Alpha alumina

Aluminum oxide, or alumina, ciystalUzes in the corundum stracture (mineralogical term) to form sapphire monociystals. We are referring here to white sapphire, like the one used for the manufacture of scratchproof glasses for watches, whereas in gemmology sapphire is the blue sapphire, which is alumina with a little Ti mby being alumina colored in red by Cr +. This is the alpha variety (Al203-a), which is the stable variety, but we will see that the preparation of alumina involves various metastable varieties hydrated aluminas and transition aluminas. 

The strong iono-covalent bonds and the rhomboedric structure combine to restrict the dislocation movement [HEU 84] the sapphire monociystal remains brittle until very high temperatnres and the basal slip (0001) 1120 is only possible beyond 900°C. The plasticity of the polycrystal comes primarily from the 

Alumina is one of the best electrical insulators, hence its dielectric applications. Electric conduction depends primarily on impurities which act as acceptors (for example, Mg, Fe, Co, V or Ni) or as donors (for example, H, Ti, Si, Zr or Y), the balance between electronic and ionic contributions depending on the temperature and the partial pressure of oxygen [KRO 84]. Among the impurities, sodium deserves a particular mention because the synthesis of alumina by the Bayer process brings into play sodic mediums sodium aluminate (Na2011Al203), also called beta alumina, is an ionic conductor with very high conductivity, which is why this compound is envisaged for solid electrolyte applications [WES 90]. 

With an optical gap of about 9.9 eV, pure alumina is transparent in the visible and colorless domain. Like the electrical properties, the colored effects depend primarily on the impurities. Sapphire and raby have been the subject of numerous studies [BUR 84] and it is esthetically and scientifically interesting to mention the alexandrite effect (from the name of a chrysobeiyl (Al2Be04) containing a little Cr +) which is green in daylight and red if illnminated by an incandescent lamp [NAS 83]. 

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The effect of bath additives on the electrocodeposition of alumina-copper has been studied. Chloride ion was found to significantly inhibit incorporation of alpha alumina in an acidic copper bath [27, 51], whereas thallium ions, cesium ions, and tetra-ethylene pentamine were promoters [25, 32]. 

X-ray diffraction conducted on the codeposited powder revealed that the deposit obtained from a suspension of gamma alumina, which had been partially converted to the alpha phase, contained both phases of alumina. Whereas, the powder codeposited from a suspension having a 50 50 mixture of alpha to gamma alumina powder, consisted only of the alpha phase. Using a parallel plate electrode configuration, Chen et al. [31] concluded that only alpha alumina can be codeposited. Chen also observed a difference in codeposition with copper when using two different phases of the titanium oxide particle system rutile readily codeposited but anatase titania did not... 

The work by Chen et al. also resulted in no measurable incorporation with particle diameters of 0.05 and 0.02 pm gamma alumina in copper [31]. When the 0.02 pm gamma particles were calcined to obtain a mix of gamma and alpha alumina, codeposition increased to 2.9 vol.%, under the same codeposition conditions. Furthermore, when the 0.02 pm gamma powders were completely converted to the alpha phase of alumina, incorporation rose to 3.3 vol.% [31]-... 

X-ray diffraction of the Buehler-supplied gamma alumina powder used in a RCE study showed the powder to be actually a combination of alpha and gamma phase alumina [28], However, the relatively high particle incorporation obtained with this alpha-gamma alumina cannot be attributed to the partial alpha crystallographic character of the powder, since the pure alpha alumina codeposited to a lesser extent. 

Fig. 7 Creep-resistance of the eutectic fiber consisting of interpenetrating phases of alpha-alumina and yttrium-aluminum-garnet (YAG) up to very high temperatures compared with other types of oxide fibers... 

In this way it has been shown that for Pd particles on single crystal alpha-alumina films and for Au particles on magnesium oxide smoke crystals there may be wide variations of behavior depending on factors which are not immediately apparent. In some cases, the particles are single crystals and show a strong correlation between their orientations and the orientation of the substrate. 

Terpstra, R. A., B. C. Bonekamp and H. J. Veringa. 1988. Preparation, characterization and some properties of tubular alpha alumina ceramic membranes for microfiltration and as a support for ultrafiltration and gas separation membranes. Desalination 70 395-404. 

Commercial catalysts. Two commercial catalysts made by Katalistiks b.v. were used. The first, EKZ-, was steam aged at 750 C for 18 hrs prior to use and the second, an EKZ-2 equilibrium catalyst from a European refinery, was heated at 300°C in air for 3 hrs prior to use. Alpha alumina, heated at 300 0 in air for 3 hrs, was used in order to estimate the contribution to conversion from thermal cracking. 

Vapor Phase Hydrogenation of Acetic Anhydride Acetic anhydride was pumped into an evaporator where it was mixed with hydrogen. The temperature of anhydride-hydrogen mixture was raised to the reaction temperature in a preheater zone, made of a 2 feet bed packed with 2 mm glass beads. The reaction took place in a 2 feet catalyst bed packed with 1 m.m. alpha-alumina coated with 0.5% Pd. The effluent was condensed and analyzed by G.C. 

Y. Wang, H. Shaw, and R. J. Farrauto, "Catalytic Oxidation of Trace Concentrations of Trichloroethylene over 1.5% Platinum on alpha-Alumina," in Ref. 3. 

The sulfur oxidation is carried out at pressure higher than 8 atm and below 180 °C, with a proprietary supported-Mo oxide-based catalyst, for example, an alpha alumina-supported MgMo04 catalyst, operating at 110 °C and 17 atm [59c]. All the products produced by oxidation side reactions and by hydroperoxide reduction are separated from the gas oil stream together with the sulfones. This operation may result in diesel yield loss therefore, the valorization or upgrade of this oxidized stream affects the process economics. This stream can be blended into the heating oil pool or treated in a hydrocracking unit to recover valuable products. 

The catalyst was 100 ml of American Cyanamid HDS-3A, a 1/16-inch diameter extrudate of nickel-molybdenum-alumina. It was diluted with inert, granular alpha-alumina to provide a bed depth of 18 inches in the middle section of a 0.96-inch ID vertical reactor with a 5/16-inch OD internal thermocouple well. The catalyst was progressively more dilute toward the top of the bed to minimize exothermic temperature effects, and end sections were packed with alpha-alumina to provide for preheat and cooling zones. 

No alumina, with the exception of very dense and crystalline "diaspora" yields alpha alumina at a temperature less than 1150°C, [14] The reforming experiments were done at 850°C and no diaspora alumina was detected in the fresh catalyst. 

A NiO/AIumina catalyst, prepared in the laboratory, which contains free alumina does not yield alpha-alumina under C02 reforming conditions (Figure 4). 

We believe that NiAl204-spinel may not be a good reforming catalyst as it will ultimately generate a mixture of Ni and alpha-alumina. We have previously found that such a mixture is not very effective for C02 reforming. 

Vanadyl phosphates (VPO) and multiple component molybdate (MCM) are good examples of catalysts, and alpha alumina, amorphous silica and alumino-silicates are good examples of catalyst supports that can be fabricated in the form of 45 to 150 im diameter spray dried porous spheres with attrition resistance improved by a relatively thin peripheral layer rich in amorphous silica, amorphous alumina, or phosphorus oxides. The hard phase component or precursor is selected in each case so that it will not interfere with the catalytic performance of the catalyst. 

To test the validity of the theoretical analyses of the kinetics of formation of alumina ceramics, Wagh et al [17] conducted an experimental investigation of alumina and phosphoric acid slurry. They used a mixture of coarser and finer alpha-alumina to obtain good packing. The coarser alumina was supplied by Reynolds Chemical Company. Its mean particle size was 0.96 mm. Alumina from Fisher Scientific, sieved to a mean particle size of 0.08 mm, was combined with this powder and 50 wt% phosphoric acid solution to produce a mixture of AI2O3 and H3PO4 with a weight ratio of five. 

U.S. Department of Labor, Occupational Safety and Health Administration. 2005. Chemical sampling information, alpha alumina (total dust). Washington, DC. OSHA. 

In addition to bulk density, particle crystallinity is another physical property that can be related to its hardness and potential effectiveness in providing the mechanical force in CMP. For example, alpha alumina abrasive has a higher hardness, thereby producing higher removal rate in comparison to its gamma crystalline counterpart [80]. As expected, the number of defects is also higher for alpha alumina due to the inherent hardness [81]. 

To illustrate the capability of this instrument, two high-purity alpha aluminas having log-normal distributions were analyzed separately and as a 50 50 mixture. Results clearly demonstrated the ability of this technique to resolve the original component distributions from a mixture of the two powders [251]. 

Figure 3.4. Effect of slip pH on mean pore diameter of alpha-alumina membrane fTerpstra and Elferink, 1991]...

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