Charge carriers, activation energy

Table 16.6 The values of specific electrical resistance /O300K and charge-carrier activation energy Ea at room temperature of 3-D-pol5merized specimens of Cgo, obtained at pressures of 11.5-12.5 GPa and temperatures of 820-1500 K.

The Oid(7) and dependences obtained for PATAC are fairly well fitted by equation 15 with hcOpi, = 0.18 eV and Eq. 14 with E = 0.061 eV, respectively (Fig. 6). The break in the 03d(7) curve can be attributed to a change in the conformation of the system. The energy necessary for the charge-carrier activation hopping between polymer chains is close to the energy of activation of the polymer chain librations (see below). This fact is evidence of the interference of charge transfer and macromolecular dynamics processes in the polymer. 

In molecular doped polymers the variance of the disorder potential that follows from a plot of In p versus T 2 is typically 0.1 eV, comprising contributions from the interaction of a charge carrier with induced as well as with permanent dipoles [64-66]. In molecules that suffer a major structural relaxation after removal or addition of an electron, the polaron contribution to the activation energy has to be taken into account in addition to the (temperature-dependent) disorder effect. In the weak-field limit it gives rise to an extra Boltzmann factor in the expression for p(T). More generally, Marcus-type rates may have to be invoked for the elementary jump process [67]. 

Electrical conductivity measurements revealed that ionic conductivity of Ag-starch nanocomposites increased as a function of temperature (Fig.l7) which is an indication of a thermally activated conduction mechanism [40]. This behavior is attributed to increase of charge carrier (Ag+ ions) energy with rise in temperature. It is also foimd to increase with increasing concentration of Ag ion precursor (inset of Fig.l7). This potentiality can lead to development of novel biosensors for biotechnological applications such as DNA detection. 

The corrosion of iron occurs particularly rapidly when an aqueous solution is present. This is because water that contains ions provides an oxidation pathway with an activation energy that is much lower than the activation energy for the direct reaction of iron with oxygen gas. As illustrated schematically in Figure 19-21. oxidation and reduction occur at different locations on the metal surface. In the absence of dissolved ions to act as charge carriers, a complete electrical circuit is missing, so the redox reaction is slow, hi contrast, when dissolved ions are present, such as in salt water and acidic water, corrosion can be quite rapid. 

A typical featnre of semicondnctor electrodes is the space charge present in a relatively thick surface layer (see Section 10.6), which canses a potential drop across this layer (i.e., the appearance of a snrface potential %). This potential drop affects the rate of an electrochemical charge-transfer reaction in exactly the same way as the potential drop across the diffnse EDL part (the / -potential) hrst, through a change in carrier concentration in the snrface layer, and second, throngh a change in the effect of potential on the reaction s activation energy. 

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