Alumina spinels

Although the fine structure hypothesis of the promoter action of alumina on iron appears well founded, it is conceivable that, in addition to the conservation of the iron surface, the beneficial effect of alumina involves also a modification of the valence forces at the borderlines between alumina (or the iron-alumina spinel A1203 FeO), and the crystallites of metallic iron (49). 

Wilson, D.R., Evans, R.M., Wadsworth, I. and Cawley, J., Properties and applications of sintered magnesia alumina spinels , UN1TECR 93 CONGRESS, Sao Paulo, Brazil, 1993. 

Evans, R.M., Magnesia-alumina spinel raw materials production and preparation , Am. Ceram. Soc. Bull., 1993 72(4), 59-63. 

Cortes, G., Magnesia alumina spinels for the refractory industry , Ceramic Technology International, 1994 109-12. 

In addition to the initial work in the alumina and mullite matrix systems previously mentioned, SiC whiskers have also been used to reinforce other ceramic matrices such as silicon nitride,9-13 glass,14 15 magnesia-alumina spinel,16 cordierite,17 zirconia,18 alumina/zirconia,18 19 mullite/zirconia,18-21 and boron carbide.22 A summary of the effect of SiC whisker additions on the mechanical properties of various ceramics is given in Table 2.1. As shown, the addition of whiskers increases the fracture toughness of the ceramics in all cases as compared to the same monolithic materials. In many instances, improvements in the flexural strengths were also observed. Also important is the fact that these improvements over the monolithic materials are retained at elevated temperatures in many cases. 

The dynamic membranes originally developed by Union Carbide are protected by three core patents U.S, 3977967, 4078112, and 4412921 (Trulson and Litz, 1976 Bibeau, 1978 and Leung and Cacciola, 1983) and their foreign equivalents. Those patents cover a broad range of metal oxides such as zirconia, gamma alumina, magnesia>alumina spinel, tantalum oxide and silica as the membrane materials and carbon, alumina, aluminosilicates, sintered metals, fiberglass or paper as the potential porous support materials. However, their marketed product, trade named Ucarscp membranes, focused on dynamic membranes of hydrous zirconium oxide on porous carbon support. 

Thus, the secant relationship (4.20) with the angular definition modified for aluminosilicates (MacKenzie et al. 1985) has been used to shed light on the possible existence of a Si-Al spinel suggested to form when the clay mineral kaolinite is heated to 980°C. On the basis of the known structure of the closely-related 7-alumina spinel, the T-O-T angle of the postulated tetrahedral Si site in such a structure predicts a chemical shift of - 79 ppm for this site. Such a resonance would normally be masked by the broad resonance of the amorphous Si02 also present, but when this material was selectively removed by leaching with KOH solution, the predicted Si peak was detected (MacKenzie et al. 1996) (Figure 4.13). Measurements of the relative intensity of this... 

Figure 4.13. NMR evidence for the presence of Si in the 7-alumina spinel formed from kaolinite at 980°C. After leaching out most of the uncombined Si02 from the sample with KOH solution, the resonance at — 77.5 ppm indicates the presence of Si in the tetrahedral sites of the Al-rich spinel. Adapted from MacKenzie etal. (1996) by permission of copyright owner.

M.W. Vance, et al., Influence of spinel additives on high-alumina spinel castables. Am. Ceram. Soc. Bull. 73(11), 70-74 (1994). 

Before leaving the topic we should mention that this is not the only way to prepare glass-ceramics. Another group of these materials used for dentistry go by the label In-Ceram . The material is prepared as a porous crystalline ceramic (alumina, spinel, and zirconia have all been used) and then infiltrated with an La-rich glass. The resulting glass-infiltrated ceramic can be quite translucent, can be colored to match other teeth, and is mechanically tough (for a ceramic). It can be coated with porcelains for improved appearance. 

A typical jump test is shown in Fig. 9.44 of flow stress versus strain. The stress temperature is 1450 °C. The slopes of each line in Fig. 9.44b yield for m 0.5, meaning that the stress exponent of the strain rate is 2 this indicates the superplastic behavior of the zirconia-alumina-spinel composite under the test conditions of temperature and strain rate. In order to determine the activation energy, a plot of strain rate versus the inverse absolute temperature must be made (as in Fig. 9.44c). The average activation energy of PS-HEBM-SPS is 945 kJ/mol, which is much higher than that of the composite processed from nanopowder mixtures (622 kJ/mol). This should represent GBS, if the concept of superplasticity is the dominant mechanism of deformation. Table 9.1 summarizes the strain rates and various temperatures of two and/or three specimens. PS-SPS appears in the Table 9.1 as PS-SPS and is listed under column C. For the purpose of comparison, the flow-stress results for nanopowder mixtures are also listed in Table 9.1 and are smaller than those processed from PS powders with/without HEBM. 

Soot precursors and residual methane are converted by steam reforming and shift reactions in the catalyst bed. These reactions are in equilibrium in the gas leaving the catalyst bed and the ATR reactor. The catalyst size and shape is optimised to have sufficient activity and low pressure drop to achieve a compact reactor design. The catalyst must be able to withstand the high temperature without excessive sintering or weakening and it should not contain components volatile at the extreme conditions. A catalyst with nickel on magnesium alumina spinel has proven to fulfil these requirements [107] [111]. 

Catalysts used for steam reforming are oftenbased on Ni/NiO or Co formulations supported on materials such as magnesium alumina spinel (Rostrap-Nielsen, 1993). These compositions can be combined with alkah or alkali-earth compounds to reduce methanation and facilitate coke gasification to avoid carbon deposition. 

Hetero-phase additions can influence both the transition alumina sequence and the temperature at which alpha alumina forms. For example, magnesium oxide additions introduced via addition of a magnesium nitrate salt to the boehmite precursor sol induces the formation of eta alumina after heating to 500°C rather than gamma alumina. This eta phase has a very diffuse X-ray diffraction patem at 500°C which sharpens with heating to 1000°C. The magnesium alumina spinel precipitates as a distinct fine crystal upon finther heating. 

Figure 2.3. High-resolution scans over characteristic spinel reflections of some catalyst precursors using the Guinier transmission geometry and monochromated Co radiation. The graph indicates the dependence of the spinel lattice parameter determined from the dependence of the spinel lattice parameter determined from the (440) reflection on the aluminum content built into the lattice. Note that all catalyst samples contain nominally the same amount of aluminum. Lattice parameters above the line indicate the presence of an excess of calcium besides aluminum. The data Ml-3 are pure alumina spinel samples.

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