What is the relationship between polarisation and total energy?
Polarisation is the derivative of the total energy wrt electrical field.
What are electrets?
Where are they used?
- specially processed dielectrics that retain excess internal (“frozen in”) polarisation
- electrets have a semi-permanent electric field
- equivalent to magnets
- uses: electret microphone - telephones, headsets, video recorders, N95 masks (trapping particles)
What are ferroelectrics?
- ionic crystals or polymers that exhibit a spontaneous dipole moment, which can be reversed by an externally applied field (analogous to ferromagnetic materials),
- often have very high dielectric constants!!
Examples of gas dielectrics
- air (1)
- nitrogen
- sulfur hexafluoride (SF6), nonreactive (3200 years lifetime!), silicon etchant (cuts layers in semiconductor manufacture), circuit breakers, switchgear (electrical disconnect switches)
Examples of solid dielectrics
Where are they used
- porcelain
- glass
- most plastics
- industrial coatings
Often used in electrical engineering.
What are piezoelectrics?
Dielectrics that generate potential difference in response to mechanical stress.
What is the difference between corona discharge and static discharge?
What colour is corona discharge?
- Corona is constant, slow, low-energy; static is a rapid, high-energy (high-noise) process.
- Corona discharge (e.g. air around high voltage sharp point / thin wire) has a faint steady bluish glow.
What is the name of the stress due to incomplete basis (increase ENCUT)?
Pulay stress.
List 4 types of dielectric processes, together with the limiting frequencies
- electronic - 1e15 Hz
- ionic (atomic) - 1e10–1e13Hz (1e4–1e7MHz)
- dipolar (oriental) - 1e3–1e9 Hz
- interfacial - 1e3Hz
- free carriers - even slower?
What is Neel temperature?
When Neel (first name?) received Nobel Prize?
Temperature at which anti-ferromagnetic state becomes paramagnetic (higher-T state).
Louis Neel received Nobel prize in 1970 for work on AF materials.
What is Curie temperature?
Temperature at which ferromagnets (or ferrimagnets) turn into paramagnets (higher-T state).
What is spin glass?
State in which magnetic spins are frozen in no apparent order.
What are the highest frequencies at which ions still contribute to polarisation in dielectrics?
1E10-1E13 Hz
Ions respond to infrared light
