Conductivity measurements anisotropy

Electrical conductivity measurements [41] along the i-axis and in the a-c plane reveal an extreme anisotropy single crystals after the currents had reached a steady value to avoid the effect of ionic... 

The room-temperature conductivities obtained for a number of PPy films appear in Table 12.3. We have applied both the linear four-probe and van der Pauw methods to the measurement of conductivity. The linear four-probe method has certain advantages the samples (narrow strips of film) are easily obtained and any anisotropy of the conductivity (within the film plane) can be investigated. The major source of error in conductivity measurements is the thickness measurement. The latter is conveniently done using a micrometer when the film thickness is greater than about 20 /zm. (Accurate thickness measurements of very thin films require the use of other techniques, such as electron microscopy.) It is advisable to make a number of conductivity measurements using specimens from different regions of a given film. 

The difficulty of such measurements is also seen in the investigations by Servet e/ al. [46] which correlate their X-ray data on differently prepared a6T films (compare Section 3.4) with conductivity data. Although the structural order increases with increasing substrate temperature a decrease in the absolute value is obtained from 6xlO S/cm for 7 s b = 77K to 1.2xl0 S/cm for Tsub = 553 K (conductivity measured parallel to the substrate surface) which is attributed to the desorption of impurities from the substrate which can act as dopants. The anisotropy, however, is increased as expected and the field effect mobility remains nearly constant. 

Among the early examples of the successful use of electric fields to probe ionic structures and electrical and optical anisotropies are the linear polyelectrolytes. Basic information about macromolecular dimensions, size, and shape have been derived from the relaxation of field-induced changes in optical properties and in electrical parameters of the electrically and optically anisotropic systems. The analysis of electric conductivity measurements has demonstrated that linear polyelectrolytes are electrically anisotropic. It was established that the extremely large dipole moments, which the electric field produces by displacement of the counterion atmosphere parallel to the long axis of the polyions, are responsible for their orientations in the direction of the external field. 

The resistivity data in the table suggests that in-situ polymerization of aniline on fabrics has a poor anisotropy in electrical conductivity, indicating the disordered nature of polymer to some extent. The a.c. conductivity measurements of the PAn-grafted fabric show an increase in a.c. conductivity with frequency but the amplitude is small, again indicating the disordered nature of the polymer. 

Most of the conductivity measurements were performed on the SPI acidic form since it is the relevant value for the fuel cell application. Nevertheless, a few data were obtained on neutralized forms [141,146,159]. Rollet et al. used sodium and tetramethylammonium ions to study the transport processes within the membranes and a transport anisotropy was clearly observed from longitudinal and transversal measurements [146]. 

The ionic conductivity of LiFeP04 single crystals was measured to be up to four orders of magnitude lower than the electronic conductivity. The anisotropy already observed for the electronic conduction is reproduced for ionic transport. Similar values were found for b- and c-direction whereas the a-axis shows much less conductivity values (Fig. 8.4). 

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