6. Electrical and Multi-ferroic Materials Flashcards
What feature in materials increases the electrical effects observed?
Increasing asymmetry (causes asymmetrical charge distribution)
What equation gives the effective dipole per unit cell? (analogous to the magnetic dipole moment)
q*r
q- charge on each ion
r- displacement between positive and negative charge centres
What equation gives the Polarisation P? (analogous to the Magnetisation)
P = Sum of electric dipoles / Volume
What are pyroelectric materials?
Materials that have an intrinsic dipole and no centre of symmetry, meaning they are certain crystals which are naturally electrically polarized and as a result contain large electric fields. (note: although they have intrinsic dipoles they are normally distorted and no overall polarisation is seen when no external field is applied)
Pyroelectricity can be described as the ability of the material to generate a temporary voltage when they are heated/cooled
What do you call a material where the electric dipoles are aligned without an external field?
Ferroelectrics (by analogy with ferromagnets)
What is the piezoelectric effect?
The production of a change in dielectric polarisation by the application of a mechanical stress. Conversely the application of an electric field to a piezoelectric material will cause it to strain mechanically
What is pyroelectricity
The release of electric charge that occurs in pyroelectric materials as their temperature changes
What is the Curie temperature?
The temperature at which the material transforms to a polar structure of lower symmetry
Where is the greatest charge per unit temperature rise generated, and whats the issue with this in reality?
At the Curie temperature, Tc.
Unfortunately, at Tc ferroelectric materials de-pole so practical devices are designed to work at much lower temperatures.
What does passive operation of piezoelectric materials mean?
When applied stress gives a change in polarisation (charge distribution)
What does active operation of piezoelectric materials mean?
When an applied field gives a strain
How is net polarisation achieved in practical applications of ferroelectric materials (Ceramics)?
Ps originates from a structural transition which results in a displacement of the positive and negative centres of charge symmetry.
The polar axes are randomly oriented in different domains before poling, hence the material
cannot exhibit polar properties overall. Poling by the application of and external applied
electric field Ep aligns the individual polar axes to yield a net overall polarisation.
