Ferroelectric Materials – Theory, Properties and applications

Ferroelectric materials are extensively used in capacitors and almost all types of switches. The various Ferroelectric materials, their theory, properties and applications are described in this article.

What is Ferroelectricity?

Ferroelectricity is the phenomenon where spontaneous electric polarization of the material takes place. The reverse electric polarization is possible by applying an electric field. Ferroelectricity was discovered in 1920 and Rochelle salt (scientific name-Potassium Sodium Tartrate) exhibited the properties of sudden electric polarization. Since its discovery, various uses have been devised and today, Ferroelectricity is used in various fields of electronics. The materials exhibiting the phenomenon of Ferroelectricity are called Ferroelectric Materials. Ironically, the Ferroelectricity or the Ferroelectric Materials do not have any connection to Iron(ferrite).

Ferroelectric Materials – Crystal Symmetry

The lattice structure in Ferroelectric Materials shows crystal symmetry. Crystal symmetry means the crystal can be grouped together into microscopic symmetry types. Microscopic symmetry types are introduced from microscopic symmetry elements. These elements combine together to form a point group. In a Ferroelectric Material, there are about 32 point groups. Out of the 32 point groups, 21 point groups do not have a center of symmetry. These non-symmetric point groups show piezoelectric effect along the directional axes. Polarization in the Ferroelectric Materials is directly related to the piezoelectric properties of the materials.

Ferroelectric Materials – Spontaneous Polarization

Spontaneous polarization in Ferroelectric Material is defined as ability to polarize at a fast rate. It is the value of the charge per unit area on the perpendicular surface. Out of the 21 non-symmetrical point groups as explained above, only 10 point groups exhibit the spontaneous polarization property. The reason is that cancellation of the electric moments along different axes.
Pyroelectric Effect: The spontaneous polarization in Ferroelectric Material is directly proportional to the temperature. This phenomenon is called as Pyroelectric effect.

Ferroelectric Materials – Hysteresis Loop

The plot of polarization in Ferroelectric Material to the applied electric field is called as the Hysteresis loop.

Hysteresis loop

As shown in the hysteresis loop, the direction of the polarization is reversed as the applied electric field is reversed. This is the basic difference between the pyroelectric materials and the Ferroelectric Materials. The pyroelectric materials do not posses polarization reversal and hence they are different from Ferroelectric Material.
The theory behind polarization reversal is the domain reversal. The electric dipoles are formed within the Ferroelectric Material when an electric field is applied. But as the field is reversed, the direction of these electric dipoles reverses. This reversal in the direction results in polarization reversal. This is the basic fundamental principle behind the theory of Ferroelectric Material.

Ferroelectric Materials – Curie Point

Curie point is the temperature below which the material shows Ferroelectric property. Thus, if the temperature of the material is below the Curie point, only then the material exhibits the Ferroelectric property. Depending upon the type of the crystal, there are various transitions for tetragonal, orthorhombic and rhombohedral crystals.

Ferroelectric Materials – Types

1. Barium Titanate (BaTiO3)
2. Lead Titanate (PbTiO3)
3. Lead Zirconate Titanate
4. Lead Lanthanum Zirconate Titanate
5. Lead Magnesium Niobate

Ferroelectric Materials – Applications

1. Capacitors – A capacitor is an electronic component used for a number of electronic circuits. It consists of 2 electrodes with a dielectric material in between. A Ferroelectric Material is preferred for the manufacturing of a capacitor. Capacitors are generally formed by the Barium Titanate (BaTiO3) material.

2. Storage memories – SRAM and DRAM are volatile. EEPROM and CMOS require a battery backup and are very costly. Ferroelectric Random Access Memory is non-volatile as well as cheaper. FRAM is reliable as well.

3. Waveguides – A waveguide is a device which controls the propagation of light within the device. It carries the electromagnetic wave. The material used inside a waveguide is a Ferroelectric Material.

4. Optical Memory Display – Latest optical memory displays technologies are devised using the Ferroelectric Material. PLZT is most preferred Ferroelectric Material for this purpose.

5. Displacement Transducers – The Ferroelectric Material is used in pressure sensors. These pressure sensors are also termed as displacement transducers.

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