Ferroelectricity

FERROELECTRICITY

When a dielectric material is subjected to an external electrical field, the dipole moments tend to orient themselves in the direction of the applied electrical field. This results in a finite polarisation. The polarisation increases linearly with the applied electrical field.

When the electrical field is removed the polarisation becomes zero, because the dipole moments go back to the random orientation configuration.

There are certain materials which exhibit electric polarisation even in the absence of the applied electrical field. They are known as Ferroelectric materials. These are permanent dipole moment in each atom or molecule.

Materials which exhibit electric polarization even in the absence of the applied electric field are known as ferroelectric materials.

The dielectric constant (ε_r) of these materials is some three orders of magnitude larger than the ordinary dielectrics.

Ferroelectricity is a result of dielectric hysteresis. Since these materials exhibit hysteresis effects, similar to those observed in ferromagnetic materials, they are called ferroelectric materials.

So they are called as ferro dielectric materials.

Ferroelectricity refers to the creation of enormous value of induced dipole moment in a weak electrical field as well as existence of electric polarisation even in the absence of applied electrical field.

Examples:

1. Barium titanate (BaTiO₃)

2. Potassium dihydrogen phosphate (KH₂PO₄)

3. Ammonium dihydrogen phosphate (NH₄H₂PO₄)

4. Lithium Niobate (LiNbO₃)

5. Rochelle salt (NaKC₄H₄O₆ 4H₂O)

Dielectric hysteresis:

Most important feature of exhibit ferroelectrics is that they hysteresis similar to thatferromagnetic materials.

Lagging of polarisation behind the applied electric field is called dielectric hysteresis. Ferroelectricity is a result of dielectric hysteresis. (Fig. 1.26)

A ferroelectric is highly dependent on temperature. When a ferroelectric is heated, at a temperature called Curie temperature (θ), the ferroelectric becomes a paraelectric.

Figure 1.27(a) shows the temperature dependence of polarization of a ferroelectric. A ferroelectric shows domain structure where, in each domain, all the dipoles are aligned in the same direction. The ferroelectric susceptibility (X_e) can be written as

X_e = C / T- θ ...(1)

where C is called Curie constant and θ is called the Curie temperature. Equation (15) is called Curie-Weiss law.

Properties of ferroelectric materials

1. These materials have permanent dipole moment in each atom or molecule

2. Ferroelectric materials can be easily polarised even by very weak electrical field

3. High value of dielectric constant

The ferroelectric materials have got very high dielectric constants roughly 3 times greater than that of ordinary dielectric.

4. Dependence of the dielectric constant on temperature

The dielectric constants of ordinary dielectrics do not change much with temperature. In the case of ferroelectric crystal, ε_r exhibits one or more sharp maxima where ε_r reaches a value of several thousands. The temperature at which these maxima occur are called curie temperature.

5. Ferroelectric is a non linear dependence of its polarisation on the external electrical field. In ordinary dielectrics, the polarisation P varies linearly with 'E'. Thus, ordinary dielectrics are often called as linear dielectrics But the relation between P and E is complex in nature and therefore non linear. From this point ferroelectrics are know as non – liner dielectrics.

6. Ferroelectric materials posses spontaneous polarisation which is a polarisation that persists when the applied electrical field is zero.

7. They exhibit domain structure as in the case of ferromagnetic materials.

Application of ferroelectric materials

1. ferroelectric materials are used to make pressure transducers, ultrasonic transducers, microphones and gas filters.

2. Some ferroelectric semiconductors like BaTiO₃ - SrTiO₃, BaTiO₃ – PbTiO₃ and SrTiO₃ - PbTiO₃ are used to make resistors which are used to measure and control temperature like thermistors.

3. Electrets are ferroelectric materials and are electrostatic analog of permanent magnets. Electrets posses a gross permanent electric dipole moment. Electrets are used in capacitor, microphones and gas filters to capture submicron particles by electrostatic attraction. Further, electret bandages are used over the fractured bones to speed up the healing process.

4. Capacitors: Ferroelectric ceramics are used in the manufacture of capacitors. Due to large dielectric constant of the ferroelectric materials, they are used for storing electric charges in electrical / electronic circuits.

5. In optical communication, the ferroelectric crystals are used for optical modulation.

6. They are used as frequency stabilizers and crystal controlled oscillators.

7. The high dielectric constant of ferroelectrics makes them to manufacture small sized capacitors of high capacitance and to store electric energy.

8. They can be used as memory devices in computers.

9. In optical communication, the ferroelectric crystals are used for optical modulation.

10. They are used in electrotransducers such as microphones.

11. Ferroelectric crystals exhibit piezoelectric property, which can be used to produce ultrasonic waves.

12. Piezoelectric acoustic transducers and pyroelectric infrared detectors are the devices based also on ferroelectrics.