Mixed metal oxides electrical conductivity

The vast majority of metal complexes in the solid state are insulators and do not exhibit any interesting electrical conduction properties because the metal atoms are surrounded by insulating ligands which prevent the passage of carriers from one site to another. This review will be limited to a discussion of the electrical conduction properties of coordination compounds, and will not include simple inorganic compounds with high electrical conductivity such as mixed metal oxides, (3-alumina and TaSe3. 

A superconductor is a material which conducts electricity without resistance and the exclusion of the interior magnetic field (Meissner effect) below a certain critical temperature Tc- Superconductivity occurs in a wide variety of materials, including elements, various metallic alloys and some heavily-doped semiconductors. Mixed metal oxides belong to the class of high-temperature superconductors (Tq > 30 K). 

But conducting plastics were not the only oddball substances that had started to pique the curiosity of researchers hunting for better ways to transport electrical current, not to mention trying to surpass the 23° K Tc that seemed as impassable as the sound barrier had been to fast flight. There was another group of materials still to be looked at, the mixed metallic oxides substances with the mechanical and physical properties of ceramics. They weren t exactly the sort of thing one might expect to find at the center of attention in a modern physics laboratory. 

The perovskite-type catalysts (ref.l), other non noble metal complex oxides catalysts (ref.2), and mixed metal oxides catalysts (ref.3) have been studied in our laboratory. The various preparation techniques of catalysts (ref.4 and 5), the adsorption and thermal desorption of CO, C2H5 and O2 (ref.6 and 7), the reactivity of lattice oxygen (ref.8), the electric conductance of catalysts (ref.9), the pattern of poisoning by SO2 (ref. 10 and 11), the improvement of crushing strength of support (ref. 12) and determination of the activated surface of complex metal oxides (ref. 13) have also been reported. 

Mioced-metal anodes also utilize titanium, niobium, and tantalum as substrate materials. A film of oxides is formed on these substrates, with protective properties similar to the passive film forming on the substrate materials. The important difference is that whereas the natural passive film is an effective electrical insulator, the mixed metal oxide surface film passes anodic current. The product forms are similar to those of the platinized anodes. These anodes are typically used with carbonaceous backfill. Electrode consumption is usually not the critical factor in determining anode life rather the formation of nonconductive oxides between the substrate and the conductive surface film limits effective functioning. Excessive current densities accelerate the buildup of these insulating oxides to unacceptable levels. 

Aluminum metal is produced from aluminum oxide by electrolysis using the Hall-Heroult process, whose story is detailed in our Chemical Milestones Box. The melting point of AI2 O3 is too high (2015 °C) and its electrical conductivity too low to make direct electrolysis commercially viable. Instead, AI2 O3 is mixed with cryolite (Na3 AlFfi) containing about 10% CaF2. This mixture has a melting point of 1000 °C, still a high temperature but not prohibitively so. Aluminum forms several complex ions with fluoride and oxide, so the molten mixture... 

Many types of oxide layers have a certain, not very high electrical conductivity of up to 10 to 10 S/cm. Conduction may be cationic (by ions) or anionic (by or OH ions), or of the mixed ionic and electronic type. Often, charge transport occurs by a semiconductor hole-type mechanism, hence, oxides with ionic and ionic-hole conduction are distinguished (in the same sense as p-type and n-type conduction in the case of semiconductors, but here with anions or cations instead of the electrons, and the corresponding ionic vacancies instead of the electron holes). Electronic conduction is found for the oxide layers on iron group metals and on chromium. 

The above mechanistic aspect of electron transport in electroactive polymer films has been an active and chemically rich research topic (13-18) in polymer coated electrodes. We have called (19) the process "redox conduction", since it is a non-ohmic form of electrical conductivity that is intrinsically different from that in metals or semiconductors. Some of the special characteristics of redox conductivity are non-linear current-voltage relations and a narrow band of conductivity centered around electrode potentials that yield the necessary mixture of oxidized and reduced states of the redox sites in the polymer (mixed valent form). Electron hopping in redox conductivity is obviously also peculiar to polymers whose sites comprise spatially localized electronic states. 

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 ... 

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