Electrical conductivity loss

On the other hand, p-type doped polymers may be expected to be stable but prone to the degradative effect of counter-ions. Druy el al. [29] demonstrated the counter-ion dependence of initial electrical conductivity loss in polyacetylene where the electrical conductivities of the samples doped with I2 and CIO4 were maximally affected, due to the strong oxidizing ability of these dopants. Somewhat complementary results have also been reported by various other workers [30-32]. 

In a different work, Hahn ei al. [102] reported that the thickness does play an important role in electrical conductivity loss in BF4 doped polypyrrole on storage. A decrease of three orders of magnitude was observed in thin films (<1 micron) over a period of three months, whereas thicker film (3-5 micron) showed one order of magnitude decrease in two month s storage in ambient atmosphere. However, thin films reached a stable electrical conductivity of approximately 2 x 10 ohm cm after about three months, similar to what has been reported for re-doped materials [41]. 

Table 16.14. Apparent rate constants of electrical conductivity loss for polythiophene containing various counter-ions under extremely dry conditions in the temperature range of iOF-lO C. Adapted from Bull Electrochem. 10(11/12), 508 (1994), with permission of C.E.C.R.I. (India).

Table IV. Apparent Rate Constants of Electrical Conductivity Loss for Polythiophene that Contains Various Counterions...

In fact, the electrical properties have many time and temperature dependent characteristics in common with the mechanical properties. The significant measure of the charging and polarisation (dielectric) behaviour of a polymer insulator is its permittivity. This can be thought of as a parallel to mechanical compliance where the stress is replaced by the electric field or voltage, and the strain by the charge movement or polarisation. Then the equivalent to mechanical loss is electrical conductance loss or dissipation. 

In general, the electrical contact resistance between the current collector and the diffusion media (PEFC and AFC) or catalyst layer is the largest electrical conductivity loss. In PEFCs, a common through-plane resistivity is around 0.07 cm, with a contact resistance of 0.002 cm that depends on compression pressure from the lands and Teflon content in the... 

Whilst the conductivity of these polymers is generally somewhat inferior to that of metals (for example, the electrical conductivity of polyacetylenes has reached more than 400 000 S/cm compared to values for copper of about 600 000 S/cm), when comparisons are made on the basis of equal mass the situation may be reversed. Unfortunately, most of the polymers also display other disadvantages such as improcessability, poor mechanical strength, poor stability under exposure to common environmental conditions, particularly at elevated temperatures, poor storage stability leading to a loss in conductivity and poor stability in the presence of electrolytes. In spite of the involvement of a number of important companies (e.g. Allied, BASF, IBM and Rohm and Haas) commercial development has been slow however, some uses have begun to emerge. It is therefore instructive to review briefly the potential for these materials. 

The modern procedure to minimise corrosion losses on underground structures is to use protective coatings between the metal and soil and to apply cathodic protection to the metal structure (see Chapter 11). In this situation, soils influence the operation in a somewhat different manner than is the case with unprotected bare metal. A soil with moderately high salts content (low resistivity) is desirable for the location of the anodes. If the impressed potential is from a sacrificial metal, the effective potential and current available will depend upon soil properties such as pH, soluble salts and moisture present. When rectifiers are used as the source of the cathodic potential, soils of low electrical resistance are desirable for the location of the anode beds. A protective coating free from holidays and of uniformly high insulation value causes the electrical conducting properties of the soil to become of less significance in relation to corrosion rates (Section 15.8). 

D. G. Gadian, F. N. H. Robinson 1979, (Radiofrequency losses in NMR experiments on electrically conducting samples),/. Magn. Reson. 34, 449. 

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