The phenomenon of ferroelectricity was discovered in 1921 by J. Valasek who was investigating the dielectric properties of Rochelle salt (NaKC4H4O6.4H2O). Barium titanate (BaTiO3) was discovered to be ferroelectric in 1944 by A von Hippel and is perhaps the most commonly though of material when one thinks of ferroelectricity. While there are some 250+ materials that exhibit ferroelectric properties, some of the more common/significant materials include:
• Lead titanate, PbTiO3
• Lead zirconate titanate (PZT)
• Lead lanthanum zirconate titanate (PLZT)
What Properties do Ferroelectric Materials Possess?
Pyroelectric Properties and Spontaneous Polarisation
All ferroelectric materials are pyroelectric, however, not all pyroelectric materials are ferroelectric. Below a transition temperature called the Curie temperature ferroelectric and pyroelectric materials are polar and possess a spontaneous polarization or electric dipole moment. However, this polarity can be reoriented or reversed fully or in part through the application of an electric field with ferroelectric materials. Complete reversal of the spontaneous polarization is called “switching”.
The non-polar phase encountered above the Curie Temperature is known as the paraelectric phase.
The direction of the spontaneous polarization conforms to the crystal symmetry of the material. While the reorientation of the spontaneous polarization is a result of atomic displacements.
The magnitude of the spontaneous polarization is greatest at temperatures well below the Curie temperature and approaches zero as the Curie temperature is neared.
Since all pyroelectric materials are piezoelectric, this means ferroelectric materials are inherently piezoelectric. This means that in response to an applied mechanical load, the material will produce an electric charge proportional to the load. Similarly, the material will produce a mechanical deformation in response to an applied voltage.
Properties including the piezoelectric, dielectric and electrooptic co-efficients may vary by several orders of magnitude in the narrow temperature band around the Curie temperature. Especially when compared to other temperature ranges, the changes to these co-efficients is much more gradual.
The piezoelectric co-efficient is much greater in the region of the Curie temperature. Other properties such as dielectric strength and electrooptic properties also change more markedly in the region of the Curie temperature when compared to other temperature ranges.
Applications for Ferroelectric Materials
• Non-volatile memory
• Piezoelectrics for ultrasound imaging and actuators
• Electro-optic materials for data storage applications
• Switches known as transchargers or transpolarizers
• Oscillators and filters
• Light deflectors, modulators and displays