Conductivity in metal oxides

Semiconductors for practical applications are often doped, mainly with the aim to improve the conductivity. In metal oxide photoelectrodes, shallow donors and acceptors are almost always necessary because of the low intrinsic charge carrier mobilities. The conductivity of the material is given by a = nefif, + so... 

A consequence of the trapping of electronic defects is that materials become insulating at low temperatures. Defect association can also occur between oxygen vacancies and acceptor dopants, as it was observed in Y-doped zirconia, where Vq are trapped by defects at low temperature, which led to a drastic decrease in oxide ion conductivity [6-8]. Similarly, protonic defects are likely to form associations with acceptor dopants, which hence would limit protonic conduction in metal oxides [9]. 

We now proceed by describing the diffusion of defects, and hence their contribution to conductivity in metal oxides. Pick s first law can be used to relate a diffusive flux to a concentration gradient. 

The electronic conductivity of metal oxides varies from values typical for insulators up to those for semiconductors and metals. Simple classification of solid electronic conductors is possible in terms of the band model, i.e. according to the relative positions of the Fermi level and the conduction/valence bands (see Section 2.4.1). 

A determined search for superconductivity in metallic oxides was initiated in mid-summer of 1983 at the IBM, Zurich Research Laboratories in Riischliken, Switzerland. This research effort was an extension of previous work (145) on oxides, namely, Sr1.xCaxTiOs, which exhibited some unusual structural and ferro-electric transitions (see Section 2.2a). During the summer of 1983, the superconductivity research was focussed on copper-oxide compounds. Muller had projected the need for mixed Cu2+/Cu3+ valence states, Jahn-Teller interactions (associated with Cu2+ ions), and the presence of room temperature metallic conductivity to generate good superconductor candidates. These researchers then became aware of the publication by Michel, Er-Rakho, and Raveau (146) entitled ... 

One of the most widely used materials for the fabrication of modern VLSI circuits is polycrystalline silicon, commonly referred to as polysilicon. It is used for the gate electrode in metal oxide semiconductor (MOS) devices, for the fabrication of high value resistors, for diffusion sources to form shallow junctions, for conduction lines, and for ensuring ohmic contact between crystalline silicon substrates and overlying metallization structures. 

None of the A2B04 oxides seems to exhibit a true metallic behavior down to the lowest temperatures. Most of these oxides show activated conduction and even those phases that have been considered to be metallic (e.g., La2Cu04) exhibit conductivities of the order of 10 ohm-1 cm-1. This is much less than the conductivities found in metallic oxides of perovskite structure (e.g., La NiC>3 or LaSrCo206 with cr of 103 ohm-1 cm-1). It is not clear whether the absence of true metallic conductivity in A2B04 oxides has something to do with localization in two dimensions 85, 86). 

Catalysts are not absolutely essential in paraffin oxidations but their use can have significant advantages such as shifting of the relative magnitude of the various steps of uncatalyzed reactions. Perhaps it should be noted in passing that commercial oxidations conducted in metal equipment always have some adventitious corrosion ions present, so the term an uncatalyzed reaction implies only that no catalyst was deliberately added. 

A new family of high conductivity, mixed metal oxides having the pyrochlore crystal structure has been discovered. These compounds display a variable cation stoichiometry, as given by Equation 1. The ability to synthesize these materials is highly dependent upon the low temperature, alkaline solution preparative technique that has been described the relatively low thermal stability of those phases where an appreciable fraction of the B-sites are occupied by post transition element cations precludes their synthesis in pure form by conventional solid state reaction techniques. 

The recent interest in nitryl hexafluoroarsenate, [N02][AsFJ, as an oxidizing agent has emphasized the need for a simple, one-step, high-yield synthesis of fliis compound. Previous syntheses have involved the initial preparation of NOjF and subsequent reaction with AsF, the use of HF with HNO3, ClNOj, or nitrate esters the reaction of NOj, BrFj, and S2Of the use of PNOj " or metathetical reactions from other [AsF ] salts. These reactions generally are conducted in metal cylinders or quartz vessels. The method reported here involves the direct reaction of NO2, Fj, and AsFj in a Pyrex vessel and provides a pure product. 

Figure 6.8 XRD patterns of PAPSA/V2O5 and V2OS. (Reprinted with permission from Synthetic Metals, Assembly of conducting polymer/metal oxide multlayer in one step by C.-O. Wu, Y.-C. LiuandS.-S. Hsu, 102, 1-3. Copyright (1999) Elsevier Ltd)...

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