Aluminas corundum

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. 

Reasoning by analogy, it would appear not unlikely that native oxides, such as alumina (corundum, ruby, sapphire), or iron sesquioxide (haematite), may be in reality an aluminate of alumina, A1(A102)3, or ferric ferrite, Fe(Fe02)3.. . ... 

Figure 2.26. Representation of the structure of a-alumina (corundum), (a) Ap+ ions (filled circle) are shown to occupy the octahedral sites between the hep layers of ions (open circle), (b) The stacking sequence of Al + ions as viewed in the direction of the arrow in (a). Reprinted from Greenwood, N. N. Earnshaw, A. Chemistry of the Elements, 2nd ed.. Copyright 1998, with permission from Elsevier.

Aluminum Oxide (Alumina, ct-Alumina, Corundum, Alandum). AI2O3, mw 101.96, v hard white cryst powd, mp 2045°, bp 2980°, d 3.965g/cc. Insol in w, V si sol in aq alkaline solns. Found in nature as a mineral such as bauxite. Lab prepn is by heating aluminum hydroxide to above 1100°. 

Bauxite aluminum oxide (alumina, corundum, emery, ruby, sapphire) aluminum sulfate alum ammonium alum. Aluminum chloride aluminum acetate. Uses of aluminum compounds. 

Abbattista et al. (26) found that phosphorus addition prevents crystallization of the y-alumina phase and the transformation from y- to a-alumina in the system AI2O3 —AIPO4 (Fig. 23). More precisely, Morterra et al. (77) reported that phosphates do not affect the phase transition from low-temperature spinel alumina (y-alumina) to high-temperature spinel aluminas 8 and 6 phases) but delay the transition of 8 and 9 to a-alumina (corundum). Stanislaus et al 46) also reported that phosphorus significantly improves the thermal stabihty of the y-alumina phase in P/Al catalysts. However, the same authors found that the positive effect of phosphorus seems to be canceled in the presence of molybdenum due to the formation of aluminum molybdate. Thermal treatments of MoP/Al catalysts at temperatures >700°C result in a considerable reduction of SSA and mechanical strength. The presence of phosphorus does not prevent the reaction between the molybdenum oxo-species and alumina since the interaction between molybdates and phosphates is weak. The presence of nickel does not obviously affect the positive effect of phosphorus in terms of thermal stability 46). On the other hand, Hopkins and Meyers 78) reported that the thermal stability of commercial CoMo/Al and NiMo/Al catalysts is improved by the addition of phosphorus. 

The (0001) surface of sapphire (a-alumina, corundum) is one of the most widely used substrate for the growth of metal, semi-conductor or high-temperature superconductor thin films. It is also used as a substrate in silicon on sapphire (SOS) technology. Moreover, its initial state is known to play a role on the overlayer properties [50]. 

Another form of alkali metal attack on the hot faces of refractory linings involves their high temperature reaction with various components of the brick to form expansive crystalline phases which cause brick to bloat on their hot faces and, subsequently, erode or spall. An example Is the case of alumina brick exposed to sodium at temperatures from about 1700°F to 3000°F. Although sodium does not form a low temperature melt with alumina, it reacts with the alpha phase of alumina, corundum, to form beta alumina, sodium aluminate. Beta alumina has a much greater volume than the very dense corundum and, therefore, disrupts the brick bonding matrix, causing eventual bond failure. 

Irradiated alumina, usually as a-alumina (corundum), has been examined optically and by ESR (102-105), but none of the several features found has been definitely identified with radiation-induced catalytic or adsorptive properties. The surface of a finely divided alumina may be y-alumina even when the bulk has the corundum (a) structure (84). 

Table 2 Structures of stable alumina (corundum) and unstable aluminas...

Figure 6.19 (a) STEM image of a cross section of a plasma-sprayed titania (brookite) bond coat-hydroxyapatite topcoat assembly. On the substrate side, embedded alumina (corundum) particles are visible. The sample was produced by focused ion beam (FIB) cutting to conserve the true geometry of the coating interfaces. The black strip at the top surface consists of platinum deposited by decomposing an organic compound to... 

A review of First Principles simulation of oxide surhices is presented, focussing on the interplay between atomic-scale structure and reactivity. Practical aspects of the First Principles method are outlined choice of functional, role of pseudopotential, size of basis, estimation of bulk and surface energies and inclusion of the chemical potential of an ambient. The suitability of various surface models is discussed in terms of planarity, polarity, lateral reconstruction and vertical thickness. These density functional calculations can aid in the interpretation of STM images, as the simulated images for the rutile (110) surface illustrate. Non-stoichiometric reconstructions of this titanium oxide surface are discussed, as well as those of ruthenium oxide, vanadium oxide, silver oxide and alumina (corundum). This demonstrates the link between structure and reactivity in vacuum versus an oxygen-rich atmosphere. This link is also evident for interaction with water, where a survey of relevant ab initio computational work on the reactivity of oxide surfaces is presented. 

Aluminum oxide Al203 Adamantine span alumina alumina corundum emery. 

Synonyms/Trade Names Alumina, Aluminum oxide, Aluminum trioxide [Note a-Alumina is the main component of technical grade alumina. Corundum is natural AI2O3. Emery is an impure crystalline variety ofAl203.] ... 

Figure 6.17 Atomic arrangement on primary slip plane of a-alumina (corundum).

The slip system for a-alumina (corundum) is given in Table 17.1. The primary slip plane is the basal plane, (0001) the slip direction is <1120>. The arrangement of atoms on the slip plane was shown in Figure 12.12. For... 

Aluminum oxide (AI2O3, alumina, corundum) is the most widely used inorganic chemical for ceramics and is produced from the mineral bauxite using the Bayer process. Bauxite is a mixture of hydrated aluminum oxide with iron oxide (Fe203), silica (SiOi), and titania (TiOi) impurities. It results from the decay and weathering of aluminous rocks, often igneous, under tropical conditions. Like kaolin, bauxite occurs as both primary deposits and secondary deposits. 

Aluminium oxide occurs in two main forms a-alumina corundum) and Y-AI2O3 activated alumina). The solid state structure of 0t-Al2O3 consists of an hep array of ions with cations occupying two-thirds of the octahedral interstitial sites. ot-Alumina is extremely hard and is relatively unreactive (e.g. it is resistant to attack by acids). Its density (4.0gcm ) exceeds that of Y-AI2O3 (3.5gcm ) which has a defect spinel structure (see Box 13.6 and Section 21.10). The a-form is made by dehydrating... 

Aluminium oxide occurs in two main forms a-alumina corundum) and Y-AI2O3 activated alumina). The solid... 

The XRD data in Figure 17 show that no ciystalline phases were detected in sulphuric acid-derived PAA samples heated to 800 °C, comparable with the phosphoric PAA data in Figure 13. Step-wise heating to 900 °C and 1000 °C resulted in the formation of ccp y-alumina. At 1100 °C, the sample consisted mainly of 8-alumina, with a small amount of a-alumina (corundum). At 1200 °C only a-alumina was observed in the heated PAA... 

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