Mullite structure

Figure 7.6. A. Na MAS NMR spectrum of the mullite-structured compound NaAl90 4 acquired at 8.45 T. B. Na DOR spectrum of the same compound acquired at 8.45 T. C. Observed and simulated 8.45 T MAS NMR spectrum of NaAl90 4 showing the separate simulated lineshapes of the two crystalline Na sites. The Gaussian lineshape on the left is from the residual amorphous precursor gel phase. From MacKenzie et al. (2001), by permission of the copyright owner.

In the presence of steam (at 760°C), HY crystals containing only about 2% V collapse, forming mullite, tridymite, and a vanadia-like phase. Incorporation of V into the mullite structure is believed to occur [21]. 

Table 1 Wyckoff positions and coordinates of atom sites for the orthorhombic mullite structure with space group Pbam (No. 55)...

The structure of mullite is similar to that of the sillimanites, consistent with the fact that they decompose to form mullite at high temperatures and 1 atm pressure. It has been suggested that the double A106 octahedral chain structure is preserved during the decomposition. The mullite structure is, however, somewhat complicated by its extensive stability over a wide range of stoichiometries. The composition of mullite can be expressed as... 

The combination of silica with alumina can retain transitional forms of alumina, as in the Altex fiber but the combination in the Nextel 480 fiber gives a mullite structure whereas the combination in the Nextel 720 fiber gives a mullite structure in which a-alumina grains are embedded. All three fibers, however lose strength above 1100°C, as shown in Figure 9. The fibers show very different creep behavior, as can be seen from Figure 10, with the Nextel 720 fiber showing the lowest creep rate of all oxide fibers. 

For technical information on clay, alumina, and mullite Structural Oxides I AI2O3 and Mullite . In Lee WF, Rainforth WM, eds. Ceramic Microstructures. Chapman and Hall, 1994. 

Use of the ceramic honeycomb packing structure in the recuperator keeps fuel and air substantially isolated as they travel through the recuperator. Various ceramic materials such as cordierite, mullite, alumina and silicon carbide can be used to fabricate honeycomb beds. While metallic materials have the potential to be used in honeycomb bed, corrosion resistance is a major issue... 

The nebulization was also employed to generate composite powders for specific applications, such as in ceramics, by hydrolyzing with water vapor droplets containing Al(5ec-OBu) and silicon methoxide in the atomic ratio Al/Si = 3. This ratio of alkoxides was chosen in order to produce mullite, which was achieved by calcination of the resulting amorphous particles at rather high temperatures (up to I400 C) (52). In another approach a mixed Al-Mg-Si ethoxide was first synthesized, and then nebulized and hydrolyzed as usual (77). Depending on the experimental conditions, the powders calcined at 500 C exhibited structures of pure cordierite, or mixed with forsterite. In all of these described cases the nebulization yielded spherical but polydisperse particles. 

Most structural PMCs consist of a relatively soft matrix, such as a thermosetting plastic of polyester, phenolic, or epoxy, sometimes referred to as resin-matrix composites. Some typical polymers used as matrices in PMCs are listed in Table 1.28. The list of metals used in MMCs is much shorter. Aluminum, magnesium, titanium, and iron- and nickel-based alloys are the most common (see Table 1.29). These metals are typically utilized due to their combination of low density and good mechanical properties. Matrix materials for CMCs generally fall into fonr categories glass ceramics like lithium aluminosilicate oxide ceramics like aluminnm oxide (alnmina) and mullite nitride ceramics such as silicon nitride and carbide ceramics such as silicon carbide. 

It is believed that, when steaming the gel at high temperatures, the V0+i attacks and breaks the Si-O-Al bonds promoting mullite formation and the collapse of the gel macroporous structure (3). The XRD pattern in Fig. 2B shows that mullite formation in the gel can be observed with only 1.5% V and when this occurs, there is a 81% decrease in surface area, Table 1. Mullite level increased with V-loadings, see Fig. 2. Data in the literature (20) indicates that when the steaming temperature is decreased to 730 C from 760 C (as in the present work) gel stability to V improved and only a 23% reduction in surface area was observed in a similar gel loaded with 1.5% V. Aluminosilicate gels are clearly less resistant than aluminas to V attack at hydrothermal conditions, Table 1. 

FIGURE 40 SEM images of acicular mullite monolith note the open structure. 

MuUite is another important crystalline ceramic that is an oxide It is a solid solution of alumina and silica in the compositional range 71-75 wt% alumina. Mullite is represented by the formula, 3 Al203.2Si02. It has excellent strength and creep resistance as well as low thermal expansion and conductivity. For more detailed information on the structure and properties of mullite the reader is referred to Schneider et al. (1994). Table 6.2 provides a summary of the properties of mullite. 

Kaolinite is transformed into X-ray amorphous state when activated in air. According to authors [14,15], amorphization involves the destruction of bonds between tetrahedral and octahedral layers inside the package, till the decomposition into amorphous aluminium and silicon oxides. Other researchers [ 16,17] consider that amorphized kaolinite conserves the initial ordering of the positions of silicon atoms while disordering of the structure is due to the rupture of A1 - OH, Si - O - A1 bonds and the formation of molecular water. Endothermic effect of the dehydration of activated kaolinite is shifted to lower temperatures while intensive exo-effect with a maximum at 980°C still conserves. When mechanically activated kaolinite annealed at 1(X)0°C, only mullite (3Al20j-2Si0j) and X-ray amorphous SiOj are observed. In this case, the phase with spinel structure which is formed under thermal treatment of non-activated kaolinite is not observed thus, mechanical activation leads to the formation of other phases. 

The heating of the mentioned mixture leads to the formation of the phase with spinel structure. This phase is transformed into mullite at 1100°C instead of 1400°C in conventional thermal method. Since synthesis temperature is much lower, thus formed mullite is in finely dispersed state... 

In industry, cordierite is usually obtained by calcination of the mixtures containing talc, kaolinite and silica at 1300-1450°C for 20-60 h. The product contains the impurity phases spinel, mullite, clinoenstatite, etc., that worsen the exploitation characteristics of cordierite. Since the mentioned minerals contain structural water, chemical interaction between them during mechanical activation can be considered from the viewpoint of soft mechanochemical synthesis. Mechanical activation of this mixture does simplifies the interaction between its components. It is sufficient to heat this mixture for 2 h at a temperature of 1260°C to obtain practically homogeneous cordierite without impurity phases (Fig. 7.2) [2-9]. 

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