Corundum phase

Metallic silver is clearly evidenced by XRD beside mullite and corundum phases. The crystallite sizes are reported in Table 2 as well as the specific surface area. After the evaluation tests, we can observe a strong decrease of the intensity of the silver diffraction peaks disclosing a loss of the active phase during the hydrogen peroxide flow. In the case of sample Bl, an important weight loss is observed (- 8.1 %, Table 1), whereas for sample C, the weight loss is much lower (- 0.9 %) and a part of metallic silver transforms into silver oxide. 

A. Band between 3800 and 3785 cm This band has a low intensity but is virtually present in the spectra of aU activated alumina samples, except for those of the corundum phase of alumina (476). There is consensus that this band corresponds to type I hydroxyls. 

Band at 3780-3760 cm. This band is usually more intense than the A-band. Busca (673) reported that the respective hydroxyls are the most reactive ones on alumina. This band is also not observed in spectra of the corundum phase (476). 

RBAO involves a similar production route, but in this case the aluminum metal powder is a minor component in a powder mix that contains a substantial fraction of alumina. Oxidation of the aluminum metal by gas percolation yields a metastable oxide phase, which is replaced at higher temperatures by the stable corundum phase. Again, the large specific volume of the oxide product reduces the volume fraction of porosity, but in this case the thermal cycle actually sinters the product, and for the optimized powder formulation, expansion during oxidation of the metal at a moderate temperature is followed by the sintering contraction at a higher temperature to yield a dense alumina product whose final shape and volume is still very close to that of the green powder preform. 

The onset of a-AbQs (corundum) formation is seen in the major XRD reflections by 1195 °C, which, unusually, is ahead of its visibility by NMR. However, the proximity of the strong AlOg NMR signal at 5 ppm arising from octahedral 6-AI2Q3 is certain to have obscured any early observation of corundum crystallisation. By 1340 °C the corundum phase is dominant, accompanied by residual O-AbCb and the minor recrystallised AIPO4 phase. 

At 1228 °C the alumina undergoes a phase transition to the stable corundum phase. The measured heat for this transition (151 J/ g) is in reasonable agreement with the theoretical value for the 8 —> a transition (132 J/g (Chase, 1998)). 

Aluminum oxide, A1203, is known almost universally as alumina. It exists with a variety of crystal structures, many of which form important ceramic materials (see Section 14.22). As a-alumina, it is the very hard, stable, crystalline substance corundum impure microcrystalline corundum is the purple-black abrasive known as emery. Some impure forms of alumina are beautiful, rare, and highly prized (Fig. 14.25). A less dense and more reactive form of the oxide is y-alumina. This form absorbs water and is used as the stationary phase in chromatography. 

Alumina - Alumina forms a variety of oxides and hydroxides whose structures have been characterized by X-ray diffraction (16). From the catalytic viewpoint y-alumina is the most important. This is a metastable phase that is produced from successive dehydration of aluminum trihydroxide (gibbsite) to aluminum oxide hydroxide (boehmite) to y-alumina, or from dehydration of boehmite formed hydrothermally. y-alumina is converted into a-alumina (corundum) at temperatures around 1000 C. 

The growth of vanadium oxide overlayers on Rh(l 11) converges after a number of intermediate stages to the formation of a three-dimensional bulk-like epitaxial V203 film [90], which is oriented with the (0 0 01) plane of its corundum structure parallel to the Rh(l 1 1) substrate surface. The V203 phase is the thermodynamically stable... 

Af-dipropyl-p- toluidine, 2 550t a, a -dinitroanthraquinones, 9 315—316 a-alumina, 2 406t 14 103. See also Corundum transition to, 2 403 a-aluminum-iron—silicon alloys, 2 317 intermetallic phases, 2 316t a-aluminum oxide-hydroxide. 

The third and the most common type is complex phase transformations, including the following (i) some components in a phase combine to form a new phase (e.g., H2O exsolution from a magma to drive a volcanic eruption the precipitation of calcite from an aqueous solution, Ca + + COf calcite the condensation of corundum from solar nebular gas and the crystallization of olivine from a basaltic magma), (ii) one phase decomposes into several phases (e.g., spinodal decomposition, or albite jadeite + quartz), (iii) several phases combine into one phase (e.g., melting at the eutectic point, or jadeite +... 

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