Electron particle current

Any electrons which reach the oxide—oxygen interface will be quickly utilized in the formation of 0" ions, and similarly, any positive holes which manage to reach the metal—oxide interface will be quickly annihilated by electrons from the parent metal. The asymmetrical surface reactions thus lead us to expect widely different electron concentrations in the oxide at the two interfaces of the oxide. This difference in electron concentration is equivalent to a difference in the chemical potential for the electronic species at the two interfaces. As in the case of the ionic defect species, such differences in concentration (and chemical potential) can be expected to produce particle currents of the defect species in question. If the primary electronic defects are excess electrons, then we can expect an electron particle current from metal to oxygen if the primary electron defects are the positive holes, then we can expect a positive-hole particle current from oxygen to metal. Of course, an intermediate situation is also possible in which electrons flow from the metal towards the oxygen while simultaneously a positive-hole current flows from the oxygen towards the metal, with recombination [8] (partial or total) occurring within the oxide film. 

The anion vacancy currents are illustrated in Fig. 22. The coordinate system used for this case is again the one illustrated in Fig. 18. The anion vacancy current and corresponding electron particle current through each oxide layer are positive in sign and increase in the order i — 1, i, i+ 1,, ... 

Fig. 42 —AES surface survey of elements in the disk substrate surface after polishing. The slurry contains 6 wt % Si02 particles with a diameter of 30 nm, 1 wt % oxidizer and 2 wt % lubricant in Dl water, and pH value of the slurry is 1.8. (a) Elements in the disk surface, (b) deep distribution of the elements. (The contents of elements and their deep distribution in the polished surface were analyzed by using a PHI 680 auger nanoprobe under determining conditions as follows ion beam current of 1 u,A, ion beam voltage of 2 kV, electron beam current of 10 nA, electron beam voltage of 10 kV and scan area of 20 fj.m by 20...

Analysis of individual catalyst particles less than IMm in size requires an analytical tool that focuses electrons to a small probe on the specimen. Analytical electron microscopy is usually performed with either a dedicated scanning transmission electron microscope (STEM) or a conventional transmission electron microscope (TEM) with a STEM attachment. These instruments produce 1 to 50nm diameter electron probes that can be scanned across a thin specimen to form an image or stopped on an image feature to perform an analysis. In most cases, an electron beam current of about 1 nanoampere is required to produce an analytical signal in a reasonable time. 

In general, a 1 MeV beta particle will eject approximately 50 electrons per centimeter of travel, while a 0.05 MeV beta particle will eject approximately 300 electrons. The lower energy beta ejects more electrons because it has more collisions. Each electron produced by the beta particle, while traveling through air, will produce thousands of electrons. A current of 1 micro-ampere consists of about 1012 electrons per second. 

It is characteristic of metal electrodes that the reaction current of redox electron transfer, under the anodic and cathodic polarization conditions, occurs mostly at the Fermi level of metal electrodes rather than at the Fermi level of redox particles. In contrast to metal electrodes, as is discussed in Sec. 8.2, semiconductor electrodes exhibit no electron transfer current at the Fermi level of the electrodes. 

Fig. 8-8. Diffusion of redox particles before and after electron transfer at metal electrode i (i ) = cathodic (anodic) electron transfer current, t fr (>dfr) = cathodic (anodic) difiusi

Figure 8-11 shows as a function of electron energy e the electron state density Dgdit) in semiconductor electrodes, and the electron state density Z e) in metal electrodes. Both Dsd.t) and AKe) are in the state of electron transfer equilibrium with the state density Z>bei)ox(c) of hydrated redox particles the Fermi level is equilibrated between the redox particles and the electrode. For metal electrodes the electron state density Ai(e) is high at the Fermi level, and most of the electron transfer current occurs at the Fermi level enio. For semiconductor electrodes the Fermi level enao is located in the band gap where no electron level is available for the electron transfer (I>sc(ef(so) = 0) and, hence, no electron transfer current can occur at the Fermi level erso. Electron transfer is allowed to occur only within the conduction and valence bands where the state density of electrons is high (Dsc(e) > 0). 

Fig. 8-13. Electron transfer via the conduction band and hole transfer via the valence band >ox (Dveo) = state density of oxidant (reductant) particles x - distance from an interface i ( ) = anodic (cathodic) current in (ip) = electron (hole) current.

Fig. 8-41. Electron transfer reaction of hydrated redox particles in equilibrium on a metal electrode covered with a thick film (F, solid curve) and with a thin film (F, dashed curve) >cs = electron transfer current via the conduction band >scl = tunneling electron current through a depletion layer in the conduction band >vb = hole transfer current via the valence band.

Figure S-4S shows the polarization curves observed, as a function of the film thickness, for the anodic and cathodic transfer reactions of redox electrons of hydrated ferric/ferrous cyano-complex particles on metallic tin electrodes that are covered with an anodic tin oxide film of various thicknesses. The anodic oxide film of Sn02 is an n-type semiconductor with a band gap of 3.7 eV this film usually contains a donor concentration of 1x10" ° to lxl0 °cm °. For the film thicknesses less than 2.5 nm, the redox electron transfer occurs directly between the redox particles and the electrode metal the Tafel constant, a, is close to 0.5 both in the anodic and in the cathodic curves, indicating that the film-covered tin electrode behaves as a metallic tin electrode with the electron transfer current decreasing with increasing film thickness.

A curreni of electricity In an electronic conductor is due to a stream of electrons, particles of. subatomic size, and ihe current causes no net transfer of niatrer. The flow is. therefore, in a direction contrary to what is conventionally known as Ihe direction of the current. In electrolytic conductors, flic carriers are charged particles of atomic or molecular size called ions, and under a potential gradient, a transfer of matter occurs. 

Except for the very early stages of surface layers, it is justified during practically the entire observable course of metal oxidation to assume that the concentration n. of ionic and electronic defects is small compared to the concentrations N, Ng or Njyj of the lattice constituents A and B or the lattice molecules. Consequently the particle currents will not contribute appreciably to a time-variation of defect concentrations, but almost exclusively to the layer formation. For defects we may thus write... 

If the velocity distribution F(5, x, t) is known, important macroscopic properties of the electrons can be calculated by appropriate velocity space averaging over the distribution. To give some examples, the density n(x, t), the density of the mean energy u (x, t) and the vectorial particle current density j x, t) of the electrons are given by the averages (Desloge, 1966)... 

As can be seen from the power and momentum balance, Eqs. (46) and (47), a temporal evolution of the mean energy density u, t) and/or of the particle current density j t) is initiated if the instantaneous compensation of the respective gain from the field and the corresponding total loss in collisions is disturbed for any reason. Generally, by collisional dissipation, the electron component tries to reduce these disturbances and to again establish the compensated state in both... 

By using the same approach and an adequate choice of the boundary condition, the spatial evolution of the electron kinetic quantities can be analyzed in all those space-dependent electric fields that do not reverse their direction with growing z. In such studies, nonconservative inelastic electron collisions can also be included and will cause, in accordance with the consistent particle balance, Eq. (56), a spatial evolution of the particle current density y (z) also. 

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