Metal oxides electrical conductivity

In n-type metal oxides, electrical conductivity arises by means of quasi-free electrons that exist because of an excess of electrons present in the lattice. N-type metal oxides are generally not active oxidation catalysts, although vanadium pentoxide (V Oj) is a notable exception. P-type metal oxides are electron-deficient in the lattice and conduct electrons by means of positive holes. These oxides are generally active oxidation catalysts. Insulators have very low electrical conduaivities because of the strictly stoichiometric metal-oxygen ratio in the lattice and very low electron (or positive hole ) mobility and are generally not active catalysts. However, insulators are often used as catalyst supports. 

Va.na.dium (II) Oxide. Vanadium(II) oxide is a non stoichiometric material with a gray-black color, metallic luster, and metallic-type electrical conductivity. Metal—metal bonding increases as the oxygen content decreases, until an essentially metal phase containing dissolved oxygen is obtained (14). 

We shall briefly discuss the electrical properties of the metal oxides. Thermal conductivity, electrical conductivity, the Seebeck effect, and the Hall effect are some of the electron transport properties of solids that characterize the nature of the charge carriers. On the basis of electrical properties, the solid materials may be classified into metals, semiconductors, and insulators as shown in Figure 2.1. The range of electronic structures of oxides is very wide and hence they can be classified into two categories, nontransition metal oxides and transition metal oxides. In nontransition metal oxides, the cation valence orbitals are of s or p type, whereas the cation valence orbitals are of d type in transition metal oxides. A useful starting point in describing the structures of the metal oxides is the ionic model.5 Ionic crystals are formed between highly electropositive... 

The only new chemistry concerns electrochemical oxidation of the tetrathiafulvene derivative 41 to the radical cation perchlorate 42 (Equation 1) <2005MCL575>. The salt 42 was formed electrochemically as a dense thin film on the electrode surface and shown to be a conducting cation-radical salt that behaves like an organic metal. The electrical conductivity shows an interesting variation with temperature which may be related to a phase transition at 102K <2005MCL575>. 

Both elements form dioxides, M02, that have distorted rutile structures in which there are M M bonds. Re03 is an important oxide but Tc03 is not known to exist. The structure of red Re03, shown in Fig. 18-D-l, is also adopted by Cr03, W03, and others and is very similar to the perovskite structure. Re03 has metallic type electrical conductivity due to delocalization of the Re 1 electrons in a conduction band of the solid. 

Bismuth is a rather rare element in the earth s crust, but its oxides and sulfides appear at sufficient concentrations as impurities in lead and copper ores to make its recovery from these sources practical. Annual production of bismuth amounts to several million kilograms worldwide. Although elemental bismuth is a metal, its electrical conductivity is quite poor and it is relatively brittle. The major uses of bismuth arise from its low melting point (271.3°C) and the even lower melting points of its alloys, which range down to 47°C. These alloys are used as temperature sensors in fire detectors and automatic sprinkler systems because, in case of... 

In order to discuss the relation between the electrical conductivity of oxide and the grain size of metal, the electrical conductivity of pure metal should first be considered. A correction coefficient r, which relates the electrical conductivity of pure metal to that of the oxide, can then be introduced, as follows ... 

In many crystals there is sufficient overlap of atomic orbitals of adjacent atoms so that each group of a given quantum state can be treated as a crystal orbital or band. Such crystals will be electrically conducting if they have a partly filled band but if the bands are all either full or empty, the conductivity will be small. Metal oxides constitute an example of this type of crystal if exactly stoichiometric, all bands are either full or empty, and there is little electrical conductivity. If, however, some excess metal is present in an oxide, it will furnish electrons to an empty band formed of the 3s or 3p orbitals of the oxygen ions, thus giving electrical conductivity. An example is ZnO, which ordinarily has excess zinc in it. 

Further improvements in anode performance have been achieved through the inclusion of certain metal salts in the electrolyte, and more recently by dkect incorporation into the anode (92,96,97). Good anode performance has been shown to depend on the formation of carbon—fluorine intercalation compounds at the electrode surface (98). These intercalation compounds resist further oxidation by fluorine to form (CF ), have good electrical conductivity, and are wet by the electrolyte. The presence of certain metals enhance the formation of the intercalation compounds. Lithium, aluminum, or nickel fluoride appear to be the best salts for this purpose (92,98). 

Polymers. Ion implantation of polymers has resulted in substantial increases of electrical conductivity (140), surface hardness (141), and surface texturing (142). A four to five order of magnitude increase in the conductivity of polymers after implantation with 2 MeV Ar ions at dose levels ranging from 10 -10 ions/cm has been observed (140). The hardness of polycarbonate was increased to that of steel (141) when using 1 MeV Ar at dose levels between 10 -10 ions/cm. Conductivity, oxidation, and chemical resistance were also improved. Improvements in the adhesion of metallizations to Kapton and Teflon after implantation with argon has been noted (142). 

Niobium is used as a substrate for platinum in impressed-current cathodic protection anodes because of its high anodic breakdown potential (100 V in seawater), good mechanical properties, good electrical conductivity, and the formation of an adherent passive oxide film when it is anodized. Other uses for niobium metal are in vacuum tubes, high pressure sodium vapor lamps, and in the manufacture of catalysts. 

For many electronic and electrical appHcations, electrically conductive resias are required. Most polymeric resias exhibit high levels of electrical resistivity. Conductivity can be improved, however, by the judicious use of fillers eg, in epoxy, silver (in either flake or powdered form) is used as a filler. Sometimes other fillers such as copper are also used, but result in reduced efficiency. The popularity of silver is due to the absence of the oxide layer formation, which imparts electrical insulating characteristics. Consequently, metallic fibers such as aluminum are rarely considered for this appHcation. 

Heterogeneous Photocatalysis. Heterogeneous photocatalysis is a technology based on the irradiation of a semiconductor (SC) photocatalyst, for example, titanium dioxide [13463-67-7] Ti02, zinc oxide [1314-13-2] ZnO, or cadmium sulfide [1306-23-6] CdS. Semiconductor materials have electrical conductivity properties between those of metals and insulators, and have narrow energy gaps (band gap) between the filled valence band and the conduction band (see Electronic materials Semiconductors). 

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