Alkaline earth aluminates

Table 3.4 Comparison of the properties of alkaline earth aluminate and zinc sulfide phosphorescent pigments...

Materials which have extremely low volatility and the potential to serve therefore as non-reactive substrates and supports include zirconia, titania, and alumina. The rare earth oxides, represented by lanthana and ceria, also show very low volatility and can be expected to resist vapor-transport-assisted sintering and corrosion. Of the alkaline earth oxides, MgO is superior to all others, although the alkaline earth aluminate complex oxides can be much more stable (e. g., by a factor of 1000 for BaO) than the alkaline earth oxides. 

Glasses and ceramics based on the Al203-based systems have important applications as ceramic materials, optical materials, and biomedical materials. Aluminate materials include alkaline earth aluminates, such as those in the CaO-A Cf system, which are refractory cousins of hydrous Portland cement [1-3]. Calcium aluminates have a role as both traditional ceramic and cement materials and are used for example as refractory cements however, calcium aluminates are also important for more novel applications... 

Aitasalo T, Holsa J, Jungner H, Lasmsaari M, Niittykoski J (2001) Mechanisms of persistent luminescence in Eu, RE " doped alkaline earth aluminates. J Lumin 94—95 59-63... 

Blasse G, Aguilar M (1984) Luminescence of natural calcite (CaCOa). J Lumin 29 239-241 Blasse G, Grabmaier B (1994) Luminescent materials. Springer, Berlin/Heidelberg/New York Blasse G, Wanmaker J, Vrigt J, Bril A (1968) Fluorescence of Eu " activated alkaline-earth aluminates. Philips Res Rpts 23 201-209... 

Chemical effects include stable compound formation and ionization, both of which decrease the population of free atoms in the sample vapour and thereby lower the measured absorbance. Examples of compound formation include reactions between alkaline earth metals and oxyanions such as aluminates, silicates and phosphates, as well as the formation of stable oxides of aluminium, vanadium, boron etc. 

At least four different explanations have been proposed to account for parabolic kinetics. The oldest and best established is the "protective-surface-layer" hypothesis. Correns and von Englehardt (6) proposed that diffusion of dissolved products through a surface layer which thickens with time explains the observed parabolic behavior. Garrels ( 12, 1 3) proposed that this protective surface consists of hydrogen feldspar, feldspar in which hydrogen had replaced alkali and alkaline earth cations. Wollast (j>) suggested that it consists of a secondary aluminous or alumino-silicate precipitate. In either case, a protective surface layer explains parabolic kinetics as follows If the concentration of any dissolved product at the boundary between the fresh feldspar... 

Tanaka et al. reported that gasoline POX over Rh, Pt, and Pt-Rh is promoted by alkali (Li) and alkaline earth metals (Ba, Ca, K) supported on magnesium aluminate spinel. The catalysts were tested isothermally at 800°C at an air to fuel ratio of 5.1 and GHSV 50,000 h Li, Mg and MgLi promoters were added to Pt supported on MgAl204 spinel. All catalysts produced similar H2 and CO reformate concentrations of 23 and 25 vol%, respectively. There was a discernable difference in the carbon deposition. The unpromoted Pt catalyst showed carbon levels of 0.02 wt% carbon, where the alkali and alkaline earth promoted Pt catalysts had carbon levels of 0.01 wt%. 

Alkali and alkaline-earth aluminosilicates are insoluble, and Muller et al. (138) resorted to tetramethylammonium (TMA) aluminosilicates to measure 27A1 chemical shifts in aluminosilicate solutions. Solutions with different Si/Al ratios and the pure TMA aluminate solution were studied. The molar ratio TMAOH Si Al varied from 3 0 2 to 9 6 2. Theoretically there are 15 distinct Q"(mSi) units with Q = Al (n 0-4 m 0-n). However, dimeric aluminate anions are found only in very concentrated solutions and even then in very small quantities, which led the authors to suggest that the Loewenstein rule is obeyed in aluminate and aluminosilicate anions. The exclusion of Al—O—Al linkages limits the number of possibilities to five... 

The methods of mechanochemical synthesis of various aluminates, including double compounds of alkaline, alkaline earth and transition metals, and aluminium, have been developed. The data on their synthesis, based on aluminium hydroxide, are presented according to this sequence. 

Cement binders are based on silicates, aluminates, aluminium silicates, aluminium ferrites of alkaline earth metals (mainly calcium). The hydration of these compounds or their mixtures with the formation of... 

Led by the efforts of Westerhausen and coworkers, a variety of heavy alkaline earth metal zincates or aluminates under formation of alkaline earth metal-carbon contacts have been prepared. Examples include the reaction of trialkylalanes with donor-free calcium i)fr-amides to afford dimeric aluminates, shown in Figure 30. Noteworthy is the planar Ca2N2 center of this complex, in addition to close calcium-aluminum contacts, which may indicate the presence of three-center, two-electron bonding. 

Chemical interferences occur when the analyte forms a thermally stable compound with a molecular or ionic species present in the sample solution. Examples include the suppression of alkaline earth metals due to the presence of phosphate, silicate or aluminate in the sample solution in the air-acetylene flame. The most well-known example of this is the absorption signal suppression that occurs for Ca at 422.7 nm owing to increasing amounts of phosphate. This signal suppression is due to the formation of calcium pyrophosphate, a thermally stable compound in the flame. 

Certain oxides, notably those of the alkali metals, the alkaline-earths, and lead, form well-defined aluminates and ferrites with general formulae M2O. w(Al, Fe)203 or MO. n(Al, Fe)203. Some of the simpler ferrites , for example, LiFe02, NaFe02,... 

High volatility limits the selection of many otherwise useful materials. Use of silica, all alkali, and alkaline earth metals higher than magnesium as components in any substrate or supporting material cannot be recommended. Catalysts with the least amount of volatility include palladium and iron oxides. It may also be possible to form complex oxides with other transition metal oxides, such as Mn, Ni or Co, which could have an acceptable activity and stability. For example, the formation of transition metal aluminates could lower volatility and increase sintering resistance, but at the expense of decreased activity. 

Use of a support made of an alkaline earth metal compound [3 If]. Tribasic calcium phosphate, calcium carbonate, calcium fluoride, strontium titanate and magnesium aluminate are optimal supports. 

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