Electrical polarization orientational

The most important materials among nonlinear dielectrics are ferroelectrics which can exhibit a spontaneous polarization PI in the absence of an external electric field and which can spHt into spontaneously polarized regions known as domains (5). It is evident that in the ferroelectric the domain states differ in orientation of spontaneous electric polarization, which are in equiUbrium thermodynamically, and that the ferroelectric character is estabUshed when one domain state can be transformed to another by a suitably directed external electric field (6). It is the reorientabiUty of the domain state polarizations that distinguishes ferroelectrics as a subgroup of materials from the 10-polar-point symmetry group of pyroelectric crystals (7—9). 

A paraelectric substance is not polarized macroscopically because the dipoles are oriented randomly. However, they can be oriented by an external electric field (orientation polarization). The orientation is counteracted by thermal motion, i.e. the degree of polarization decreases with increasing temperature. 

The general problem of the orienting effect of a static electric field (orientation of polar molecules) was first considered by Debye [6, 7], Frolich [8], and more recently Bottcher [9,10]. 

Relaxation processes are probably the most important of the interactions between electric fields and matter. Debye [6] extended the Langevin theory of dipole orientation in a constant field to the case of a varying field. He showed that the Boltzmann factor of the Langevin theory becomes a time-dependent weighting factor. When a steady electric field is applied to a dielectric the distortion polarization, PDisior, will be established very quickly - we can say instantaneously compared with time intervals of interest. But the remaining dipolar part of the polarization (orientation polarization, Porient) takes time to reach its equilibrium value. When the polarization becomes complex, the permittivity must also become complex, as shown by Eq. (5) ... 

The electric polarization of the solvent has three components electronic, atomic (i.e., translational and vibrational), and orientational. The polarization of a nonpolar solvent is almost entirely electronic this leads to e 2. Polar solvents can have much larger dielectric constants, e.g. is 13.9 for 1-pentanol, 37.7 for methanol, and 78.3 for water.50... 

For Laurent, who used metaphors of the chemical tree and the biological nucleus, the type was an ordering principle of extraordinary power. Berzelius was suspicious of this orientation because it was at odds with his theory of electrical polarities between elementary substances. 53... 

Polypeptides are electrically polar, carrying permanent dipoles at the planar CO-NH groups of the backbone chain and generally at some atomic groups of the side-chains. Because of the vector nature of dipoles, we must speak of the mean-square dipole moment, averaged over all possible conformations of the backbone chain and all accessible orientations of the side-chains when the dipolar nature of a polypeptide in solution is considered. The of a polypeptide thus may depend on what conformation the molecule assumes in a given solvent. 

Electret. The electrical equivalent of a permanent magnet. When a block of dielectric material, such as carnauba wax, is allowed to be solidified in a strong electric field it acquires a permanent state of electrostatic polarization (orientation of molecules) in the direction of the field. 

This account has summarized several of our approaches to the preparation of electric-field-aligned chromophoric polymers for second order NLO applications. Molecular design has been employed wherever possible to arrive at structures that probe particular aspects of the polar orientation issue. The rich variety of accessible organic structures has enabled us to consider the orientation problem from a variety of points of view, and to indicate by example the manner in which multifunctional organic synthesis may play a role in the fabrication of oriented materials. 

However, the temperature, at which the maximum of the initial scattered light occurs, seems to be related to the scattering angle 9S and thus to the period Ag , respectively. Figure 9.14(b) shows the correspondence between the temperature Tm of maximum intensity Ig and the spatial period Agn. A spatial disorder of the smallest polar structures occurs at Tm = 45 °C, while the spatial orientation of the largest structures remains stable up to Tm = 60 °C. Such big dispersion of the thermal decay of polar structures over Agn unambiguously illustrates the relaxor behavior of sbn. At the same time it is a key point to understand the bandwidth in the determination of the phase transition temperature Tm in sbn from different methods. For example, in sbn doped with 0.66 mol% Cerium, Tm detected from the maximum of the dielectric permittivity e at 100 Hz (e-method) equals Tm = 67 °C [20], Determination of Tm from the inflection point of the spontaneous electric polarization P3... 

When an electric field is applied to an ideal dielectric material there is no long-range transport of charge but only a limited rearrangement such that the dielectric acquires a dipole moment and is said to be polarized. Atomic polarization, which occurs in all materials, is a small displacement of the electrons in an atom relative to the nucleus in ionic materials there is, in addition, ionic polarization involving the relative displacement of cation and anion sublattices. Dipolar materials, such as water, can become polarized because the applied electric field orients the molecules. Finally, space charge polarization involves a limited transport of charge carriers until they are stopped... 

When a substance is placed in an electric field, such as exists between the plates of a charged capacitor, it becomes to some extent electrically polarized. The polarization results at least in part from a displacement of electron clouds relative to atomic nuclei polarization resulting from this cause is termed electronic polarization. For molecular substances, atomic polarization may also be present, owing to a distortion of the molecular skeleton. Taken together, these two kinds of polarization are called distortion polarization. Finally, when molecules possessing permanent dipoles are present in a liquid or gas, application of an electric field produces a small preferential orientation of the dipoles in the field direction, leading to orientation polarization. 

Fig. 20 Mechanisms of polarity inversion in a dielectric, (a) Electric polarization (b) ionic polarization (c) orientation polarization.

If /ci 4= 0, the state of lowest free-energy density has a finite splay, sq = kilku. This can only exist when the molecules are distinguishable end from end, and there is polarity along L in their preferred orientation. Then, almost inevitably, the molecules have an electric dipole moment, and therefore, unless the material is an electric conductor, the condition V L 4= 0 implies V P 4= 0 (where P is the electric polarization), so that finite splay produces a space-charge. As a second consideration, it is not geometrically possible to have uniform splay in a three-dimcnsionally extended region. The simple cases of uniform splay are those in... 

The clearest examples of the influence of polar constituents usually involve materials in which no more than one component has a large dipole moment, such as a relatively nonpolar CTM doped into nonpolar and polar polymers, a nonpolar polymer doped with relatively nonpolar and polar CTMs, or a relatively nonpolar CTM and nonpolar polymer additionally doped with a highly polar additive. Similar effects are sometimes seen when a polar polymer is doped with CTMs having various dipole moments [56g-j, 64p]. Often, however, the mobility in a highly polar CTM is independent of, or only weakly affected by, the nonpolar or polar character of the host polymer [68a-c]. Often, too, for a highly polar CTM, a is independent of CTM concentration, contrary to what one would expect on the basis of Eq. (10). A possible explanation is the tendency of neighboring dipoles to have antiferro-electrically correlated orientations [61a]. 

Stracke, A., Bayer, A., Zimmermaim, S., Wendorff, J. H., Wirges, W., Bauer-Gogonea, S., Bauer, S., and Gerhard-Multhaupt, R. Relaxation behaviour of electrically induced polar orientation and of optically induced non-polar orientation in an azo-chromophore side group polymer. J. Phys. D-Appl. Phys. 1999, 32, pp. 2996-3003. 

Electret. The electrical equivalent of a permanent magnet. When a block of dielectric material, such as carnauba wax, is allowed to be solidified in a strong electric field it acquires a permanent state of electrostatic polarization (orientation of molecules) in the direction of the field. Swann (Ref 3) traced the mathematical consequences on the assumption that an electret consists of a) a distribution of polarization which decays with time and b) a distribution of surface volume chge which disappears acedg to ohmic conductivity having no relation to the decay of polarization Refs 1) Hackh s Dict(1944), 296-L 2) F. 

The 90 geometry of Figure 6.25 does not require a polarizer to determine p but does require rotation of the laser polarization. The intensity (/j) is determined with the laser electric field oriented on the y axis, with the observation axis along the x axis. Then the laser electric field is rotated 90 (usually with a quarter wave plate) to position it parallel to the x axis. The observed intensity is now /x, and p may be calculated directly from the two spectra. 

---