Alumina based materials

Although beryllium oxide [1304-56-9] is in many ways superior to most commonly used alumina-based ceramics, the principal drawback of beryUia-based ceramics is their toxicity thus they should be handled with care. The thermal conductivity of beryUia is roughly about 10 times that of commonly used alumina-based materials (5). BeryUia [1304-56-9] has a lower dielectric constant, a lower coefficient of thermal expansion, and slightly less strength than alumina. Aluminum nitride materials have begun to appear as alternatives to beryUia. Aluminum nitride [24304-00-5] has a thermal conductivity comparable to that of beryUia, but deteriorates less with temperature the thermal conductivity of aluminum nitride can, theoreticaUy, be raised to over 300 W/(m-K) (6). The dielectric constant of aluminum nitride is comparable to that of alumina, but the coefficient of thermal expansion is lower. 

Table 12.2 Temperature of onset of PdO decomposition (To) during the heating ramp and that of re-formation of PdO from Pd (Tr) during cooling in thermal cycling in air for PdO supported on different alumina-based materials.

Alumina based materials are well-known catalytic materials and catalytic supports. HFEPR at 130 GHz has been used to show27 that reliable values for the... 

Alumina offers another alternative to silica because of its inherent higher pH stabihty. However, in contrast to its extensive use as a medium in column chromatography for purification purposes, or for separations in the normal-phase mode, there are still relatively few reports concerning alumina-based materials in the reversed-phase mode. 

Surface interaction of fluorine with silica- and alumina-based materials. 

Chemical composition silica-based and silica-alumina-based materials, chrome, magnesia, chrome-magnesia, spinel, SiC, materials containing carbon (more than 1% carbon or graphite), and special materials (containing other oxides or materials such as zircon, zirconia, Si3N4, etc.)... 

At 1500°F, the steam-coke reaction on alumina occurs approximately three to six times faster than on silica-alumina type substrates. Comparison of results from Runs 56 and 58 indicates the extent of reproducibility of the experiments. In comparison with this reproducibility and in consideration of the observed consistency, the increased gasification rates on the alumina substrates are significant. All alumina-based materials tested, viz., y-alumina, calcined Catapal, and bauxite, showed equally high activities. 

The activation energies for coke gasification on the three substrates shown in Figure 4 were 33 Kcal/mole for the three alumina-based materials and 54 Kcal/mole, for the silica-alumina catalysts. The increased activity and lower activation energy for the coke deposited on the aluminas (compared to that on the silica-aluminas) cannot be due to a direct catalytic effect of alumina on the gasification reaction, but rather to an indirect effect of the alumina that controls the nature and structure (surface area and structural disorder) of the coke during its deposition. 

Table 4.1 further shows essential mechanical properties of several products of CeramTec s BIOLOX family of alumina-based materials for femoral heads of hip endoprostheses as well as those of BIONIT manufactured by Mathys Orthopadie GmbH (Bettlach, Switzerland). It is evident that decreasing the grain size of the ceramic precursor powders increases both the flexural strength and the fracture toughness of the material dramatically. 

Disperal . [Condea Chemie GmbH] ffighly dispersible aluminas base materials for abrasives and high tenqi.-resistant ceramics, as auxiliaries for separation processes. 

Several materials have been proposed as alternatives to quartz gravel pack materials and have been tested for stability under steam injection conditions. Phenolic resin-coated gravel has been used in thermal wells (79). The coated grains agglomerate with temperature to form a consolidated pack that does not require a screen. Alumina-based materials have been reported to be stable in the high pH environment to over 300 °C (78), but others (80) found that dissolution of these materials can occur. Weaver and Knox (80) proposed an alternate material, SRG, whose chemical formulation is not revealed. A steel wool filter has been used as an alternative to gravel packs in thermal wells (58, 59, 81, 82). 

The N2 adsorption-desorption isotherms were collected on an ASAP 2010 analyser (Micromeritics). Prior to analysis, the samples were degassed (psurface area were estimated fi om a t-plot (Harkin-Jura) analysis of the adsorption curve (0.3 nm < t < 0.5 nm, with t being the statistical thickness). The pore size distribution was calculated from the desorption branch of the isotherm using the B JH model. 

At the DLR Institute of Materials Research an all-oxide CMC (WHIPOX = wound highly porous oxide) consisting of alumina fibres Nextel 610 or mullite based fibre (Nextel 720, both 3M) and an alumina or mullite matrix, respectively, has been developed in recent years [1,2]. Mullite-based CMCs typically offer higher creep stability than alumina-based composites but display lower thermal conductivity. Lower thermal stability of alumina-based materials, however, can be accepted, since service temperature is significantly lower for these materials as a result of the better cooling efficiency. Therefore material development was focused on alumina... 

The heating of the fiber up to 1000 - 1200 °C decreased the value of specific surface because of sintering of oxide particles. The fibrous alumina-based materials annealed above 1400 °C become more sintered, their porosity did not exceed 30 %, the specific surface decreased down to 5 m2/g, and the grain sizes were 100-150 nm. At the same time, the strength of this material was enhanced. 

The presence of hydroxyl groups on alumina-based materials is also evident from the sphtting of the XPS Ols signal, which is usually characterized by a contribution near 530 eV that is assigned to oxide species and a second contribution at higher energy that is assigned to hydroxide species (278). 

From the point of view of the volume of production, polycrystalline alumina is the material most frequently used as ceramics for structural applications. However, in comparison with for example, silicon nitride, where the influence of various additives on microstructure and properties has been well characterized and understood, and despite several decades of lasting research effort, alumina remains a material with many unknown factors yet to be revealed. Alumina-based materials can be divided roughly into three groups ... 

Naik B, Ghosh NN (2009) A review on chemical methodologies for preparation of mesoporous silica and alumina based materials. Recent Pat Nanotechnol 3 213-224... 

Beryllium oxide materials are particularly attractive for use in power vacuum tubes because of their electrical, physical, and chemical properties. The thermal conductivity of BeO is approximately 10 times higher than that of alumina-based materials. Figure 5.35 compares the thermal conductivity of BeO to that of alumina and some alternative materials. As the chart iUustrates, BeO has a lower dielectric constant and coefficient of thermal expansion than alumina, ft is, however, also sHghtly lower in strength. 

As measured by the heats of interaction, it is seen that the surface of each of these commercial alumina-based materials is quite different, and each is heterogeneous in its interaction with ammonia. There are some sites which bind very strongly (>20 kcal/mol), while others only physically interact with Van der Waals forces (<10 kcal/mol). Both SP-100 and SELEXSORB-NH3 are aluminas whose surfaces have been chemically modified. 

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