Spinel phase compositions

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... 

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. 

Both powders collected from the reactor wall and from the reactor bottom showed similar diffraction patterns (Fig. 5.) indicating the presence of spinel as the dominant phase (amount of sample C was negligible in these experiments). Composition of the spinel phase was estimated from the lattice parameters (a), assuming, that the increase of lattice parameter due to Zn incorporation is proportional to the Zn concentration. From these calculations the compositions of sample R and sample RB were Zno.7Fe2.3O4 and Zno.4Fe2.6O4, respectively. Consequently, some of the Zn content of the starting mixture could not build into the spinel structure and left the system through the exhaust in the form of very fine ZnO powder. 

Iron vanadate, FeV04, is a prospective material for lithium rechargeable batteries and in catalysis. In [90] mechanical coactivation of iron and vanadium oxides was used to prepare intimate nanoscale mixture, similar to those prepared by soft chemistry. Reduction of this mixture at the same temperature and oxygen partial pressure conditions as of soft chemistry products (500°C and 10 Pa) leads to formation of a nanometric vanadium ferrite with the only spinel phase. The characterization of the powders thus prepared was perfomed by X-ray diffraction, SEM, IR spectrometry, thermogravimetry and colourimetry. It was shown that the homogeneity of grain size and chemical composition is achieved if the initial oxides have similar grain size. 

Figure 7 Normative spinel Iherzolite mineral abundances (wt.%) versus rock Mg for a set of reconstructed abyssal peridotite compositions. Reconstructions were made using mineral modes and phase composition data (see text Niu et al. (1997), Baker and Beckett (1998), and Asimow (1999) for details). Circles are from Baker and Beckett (1998), and filled circles are reconstructions based on site-averages, whereas open circles are reconstructions based on single thin section modes. Crosses are from Asimow (1999), and are reconstructions based on single thin section modes after the method of Niu et al (1997).

Nanoparticles may also be important within planetary interiors. For example, phase transitions within the deep Earth may generate materials that are composites of nanoparticles (e.g., within the spinel phase at the olivine-spinel transition at the 400-km discontinuity). These grain sizes may affect both kinetics and rheology (e.g., of ice in planetary interiors Stern et al. 1997). Chemical reactions in the deep Earth, perhaps between metal and silicate near the core-mantle boundary, may be impacted by nanocrystals. 

Flexibility of the bulk/surface structure and reaction media effect. For such systems as manganese oxides, copper oxides, spinel iron oxides Fe304-y-Fe203 [4, 5, 24, 25 ], reaction media effect at enhanced temperatures (up to 400 °C ) and at prolonged (up to 10 h) exposures in reaction mixtures was found to remove all initial differences in the phase composition and defect structure. All extended defects were washed out due to interaction with a flux of point defects created by reaction media. As a result, a constant level of the catalytic activity was achieved for these oxide systems demonstrating apparent structural insensitivity of the reaction of CO oxidation. Hence, in this case, great flexibility of the oxide bulk structure allows to reach the same true steady state of the catalyst. 

Implantation of Cu ions into polycrystalline a-alumina results in another phase composition [7]. After llOkeV Cu implantation and 800-1000"C annealing monoclinic CuO and spinel phase CUAI2O4 were observed. Moreover, after aimealing at 1200 C the whole quantity of copper completely disappeared and the alumina surface did not display any precipitates [7]. In our case strong diffusion of... 

However, for samples 2-5, the amounts of CuO detected by quantitative XRD analysis (18) approach a maximum of 50% of the value calculated on the basis of a phase composition CuO + stoichiometric spinels, showing the existence of a consistent fraction of copper ions which escape XRD detection, probably present inside the spinel-type phase or strongly interacting with it (19). 

The seismic discontinuity occuring in the earth s mantle at a depth of 670 km is attributed to the phase transition from the spinel phase to the perovskite phase, and it is this discontinuity which marks the separation between the upper and lower mantle. Calculations were carried out of this phase transition in the MgSi03 and Mg2Si04 systems, which approximately model the composition of the mantle... 

The spinel-monoclinic transition of Cr2FeS4 was observed at sulfur pressures of 10 -10 Pa at 800°C and at 10°-10° Pa at 900°C [1987Nar]. The results of high-temperature XRD measurements and the phase composition data indicate the occurrence of a reversible solid-phase reaction Cr2FeS4 + Fe dCro.sFco.sS, coexisting with the first-order stmctural transition in Cr2FeS4 [1993Sok]. 

The transparent samples were characterized in terms of chemical composition, phase composition, density, hardness, flexural strength, fracture toughness, and optical transmission. Biocompatibility was evaluated with in vitro cell line experiments for cytotoxicity, apoptosis, and genotoxicity. Experimental results indicated that the transparent spinel samples exhibited promising physico-chemical, mechanical, optical properties, and excellent biocompatibility for applications as orthodontic brackets. A piece of the transparent ceramics is shown in Fig. 10.35, together with some tooth samples [182]. 

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