Spinel formation reactions

Of all solid state reactions, the formation of oxide spinels is at present the most thoroughly investigated [4, 5, 33]. The first reason for this is the relatively simple crystallographic structure of the spinel lattice. Essentially, this consists of a nearly close-packed face-centred-cubic sublattice of oxygen ions. The tetrahedral and octrahedral interstices of this sublattice are filled in a certain way by the cations. The second reason is that spinels are technically very interesting substances, and one would like to be able to find optimal methods for their preparation. For instance, ferrites are used as control or circuit elements in the electronics industry, and chromite brick is used as cladding in ovens which are used for the production of steel. Therefore, the formation of spinels will now be discussed in detail as a model of a classical solid state reaction. 

In Fig. 6-3 is shown a number of possible reaction mechanisms. Included here are mechanisms in which the oxygen is transported through the gas phase and local electrical neutrality is maintained by means of electronic semiconduction in the reaction product. 

In practice, one can say that the transport coefficients of the individual ions are generally rather different from one another. In oxide spinels, the diffusion of oxygen is negligible compared to the cationic diffusion. Therefore, we can eliminate a number of the mechanisms shown in Fig, 6-3. Furthermore, if ideal contact is maintained at the phase boundaries so that the gas phase cannot enter, then the only remaining probable reaction mechanism is the counterdiffusion of cations. In this mechanism, the two cation fluxes in the reaction product are coupled through the condition of electroneutrality. 

The spinel formation reaction AO + B2O3 = AB2O4 is a heterogeneous reaction. Therefore, material must be transported across the phase boundaries and through the reaction product. As the thickness of the reaction layer increases, the resistance to diffusion of the 

Jc is known as the practical reaction rate constant. The parabolic growth of NiAl204 from NiO and AI2O3 is shown graphically in Fig. 6-4. 

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Hulbert [77] discusses the consequences of the relatively large concentrations of lattice imperfections, including, perhaps, metastable phases and structural deformations, which may be present at the commencement of reaction but later diminish in concentration and importance. If it is assumed [475] that the rate of defect removal is inversely proportional to time (the Tammann treatment) and this effect is incorporated in the Valensi [470]—Carter [474] approach it is found that eqn. (12) is modified by replacement of t by In t. This equation is obeyed [77] by many spinel formation reactions. Zuravlev et al. [476] introduced the postulate that the rate of interface advance under diffusion control was also proportional to the amount of unreacted substance present and, assuming a contracting sphere (radius r) model... 

In a study of the spinel formation reaction CoO + A1203 = C0AI2O4... 

Further investigations of spinel formation reactions are to be found in the literature [1], but the above representative selection illustrates a number of typical features of these rate processes. Following migration of cations from one constituent onto the surfaces of the other, the process is limited by the rate of diffusion across a barrier layer. While obedience to a particular kinetic expression is sometimes reported, the data available are not always sufficiently precise to enable the fit found to be positively... 

While many reactions are undoubtedly of these three types, few kinetic studies are available. There are, however, close resemblances between specific rate processes in which a gas is evolved and the more intensively studied spinel formation reactions (Sect. 4.1). 

Inert markers have been used to obtain additional information regarding the mechanism of spinel formation. A thin platinum wire is placed at the boundary between the two reactants before the reaction starts. The location of the marker after the reaction has proceeded to a considerable extent is supposed to throw light on the mechanism of diffusion. While the interpretation of marker experiments is straightforward in metallic systems, giving the desired information, in ionic systems the interpretation is more complicated because the diffusion is restricted mainly to the cation sublattice and it is not clear to which sublattice the markers are attached. The use of natural markers such as pores in the reactants has supported the counterdiffusion of cations in oxide spinel formation reactions. A treatment of the kinetics of solid-solid reactions becomes more complicated when the product is partly soluble in the reactants and also when there is more than one product. 

Aksel, C., Spinel formation, reaction conditions and densification properties of magnesia-spinel composites , Key Eng. Mater, 2004 264-8 1071 -4. 

The spinel formation reaction can be represented by the chemical equation... 

Fig. 3.3 Reaction mechanisms and corresponding net phase boundary reactions for the spinel formation reaction AO B2O3 = AB2O4. Reproduced with permission from [47]. Cop5urght 2007, Springer...

Effective mineralizers for spinel formation reactions include fluorine- and boron-containing compounds. The former includes LiF, NaF, AIF3, Na3AlF6<... 

Effect (ff Preformed Spinel Addition on Physical Properties. In AI2O3-spinel castables, preformed spinel is used, so the volume expansion associated with the spinel formation reaction that occurs in Al203-Mg0 castables is eliminated. As aresult, compared with Al203-Mg0 castables, Al203-spinel castables usually show better thermal shock resistance. 

In Al203-MgO castables, MgO (instead of preformed spinel as in AI2O3-spinel castables) is used. Due to the volume expansion associated with the spinel formation reaction from MgO and AI2O3, Al203-Mg0 castables show different microstructures and properties to those seen in AI2O3-spinel castables. 

The thermal expansion behavior of Al203-Mg0 castables is related to the spinel formation reaction, which is affected by temperature and MgO content. The starting temperature for formation of in-situ spinel is I000°C, and the spinel formation rate increases significantly with increasing temperature to... 

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