Electrode point

The early pioneers also include Benjamin Franklin and Charles de Coulomb. Franklin studied the effect of point electrodes in drawing electric currents. Coulomb discovered that a charged object gradually loses its charge i.e., he actually discovered the electrical conductivity of air. Coulomb s importance for the development of electrostatic air-cleaning methods is great, mainly because the present theories about electric charges and electric fields are based on his work. 

The first experiment with the electrostatic gas cleaning was made in 1824, when Hohlfeld show that a fog was cleared from a glass jar which contained an electrically charged point electrode. Similar demonstrations were published in the 19th century, an example being the precipitation of tobacco smoke in a glass cylinder by Guitard (1850). 

In practice, it is found desirable to have a small pip in the centre of the graphite electrode and to place the powder in a depression around it. The arc tends to pass between the upper pointed electrode and the small projection. 

Kleinfeld, M. Wiemhofer, H.-D. 1988. Chemical diffusion coefficients and stabihty of CuInS2 and CuInSe2 from polarization measurements with point electrodes. Solid State Ionics. 28-30 1111-1115. 

In their work74 on the ECT of melanoma tumors in hamsters, Schauble et al. (U.S.A.) implanted a 1.5 V battery as the power source. A stainless steel point electrode was placed in the tumor, either as anode or as cathode and a counter electrode made of plane wire mesh was applied to the skin of the chest. Three different current levels were used 3 mA 0.5 mA 1 pA. The two higher currents produced necrosis of the tumor at the dose leve of lh/day for 4 consecutive days. A more pronounced effect was observed with anode in the tumor and cathode outside it. 

Fig. 9.1 Transient behavior of a YSZ-Pt point electrode undergoing anodic (0.1 V versus air) and cathodic (—0.15 V versus air) step changes in voltage (Bay and Jacobsen, 1997).

Note that the configurations in Figure 20 involve point electrodes. To initiate a pulsed discharge in water, it is necessary to have a high-intensity electric field (107-109 V m ) at the tip of the electrode. Proper insulation of the electrode is essential, because water is much more conductive than air. A small protrusion of the point electrode from the insulator surface... 

Figure 20 Schematic of two configurations for a pulsed corona reactor (a) ground electrode submerged in liquid and (b) ground electrode suspended above the liquid surface. The immersed electrode in both cases is a point electrode (reprinted with permission from Grymonpre et al, 2004 Copyright 2004 American Chemical Society). For a variety of other reactor configurations, see Locke et al. (2006).

A final topic to be discussed in this section is the direct injection of electrons into a liquid by the use of nanowires. The difference with the discharge processes described above for the point electrodes is twofold the use of nanostructured electrodes and the generation of solvated electrons, instead of the initiation of a discharge. Solvated electrons have a very short... 

Figure 30. Ionic space charge effects at grain boundaries in Frenkel disordered materials, (a) Theoretical profiles if u, > u .120 (b) The enhanced grain boundary conductivity can be verified by point electrode impedance spectroscopy.121 The number given are in units of nS / cm and refer to room temperature.

Unlike the case of volume resistivity, the resistance never becomes independent of electrode separation. Again, measurement with finely pointed electrodes will be over-sensitive to the precision of contact. 

In order to predict absolute dielectric strengths we need to have more detailed information than is yet available about electronic states and mobilities in polymers. For the present we can only conclude that there is satisfactory agreement between the form of the theoretical results, based on a rather general electronic model, and the best experimental results. To the extent that the model is a very reasonable one, we can say that we can understand intrinsic breakdown behaviour. Measurement of pre-breakdown currents, especially with pointed electrodes which impose regions of very high field strength at their tips when embedded in the material, suggests that electronic carrier production either by injection from the electrodes (Schottky emission) or from impurities (Poole-Frenkel effect) may play a part in the breakdown process. More work is required, however, before this can be fully understood. 

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