Alumina scales transformation

The morphology of alumina scales on NiAl depends on the oxidation temperature and oxidation time. At lower temperatures (<1400 K). the metastable oxide phases 7-and 0-Al2O3 are formed initially. These metastable phases transform into a-AFO, during a transient time which decreases with higher temperature (> 1200 K) [24—26J. 

Continuing the theme of precursor-oxide relationships, further up the scale of difficulty is the basic aluminum sulfate-alumina case. In addition to the relative complexity of the decomposition of the basic sulfate( ) to the primary alumina product which is amorphous to X rays, there are the further stages of recrystallization to //-alumina and transformation to a-alumina to be considered. A limited study by electron microscopy and diffraction has been made of the conversion of the basic sulfate to a-alumina, and the few observations so far available combined with the structural essence of the work cited above provide a useful sketch of the subject system. 

In addition to large-scale industrial applications, solid acids, such as amorphous silica-alumina, zeolites, heteropoly acids, and sulfated zirconia, are also versatile catalysts in various hydrocarbon transformations. Zeolites are useful catalysts in fine-chemical production (Friedel-Crafts reactions, heterosubstitution).165-168 Heteropoly compounds have already found industrial application in Japan, for example, in the manufacture of butanols through the hydration of butenes.169 These are water tolerant, versatile solid-phase catalysts and may be used in both acidic and oxidation processes, and operate as bifunctional catalysts in combination with noble metals.158,170-174 Sulfated zirconia and its modified versions are promising candidates for industrial processes if the problem of deactivation/reactivation is solved.175-178... 

Some of the earliest catalysts for this transformation were heterogeneous metal oxides (typically of tungsten, molybdenum or rhenium) on a support such as silica or alumina. These are still the catalysts employed for all current large scale industrial processes. Ill-defined homogeneous systems comprising alcohol solutions of a metal halide in conjunction with a promoter were also employed extensively in the early years, and are currently still used for the... 

In other work, they showed that the stress-induced transformation of tetragonal to monoclinic zirconia is limited to nanometer-scale regions in these nanolaminates.In Secs. 6.2.2.2, we discuss our own theoretical rationalization of these observations. The role of alumina seems merely to confine the size of Zr02 crystallites formed. Our own calculations on Zr02-Al203 (discussed in Secs. 6.2.2.2) are consistent with these observations. 

Even if this conditions represent an important improvement, this technology is not applicable on an industrial scale as the catalyst deactivates irreversibly by fluorination (ref. 12). Moreover, if the use of fluorinated aluminas leads to the improvement of initial selectivity, it can not avoid the catalyst deactivation which is transformed during the course of the reaction to aluminium fluoride, a non selective catalyst (ref. 12). 

The areas concerning monolithic intermetallics which have been studied in recent years are (i) the formation of mctastable aluminas, and their transformation to stable a-alumina, (ii) the formation of interfacial voids and scale adherence and how these are influenced by reactive elements and sulfur, and (iii) accelerated oxidation at intermediate temperatures. Additionally the applications oriented areas of (iv) coatings, (v) oxidation of composites, and (vi) life predictions have received attention. 

Phase transformation from 0 to a results in an a-Al203 scale ridge network (Hindam and Smeltzer, 1980b Doychak et al., 1989 Doychak, 1994) that, for the sake of clarity, is referred to as extrinsic (Fig. 6- 10b, c). Extrinsic ridges form on any NiAl alloy that doesn t immediately transform to a-Al203 (a delayed phase transformation). Because the transition aluminas grow epitaxially, the ridge network varies with the orientation of the substrate. On a polycrystalline substrate. 

Several types of phase transformations can occur during aging. We have already mentioned mlcrosyneresis, in which solid phase separates from the liquid on a local scale. There may also be segregation of the liquid into two or more phases. For example, in base-catalyzed hydrolysis of silicon alkoxides, there may be isolated regions of unreacted alkoxide [14]. When a gel of that type is soaked in pure water, it turns white and opaque [15], apparently (as explained in Section 2) from segregation of droplets of partially reacted alkoxide. Aging may also lead to crystallization, as in the precipitation of nitrate crystals from alumina gel made from Al(NO)3 [16],... 

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