A-Alumina phase

This transition produces an isomorphous phase and the resulting y-alumina has the same morphology and texture as its boehmite precursor. With increasing temperature and time the mean pore diameter increases gradually and other phases appear (S-, 6-alumina). Due to the broad XRD lines, the distinction between y- and S-alumina cannot be made 6-alumina occurs at about 900°C while the conversion to the chemically very stable a-alumina phase takes place at T> 1000°C. Some typical results for alumina membranes synthesized without binders are given in Table 2.4. When PVA was used as a binder, thermogravimetric analysis showed that, provided the appropriate binder type was used, the binder could be effectively removed at T > 400°C. The ash residue is of the order of 0.01 wt.%. Mean pore size and... 

In order to sustain this reaction at the sink side of the PEVD system, a source is required at the other side of the substrate (anode) to supply sodium. Otherwise, depletion of sodium in the Na" -P"-alumina solid electrolyte will lead to an a-alumina phase buildup at the anode that will block the ionic transport path of the PEVD system. The electrolytic properties of the solid electrolyte in this PEVD system will then be lost. Elemental sodium, for instance, could be the source giving the following anodic reaction ... 

Chromium impurities in the a-alumina phase fluoresce under laser irradiation of appropriate wavelength. For materials exhibiting the piezo-spectroscopic effect, the wave number, v, of the fluorescence line depends on the stress state. In the case of alumina, the line shift, Av, is approximately proportional to the stress for stresses up to a few GPa... 

It has also been found that the presence of chromia aids in a - alumina formation, as well as limits the phase transformations during heating to temperatures below 1200 (Marple et al., 2001 Chraska et al, 1997). Therefore, chromium nitrate was added to the aluminum nitrate precursor to stabilize the a - alumina phase. 

The other structures of interest are those ivith the basic formula M2O3, O/M = 1.5. There are tv o of these structures found in CICPs, one being corundum, named after the a-alumina phase of AI2O3, and the other is hematite named after the mineral Fe203. There is a slight difference in spatial geometry betiveen the tv o structures, but both are quite similar. Metal ions are trivalent and octahedrally coordinated in both. 

Perhaps the closest analogy to the present system in the 600-650 "C range is the y- to a-alumina phase transformation studied by Clark and White ( ), which also empirically obeyed a first-order relationship and which finally slowed down upon nearing completion. This process also had a high activation energy (79 kcal/mole). 

An experimental sol-gel 1.9% silica-doped alumina fibre has been studied before and after the n to a-alumina phase change. The initial fibres contained roughly equal amounts of randan pores and pores aligned parallel to the fibre axes, but only the former were eliminated during the phase change. The product a-alutnina fibre has therefore 90% axially-aligned pores and should have an excellent specific modulus. 

Silica-dQped sol-gel aligned fibre tews were made by a proprietory process with a mean flhre diameter of Sym. They viere finished at 900 °c in order to produce the mesoporous r -aluraina phase. They were then further calcined at 1300°C in a muffle furnace in order to convert the fibres to the a-alumina phase. Powder XRD patterns were determined to confirm the phases. 

Dispersion-strengthened copper is made by dispersing a thoria or alumina phase through copper powder. The resulting P/M product retains its strength at elevated temperatures. It is used, for example, as the conductor or lead wine that supports the hot filament inside incandescent lamps. 

Fig. 11. Micrographs of (a) a hot-pressed alumina—TiC ceramic showing a white TiC phase and a dark alumina phase (3) and (b) a fracture surface of an...

Direct Hydration of Ethylene. Hydration of ethylene to ethanol via a Hquid-phase process cataly2ed by dilute sulfuric acid was first demonstrated more than a hundred years ago (82). In 1923, the passage of an ethylene-steam mixture over alumina at 300°C was found to give a small yield of acetaldehyde, and it was inferred that this was produced via ethanol (83). Since the late 1920s, several industrial concerns have expressed interest in producing ethanol synthetically from ethylene over soHd catalysts. However, not until 1947 was the first commercial plant for the manufacture of ethanol by catalytic hydration started in the United States by Shell the same process was commerciali2ed in the United Kingdom in 1951. 

Alumina is used because it is relatively inert and provides the high surface area needed to efftciendy disperse the expensive active catalytic components. However, no one alumina phase possesses the thermal, physical, and chemical properties ideal for the perfect activated coating layer. A great deal of research has been carried out in search of modifications that can make one or more of the alumina crystalline phases more suitable. Eor instance, components such as ceria, baria, lanthana, or 2irconia are added to enhance the thermal characteristics of the alumina. Eigure 6 shows the thermal performance of an alumina-activated coating material. 

The proposed proeedure are detailed next eaeh SPMD was mierowave-assisted extraeted twiee with 30 mL hexane aeetone, and irradiated with 250 W power output, until 90°C in 10 minutes, being this temperature held for another 10 minutes. Clean-up of extraet was performed by aeetonitrile-hexane partitioning eoupled by a solid-phase extraetion with a eombined eartridge of 2 g basie-alumina (deaetivated with 5% water) and 0.5 g C. ... 

Microwave extraction realized at 120 °C for 30 min with Hexane -Acetone (3 2 V/V) as the extraction solvent was identified as the most effective extraction procedure for isolation of TPH from biotic matrices. The aim of this research is to develop a silica gel and alumina fractionation procedure for plant sample extraction. Column chromatography with two solvents (chloroform and hexane dichloromethane) as a mobile phase were used for clean-up of extract. In this research the efficiency of recovery received from chloroform as a mobile phase. 

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. 

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