Spinels sodium 3-alumina

The structure of / -alumina is shown in Fig. 5. The aluminum and oxygen ions (green and red, respectively) form spinel blocks. The mobile sodium ions (blue) are located in layers between them. The spinel blocks are connected to each other by oxygen ion bridges within the conducting layer. 

Figure 5. Schematic representation of the ft -alumina structure. The aluminum (green) and oxygen (red) ions form spinel blocks which are separated from each other by oxygen bridges. The mobile sodium ions (blue) are located in the layer. The unit cell is indicated.

Fig. 7.17 Relation of the spinel structure (left) io the structure of sodium beta alumina (right). The sodium ions are free to move in the open spaces between spinel blocks, held apart by Al—O—Al pillars in the "parking garage structure. [In part from Wells. A. F. Structural Inorganic Chemistry, 5th ed. Oxford University Oxford, 1984. Reproduced with permission. ...

Another example of this type of intercalation compound is sodium beta alumina where the sodium ions are free to move between the spinel layers. The sodium ions can be replaced by almost any +1 cation such as Li. K, Rb+, Cs. NHJ, H 0 Tl+, Ga+, NO+, etc. The conductivity of these materials varies with the size of the ions moving between the fixed-distance (A)—0—Ai) layers. 

Fig. 8.6 Schematic diagram of -alumina showing loosely packed planes containing mobile sodium ions situated between spinel blocks of Al3+ and O2- ions...

Sintering is a critical step with schedules extending over typically 24 h and a peak temperature of approximately 1600 °C. At this temperature there is an appreciable sodium vapour pressure over beta aluminas and special precautions are taken to maintain the required composition. The component may be enclosed in a sealed spinel (MgAl204) saggar which can be re-used. 

Like diffusion, ionic conduction is a thermally activated process. Low activation barriers are, therefore, achieved in the same manner. Geometric features, such as open channels, result in larger diffusivities (easier ion movement) because this lowers the magnitude of the AH terms in Eqs. 6.38 and 6.39. For example, in /3 alumina, the sodium ions are located in sparsely populated layers positioned between spinel blocks. Accordingly, they diffuse through these channels easily owing to the large number of vacancies present. 

The silver analogue of /3-alumina appears to be quite different. Crystals studied by Roth contained about 25% more silver than the idealized formula AgAliiOi7 requires. The silver is to be found in the planes between the spinel layers, and the excess charge appears to be balanced by the insertion of extra oxygen atoms into this plane as well. This model is very different from that proposed for the sodium compound. 

In the /i"-alumina structure, the phase is stabilized at high temperatures by small amounts of monovalent (e.g., Li20) or divalent (e.g., MgO, ZnO, NiO) oxidesIn these stabilized structures, the cation dopant substitutes directly for trivalent aluminum ions in the spinel block (i.e., LiXi, MgAi) and is electrically compensated by additional sodium ions (Nai) in the conduction plane. 

Figure 7.5 Costal structure of sodium p-alumina [57] one of the two Na + -conducting planes, together with the two adjacent spinel blocks. Na ions = gray AIO4 tetrahedra and AIO, octahedra = white.

Cation-conducting electrolytes (see Chapter 7) are also used in electrochemical sensors, one of the most widely used being 3 alumina [55, 56]. The 3 alumina structure consists of relatively densely packed spinel blocks that are separated by less densely packed planes through which ionic conduction occurs. The most common example is sodium 3 alumina, which is a Na+ ion conductor, although the sodium can be exchanged with other ions to create electrolytes that conduct other cations [57], as well as other species, such as O [58] (see also Chapter 8). 

The discovery of a solid conductor of sodium ions by Kummer and Weber made possible the construction of sodium-sulfur cells which utilize molten or dissolved reactants separated by the ceramic electrolyte j3-(cf. Fig. 12), or, usually, j3"-alumina. The latter ceramic has a three Al-0 spinel block structure, a molar ratio of Al203-Na20 = 5, and contains 1-4% of MgO or Li20. The resistivity of the polycrystalline material at 350°C is about 5 H cm, -4 times lower than that of alumina. Other recently reported solid Na ion conductors containing phosphorus oxides do not seem to be stable in contact with sodium at elevated temperatures. ... 

The electrical and/or magnetic properties of a number of mixed oxides of A1 and other metals including members of the spinel family (see Box 12.6) and sodium P-alumina (see Section 27.3) have extremely important industrial applications. In this section, we single out Ca3Al205 because of its role in cement manufacture, and because it contains a... 

Evidence of spinel structure within some of the alumina and titania catalysts was established by x-ray powder techniques. Samples were removed from the reaction bed of some of the runs where high yields of sodium sulfate had been obtained using either alumina or titanium dioxide as catalyst. The presence of compounds having spinel type structure was established by x-ray powder diffraction. The amount of spinel was variable and of small quantity because neither the proportions of necessary ions nor the reaction conditions were ideal for spinel formation. 

Klissurski et al. [87] have examined the combustion of acetone, toluene and styrene by zinc-cobalt spinel oxides supported on alumina. Catalysts were prepared by co-precipitation with sodium carbonate from a mixed zinc/cobalt nitrate solution at pH 9. The supported catalyst was prepared by deposition of the precursor on Y-AI2O3 from a suspension in dimethylformamide and water. The supported precursor was dried at 150°C and calcined at 300°C to produce the catalyst. The bulk and supported catalysts both showed the formation of zinc cobaltite spinel structures which were thermally stable. Microreactor studies at 15,000 h- space velocity showed that the components of a mix of acetone, toluene and styrene were destroyed at 225°C, 280°C and 350°C respectively. The VOC concentrations were not specifically expressed but it is assumed that they... 

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