Spinel phases

Hammetter [91H01] has carried out DTA analysis of a shock-modified but unreacted 2A1 -t- Fe203 mixture and found a preinitiation event, indicating the presence of shock-produced Hercynite (spinel phase FeAl204). These preinitiation events observed in thermal analysis provide direct evidence for the influence of mechanical, shock-induced mixing in controlling solid state chemistry. 

Table 1.4 Spinel phases encountered in alloy oxidation...

A number of authors have reported studies of the oxidation of low-alloy steel in steam. The general observations indicate strong similarities with oxidation in air, the kinetics being typically parabolic and the scales typically comprising FcjO, Fej04, FeO and spinel phases, dependent upon alloy composition, temperature and oxygen partial pressure of the environment... 

The segregation or demixing is a purely kinetic effect and the magnitude depends on the cation mobility and sample thickness, and is not directly related to the thermodynamics of the system. In some specific cases, a material like a spinel may even decompose when placed in a potential gradient, although both potentials are chosen to fall inside the stability field of the spinel phase. This was first observed for Co2Si04 [39]. Formal treatments can be found in references [37] and [38],... 

One pecuhar oxygen buffer reaction is the well-known thermobarometric equation of Buddington and Lindsley (1964), which furnishes information on both T and the activity of molecular oxygen. The reaction uses the equilibrium among titanomagnetite (or ulvospinel ), magnetite, hematite, and ilmenite components in the hemo-ilmenite and spinel phases ... 

When supported on kaolinite, Ni observability is independent of the thermal pretreatment used to age the catalyst however, its speciation or interaction with this clay change after steaming. In fact, whereas 2% Ni on kaolin is approximately 60% reducible after calcination, the steamed sample is not, probably because of the formation of a stable Ni-silicates or even a surface spinel phase like Ni-aluminate. 

With M/(Fe + M) >0.15, a spinel phase (MFe204) formed in all cases and when this ratio exceeded 0.33, Cu and Ni precipitated as separate phases (Tab. 14.3). Formation of a spinel phase requires a threshold level of in the system. It is considered that the spinel phase nucleates in the water layer adsorbed on or adjacent to, the surfaces of the ferrihydrite particles and that these nuclei grow by addition of soluble M-Fe-hydroxo complexes released by the dissolving M-ferrihydrite (Cornell Giovanoli, 1987, 1989 Giovanoli Cornell, 1992). Tronc et al (1992) suggested that when the ratio is very low, a different mechanism operates a mixed valence... 

The oxide film formed in dry air at room temperature consists of a spinel phase, probably a solid solution of magnetite and maghemite. Such films form on magnetic tapes. They are around 1.5-2.0 nm thick, and in a dry atmosphere, can provide indefinite protection (e.g. the Delhi pillar). Ali and Wood (1969) found that with time and at a relative humidity of 46%, some hematite developed as well. At higher temperatures (200-300 °C) well defined duplex films with an inner layer of magnetite... 

Since the spinel phase must be prepared at low temperatures (by hydrothermal synthesis or by careful oxidation of magnetite at a temperature T < 300 C, for example) it has been widely suspected that some incorporation of hydrogen is needed to stabilize it. However, Schrader and Buttner have shown that pure y-Fe203 does exist, and Coey et al. have been able to prepare Fe3 j04 in the compositional range 0 < x < 0.08 by quenching non-stoichiometric magnetite prepared at 1450 °C. There is no evidence that hydrogen is needed to stabilize the system. 

Mixed metal amorphous and spinel phase oxidation catalysts derived from carbonates... 

Many spinel phases have been used as colorants for ceramics and porcelain enamels. The most important pigments based on spinel phases used in paints, plastics, and building materials are given in Table 25. 

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