Spinel Structured Oxide

Two spinel structure oxides in the ratio 0.25NiAl2O4 0.75MgFe2O4 are heated. The formula of the resulting solid solution is ... 

The weight gain (oxide thickness) peaked at a density of 20 kg/m (8 MPa) (Fig. 4.7) for four alloys exposed to SCW for 1000 h at 625°C at 8 and 29 MPa (static autoclave) and at 0.1 MPa (flowing steam in a tube furnace). 310 SS formed a Cr-rich oxide layer with the spinel stmcture, with an underlying recrystallized y layer depleted in Cr (Fig. 4.8 Table 4.2). The spinel-structure oxide had essentially the same lattice parameters at each pressure the inner oxides had roughly similar compositions (Table 4.2), while the outer oxide compositions showed a pressure (density) dependence. 

Negative temperature coefficient (NTC) resistors for example, spinel structure oxides, manganese(ll. 111) oxide - Mn304 Positive temperature coefficient (PTC) resistors for example, aluminum oxide AI2O3, magnesium oxide - MgO... 

Chromates III). Mixed oxides, e.g. FeCr204, having spinel structures and prepared by solid state reactions. 

Titanium IV) oxide, T1O2. See titanium dioxide. Dissolves in concentrated alkali hydroxides to give titanates. Mixed metal oxides, many of commercial importance, are formed by TiOj. CaTiOj is perovskite. BaTiOa, per-ovskite related structure, is piezoelectric and is used in transducers in ultrasonic apparatus and gramophone pickups and also as a polishing compound. Other mixed oxides have the il-menite structure (e.g. FeTiOj) and the spinel structure (e.g. MgjTiO ). 

Zirconates and hafnates can be prepared by firing appropriate mixtures of oxides, carbonates or nitrates. None of them are known to contain discrete [M04]" or [MOs] ions. Compounds M ZrOs usually have the perovskite structure whereas M2Zr04 frequently adopt the spinel structure. 

The lithium-titanium oxides are prepared by heating a mixture of anatase (Ti02) and LiOH at a high temperature. The product heated at 800-900 °C has a spinel structure of Li4/3Ti5y304. When the charge and discharge cycles are performed... 

We distinguish electrodes consisting of simple oxides, from those consisting of complex oxide systems. The latter include cations of different metals or cations of a given metal in different valence states. An example for the latter type is cobalt cobaltite C03O4 (a spinel structure) containing Co and Co ions. 

Complex Base-Metal Oxides Complex oxide systems include the mixed oxides of some metals which have perovskite or spinel structure. Both the perovskites and the spinels exhibit catalytic activity toward cathodic oxygen reduction, but important differences exist in the behavior of these systems. 

In the following, we start by assuming purely ionic structures. In spinel the oxide ions form a cubic closest-packing. Two-thirds of the metal ions occupy octahedral interstices, the rest tetrahedral ones. In a normal spinel the A ions are found in the tetrahedral interstices and the M ions in the octahedral interstices we express this by the subscripts T and O, for example Mgr[Al2](904. Since tetrahedral holes are smaller than octahedral holes, the A ions should be smaller than the M ions. Remarkably, this condition is not fulfilled in many spinels, and just as remarkable is the occurrence of inverse spinels which have half of the M ions occupying tetrahedral sites and the other half occupying octahedral sites while the A ions occupy the remaining octahedral sites. Table 17.3 summarizes these facts and also includes a classification according to the oxidation states of the metal ions. 

Nearly no eddy current losses occur in electrically insulating magnetic materials. This is one of the reasons for the importance of oxidic materials, especially of spinels and garnets. Another reason is the large variability of the magnetic properties that can be achieved with spinels and garnets of different compositions. The tolerance of the spinel structure to substitution at the metal atom sites and the interplay between normal and inverse spinels allow the adaptation of the properties to given requirements. 

It is known that some spinel-structured 3d-metal oxides are good catalysts for many processes involving electron transfer [12]. However, their low conductivity does not allow for the direct use in the electrode of the battery, and grafting them onto the carbon matrix is also very difficult technical problem. It was found recently that this problem could be solved indirectly, creating the spinel catalytic centers on the surface of carbon by means of adsorption of some 3d-metal complexes on the graphite surface followed by subsequent pyrolysis at certain temperatures [13,14],... 

Only two oxides of cobalt have been characterized, CoO and Co304 (which is actually ConConl04). The latter has a structure in which Co2+ ions are located in tetrahedral holes and Co3+ ions are located in octahedral holes of a spinel structure. Decomposition of either Co(OH)2 or CoC03 produces CoO, and decomposition of Co(N03)2 can be used to produce Co304. 

Acceptor doping, as in lithium oxide doping of nickel oxide, produces p-type thermistors. The situation in nickel-oxide-doped Mn304 is similar but slightly more complex. This oxide has a distorted spinel structure (Supplementary Material SI), with Mn2+ occupying tetrahedral sites and Mn3+ occupying octahedral sites in the crystal, to give a formula Mn2+[Mn3+]204, where the square parentheses enclose the ions in octahedral sites. The dopant Ni2+ ions preferentially occupy... 

Also, other metal oxide coatings are possible, for example, electrochemically deposited manganese dioxide. Moreover, further electrocatalytically active oxides are research objectives, for example, oxides with spinel structure such as CoMu204 [36]. 

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