ynylene, polypyrole, and polyaniline. Some other polymers such as polyphthaocyanine can be cross-linked by special curing processes to raise its conductivity Dielectric behavior A dielectric is a material separating two charged bodies. For a material to be a good dielectric, it must be an electrical insulator. Thus, any insulator is also called a dielectric. Dielectric materials are used in capacitors, devices used to store the electric energy. When a voltage is applied between two parallel plates in a vacuum, one plate becomes positively charged and the other negatively charged, with corresponding field directed from the positive to the negative. Then the energy stored, Q, is given by: Q = CV... (15) where C is the capacitance (units: coulombs per volt or farads [F]). It is dependent on the geometry of plates as follows: A C = ε 0.. (16) l where A area of the plates, l distance between them, and ε 0 permittivity of vacuum (8.85 X F/m). If a dielectric material is inserted between the plates, A C = ε. (17) l ε permittivity of the medium, which will be greater in magnitude than ε 0. The relative permittivity ε r, also called dielectric constant, is defined as
10 ε ε r =...(18) ε 0 which is greater than unity, and presents the increase in charge storing capacity by insertion of the dielectric medium between the plates. This is result of polarization orientation of permanent or induced dipoles under externally applied electric field. Polarization causes positive charge to accumulate on the bottom surface next to the negatively charged plate, and causes negative charge to accumulate toward the positively charged plate on the top. This tends to decrease the effective surface charge density on either plate. The expected decrease in effective charge corresponds to the polarization, of the material which equals the induced dipole moment per unit volume of polarizable material. The magnitude of the charge per unit area on either plate, called the dielectric displacement or flux density or surface charge density, D, is defined as: D = εξ. (19) where ξ electric field strength, ξ = V / l.. (20) In the presence of the dielectric, surface charge density can also be presented as D = εξ = ε 0 ξ + P (21) where polarization is defined as, P ε ( ε 1) ξ... (22) = 0 r the increase in charge density above that for a vacuum because of the presence of the dielectric. Figure 14-5: Schematic representation of capacitor and polarization phenomenon.
11 The polarization may also be thought of as the total dipole moment per unit volume of the dielectric material or as a polarization electric field within the dielectric that results from the mutual alignment of the many a atomic or molecular dipoles. Polarization of a dielectric material may be caused by at least one of four major types of polarization: electronic, ionic (atomic), orientation, and space charge (interfacial). Electronic polarization arises because the center of the electron cloud around a nucleus is displaced under an applied electric field. Ionic polarization occurs in ionic materials because an applied field acts to displace cations in the direction of the applied field while displacing anions in a direction opposite to the applied field. Orientation polarization can occur in materials that possess permanent electric dipole moments. These permanent dipoles tend to become aligned with the applied electric field, but entropy and thermal effects tend to counter this alignment, thus it is highly temperature dependent. Space charge polarization results from the accumulation of charge at structural interfaces in heterogeneous materials. Such polarization occurs when one of the phases has a much higher resistivity than the other. The total polarization subjected to an alternating electric field depends upon the ease with which the permanent or induced dipoles can reverse their alignment. The time required for dipole reversal is called the relaxation time, and its reciprocal is called the relexation frequency. As the frequency of the applied electric field approached the relaxation frequency