Spinel thermal properties

Magneisum ferrite spinel. MgFe204 is one a representative of soft ferrites extensively used for high-frequency applications. Fully dense bulk ferrites have been synthesized by low-temperature sintering of fine powders, by thermal decomposition and by co-precipitation. Preparation from ultra-fine powder is more advantageous since the composition can be more easily controlled, and the electrical and thermal properties are improved as a result of the reduced grain size. 

In previous papers [18,19,39], a model was proposed for Ni/Al mixed oxides obtained from HT precursors, involving the formation of NiO (containing a very small amount of Ap ions) and Ni-doped alumina phases, which strongly interact with a spinel-type phase present at their interface. The spinel-type phase, probably a non-stoichiometric-type, is responsible for the thermal properties of the calcined precursors, hindering the growth and sintering of the NiO crystallites. Likewise, the reduction of the NiO phase is hindered by the presence of the... 

Based on their structure, cathodes can be separated into three basic families layered oxides, spinels, and olivine phosphates. The three families differ greatly in their cell performance as well as in thermal properties. Layered oxide cathodes typically have higher specific capacities than spinels or phosphates. As a consequence of their structure, layered oxide materials are more energetic upon thermal runaway events due to the ease with which oxygen is liberated from their framework upon decomposition. 

The only stable oxide lattices that have so far been of value as mixed metal oxide pigments are those with spinel, rutile, and hematite structures. These lattices possess not only good thermal and chemical stability, but also have a high refractive index which is important for good optical pigment properties. 

M(lI)AlSn-LDHs with M(II) being Mg, Ni or Co were synthesized by a coprecipitation method. The influence of Sn on the thermal transformations and redox properties were investigated in detail using XRD, TG/DTA, SEM, TPR, 1 l9Sn-MAS NMR and UV-visible diffuse-reflectance (DR) spectroscopy methods. Some of these samples calcined at 450 °C were tested as catalysts in the partial oxidation of methanol (POM) reaction. In this paper we discuss briefly the effect of Sn-incorporation on the structural features and reducibility of CoAI-LDH. The catalytic performance of Co-spinel microcrystallites derived from CoAl-, and CoAlSn-LDHs was also evaluated. 

Spinel oxides are important industrial catalysts, possessing good thermal and chemical stability and an ability to maintain catalytic activity at high temperatures. The study reported here is concerned with the properties ofZn(II)-Cr(III) spinel oxides prepared from a carbonate containing LDH precursor. For this study the spinel oxide was separated, by treatment with dilute mineral acid, from the oxide mixture (ZnO and ZnC O, ) obtained by the thermal decomposition of LDH. For comparison, Zn(II)-Cr(lII) spinel oxides were also prepared by more standard synthesis methods. Structural and textural properties were evaluated, using PXRD, TG/MS, FT-IR and N2 adsorption. 

Although the thermal decomposition of Zn-Cr-C03-LDHs has, therefore, been studied in detail, a specific characterisation of the actual spinel oxides obtained from the LDHs, separated from the mixed oxide phase, has not been reported either for Zn-Cr-LDHs or for other cation combinations. Although the structure of the spinel oxides formed from LDHs has been reported occasionally, the effect of separating the spinel phases from the whole thermal decomposition product in the properties of the spinel oxide has not been evaluated. Here we report on the properties of the spinel oxides produced by the thermal decomposition of a Zn-Cr-C03-LDH. For comparison, spinel oxides phases were synthesised by the two other methods described above, and treated in a similar way to those obtained by LDH decomposition. 

Spinels provide another possibility for oxide cathodes [8]. Patents have been filed covering the thermally prepared spinels of Ni, Co, Fe and Mn [482]. However, very few details are available. Pure Co304 is easily reduced cathodically with dramatic changes of its properties [483]. Additives are necessary to slow down the cathodic reduction [8]. Studies in the field of catalysis can guide in the search for additives. Among others, La, Zr and A1 are expected to be the most efficient ones [484, 485]. No data are presently available to test this prediction. 

Mechanical properties and thermal shock behaviour of magnesia-spinel composite refractory materials... 

Aksel, C., Thermal Shock Behaviour and Mechanical Properties of Magnesia-Spinel Composites , PhD Thesis, Department of Materials Engineering, University of Leeds, Leeds, UK, 1998. 

Aksel, C. and Riley, F.L., Effect of particle size distribution of spinel on the mechanical properties and thermal shock performance of MgO-spinel composites , J. Eur. Ceram. Soc., 2003 23(16) 3079-87. 

The unique basic properties of HT make them very useful for catalytic purposes. However, notably, what often is indicated as a hydrotalcite is not really this compound with a layered stmcture. During thermal treatments a HT transforms first into an amorphous oxide and then, at higher temperatures, into a crystalline spinel-like oxide. Therefore, when the sample is calcined above 300 °C it is instead present as an amorphous oxide or as a crystalline spinel-like oxide. HT derived oxides, however, have the peculiar characteristic of reconstmction during catalytic reactions, returning to a stmcture resembling the starting HT stmcture (memory effect), which often is quite relevant in determining the catalytic performance [259-261]. 

Hydrotalcite-like compounds (HTlcs) have attracted much attention in recent years as catalyst precursors and catalyst support. This is due to (i) their ability to accommodate a large variety of bivalent and trivalent cations (ii) the homogeneous mixture of the cations on an atomic scale and (iii) the formation of thermostable mixed oxides, often denoted as ex-HTlcs, with high surface area upon decomposition. The first two properties are a result of the precursor while the last property appears to be related to the decomposition mechanism. The transitions in the structural properties of Co-based hydrotalcites upon high temperature treatments have been extensively studied in our group.15,16 In the first decomposition step, water is removed from the structure. This transition is followed by dehy-droxylation and decarbonation, as well as carbonate reorganization in the interlayer. Thermal treatment in air finally leads to a solid solution of cobalt spinels (Co(Co, A1)2C>4). Mixtures of CoO and C0AI2O4 are formed upon treatment in inert. 

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