Dielectric Properties of Solids:
A solid which does not allow the net flow of electric charge on applying an electric field (in spite of presence dipoles) is called a dielectric solid. It is because the electrons are held strongly by individual atoms in insulators and do not move in an electric field. However, due to shifting in charges, dipoles are produced which result in polarisation. Two possibilities in which the alignment of dipoles occur are-
- Dipole moments cancel each other (compensatory manner).
- Alignment of dipoles in such a manner that there is a net dipole moment. There may be no dipoles in the crystal but only ions are present.
Depending upon the alignment of dipoles, the crystals have very interesting properties such as-
- Piezoelectricity- The crystals in which the dipoles may align themselves in an ordered way, having some resultant dipoles are called the piezoelectrics. Such crystals exhibit piezo-electricity or preserve electricity. On applying mechanical stress these crystals get deformed due to displacement of dipoles and produce electricity. Conversely, on applying electric filed there is atomic displacement which causes mechanical strain. Thus, these crystals are used as pick up in record players where they produce electric signals by the applying of pressure. They are also used in microphones, ultrasonic devices, sonar detectors, etc. Example- Titrates of Barium and Lead, Zirconate, etc.
- Pyro-electricity- The property by virtue of which these crystal may produce electric charges on opposite faces on heating is called pyro-electricity. “Pyro” means heat.
- Ferroelectricity- In some piezo crystals, the dipoles are permanently aligned in a particular direction even in the absence of electric field. Such solids are called ferroelectric. Applying an electric field can change the polarization in such solids. Example- ferroelectric solids are Sodium, Potassium, Tartarate (Rochelle Salt), Barium Titanate (BaTiO3) and Potassium dihydrogen phosphate (KH2PO4). if the alternate dipoles are in the opposite direction so that the dipoles is Zero, the crystal is called anti-ferroelectric. Lead Zirconate is anti-ferroelectric solid.
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