FERROELECTRICITY VERSUS PIEZOELECT- RICITY

Ferroelectric materials and piezoelectric materials can be compared with the help of following points :

a) In piezoelectricity, the crystal is polarized by the application of an external stress, whereas in ferroelectricity the source of polarization is the dipole interaction energy itself.

b) Both the phenomena occur in non-centrosymmetrical crystals which are 20 in number. Piezoelectricity occurs in all the 20 crystals, whereas ferroelectricity only in 10, namely, those which provide a favourable axis of a polarity.

c) All ferroelectrics are therefore piezoelectric, but all piezoelectrics are not ferroelectric. For example: tourmaline is piezoelectric, but not ferroelectric.

d) The piezoelectric coefficient is the ratio of the set-up charge to the stress applied along crystallographic axis. The ferroelectrics have very large piezoelectric coefficients.

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Applications of ferroelectric materials

ferroelectric materials have following applications:

(a) As Capacitors: Ferroelectric materials can be used to make capacitors.

(b) As Varactors: The value of dielectric constant for ferroelectric materials can be changed depending upon the change in the magnitude and directions of electric field. If it happens, then they can be made to behave like varactors.

(c) As Radio frequency power amplifiers: Change in electric field changes dielectric constant and this is then changes the power supply. Change in the field modulate the frequency and it means that the ferroelectric capacitors can be used as radio frequency power amplifiers.

(d) As computer memories:.Very thin ferroelectric crystals can be

used for crystal memories.

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Types of ferroelectric materials

Ferroelectric materials can be classified on the basis of their chemical composition and structure as :

(a) Rochelle salt: The first solid to show ferroelectric properties is Rochelle salt. Rochelle salt is the sodium-potassium salt of tartaric acid (NaKC4H4O6.4H2O). It has the property of being ferroelectric only in the temperature range between -18 C and 23 C, which means it has two transition temperatures.

(b) Dihydrogen  phosphates  and  arsenates:  Busch and Scherier discovered  ferroelectric  properties in KH2PO4 in 1935  which  is an example  of  the  dihydrogen  phosphate and arsenate of the alkali metals. The  spontaneous polarization of this material a  function of  temperature.

In this case, there is only one Curie temperature = 123 K

(c) Oxygen Octahedron group (Barium Titanate): The best known ferroelectric material is barium titanate (BaTiO3) and it is a representative of the oxygen octahedron group of ferroelectric

material. The reason for this name is that the above the Curie temperature that is at 120 C, BaTiO3 corresponds to the cubic structure.

When the temperature is decreased through the critical temperature of 120 C, the material becomes spontaneously polarized and at the same time the structure changes.

BaTiO3 has two more transition temperatures; one at 5 C, and other at -80 C. There will be a change in the crystal structure of the material associated with there ferroelectric transitions.

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