Hello everyone. In this lecture, we are going to talk about the one important electrical property of ceramics, it is dielectrics. As shown here, the metal has general bandgap, and semiconductor has the small bandgap. An insulator has the large band gap. Basically, the ceramics are insulators, but by some compositional tuning, we can make semiconducting and sometimes conducting ceramics. So as shown here, electronic ceramics can be divided into three categories. One is dielectrics, second one is semiconductors, and third one is conductors. The dielectric ceramics such as barium titanate, strontium titanate, calcium titanate, zirconia, silica, alumina, beryllium oxide, can be used for capacitor. Semiconducting ceramics such as doped [inaudible] , tin oxide, zinc oxide, titanium oxide, tungsten oxide, indium oxide, iron oxide, copper oxide, nickel oxide, cobalt oxide, chrome oxide, manganese oxide can be used as an active material for gas sensors. Conducting ceramics such as the doped tin oxide, the ITO, and doped zinc oxide, AZO, can be used as the transparent electrode due to their high electrical conductivity and high optical transmittance. Dielectric materials are solids with energy bandgap of three electron volt or more, so they are insulators. Almost impossible to excite the electrons from the valence band to conduction band by an applied field. So generally, dielectrics are also called as insulators. They allow movement of some electrons at abnormally high temperatures and they are non-metallic materials of high specific resistance. They have change in insulation resistance due to moisture, temperature, electric field change. For example, glass, ebonite, mica, rubber, wood, paper are dielectric materials. The only difference between a dielectric and an insulator is related with their applications. If we use dielectric material as electrical insulating application, they are insulators. But if we use this material as electrical energy storage material, they are dielectrics. Dielectrics can be divided into two categories. One is active dielectrics. When a dielectric material is kept in an electric field, if it actively accepts the electricity, this is an active dielectric material and can be adapted to store the electrical energy in it. A second type of dielectric is passive dielectrics. They can restrict the flow of electric energy in them, so they are insulators. Let's think about the properties of dielectric ceramics. They should have large energy bandgap more than three electron volt. So electrons are tightly bound to their parent nuclei, so there is no free electrons to carry the current. So their electrical conductivity is very low, and they also have negative temperature coefficient of resistance. An electric dipole is a system consisting of two equal and opposite charges of plus q and minus q, as shown here, separated by a distance d. This is an electric dipole. Dipole moment p is the product of the magnitude of the charge q and distance between two charges d, that makes dipole moment. The p is given by qd. You should know some fundamental definitions which is related with dielectric ceramics. The permittivity is a dielectric property of a medium which indicates easily polarizable nature of a material. Dielectric constant is a dielectric characteristic of a material, a measure of polarization of the dielectrics, and the ratio between absolute permittivity of the medium and the permittivity of free space. Polarization is the process of producing electric dipoles inside the dielectric material by the application of an external electrical field, and polarizability is a ratio of average dipole moment to the electrical field applied. So there are four different types of mechanisms of polarization. The first, electronic polarization, atom or atomic polarization. If we apply the electric field, we can induce the displacement of charges of the electrons. The second, the ionic polarization. The material must have some ionic character, so automatically has internal dipoles and built-in dipoles can cancel each other. If we apply an external electric field, we can induce net dipole by slightly displacing the ions as shown here. The third, orientation polarization. The material must have natural dipoles which can rotate freely. So randomly oriented dipoles in thermal equilibrium result in no net polarization, but if we apply the external electrical field, this aligns dipoles and results in induced polarization. The fourth, the interface space charge polarization, which is related with the charged surfaces and boundaries, so become oriented to some degree in an external field makes polarization. Dielectric materials include piezoelectric material, and piezoelectric materials include pyroelectric materials, and pyroelectric materials include paralectric and ferroelectric materials. The piezoelectrics is the charge generation by mechanical field. Pyroelectrics is the charge generation by thermal field. You should know the difference between pyroelectrics and thermoelectrics. The thermoelectrics is the electric energy generation due to temperature difference and pyroelectrics is a charge generation by electrical field. So among 32 crystalline classes, 20 classes are piezoelectric and among them, 10 classes are pyroelectric and among them, several materials such as barium titanate and lead titanate can show the ferroelectric behavior. The piezoelectric is generating an electric charge when subjecting it to applied stress, and also generating a mechanical strain in response to an applied electrical field. The force is applied to a segment of the material. This can lead to the appearance of electrical charge on the surfaces. Specific depletion of electric charge in the unit cell of a crystal can be obtained as shown here. We can find two different effects in piezoelectrics. The first is the direct piezoelectric effect, which can be related with the piezoelectric generator. The compression is the decrease in volume resulting in voltage with the same polarity, and tension is increase in volume results in voltage with opposite polarity. The second is the inverse piezoelectric effect. If we apply voltage with the same polarity, we can induce the material expansion. If we apply the voltage with opposite polarity, the material should be contracted. So one important material of piezoelectric is PZT, the lead zirconate titanate, the titanate lead zirconium oxide. MPB composition, MPB is the morphotropic phase boundary at the phase boundary between rhombohedra, lead zirconate, and tetragonal lead titanate. So at this Morphotropic phase boundary, the PZT shows the highest piezoelectric properties. So things like piezoelectric material can be used for piezoelectric generator, sensors, actuators, and transducers. Paraelectric is an ability become polarized under an applied electric field. So there is no permanent electric dipole and removal of the field result in zero polarization. The mechanisms include distortion of individual ions and polarization of molecules of combinations of ions or defects. Thank you.
