Ferroelectricity and Applications

Ferroelectricity is a physical property of a material that exhibits a spontaneous electric polarization or permanent electric dipolemoment even in the absence of electric field and which can be reversed by the application of an external electric field.

A ferroelectric material have a spontaneous dipole moment that can be switched in an applied electric field, i.e. spontaneous switchable polarisation. This is found when two particles of charge q are separated by some distance r, i.e.

Fig 1.1

The electric dipole moment, μ = q.r

The ferroelectricity phenomenon was discovered in 1921 by J. Valasek who was investigating the dielectric properties of Rochelle salt (NaKC₄H₄O₆. 4H₂ O). The most commonly used ferroelectric is Barium titanate (BaTiO₃) which was discovered in 1944 by Avon Hippel. There are some more than 250 materials that exhibit ferroelectric properties. Some of the important materials are :

• Lead titanate, PbTiO₃
• Lead zirconate titanate (PZT)
• Lead lanthanum zirconate titanate (PLZT)
• KDP [potassium dihydrogen phosphate], etc.,

Properties of Ferroelectric materials

1. Ferroelectric materials are polarized even in the absence of electric field. ie., Spontaneous Poalrisation.
2. They show hysteresis effect as shown in graph.
3. The ferroelectric materials have very high dielectric constants (three times) than ordinary dielectric materials.
4. Only Below a transition temperature called the Curie temperature ferroelectric materials show spontaneous polarization. At Curie temperature the ferroelectric become paraelectric.
5. Ferroelectric materials have domain structure like ferromagnetic materials.
6. Ferro electric materials exhibit piezoelectricity and pyro electricity.

Fig 1.2

Applications for Ferroelectric Materials

• Capacitors
• Non-volatile memory
• Piezoelectrics for ultrasound imaging and actuators
• Electro-optic materials for data storage applications
• Thermistors
• Switches known as transchargers or transpolarizers
• Oscillators and filters
• Light deflectors, optical modulators and displays
• Microphones, sonar devices

Some of the dielectrics and its application

Dielectric materials can be solids, liquids, gases or high vacuum

Solid dielectrics are perhaps the most commonly used dielectrics in electrical engineering, and many solids are very good insulators.

Some examples include porcelain, glass, and most plastics. Air, nitrogen and sulfur hexafluoride are the three most commonly used gaseous dielectrics.

Industrial coatings such as parylene provide a dielectric barrier between the substrate and its environment.

Mineral oil is used extensively inside electrical transformers as a fluid dielectric and to assist in cooling.

Dielectric fluids with higher dielectric constants, such as electrical grade castor oil, are often used in high voltage capacitors to help prevent corona discharge and increase capacitance.

Some dielectrics can generate a potential difference, when subjected to mechanical stress, or change physical shape if an external voltage is applied across the material. This property is called piezoelectricity. Piezoelectric materials are another class of very useful dielectrics.

Some ionic crystals and polymer dielectrics exhibit a spontaneous dipole moment which can be reversed by an externally applied electric field. This behavior is called the ferroelectric effect. These materials are analogous to the way ferromagnetic materials behave within an externally applied magnetic field. Ferroelectric materials often have very high dielectric constants, making them quite useful for capacitors.