Dielectrics- Ferroelectric Domains Flashcards
How many different dipole directions are there in BaTiO3?
6 because there are 6 equivalent directions for the Ti to displace
Equation for dipole interaction
E is proportional to d1d2/r^3
Means neighbouring cells align in same direction
Polarisation définition and formula
P=Σμ/V
Total dipole moment per unit volume
Why do domains form in alternative directions?
Lots of aligned cells generate a domain which makes a large dipole in one direction. This generates a counteracting electric field that encourages the individual dipoles to swap direction. Dipoles still want to maintain local alignment so alternative direction domains form (twinning). Nearby dipoles align with no overall net dipole
How does polarisation of BaTiO3 change the lattice?
Now a=3.99Å and c=4.01Å. The cell is no longer commensurate with another polarised cell at 90° to it
How is mechanical strain created from polarisation?
There is distortion at domain walls in order to keep the crystals together. The distortion from ideal positions creates a mechanical strain
How does a crystal minimise strain caused by polarisation?
Minimise 90° domain walls as much as possible (just small ones near edge of material). Mainly use 180° orientation which minimises the strain created
What is required for polarisation switching?
Requires energy to alter the relaxation of the ions in the cell. An electric field provides this energy to move the ions.
How ca polarisation be locked into a material?
Aligned domains want to make a twin (rotate 180°) but must first create a 90° twin to do so. This would cause a large strain so there is a high energy barrier to overcome in the absence of an applied field
Hysteresis loop of polarisation
Like that for magnetic materials. Starts at origin. Curves up then levels off to saturated polarisation. Shallow gradient line to y-axis at the remanent polarisation. Then curves down to coercive field at x-axis. Rest of graph is like normal hysteresis loop
Explain stagesof hysteresis loop of polarisation
Net polarisation of naturally cooled ceramic is 0 as domains orientate themselves to reduce electrostatic energy within solid. Field applied gradually shifts cells to align with field. At Psat, the maximum level of 90 and 180° domain wall motion has occurred. Removal of field results in domain relaxation to remove internal strain. Reverse field (Ec) required to return P to 0 from Pr.
Units of Psat, Pr and Ec
Psat Cm^-2
Pr Cm^-2
Ec Vm^-1
What is poling?
Application of large DC electric field (e.g 10kV/mm) in an oil bath (so electricity can’t move) to temperature just below Tc (10-20 below). Aligns the dipoles in similar orientations to induce a net polarisation within a ceramic.
How does strength of applied electric field in poling affect the polarisation?
Large Eapp means 90° domain walls move to give component of polarisation in the direction of field. Removing this Eapp leaves solid with net polarisation (poled ceramic). Small Eapp means 180° domain walls move to give component of polarisation in field direction.
Ways to depole a ceramic
Apply a large force
Heat to above Tc
Apply a reverse field Ec
