Types of Polarization

polarization is the alignment of permanent or induced atomic or molecular dipole moments with an externally applied electric field. There are three types or sources of polarization electronic, ionic, and orientation. Dielectric materials typically exhibit at least one of these polarization types, depending on the material and the manner of external field application. 

Figpre 18.32 (a) Electronic polarization that results from the distortion of an atomic electron cloud by an electric field, (b) Ionic polarization that results from the relative displacements of electrically charged ions in response to an electric field, (c) Response of permanent electric dipoles (arrows) to an applied electrie field, producing orientation polarization. (From O. H. Wyatt and D. Dew-Hughes, Metals, Ceramics and Polymers, Cambridge University Press, 1974. Reprinted with the permission of the Cambridge University Press.) 

Total polarization of a substance equals the sum of electronic, ionic, and orientation polarizations 

The total polarization P of a substance is equal to the sum of the electroruc, ionic, and orientation polarizations (P, Pj, and P , respectively), or 

It is possible for one or more of these contributions to the total polarization to be either absent or negligible in magnitude relative to the others. For example, ionic polarization does not exist in covalently bonded materials in which no ions are present. 

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Now let us examine the molecular origin of Molecular polarity may be the result of either a permanent dipole moment p or an induced dipole moment ind here the latter arises from the distortion of the charge distribution in a molecule due to an electric field. We saw in Chap. 8 that each of these types of polarity are sources of intermolecular attraction. In the present discussion we assume that no permanent dipoles are present and note that the induced dipole moment is proportional to the net field strength at the molecule ... 

These types of polar monomer provide sites for hydrogen bonding which increase the cohesive strength of the PSA because of strong inter-chain interaction, and they can also allow for hydrogen bonding or other polar interactions with some substrates. 

The amount of polar monomer one would copolymerize with the alkyl acrylate monomer(s) very much depends on the type of polar monomer and the desired change in rheological properties one would like to achieve. Strong hydrogen bonding monomers, such as acrylic acid, methacrylic acid, acrylamide, or methacrylamide are typically used at levels of 12% or less of the total monomers. 

Normal phase a. Type of polar solvent b. Coneentration of polar solvent e. Aeid and base as modifiers d. Temperature... 

As shown in Fig. 8, three types of polarization exist during the discharge of porous MnOa. The battery active EMD or CMD (chemical manganese dioxide) is highly porous and the concentration polarization due to the pH change, r (ApH), is very important. Kozawa studied the three types of... 

Figure 8. Three types of polarization of Mn02 (1) J]c (H+ solid), due to proton diffusion in solid (2) rja, due to the solution-solid interface (3) 7/t (ApH), due to a pH change of the electrolyte in the pores.

Consider now an encounter (F) radical pair formed from two free radicals. Since there are three components to the triplet state, T+i, To and T j, and only one singlet component, S, the encounter of two free radicals having uncorrelated spins leads to a statistical distribution of T and S radical pairs. However, some of the S radical pairs will react without undergoing T-S mixing, and this has the effect of increasing the relative number of T radical pairs. Consequently the F-pairs will give the sam e type of polarization as the T-pairs, but the degree of polarization will be less. 

This type of polarization appears to be general in such systems and it has been proposed that it arises because of T i-S mixing. An interesting and important addif ional observation comes from a study of the reaction of isopropyl chloride with sodium naphthalene in a field of 60 G. Net emission in the products is predicted for T i-S mixing and multiplet polarization for Tq-S mixing. Since no multiplet polarization can be detected, it would seem that Tq-S mixing is suppressed in this system (Garst et al., 1971). Presumably, the naphthalene radical anion... 

Electrode reactions are heterogeneous since they occur at interfaces between dissimilar phases. During current flow the surface concentrations Cg j of the substances involved in the reaction change relative to the initial (bulk) concentrations Cy p Hence, the value of the equilibrium potential is defined by the Nemst equation changes, and a special type of polarization arises where the shift of electrode potential is due to a change in equilibrium potential of the electrode. The surface concentrations that are established are determined by the balance between electrode reaction rates and the supply or elimination of each substance by diffusion [Eq. (4.9)]. Hence, this type of polarization, is called diffusional concentration polarization or simply concentration polarization. (Here we must take into account that another type of concentration polarization exists which is not tied to diffusion processes see Section 13.5.)... 

Other types of polarization are caused by specific features in the various steps of the electrochemical reaction that produce a potential shift relative to the effective equilibrium potential (i.e., that which already accounts for the prevailing values of surface concentrations). These types of polarization, which may differ in character, are jointfy termed activation polarization. The value of activation polarization is sometimes called the overvoltage (this term should be reserved for the complete ceU see Section 2.5.2). 

When concentration changes affect the operation of an electrode while activation polarization is not present (Section 6.3), the electrode is said to operate in the diffusion mode (nnder diffusion control), and the cnrrent is called a diffusion current i. When activation polarization is operative while marked concentration changes are absent (Section 6.2), the electrode is said to operate in the kinetic mode (under kinetic control), and the current is called a reaction or kinetic current i,. When both types of polarization are operative (Section 6.4), the electrode is said to operate in the mixed mode (nnder mixed control). 

The kinetic and polarization equations described in Sections 6.1 and 6.2 have been derived under the assumption that the component concentrations do not change during the reaction. Therefore, the current density appearing in these equations is the kinetic current density 4. Similarly, the current density appearing in the equations of Section 6.3 is the diffusion current density When the two types of polarization are effective simultaneously, the real current density i (Fig. 6.6, curve 3) will be smaller than current densities and ij (Fig. 6.6, curves 1 and 2) for a given value of polarization. 

Often, it is claimed that at a given current density, the total polarization AE is the sum of pure concentration polarization AEj and pnre activation polarization AE. This is true only in the region of low polarization, where the valnes of polarization are proportional to current density. In other regions it is not trne. In fact, the total polarization defined by Eq. (6.39) (Eig. 6.6, cnrve 3) is larger than the sum of the individual types of polarization, which for the same cnrrent density are defined by Eqs. (6.13) and (6.28) (curves 1 and 2). This is dne to the fact that concentration changes affect activation and concentration poiarization in different ways. 

Therefore, the other types of polarization do not change their configuration when the electron changes its spatial position. This polarization is called slow or inertial polarization, (= P the subscript slow is omitted below). It reacts only to the average position of the transferable electron (in the donor or in the acceptor). The inertial polarization includes all other components and part of the electronic polarization ... 

To quantitate this interaction, several types of polar fragments need to be defined ... 

Figure 18.10 Three types of polarization curves typically manifested by simple Fe and Co porphyrins and cofacial metaUoporphyrins (simulated voltammograms).

The constants characterizing the electrode reaction can be found from this type of polarization curve in the following manner. The quantity k"e is determined directly from the half-wave potential value (Eq. 5.4.27) if E0r is known and the mass transfer coefficient kQx is determined from the limiting current density (Eq. 5.4.20). The charge transfer coefficient oc is determined from the slope of the dependence of In [(yd —/)//] on E. 

Table 6.2 Bond Moments for Some Types of Polar Bonds. ...

If only one type of polar molecule is present, the interaction energy can be expressed as... 

Optical Activity Certain media are optically active, which means that they interact with plane polarized light by changing the direction of polarization. A simple explanation of this effect is that optically active material interacts differently with left and right circularly polarized light respectively, for instance by exhibiting different indices of refraction with respect to these two types of polarized light. If this is the case one or the other of the circular components of plane polarized light would lag behind in that medium. 

In one study, various distinct types of polar modifiers to n-hexane were tested for 3-chloro-l-phenylpropanol (3CPP) and 1-phenylpropanol (IPP) enantiomer separation [53]. Thereby, alcohol modifiers turned out to be more effective displacers of the solutes from the adsorption places on the sorbent surface, yet aprotic polar modifiers provided higher separation factors (with ethyl acetate in n-hexane affording the best separations for these chiral alcohols). It is evident, though, that the optimal choice of polar modifier is strongly solute dependent and can therefore not be generalized. 

Figure 6. Typical discharge curve of a battery, showing the influence of the various types of polarization.

We have already introduced the concept of ionic polarizability (section 1.8) and discussed to some extent the nature of dispersive potential as a function of the individual ionic polarizability of interacting ions (section 1.11.3). We will now treat another type of polarization effect that is important in evaluation of defect energies (chapter 4). 

The remaining types of polarization are absorptive types with characteristic relaxation times corresponding to relaxation frequencies. Debye, in 1912, suggested that the high dielectric constants of water, ethanol, and other highly polar molecules were due to the presence of permanent dipoles within each individual molecule and that there is a tendency... 

The polarizations noted above are the major types found in homogeneous materials. Other types of polarization, called interfacial polarizations, are the result of heterogeneity. Ceramics, polymers with additives, and paper are considered to be electrically heterogeneous. 

The relaxation time required for the charge movement of electronic polarization E to reach equilibrium is extremely short (about 10 s) and this type of polarization is related to the square of the index of refraction. The relaxation time for atomic polarization A is about 10 s. The relaxation time for induced orientation polarization P is dependent on molecular structure and it is temperature-dependent. 

Polarization. The net current flow produced in a cell results in a deviation of each half-cell potential from the equilibrium value listed in Table 3.3. This deviation from equilibrium is termed polarization, the magnitude of which is given the lowercase greek symbol eta, q and is called the overpotential, E-E°. There are two primary types of polarization activation polarization and concentration polarization. 

The second type of polarization, concentration polarization, results from the depletion of ions at the electrode surface as the reaction proceeds. A concentration gradient builds up between the electrode surface and the bulk solution, and the reaction rate is controlled by the rate of diffusion of ions from the bulk to the electrode surface. Hence, the limiting current under concentration polarization, ii, is proportional to the diffusion coefficient for the reacting ion, D (see Section 4.0 and 4.3 for more information on the diffusion coefficient) ... 

We can further describe the polarization, P, according to the different types of dipoles that either already exist or are induced in the dielectric material. The polarization of a dielectric material may be caused by four major types of polarization electronic polarization, ionic (atomic) polarization, orientation polarization, and space-charge (interfacial) polarization. Each type of polarization is shown schematically in Figure 6.24 and will be described in succession. In these descriptions, it will be useful to introduce a new term called the polarizability, a, which is simply a measure of the ability of a material to undergo the specific type of polarization. 

Figure 6.24 Schematic illustration of different types of polarization. From W. D. Kingery, H. K. Bowen and D. R. Uhhnann, Introduction to Ceramics. Copyright 1976 by John Wiley Sons, Inc. This material is nsed by permission of John Wiley Sons, Inc.

The final type of polarization is space-charge polarization, sometimes called interfacial polarization, and 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, and it is found in a variety of ceramic materials, especially at elevated temperatures. The space-charge polarization, P c, has a corresponding space-charge polarizability, The two are related via a relationship of the form for the other types of polarization. 

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