ELECTRICAL POLES

Assume that electrical conductors, such as aqueous solutions containing charged particles known as ions, exists in a static electrochemical system and that there exists an irrfinitesimal current flow. Thus, an electric field strength vector E at a point P in space and an inner electric potential f of the system are defined below, respectively [1] 

In fact, Q depends on the type of ions, which are treated hereafter as poles, and it is related to Faraday s constant F. Both F and Q are defined by 

The Cartesian electric field strength components in a three-dimensional scheme are defined as fallows 

Equation (2.3) gives the definition of the monopole potential due to a point charge Q. This is a simple definition of an internal potential between two points located at a relatively short distance. Otherwise, — 0 as r — oo. (3n the other hand, when — c as r — 0 an electric potential singularity can be established for in the order of r as predicted by eq. (2.3). 

Consider an electrolyte or a metal as a single electrical conductor phase in equilibrium. In this case, current flow does not occur and the electric field at aU points in the phase is zero. In fact, a non-infinitesimal current is an irreversible process since heat is generated by the current flowing in the phase [1]. At equilibrium, eq. (2.2b) gives Ea, = Ey = E = 0, which means that the electric potential is constant in the bulk phase so that 0 and the current is / = 0. 

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Fig. S3. High quality, biaxially stretched PVDF ferroelectric film can be electrically poled to a remanent polarization of over 9 fxCcm with the Bauer cyclic poling technique. The polarization versus electric field of a typical sample is shown.

This chapter provides an overview of the basic principles and designs of such sensors. A chemical sensor to detect trace explosives and a broadband fiber optic electric-field sensor are presented as practical examples. The polymers used for the trace explosive sensor are unpoled and have chromophores randomly orientated in the polymer hosts. The electric field sensor uses a poled polymer with chromophores preferentially aligned through electrical poling, and the microring resonator is directly coupled to the core of optical fiber. 

Piezoelectric materials are materials that exhibit a linear relationship between electric and mechanical variables. Electric polarization is proportional to mechanical stress. The direct piezoelectric effect can be described as the ability of materials to convert mechanical stress into an electric field, and the reverse, to convert an electric field into a mechanical stress. The use of the piezoelectric effect in sensors is based on the latter property. For materials to exhibit the piezoelectric effect, the materials must be anisotropic and electrically poled ie, there must be a spontaneous electric field maintained in a particular direction throughout the material. A key feature of a piezoelectric material involves this spontaneous electric field and its disappearance above the Curie point. Only solids without a center of symmetry show this piezoelectric effect, a third-rank tensor property (14,15). 

It should be stated that an electric field of < 10 V/pm was applied to the cell in order to unwind the FLC helix of 3, and the observed NLO behavior is a combination of the electric field induced SHG (EFISH) and that due to the spontaneous polar order in the phase. While other FLCs give much lower SHG efficiency with the same applied fields, and achiral smectic LC phenylbenzoates in our hands give unobservable SHG under identical conditions, we cannot completely rule out at this time the possibility that a significant amount of the response from compound 3 is due to the electrical poling. Control experiments to test for this (e.g. by SHG from compound 5 and/or racemic 3) are in progress, as are further experiments aimed at obtaining the phase-matched SHG efficiency for 3. 

This work provides synthetic routes into polymeric-organometallic NLO materials based on some new and exciting ferrocene chemistry. Electric poling and SHG studies on these new materials will allow us to contrast the relative effectiveness of the two approaches (i.e. pendant versus main-chain). 

As an alternative to surface energy matching, electrical poling can be used to orient the microdomains.32 33 One of the advantages of this method is that the thickness of the film can be thicker, and the substrate can be varied.34 When the block copolymer is allowed to phase separate on an electrode surface, the porous film can be filled via electroplating, leading to the formation of nanowires ( 15 nm in diameter) in a polymer matrix.35... 

Equation (9) defines r33 while Eq. (10) defines r13. For electrically poled polymer materials, it is common to assume that r33=3(n0/ne)4r13 [3]. However, the reader should note that this is not always the case [13,14] and is seldom the case with use of laser-assisted (photochemical) poling [15,16]. Thus, we need only consider Eq. (4) in our further discussion. However, it should be noted that devices such as the birefringent modulator depend on the difference between r33 and r13 so that both components can enter the equation for device performance. 

Fig.12. Computation by Monte Carlo methods of the first four order parameters of an ensemble of 1000 chromophores (of dipole moment 13 Debye) existing in a medium of uniform dielectric constant. At the beginning of the calculation, the chromophores are randomly ordered thus, ==O. During the first 400 Monte Carlo steps, an electric poling field (600 V/micron) is on but the chromophore number density (=10 7 molecules/cc) is so small that intermolecular electrostatic interactions are unimportant. The order parameters quickly evolve to well-known equilibrium values obtained analytically from statistical mechanics (black dots in figure also see text). During steps 400-800 the chromophore number density is increased to 5xl020 and intermolecular electrostatic interactions act to decrease order parameters consistent with the results of equilibrium statistical mechanical calculations discussed in the text. Although Monte Carlo and equilibrium statistical mechanical approaches described in the text are based on different approximations and mathematical methods, they lead to the same result (i.e., are in quantitative agreement)...

A further significant development is the observation by T. Fujiwara et al. [15] of enhanced SHG achieved through a combination of ultraviolet laser irradiation and electrical poling. A glass of composition 0.157Ge02-0.843Si02, poled at a field strength of 30 MV m 1 and simultaneously irradiated with ultraviolet laser... 

The calculation of the molar polarizabilities, often involves statistical mechanical averaging over orientational distributions of the molecules. An important example is the distribution function w caused by dipole orientation in an externally applied static electric field E° because it describes the process of electric poling of NLO-phores. To second order in the field, the dipolar contributions to this (normalized) function are given by (100),... 

Multiple CT leading to three level contributions and the possibility of ordering through electrical poling is combined with dipolar 2D NLO-phores. The establishment of structure-property relationships for this type of molecule, reviewed recently (Wolff and Wortmann, 1998), is still in its infancy because application of a single analytical method is clearly inadequate to unravel the combination of different tensor elements. It is convenient to keep the number of numerically different tensor elements as low as possible and to study planar molecules of C2v symmetry. Out of the seven /3 components that are significant for this case, only hve are independent. In addition, the components in the x-direction, dehned as perpendicular to the molecular plane (y,z) are negligibly small, so only four components remain = y y, yy and A... 

This complexity of the saturation phenomenon, though a drawbadr, is at the same time an asset from the theoretical point of view. The reader vrill have noted from the theory discussed above that electric saturation makes directly apparent the participation of reorientation of the electric poles of molecides and macromolecules. In this respect, saturation is unique, owing to the part played in it, in addition to the various molecular processes specific to other effects e.g. the Kerr effect), by entirely new processes provi ng valuable information on the interactions between molecules in condensed phases and their internal structure. We believe it... 

Liquids with large dielectric constants are sometimes called dipolar liquids (or simply polar liquids). It is interesting to note that these dipolar liquids are good solvents foi ionic crystals (containing electric " poles or charges), and that non polar liquids (benzene, etc.) are good solvents for non-polar substances. 

In general, for second row atoms, the most electronegative atoms are located at the ends of the molecule, corresponding to a minimized repulsion of negative electric poles. Similarly, metal complexes contain a central electropositive atom surrounded by more electronegative ligands. When the electronegativities of the atoms involved are similar, then isomers may occur. One example is aoa and CICIO. 

The difference of electronegativity for F and N is greater than for Cl and O. Accordingly, F2N occurs only as FNF with the two negative electric poles (F) at the ends of the molecule. 

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