Mullite gels

Mechanochemical processing has also been shown by MAS NMR to be capable of converting a diphasic mullite gel into a more homogeneous precursor by forming Al-O-Si bonds (Figure 4.29B) (Temuujin et al. 1999). 

M. Schmuecker and H. Schneider, Structural development of single phase (type I) mullite gels, J. Sol-Gel Sci. Tech. 15, 191-199 (1999). 

Acosta S., Corriu RJ.P., Leclercq D., Hubert Mutin P., Vioux A. Monophasic pre-mullite gels prepared by a non-hydrolytic process. Mater. Res. Soc. Symp. Proc. 1994 346 345-350 Alder K.I., Sherrington D.C. Synthesis and characterisation of Ti(IV) and Zr(IV)-containing elastomeric polysiloxane networks a possible route to interesting heterogeneous catalysts. J. Mater. Chem. 2000 10 1103-1111... 

Intensity of crystallization peak in mullite gel is related to the conditions of hydrolysis, which control the homogeneity of the powder. (A) Solution hydrolyzed with excess water. (B) Thin layer of solution hydrolyzed by wet air. (C) Solution stirred and slowly hydrolyzed over 90 days. From Yoldas (103. ... 

Synthesis and phase development of same mullite gels. /. Am. Ceram. Soc.,... 

Mullite, 3 AI2O3 2 SiC>2, a high-temperature ceramic being considered for use in engines, can be made by the sol-gel method. 

Finally, it is worth noting that the mullite precursors shown above can be made at the kilogram scale in a matter of hours, whereas sol-gel derived materials can require up to one month to obtain atomically mixed gels. To date, no attempts have been made to form transition metal precursors via the OOPS or the Kemmitt and Milestone processes. However, the following section suggests that there are good reasons to explore this approach. 

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. 

Oxycations of V (V02 or VO ) can easily penetrate the gel microspace and break Si-O-Al bonds causing the collapse of the gel microstructure and mullite formation even at low (1.5%) V levels. Excess V generates an x-ray amorphous, V205-like species which is highly hydrated and less susceptible to dnemical reduction than bulk... 

Typical substances that find wide use as high-area supports include silica gel and y-alumina, which can be obtained with surface areas in the range 100-800 m2/g. Materials used as low-area supports ( 1 m2/g) include a-alumina and mullite (alumina-silica). It is not easy to make general statements about the preparation of industrial catalysts because of the great variety of forms they take, but in many cases one can distinguish between the chemical operations in which the various components are assembled in the desired form, and the fabrication step in which they are made into the desired shape. The first step will be illustrated by a description of the method of preparing of silica gel and y-alumina support material [1]. 

The effect of mechanical activation on the synthesis of mullite 3Al203 2Si02 from the mixtures of activated aluminium hydroxide and silica gel was studied by Klevtsov, Mastikhin, Krivoruchko et al. [36], According to NMR, the interaction between the initial components is observed at the stage of mechanical activation, hi activated mixtures, a transition of AP from octahedron into tetrahedron and pentahedron-tetrahedron ones is observed. The formation of mullite occurs under heating of activated mixtures at 1100°C, while for unactivated mixtures at 1400°C only. Lower temperature allows to obtain the product in fine disperse state (S=50 m /g). 

The effect of mechanical activation on mullite synthesis from the mixtures of aluminium hydroxide and silica gel was studied in [32,39]. They demonstrated that under heating of nonactivated mixtures, the components undergo phase transformations similar to those observed for individual compounds. Mullite is synthesized at 1400°C. 

The authors [33-36] carried out comparative investigations of mullite synthesis depending on the nature of initial reagents. The following mixtures were used for synthesis 1) a-AbOs + silica gel 2)... 

Al(OH)j + silica gel 3) Al(OH)j + fused Si02 and 4) Al(OH)j -i- silicic acid Si02-nH20. The reagents were taken in ratios corresponding to the composition of mullite, 3Al203-2Si02. 

FIGURE 12.2 (a) Comparison of the linear shrinkage of mullite (3Al203-2Si02) micronic powder, 600°C thermally treated gel (xerogel) and pristine gel. (After Reference 87.) (b) Plot of the R.T. microhardness for mullite and Nasicon (Na3Zr2Si3POi2) as a function of thermal treatment. (After Reference 27.)... 

FIGURE 12.9 (a) SEM of a sol-gel ZrOj coated SiC fiber reinforced glass-coMiic. (b) TEM detail of the SiC-mullite coating interface see the elongated rnnUite crystals (1 = 5-10 nm). 

Colomban, P, Protonic defects and crystallization of Sol-Gel (Si,Ge) mullites and alumina, Ceramics Today — Tomorrow s Ceramics, P. Vincenzini, Ed., Mater. Sci. Monogr, 66B, 599, 1991. 

Colomban, P, and Mazerolles, L., Nanocomposites in mullite-Zr02 and mullite-Ti02 systems synthesized through alkoxide hydrolysis gel routes. Microstructure and fractography, J. Mater. Sci., 26, 3503, 1991. 

Colomban, P., Bruneton, E., Lagrange, J.L., Mouchon, E., Sol-gel mullite matrix-SiC and -mullite 2D woven fabrics composites with or without zirconia containing interphase. Elaboration and properties, J. Eur. Ceram. Soc., 16, 301, 1996. 

FIGURE 13.1 Transmission electron micrographs of mullite composition gels (a) nanocomposite and (b) homogeneons (single phase). 

Thermal decomposition of two starting materials in inert organic solvent may provide a convenient route for the synthesis of mixed oxides or precursor of mixed oxides. For example, when a mixture of aluminum isopropoxide and tetraethoxysilane (tetraethylorthosilicate) in a 3 1 ratio is decomposed in toluene, an amorphous product is obtained." Note that thermal decomposition of the former compound alone yields %-alumina, while the latter compound alone does not decompose at the reaction temperature." Mullite is crystallized by calcination of the product at 900°C." It is known that the crystallization behavior of mullite from the precursor gel depends on the homogeneity of mixing of aluminum and silicon atoms in the precursor when the precursor gel has atomic scale homogeneity, mullite is crystallized at 900°C, and the gel with homogeneity in a nano... 

---