Ceramic Formation with Iron Oxides

Wagh and Jeong [9] tested three compositions within this region, which are listed in Table 12.2. The acid was neutralized using K2CO3 to pH 2 and then the powders were mixed in the solution. The reaction was exothermic and spontaneous, but all pastes set well. During setting, a considerable amount of water was expelled from the samples. This water condensed on the surface of the samples. 

Note that the ratios Fe203 Fe given in Table 12.2 yield (Fe (aq)) = 0.1, 0.2, and 0.3. These numbers are close to the theoretically predicted range of 0-0.15 (Section 12.2). When a larger percentage of Fe was selected, the mixtures were hot due to the reaction of Fe, but did not set into ceramics. This implies that Fe content of only within or near the theoretically predicted range will form ceramics. 

The example of iron phosphate demonstrates applications of reduction mechanisms in forming ceramics of oxides that occur in higher oxidation states. There is only a very 

Scanning electron photomicrograph of iron phosphate ceramic. 

In spite of this limitation, the method is very useful, because it provides a means of forming a ceramic of one of the most common and inexpensive oxides. As discussed before, iron oxide is a component of lateritic soils and red mud, high-volume iron mine tailings, and machining swarfs. Thus, useful products of several mineral waste streams can be formed by the process described in this chapter. Development of ceramics using red mud and swarfs is discussed in Chapter 14. 

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It was shown in [18] that practically monophase fine barium hexaaluminate can be obtained by mechanical activation of a mixture of barium oxide with Y-AI2O3, which exhibits acid properties to a larger extent than a-Al203, and by consequent thermal treatments at increased temperature. The product then is grinded in the presence of water. The synthesis was shown to proceed almost completely after activation for 5 min in the AGO-2 planetary mill and thermal treatment at 1300°C for 1 h. Mechanical activation of the mixture of aluminium hydroxide with barium oxide, followed by thermal treatment at 900°C, results in the formation of the final product and a-Al203 as an admixture which remains even at 1300°C. Mechanochemical synthesis helped also to synthesize barinm hexaaluminate in which a part of aluminium cations is replaced with manganese, iron, cobalt cations. Such compounds are nsed as active ceramics in catalysis [17]. 

Nonstoichiometry may occur for some ceramic materials in which two valence (or ionic) states exist for one of the ion types. Iron oxide (wustite, FeO) is one such material because the iron can be present in both Fe " and Fe states the number of each of these ion types depends on temperature and the ambient oxygen pressure. The formation of an Fe " ion disrupts the electroneutrality of the crystal by introducing an excess -1-1 charge, which must be offset by some type of defect. This may be accomplished by the formation of one Fe " vacancy (or the removal of two positive charges) for every two Fe " ions that are formed (Figure 12.20). The crystal is no longer stoichiometric because there is one more O ion than Fe ion however, the crystal remains electrically neutral. This phenomenon is fairly common in iron oxide, and, in fact, its chemical formula is often written as Fei 0 (where x is some small and variable fraction substantially less than unity) to indicate a condition of nonstoichiometry with a deficiency of Fe. 

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