Ferro and Piezo Electricity and Applications of Dielectric Materials - Civil Engineering (CE) PDF Download

Ferro and Piezo Electricity:

Ferroelectricity:
Some dielectric materials spontaneously acquire an electric dipole moment below a certain temperature. This is referred to as spontaneous polarization. Analogy with magnetic material results in a type of dielectric materials called ferroelectric materials. Similar to ferromagnetic materials ferroelectric materials also exhibit ferroelectric hysteresis. It is a plot of polarization (P) versus Electric field strength (E). Ferroelectric hysteresis is the lagging of the polarization with respect to applied electric field strength during the positive polarization and negative polarization of the specimen.

The static dielectric constant of a ferroelectric material changes with temperature which is given by ε_r = C/T −θ , where C is a constant, T is the temperature and θ is a temperature very close to a temperature called Curie temperature (T_c). Ferroelectric materials exhibit Piezoelectricity and Pyro-electricity. Quartz, Lithium Niobate and Barium Titanate are the few examples of ferroelectric materials. Ferroelectric materials are used in pressure transducers, ultrasonic transducers, microphones, infrared detectors and capacitors.

Piezoelectric Materials:
Certain dielectric materials are electrically polarized when their surfaces are stressed. This phenomenon is called piezoelectric effect and the materials are called piezoelectric materials. The charges produced on the surface due to stressing are proportional to the applied force which is utilized in the conversion of mechanical energy into electrical energy. When crystals like Tourmaline, Rochelle salt and Quartz are sliced in a particular fashion they exhibit piezoelectric effect. In the crystal the distribution of the ionic charges about their lattice sites is symmetrical. Thus the net internal field is zero. But when the crystal is stressed the symmetry is altered due to the displacement of charges which results in non zero internal field. Piezoelectric strains are very small. Hence the corresponding electric fields are very high. For quartz a strain of the order 10⁷ the corresponding electric

field is 1000v/cm. The inverse of Piezoelectricity is called electrostriction. Electrostriction is a phenomenon of straining a crystal by applying an electric field. Hence the piezoelectric materials are also called electro-strictive materials. The piezoelectric crystals are used in electro-mechanical transducers, as oscillators to generate highly stable frequency and measurement of velocity of ultrasonic waves in solids and liquids.

Properties and Frequency dependence of Dielectric constant, Dielectric loss and complex dielectric constant)
It is found under alternating field conditions of high frequency, the dielectric constant is a complex quantity. When dielectric materials are placed in alternating field the polarization tend to reverse as the polarity changes. If the frequency of the field is low (less than 1M Hz), then the polarization can follow the alternations of the field and hence the dielectric constant remains static.
Under alternating field conditions of high frequency (Greater than 1MHz) the oscillations

of dipoles lag behind those of field. If the frequency is increased further they are completely unable to follow the alternations in the field and hence the molecular process Orientation polarization ceases due to dielectric relaxation. This occurs in the frequency range 10⁶ Hz to 10¹¹Hz. As the frequency is increased further other polarizing mechanisms start to cease one after another. The ionic polarization ceases in the frequency range 10¹¹ Hz to 10¹⁴Hz. Finally only electronic polarization remains. Hence as the frequency of the field increases the polar ization decreases and hence the dielectric constant decreases. This is known as Anomalous Dielectric Dispersion.

Dielectric Loss:
In the alternating field conditions during the rotation of dipoles they have to overcome some sort of internal friction, which is dissipated as heat by the material. This is called as dielectric loss. Complex Dielectric Constant: The complex dielectric constant is given by  . Where  determines Dielectric Loss.  determines the component of current out of phase by 900 with the field.

Important applications of Dielectric Materials
Dielectric materials find a wide range of applications as insulating materials.
(1) Plastic and Rubber dielectric are used for the insulation of electrical conductors
(2) Ceramic beads are used for the prevention of electric short circuiting and also for the purpose of insulation.
(3) Mica and Asbestos insulation is provided in electric iron in order to prevent the flow of electric current to outer body.
(4) Varnished cotton is used insulators in transformers.
(5) Dielectric materials are used in the energy storage capacitors.
(6) Piezoelectric crystals are used in oscillators and vibrators.