Electrical conductivity anisotropy

The important role played by the electrical conductivity anisotropy a a is evident if Ga decreases, U q,f) increases and diverges at a small positive value of a a when the two terms in the denominator of Eq.(7) cancel each other. If a a approaches zero or becomes negative (case F), the CH term vanishes or acts as a stabihzing term, respectively thus convection is not expected. The role of a is somewhat different an EC transition exists for vanishing and even for positive (case E). 

A non-electrochemical technique which has been employed to alter the physical characteristics of a number of polymers is that of stress orientation [26, 27], in which the material is stressed whilst being converted to the desired form. This has the effect of aligning the polymer chains and increasing the degree of order in the material, and is obviously most applicable to materials which can be produced via a precursor polymer. With Durham polyacetylene (Section 4.2.1) increases in length in excess of a factor of twenty have been achieved, with concomitant increases in order, as shown by X-ray diffraction and by measurements of the anisotropy of the electrical conductivity perpendicular and parallel to the stretch direction. 

Other physical properties. Anisotropy of thermal and electrical conductivity, coefficient of thermal expansion, elasticity, and dielectric constant may also provide information on internal structure. These properties, however, have so far been little used in structure determination, because they are less easily measured than those already considered consequently not very much experimental evidence is available for the purpose of generalizing on the relations between such properties and structural features. For further information on these subjects, see Wooster (1938), Nye (1957). 

Although the anisotropy of the complexes electrical conductivity was not mentioned, the three-dimensional nature of the complex facilitates a metal-like temperature dependence of its conductivity down to 4K where a = 105 2 lcm l (room-temperature a is a respectable 300 2 1cm 1). The absence of a metal-insulator transition down to 4K shows that the Peierls instability has been successfully avoided by increasing the interstack coupling. 

All electrooptical effects known to the present time for polymeric liquid crystals may be divided into two groups. First of all there are so called orientational effects, which are due solely to the effect of the electric field (field effect) on LC polymers, but are not a result of a current flowing. The second group of electrooptical effects is attributed to the phenomena ascribed to the anisotropy of electrical conductivity (Act) of liquid crystals. These are called electrohydrodynamic effects. 

The pronounced anisotropy of the electrical conductivity in layered compounds (8,15) suggests that the charge carriers move, on their way to the surface, predominantly within the layers, i.e. parallel to the main surface as shown in Fig. 14 (in which the relative path of the charge carriers within the layers is compressed). The random character of interlayer charge transport due to extrinsic conduction leads to a variety of possible paths, two of which are represented in Fig. 14. 

The temperature dependence of the electrical conductivity of KCP(Br) has been studied by Zeller and Beck (Figure 4).37 At room temperature, try is about 300 2-1 cm-1 and the dependence of conductivity on temperature is slightly negative with the degree of anisotropy ffg/ffi of about 105. 

Theoretical analysis indicates that occurrence of such convective instabilities depends on anisotropy of electrical conductivity and dielectric properties in the initial aligned nematic material. That is, conductivity parallel to the direction of alignment must differ from conductivity perpendicular to this direction. Calculation of the stability condition requires knowledge not only of these anisotropic electrical properties but also of anisotropic elastic and viscous properties which oppose disruption of the alignment and flow. 

Some important observations, which should apply de facto to many nematic systems containing dispersed nanoparticles, particularly those with metal or semiconductor cores, were reported in 2006 by Prasad et al. [297]. The authors found that gold nanoparticles stabilized with dodecanethiol decreased the isotropic to nematic phase transition of 4-pentyl-4 -cyanobiphenyl (5CB) almost linearly with increasing nanoparticle concentration (x p) and increased the overall conductivity of these mixtures by about two orders of magnitude. However, the anisotropy of the electric conductivity (Act = [Pg.349]

The electrical conductance shows a weaker concentration dependence above than below the CMC corresponding to a decrease in the equivalent conductance (Fig. 2.10). The transport number of the surfactant ion rises sharply at the CMC while that of the counterion may become negative. This as well as electrophoretic mobilities may yield information on micellar charge. At high concentrations, conductance anisotropies have been observed for flowing systems. This, as well as flow birefringence, is useful for the demonstration of nonspherical micelle shape. 

A direct verification of a quasi-two-dimensional electric conductivity, which is so unusual for TCNQ-based ARS, can be performed by studying the electric resistance anisotropy of [N-C2H5-Pz](MTCNQ)2 single crystals. 

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