Electronic conduction polarisation

Fig. 2-37. Typical shapes of polarisation curves of electron-conducting minerals 1- current cp-potential (reproduced with permission from Putikov, 1993).

The essence of the CPC method consists of recording and interpreting polarisation curves obtained when the polarisable electrode is an electron-conducting ore body (Fig. 2-39). One pole of the current source, electrode A, is connected to the ore body by means of a special device (e.g., in a borehole intersection through the ore body). The... 

Both current electrodes, A and B, are placed in the host rocks on ground surface (Ryss, 1973, 1983). Current, as a linear function of time, is introduced into the ground by means of these electrodes (Fig. 2-45). If there is an electron-conducting ore body at depth this current flows into the ore body at one end (the zone of cathodic polarisation) and flows out of the ore body at the other end (the zone of anodic polarisation). This results in a potential difference with different signs and values in different parts of the ore body. This double electrical layer creates a secondary electrical field in the host rocks that can be measured by means of the measuring electrodes, M and N. For this measurement electrode M is moved along a profile to successive positions M), M2,. .M whilst electrode N is placed at infinity (Fig. 2-45). For each position of electrode M the CLPC polarisation curve is recorded. This curve is dependent on current I in the... 

Since K depends on the wavefunction density at the nucleus, the effect is dominated by s-electrons which is certainly true in metals with unpaired s-electrons. If the Pauli susceptibility and electron density can be independently measured then the Knight shift will give an independent measure of the s-component of the conduction electron spin density. These shifts are positive and are much larger than chemical shift effects, some typical values being Li — 0.025%, Ag — 0.52% and Hg — 2.5%. In other metals the situation is more complicated when the s-electrons are paired but there are other electrons (e.g. p but especially d). As only s-electrons have significant density at the nucleus the effects of these other electrons are much smaller. The hyperfine fields of these electrons induce polarisation in the s-electrons that subsequently produce a shift, termed core polarisation. 

Early theories of the state of the solvated electron suggested that it was located in a cavity in the liquid where it was trapped by its polarisation of the surrounding medium. In the latest theory, the electron cavity is characterised by a loosely packed first coordination layer containing an appreciable amount of empty space. The high electron mobility in an electric field cannot be reconciled with hydrodynamic motion of the whole cavity, and instead it is proposed that the loosely packed structure allows the electron to jump or leak away, by quantum-mechanical tunnelling,No numerical estimates of electron conductances have yet been made on this model. 

These equations provide the basis for the mathematical analysis of the polarisation curves. Such analysis was carried out to gain some important information on the properties and the behaviour of sulphide melts ionic and electronic conductivities the efficiency of electrolytic decomposition and its dependence on electrolysis conditions the values of the stationary voltage and dissolution rate of electrolysis products after the current cut-off. 

The effect of the electric field upon a polymer could be to cause ionic or electronic conductance, dielectric loss or breakdown. Common polymers are good insulators, some values of volume resistivity appear in Table 2.3, but problems can arise with hydrophilic polymers such as the polyamides. Nylon 66 has a specific resistance of about 10 ft cm when dry, but on equilibration with saturated air at room temperature this substance absorbs about 8% of water and its specific resistance falls to about lO ft cm. When a polymer is placed in an electric field, the effect is to displace the centres of gravity of electronic and nuclear charges so that the material becomes dielectrically polarised. Further... 

The high conductivity of cerium-lanthanum mixed oxides and the favourable polarisability of electrodes on such solid electrolytes was already stimulating application ideas in the 1960s. But electronic conductivity of these electrolytes above 600°C was seen as a weighty problem [71]. The influence of electronic conductivity on the cell performance was investigated first by means of an equivalent circuit [40,100]. The results, shown in Figure 2.8, led to the conclusion that the ion transport number has to be greater than 0.9 if a solid electrolyte was to be successful in a SOFC [100]. 

Hole and electron conduction (minor carrier) and transport number of oxide ions in LaGaO 3-based oxides have been reported by Baker etal. [7 5], Yamaji et al. [76] and Kim eta/. [77] based on the polarisation method. Figure 4.20 shows the... 

It is possible to discriminate between these two contributions by using techniques like the Wagner polarisation cell, which is discussed in Section 1.4.4. The disadvantage of such methods is that the test electrodes replace those that would be used in a practical device application and so the behaviour of the electrode-electrolyte interfaces is significantly different. In consequence, the predicted electronic conductivity may be some orders of magnitude higher or lower than that found when the electrolyte is incorporated in a working device. 

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