FERROELECTRICS Nicole Harrison

Ferroelectric Materials zA group of dielectric materials that display spontaneous polarization. In other words, they possess polarization in the absence of an electric field.

Barium Titanate (BaTiO 3 ) zThe Ba 2+ ions are in the corners, they have a tetragonal symmetry. The O 2- ions are displaced below the centers of each of the six faces and the Ti 4+ ion is displaced upward from the unit cell center by the same amount. zThe permanent ionic dipole moment comes from the relative displacements of the oxygen and titanium ions from their symmetrical positions.

zWhen a ferroelectric material is heated above its Curie Temp. then the unit cell becomes cubic, all the ions assume symmetric positions within the cubic unit cell and ferroelectric behavior ceases. zSpontaneous polarization is a result of interactions between adjacent permanent dipoles which mutually align, all in the same direction.

Examples of Ferroelectrics zBarium Titanate, Rochelle salt, potassium dihydrogen phosphate, potassium niobate, and lead zirconate-titanate (PZT)

Outstanding Properties zThe reversibility of the permanent polarization by an electric field. Reversibility is a result of the fact that the polar structure of a ferroelectric is a slightly distorted nonpolar structure. The distortion gives rise to nonlinear dielectric behavior. zFerroelectrics have very high dielectric constants at relatively low applied field frequencies. Capacitors made from these materials can be significantly smaller than capacitors made out of other dielectric materials.

Antiferrolectric Materials z“Crystals that are isomorphous to ferroelectrics, undergo with decreasing temperature phase transitions from a state of higher symmetry to a state of lower symmetry which are connected with slight distortions of the crystal lattice. This leads to twinning. However, the twins have no net electric dipole moment. In most cases the crystal structure of the low symmetry phase can be described in terms of equivalent sublattices with equal but opposite polarization.”

Materials in Actuators zAntiferroelectric films can be used for high strain microactuators zFerroelectric liquid crystalline elastomers (FLCE) can be used for soft actuators zComposite Polyurethane-Ferroelectric Ceramics made of PZT zTwinned lithium tetraborate is used in monomorph actuators zMany Ferroelectric thin films