high frequency AC field?

Answer 10

One that can change quickly. A slow changing polarisation will fail to respond

Question 11

Atomic polarisation frequency

Answer 11

Operates at high frequencies up to about 10^15. Because there is a tiny mass of e- and they don’t move very far (still within atom)

Question 12

Ionic polarisation frequency

Answer 12

Operates at still high frequencies up to 10^13 Hz. There is a medium mass of ions and they only move a small distance from their original lattice sites

Question 13

Dipolar polarisation frequency

Answer 13

Operates at lower frequencies up to below 10^10Hz. There is a large mass of molecules and a medium distance for them to align.

Question 14

Space charge polarisation frequency

Answer 14

Operates at only low frequencies. Tiny mass of charge carriers but very large distance for them to move from one side of grain to the other