Electronic properties, passive layers

Valve metals — Metals that form a compact, electronic insulating passive layer when anodized in aqueous electrolyte, exhibiting asymmetric conductivity blocking anodic reactions, except at very high voltages. Valve metals include aluminum, - titanium, tantalum, zirconium, hafnium, and niobium. Some other metals, such as tin, may exhibit valve-metal properties under specific conditions. 

It is now well established that in lithium batteries (including lithium-ion batteries) containing either liquid or polymer electrolytes, the anode is always covered by a passivating layer called the SEI. However, the chemical and electrochemical formation reactions and properties of this layer are as yet not well understood. In this section we discuss the electrode surface and SEI characterizations, film formation reactions (chemical and electrochemical), and other phenomena taking place at the lithium or lithium-alloy anode, and at the Li. C6 anode/electrolyte interface in both liquid and polymer-electrolyte batteries. We focus on the lithium anode but the theoretical considerations are common to all alkali-metal anodes. We address also the initial electrochemical formation steps of the SEI, the role of the solvated-electron rate constant in the selection of SEI-building materials (precursors), and the correlation between SEI properties and battery quality and performance. 

Passive layers of various metals have semiconducting properties others have in-solating properties. As usual, this is a consequence of the band gap. The anodic oxides of metals like Fe, Cr, Ni, Co and Cu show semiconducting properties, whereas the valve metals like Al, Ta, Zr, Hf and Ti form electronically insolating... 

The electrochemical properties of passive layers lead to the question of their structure on a mesoscopic scale and at atomic resolution. Their barrier character with respect to metal corrosion postulates a dense, poreless film their electronic properties, in some cases, crystalline structures. The change of their properties with film aging, as in e.g. film-breakdown phenomena, support the existence of many defects that may heal with time. In many cases an amorphous structure is assumed. Some ex situ... 

In the last chapter, Strehblow provides a review of experimental methodology and theoretical concepts of passivation and passivity of metals. The topics of emphasis include growth and composition of passive layers, their structure and electronic properties, and their breakdown. Current accomplishments are discussed in detail for a selected number of key metal and alloy systems. Summarized in some detail are the most important analytical methods for elucidation of chemical composition, electronic properties and structure of passive layers. It is shown for many systems that the application of multiple combinations of electrochemical and spectroscopic methods provide many insights and confidence in the interpretation of the passive behavior of metals. 

The purpose of encapsulation is to protect electronic IC devices and prolong their reliability. Moisture, mobile ions, (eg., sodium, potassium, chloride, fluorides), UV-VIS and alpha particle radiation, and hostile environmental conditions are some of the possible causes of degradation or interaction which could negatively affect device performance or lifetime. Silicon dioxide, silicon nitride and silicon-oxy-nitride, commonly used as passivation layers have excellent moisture and mobile ion barrier properties and are, therefore, excellent encapsulants for devices. As for the... 

Two examples of intercalation reactions which can be useful in photoelectrochemistry are depicted in Fig. 23. The first one (a) concerns the already discussed passivation and neutralization of step sites, which act as traps and recombination centers for charge carriers (A in Fig. 23 a). As already shown by Parkinson and others27 partially intercalated molecules can favourably alter the electronical properties of terminating layers, thus improving the efficiency of the electrode material. The second example (b) vis-... 

Adherents to this theory have different opinions on the potential at which the film forms. Its thickness, the mechanism of formation, and, most Important, the "cause of passivity. In the earlier theories It was postulated that the passivation follows the formation of a "primary layer" of small conductivity, x lth porous character, which Is sometimes due to precipitation of metal salt on and near the electrode.(32) In the pores the current Increases, and by polarization at an "Umschlagspotentlal" (Vj, = V, Figure 1) an actual passive layer is formed. Thus the essential concept of the passivation process Is connected with the change of the properties (chemical or physical) of the primary film at a certain potential. The passive film Is free from pores and presents a barrier between the metal and the environment. It is electronically conductive and slowly corrodes In solution.(6,8,24,37)... 

---