Passive films insulating

The passive film itself could not be treated as purely insulating since the known thickness of the film should give rise to a capacitive effect that would easily be seen in the impedance diagram. In fact, the film is apparently relatively conducting, probably through proton migration. 

The passive film is composed of metal oxides which can be semiconductors or insulators. Then, the electron levels in the passive film are characterized by the conduction and valence bands. Here, we need to examine whether the band model can apply to a thin passive oxide film whose thickness is in the range of nanometers. The passive film has a two-dimensional periodic lattice structure on... 

Even if LiPFe is replaced by more thermally stable salts, the thermal stability of passivation films on both the anode and the cathode would still keep the high-temperature limits lower than 90 °C, as do the thermal stability of the separator (<90 °C for polypropylene), the chemical stability of the insulating coatings/sealants used in the cell packaging, and the polymeric binder agents used in both cathode and anode composites. 

In the discussion of E the vs pH diagram for iron in water depicted in Figure 1.70, we noted that, with application of high positive potentials, the system moves into a region of passivity and results in a reduced corrosion rate. The passive film formed should be coherent and insulating to withstand corrosion and mechanical breakdown. Upon formation of the passive state the corrosion rate is reduced. Thus by polarization and applying more positive potentials than corrosion potentials the metal attains passivity and is protected. This is the principle of anodic protection. It is necessary that the potential of passivation be maintained at all times, since deviations outside the range would result in severe corrosion. 

Passive films formed in aqueous solutions consist of an oxide or a mixture of oxides, usually in hydrated form. The oxide formed on some metals (e.g., Al, Ti, Ta, Nb) is an electronic insulator, while on other metals the passivating oxide film behaves like a semiconductor. Nickel, chromium, and their alloys with iron (notably the various kinds of stainless steel) can be readily passivated and, in fact, tend to be spontaneously passivated upon contact with water or moist air. It should be noted that passivation does not occur when chloride ions are introduced into the solution indeed a preexisting passive film may be destroyed. Many other ions are detrimental to passivity, such as Br, I, SO, and CIO, but chloride is the worst offender, because of its... 

Silicon oxides (SiOx) are the most widely used thin films in silicon microelectronic and micromechanical devices. Similar to silicon nitride (Section 5.5.4), these amorphous films exhibit dielectric properties. Silicon oxide is often utilized as part of a dielectric membrane, as a passivation or insulating layer, or as a sacrificial layer, which can be etched with hydrofluoric acid (HF)-containing etchants. Two different approaches to forming a silicon oxide thin film are... 

Passive films are either insulators or semiconductors. On metals such as Fe, Ti, Sn, Nb, and W, the passive films are n-type semiconductors with relatively high donor concentrations at 1019-102° cm-3. Some metals such as Ni, Cr, and Cu form the passive film of p-type oxides. The passive films on metallic Al, Ta, and Hf are insulator oxides. 

It is worth noting that, as far as they are less than several nanometers thick, the passive films are subject to the quantum mechanical tunneling of electrons. Electron transfer at passive metal electrodes, hence, easily occurs no matter whether the passive film is an insulator or a semiconductor. By contrast, no ionic tunneling is expected to occur across the passive film even if it is extremely thin. The thin passive film is thus a barrier to the ionic transfer but not to the electronic transfer. Redox reactions involving only electron transfer are therefore allowed to occur at passive film-covered metal electrodes just like at metal electrodes with no surface film. It is also noticed, as mentioned earlier, that the interface between the passive film and the solution is equivalent to the interface between the solid metal oxide and the solution, and hence that the interfacial potential is independent of the electrode potential of the passive metal as long as the interface is in the state of band edge pinning. 

Fig. 8 Double logarithmic plot of ionic and electronic conductivity, /C on and k, of oxide films. Dotted line ionic conductivity equals the electronic one. Arrows indicate changes due to increasing field, approaching Ufb and increasing electronic equilibrium. Passive Ti as example 1 stable passive film of Ti02 near the flat band potential 2 insulating film of Ti02 on anisotropic metal plane (xxxO) 3 as 2, but for the isotropic, close packed metal plane (0001).

Insulating Passive Films on Metals Growth, and Corrosion of Al as Example... 

The passive films on valve metals are almost perfect insulators. Those dielectric films are used in condensers [146], in tunnel junctions [147] and for insulation of semiconductors [148]. The basic requirement is the ability to withstand an electric field of 0.1-1 GVm without electric breakdown. The breakdown of passive films means a spontaneous local... 

Metals with insulating passive films (valve metals)... 

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