Magnesia Aluminate Spinel

Magnesia aluminate (MgAl204) is deposited by combining the two metal chlorides with CO2 and H2 at 950°C as followsit l 

A potential application is as an insulator coating on silicon in semiconductor devices. 

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Aksel, C., Davidge, R.W., Knott, P. and Riley, F.L., Mechanical properties of magnesia-magnesium aluminate spinel composites , in III Ceramic Congress Proceedings Book, Engineering Ceramics, Istanbul, Turkey, 2, 172-9, 1996. 

The integrated planar (IP) SOFC stack concept, for instance, applied by Rolls-Royce Fuel Cell Systems, uses an array of series-connected cells deposited by screen printing onto a flat support tube, a schematic cross section of which is shown Fig. 21.23. The support tube, an inert, porous ceramic, is fabricated from a magnesia magnesium aluminate spinel (MMA), whereby the desired coefficient of thermal expansion is adjusted by the ratio of the base materials [76]. 

Both structure and composition affect phonon conductivity in singlephase crystalline ceramics. Complex structures scatter lattice waves to a greater extent. Hence, thermal conductivity is lower in those structures. For example, magnesium aluminate spinel, consisting of two oxides, has a lower thermal conductivity than the single oxides alumina or magnesia. 

Magnesia-rich magnesiiun aluminate spinel (MgO MgAl O ) were tested as solid base catalysts for the transesterifieation of soybean oil with methanol. With oil to MeOH ratio of 1 25 and 3% catalyst at 65° C, biodiesel with 60% yield was obtained in 10 h. The enhanced catalytic activity of the catalyst... 

Wang, Y., Zhang, F., Xu, S., Yang, L., Li, D., Evans, D. G., and Duan, X. Preparation of macrospherical magnesia-rich magnesium aluminate spinel catalysts for methano-lysis of soybean oil. Chem Eng Sci 63,4306-4312 (2008). 

Alumina is incorporated as a sohd solution of the iron aluminate spinel, hercynite, in the crystal lattice. The alumina concentration should be less than the solubility of alumina in magnetite. This corresponds to a maximiun content of about 3% alumina. Any excess of alumina does not go into solid solution, and leads to a reduction in catalytic activity, particitlarly when using catalysts promoted with alumina. The presence of alumina as a structural promoter also leads to the formation of wustite and stabihzes the reduced catalyst. Small amounts of magnesia can also dissolve into magnetite and act as a promoter. The calcium component exists in the form of ferrites or alirminates by neutrahzing acidic components—such as silica—and protects the potash that activates the catalyst. 

Cunha-Duncan FW, Bradt RC. Synthesis of magnesium aluminate spinels from bauxites and magnesias. J Am Ceram Soc 2002 85(12) 2995. 

Chemical reactions between solids are used to synthesize mixed powders or complex oxides. The reactants are usually simple oxides, carbonates, nitrates, sulfates, oxalates or acetates, which are mixed according to a target compound with a given stoichiometric composition. An example is the reaction between magnesia and alumina to form magnesium aluminate or spinel, with the following reaction equation ... 

Methane or natural gas steam reforming performed on an industrial scale over nickel catalysts is described above. Nickel catalysts are also used in large scale productions for the partial oxidation and autothermal reforming of natural gas [216]. They contain between 7 and 80 wt.% nickel on various carriers such as a-alumina, magnesia, zirconia and spinels. Calcium aluminate, 10-13 wt.%, frequently serves as a binder and a combination of up to 7 wt.% potassium and up to 16 wt.% silica is added to suppress coke formation, which is a major issue for nickel catalysts under conditions of partial oxidation [216]. Novel formulations contain 10 wt.% nickel and 5 wt.% sulfur on an alumina carrier [217]. The reaction is usually performed at temperatures exceeding 700 °C. Perovskite catalysts based upon nickel and lanthanide allow high nickel dispersion, which reduces coke formation. In addition, the perovskite structure is temperature resistant. 

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